WO1996007950A1 - Optical power splitter with electrically-controlled switching structures - Google Patents

Abstract

A new class of optical energy transfer devices (11) and energy guiding devices uses an electric field to control energy propagation using a class of poled structures (36, 38) in solid material. The poled structures (36, 38), which may form gratings in thin film or bulk configurations, may be combined with waveguide structures. Electric fields applied to the poled structures through electrodes (24, 26) control routing of optical energy. Devices include splitters, parallel and Y couplers, mode converters and energy leakage attenuators.

Description

OPTICAL POWER SPLITTER WITH ELECTRICALLY-CONTROLLED SWITCHING STRUCTURES

BACKGROUND OF THE INVENTION This mvention relates to devices, particularly optical devices, for controlling propagation of energy, particularly optical beams, using electric field control. In particular, the mvention relates to devices with poled structures, including periodically poled structures, and electrodes which permit controlled propagation of optical energy in the presence of controlled electric fields applied between electrodes.

More particularly, the mvention relates to a new class of switchable energy conversion devices, energy guiding devices, filters, and bulk energy transfer devices based on the use of poled structures in solid state mateπal. In some applications, the poled structures can be switched electrically to control optical or even acoustic energy. A poled switch is especially applicable to the fields of laser control, communications, flat panel displays, scanning devices and recording and reproduction devices. Interactions with energy beams such as optical or acoustic beams can be controlled by means of applied electric fields in electro-optic (EO) or piezoelectric mateπals. An electπcally controlled spatial pattern of beam interaction is desired in a whole class of switched or modulated devices. Patterned responses can be achieved in uniform substrates using the electro-optic or piezoelectric effect by patterning the electπc field. However, Maxwell's equations for the electπc field prevent sharp field venations from extending over a large range. Some mateπals can be poled, which means their electro-optical and/or piezoelectπc response can be onented in response to some outside mfluence. In these mateπals, is possible to create sharp spatial vaπations in EO coefficient over potentially large ranges. By combining slowly varying electπc fields with sharply varying (poled) mateπal, new types of patterned structures can be fabπcated and used.

Potable EO mateπals have an additional degree of freedom which must be controlled, as compared to fixed EO crystals. Usually, the substrate must be poled into a uniformly aligned state before any macroscopic EO response can be observed. Uniformly poled substrates have been fabπcated both from base mateπals where the molecules initially have no order, and from base mateπals where the molecules spontaneously align with each other locally, but only within randomly onented microscopic domains. An example of the first type of mateπal is the nonlmear polymer. Examples of the second type of mateπal are sintered piezoelectnc mateπals such as lead zirconate titanate (PZT), liquid crystals, and crystalline ferroelectnc mateπals such as lithium mobate (LιNb0₃). Nonlmear polymer poling is descπbed in ♦

E.Van Tomme, P.P. Van Daele, R.G. Baets, P.E. Lagasse, "Integrated optic devices based on nonlmear optical polymers", IEEE JQE 2J. 778, 1991. PZT poling is descπbed for example in ♦ U.S. Patent 4,410,823, 10/1983, Miller et al, "Surface acoustic wave device employing reflectors". (Liquid crystal poling is descnbed m standard reterences. such as S. Chanαrase har. Liquid Crystals, Second Edition (1992), Cambπdge University Press, Cambndge.) Ferroelectπc crystal poling is descπbed ♦ U.S. Patent 5,036.220 07/1991, Byer et al., "Nonlmear optical radiation generator and method of controlling regions of ferroelectnc polanzation domains in solid state bodies". Examples of poled EO devices include:

♦ the beam diffractor in a polymer layer with interdigitated electrodes of S Ura. R. Ohyama, T. Suhara, and H. Nishihara. "Electro-optic functional waveguide using new polymer p-NAn-PVA for integrated photonic devices," Jpn. J. Appl. Phys. , 31, 1378 (1992) [UOS92];

♦ the beam modulator m a polymer layer with planar electrodes of U.S. Patent 5, 157,541 10/1992, Schildkraut et al. "Optical article for reflection modulation";

♦ the total internal reflection beam reflector in a lithium mobate waveguide with an electrode pair of H. Naitoh, K. Muto, T. Nakayama, "Mirror-type optical branch and switch", Appl Opt. VL, 101-104 (1978);

♦ the 2x2 waveguide switch in lithium mobate with two electrodes of M. Papuchon, Am. Roy, "Electncally active optical bifurcation: BOA", Appl. Phys. Lett. 31, 266-267 (1977); and ♦ the wye junction beam router in a lithium mobate waveguide with three electrodes of H. Sasaki and I.

Anderson, "Theoretical and expenmental studies on active y-junctions in optical waveguides", IEEE Joum.

Quant. Elect. , OEM. 883-892 (1978).

These devices use uniformly poled mateπal with vaπed electrode and optical structures.

Many of the advantages of patterned poled devices have not been recogmzed. For example, in the book by ♦ H. Nishihara, M. Haruna, T. Suhara, Optical Integrated Circuits, McGraw-Hill, New York (1989)

[NHS89], many electro-optical devices activated by vaπous electrode patterns are descπbed, but all of these devices are fabπcated on a uniformly poled substrate. The same is true of another review article, ♦

T. Suhara and H. Nishihara, "Integrated optics components and devices us g peπodic structures," IEEE J.

Quantum Electron. , QE-22, 845, (1986) [TH86], which descπbes the general characteπstics of grating coupled devices without recognizing the advantages of a poled grating as opposed to an electrode grating.

In selected instances in the literature, certain advantages of patterned poled substrates have been pointed out.

♦ A surface acoustic wave reflector with an array of domain reversals m a piezoelectπc ceramic (but no electrodes) is descnbed m U.S. Patent 4,410,823, Miller et al.; ♦ A beam steerer with tnangular domain reversed regions in LιTa0₃ is descnbed m Q. Chen, Y. Chiu,

D.N. Lambeth, T.E. Schlesinger, D. D. Stancil, "Thin film electro-optic beam deflector usmg domain reversal in LιTa0₃", CTuN63, CLEO'93 Conference Proceedings, pp 196 et. seq., Optical Society of Amenca.

♦ A Mach-Zehnder modulator with domain reversals to compensate phase differences between microwave and optical beams is descπbed in U.S. Patent 5,278,924, 01/1994, Schaffher, "Peπodic domain reversal electro-optic modulator". ♦ A Mach-Zehnαer electnc field sensor with one αomain reversed region in an electro-optic substrate is descnbed in U.S. Patent 5.267,336, 11/1993. Sπram et al.. "Electro-optical sensor for detecting electπc fields".

Use of patterned poled structures offers efficiency advantages in beam control (including generation, modulation, redirection, focussing, filtration, conversion, analysis, detection, and isolation) with applications in laser control: communications: data storage; and display. What is needed m these areas are adjustable methods for beam control with high efficiency. Due to the sharp domain transitions, higher efficiency devices can generally be obtained using pattern poled substrates to create the high frequency vaπations; the electrodes are needed to excite the patterned poled substrate, not to create the high frequency vaπations.

The polmg process in polymers is quite different from that of crystals, and results in poorly defined domain boundaπes. In crystals, there are a discrete number of (usually two) polmg directions which are stable, and poling a local region consists of flipping atoms between these alternative states. Poled regions are fully aligned, and shaφ boundaπes exist between oppositely aligned domains. In poled polymers, any molecule can be onented in any direction regardless of the poling direction. The polmg process produces only an average component of alignment within a random distπbution of individual molecules. In polymers, the poling (and the related EO coefficients) therefore have a continuous vaπation in strength and onentation. The sharp domain boundanes obtained in crystals are absent. This has a profound mfluence on the efficiency of certam types of poled device in polymers. Smce the polmg strength and direction in polymers follows the strength and direction of the local applied electπc field, it is not possible to obtain poling features with spatial dimensions any shaφer than permitted by Maxwell's equations. In polymers, there is very little advantage to be obtained from spatially patterning the poled regions instead of the electrodes.

In devices based on optical polymers, polmg is required to create an electro-optical response. The polmg is done by applying a voltage to electrodes fabπcated on the device (m the presence of heat). The entire polymer film may be poled with a uniform electrode, after which the electrodes are spatially patterned for the desired functionality. The EO performance of the device will not change much if the polmg is accomplished with the patterned electrodes, since the active region withm reach of the electπc field is still poled almost as well. The choice of whether to pole the whole layer or just the region under the electrodes is mainly by convenience in fabncation. Examples of polymer EO devices where the polmg is spatially patterned outside the active region of the device are ♦ the switched waveguides of U.S. Patent 4,867,516, 09/1989, Baken et al., "Electro-optically induced optical waveguide, and active devices compnsiπg such a waveguide", and ♦ U.S. Patent 5,103,492, 04/1992, Ticknor et al., "Electro-optic channel switch". None of these devices have the electrodes traverse multiple boundanes of a patterned poled structure.

The polmg process also changes the index of refraction ellipsoid in polymers. This fact has some desirable consequences, such as making possible waveguides fabπcated by polmg a stnpe of polable polymer as descπbed in ♦ J. I. Thackara, G F. Lipscomb, M. A. Stiller, A. J. Ticknor, and R. Lytel, "Poled electro-optic waveguide rormaiion in thm-rllm organic media. " Appl. Phys. Lett. , 52, 1031 (1988) [TLS88] and m ♦ U.S. Patents 5,006.235. 04/1991. and 5.007.696. 04/1991 , Thackara et al. "Electro-optic channel waveguide". However, it leaves a Droblem in that poled polymer boundaπes are lossy in their unexcited state (they scatter, diffract and refract). Devices in which a light beam crosses poled polymer boundaπes have the problem that although transparency may be achieved, the poled polymer must be activated electncally to produce a uniform index ot refraction. Poled crystalline devices do not have this problem because polmg does not change their index of refraction.

A solution to the problem of lack of transverse spatial definition in poled polymers was proposed in ♦ U.S. Patent 5,016,959 05/1991, Diemeer, "Electro-optical component and method for making the same", who descπbe a total internal reflection (TIR) waveguide switch m which the entire polymer film is poled, but the electro-optic coefficient of selected regions is destroyed by irradiation, creatmg unpoled regions with shaφ spatial boundanes. While the underlying molecules in these unpoled irradiated regions remain aligned, they no longer have any electro-optic response. This approach is useful in creatmg shaφ poled-unpoled domain boundaπes in polymer films. It has the disadvantage that it cannot produce reverse poled domains so its efficiency is considerably reduced compared to the equivalent crystal polmg technique.

In nonlinear frequency conversion devices, domains of different polaπty are typically peπodically poled into a nonlmear optic mateπal. but not excited by an electπc field. The poled structure penodically changes along the axis of the beam to allow net energy conversion despite a phase difference that accumulates between the two beams. This process is known as quasi-phasematching, and has been demonstrated in ferroelectncs [U.S. Patent 5,036,220, Byer et al.] such as lithium mobate, KTP, and lithium tantalate, as well as in polymers, as descπbed in ♦ U.S. Patent 4,865,406 09/1989, Khanaπan et al, "Frequency doubling polymeπc waveguide". Electrodes are not typically used in these devices, smce the phasematching occurs m the absence of an electπc field. Generalized frequency conversion m polymers is descπbed in ♦ U.S. Patent 5,061,028 10/1991, Khanaπan et al. "Polymenc waveguides with bidirectional polmg for radiation phase matching", as well as TE-TM modulation. Khanaπan et al. used patterned electrodes in both patents to pole the polymer film; the attendant loss in shaφness of the spatial pattern becomes a severe problem where more complex electrode structures are needed such as in the latter patent. Devices are known employ g peπodic structures which use electπc fields to control gratings in order to control propagatmg fields. A diffraction grating modulator is shown m ♦ U.S. Patent 4,006,963, 02/1977, Baues et al. "Controllable, electro-optical gratmg coupler". This structure is fabπcated by removing mateπal penodically in an electro-optic substrate to form a permanent gratmg. By exciting the substrate electro-optically, the fixed index grating has a greater or lesser effect, producing some tuning. This structure does not contain poled regions. The drawbacks of the Baues structure are the same as for the polymer film: the gratmg cannot be made transparent without the application of a very strong field. The current tecnnoio y tor an EO switchaole grating is snown in FIG. 1 (Pπor Art). In this structure, penodically patterned electrodes serve as the elements that define the gratmg. The underlying mateπal does not have a patterned poled structure, as hereinafter explained. An mput beam 12 is coupled mto a electro-optically active material 2 which contains an electrically controllable gratmg 6. When the voltage source 10 to the grating electrodes is off, the input beam continues to propagate through the mateπal to form the output beam 16. When the grating-controlling voltage source is switched on, an index modulation grating is produced in the mateπal. and a portion of the input beam is coupled mto a reflected output beam 14. The mateπal has an electro-optically active poled region 4 with a single domam, with the same polaπty throughout the poled structure. A first electrode 6 is lnterdigitated with a second electrode 7 on a common surface 18 of the substrate. When a voltage is applied between the electrodes, the vertical component of electπc field along the path of the beam 12 alternately has opposite sign, creatmg alternate positive and negative index changes to form a grating. The strength of the gratmg is controlled by the voltage source connected between the two electrodes by two conductors 8.

A second general problem with the existing art of EO and piezoeiectnc devices usmg uniform substrates and patterned electrodes is that the pattern of the excited electπc field decays rapidly with distance away from the electrodes. The pattern is essentially washed out at a distance from the electrodes equal to the pattern feature size. This problem is aggravated in the case of a gratmg because of the very small feature size. Pπor art gratings formed by lnterdigitated electrodes produce a modulated effect only in a shallow surface layer. EO structures interact weakly with waveguides whose dimension is larger than the feature size. While longer gratmg penods may be used in higher order interaction devices, the lack of shaφ definition descnbed above agam senously limits efficiency The minimum gratmg peπod for efficient interaction with current technology is about 10 microns. What is needed is a way to maintain the efficiency of EO devices based on small structures, despite a high aspect ratio (i.e. the ratio of the width of the optical beam to the feature size). Switchable patterned structures are needed which persist throughout the width of waveguides and even large unguided beams.

In bulk mateπal, gratings may be formed by holographic exposure and acoustic excitation. Holographic exposure is very difficult, and storage mateπals such as SBN are not yet developed to a commercial state. Acoustic excitation is very expensive to implement and to power, and requires additional components such as soft mounts and impedance matched dampmg structures. Other methods form surface gratings, including deposition techniques, mateπal removal techniques and mateπal modification techniques

(such as lndiffusion, outdiffusion. and ion exchange). What is needed is an approach capable of a large enough aspect ratio to produce bulk interaction structures, preferably with feature control at an accessible surface.

While the EO mateπal can m pπnciple be any electro-optically active mateπal, liquid crystals are a special case and have limited applicability. A light modulator based on diffraction from an adjustable pattern of aligned liquid crystal domains is descπbed in ♦ U.S. Patent 5,182,665, 01/1993, O'Callaghan et al. , "Diffractive light modulator". A light modulator based on total internal reflection modulated by liquid crystal domam formation is descπbed in ♦ U.S. Patent 4,813,771 03/1989, Handschy et al., "Electro-optic switchmg devices using feπoelectπc liquid crystals". In all of these devices, the domains must physically appear or disappear to produce the desired effect. The onentation of the molecules m the liquid crystal device changes in response to an applied field, producmg a patterned structure which interacts with light. However, liquid crystals have important drawbacks. They are of course liquid and more difficult to package, and they have a limited temperature range and more complex fabncation process than solid state devices. High aspect ratio structures cannot be made because of the decay of the excitmg field pattern with distance. The molecular onentation relaxes as soon as the field is turned off, and re-establishing the pattern takes a long time, so fast switchmg is not possible.

The structures which switch light from waveguide to waveguide in the pπor art have a high insertion loss or large channel spacing which render them unsuitable for large routmg structures. A large switchmg structure must have switchmg elements with insertion loss low enough to permit light to propagate through the structure. If a waveguide has 100 switches, for example, the switches must have less than about .03 dB insertion loss. In the pπor art this is not possible. R.A. Becker and W.S.C. Chang, "Electro-optical switchmg m thm film waveguides for a computer commumcations bus", Appl. Opt. 1$, 3296 (1979), demonstrate a multimode crossing waveguide array structure coupled via lnterdigitated electro-optic gratmg switches. This switch has an inherently high insertion loss (0.4 dB) and poor switchmg efficiency ( = 10%). U.S. Patent 5,040,864, 8/1991, J.H. Hong, "Optical Crosspomt Switch Module", discloses a planar waveguide structure which may in principle have a low insertion loss, but which requires very large crossmg junctions for efficient switchmg, and is therefore mcapable of producmg a high density switchmg array

In summary, the pnor art has shortcomings in several areas: 1) large aspect ratios of controllable patterns are needed for efficient interaction with bulk waves or small patterns; 2) shaφ domam transitions are needed for efficiency in higher order interactions; 3) transparency of domam structures is needed at zero applied field for proper unpowered operation; and 4) low insertion loss is required for arrays of switches. Poled structures contained in the above and other structures have not been fully utilized heretofore to realize practical devices.

SUMMARY OF THE INVENTION According to the mvention, a new class of optical energy transfer devices and energy guidmg devices uses an electnc field to control energy propagation usmg a class of poled structures m solid mateπal. The poled structures, which may form gratings m thm film or bulk configurations, may be combined with waveguide structures. Electnc fields applied to the poled structures control routmg of optical energy. Devices mclude splitters, parallel and Y couplers, mode converters and energy leakage attenuators. The mvention will be better understood upon reference to the following detailed descπption in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a modulator with interdigitated electrodes, according to the prior art.

FIG. 2 is a generalized embodiment of the switched grating for interacting with bulk optical beams, according to the invention.

FIG. 3 is an embodiment of a waveguide retroreflector using the switched grating.

FIG. 4 is an embodiment of an electrode configuration for the retroreflecting device with three electrodes disposed on the same face of the crystal.

FIG. 5 is an embodiment of an electrode configuration for the same device, in which two electrodes are disposed on the same face of the crystal.

FIG. 6 is an embodiment of an electrode configuration for the device, in which three electrodes with tapered separation are disposed on the same face of the crystal.

FIG. 7 is a tee embodiment of a poled crossing waveguide coupler.

FIG. 8 is an x embodiment of a poled crossing waveguide coupler. FIG. 9 is an embodiment of a poled waveguide output coupler, with output out of the plane of the waveguide.

FIG. 10 is an embodiment of a parallel waveguide poled directional coupler.

FIG. 11 is a top view schematic diagram of the an x crossing waveguide coupler with illustrations of alternative input and output mode profiles. FIG. 12 is an embodiment of an x crossing waveguide coupler with tapered coupling region geometry excited with a tapered electrode gap.

FIG. 13 is an embodiment of an x crossing waveguide coupler with generalized coupling region geometry and electrode pattern.

FIG. 14 is a bulk optics embodiment of a tunable-frequency poled electro-optic retroreflector.

FIG. 15 is a waveguide embodiment of a tunable-frequency poled electro-optic retroreflector.

FIG. 16 is a bulk optics embodiment of a tunable-frequency electro-optic retroreflector with electro-optic cladding and independent excitation of poled grating and cladding. FIG. 17 is a waveguide embodiment of a multiple frequency poled electro-optic retroreflector.

FIG. 18 is an illustration of a phase shifted poled grating.

FIG. 19 is an embodiment of a multiple period grating reflector.

FIG. 20 is an illustration of the frequency response curves of two devices with multiple periodicity and different free spectral range.

FIG. 21 is an embodiment of a twin grating tunable reflector.

FIG. 22 is a schematic illustration of an integrated etalon consisting of twin gratings with adjustable optical path length. FIG. 23 is an embodiment of a dual gratmg switchable wye junction with phase shifter.

FIG. 24 is an embodiment of a poled waveguide mode converter.

FIG. 25 is an embodiment of a waveguide router usmg the waveguide mode converter.

FIG. 26 is an embodiment of a switchable parallel waveguide resonator.

FIG. 27 is an embodiment of a three-arm waveguide etalon.

FIG. 28 is an embodiment of a ring waveguide etalon.

FIG. 29 A is an embodiment of a modulator/attenuator with controllable poled mid- structure.

FIG. 29B is an embodiment of an adjustable lens structure. FIG. 30 is an embodiment of a poled total internal reflectmg (TIR) waveguide switch with switched poled waveguide stub.

FIG. 31 is an embodiment of a dual TIR waveguide switch.

FIG. 32 is an embodiment of a TIR electπcally switched beam director with switched unpoled waveguide stub. FIG. 33 is an embodiment of a two position poled waveguide router without TIR.

FIG. 34 is an embodiment of an array of poled ΗR switches with a 50% switch packing density.

FIG. 35 is an embodiment of an array of poled ΗR switches with a 100% switch density.

FIG. 36 is an embodiment of a dual waveguide structure for high density packing architectures with permanent turning mirror and asymmetπc loss crossmg region.

FIG. 37 is an embodiment of a switched waveguide array with ΗR switches.

FIG. 38 is an embodiment of a switched waveguide array with gratmg switches.

FIG. 39A is an embodiment of an m x m commumcations switch array with system control lmes. FIG. 39B is an embodiment of a 3 x 3 switch array with WDM capability.

FIG. 40 is an embodiment of a two dimensional switchmg array with pixel elements.

FIG. 41 is an embodiment of a one dimensional switchmg array with pixel elements coupled to data tracks.

FIG. 42 is an embodiment of a switchable spectrum analyzer usmg selectable grating reflector sections and a detector array.

FIG. 43 is an illustration of a poled acoustic multilayer mterferometnc structure.

FIG. 44 is an illustration of a poled acoustic transducer.

FIG. 45 is an embodiment of a tuned coherent detector of multi-frequency light waves.

FIG. 46 is an embodiment of a low loss switchable waveguide splitter usmg a smgle poled region.

FIG. 47 is an embodiment of a low loss switchable waveguide splitter usmg multiple poled regions. FIG. 48 is an illustration of the key design elements for a 1 x 3 waveguide splitter. FIG. 49 is a multiple layer stack of active waveguide devices shown as an adjustable phased array modulator.

FIG. 50 is an embodiment of an adjustable waveguide attenuator of the prior art.

FIG. 51 is an embodiment of a multiple poled segment adjustable waveguide attenuator. FIG. 52 is an embodiment of a structure with widened bandwidth using an angle- broadened poled grating.

FIG. 53 is an embodiment of a structure with widened bandwidth using a curved waveguide.

FIG. 54 is an embodiment of an electrically controllable poled lens.

DESCRIPTION OF SPECIFIC EMBODIMENTS

This invention as claimed relates specifically to Figs 7, 8. 25, 46, 47 and 51. Other figures generally relate to the claimed invention Refemng to FIG. 2, there is shown a generalized embodiment of a device 11 of the present mvention, which is a patterned poled dielectπc device.

Essentially, this device is an electπcally-controllable stacked dielectπc optical energy redirector, or more succinctly, an electπcally-switchable mirror. In a preferred embodiment, the invention is a bulk optical reflector in a ferroelectπc crystal 20 of lithium mobate. The electπcally-controlled switchmg element is a poled gratmg 22, which consists of alternating poled domams of two types 36 and 38. A domam, which may be of any shape or size, is a physical region withm which certam mateπal properties are approximately constant. A poled domain is a region m a mateπal m which the molecular groups have a directionality and these groups are substantially aligned (or are partially aligned) m. or near, a direction called the poling direction. There are many types of domains mcludmg domams of aligned atomic structures in different directions, domains of aligned molecules or atomic structures with vaπous modified parameters such as the nonlinear activity or the electro-optic coefficient, domains of atomic structures with no preferred direction, domains defined by regions activated by different electrodes, poled regions m which the polmg direction vanes systematically across the region such as occurs m the case of polymers and fused silica poled with localized electrodes, domains of randomly onented molecules, and by extension, a random domain structure: domains of sub-domains which are randomly poled within the domam. A poled structure is a set of individual domains A patterned poled region is a region in a matenal m which the domams within the region have been poled accordmg to a spatial pattern, with more than one domain type. There may be a systematic offset between the poled pattern and the imposed pattern used dunng the polmg process, dependmg on the nature of this process. The boundanes of the pattern may also be somewhat irregular and not follow the imposed pattern perfectly, particularly if the polmg process is not under complete control. The device is descnbed as a patterned poled dielectπc because an electnc field is applied m controlling the device, so the mateπal must be a dielectπc in order to withstand the required field without damage. Typically, the poling process is also accomplished usmg an electnc field, which the matenal must also withstand. In general, we mean by dielectπc the capability of the mateπal to withstand the minimum electπc fields needed for the application. In operation, an optical input beam 40 is incident on and through the crystal, along an optical axis. The optical axis is normal to the phase front of the beam and is defined by the mean location of the propagatmg beam across its intensity profile at the phase front. The optical axis is straight in a uniform matenal, but may bend m several situations including curved waveguides, nonuniform media, and m reflective or diffractive structures. The mput beam 40 preferably has a sufficiently small spot size 21 throughout the crystal length so that it is not apertured by the crystal, causing undesirable power loss and mode conversion. In a bulk-interaction device such as is shown m FIG. 2, the domains 36 and 38 must penetrate a sufficient distance through the substrate 20 so that they overlap at least a portion of the mput beam 40. The gratmg 22 lies transverse of the put beam 40. This means the planes 34 of the gratmg 22 are transverse of the axis of the input beam 40. For two lines (or a line and a plane, or two planes) to be transverse of each other we mean that they are not parallel Since the grating is transverse of the beam 40, the beam passes through at least a portion of the structure of the gratmg 22

The optical beam 40 is deπved from an optical frequency source (not shown) and has a wavelength such that the beam is not substantially absorbed in the crystal, and such that the photorefractive effect does not distort the beam significantly The optical frequency source means may include one or more optical exciters capable of supplymg sufficient bnghtness within the wavelength acceptance of the gratmg reflector 22 to produce a useful switched output beam 44 The output beam may be coupled to other elements on the same substrate, or it may be coupled to external devices, m which case the output surface through which beam 44 emerges is preferably antireflection coated The antireflection coatmg may be a multilayer dielectπc coatmg, a smgle quarter wave layer of a matenal with almost the appropπate mdex of refraction, or a sol-gel coatmg. The exciter may be any light source mcludmg a laser, a light emitting diode, an arc lamp, a discharge, or even a filament, provided that the desired spectral bnghtness is achieved. The desired spectral bnghtness mav be supplied directly from one or more exciters, indirectly from one or more frequency converted (doubled, mixed, or parametπcally amplified) exciters, or m combmation with several of the above alternatives. Absoφtion effects will limit the wavelength to the range from about 400 to 4000 nm. The effect of the photorefractive phenomenon vanes with the configuration, the wavelength, dopants, and the poling structure, and we assume here that it has been brought under control so that any beam distortion remains within acceptable limits. The grating 22 is formed or defined by the boundaπes 34 between alternating domams of two different types. The first type of domam 36 has a different electro-optic (E-O) coefficient than the second type of domain 38, so that a uniform electπc field applied between the electrodes 24 and 26 results in different changes m the index of refraction m the two types of domams. Because the mdex of refraction changes the phase velocity of the wave, there is an impedance mismatch between the regions of different mdex or phase velocity. It is advantageous to accomplish such an mdex change with matenal m which the regions 36 have a reverse sense relative to the polmg direction of the other domam type 38 and the oπginal wafer 20, as shown by the polmg sense arrows 39, 41. By reverse sense we mean the poling direction is opposite to some reference direction. (An alternative realization of the field controllable gratmg is m an irradiated masked polymer film which has its E-O coefficient destroyed inside or outside the regions 36.) A uniform electπc field applied to the structure 22 produces a modulated mdex of refraction. The pattern of mdex modulation adds to the pre-existing mdex of refraction distnbution; the simplest configuration has no mdex modulation in the absence of the applied electnc field, and develops an mdex gratmg linearly in response to the applied field. A peπod 48 for the gratmg 22 is the distance between two domam boundaπes entirely mcludmg a region corresponding to each domain type. An alternative realization of the mdex of refraction gratmg is obtained by applymg a strain field to the poled regions. The photoelastic response of the matenal produces different mdex of refraction changes m the different poled regions. The strain field may be applied permanently by, for example, laying down a film on top of the substrate at a high temperature and then cooling to room temperature. A concentration of strain may be achieved by etching away a stripe of the film, for example. The poled elements 36 and 38 alternate across the grating 22 with no space between them. If additional domain types are available, more complicated patterns of alternation are possible with domains separated by variable distances of the different domain types. For some applications, the grating 22 is a uniformly periodic grating as shown in FIG. 2 so that the domain types contained in one period along the length of the grating 22 are reproduced in the other periods. For other applications, it is advantageous to modify the period to obtain advantages such as multiple spectral peaks or a broader spectral bandwidth. By grating we mean an array of distinguishable structures, including all possible variations of geometry and periodicity.

A periodic index grating is capable of supplying virtual .photons in an interaction between optical beams. This means the grating structure is capable of supplying momentum, but not energy, to the interaction. For an interaction to proceed, both energy and momentum must be conserved, and the grating is useful when a momentum increment is required to simultaneously satisfy the two conservation relations. The grating periodicity defines the momentum which is available to the interaction. The grating strength determines the "intensity" of the virtual photon beam. The number of periods in the section of the grating traversed by the optical beam determines the bandwidth of the virtual photon momenta which are available. Because of the bandwidth limitation, the interaction can only proceed within a specific range (or ranges) of optical frequencies. Grating devices are therefore inherently frequency selective, and typically operate around a nominal wavelength.

For example, in a simple reflection process at an angle, as illustrated in FIG. 2, the photons of the input beam 40 have the same optical frequency as the photons of the output beams 44 and 42, so energy conservation is observed. However, the momentum of the photons in input beam 40 and diverted output beam 44 are not the same; for the reflection process to occur, the change in momentum must be supplied by the grating 22 as illustrated by the vector diagram 43 associated with FIG. 2. The grating 22 supplies a virtual (with momentum but no energy) photon to the interaction to enable the conservation of momentum. The momentum vector associated with the i mode, k _; = 2τn_i/λj, is equal to the product of 2ιr times the effective index n, for that mode divided by the wavelength λ, f°^r that wave, and it points in the direction of propagation. The magnitude of the momentum vector is also called the propagation constant. In the case of a single period grating, the momentum vector k. = 2x/Λ points perpendicular to the grating surfaces, and it can have any wavelength value Λ which is present in the Fourier transform of the grating. The optical spacing (the width of the grating lines and spaces) associated with the propagation constant k. of a 50% duty cycle grating is therefore Λ/2. The frequency of interaction may also be tuned by adjusting for example the index of refraction of the optical beams, or the grating period by thermal expansion or other means. Depending on how a given device is implemented, an index structure may have a spectrum of wavelengths and vector directions which can be contributed to the interaction. Also, multiple virtual photons may be contributed to an interaction in a so-called "higher order" grating interaction. A "higher order" grating is one which has a period which is related to the required penod for momentum conservation oy division b\ an integer. The required momentum virtual photon is obtained from the harmonics of the higher order" grating. The condition that momentum be conserved by the process is commonly called the Bragg condition, so the gratings of this invention are Bragg gratings, and the incidence angle on the gratings is the Bragg angle for the m-band or resonant frequency component. This dual conservation ot energy and momentum is required for any energy beam interaction, whether the energy beam is optical, microwave, acoustic, or any other wavelike energy form consistmg of a time-vanable energy field. Only the implementation of the grating may change, to produce an impedance modulation for the different forms of energy so that the pattern of the structure can couple with the wavelike energy form. In FIG. 2, the mdex grating functions as a frequency-selective optical energy router or reflector. A beam of a charactenstic frequency withm the interaction bandwidth (capable of interacting with one or more of the virtual photons) is known as an m-band beam, while energy beams of other frequencies are known as out-of-band beams. The grating 22 has a frequency bandwidth which corresponds to the full width at half maximum of the reflection efficiency of the grating as a function of optical frequency. When the index grating is present (the grating is "on"), a beam having an optical frequency withm the bandwidth of the grating is reflected from the grating at the angle 46 around a normal 47 to the grating structure. An out-of-band beam transmits through the crystal along the same optical axis and in the same direction as the input beam, forming part of the transmitted output beam 42. An electπc field applied in the region mcludmg the grating controls the strength of the index modulation (which can also be thought of as the intensity of the virtual photons), adjusting the ratio of the power m the transmitted output beam 42 to that m the reflected output beam 44.

For a weak retro reflecting grating (which does not substantially deplete the input beam), the full width half maximum bandwidth Δλ is given by

Δλ = (l)

2.24nL

where λ = vacuum wavelength of the input beam, n — mdex of refraction of the beam, and

L = length of the grating.

For highly reflectmg gratings, the effective length is smaller than the total length of the gratmg, increasing the bandwidth.

The two types of domains may exhibit an index difference before an electπc field is applied. In this case, a permanent mdex grating accompanies the poled switchable mdex gratmg. As the electnc field is applied, the net modulation in the mdex of refraction (the gratmg strength) may be increased or decreased, dependmg on the polarity. The "grating off" situation (index grating value near zero) is then achieved at a specific value of applied field. The grating can then be turned "on" by applying any other field strength. If the polarity of the applied field is reversed, for example, an index grating is produced with twice the strength of the original permanent grating. The poled grating structure of our invention has two major advantages over the prior art.

First, the poled domain structures can have very shaφ boundaries, providing a strong Fourier coefficient at virtual photon momenta which are multiples of the momentum corresponding to the basic grating period. This is very useful in cases where it is impractical to perform lithography with the required small feature size. Second, strong index modulation gratings can be made even if the optical mode dimension is large compared to the grating period. This is not possible in a uniformly poled substrate excited by patterned electrodes, because the electric field modulation decays exponentially with distance away from the plane of the electrode array, losing most of the modulation within a distance equal to the grating period. The poling process can create poled features with an extremely high aspect ratio, or the ratio of depth of the domain to its width. Using an electric field poling technique, aspect ratios in excess of 250: 1 have been fabricated. Because we use essentially uniform electrodes, we get good electrostatic penetration; with deep domain walls, good modulation is available across the entire beam.

The grating may also be a two dimensional array of index changes, in which case the grating has periodicities in two dimensions. The virtual photon contributed by the grating can then contribute momentum in two dimensions. This might be useful, for example, in an application with several output beams from a single grating.

In the preferred embodiment, the ferroelectric crystal is a commercially-available, z-cut, lithium mobate single-crystal wafer. Other cuts, including x-, y-, and angle-cuts can also be used, depending on the poling method and the desired orientation of the poled domains. The fabrication steps include primarily poling and electrode fabrication. Prior to processing, the crystal is cleaned (for example by oxygen plasma ashing) to remove all hydrocarbons and other contaminants remaining from the polishing and handling processes. To control the poling, a mask and processing electrodes are used to create a pattern of applied electric field at the surface of and through the wafer, as described in U.S. Patent Appl. No. 08/239,799 filed May 9, 1994. The poling pattern is adjusted to produce the poled domain inversion in regions 36 during the application of the poling field. In brief, a silica layer several microns thick is deposited on the +z surface 23 of the wafer 20. This film is thinned or removed over the regions 36 where domain inversion is desired, a liquid electrode or deposited metal film is used to make a good equipotential surface over the patterned silica, and an electric field exceeding approximately 24 kV/mm is applied with the +z surface 23 at a higher potential than the - z surface 25. Using this technique, ferroelectric crystals of lithium niobate have been poled to create patterns of two domain types which are of reverse polarity (domain inversion). The magnitude of the electro-optic coefficient for the two types of domains is identical, although with a reverse polarity.

In addition to the preferred technique, domain inversion has been achieved in ferroelectrics using in-diffusion, ion-exchange, and alternate electric field poling techniques. Domain formation oy tftermaiiv-ennanceo in-diffusion nas oeen demonstrated in nthium mobate. usmg titanium. The tnanguiar snape or the inverted region limits the interaction efficiency for small domam size, however, and is usetui mainiv in waveguide devices with long peπoαs. Patterned polmg via ion exchange has been demonstrated in KTP in a sait bath containing rubidium and baπum ions, in which the potassium ions in the crystal were excnangeo for the rubidium ions. Electπc field poiing usmg alternate techniques to the preferred one nave also oeen demonstrated in both lithium mobate and lithium tantaiate. Potentially, all solid ferroeiectπc mateπals. mcludmg KTP and baπum titanate. can be poied by electnc field domain- inversion techniques. (Solid means holding its structure for a certam penod of time, such as cooled fluids, glasses, crosslinked polymers, etc.) Gratings with different characteπstics are generated by the different techniques. Electnc field polmg aligns the domains m the crystal without producmg an intrinsic change in the index of refraction, while trie ion-exchange and diffusion techniques do create a mdex change m the poled regions. A permanent mdex gratmg accompanies the switchable poied gratmg when these latter methods are used. n general, there are two types of differing domams. at least the first type of which is poled. Although oniy two types of domams are required, more complex switchable gratmg structures can be fabricated with additional types of domains. The second domam type may be reverse poled, unpoled, or poled at another angle, and it may be distinguished by possessmg a distmct electncal activity coefficient, (e.g. the electro-optic or the piezo-optic coefficient). For example, it may in some applications be cost effective to fabncate the device from unpoled lithium mobate wafers, m which case the substrate wafer is compπsed of multiple randomly onented domains. The poied domams will have a uniform onentation while the oπenution in the other domains will be random. The performance of the device will be affected by the details of the random pattern, dependmg on the type of device. As another example, the second domains may be onented peφendicular to the first or at another angle, and the difference in the electncal response can still nroduce a useful electronically controlled structure. The poled domams may also be formed m a mateπal which was previously unpoied and randomly onented on a molecular scale, such as in fused silica or polymers. The polmg process oπents the structure of the material to form the first domam type, while the second domam type consists of the unpoied or randomly onented regions in the material. In an alternate technique, the poled structure can be formed by selectively changing or destroying the electncal activity coefficient in regions corresponding to the second domain type. The orientation of the atomic structures in these regions does not need to be altered: if the electrical activity is changed in the second domam region, the domams are different. For example in nonlinear polymers, the electro-optic coefficient may be disabled by irradiation, producing regions of electncal activity where the irradiation is masked off. A similar effect has been demonstrated in lithium mobate, where proton exchange destroys the nonlinear coefficient. Modification of the electro-optic coefficient can also be achieved by optical radiation, electron bombardment, and/or ion bombardment in many other mateπals, including most nonlinear mateπals such as KTP and lithium tantaiate.

In lithium mobate. an applied field E, along the z axis of the crystal induces a change in the extraordinarv mdex or refraction on. which is eiven bv ^^"33 *-"Jl _e on, = - (2)

where r,₃ is the appropπate electro-optic nonlmear optical coefficient. Because r_?3 is the largest nonlinear constant m lithium mobate, it is best to use the change in the extraordinary mdex in practical devices. (The nonlinear constant r„ which produces a change in the ordinary index of refraction due to an applied E₃, is a factor of 3.6 smaller than r₃₃.) To use the change in the extraordinary index, the light waves must be polarized along the z axis of the mateπal. In a z-cut crystal, this polarization is called TM. (In TE polarization, the electric vector lies in the plane of the crystal surface. The only other significant nonlinear coefficient is r₁₅, which couples TE and TM waves upon the application of an electric field E, or E-.)

Because the index change induced in the poled structures is quite small (with an applied field of 10 V/μm along the z axis of a lithium mobate substrate, the mdex change on. is only l.όxlO⁵), the gratmg reflector of FIG. 2 has a strong angular dependence. The Brewster angle for a weak index change is 45°, so the gratings will totally transmit any TE polarized wave when the planes of the grating are disposed at and angle of 45° with respect to the phase front of the light beam. The device may therefore be used as a polarizer. The reflected beam will always be essentially polarized at 45° incidence. If the reflection coefficient for the TM wave is high, which can be arranged with enough grating periods and a high applied field, the extinction ratio of the polarizer can also be very high in the forward direction. At normal incidence, of course, there is no difference in reflection between the two polarizations due to this effect (although there are differences due to other effects such as the different electro-optic coefficients described above). A total internal reflection device operating at grazing incidence is far from Brewster's angle and has little difference in reflection due to this effect. The wafer matenal can be any polable solid dielectπc material, including ferroelectrics, polymer films, and some amoφhous materials such as fused silica which can also be poled for producing many useful devices according to the mvention. The poled material may also be a thin film deposited on a substrate of a second material. Many of the polable thin films, such as fused silica, lithium niobate, potassium niobate, barium titanate, zinc oxide, II-VI materials, and vaπous polymers, have been successfully deposited on a substrate. A wide variety of substrates have been used, including MgO, silicon, gallium arsenide, lithium niobate, and vaπous glasses, including quartz and fused silica. For the domains to be electronically switchable, they must consist of electro-optic materials, which are materials having an index change induced by an applied electric field.

After the poling step, the liquid electrode mateπal and silica masking film are preferably removed. Referring again to FIG. 2, a first electrode 24 and a second electrode 26 confront the dielectric material in order to provide a means to create the electric field which controls the grating. (Confronting a mateπal means placed close to the material but not necessarily touching, approximately aligned to the surface of the material but not necessarily with a constant gap dimension, and includes situations with additional material of varying dimensions placed on top of the matenal.) The electrodes 24 and 26, consistmg of an eiectncaily-conouctive mateπai. _;re Drereraoiv laid out on opposins surfaces of the crystal in a spatially delimited manner usmg standard deposition techniαues. These electrodes are referred to as being on opposing pianes even tnougn me surfaces may oe curved and/or non-parailel as part of a larger geometry. The electrodes may be formed by anv matenal that provides sufficient transport of electrical charge to achieve an adequate field strength to activate the poied gratmg in a time consistent with the application. For example, the electrodes could alternatively consist of metals sucn as aluminum, gold, titanium, chromium, etc., conductive paint, epoxy, semiconducting mateπal. or optically transparent mateπals such as oxides of indium and tm. and liquid conductors such as salt solutions. They may also confront the surfaces 23 and 25 with a gap filled with air. an optically transparent buffer layer, and/or other matenal. Only one electrode is required smce a potential voltage difference can be created between that electrode and any potential reference such as an extenor ground plane, a second electrode, or multiple electrodes. The electrodes are the eiectπc field creatmg means because the application of a voltage to an electrode establishes an eiectπc field pattern which is determined by the electrode. A voltage and current supply is of course also needed. The eiectrooes are placed so that the control eiectπc field is applied through the active volume of the mvention. wruch may consist of a pattern poled region or a gratmg.

In the case of metallic electrodes, it may be best to incoφorate a coatmg deposited below the electrode, to reduce the optical loss which occurs when a portion of the guided wave mode extends to the metallic electrode. The coatmg should be thm enough to maintain high electnc field at the surface in the case of multiple electrodes mounted on the same surface, but thick enough to reduce the optical loss. Another coatmg is also useful above the electrodes to reduce the probability of breakdown.

A voltage control source 32 (or potential source) provides the electncal potential to drive the electrodes through connections 30 to activate the gratmg. The activated electrodes are polarized relative to each other according to the polantv of the applied voltage. The voltage of the source produces a large enough electπc field through the poied regions to switch a significant amount of light mto the switched output beam 44. The voltage of the source is vanable to provide a means to control the ratio of power m the two output beams. Substantially all of the mput beam may be reflected with a long gratmg if the electnc field is sufficiently high, forming an electrically activated mirror. For lower electric fields, the gratmg forms a partial reflector. The voltage control source may be a battery, an electrical transformer, a gas powered generator, or any other type of controllable source of electncal current and potential. The control means 32 may also incorporate a controller which generates a time dependent voltage, and which supplies the current to change the voltage on the electrodes 24 and 26 at the frequencies required by the application. The control means 32 may also have multiple outputs capable of controlling multiple devices, and which might be sequenced temporally according to some pattern. The source 32 may have control inputs for manual or electronic control of its function by computer or by another instrument. In order to avoid unnecessary repetition, it should be understood that the variations described in reference to FIG. 2 appiy to the embodiments descnbed below, and that the vaπations descπbed in reference to the figures below also apply to FIG. 2. Referring now to FIG. 3, a guided-wave embodiment of the present mvention is shown. Specifically, this embodiment is an electrically-controlled, frequency-selective waveguide retroreflector. All of the optical beams in this device are confined in two dimensions by an optical waveguide 64, which traverses one surface of the polable dielectric material that forms the substrate 60 of the device 61. A waveguide is any structure which permits the propagation of a wave throughout its length despite diffractive effects, and possibly curvature of the guide structure. An optical waveguide is defined by an extended region of increased index of refraction relative to the surrounding medium. The strength of the guiding, or the confinement, of the wave depends on the wavelength, the index difference and the guide width. Stronger confinement leads generally to narrower modes. A waveguide may support multiple optical modes or only a single mode, depending on the strength of the confinement. In general, an optical mode is distinguished by its electromagnetic field geometry in two dimensions, by its polarization state, and by its wavelength. The polarization state of a wave guided in a birefringent material or an asymmetric waveguide is typically linear polarized. However, the general polarization state may contain a component of nonpaπulel polarization as well as elliptical and unpolarized components/particularly if the wave has a large bandwidth. If the index of refraction difference is small enough (e.g. Δn= .003) and the dimension of the guide is narrow enough (e.g. W=4 μm), the guide will only confine a single transverse mode (the lowest order mode) over a range of wavelengths. If the waveguide is implemented on the surface of a substrate so that there is an asymmetry in the index of refraction above and below the waveguide, there is a cutoff value in index difference or waveguide width below which no mode is confined. A waveguide may be implemented in a substrate (e.g. by indiffusion), on a substrate (e.g. by etching away the surrounding regions, or by applying a coating and etching away all but a strip to define the waveguide), inside a substrate (e.g. by contacting or bonding several processed substrate layers together). In all cases, we speak of the waveguide as traversing the substrate. The optical mode which propagates in the waveguide has a transverse dimension which is related to all of the confinement parameters, not just the waveguide width.

The substrate is preferably a single crystal of lithium niobate, forming a chip which has two opposing faces 63 and 65 which are separated by the thickness of the wafer. The opposing faces need not be parallel or even flat. The waveguide is preferably formed by a well-established technique such as annealed proton exchange (APE) on face 63. Alternatively, ions other than protons may also be indiffused or ion exchanged into the substrate material. The APE waveguide increases the crystal extraordinary refractive index, forming a waveguide for light polarized along the z-axis. For a z-cut crystal, this corresponds to a TM polarized mode. Waveguides formed by alternate techniques, such as titanium in¬ diffusion in lithium niobate, may support both the TM and TE polarizations.

Preferably, the waveguide is designed to support only a single lowest order transverse mode, eliminating the complexities associated with higher order modes. The higher order transverse modes have different propagation constants than the lowest order mode, and higher scattering loss, which can be problems in some applications. However, multimode waveguides might be preferred for some applications, such as for high power propagation. One alternative configuration is to excite the gratmg oy applying pressure rather than by directly applying an eiectπc field. The effect of an applied Dressure is indirectly the same: by the piezoeiectnc effect, tne applied stress produces an electπc field, which in turn cnanges the mdex of refraction of the domains. However, no sustaining energy need be appiied to maintain the stress if the structure is compressed mechanically, for example. This alternative, like the others mentioned herein, jppiy also to the other similar realizations of the mvention descπbed below.

Once the waveguide dimensions are determined, a photomask for the waveguide is generated and the pattern is transferred to a masking mateπal on the substrate, by one of many well known lithographic processes. The mask material may be SiO-, tantalum or other metals, or other acid resisting mateπals. To fabncate an APE waveguide, the masked substrate material is immersed in molten benzoic acid to exchange protons from the acid for lithium ions in the crystal. The resulting step index waveguide may then be annealed for several hours at around 300 °C to diffuse the protons deeper mto the crystal and create a low-loss waveguide with high electncal activity coefficients.

In addition to i -diffusion and ton exchange rwo-dimensional waveguides, planar and two dimensional ndge or stπp-loaded waveguides can be formed. Planar waveguides may be formed by depositing the electπcally active material on a substrate of lower mdex. Deposition techniques for waveguide fabπcation are well-known and include liquid phase epitaxy (LPE), molecular beam epitaxy (MBE), flame hydrolysis, spinning, and sputtering. Ridge waveguides can be formed from these planar guides by usmg processes such as lift-off, wet etch, or dry etch such as reactive ion etching (RIE). Planar guides can also be used in the present mvention. particularly in devices using a variable angle of diffraction off the gratmg.

The gratmg 62 in this embodiment is disposed normal to the optical waveguide 64 which traverses the substrate. The gratmg is composed of a first type 66 and second type 68 of domam, which do not necessanly extend through the substrate. For example, when the active matenal is poled using in- diffusion or ion exchange, the inverted domams 66 typically extend to a finite depth in the mateπal. The partial domains may also be formed when the poling is achieved by destroying the electrical activity of the material (or reducing the electro-optic activity) by a technique such as ion bombardment or UV irradiation.

The optical input beam 80 is incident on and is coupled into the waveguide. Coupling refers to the process of transferring power from one region mto another across some kind of generalized boundary such as across an interface, or between two parallel or angled waveguides, or between a planar guide and a stπpe guide, or between smgle mode and multimode waveguides, etc. When the grating is on, a portion of the input beam is coupled back into a retroreflected output beam 82. While the retroreflection of the grating need not be perfect, i.e. the gratmg may reflect the light to withm a few degrees of the reverse direction, the waveguide captures most of this light and forms a perfectly retroreflected beam. The imperfection of the retroreflection results m a coupling loss of the retroreflected beam mto the waveguide

64. When the gratmg is off (when the controlling electrical field is adjusted to the "off" position in which the mdex gratmg has a minimum value near zero, typically at zero field), the mput beam continues to propagate in the same direction through the waveguide to form a transmitted output beam 84. As in the bulk device, the strengtn of the gratmg can oe vaπeα with tπe voltage source 76 to control the ratio of the power in the two output beams.

A first electrode 70 and second 72 electrode confront opposing laces of the dielectπc mateπal 60. The substrate is a dielectπc because it is capable of withstanding an applied electnc field without damage, but it need not be a perrect insulator as long as the current flow does not adversely affect the performance of the device. The electrodes may ce formed of any eiectπcaily conductmg matenal. There must also be a means for creatmg an eiectπc field through the dielectπc mateπal usmg the first electrode structure.

The electrodes bridge at least two of the elements of the first type of poled structure that forms the gratmg. This means the eiectπc field produced by the electrodes penetrates into at least the two elements. Thus, these elements can be activated by the field. Two wires 74 preferably connect the voltage control source 76 to the two electrodes to provide an electπc field in the region formed by the intersection of the waveguide 64 and the poled structure 62. The wires may be formed from any material and in any geometry with sufficient conductivity at the operating frequency to allow charging the electrodes as desired for the application. The wires may be round, flat, coaxial cables, or integrated lead pattern conductors, and they may be resistors, capacitors, semiconductors, or leaky insulators.

Alternately, the electrodes can be arranged in any manner that allows an electric field to be applied across the electπcally active mateπal. For example, the electrodes may be interspersed in different layers on a substrate, with the active material between the electrodes. This configuration enables high electnc fields to be produced with low voltages, and is particularly useful for amoφhous active mateπals. such as silica and some polymers, which can be deposited over the electrode material.

The poied structure 62 is preferably deeper than the waveguide so that the intersection between the waveguide 64 and the poled structure 62 has the transverse dimensions of the mode in the waveguide and the longitudinal dimensions of the gratmg. FIGS. 4. 5 and 6 show alternate electrode configurations m which the electrodes are disposed on a common face of the dielectπc mateπal 189. These configurations are especially useful for embodiments of the present invention that use a waveguide 180 to guide an optical beam, since the same- surface electrode configurations permit high electric fields at low voltage. These electrode structures are of particular interest for low voltage control of the gratmg 182 because of the proximity of the electrodes to the section of the waveguide which traverses the gratmg. In the electrode configuration 186 depicted in

FIG. 4, the first electrode 170 and second electrode 172 confront the dielectπc matenal on the same surface. These electrodes are referred to as being on a common plane even though the surface may be curved as part of a larger geometry. The first electrode is placed above a portion of the waveguide that contains several grating elements, each of which consists of alternate regions of a first type of domam 184 and a second type of domam 185. The second electrode is positioned around the first electrode. The distance between the electrodes along the waveguide is approximately constant along the axis of the waveguide for cases where a uniform field along the axis of the waveguide is desired. The electrode spacing may also be vaπed to taper the field strength, as shown schematically m the device 188 of FIG. 6. voltage source ι74 connected oetween tne two eiectrooes αistioseα as snown in FIG 4. is capable or generating electnc fields between tne eiectrooes The eiectπc field vectors 176 have their largest component perpendicular to the suπace ot the mateπal, in tne region or the eiectncaily-active waveguide. For a z-cut ferroelectπc crystal sucn as lithium niooate, tnis eiectπc field structure activates the largest electro-optic coefficient r-.,, creatmg a cnange in mdex tor a TM poiaπzeo optical beam. For an applied eiectnc field of 10 V/μm ana an ootical beam witn a waveiengtn or 1.5 urn m lithium mobate, the strength of a first order gratmg is 40 cm '.

A means 178 for contacting the eiectrooes to a voltage source is required for each of the electrode configurations. To form this means, an eiectπcailv conducting mateπal. such as a wire, is electncally contacted between the electrodes on the device and the terminals of the potential source. In all electrode configurations, each electrode typicailv has a section, or pad, or contact, to which the wire is contacted. The pads are prereraoly or large enough size to reduce placement tolerances on the electncal contact means for easier bonding. The wire can then De contacted to the pads usmg a technique such as wire bonding by ultrasonic waves, neating, or conductive epoxy. Alternately, a sonng-loaded conductor plate can be oiaceα m direct contact with the electrode to make tne required electncal connection to the voltage source. In the figures, the electrodes are typically large enough and function as the contact pads by themselves.

Another realization 187 of the same-surface electrode structure is shown m FIG. 5, wherein the first electrode 171 and second electrode 173 are placed on either side of the optical waveguide. When an eiectπc potential is applied across the two electrodes positioned in this manner, the electπc field vectors 177 have their largest component parallel to the substrate surface. For a z-cut ferroelectπc crystal, the electro-optic coefficient that creates a change m mdex tor a TM polaπzed optical wave and the applied electπc field is r,₃. For an applied eiectπc field of 10 V/μm and an optical beam with a wavelength of 1.5 μm m lithium mobate. the first order gratmg coupimg constant is 12 cm ' .Alternately, tor TE waveguides the active electro-optic coetficients are switched for the two configurations. For an electπc field vector peφendicular to the surface of the chip, the appropπate coefficient is r_l3, while for an electπc field vector parallel to the surface of the chip, the electro-optic coefficient used is r₃₃. Similar situations apply for x- or y-cut crystals, or intermediate cuts.

As a further vanation of the configuration of FIG. 5, the electrodes are asymmetrically arranged so that one electrode approximately covers the waveguide 180 and the other electrode is displaced somewhat to the side. In this configuration, the strong vertical field mduced under the edges of the adjacent electrodes is made to pass predominantly through the waveguide region under one of the electrodes.

In FIG. 6, the electrodes 175 and 179 have a separation from the center electrode 181 which is tapered. When a voltage is applied across these electrodes, this configuration produces a tapered field strength, with the strong field towards the πght and the weaker field towards the left. By "tapered" we mean that any parameter has a generalized spatial vanation from one value to another without specifying wnether me vanation is linear or even mono tonic: the parameter may be a gap, a width, a density, an index, a thickness, a duty cycie. etc. The mdex cnanges induced in tne poied domains towards ' the left of the waveguide 180 are therefore weaker than the mdex cnanges induced towards the πght. This might be useful, for example, to obtain a very narrow bandwidth total reflector where it is needed to extend the length of the interaction region. In non-normal incidence angie devices, such as shown m FIG. 7 and FIG. 8. the taper might be useful to optimize the coupling or a specific mput mode mto a specific output mode.

In all electrode configurations, the voltage applied can range from a constant value to a rapidly varying or pulsed signal, and can be applied with either polarity applied between the electrodes. The value of the voltage is chosen to avoid catastrophic damage to the electπcally-active material and surrounding materials in a given application.

When a constant electric field is applied across materials such as lithium niobate, charge accumulation at the electrodes can cause DC drift of the eiectπc field strength with time. The charges can be dispersed by occasionally alternating the polantv of the voltage source, so that the electric field strength returns to its full value. If the time averaged electnc field is ciose to zero, the net charge drift will also be close to zero. For applications sensitive to such dπft. care shouid be taken to minimize the photorefractive sensitivity of the mateπal. such as by ln-diffusion of MgO, and operation is preferably arranged without a DC field.

Surface layers are useful for preventing electπc field breakdown and lossy optical contact with the electrodes. Losses are particularly important for waveguide devices, since the beam travels at or near the surface, while breakdown is most critical when electrodes of opposite polarity are placed on the same surface. This concern applies to the poling of the active material as well as to the electro-optic switching. The largest vector component of the electπc field between two same-surface electrodes is parallel to the surface of the material. Both the breakdown problem and the optical loss problem can be considerably reduced by depositing a layer of optically transparent material with a high dielectric strength between the guiding region and the electrodes. Silicon dioxide is one good example of such a material.

Since there is also a potential for breakdown in the air above and along the surfaces between the electrodes, a similar layer of the high-dielectric-strength material can be deposited on top of the electrodes.

FIG. 7 and FIG. 8 show two embodiments of a electrically-controlled frequency-selective waveguide coupler. In FIG. 7, a pair of two-dimensional waveguides traverse one face of a dielectric matenal. and intersect at an angle 118 to make a tee, forming a three-port device. A gratmg 100, consistmg of a first type 104 and second type 102 of domains, is disposed at an angle to the two guides m the intersection region between them (the volume jointly occupied by the optical modes m the two waveguides . The peak index change m the intersection region is preferably equal to the peak index change in the waveguides. This is done if the fabrication of the tee structure is accomplished m one step (be it by indiffusioπ. ion exchange, etching, etc.). In the alternative approach of laying down two waveguides in subsequent steps, which is most convement in the crossing waveguide geometry of FIG. 8. the peak index change in the intersection region is twice the index change in the waveguides, which is not needed. As always, the penodicity and angie of the gratmg is chosen such that the reflection process is phase matched oy the momentum or a virtual pnoton withm tne oanowiαth 01 the gratmg. ror optimal coupling between ' an m-band mput beam in the first waveguide and an output oeam 114 in tne second waveguide 108, the angle of incidence or the mput beam is eαuai to tne angie or diffraction orf the gratmg. In this case, the bisector of the angie between the two guides is normal to the domam boundaπes of the gratmg in the plane oi the waveguide.

-»_α incut beam 112 is mcident on and is couDled mto the first waveguide 106. A first electrode 120 ana second electrode 122 are laid out on the same face of the dielectπc material so that an eiectπc field is created in the intersection region between the waveguides, when a voltage source 124 connected to the two electrodes by conductors 126 is turned on. The electπc field controls the strength of the gratmg m the intersection region via the electro-optic effect, coupling the m-band beam from the first waveguide into the second waveguide to form a reflected output beam 114. With the gratmg turned off, the mput beam contmues to propagate predominantly down the first waveguide segment to form a transmitted output beam 116 with very tittle loss. .Alternately, counter-propagating beams can be used in ihe waveguide so that the mput beam enters though the second waveguide 108. and is switched mto the output waveguide 106 by interacting with the gratmg.

In single mode systems, the gratmg strength is preferably spatially distributed in a nonuniform manner so that a lowest order Gaussian mode entering waveguide 106 is coupled into the lowest order Gaussian mode of waveguide 108. The gratmg strength can be modulated by adjusting the geometry of the electrode, by adjusting the gaps between the electrodes, and by adjustmg the duty cycle of the gratmg. The bandwidth of the gratmg may also be enhanced by one of a number of well known techniques such as chiφing, phase shifting, and the use of multiple penod structures.

The size of the coupling region is limited, in the geometry of FIGS. 7 and 8 by the size of the intersection region between the guides where their modes overlap. To obtain a high net interaction strength for a given eiectπc field strength, it is desirable to mcrease the size of the waveguides to produce a larger intersection. However, large waveguides are multimode. which may not be desirable for some applications. If adiabatic expansions and contractions are used, the advantages of both a large intersection region and single mode waveguides can be obtained simultaneously. The input waveguide 106 begins as a narrow waveguide and is increased in width adiabatically as the intersection region is approached. The output waveguide 108 has a large width at the intersection to capture most of the reflected light, and it is tapered down in width adiabatically to a narrow waveguide. The idea of adiabatic tapenng of an mput and/or an ouφut waveguide can be applied to many of the interactions descnbed herein.

Referring to FIG. 8, the two waveguides 136 and 138 intersect at an angle 158 to make an x intersection, forming a four-port device. This device is a particularly versatile waveguide switch, smce two switching operations occur simultaneously (beam 142 into beams 146 and 148. and beam 144 mto beams 148 and 146). The gratmg 130, consisting of a first type 134 and second type 132 of domains, is disposed at an angle to the two guides m the intersection region between them. The angie of the gratmg is preferably chosen such that the bisector of the angle between the two guides is normal to the domam boundaπes of the gratmg, in the plane of the waveguide. A first incut beam 142 is incident on and is coupieo mto the first wavegmde 136 and a second mput beam 144 is coupled mto the second waveguide 138. A first electrode 150 and second electrode 152 are laid out on the dielectπc matenal so that an eiectπc field is created m the intersection region between the waveguides, when a voltage source 154 connected between the two electrodes is turned on. The electπc field controls the strength of the index gratmg the intersection region through the electro-optic effect. When the gratmg is on. a portion of the m-bano component or the first mput beam is coupled from the first waveguide to the second waveguide to form a first output beam 146. At the same tune, a portion of the m-band component of the second mput beam rrom the second waveguide is coupled mto the first wavegmde to form the second output beam 148. In addition, the out-of-band components of the two beams, and any unswitched components of the m-band beams, contmue to propagate down their respective waveguides to form additional portions of the appropπate output beams. Thus, for two beams with multiple optical frequency components, a smgle frequency component in the two mput beams can be switched between the two output beams.

The waveguide may only be a segment, in which case it is connected to other optical components located either off the substrate, or integrated onto the same substrate. For example, the waveguide segment could be connected to pump lasers, optical fibers, crossmg waveguides, other switchable gratings, mirror devices, and other elements. An array of crossmg waveguide switches would compπse an optical switchmg network.

In FIG. 9, a further embodiment of the waveguide coupling switch is shown. The domam walls of the gratmg are now disposed at a non-normal angle to the surface 157 of the crystal 158, so that the mput beam 159 in waveguide 160 is reflected out of the plane of the crystal to form a reflected output beam 161. As before, an unreflected beam continues to propagate through the waveguide to form a transmitted output beam 162. An optically transparent first electrode 163, which can consist of indium tm oxide, is disposed on one face of the dielectπc matenal 158. over a portion of the gratmg that crosses the waveguide. A second electrode structure 164, which may be optically absorbing, is disposed on the matenal. As m all cases descπbed in this disclosure, the second electrode may be arranged in one of many alternate configurations: surrounding the first electrode as m FIG. 7, on opposite sides of the matenal 158 as on FIG. 2, tapered similar to the configuration shown in FIG. 6. The electrodes are connected with two wires 156 to a voltage source 154, which controls the power splitting ratio of the ln- band beam between the transmitted beam 162 and the reflected beam 161. Alternately, the electrode configuration could be as shown m FIG. 5 , in which case both electrodes may be opaque.

Referring agam to FIG. 9, the domam walls are preferably formed by electnc field polmg of a ferroelectπc crystal which is cut at an angle to the z-axis 165. Smce the electnc field poled domains travel preferentially down the z axis, polmg an angle-cut crystal by this technique results m domam boundanes parallel to the z axis, at the same angle to the surface. The angle 166 of the cut of the crystal is preferably 45° so that light propagating m the plane of the crystal may be reflected out of the substrate normal to the surface of the matenal (any angle may be used). The domains shown m FIG. 9 are planar, but can also be configured in more general configurations. A planar gratmg will produce a flat output phase front from a flat input phase front. If the device shown is used as a bulk reflector without the waveguide, a coilimated input beam will produce a collimated output beam. The device is useful as a bulk reflector for example if a beam is incident from outside the device, or if the waveguide is brought to an end within the device with some distance between the end of the waveguide and the poled reflector. In some cases, however, it may be desirable to produce a curved ouφut phase front from a collimated beam, as in the case of some applications requiring focussing, such as reading data from a disk. By patterning a set of curved domains on the upper surface of the substrate illustrated in FIG. 9, a set of curved domains may be poled into the bulk of the material since the domain inversion propagates preferentially along the z axis. A concave (or convex) set of domains may therefore be formed which create a cylindrical lens when excited by a field. Wedges and more complicated volume structures oriented at an angle to the surface may be formed by the same process.

In an alternate method, a z-cut crystal can be used as the substrate if the poling technique causes the domain boundaries to propagate at an angle to the z-axis. For example, titanium (Ti) in- diffusion in a z-cut crystal of lithium mobate produces triangular domains that would be appropriate for reflecting the beam out of the surface of the crystal. The angle of the domains formed by in-diffusion with respect to the surface is typically about 30°, so that an input beam incident on the grating will be reflected out of the surface at an angle of about 60^s to the surface of the crystal. The output beam may then be extracted with a prism, or from the rear surface (which may be polished at an angle) after a total internal reflection from the top surface. The electrode structure shown excites both an E₃ component, and either an E, or an E_? component. A TM polarized input wave 159 experiences an index change which is a combination of the extraordinary and the ordinary index changes.

In FIG. 10 there is shown an embodiment of a switchable waveguide directional coupler. A first waveguide 204 is substantially parallel to a second waveguide 206, over a certain length. While the beams propagate adjacent each other and in a similar direction, their central axes are displaced. The central axes are never brought coaxial so that the waveguides do not intersect. However, the waveguide segments are in close proximity in a location defined by the length of the coupler, so that the transverse profiles of the optical modes of the two waveguides overlap to a large or small extent. The propagation of the two modes is then at least evanescently coupled (which means the exponential tails overlap). The evanescent portion of the mode field is the exponentially decaying portion outside the high index region of the waveguide. The propagation constant associated with a mode of each of the two waveguides is determined by k - 2ιrn_cΛ/λ in the direction of propagation. The effective index n__ff is the ratio of the speed of light in a vacuum to the group velocity of propagation, which varies according to the mode in the waveguide. The value of n__ff is determined by the overlap of the mode profile with the guided wave structure.

Preferably, the width of the two waveguides, and thus the propagation constants of the modes in the two waveguides, are different, so that coupling between the modes is not phasematched when the grating is off. (The index of refraction profiles of the two waveguides may also be adjusted to create different propagation constants. I With the gratmg on. ;mv mput beam 210 in the first waveguide will contmue to propagate m that waveguide to form a transmitted output beam 214 exiting the first waveguide 204. When the gratmg is on. the gratmg makes up tne difference in the propagation constants of the two waveguides so that coupling between the two modes is phasematched. and an m-band output beam 212 exits the second waveguide 206. To optimize the coupling, the gratmg penod Λ is chosen so that the magnitude of the difference of the propagation constants in the two waveguides is equal to the gratmg constant (within an error tolerance). The propagauon constants of the two waveguides may alternately be chosen to be equal, so that coupling between the two waveguides occurs when the grating is off. In this case, turning the gratmg on reduces the coupling between the two guides. The strength of the grating determines a coupling constant, which defines the level of coupling between the two waveguides. Along the length of the interaction region of the two waveguides, the power transfers sinusoidally back and forth between the guides, so that coupling initially occurs from the first waveguide to the second, and then back to the first waveguide. The distance between two locations where the power is maximized in a given waveguide mode is known as the beat length of the coupled waveguides. The beat length depends on the strength of the gratmg.

A first electrode 220 and second electrode 222 are positioned on the material surface to create an eiectπc field across the gratmg region 202 when a voltage is applied between the two electrodes. A voltage source 226 is connected to the two electrodes with an electπcally conductive material 224. The strength of the gratmg, and thus the beat length between the two waveguides, is controlled by the voltage applied across the gratmg.

The propagation constants of the two guides are strongly dependent on wavelength. Since the momentum of the virtual photon is essentially or dommantly fixed (i.e. determined by parameters which are not varied in an application), power is transferred to the second waveguide only m the vicinity of a smgle frequency with a frequency bandwidth depending on the length of the coupling region. Depending on the gratmg strength, an adjustable portion of the m-band mput beam exits the second waveguide as the coupled output beam 212, while the out-of-band portion of the mput beam exits the first waveguide as the transmitted output beam 214 along with the remainder of the m-band beam.

The coupling between the two modes can be controlled electro-optically by several means, mcludmg changing the strength of the coupling between the modes, increasing the overlap of the modes, or changing the effective index of one of the waveguides. Electro-optically controlled coupling, described above, is the preferable method. In order to couple efficiently between the modes in the two waveguides, the mput beam is forward-scattered, which requires the smallest gratmg penod.

The coupling gratmg can alternatively be implemented as a combination of permanent and switched gratings as described above m conjunction with FIG. 2. Here we give a detailed example of how this can be done. After forming the desired penodic domams, the substrate can be chemically etched to form a relief gratmg with exactly the same penod as the poled structure. For the preferred matenal of lithium mobate. the etch can be accomplished without any further masking steps, smce the different types of domams etch at different rates. For example, hydrofluoπc acid (HF) causes the -z domains of lithium mobate to etcn significantly ( > 100x) raster man tne - z domains. hus oy immersing the z-cut crystals m • a 50% HF solution, the regions consistmg or the first type of domam are etched while the regions consistmg of the second type of domam essentially remain unetched. This procedure produces a permanent coupling gratmg which can be used on its own to produce coupling between the two waveguides. After the electrodes are applied, the poied grating can be excited to produce an additive mdex of refraction grating which is supeπmposed on that of the etched substrate. The etch depth may be controlled so that the effective mdex change mduced by the permanent etched gratmg can be partially or wholly compensated by the electro-optically mduced gratmg when the electrodes are excited at one polantv, while the index grating is doubled at the other excitation polaπty. A push-pull grating is thereby produced whereby the gratmg can be switched between an inactive state and a strongly active state.

An etched gratmg is also useful when the etched region is filled with an electro-optical mateπal, such as a polymer or an optically transparent liquid crystal, with a high electro-optic coefficient and an mdex close to that of the substrate. Preferably, the filled etched region extends down into the optical beam. When a voltage is applied across the filled etched region, the mdex of the filler material is also vaπed around that of the rest of the waveguide.

Alternately, the overlap of the modes m the two waveguides can be electro-optically modified. For example, the region between the two waveguides could have its refractive mdex raised. This reduces the confinement of the waveguides, and spreads the spatial extent of the individual modes towards each other, increasing the overlap. To implement this approach, the region between the two waveguides may be reverse poled with respect to the polarity of the substrate traversed by the waveguides.

If the electrode extends across both the waveguides and the intermediate region, an applied voltage will increase the index of the area between the waveguides while decreasing the index within the two waveguides. The resulting reduction m mode confinement thus mcreases the overlap and the coupling between the two modes. Care must be taken not to mduce undesirable reflections or mode coupling loss m the waveguides, which might occur at the edge of the poled region. These losses can be minimized, for example, by tapering the geometry of the poled regions or of the electrodes so that any mode change occurs adiabatically along the waveguide, minimising reflections. An adiabatic change means a very slow change compared to an equilibrium maintaining process which occurs at a definite rate. In this case, it means the change is slow compared to the rate of energy redistribution which occurs due to diffraction within the waveguide and which maintains the light in the mode characteristic to the waveguide.

A third means to change the coupling between the two waveguides is to change the effective mdex of one of the waveguides relative to the other. Thus, the propagation constant of the guide is changed, which in turn alters the phasematching condition. This effect may be maximized by poling one of the waveguides so that its electro-optic coefficient has the opposite sign from that of the other waveguide. In this case, the coupling gratmg may be a permanent or a switched gratmg. A first electrode covers both waveguides and the region between them, while a second electrode may be disposed on both sides of the first electrode. .An electnc field applied between the two electrodes causes the propagation constant of one waveguide to increase, and that of the other waveguide to decrease, thus maximizing the difference in propagation constants. Tne rating coupling process is maximally erficient only at a particular difference in propagation constants. By tuning the applied voltage, the phasematching may be adjusted as desired. This effect can be used to create a wavelength tunaole filter.

The parallel waveguides shown m FIG. 10 may be nonparallel, and the waveguides may not even be straight. If it is desired, for instance, to spatially modify the mteraction strength between the waveguides, this end can be accomplished by spatially adjusting the separation between the guides. These modifications may also, of course, be applied to the subsequent embodiments of parallel waveguide couplers described herein.

Referring to FIGS. 12 and 13 there are shown alternate embodiments of the crossmg waveguide coupler for controlling the profile of the reflected beam. In each embodiment, the area covered by the grating does not extend entirely across the intersection region of the two waveguides. The motivation for these grating structures is best understood with reference to FIG. 11. Depending on how it is configured, the power coupling structure 282 may distort the spatial profile of the mode 284 it couples mto the ouφut waveguide. A power coupler which is uniform m space and which uniformly covers the entire intersection region 280 between two waveguides disposed at a large angle to each other such as 90° will produce an output beam profile such as assymmetπc profile 286. The power m the input beam decreases as it passes through the power coupling structure or gratmg. In the case of a right angle intersection, the near field profile of the reflected beam matches the monotonically decreasing power in the input beam. The disadvantage with the nonsymmetric profile 286 lies in single mode structures where only a fraction of the coupled power will remain in the waveguide. Much of the power will be lost from the guide.

For single mode devices, a structure is needed which couples power into the Gaussian-like spatial configuration 288 of the lowest order mode of the output waveguide. To accomplish this goal, the region 282 must be extended out mto the evanescent tails of the guided modes, and the net interaction must be modulated, either geometrically or by spatially adjustmg the local strength of the power coupling gratmg. FIGS. 12 and 13 show ways to accomplish this end with geometrical arrangements of gratings. It is also possible to accomplish this end by spatially modulating the "duty cycle" of the gratmg within the power coupling region 282, by changing the order of the grating in selected regions, and in the case of electrically controlled coupling, by tapering the strength of the applied electric fields (by adjusting electrode spacing as illustrated in FIG. 6, or by adjusting the electrode duty cycle in the case of grating electrode structures). The duty cycle of a grating means the fraction of each penod which is occupied by a given domam type; the duty cycle may vary with position.

In FIG. 12, a device 300 with a modified gratmg structure is shown, m which the gratmg area 310 covers part, but not all of the rectangular intersection region of the two normal guides 316 and 318. With the gratmg unactivated, the input beam 302 passes through guide 316 undeflected to exit as output beam 308. The dimensions of the intersection region match the widths 304 and 305 of the two waveguides. The presence of a small region of power coupling structure at any point in the intersection region will result in local coupling between a given transverse segment of the beam profile in an mput waveguide mto a given transverse segment or the oeam prorne in an output waveguide. Tne reflected beam profile is constructed from the propagated sum or these phased-coupled contπbutions. The gratmg region 310 depicted is tnanguiar in snape. with the po ts or the tπangie 311, 312. and 313. The shape of the gratmg region can oe modified from the tnanguiar. and the local gratmg strength can be modulated. The exact shape of the gratmg region which optimizes smgle moαe couplmg characteπstic between the waveguides can be calculated with an established waveguide propagation technique, such as the beam propagation method.

A further embodiment of a smgle-mode couplmg gratmg device 340 is shown m FIG. 13. The gratmg region 350 is a double convex shape, with one pomt at corner 351 common with waveguides 346 and 348 and beams 330 and 342, and the other pomt on opposite corner 352, common with both waveguides and beams 342 and 332. This structure has the advantage of reflectmg most of the power in the middle of the beam, where the optical tensity is the highest, and thus better couples the power between the lowest order modes m the two waveguides 346 and 348. The optimal shape of the gratmg region again depends on the couplmg constant of the gratmg. Referring to FIGS. 12 and 13, a first electrode 320 is disposed on the same surface of the substrate as the waveguide, over the grating region, and a second electrode 322 is disposed on the same surface around the first electrode. The distance between the two electrodes may be constant as illustrated in FIG. 13. or it may be tapered as illustrated m one dimension m FIG. 12. A voltage control source 324 is connected with two wires 326 to the two electrodes. An electπc field can thus be applied through the gratmg region to activate one of the electro-optic coefficients and change the couplmg between the mput beam and the output beam.

For purposes of illustration. FIG. 12 also shows a tapered mput waveguide segment 287 and a tapered output segment 289. An mput beam 285 expands adiabatically through the tapered segment 287 to increase the intersection area and thereby increase the total reflection from gratmg 310. The gratmg is capable of reflectmg the now-expanded beam 285 toward the ouφut beam 308. If desired, the ouφut waveguide may also contain a tapered segment 289 to reduce the witdth of the ouφut beam. (Alternatively, the output beam may be kept wide if desired for later beam switchmg interactions.)

The gratmg may extend beyond the intersection region of the two waveguides. A gratmg extended along the mput waveguide enables residual transmitted light after the intersection region to be removed from the waveguide, typically mto radiation modes. The extended gratmg minimizes crosstalk between optical channels in switchmg arrays, m which an individual waveguide may have more than one signal channel propagatmg along its length.

Specifically contemplated by the mvention is a means for tuning the gratmg Several embodiments m which tumng is achieved are shown in FIGS. 14-17 Referring to FIG. 14, there is a bulk optical device 400 m which the strength and center wavelength of a normal mcidence reflection gratmg are controlled by a smgle voltage source 426. This device consists of a patterned poled gratmg region 410, which is electro-optically activated by two electrodes 420 and 422 on opposing surfaces of the mateπal and connected to 426 by conductors 424. The strength and the center frequency of the gratmg are tuned simultaneously by appiymg a s gie voltage oetween the two eiectrooes or the device. The average retractive mdex of the gratmg changes with the applied electπc field, causing a cnange m the center wavelength of the gratmg that is proportional to the electπc field. The average index is calculated over a smgle penod of the gratmg in a peπodic gratmg, by summing the weighted mdex changes m the vaπous types of domains. The weightm factor is the physical length 416 and 418 of eacn domam type, along the optical path of the mput beam 404. The condition for frequency tumng is that the weighted sum must not equal zero so that the average mdex changes as a result of the electnc field.

The product of the mdex of refraction and the physical distance traversed by an optical beam is known as the optical distance. (The mdex of refraction is replaced by the effective mdex of refraction for waveguide devices.) A 50% duty cycle is obtained m a gratmg with two types of domam if the average optical distance across the two types of domains is substantially equal (approximately equal within the error range determined by the needs of the application). The average is taken over many subsequent domains to allow for the possibility ot a chirped, nonpeπodic, or other more general type of gratmg. In general the domains may have different indices of refraction as well as different electro-optic coefficients. The general condition tor tumng is expressed m terms of the physical distance travelled m the different types of domains. For each domam, the total optical phase advance is given by the optical distance travelled (times 2τ/λ). However, the change m the phase advance is given by the product of the applied electπc field, the appropnate electro-optic coefficient, and the physical distance (times 2τ/λ). The average change m mdex of refraction expeπenced by the wave is equal to sum of the changes in phase advance m all domains traversed by the optical wave within a section of the mateπal of length t (times λ/2τf ). This change in average mdex determines the change m the peak mteraction wavelength according to δλ/λ = δn/n. The gratmg strength is changed simultaneously with the wavelength m this structure, but such simultaneous change may be undesirable. The structure may be designed so that the operating pomt about which tumng is accomplished maintains a sufficiently high gratmg strength for the application across the entire wavelength tumng range. Or, a separate tuning structure may be used as is descπbed below reference to FIGS. 16 and 17.

The change in the average refractive mdex can be achieved by many different means. One alternative is that of randomly non-electro-optically active domains 414 alternating with electro- optically active domains 412. The electro-optically active regions are poled domains, while the non- electro-optically active domains may be randomly poled or unpoled or radiation-disabled. Thus, the electnc field causes an average increase m the mdex Δn,,. across the gratmg. In the poled-random configuration of FIG. 14, ^Δn.._t is equal to the product of the mdex change in the active domains 412 times the duty cycle. The duty cycle is equal to the length 418 divided by the sum of the lengths 418 and 416. The tunabihty that can be achieved usmg this techmque is λΔn,_v,/n in a poled-random structure, where λ is the optical wavelength, and n is the oπginal (effective) mdex of the mateπal. Assuming a wavelength of 1.55 μm and a 10 V/μm electnc field lithium mobate, the tumng range for a 50% duty cycle structure is 1.1 run. When me mput beam 404 is wiinin tne Danαwidth or the grating, tne gratmg couples the ream mto a retroreflecting output beam 402; otherwise the input oeam toπns a transmitted output beam 406. Contrast this behavior with that or a 50% duty cycle gratmg vvnere tne two domain types nave the same eiectro-opt c coefficients but opposite polantv. as in the case of domam inversion. In this latter case, i there is no cnange m the average mdex of refraction smce tne cnange in mdex of the first domam type cancels with the change m index of the other domain type. A 50% duty cycle domam reversal gratmg does not tune its center frequency.

An alternate means to achieve an average effective mdex change m domam reversed gratings is to use a non-50 % duty cycle for the poled domam area- with unequal lengths 416 ≠ 418. The 0 tunabihty that can be obtamed usmg this techmque is (2D-l)Δnλ/n, where D is the duty cycle of the largest domam type (D > 0.5). For example, with a 75 % duty cycle, a wavelength λ of 1.55 urn, and a 10 V/μm electπc field in lithium mobate, the tumng range is 0.54 n . The domam reversed gratmg is also stronger than a gratmg m which the second domam type is not electro-optically active.

In FIG. 15. a waveguide device 440 usmg the same average mdex effect is shown. In 5 this case, the average effective index of the waveguide 442 in the gratmg region 450 changes with the applied electπc field, causing a change m the center wavelength of the gratmg. A voltage control source 466 is used to apply an electnc field between a first electrode 460 and second electrode 462, which are preferably placed on the same surface of the mateπal. The average effective mdex can be achieved by a vaπety of geometπes. including non-electro-optically active domains or a domam reversal grating with a non-50 % duty cycle. When the mput beam 445 is withm the bandwidth of the gratmg, the grating couples the beam mto a retroreflecting output beam 444; otherwise the mput beam forms a transmitted output beam 446.

A means to enhance the tunabihty of a gratmg m a waveguide device 480 is to overlay a second electro-optic matenal 482 on the waveguide to form a cladding, as shown in FIG. 16. The cladding should be transparent to the wave propagating m the waveguide and it should be electπc field-sensitive to enable adjustable modification of its mdex of refraction. The average effective mdex is determined partly by the mdex of refraction of the cladding. The second mateπal may have a higher electro-optic coefficient than the substrate. Liquid crystals and polymers are good examples of mateπals which can be used as cladding. The index of the cladding is preferably close to that of the guiding region so that a large portion of the guided beam propagates in the cladding.

For this embodiment, a first electrode 502 is surrounded by a second electrode 504 on the substrate, for applymg an electnc field across the poled grating 490. Preferably, the electrodes are placed below the cladding, directly on the substrate. If the first electrode 502 is positioned directly above the waveguide 484 as shown m FIG. 16, it must be made of an optically transparent matenal. The electrodes may also be disposed to either side of the waveguide 484, in which case they need not be transparent. A third electrode 506 is positioned on top of the cladding, above the waveguide and the first electrode. For this embodiment, the center wavelength and strength of the grating are separately controllable. The gratmg strength is controlled bv a first voltaee source 510. connected bv two wires 513.514 to the first and second electrodes, while the center wavelength of the gratmg is controlled bv a second voltage source 512, connected between the first and third electrodes with two wires 514 and 515 In an alternate electrode configuration, only two electrodes are used, both of which are preferably positioned on top of the claddmg mateπal so that their mduced field penetrates both the claddmg matenal above the gratmg, and the gratmg structure itself. A smgle voltage source then controls both the center wavelength and the gratmg strength, but not independently.

The amount of tunabihty that can be achieved with an electro-optically active claddmg depends on what portion of the guided beam propagates in the claddmg. If the two indices are relatively close so that 10% of the beam propagates m the claddmg, then the average change m the effective mdex of the guided mode is equal to 10% of the change m mdex of the claddmg. For a claddmg mdex change of

0.1, the tunabihty is on the order of 7 nm.

FIG. 17 shows an embodiment of a discretely tunable gratmg device 520, which consists of several individually controllable gratmgs 530, 532, 534. The gratmgs m seπes, with all gratmgs in the path of the mput beam 522, and forward 523 and reflected 524 beams. Each individual gratmg m the structure may also be continuously tunable over a small range. Each gratmg m FIG 17 has a first electrode 542 and a second electrode 544, which are connected to a voltage controlling network 552 with wires. The gratmgs can be switched on one at a time, so that only one wavelength m a small passband will be reflected at a time, or multiple gratmgs can be switched on simultaneously to create a programmable optical filter, with a center wavelength and bandwidth which are separately controlled. The gratings themselves may be implemented with the vaπations descπbed above, mcludmg the possibility of multiple penods in each gratmg.

The structure can be realized either m the bulk or as a waveguide device. In the latter case, an optical waveguide 528 is fabπcated on the substrate so that the waveguide intersects the poled gratmgs. The poled domams 536 may extend only through the waveguide and do not necessaπly extend all the way through the matenal. Both electrodes are preferably (for higher field strength) deposited on the same face of the substrate as the waveguide. The second electrodes of all the gratmgs may be connected as shown to minimize the number of electncal connections.

Alternately, the individually-addressable gratmg structure can be a bulk device, m which case the waveguide 528 is omitted, and the poled regions 530, 532 and 534 are optimally fabπcated with sufficient depth to overlap with the propagatmg optical mode. The two electrodes for controlling each gratmg are then optimally positioned on opposing taces of the mateπal to optimize the field penetration, as shown for example m FIG. 2 for a smgle gratmg. Cross excitation between adjacent gratmgs caused by fringing of the electπc fields between the electrodes can be minimized by separating the grating-electrode groups by an amount comparable to the substrate thickness, or by add g interspersed fixed-potential electrodes.

An alternate means for tumng the gratmg is to vary the temperature of the active matenal. The tumng occurs because of two effects: thermal expansion and the thermo-optic effect. For different mateπals, either one of these two effects may dominate thermally mduced tuning. In lithium mobate, the larger effect is thermal expansion, tor wnicn the largest l -axisi expansion coerficient ΔL/L is + 14 x 10* °C"'. while the thermo-optic coerπcient tor tne ordinarv axis Λn_;n is - 5.6 x 10"° °C"'. For a temperature range of 100 °C. :he combination of these two effects gives a total wavelength tuning range of 2.6 nm. For many purooses. it is desirable to create poied gratmgs with a generalized frequency content. Multiple mteraction peaks may oe desired for example, or simply a broadened bandwidth of interaction. To accomplish this end. some way is needed to determine the pattern of poied region boundanes which corresponds to a given mathematical function containing the desired frequencies. FIG. 18 illustrates the results of the process in the case of a smgle frequency containing arbitrary phase shifts. Referring now to FIG. 18, optical phase shifts 564 and 565 can be incorporated at one or more positions along a sinusoidal function 560 to modify its wavelength structure. The mean level of the function is given by the straight line 561. Also shown is the corresponding squared wave function 562 with identical phase shifts, as can be achieved by a typical poling process. To achieve the translation of the continuous function mto the square wave function, the regions 570 where the curve 560 exceed the average 561 sine wave corresponds to one type of domam. while the regions 572 where tne curve 560 falls below 561 corresponds to a second type of domam. The Founer transform of the square wave curve 562 will have the same frequency components as the transform of the sinusoidal function 560 in the low frequency range below the harmonics of the s e wave frequency. This approach works for any type of generalized frequency distπbution as long as the bandwidth does not exceed a small fraction of the carrier frequency.

A phase shifted grating may be implemented in any of the devices described herein such as in FIG. 2 for example, where the location of the domam walls 34 in the gratmg 22 can be determined by the pattern 562 of FIG. 18 rather than a periodic function. The phase shifted pattern can be controlled with a polmg mask incoφorating the desired pattern.

.Arbitrary multiple penod gratmgs can be specified usmg a similar techmque. Each penod present m the gratmg is represented in a Fourier seπes (or integral) by a corresponding sign wave of the desired amplitude. All waves are added together to form a resultant wave. The positive portion of the resultant wave corresponds to one type of domain, while the negative portion corresponds to the second type of domam. The number of superimposed gratmgs can in principle be scaled up to any number, • limited in practice by the minimum attainable feature size.

FIG. 19 shows an alternate way of fabricating a supenmposed multiple-period grating device 580. A two gratmg waveguide structure is depicted, with a switchable smgle penod poled gratmg

582. and a permanent relief gratmg 584 interacting with a smgle beam in a waveguide. A coatmg 588 is shown deposited on top of the relief grating to reduce the loss which occurs when the evanescent tail of the guided wave mode overlaps with the metallic electrode. This coatmg is an important design optimization element for all of the elements descπbed herein, and should be appiied between each electrode structure and adjacent optical waveguides. A coatmg is also useful above the electrodes in all of the elements descπbed herein to reduce the probability of breakdown.

The electncallv controllable gratings in the suoeφeπod structure are switched by a smgle pair of electrodes 602 and 604. connected by wires 606 to a voltage control source 608. The first electrode 602 is preferably centered over the waveguide, while the second electrode 604 runs parallel to th first, on either side of the waveguide. The device depicted is a waveguide device, with a waveguide 586 confining the input beam 590, as weil as the transmitted ouφut beam 592 and the reflected ouφut beam 594. The multiple period gratmg structure can be configured in many ways. For example multiple independent peaks m the frequency spectrum can be useful as a multiple frequency feedback mirror. Two operations (e.g.., phasematching and reflection) can be achieved in a smgle grating which incorporates the proper two periods for enabling the processes. As a final example, the grating can be fabπcated with the phase and amplitude of its components adjusted for equal effect on the two polarization modes, making a polarization insensitive component.

Another useful modification of a peπodic structure is a chiφed penod. Along the length of the gratmg structure, the period can be gradually mcreased or decreased, so that the center wavelength vanes from one end of the gratmg to the other. Thus, the wavelength bandwidth of the gratmg is broadened over that of a constant penod gratmg. The chiφing across the gratmg is not necessarily linear: many different wavelength reflection profiles m frequency space (e.g.., square wave, Lorentzian) can be achieved, depending on the variation in the chiφ rate. As described above, the duty cycle and/or the strength of the exciting electric field can also be spatially adjusted to modify the strength of different portions of the chirped grating. The duty cycle of the gratmg can be controlled by the mask as desired. The electπc field strength can be controlled by adjusting the separation of the electrodes as shown for instance in FIG. 6.

A wide spectrum tunable device can be realized in a structure containing two separate gratings which have a multiple peak structure, as shown m FIGS. 21 and 22. FIG. 20 demonstrates the basic principle of these devices and depicts the multipeak comb transmission (or reflection) profiles 620 an 622 as a function of the optical frequency for two such gratmgs. The first gratmg profile 620 has transmission peaks separated by a first penod 626, while the second gratmg profile 622 has peaks separate by a second penod 624 that is slightly different from the first. The key idea is for the device to operate only at a frequency determined by the overlap of peaks from both curves (frequency μ,). Tumng is achieved by tuning the comb of transmission peaks of the gratmgs with respect to each other. Different transmission peaks in the two combs will overlap each other in vaπous ranges of the relative frequency shift, so that the net transmission of the combined gratmgs jumps discretely over a much wider wavelength range than can be achieved with only thermal or electro-optic tumng. In the example of FIG. 20 where th peak separations differ by 10%, if the frequency of the first grating is increased by 10% of the frequency separation 626, the next higher frequency peaks will supeπmpose, resulting m an effective frequency shift ten times larger than the tuning amount. In FIG. 21, a guided wave embodiment of the device is shown, m which two gratings 65 and 652 are placed over a smgle waveguide 642. An mput beam 644 is partially reflected mto beam 643 and transmitted as beam 645. A first electrode 666 and second electrode 668 are positioned around the first gratmg 650 so that a first voltage source 662 connected to the electrodes activates that gratmg. A third electrode 664 is positioned, aion with the second electrode, arouno the second gratmg 652. The second gratmg is controlled by a second voltage source 660 connected to the second and third electrodes. In the preferred embodiment, each gratmg is a multiple oeaκ structure as descnbed in FIG. 20. and the device forms a frequency-noo-tuned reflector. According to the curves of FIG. 20. the gratings are configured as broadband reflectors, reflectmg essentially ail the incident radiation frequencies except a comb of equally spaced frequencies where the transmission is high. The cascaded gratmgs will therefore reflect all frequencies m the frequency range illustrated in FIG. 20. except where the two transmission peaks overlap at v,. Provided that the reflections of the two gratmgs are arranged to add in phase in the reflected beam 643. the transmitted spectrum will be essentially equal to the product of the two transmission curves 620 and 622. When the center frequency of one of the gratmgs is tuned, the single transmission peak at v will hop to the next adjacent peak, and then the next, and so on. Such a structure is particularly useful as an eiectncally tuned receiver m. for example, a wavelength-division-multiplexed (WDM) communication system. The receiver can be configured to detect only incoming light in a specific band, while being insensitive to light at other frequencies. As seen above, a gratmg structure can be shifted by about 0.5 nm. assuming a 10 V/μm field m a domam inverted gratmg with duty cycle of 75 %. This continuous tuning range can be used to produce discontinuous tumng in the structure 640 across perhaps 100 bands in a 50 nm range, if the width of the individual frequency peaks 628 are narrower than about 1/ 100th of the frequency separation.

Note that if the frequency of the mput light is known to lie only within the transmission bands of curve 620 in FIG. 20, for example, the device can be realized with only a smgle grating structure with the transmission spectrum of curve 622, using essentially the Moire effect. By tumng the center frequency of the spectrum 622, any one of the desired bands can be selected while reflectmg the rest. The tee structure of FIG. 7 is then particularly interesting in this context: the mput beam 112 containing multiple frequency components is then split by the gratmg structure 100 (configured for tumng as described herein) mto a smgle transmitted beam 116 which can be detected or otherwise processed, and a reflected beam 114 which contains all the other frequency components. The power contained in beam 114 is not lost, but can be routed to other nodes in a commumcations network, for example.

Other variations can be formed of this basic structure, wherein, for example, the spectra of FIG. 20 are the reflection curves of the individual gratings instead of the transmission curves. In this case, the structure acts as an etalon when the frequencies of the reflection peaks align with each other, with reflectivity according to the relative phase of the reflected waves. Otherwise, the net reflection of the compound structure is essentially the sum of the reflection curves of the two individual structures.

It may be important to optimize the relative phase of the two reflections by adjustmg the optical path length 653 between the two gratmgs. The relative phase can be controlled by using an electro- optic structure (as shown for example in FIG. 22) between the two gratmg entrances 654 and 655 to adjust the optical path length 653. For a lithium niobate crystal and an mput wavelength of 1.5 μm, an activated distance between the gratmgs of at least 250 μm is required to adjust the relative phase between the two beam of up to ±ir, (usmg a z-axis applied field of 10 V/_ιm). The strength of one of the gratmgs (but not its frequency) may optionally be controlled via a field applied at its electrode if the gratmg is not designed for tumng (its average mdex of refraction is configured to be independent of the applied voltage). If both gratmgs are tuned together, narrow range continuous tumng results. As an alternative or supplement to electronic excitation, the phase of the two reflections and the peak wavelengths of the gratmgs can all be varied together through thermal or mechanical control of the chip.

FIG. 22 shows schematically two gratmg reflectors 633 and 634 separated by a phase shifter secuon 635 and forming an integrated etalon 640 hav g a charactenstic free spectral range (FSR). (The structure 630 is essentially the same as that of FIG. 21 , with the addition of the phase shifter section, which consists of electrodes capable of actuating a region of electro-optic matenal traversed by the waveguide 636.) For simplicity, we consider the case of umform smgle-peπod gratmgs, but the mdividual gratings may generally be more complex structures. The gratmgs may be fixed or electromcally actuatable. The reflections off the two gratings can be made to add m phase for a beam at a reference frequency by adjusting the voltage applied to the phase shifter section 635. A beam at a second frequency will also add in phase if the frequencies of the two beams are separated by a multiple of the FSR. Smce the FSR is inversely proportional to the optical path length between the two gratmgs, choice of the path length determines the density of the reflection peak structure of the etalon device. As an example, two short high reflecting gratmgs separated by 220 μ m lithium mobate can have gratmg reflection peaks separated by a multiple of 1 nm. The multiple peak structures 620 or 622 descπbed m FIG. 20 can each be implemented as an integrated etalon. A dual gratmg wye junction embodiment is shown m FIG. 23, m which the two gratings

690 and 692 extend across two separate waveguides 682 and 684. In general a wye junction has an mput and multiple output waveguides which may lie in a plane or m a volume. The two waveguides are connected to the first waveguide 686 with a wye junction 688. The power m the optical mput beam 691 is split between the second waveguide 682 and third waveguide 684 so that approximately 50% of the mput beam 691 is incident on each of the gratmgs. The two gratmgs may have a simple reflection structure, or they may have a seπes of high reflection peaks. The gratmgs may be permanent, or they may be electromcally adjustable, m which case electrodes 694 and 696 are provided for exciting the gratings. A common electrode 698 is then provided across the wafer (or alternately on the same surface as the waveguides, adjacent to the other electrodes similarly to FIG. 21). The relative optical path length of the two branches of the waveguide can be adjusted by the electrode 689 which is disposed on one waveguide over a region of electro-optic activity. By adjustmg the voltage on the phase adjusting electrode 689, the two reverse-propagating reflected beams may be ad_justed to have the same phase when they meet at the wye junction. The reflected modes superpose and form a wave front profile which may have a phase discontinuity in the center, dependmg on the relative phase of the two waves. As the combined wave propagated, the spatial concentration of the optical mode in the region of the guide is strongly affected by the phase shift. If they have the same phase, the profile forms a symmetπc mode which couples efficiently into the lowest order mode of the mput waveguide to form the retroreflected ouφut beam 693. Two reflected beams which add out-of-phase at the wye junction will have very iow coupling mto any symmetπc mode (sucn as the lowest oroer mooe) of waveguide 686. If the waveguide 686 is smgie mode, this reflected energy will be rejected from the waveguide. Thus, by adjustmg the optical path length or one of the arms of the wye with the electrode 689. the reflection can be rapidly adjusted from almost 100% to a value very ciose to zero. Furthermore, if the gratmgs are implemented as electromcally tunable reflectors in one of the tunable configurations descπbed herein, the modulated reflection property can be shifted mto different regions of the spectrum.

Referring to FIG. 24. there is shown a switchable waveguide mode converter 720 using a poled gratmg 722. The waveguide 730 preferably supports both an mput mode and an output mode, which may be two transverse modes or two modes of polaπzation (e.g. TE and TM). The two modes in the waveguide typically have different propagation constants, which are determined by the effective indices of the modes. The gratmg 722 is excited electπcally by electrodes 740 and 742, coupled to the source of electrical potential 744 by the connections 746. The gratmg period Λ (724) is chosen so that the magnitude of the difference of the propagation constants m the two waveguides is equal to the gratmg constant 2xn/Λ. When the gratmg is on. the gratmg makes up the difference in the propagation constants of the two waveguides so that couplmg between the two modes is phasematched. The gratmg strength and the device mteraction length in the gratmg should be set to optimize the flow of power from the input mode into the ouφut mode. The net rate of power conversion from one mode into the other is determined by the strength of the electro-optic coefficient (r₃, m lithium mobate) and by the strength of the electnc field.

For two transverse modes, the coupling depends on the spatial overlap of the two modes in the presence of the grating structure, and on the strength of the gratmg. The two modes may be orthogonal by symmetry, so that even if the modes are phasematched. there will be no conversion in a symmetπc structure. In this case, the phasematching structure itself can be made asymmetric to eliminate the problem. In the preferred embodiment of FIG. 24, the asymmetry can be introduced via the electric fields which excite the poled structure. The vertical component of the electπc field reverses sign midway between the two electrodes 740 and 742. It is best to center the electrodes on the waveguide to optimize mode conversion between transverse modes of different symmetry. The reverse is true when coupling transverse modes of the same symmetry: now the phasematching structure should be made symmetric to optimize the conversion. Several alternative approaches can also be used. A three electrode structure has a symmetπc vertical component of the electric field and an asymmetric horizontal field. The hoπzontal field can be used in conjunction with one of the hoπzontally-coupled electro-optic coefficients to couple modes of different symmetry. Or, the poled structure may have a phase reversal plane that essentially bisects the waveguide, m which case a symmetric component of the electπc field can be used to couple modes of different symmetry (vertical field in the case of three electrodes, hoπzontal field in the case of two). Since the propagation constants of the two modes are strongly dependent on wavelength, the beat length of their interaction also depends on the wavelength. Thus, for a given length of the couplmg region between the two modes, the power coupled into the second mode is frequency-sensitive. The coupling has a frequency bandwidth associated with it. For a given gratmg strength, a portion of the m-band mput beam is coupled mto the output mode wnich exits as the coupled output beam, wnile the remainder of the mput beam exits the first waveguide as the transmitted output beam.

The structure shown in FIG. 24 can also be used to couple between TE and TM poianzed modes. The electro-optic coefficient r«, enables couplmg between the two orthogonal poiaπzations m a lithium niobate crystal, for example. As before, the penod of the gratmg is chosen so that the grating constant is equal to the difference m propagation constants between the two mooes. The mteraction length is chosen to optimize the power transfer.

A waveguide, such as a titanium-indiffused waveguide which supports both TE and TM modes, is used m applications where both poiaπzations can enter or leave the converter. A waveguide such as a proton exchanged waveguide which supports only one polaπzation (TM m z-cut lithium mobate substrates or TE m x- or y-cut) can be useful m applications where only a smgle polaπzation is desired. Such a one-polaπzation waveguide can act as a very effective filter tor the other polanzation. The wrong polanzanon component will rapidly disperse away from the waveguide due to diffraction, leavmg only the guided polanzation in the waveguide. For example, the proton exchanged ouφut waveguide 731 may act to guide only the mput polaπzation or onl^y the ouφut polanzation, as desired. This device can be used as an optical modulator with excellent transmission and extmction if the gratmg couplmg is strong, and the mteraction length and electnc field are selected correctly. A modulator configured with a proton exchanged waveguide will transmit essentially all of the correctly poianzed mput light, and produce very low transmission of light which is coupled mto the perpendicular poianzed mode. Alternately, the input waveguide may be titanium-indiffused to accept either polanzation at the mput. The mdex profiles that form the waveguides for the two beams are preferably similar so that the profiles of the TE and TM modes overlap well, and the couplmg efficiency is maximized.

To activate the r«, coefficient, an electπc field is applied along the Y or the X axis of the crystal. The electrode configuration that will achieve the appropπate field direction depends on the cut of the crystal. For a z-cut crystal with a waveguide onented along the x axis, the first electrode and second electrode can be placed on either side of the waveguide. Alternately, for a y-cut crysul with a waveguide onented along the x axis, the first electrode can be placed directly over the waveguide, with the second electrode on either side of the waveguide, parallel to the first electrode.

Smce the poled domams m the gratmg 722 can be made to extend through a bulk substrate (such as 0.5 mm thick or more), the structure of FIG. 24 is also useful for a controllable bulk polanzation converter. In this case the waveguide 730 is unnecessary, and the electrodes are optimally configured on either side of a thm bulk slab of poled mateπal.

Referring to FIG. 25, there is shown a switched beam director 700 incorporating a wye power splitter 702 and a transverse mode converter 704. The mode converter works in a similar way to the transverse mode converter descπbed above in relation to FIG. 24. The gratmg structure 706 phase matches energy conversion from the lowest order (symmetπc) mode incident m waveguide 708 mto the next higher order (antisymmetric) mode of the waveguide. The length and strength of the mteraction region where the waveguide and the gratmg structure overlap are chosen to convert approximately half of the mput single symmetnc mode power mto a higher order antisymmetπc mode. Furthermore, the optical path length between the gratmg mode converter section 704 and the wye splitter 702 is chosen so that the phase of the two modes adds constructively at one of the branches 712 of the wye and destructively at the other branch 713. The result is that the power is routed primaπly into the waveguide 712 with the constructive interference, with very little power leakage mto the other waveguide 713. In this condition, any reverse propagating power in the guide 713 is essentially excluded from coupling into a reverse propagating mode in the guide 708 after the mode coupler 704. The device forms an efficient power router in the forward direction and an isolating structure in the reverse direction.

By adjusting the optical path length between the grating mode converter section 704 and the wye splitter 702, it is possible to switch the output power from guide 712 to guide 713. This is done by adjusting the relative optical path length for the lowest order mode and the higher order mode so that the two modes slip phase by T relative to each other, now producing constructive interference in the guide 713 and destructive interference in the guide 712. The relative path length adjustment can be achieved in the path length adjustment section 705 by exciting the electrode pair 711 and 709 with the voltage source 714, changing the index of refraction under the electrode 711 via the electro-optic effect in the substrate

703, which is preferably lithium mobate (but may be any electro-optic material with transparency for the waves such as lithium tantaiate, KTP, GaAs, InP, AgGaS_?, crystalline quartz, etc.). The propagation distance of the waveguide 708 under the electrode 711 is selected, along with the excitation voltage, to enable changing the relative phase of the two modes by at least the desired amount. The grating 706 may be a permanent grating fabricated by any of the techniques known in the art. However, to optimize the functioning of the device, it is desirable to have almost exactly equal power in the symmetric and the antisymmetric modes. It is difficult to achieve sufficient control in existing fabrication techniques to achieve this goal, and it is therefore desirable to have some adjustment in the grating strength. This adjustability can be achieved with the use of at least some poled grating sections, excited by the electrodes 709 and 710, which are driven by the power supply 715, and which can be used by themselves to accomplish the desired mode conversion, or to adjust the strength of a combined poled- permanent grating.

The input waveguide 708 is best implemented as a single mode waveguide incorporating a (preferably adiabatic) taper 701 to permit guiding of the two modes between the transverse mode coupler 704 and the wye splitter 702. The waveguides 712 and 713 are both preferably single mode. While any order modes may be used in the device as long as their symmetry is opposite, it is most desirable for interconnection purposes to work with the lowest order mode at the input and output legs. The intermediate excited mode is less critical, and could be, for instance, a higher order antisymmetπc mode. FIG. 26 shows a parallel waveguide switchable resonator 750 in which an mput waveguide 752 is coupled to a parallel waveguide 754 along an mteraction region 753. Grating reflectors

755 and 756 are disposed across the waveguide 754 in such a way as to retroreflect light propagating in the guide. The pair of separated reflectors and the waveguide 754 form an integrated etalon coupled to the input waveguide 752. The length of the coupling region 753 and the separation of the parallel waveguides in the coupling region are cnosen so that a certam desired fraction T of the mput beam 757 is coupled mto the waveguide 754. The light coupled mto the etalon structure 754, 755. and 756 resonates between the reflectors 755 and 756, and couples out mto two pnnicipal output channels: the forward propagating wave 759 and the reverse propagating wave 758 in waveguide 752. The same fraction T of the power circulatin in the etalon couples mto each of the two output channels 758 and 759.

As for any etalon, the integrated etalon has a frequency acceptance structure compπsed o multiple peaks m frequency space with width dependent on the loss of the resonator, and separation equal to the free spectral range. If the optical frequency of the mput beam 757 matches one of these resonant frequencies, the power circulatmg the etalon will build up to a value P_^ determined by P_M = P„.T (T+I 2)² where ?„. is the mcident power 757 in the waveguide 752, T is the loss of the etalon not mcluding the output couplmg mto the forward propagatmg wave 759 and the reverse propagating wave 75 in waveguide 752, and we have assumed weak couplmg and low loss. The ouφut coupled wave from the etalon which propagates m the reverse direction in waveguide 752 forms the reflected wave 758. The reflected power m beam 758 is equal to P„_f = P^l + r/2T)² on the peak of the resonance. When T ► 172, essentially all of the mcident power is reflected. The output coupled wave from the etalon which propagates in the forward direction m waveguide 752 is out of phase (on a cavity resonance) with the uncoupled portion of the mput wave 757, and the two beams destructively interfere, producing a low amplitude output beam 759. Because the two beams have unequal amplitude, the residual power P^,, = P_ωc/(l -r-2T r)^ϊ in the output beam 759 is not quite zero, but it can be very close. If the coupling T is made very large compared to the loss V of the etalon, the transmission of the device is greatly suppressed

(by 26 dB if T = IOJ"). This structure then acts as a very low loss reflector at a comb of frequencies separated by the FSR.

The device can be switched by changing the optical path length between the two reflector 755 and 756. Electrodes 761 and 762 are disposed to produce an electnc field through the waveguide 754 between the mirrors 755 and 756. The electrodes are excited with a voltage source 763, changing the effective index of the substrate under the electrode 761 via the electro-optic effect, thereby changing the optical path length between the mirrors and shifting the resonances of the integrated etalon. If the resonances are shifted by more than either the width of the resonances or the frequency bandwidth of the incident beam, the reflection will drop to zero, and the transmission will nse to essentially 100% as the circulating power within the etalon is suppressed to approximately P„T/4.

The gratings 755 and 756 may be permanent gratmgs, or they may be poled gratmgs excited by electrodes as shown m previous diagrams and discussed above. If the gratmg 756 is a poled gratmg, the device may also be switched by switching it off. With gratmg 756 off, i.e. not reflectmg, the loss to the mcident wave 757 is equal to the couplmg constant T, but now the comb structure is eliminated instead of just being frequency shifted as by the electrode 761. The difference m switching function between these two modes of operation may be significant with for example a broadband input signal where it is necessary to switch off the reflection rather than just change its frequency. For a smgle frequency mput beam, the reflection can be switched equally well by changing the path length with electrode 761 or by spoiling the Q of the resonator by switchmg off the mirror 756. However, if the reflectivity of the mirror 756 is retained and only the frequency spectrum of the eulon is shifted with the electrode 761 , other frequency components of a broadband input wave would be reflected, and this might be highly undesirable in some applications. The power P^ which buiids up m the eulon can be quite large if T and T are small, and can be useful in applications such as second harmonic generation, for example. In this application, a quasi- phasematched (QPM) periodic poled structure in a section of the lithium mobate substrate is incorporated into the resonator between, say, the mirror 756 and the interaction region 753, or possibly within the mteraction region itself. One of the resonant frequencies of the eulon is then tuned to coincide with the phasematching frequency for the QPM frequency doubler. The power buildup which occurs enhances the frequency conversion efficiency of the device as the square of the buildup factor P^P^. The high reflection which occurs at this frequency can also be used to injection lock the pump laser to the desired frequency if the FSR is large enough that the other resonant modes are not injection locked simultaneously. The linear integrated etalon geometry described above in reference to FIGS. 21 and 22 can also be used to accomplish the same purposes.

To optimize the power building up in the eulon between the reflectors 755 and 756, the losses in the resonator must be minimized. The coupling of FIG. 26 cannot be "impedance matched", in analogy to the process known in the art of bulk buildup cavities, where the input coupling into the resonator is adjusted to cancel by destructive interference the portion of the incident beam which is not coupled into the cavity. This is the condition of the etalon transmission interference peak. As described above, what happens in the integrated structure is that the transmitted beam can be nearly cancelled while the power builds up in the coupled resonator, but a strong reflected wave emerges. The reflected wave may be eliminated in a ring waveguide structure, as is illustrated in FIGS. 27 and 28.

An ouφut 751 proportional to the power circulating within the eulon may be taken through the gratmg 756, if desired, or alternately through the gratmg 755.

In FIG. 27, a three-arm eulon 760 is shown with an input waveguide 752, a parallel waveguide coupling region 753, a ring resonator formed by three waveguide segments 764, 765, and 766, three grating reflectors 767, 768, and 769. The optical path length adjustment section formed between the electrodes 761 and 762 is optional. The grating reflector 767 is disposed to optimally reflect the power arriving from waveguide 764 into the waveguide 765. In a single mode system, the spatial configuration of the grating (and its electrodes if any) is designed to couple from the lowest order mode of waveguide 764 into the lowest order mode of 765. The gratings 768 and 769 are similarly configured to optimize the power flow from waveguide 765 into waveguide 766, and then into waveguide 764 again, forming a Fabry- Perot resonator with a determinate optical path length, FSR, optical loss coefficient, and coupling T with the input waveguide 752. Now, impedance matching is possible, and is accomplished when the coupling coefficient T equals the toul round-trip loss coefficient of the resonator less the output coupling loss, principally in the coupling region 753. If a phase matched frequency doubler is disposed within the resonator, the converted power out ot the tundamenul Ireαuencv beam circulatmg in the resonator does count as one of the losses m the total round-tπp ioss.

If an mput beam 757 is mcident on the device with a frequency equal to one of the resonances of the three-arm eulon. power will couple across the parallel waveguide mteraction region mto the eulon and build up to a circulatmg power ot P_nrc = P„.T (T- T)². Because of the nng structure, the power will circulate pπmaπly in one direction, rrom waveguide 764 to waveguides 765, 766, and back to 764. There is now only a smgle ouφut coupled wave trom the eulon onto the waveguide 752, and it propagates m the forward direction. The ouφut coupled wave interferes destructively with the remainder of the mput wave 757, forming a weak transmitted wave 759. The transmitted power P^. m the output beam 759 is given by P,^, = P^l-rvT)² / (1 + I7T)², and can be brought to zero if T = T, which is the impedance matched condition. In this case, all the mcident power flows mto the resonator. In the impedance matched condition, the two beams have equal amplitude, and the transmitted power drops to zero. There is essentially no reflected power m beam 758 except tor reflections from discontinuities m the waveguide 752, which can be mmimiTwl y good design The gratmg 767 or any of the other gratmgs may be configured as a switchable gratmg, in which case the quality Q of the eulon may be spoiled by turning off the gratmg, eliminating the comb structure entirely but leaving some optical loss due to power coupled mto the waveguide 764. An output beam 751 may be taken m transmission through the gratmg 768. and/or through the gratmgs 767 or 769. FIG. 28 shows a nng waveguide eulon 770. As before, the mput waveguide 752 is coupled to a waveguide 772 in a parallel mteraction region 753. The mteraction region 753 mcludes a grating FIG. 28 (although it is not required) to emphasize that gratmg couplmg is a useful option m the eulon geometry of FIGS. 26, 27, and 28. The waveguide 772 follows a curved closed path (with any geometry mcludmg potentially multiple loops with crossmgs), feeding a portion of the power emergmg from section 753 back mto the mteraction region 753. As before, electrodes 761 and 773 are supplied to allow the optical path length, and hence the FSR to be adjusted, although m this case they are shown disposed on the same face of the substrate. A straight section 771 is provided where certam cπtical functional components may be fabπcated, accordmg to the application of the eulon structure. If the eulon device 770 is used for frequency doublmg, it would be advanugeous to insert the frequency doubling structure mto a straight section such as 771 of the nng, but provision must be made to couple the frequency converted light out of the nng waveguide.

The functioning of the device 770 is otherwise similar to that of the device 760. While the device 760 may consume less surface area on a substrate, the device 770 may have lower optical loss in the eulon, particularly if the diameter is one cm or larger.

The devices 760 and 770 can function as buildup cavities for frequency doublmg m which the feedback mto the optical source is minimal. They can also switch the transmission of a given frequency without retroreflection, which is useful m applications mcludmg optical commumcations.

In WDM communications, many commumcations channels separated by their optical wavelength may be earned on the same optical fiber. To detect a cnannel, the light m the desired wavelength region must first be separated rrom the remaining cnanneis wnic are routed to other destinations. This separation function is performed by a c annei droopmg filter. A channel dropping filter is a communications device which is used in a wavelength division multiplexed (WDM) environment. It is desired to multiplex several channels across a single transmission fiber by carrying the channels on different wavelengths. A cnticai component m such a system is a channei droppme filter which allows the extraction of a smgle channei for routmg or detection purooses. The ideal filter will extract essentially all of the light in a channei with good extinction ratio, so that the same wavelength may be used later in the network without undesirable crosstalk. It must have very low insertion losses for the out-of-band components because multiple channel dropping filters may be installed on any given line. Preferably, it should be switchable so that a channel may be dropped at a destination location, and after the communication is finished, the channei may continue past that location to another destination. The inverse of the channel dropp g filter is the channei adding filter which adds a channel to a fiber without significantly affecting the power propagatmg m the other channels. Transmission and reflection filters have been analyzed in detail [HL91, KH087]. Several of the above structures may be used for channel droppmg filters, including the devices described in reference to FIGS. 7, 10, 26, 27 and 28.

The grating coupled waveguide tee of FIG. 7 is a channel droppmg filter with low loss for the out-of-band components. With prior art gratings, this configuration has difficulty with crosstalk, since achieving 99.9% outcoupling for the in-band component requires a very long grating. The coupling strength of our periodic poled gratings is significantly increased over the prior art, due to the ability to use higher order gratings with shaφ interfaces which extend entirely across the waveguide. Whereas the prior art is limited to shallow waveguides to optimize the overlap between the necessarily shallow grating and the waveguide, we are able to use the lower loss waveguide configuration with essentially equal depth and width because our grating structure extends entirely across the depth of the waveguide. This structure can also be used as a channel adding filter. The device of FIG. 10. if the gratmg is configured as descπbed m Haus et al. "Narrow band optical channel droppmg filters" J. Lightwave Technol. J_0, 57-62 (1992), is also a channel dropping filter. Our contribution in this case is only the poled gratmg coupling technique, which enables strong coupling between the waveguides in a short distance, and which relieves fabrication difficulties in permitting efficient higher order gratings to be produced. The devices 750, 760 and 770 can be used as channel droppm filters by tumng a resonance of the eulon to the frequency of the channel to be extracted from the input waveguide 752. If the integrated e lon is nearly impedance matched, essentially all the power at the resonant frequency is transferred into the eulon. In the nng geometries of FIGS. 27 and 28, the transmitted and reflected powers in the waveguide 752 can be reduced to any desired level, minimising crosstalk. The light corresponding to the desired channel is completely extracted (dropped) from the mput waveguide, leaving neither reflections or transmissions. In the linear geometry of FIG. 26, some light is lost to reflection, which does not significantly reduce the detection efficiency, but which may cause crosstalk problems m a commumcations network. The signal earned by the light can be detected by placing a detector over a waveguide segment or the etalon and counted to the light in the waveguide Or. the detector can be coupled to one of the output waveguides such as 754 in FIG. 26, 764, 765. or 766 in FIG. 27, and 794 in FIG. 28. In the case of the device 760. the outcouplmg can be accomplished by adjustmg the reflection of one of the resonator gratmg reflectors 767, 768 or 769 so that a small portion of the circulatmg power is coupled out mto the continuation segments of the waveguides as snovvn for ouφut beam 751. Those continuation waveguide segments may also be connected to ports ot other devices, which may be either discrete devices or integrated on the same substrate. In the case of the device 770, a parallel waveguide output coupler (with or without gratmg) may be placed m the straight section 771 of the nng. Although only a fraction of the circulatmg power may be outcoupled at these ports, the total outcoupied power may be very close to 100% of the channel power entermg the waveguide 752 due to the buildup which occurs m the eulon. Output couplmg is shown with an adjacent waveguide 794, producmg the output beam 751.

The nng geometnes excel m terms of extmction ratio (which is high when the light separation efficiency is high) and low crossulk because they can be adjusted to have almost toUl transfer of power mto the eulon. All of the eulon devices can be designed with very low msertion loss for the out-of- band beams. All of the devices of FIGS. 26-28 are switchable by means of the phase shifting electrodes

761, and 762 (and 763 m FIG. 28).

As descnbed before, the optical path length may be adjusted usmg electrode 761 to shift the frequency of the integrated etalon resonances. The desired channel may be selected this way directly. Or, multiple channels may be selected by this techmque usmg the approach descnbed above m reference to FIGS. 20, 21, and 22; if the FSR of the eulon is selected to be slightly different from the channel separation, the Moire effect is used to select widely spaced channels with a minimum of continuous tu ng. (A good choice is to make the FSR equal to the channel spacing plus a few times the frequency width obtained when convolvmg the channel bandwidth with the eulon resonance bandwidth).

As a vanation on the structures 750, 760, and 770, the coupling region 753 may be implemented as a grating-assisted coupler as descnbed above in reference to FIG. 10. This has the advanuge, m the poled-grating implemenution, that the couplmg fraction T can be adjusted. Particularly for the nng resonator designs 760 and 770, an aαjusuble couplmg is useful to achieve impedance matching. As a further vanation. the electrodes may be implemented on the same face of the substrate, as descπbed above to obtain lower volUge excitation. The structures of FIGS. 27 and 28 may also be used as efficient channel addmg filters if the signal to be added to the output beam 759 is brought m on the waveguide 766, for example, or if it is coupled mto the straight section 771 via the waveguide 794. These mput interactions will preferably be impedance matched.

Referring now to FIG. 29A, there is shown a waveguide modulator/attenuator 800 usmg a poled segment 806. The function of the poled segment 806 is to (switchably) collect the light emitted from an mput waveguide segment 802 and launch it mto an output waveguide segment 804 when switched on. In this device, an mput light beam 820 is coupled mto the input waveguide 802. A poled segment 806 is positioned between the mput segment and the output waveguide segment 804 The input and output waveguide segments are preferably permanent waveguides which may be fabπcated by any of the standard techniques mcludmg mdiffusion and ion exchange. The segment 806 is preferably a reverse poled region within a umformly poled substrate so that there is essentially no difference in mdex of refraction and hence no waveguiding effect when the electπc field is off. The segment 806 is a waveguide segment as shown in FIG. 29A. (It may alternatively be configured m several geometπcally different ways such as a positive lens structure, a negative lens structure, or a compound structure for relaying light between many such elements: see FIG. 29B.) The segment 806 is turned on by appiymg an electπc field through the segment. The electnc field changes the index of refraction of the poled segment and surrounding regions. Because the segment 806 is poled differently (preferably reverse poled) from the substrate material, the index of the segment can be raised relative to the surrounding material by applying the correct field polarity, forming a waveguide. The mdex inside the boundary of the waveguide may be increased, or the index at and outside the boundary may be depressed. When the poled segment is on, a contmuous waveguide is formed, joining the mput and output segments. This is achieved by butting the waveguides together, aligning them to the same axis, and adjustmg the width of the poled segment so that its transverse mode profile optimally matches the mode profile of the mput and output waveguides 802 and 804.

With the poled segment off, the mput beam is not confined m the poled region, so that the beam expands substantially by diffraction before it gets to the output waveguide segment. If the separation of the input and output waveguide segments is much greater than the Rayleigh range of the unguided beam, so that the beam expands to a dimension much larger than that of the output waveguide, only a small portion of the input beam will be coupled into the ouφut waveguide segment to form the output beam 822.

By adjusting the length of the segment 806 relative to the Rayleigh range, the amount of power transmitted in the off condition can be reduced to the desired degree.

The location of the ends of the poled segment 806 are adjusted relative to the locations of the ends of the mput and output waveguides to minimize the loss caused by the discontinuity. Because the permanent waveguides have a diffuse boundary, the poled waveguide has a discrete boundary, and the mdex change in the switched segment adds to the pre-existing mdex, it is desirable to leave a small gap on the order of half the diffusion length between the lithographically defined boundary of the waveguides 802 and 804, and the ends of the poled segment 806. To further reduce the reflection and other loss at the junction between waveguides 802 and 806, it is also advanugeous to Uper the onset of the index change in the segment 806 by either making the exciting electrode 810 slightly shorter than the segment 806 or by tapenng the electrode width near its end, in both cases taking advanuge of the reduction of the electric field by the fringing effect.

One distinguishing aspect of this configuration is that the reflected power can be minimized in both the on and the off conditions. With the switch off, the reflection is dominated by the residual reflection at the end 803 of the waveguide 802. This reflection may be minimized by Upenng the reduction of the mdex difference along the length of the waveguide. The reflection from the end 805 of the waveguide 804 is suppressed by the square of the "off" transmission. In the "on" condition, the reflection is rninimized by also Upering the index difference of the structure 806 along the direction of propagation, creating a smooth boundary rather than a shaφ interface.

The boundaries of the excited poled region confine the beam laterally when they are activated because of the increase in the index of refraction within the boundaries. If the depth of the poled region equals the depth of the waveguides 802 and 804, the beam is also confined in the vertical direction by the poled segment boundaries. However, it is difficult to control the depth of the poling in a z-cut lithium niobate wafer. It is easiest to pole a deep domain, and take one of several alternative measures to obtain confinement in the vertical dimension. The preferred approach is to arrange the electrodes so that the amplitude of the electric field falls off in the vertical dimension. This is achieved by the same-side electrode configuration shown in FIG. 29A, but not with electrodes placed on opposite sides of the substrate. The penetration depth of the electric fields can be reduced by narrowing the gap between the two electrodes and by reducing the width of the overall electrode structure.

In addition or as an alternative, a weak permanent waveguide can be fabricated in the volume between the input and output waveguides, which is insufficient to convey much energy by itself, but which in combination with the index elevation produced in the poled segment 806 can optimally confine the light in two dimensions to convey essentially all the light into the output waveguide 804. This can be done, for example, by adjusting the permanent index change (relative to the substrate) within the segment to about 0.6 of the index change in the waveguides 802 and 804. If the "on" index change in the segment 806 is adjusted to about 0.5 of the same value, the combined index change is sufficient to achieve reasonable guiding while the permanent index change is insufficient. In the "on" condition, the mode is confined in both transverse dimensions even though the switched index change produced in the poled region may be considerably deeper than the desired waveguide dimension: the effective depth of the "on" waveguide is mainly determined by the permanent index change. The weak waveguide may be fabricated in a second masking step, or it may be fabricated in the same masking step with a narrower mask segment defining the weaker waveguide segment.

As a related alternative, the region between the input and output waveguides may be a planar waveguide, in which case the propagating mode can at minimum diffract in one dimension. Switching on a poled section will in this case add the needed transverse confinement despite having a deeper index change than the planar waveguide. Since in both cases the confinement of the waveguide in the two dimensions is achieved by two independent techniques, switchable waveguides of essentially any aspect ratio (the ratio of the waveguide width to depth) can be formed. Both the planar and channel waveguides can be fabricated by the same techmque, which is preferably the annealed proton exchange process. Separate proton exchange steps may be used to define the planar guide and channel waveguide. The waveguide fabrication process is completed by annealing, during which the index changes are diffused down to the desired depth, and the optical activity of the material is restored. Preferably, the two sets of guides are annealed for the same length of time, although one set can be made deeper by partially annealing before the second proton exchange step is performed. An important alternative is to use a mil. unirorm permanent waveguide traversing the poled segment 806, and to use the electπcaily excited segment to rum off the guidmg. In this case, the polaπty of the field is chosen to depress the mdex in the poled region, and the depth of the poled region can be very large (m fact this has some advantage in terms of mode dispersal). This type of switched waveguide is normally on (i.e. transmitting), and requires the application of an electπc field to switch it off. There are advanuges to both normally-on and normally-off switch configurations m terms of their behavior dunng a power failure, so it is important that this mvention is capable of providing both modes. To switch the waveguidmg off m the segment 806. an mdex change is desired which is approximately equal and opposite to the mdex change mduced m the permanent waveguide. The effect of the vanation with depth of the electπc field on the "off^* sUte is quite small because it is sufficient to suppress the majonty of the waveguide in order to strongly disperse the light.

Confinement can be achieved m both dimensions without the need of a planar waveguide, by a finite-depth polmg techmque. Several polmg techniques (such as for example tiumum-mdiffusion m lithium mobate and lithium tantaiate and ion exchange in KTP), produce polmg to a finite depth, which can potentially be optimized to form a poled channel waveguide with a particular depth. These techniques, however, produce an mdex change along with the polmg, forming somewhat of a permanent waveguide dependmg on the processing parameters. Dependmg on the strength of this mdex change, the poled waveguide segment may be fabπcated m either the "normally on" or the "normally ofr configuration. Preferably, the electnc field is created m the poled region by applymg a volUge across two electrodes, which are laid out on the same face of the crysul as the polled waveguide segment. A first electrode 810 is laid out over the poled region, while the second electrode 812 is placed in proximity to one or more sides of the first electrode. For a z-cut crysul. this configuration activates the d₃₃ electro-optic coefficient of the substrate. A volUge source 816 is electπcaily connected via two wires 814 to the electrodes to provide the dπvmg volUge for the device. This device can be used as a digital or nonlmear analog modulator. A full-on volUge is defined to be the volUge at which the loss across the poled region is the lowest. The off volUge is defined as that volUge which reduces the couplmg to the output waveguide segment to the desired extent. By continuously varying the volUge between the on and the off vol ges, the device can be used as either an analog modulator or a vanable attenuator.

In an alternative structure, the structure 806 forms a switched curved waveguide, which agam aligns with the mput 802 and output 804 waveguides. The mode of such a structure is called a

"whispering gallery" mode in the extreme case where the curvature is small and the mode confinement on the mside edge becomes mdependent of the inside waveguide edge. For larger curvatures, the mode is a modified whispering gallery mode where some confinement is provided by the inside edge of the waveguide. The poled structure provides an advanuge in addition to the switchability, namely that the shaφ mdex of refraction transition on its outside wall greatly improves the confinement of the modified whispering gallery mode which propagates in the curved waveguide. The mput and ouφut waveguides need not be coaxial or parallel m this case, potentially increasing the forward isolation m the switched-off condition. If the mput and output waveguides are arranged along axes at an angle to each other, the structure 806 may be a curved waveguide segment with a single radius of curvature or a upered radius of curvature, used to optimally couple power between them when the curved waveguide structure 806 is turned on.

FIG. 29B shows an alternative structure 801 which is a switched lens modulator/attenuator m which the pπsmatic structure of segment 806 is modified into a lenslike structure in which the product of the local optical path length and the local (signed) mdex change is reduced quadratically with transverse distance away from the axis of the guides 802 and 804. The lenslike structure is placed such that it concentrates or refocuses the beam 821 emerging from the end 803 of the mput waveguide 802 mto the end 805 of the output waveguide 804. The optical wave is allowed to diffract away from the end 803, and passes through the lenslike structure 807. Note that in this structure, multiple elements may be placed adjacent each other, increasing the net focussing effect. The index of refraction withm the regions 807 is increased to obtain a focussing effect. If the surrounding region is poled in a reverse direction to the regions 807, or if the electro-optic coefficient of the suπounding region is otherwise opposite to that of die regions 807, the spaces between the lenses also act as focussmg regions. (The negative lens shape formed by the regions between the lenses 807, excited to a lower index value, acts as a converg g lens structure.)

The electrode 810 is placed over the structure 806 with electrodes 812 being placed outside the structure but adjacent the electrode 810 with a gap as desired. When the electrodes are not actuated, the beam contmues to diverge, and very little power is refocussed into the waveguide end 805. When the switch is on, the beam is refocussed, and a fraction of the power contmues through the guide 804. Vertical confinement is needed for efficient power collection in the on state, while it is undesirable in the off sUte.

Vertical confinement may be provided as needed by, for example, providing a umform planar waveguide 835 across the entire surface on which the structures are patterned. Vertical confinement may also be provided by the lenslike structure 806 if it is poled deep into the substrate, and the electnc field reduction as a function of depth is Uilored to collect and refocus the energy back to the waveguide end 805. The structure of FIG. 29B may of course also be used in other contexts which may not have one or both waveguides 802 and 804.

Referring to FIG. 30, there is shown a poled total internal reflectmg (TIR) optical energy redirector 830 usmg a poled waveguide segment. This figure illustrates both a poled ΗR reflector for high switched reflection combmed with a poled waveguide segment for low msertion loss. An mput waveguide 832 extends entirely across the device. A poled region 836 extends across the waveguide at an angle 848, forming a TIR interface for the beam propagatmg in the guide when the poled region is electro-optically activated. A portion of the poled region also forms a poled waveguide segment 837 that is connected to an output waveguide segment 834. The poled waveguide segment and the output waveguide segment are both laid out at twice the angle 848 with respect to the mput waveguide. A volUge source 846 provides the electncal activation for the switch, and is connected to it through two wires 844.

The poled region 836 is defined by six vertical faces accordmg to the diagram, with one face traversing the waveguide 832 at a shallow angle 848 equal to the ΗR angle and less than the cntical angle for total internal reflection for a desired electrode excitation. This face is the ΗR reflectmg mterface. The next three consecutive vertical faces ot the poied region enclose a projection outside of the waveguide 832. The projection is a switchable waveguide segment. The placement ot the next two vertical faces is not cπtical, and may follow tne waveguide boundaries and cross it at 90°

The domains (836 and the region of the substrate outside 836) are charactenzed by a quiescent index of refraction distnbution, which is the spatial distπbution of the mdex in the absence of applied electπc field. When an exciting electπc field distπbution is applied through the domams, they will have an excited mdex of refraction distπbution which is different from the respective quiescent distπbution. The excited distnbution will also have a range according to the accessible range of the applied electnc field. The advanuge of juxuposing two domain types near one another is that the electnc response may be opposite m the two domams, producing a transition with double the change m index across the region of juxtaposition. In the case of mdex or refraction changes, the transition forms a reflection boundary with larger reflection than would be attained with a single domain type.

When the switch is on, an input beam 851 that is coupled mto the waveguide reflects off the TIR interface, propagates down the poled waveguide segment, and passes into the output waveguide segment 834 to form a deflected output beam 854. When the switch is off, the mput beam propagates through the poled interface and continues through the input waveguide to form an undeflected output beam 852. Because the mdex change at the TIR mterface is low, the reflection m the off sUte is very low. Because the permanent waveguide segment 834 is separated by several mode exponential decay lengths from the guide 832, the power lost due to scatter as the beam passes by the switching region is also extremely low. An "ofr switch is essentially invisible to the waveguide, producing extremely low loss m the mput guide. The additional loss of the switched region in the off sUte compared to an equal length of unperturbed waveguide is called the msertion loss. Low insertion loss is especially desirable when the mput waveguide is a bus with many poled switches.

The angle θ (848) ot the poled interface with respect to the input waveguide must be less than the maximum or cπtical ΗR angle θ_c, as deπved from Snell's law:

θ ≤ θ_c = cos^_1(l - 21Δw| |Δ/z| (3) n n

where θ = TIR angle (between the waveguide and the polmg interface), n = mdex of refraction of waveguidmg region, and

Δn = electro-optic change m index on each side of polmg boundary Smce the mdex change occurs on each side of the poling boundary with opposite sign, the effective mdex change is 2Δn. This expression assumes slowly varying (adiabatic) changes in the index away from the boundary. Due to the doublmg m the effective mdex change, the maximum switchmg angle that can be achieved with a poled ΗR switch is mcreased by /2 over the pπor art switches with a pair of electrodes ana no poled mterface. This is a very significant increase since it increases the maximum packing density of switch arrays which can be achieved using a TIR switch.

The cπtical angle 0_C depends on the polarization of the input beam because the mdex change Δn depends on the polanzation. In z-cut lithium niobate. for example, with a vertical field E,, the TM wave is sensitive to the change the extraordinarv index or refraction through the r₃₃ and the TE wave to the change m the ordinary mdex through r,,. Since r„ r,_Jt it is far easier to switch TM waves. Use of annealed proton exchanged waveguides is very convenient because they guide only waves poianzed m the z-direction. In x-cut y-propagating (or y-cut x-propagating) lithium niobate, on the other hand, the TE wave has the higher change m index. Note that in this case, the electrode configuration must be changed to produce a field component in the z direction in the plane of the substrate, instead of m the vertical direction.

The design angle for actual TIR switches must be chosen after optimizing several factors. The mode to be switched includes two angular distributions (in the waveguide fabncation plane and out of the plane) which can be different if the widths of the waveguide in the two planes are different. The angular content bφ of the mode a given plane covers approximately bφ = ± λ/πw₀ where w„ is the 1/e² mode waist m that plane. We wish most of the light to be reflected at the TIR interface, so the angle of mcidence must be less than the cntical angle θ_c by approximately the angular content bφ in the plane of the switched waveguides. The angular content bφ is inversely related to the waist size, but so is the packing density which we wish to optimize. The angular content of the mode in the direction out of the plane of the waveguides also must be taken mto account because it also contnbutes to the effective mcidence angle, although in a geometncally more complex way.

An alternative way of producing a TIR switch is with a stram field instead of or in addition to the electπc field. The stram field is most conveniently implemented in a permanent fashion; the electπc field is most useful for producmg changes in the reflection. An onented strain field applied at a domam boundary produces different changes in the index of refraction, v ia the photoelastic effect, in the two domains, resultmg in an index of refraction mterface. As mentioned above in reference to FIG. 2, the stram field may be produced by heating the sample to a high temperature, depositmg a film with a different coefficient of thermal expansion, and cooling to room temperature. A pattern applied to the film by etching away regions such as stπps will produce a strain field about the gap m the film. This stram field can then be used to actuate an index of refraction difference at domam boundaπes. If the applied film is a dielectπc an electπc field may be applied through it to the poled regions provided that the deposition of electrodes does not undesirably change the stram field. The film is preferably a film with low optical absoφtion so that it can be contacted directly to the substrate instead of being spaced by a buffer layer.

The poled region mcludes a portion of the input waveguide and has an mterface normal to the propagation axis of the waveguide. The portion of the mput waveguide that contains the ΗR mterface crossmg defines the length of the switch:

where θ is previously defined, I = Wfcθt(θ) - — ⁽ ) θ

L = length of the switch measured along the input waveguide, and

W = width of the waveguide

Thus, in order to minimize the size ot the switch, the width of the waveguide must be made as small as possible. For space-cntical applications, it is preferable that the waveguide segments be smgle mode. As a numencal example, if the width ot the single-mode waveguide is 4 μm. the maximum mdex change Δn is

0.0015. and the mdex of refraction is 2.16, then the TIR angle θ is 3° and the length of the switch L is 76 μm.

The poled waveguide segment torms an angle with respect to the input guide equal to 20, which is the deflection angle of the TIR intertace. In order to efficiently modematch the beam reflectmg off the TIR intertace into the poied waveguide segment, the poled segment should have nearly the same transverse mode profile as the input waveguide. Efficient mode matching can be achieved by selecting the proper combination ot width and index difference of the poled waveguide. The poled waveguide segment intersects the mput waveguide along the latter half of the side of the waveguide occupied by the switch mterface. The exact dimensions and placement of the waveguide are determined to optimally match the near field mode profile emerging from the toUl internal reflection process to the mode of the waveguide m terms of direction of propagation and transverse profile. The same is true of the match between the poled waveguide segment and the permanent waveguide segment 834, similarly to what was descπbed above m reference to FIG. 29 A.

The permanent waveguide segment is essentially a continuation of the poled waveguide segment. The length of the poled segment depends on optimizing losses in the input waveguide and the switched waveguide. In order to avoid scatteπng interaction between the undeflected beam in the mput waveguide when the switch is off, the permanent waveguide segment must be separated by some distance (at least an optical wavelength) from the input guide. For a bus waveguide with many switches, the loss m the mput guide must be reduced to a value related to the inverse of the number of switches. The modal profile of a beam m the input guide extends a certain distance beyond the lndiffused edge of the guide, where it decays exponentially. If the permanent segment is separated from the mput guide by several of these exponential decay constants, the loss can be reduced to an accepUble level for a bus waveguide.

The length of the poled segment affects the loss in the reflected beam as well. The poled waveguide segment may have higher losses per unit length than an indif fused waveguide, due to higher wall roughness. In addition, there are the above mentioned mode conversion losses at each end of the waveguide, which are minimized by optimally matching the mode profiles. If the poled segment is short (on the order of the Rayleigh range of the beam), the transmitting beam does not substantially convert mto the mode of the poled segment, thus reducing the coupling losses. The optimal length of the poled segment depends on the relative loss that is tolerable m beams in the mput waveguide and the switched waveguide. As m the case of the waveguide segment modulator/ attenuator snown m FIG. 29A, there is a need for vertical confinement of the mode in the switched waveguide segment 837 The same options descπbed there can be implemented here. Shown in FIG 30 is a planar waveguide 835 which confines the beam in a plane parallel to the surface of the substrate Since the planar waveguide is umform, its presence does not affect the loss of the waveguide switch junction in its off sUte In place of the planar waveguide, or in some combination, the other alternatives mav also be implemented, mcludmg ilormg the depth of the electnc fields to obtain vertical confinement, using short depth poling, using a partial waveguide which is augmented by the field induced index change, and usmg a full permanent waveguide which is turned off by a field activated poled region The latter two alternatives have the disadvantage that the loss to the beam through waveguide 832 is higher due to the adjacent mdex discontinuities

Hoπzontal confinement is also an issue in optimizing the switching region. If high switched efficiency is desired, it is preferable to have a large TIR reflection angle. The left half of the mput wave 851 reflects first off the interface 838, forming the πght half of the reflected wave. However, after reflection, the nght half of the reflected wave is unconfined in the transverse dimension until it arπves mto the waveguide segment 837. Dunng its unconfined passage, it will expand by diffraction, reducmg the fraction ot the beam power which couples into the output waveguide 834 This effect degrades the efficiency of the switch in its on position. However, the mean unguided distance is limited to approximately the waveguide width divided by four times the sine of the angle 848 The πght half of the mput wave remains confined after it passes the waveguide segment 837 until its reflection off the mterface 838 because of the permanent index change due to the πght hand side of the waveguide 832. It then matches well mto the output waveguide 834. Both portions of the input beam 851 suffer an undesired reflection from the side of the waveguide 832 after reflecting from the TIR surface 838. Smce this surface is at the same angle to the axis of propagation of the beam as the surface 838 was. but with only a fraction of the mdex difference, there only be a partial, not a total, reflection from this surface which also adds to the loss of the switch.

The electrode design is a cπtical aspect of this switch, in order to optimize the efficiency of the reflector and minimize the loss of the waveguide Preferably, two electrodes are used to activate the switch. A first electrode 840 is placed over the TIR interface 838, while a second electrode 842 is placed alongside the first electrode, adjacent to that mterface. The main parameters for optimization are the separation of the two electrodes and the distance between the edge of the first electrode and the polmg boundary, which may or may not overlap. The spacmg between the two electrodes influences the volUge required to activate the device, as well as the width of the electπc field pattern which penetrates the substrate and produces the mdex change profile. Electrodes that are spaced further apart require higher volUges, but create an electπc field that extends deeper into the substrate than closely spaced electrodes. The electnc field penetration depth is cπtical to obtaining a large net reflection. Because the fields get weaker the farther they are away from the electrodes, the mduced mdex change at the polmg boundary also drops with depth, as does the TIR angle. At a certain depth called the effective depth, the mdex change becomes insufficient to maintain total reflection for the central ray of the optical beam at the angie ot the switch structure. Since the reflection drops rapidlv with index chance at values below the mmimum ΗR value, the TIR mirror essentiaiiv stops runctioning at this depth. For high net reflection mto the guides 837 and 834, the device design snouid be adiusted to create an effective depth below the majonty of the field profile in the guide 832. The second important operating parameter influenced by the electrode design is the penetration of the evanescent fields of the reflecting wave bevond the TIR interface 838 Although no power may be transmitted beyond the TIR interlace in the "on' condition, the electromagnetic fields penetrate the ΗR surface by a distance on the order of a wavelength. There will also be spatial dependence of the applied electπc field beyono the TIR surface, the field strength being reduced (and in fact inverted) in regions closer to the other electrode 842. The index change is therefore reduced beyond the TIR interface. Care must be taken that the evanescent fields decay to a negligible value before substantial vanation in the field occurs, or power will leak through the TIR interface. Optimally, the first electrode will overlap the pohng interface by a distance cnosen ior maximum index change and for sufficient constancy of field beyond the intertace 838 The first electrode also extends across the poled waveguide segment 836, and possibly mto adjacent areas. The shapes of the two electrodes exciting this region 836 are determined by optimizmg the power flow through the waveguide segment and into the permanent waveguide 834. Other electrode structures can be used to modify the strength of the electπc field in the poled region. If, for example, the second electrode is extended around the first to form a U shape, the electπc field under the first electrode is increased on the average, but it forms somewhat ot a two-lobed waveguide, which may not provide an ideal mdex profile.

The TIR switch is an optical energy router and can also be used as a modulator. If the volUge source is contmuously vaπable, then the modulator is analog, with a nonlmear relationship between volUge and reflectivity. As the applied voluge is increased, the depth of the total reflectmg mterface is mcreased, producmg a contmuously adjustable reflection out of the wave 851 into the wave 854. The modulator can be used m reflection or transmission mode, depending on whether the transmission should go to zero or 100% when the volUge is removed. For special nonlinear applications, the nonlineanty of the reflection and transmission coefficients as a function of volUge, such as where the receiver is loganthmically sensitive to the level of the signal, might be useful. FIG. 31 shows a TIR switch with two TIR reflectors. If it is desired to mcrease the angle between the output waveguide 834 and the mput waveguide 832, a second ΗR mterface 839 may be added. The angle between the mput waveguide 832 and the output waveguide 834 is doubled relative to that of FIG. 30, and may be doubled again and again by adding additional ΗR interfaces. The interface 839 is created at an angle 849 relative to the interface 838 equal to twice the angle 848. (Subsequent TIR interfaces, if any are added, should be added at the same angle 838 relative to the previous ΗR mterface.)

The switched waveguide portion 837 of FIG. 30 is no longer required since the dual ΗR mirror structure brmgs the light so far away from the mput waveguide 832 that the permanent waveguide 834 may be butted directly agamst the end of the poled region 836 without contπbuting significant loss to the waveguide 832. Again, vertical confinement is provided in the poled segment 836. The poied segment 836 and the ouφut waveguide 834 are configured and aligned so that the field profile propagating m the cham of ΗR and waveguide segments optimally matches the local lowest order mode field profile of the input waveguide 832. After the TIR reflectors, the deflected beam is matched into a permanent waveguide 834 to form the output beam 854 when the switch is on

The shape of the inside boundary of the poled region outside the mput waveguide 832 is defined by the reflection of the mput waveguide through the TIR minors, one after the other. This defimtion of the inside boundary achieves optimum guidmg of the inside edge of the waveguide mode while it is reflectmg from the two ΗR mirrors. FIG. 32 shows a TIR switched beam director with a poled TIR switch 831 with an electrically switched waveguide segment. In this structure, the region 836 is reverse poled, lies behind the mterface 838, and is excited as before by a pair of electrodes 840 and 842, which are activated by volUge source 846 and connected via conductors 844. The polaπty of the excitation is again selected to produce a negative mdex change coming from the direction of the input beam 851 When the switch is on, the beam is reflected off the TIR interface 838 towards the permanent waveguide 834, but unlike m FIG. 30, there is no poled waveguide segment joining them Instead, the electrode 842 is extended over the intermediate region between the mput waveguide and the output waveguide 834. A couplmg waveguide segment may be formed by applymg an electnc field to a region between a lateral boundary of the segment of the mput waveguide 832 comaining the ΗR reflection boundary and an mput boundary of the output waveguide 834. The three dimensional distnbution of the electnc fields is determined, as always, by the shape of the electrodes and Maxwell's equations The electnc fields produced by that electrode produce a positive mdex change through the electro-optic effect, providing the desired switched waveguidmg section. As descπbed elsewhere, this waveguide segment is also configured and aligned to optimize the couplmg of the mput mode 851 mto the output mode 854. As an alternative in this and any of the TIR switch lmplemenUtions, the output wavegmde may oπgmate at the mput waveguide with negligible gap. This alternative has higher msertion loss m the switch off (straight through) configuration, but it has a simpler structure.

Referring to FIG. 33, there is shown a two position waveguide router usmg a poled segment, which is not based on toUl internal reflection. The poied region 866 forms an electncally exciuble waveguide segment which crosses the mput waveguide 862 at a small angle. When the field is applied, the mdex m the segment 866 is mcreased, while the mdex in the adjacent region m the mput waveguide is decreased. Thus, the mput beam 880 is at least partially coupled into the poled waveguide segment. When the switch is off, the mput beam contmues to propagate through the mput guide to form an unswitched output beam 882. The small angle may be Upered adiabatically, forming a low loss waveguide bend, if it is desired to switch all or most of the mput light into the output guide 864 to exit the device as the switched output beam 884.

At least two electrodes are used to apply an electnc field across the poled region to activate the waveguide. A first electrode 870 is positioned above the poled waveguide segment, while a second electrode 872 is positioned adjacent to the first electrode. The second electrode 872 is adjacent to the first electrode and may be placed on ootn siues or the poled waveguide segment, in order to achieve a high power splitting ratio. As before, the electrodes are excited by the power supply 846 through conductors 844, and a planar waveguide 835. or the eiectπc field falloff with depth, or one of the other approaches described herein is used to obtain vertical confinement for the switched propagatmg modes. Referring to FIG. 34. several poled TIR switches are placed side by side to form an array

900. The poled regions 912 and 914 forming the TIR interfaces are placed one after the other along the waveguide 910. Each poled region has the same crysul oπenution, with the z axis of the crysul in the regions 912 and 914 reversed relative to that of the remainder of the crysul. The other aspects and many variations of this configuration have been descπbed above in reference to FIG. 30. Each of the switches are individually activated using a multi-output volUge control source

926, which is connected to the electrodes with wires 928. When all switches are off, the mput beam 902 propagates down the input waveguide 910 to form an unswitched output beam 904 with negligible loss. If the first switch is on. then the input beam reflects off the first TIR interface to form a first reflected output beam 908 in waveguide 916. If the first switch is off and the second on, the input beam reflects off the second TIR interface to form a second reflected output beam 906 in waveguide 918, and so on for the subsequent switches. This multiple switch structure can be extended to n switches.

An electrode is laid out over each TIR interface as described above. One or more of the electrodes 920, 922. and 924 serve as the cathode for one switch and the anode for another. For example, a volUge is applied between the second electrode 922 and the first and third electrodes 920 and 924 to activate the second switch fonmng the output beam 906. An electrode 922 that acts as both an anode and a cathode should preferably extend adjacent to the TIR interface of the pπor poled segment 912 while also covering the TIR interface of one poled region 914 and one waveguide segment of one poled region 914. Only a portion of the structure is shown, with two complete poled segments 912 and 914 and one complete electrode 922. This structure can be replicated into n switches by aligning duplicate complete electrodes and poled segments.

In order to avoid crossulk in the channels, the volUge on the electrodes may be applied in such a manner that the input beam does not see any electro-optic index changes until it enters the region of the activated switch. For example, to activate the TIR interface of the second poled region 914, a volUge may be applied between electrodes 922 and 924, keeping the same potential on electrodes 920 and 922 and pπor electrodes.

Although the toUl length of the poled regions is longer than L, the distance occupied along the waveguide by a given region is equal to L by definition. A linear array of TIR switches with a 100 % packing density would therefore have new poled regions surting every distance L. This is called 100% packing density because at this density the adjacent regions just barely touch each other at the inside comer of the poled region in the waveguide. Having adjacent regions touch each other is disadvanUgeous because some of the light guided in the previous poled structure can leak out into the next poled structure where the structures touch. We have noted above that the comer which touches the preceding poled region is formed . by two vertical faces of the poled region whose placement is not cntical. By movmg these faces so that the width of the poled region is thinned on the side of this inside co er, it can be arranged that the regions no longer touch each other, reducing the leak of optical energy. For example, the inside comer can be moved to the middle of the waveguide by halvmg the length of the face which traverses the waveguide at 90°.

The face which used to parallel the waveguide now parallels the TIR interface, and becomes a cπtically positioned surface. We call the poled regions with this geometry "dense packed" poled regions. (There are other ways the objective of minimizing the light leak may be accomplished, such as adding a seventh vertical face between the two noncπtical faces, but the alternative just described has another advanuge in dense packing.)

FIG. 35 shows a configuration wherein the lmear density of switches is be doubled by using the dense packed geometry for the poled region and reversing the polaπty of the adjacent poled regions. The interfaces of the poled regions transverse to the waveguide are now identical but for a translation along the axis of the waveguide. The poied regions will therefore sUck solidly along the waveguide, doubling the switch density. In fact, only the reverse poled region is fully spatially defined, smce the other region has the same poling direction as the substrate (in the optimal case where the substrate is fully poled). Two regions 952 and 954 of reverse poling are shown in FIG. 35. The ΗR interfaces can be thought of as the first face or the input face and the second face or the output face of the poled region where unswitched light travelling m the waveguide 950 potentially enters or leaves, respectively, the unexcited poled region.

The TIR mterface for the output beam 946 is formed between the poled substrate and the first (mput) face of the reverse poled region 952, and is excited by electrode 966. The ΗR interface for the output beam 947 is formed between the second (output) face of the reverse poled region 952 and the poled substrate, and is excited by electrode 967. The TIR interface for the output beam 948 is formed between the poled substrate and the first face of the reverse poled region 954, and is excited by electrode

968. The ΗR interface for the output beam 949 is formed between the second face of the reverse poled region 954 and the poled substrate, and is excited by electrode 969. The electrodes extend above the respective TIR interfaces, and along the switched waveguide segments which connect to the permanent output waveguides 956, 957, 958, and 959. Preferably, one or more of the electrodes 966, 967, 968, 969 and 970 serve as the cathode for one switch and the anode for another. Each electrode therefore extends parallel to and along the full length of the ΗR mterface of the previous switch.

Each of the switches is individually switchable by applying electric fields with volUge source 926 via conductors 928. When all switches are off, the input beam 942 propagates down the bus waveguide 950 to form an unswitched output beam 944. When the first switch is on, the input beam reflects off its respective ΗR interface to couple mto the first output waveguide segment 956 to form a first reflected output beam 946. For the subsequent switches, the input beam reflects off the respective subsequent Η interface to couple into a waveguide segment 957, 958, or 959 to form a reflected output beam 947, 948, or 949. The volUge on the electrodes is typically set so that there is no optical interference from adjacent switches, all preceding switches are olf This can be accomplished for example by maintaining all the preceding electrodes at the same potential as the switched electrode This multiple switch structure can be extended to n switches

It is desirable to extend the upstream end of the dense packed poled regions significantly beyond the edge of the mput waveguide 950, maintaining the angle of the vertical surfaces with respect to the waveguide. This extension captures the full exponential Uil of the input waveguide mode, and pushes the remaining noncπtically positioned surface of the extended dense packed poled region out of the waveguide 950, thereby dimmishmg the optical loss (Upstream and downstream are defined m relation to the direction of propagation of the mput beam 942.) If the switched waveguide segment of the poled regions is designed as descπbed above in reference to FIG. 30, the separation of the ouφut waveguides becomes equal to their width m the highest density packing, so that they merge into a planar waveguide While a planar output waveguide may be useful for some applications, the output waveguides mav be separated using a second poled ΗR mterface withm each switch. The use of two TIR interlaces in a switch has been descπbed in reference to FIG. 31 Note that in the case of FIG. 35, the geometry of the poled region is slightly different to accomplish the stacking The "output waveguide" section of the extended dense packed poled regions is routed about the end of the first ΗR mterface to an angle 3Θ relative to the mput waveguide 942, maintaining the parallelism of its faces. This "output waveguide" section therefore becomes a second ΗR reflector segment. The width of the second TIR reflector segment is about 50% larger than the mput waveguide. The mode propagatmg in the second TIR reflector segment is unconfined on its inner side for a distance of about 2W/sιn0 where W has been defined as the waveguide width. Any diffraction which occurs on this side will result m reduced power coupling into the output waveguides 956-959. It is desirable to keep this distance less than about a Rayleigh range. In the case of a 4 μ wide waveguide operating at a ΗR angie of 4.5°, the total unconfined distance is about 100 μm, which is approximately equal to the Rayleigh range for a blue beam. One solution to optimizmg the performance of an array of such switches lies m addmg a permanent reduction of mdex of refraction (without degrading the electro- optic coefficient) m a strategic location withm the second TIR reflector segment. This strategic location is the zone bounded by the mside wall of the extended dense packed poled region, and by the mside wall of the poled region 836 as defined in reference to FIG. 31. The permanent index of refraction reduction defines a permanent waveguide boundary at the optimal location for confinement of the mode as it is reflectmg from the two successive ΗR mirrors The added index reduction Upers to zero as it approaches the mput waveguide, and the loss added to the mput waveguide can be reduced sufficiently by truncating the mdex reduction region sufficiently far from the guide. The index reduction also does not mterfere with the TIR function of the previous TIR mterface (mdeed, it helps).

Thus, the switched beam reflects from two consecutive TIR interfaces, doublmg the total deflection angle of the switch to 40. By doubling the output angle, space is now made available for output waveguides ot width equal to the input waveguide, with a separation equal to their width in the densest configuration.

The output waveguides connect to the poled region in FIG. 35 at the final comer of the second ΗR reflector, at an angle θ relative to the second TIR interface and optimally aligned to collect the light reflectmg off the second TIR mterface. Preferably, the two TIR reflectors for a given switch are connected without an intervening waveguidmg segment. This minimizes the path length that the deflected beam must travel in the poled waveguide, which may have a higher loss than a permanent channel waveguide due to wall roughness and asymmetry.

In an alternate poling boundary structure, the boundary between two adjacent poled regions may be a curved TIR structure. The mode of such a structure is agam a whispering gallery mode, possibly modified by some confinement on the inside boundary of the waveguide. The radius of curvature of the polmg boundary is made small enough so the whispenng gallery mode matches well with the waveguide mode for large power couplmg between the two types of guide, yet large enough for practical toUl mtemal reflection to occur for the distnbution ot angles within the mode. FIG. 36 shows a dual crossmg waveguide structure 980 for higher packing density. This structure incorporates two innovations: an asymmetπc loss waveguide cross 997, and 90° mirrors 976 and 977. The density is increased with the addition of a second input waveguide 982 parallel to the first mput waveguide 984, on the same surface ot the substrate 981, effectively doubling the packing density. The switchmg elements 983 and 985 have been illustrated schematically as one of the vaπants of the poled TIR switch descnbed above, but can alternatively be any integrated optic switch descnbed in the literature, so we do not descπbe the switch m deUil here or in the FIG. 36. (The switches may also be implemented in alternate ways descπbed herein such as the grating switches descπbed in reference to FIG. 7, the coupler descnbed m reference to FIG. 10, the splitter descπbed in reference to FIG. 25, and the guiding switch descπbed in reference to FIG. 33.) A first mput beam 992 propagates down the first waveguide, while a second mput beam

994 propagates down the second waveguide. The two beams may oπginate from distinct sources or from the same optical source via an active or passive splitter. When the coπesponding switch is off, the mput beam 992 and 994 propagate through to form the undeflected output beam 993 and 995, respectively. If the corresponding switch is on, the first mput beam 994 is deflected into the output beam 996, while the second mput beam 992 is deflected mto the output beam 998.

In the asymmetπc waveguide cross 997, two waveguides cross each other with the mdex of refraction profiles adjusted to minimize the loss m one guide at the expense of somewhat higher loss m the other. The crossmg guides are laid out at a large angle with respect to each other (herein illustrated at 90°), m order to minimize the crossing loss. Referπng to the geometry of FIG. 36, the second deflected beam 998 crosses over the first waveguide 984 (in this case so that the switched output light beams can propagate in parallel output waveguides 986 and 988). The waveguide 988 is broken at the crossmg pomt with the waveguide 984, leavmg the gaps 990 and 991. This is done to minimize the loss in the waveguide 984, producmg an asymmetπc loss structure with higher loss in waveguide 988 than in waveguide 984 m the crossmg region. For later convenience, we sa that the asvmmetπc cross points" along the waveguide with lower loss. The asvmmetπc cross 997 points aiong the waveguide 984. If the gaps 990 and 991 are wider than several exponential decav lengths for the mode in the guide 984. the cross structure will provide essentially no additional loss to the waveguide 984. A large number of asvmmetπc cross structures may then be sequenced pointing along the waveguide 984 to produce a low loss waveguide crossmg many others. The gaps 990 and 991 will produce some reflection and scatter to the beam 998 propagating m the broken waveguide 988. and the width of the gap may be minimized subject to the combined constraints of desired low loss m the two waveguides. To minimize the optical loss from the beam 998 propagatmg in the waveguide 988 at the cross structure, the index profile transverse to the axis of propagation of the guide may be modulated or pered along the axis of the guide The goal is to mainuin very low loss in the waveguide 984 while minimizing the loss in 988. This puφose is achieved if the index of refraction change m the regions adjacent to the guide 984 is small and slowly varying compared to the mdex of refraction change of the waveguide 984 itself. (All index of refraction changes referred to are relative to the substrate.) The loss in the second waveguide has two components- one due to reflection from the mdex discontinuities, and one oue to diffractive spreading The reflection loss is determined by the magnitude of the index change the waveguides, and by its Uper profile at the ends and sides of the waveguides. For example, if the index change at the core of the waveguides is the same in both at Δn = .003, the net reflection loss at the four interfaces will be less than 5%, neglecting corrections due to the exact mdex profiles which can reduce the reflection. The diffractive loss is even lower because the gap width is typically much less that the free space Rayleigh range. If, for example, the narrowest mode dimension is the depth, at 2 μm. then the Rayleigh range is 55 μm, assuming an index in the mateπal of 2.2 and a wavelength of 0.5 μm. The diffractive loss at each gap is less than 1 %, assuming a 3 μ wide gap. If the waveguide depth is 4 μm, the diffractive loss is substantially smaller The diffractive loss may be minimized by mcreasing the dimensions of the waveguide relative to the gap size.

In general, the "gaps" 990 have an mdex of refraction distπbution adjacent the crossmg region. This mdex of refraction distπbution is defined relative to the index of refraction of the substrate. The mdex of refraction the gaps may Uper from a value equal to the index of refraction distπbution of the waveguide 988 to another value adjacent the crossmg region. The important part of the crossmg region is the volume withm which propagates the optical mode of the waveguide 984. To minimize the loss m the waveguide 984, the mdex of refraction adjacent the crossmg region in this important part is much smaller than the mdex of refraction distnbution withm the waveguide 984.

The crossed waveguide geometry with asymmetπc optical loss may be combmed m many geometric vaπations. For example, three or more mput waveguides may be used with multiple crossmg pomts where switched output waveguides traverse input waveguides. The selection of preferred waveguides, preferred in the sense of hav g its loss minimized at the crossing point, can be also done in many ways. We have discussed an example in which the preferred guides are parallel. However, in a more complex system, there may be preferred guides which cross each other as well as crossmg unprererred guides. The selection or how to accomplish the crossings or the preterreo guides depends on the application. The waveguide crossmg stmctures in a device may be any combination of asymmetπc loss crossings and symmetnc loss crossmgs where the gap widths are zero.

For switches that deflect the beam at a small angle (such as a TIR switch), additional beam turning means such as 976 and 977 may be provided, in order to achieve the desired large angle of intersection at the waveguide cross. The beam turning means 976 and 977 is preterably a vertical micromirror, and is uisUlled at a fixed position. Each micromirror is formed by removmg the substrate mateπal within its volume, leaving a flat vertical surface (preferably with low roughness) adjacent to the waveguide and onented at such an angle so as to direct the reflected light optimally down the ouφut waveguide 986 or 988. The rmcromirrors may be fabπcated using conventional processing techniques, mcludmg laser ablation with, for example, a high power excimer laser or ion beam etching, both of which might define the mirror geometry with the aid of a mask. The volume may be filled with a low mdex, low loss mateπal such as aluminum oxide or silica to prevent contamination of the mirror surface, and to maintain the toul internal reflection property of the mirror The angle of the micromirror relative to an mput of one of the waveguides is preferably adjusted to provide toUl internal reflection. The thickness of the micromirror volume in the direction normal to its reflective surface is preferably much greater than a wavelength of light in order to minimize leakage through the micromirror volume of the evanescent tail of the reflected light wave. The angle relative to the other waveguide is adjusted so that the mean propagation direction of the reflected beam is parallel to the central axis of the other waveguide. The location of the micromirror is adjusted to optimize the couplmg of the light from one waveguide to the other. The location of the mirror m the junction region is preferably adjusted so that the "centers of gravity" of the two beam profiles illuminating the mirror surface are at the same place. The length of the mirror transverse of the mcident and reflected beams is greater than about twice the width of the waveguide to reflect essentially the entire mode, mcludmg the exponentially decreasing intensity in the beam uils. Light input from one of the waveguide modes diffracts through the waveguide junction region to the micromirror, reflects, and diffracts back through the waveguide junction region at the reflected angle before coupling mto the output waveguide mode. The junction region between the two waveguides m the vicinity of the mirror is optimally kept small compared to the Rayleigh range of the unconfined beam, which can be accomplished with waveguides havmg widths m the 2 to 5 micron range.

The structure of FIG. 36 makes possible a large interdigiuted array of switched light distnbution waveguides. The entire structure 980 may be replicated many times along a pair of mput waveguides, producmg a set of mterleaved output waveguides with a simple pattern of altematmg parenUge (in this context, parenUge means deπvmg optical power from a specific "parent" mput waveguide). Each mput waveguide may be connected to a large number of output waveguides as long as the switchmg elements have a very low insertion loss, as is the case for the elements listed above and descπbed herein. Because of the asymmetπc cross structures, addmg more input waveguides above the others (with additional switches, micromirrors, asymmetπc waveguide crosses, and interleaved output waveguides) does not significantly increase the loss or the lower input wav eguides or arrect their aointv to distπbute light over a long distance to many output waveguides. It will increase moderately the optical source power required for each additional mput waveguide in order to deliver the same power to the end of their respective ouφut waveguides. As many input waveguides as desired may be used in parallel to distπbute a potentially large toUl power of light. Their output waveguides may be interleaved in many alternative patterns usmg the approach of FIG. 36. The same result mav be achieved using grating reflectors m the place of the TIR switches. If the grating reflectors are onented at a large angle to the mput waveguides, the micromirrors are also no longer needed.

The structure oescπbed in tne previous paragraph is a one-to-many architecture m that it has one ouφut per switch with a multiplicity of switches per input. There is no way to connect many inputs mto the same output. What is needed is a many-to-one architecture. The many-to-many configuration is then obUined by combining the one-to-many and the many-to-one configurations.

FIG. 37 shows an aπay 1060 of waveguides with TIR switches arranged m a many-to-one configuration. In the structure snown. two Input waveguides 1072 and 1074 switch two input beams 1062 and 1064 into an output beam 1070 in one output waveguide 1076. The input TIR switches 1090 and

1092, and the output switches 1094 and 1096 have been descπbed before in reference to FIGS. 30 - 32 and 36, so they are shown only schematically, leaving off many elements (such as the electrodes, the contacts, the power supply, the controller, the vertical confinement means, the depth of the poled regions, the type of output waveguide confinement) for claπty. The input TIR switch is arranged with the beam propagatmg m a forward sense as descπbed in reference to FIG. 36, while the output TIR switch is arranged with the beam propagatmg m a reverse sense. The switches 1090 and 1092 are switched at substantially the same time, as are switches 1094 and 1096, because both are required to accomplish injection of power mto the output waveguide 1076. As descπbed in reference to FIG. 36, when the switches 1090 or 1094 are on, a fraction of the beams 1062 and 1064 are switched, respectively, into the waveguide 1078 or 1084. The remainder of the mput beams propagates along the extension of the input waveguide into an output path as beams 1066 or 1068, which may be used in some other component or brought into a beam dump for absoφtion or scatter out of the system. Micromirrors 1082 and 1088 are provided to reflect the beams from waveguides 1078 and 1084 into the waveguides 1080 or 1086, respectively. In their on condition, TIR switches 1092 or 1096 receive the beams propagating m the waveguides 1080 or 1086, respectively, fonmng the output beam 1070. If it is desired to switch the beam 1062 into the output beam 1070, clearly the switch 1096 and all subsequent switches must be off. (It would otherwise reflect much of the desired beam out of the waveguide 1076.) A similar constraint applies for all the other switched beams in multiple switch arrays.

The substrate 1098 is processed as descnbed herein to produce the stmctures illustrated. When the switches 1090 or 1094 are off, the input beam propagates through the switching regions 1090 or

1094 with negligible loss, traverse the waveguide 1076 (in an asymmetπc cross if desired), and emerge as output beams 1066 or 1068, respectively, possibly for use as inputs to additional switches. Additional input waveguides may also be provided, coupling mto the waveguide 1076 (or - not coupled, as desired), in a modified repetition of this structure in the direction of the output beam 1070. Additional output waveguides may also be provided, coupled if desired to the input waveguides 1072 and/or 1074, in a modified repetition of this structure in the direction of the beams 1066 and 1068. FIG. 38 shows an array 1210 of gratmg reflectors in a many-to-many configuration. In the structure shown, two input waveguides 1222 and 1224 switch two input beams 1212 and 1214 into two output beams 1220 and 1221 in two output waveguides 1226 and 1228 which abut or encounter the input waveguides. The grating switches 1230, 1232, 1234, and 1236 conuining the gratings 1238, 1240, 1244, and 1246 have been described before in reference to FIGS. 7, 8, 12, and 13, so they are shown only schematically, leavmg off many elements (such as the electrodes, the contacts, the power supply, the controller, the vertical confinement means, the depth of the poled regions, the Upering of the poled regions or electrode spacing) for clarity. When the switches 1230 or 1232 are on, a fraction of the beam 1212 is switched into the output beams 1220 or 1221, respectively. The remainder of the input beam propagates along a continuation of the input waveguide into an output path as beam 1250, which may be used in some other component or brought into a beam dump for absoφtion or scatter out of the system. When the switches 1234 or 1236 are on, a fraction of the beam 1214 is switched into the output beams 1220 or 1221, respectively. The remainder of the input beam propagates along a continuation of the input waveguide into an output path as beam 1252, which may be used in some other component or brought into a beam dump for absoφtion or scatter out of the system. It should be understood that the stmctures admit to bi- directional propagation.

The substrate 1248 is processed as described herein to produce the stmctures illustrated. When the switches are off, the input beams propagate through the switching regions (in which the waveguides may be configured as an asymmetric cross if desired), and emerge as output beams 1250 and 1252, respectively, possibly for use as inputs to additional switches. The waveguides may cross each other in simple large-angle junctions as shown, or the junctions may be asymmetric crosses, which do not substantially affect the placement of the gratings 1238, 1240, 1244, and 1246. Note that the gratings may in fact be parts of a single large grating which covers the substrate and which is only activated in the regions of the different switches by the desired electrodes. If the gratings are constructed from poled domains, for example, this allows the entire substrate to be poled for the gratings, which may be simpler in production. Alternatively, the gratings could be arranged in stripes or other groupings.

Additional input waveguides may also be provided, coupling into the waveguides 1226 or 1228 (or not coupled, as desired), in a modified repetition of this structure in the direction of the output beams 1220 and 1221. Additional output waveguides may also be provided, coupled if desired to the input waveguides 1222 and/or 1224, in a modified repetition of this structure in the direction of the beams 1250 and 1252.

FIG. 39A shows schematically an example application of the alternative switch arrays in the n x n communications routing application. In this application, the optical power in n input optical channels is to be routed to n output optical channels with minimal loss and minimal crossulk. A controller sets up an addressable path between one channel anu another. A simple square array is formed by repeatmg the structure of FIG. 38 until n inputs are arrayed on the left and n outputs are arrayed on the bottom, with switches at all n^: of the waveguide intersections. The intersection angle may be any convenient angle. In this structure, the switching of any channel into any other is accomplished by activating one of the switches. The light beams cross each other at the waveguide crosses with a small amount of crossulk which can be reduced by optimizing the waveguide geometry. This structure is capable of independent one-to-one connections between any input and any output. Note also that the connections are bidirectional so that a communications channel can be used equally well, and in fact simultaneously, in both directions. The switches are shown as implemented with gratings for specificity, but they may be implemented with dual TIR switches as described in reference to FIG. 37 by replicating the structure of FIG. 37 forming the n x n inputs and outputs, or with any other optical switching technique now known or yet to be discovered. Note that in the case of the TIR switches, the optical daU path does not pass through the vertex of the intersection between the input and the output waveguide. Instead, it passes through another waveguide near the intersection. According to the specific geometry of the switch, the input and output waveguides may intersect at a large angle as shown in FIGS. 37, 38 and 39, or at an oblique angie. The fixed reflectors 1088 and 1082 in the dual-TIR switching geometry may not be required in the case of the obliquely intersecting waveguides.

In this simple square geometry with n parallel input waveguides, there will be one input waveguide which can be connected into the closest output waveguide with a single switch, forming a best case connection with lowest loss. At the other extreme, there will be one waveguide which must traverse

2(n-l) waveguide crosses to be switched into the farthest output waveguide. This worst case connection will have much higher loss then the best case connection. To reduce the maximum insertion loss of the switch array structure, asymmetric cross junctions may be used as described in reference to FIG. 36. The loss of the worst case connection will be best helped with every waveguide cross :t traverses being an asymmetπc cross pointing in the direction of propagation of the light along either the input or the ouφut waveguide. This structure is clearly not generaiizable to the inner waveguides because use of asymmetric switches in the intermediate junctions will help some switching paths at the expense of others. What is needed is an algorithm for selecting the optimal direction for the asymmetric crosses. A good way to dispose the asymmetric crosses is for roughly half of the crosses to point in each direction. Observe that the n(n-l) crosses on the upper left of the diagonal (but not including the diagonal) are predominantly used to distribute energy to the right. These crosses therefore should point along the direction of the input waveguides, while the crosses on the lower right should point in the direction of the output waveguides. In a bidirectional structure, the crosses on the diagonal should be simple symmetric crosses, herein called the simple diagonal arrangement of the asymmetric crosses. Other arrangements may be used according to different usage patterns, but this is a good general puφose arrangement.

A n x m (where n > m) arrangement will permit full connectivity only between m "input" lines and m "output" lines. Here, "input" and "output" are only used for identification purposes smce all lines are bidirectional. The additional n-m "system" lines may be useful for system control in both momtormg and broadcast functions If line A wisnes connection to line B, for example, it would send system requests for that function until answered. Line m + 3. for example, might be dedicated to scanning all the "mput" l es for system reαuests (To provide a similar line to monitor tne Output" lmes. a larger matπx of lmes is required, such as the n x n matπx shown in FIG 39A where m lines are dedicated to users m a sub group of m x m lines A line such as line n-2 may then be used to monitor the "output" lmes.) In monitonng, the system will turn on successive gratings corresponding to the "mput" or "output" lmes, and detect whether the lme is active. Some power will be switched into the monitor detector by the successively switched-on gratmgs m lme with the monitor detector if any one of the monitored lmes is active. An active lme will have an activated reflector connecting it to another selected line. However, the activated reflector will leak some power through to form a beam which can be detected by the monitor detector. When the monitor detector connected to line m+3 in this example switches on the switch 1255 (drawn as a gratmg switch for specificity) and receives the request from line A. the control system will have to check whether line B is busy When the connection is made to line n-2 through switch 1253, the residual beam which leaks past the line B connection switch will alert the system that line B is active. If no activity is sensed, a system request can be sent to both lines A and B (possibly through the same mo tor lme if it has multiplexed send/receive capability, or possibly through a separate system lme), and the switch 1254 can be closed to establish the connection.

The broadcast function is not feasible from lines within the basic m x m switching block which is used for one-to-one connections, because even partially turning on the required row of switches corresponding to all the outputs from a given input would interfere with some of the already established and potentially active commumcations connections between other channels. Broadcast is best accomplished from the system lmes which are "outside" of the m x m switching block illustrated in FIG. 39A (The "mside comer" of the geometry is the best case waveguide connection with the lowest loss, between lmes 1 on the "mput" side and lme 1 on the "output" side.) Line C is shown to be actively connected to most or all of the "output" lmes m FIG. 39 A by means of gratings 1256 as an example of broadcast. The switches

1256 on lme C must be only partially turned on so that sufficient power is delivered to each "output" lme. A similar protocol may be used to prevent collisions between channels in the case of broadcast as in the case of simple commumcations connection. Broadcast connections would only be set up with inactive channels, and the system can group channels together and/or wait for individual channels to permit broadcast to them.

To increase switchmg efficiency, the waveguides may be large multimode waveguides, which in the case of a smgle mode communications network will be connected to the smgle mode mput and output ports 1 through m with adiabatic expanders descnbed elsewhere herein.

The entire structure descnbed above in reference to FIG. 39A is useful as an asynchronous transfer mode (ATM) switch, or m any point-to-point switched communications application.

One useful vanation of the structure is for multiple wavelength operation in a WDM network. Wavelength selective optical switches can be implemented as descπbed herein by usmg poled gratmg switches, or by usmg tunable fixed gratings which tune into and out of a specified commumcations band. In a WDM network, the desire is to switch a specific wavelength between channels without affecting other wavelengths which may be travelling (bidirectionally) in the same channel With a tunable switch which can select a frequency of reflection while essentially transmitting the other set of frequencies in the WDM spectmm, the simple geometry of FIG. 39A is appropπate. However, if a switched gratmg is used which has a smgle frequency of operation, separate connection paths are necessary for each wavelength.

FIG. 39B shows a switched WDM communications network 1260 with separate paths for each frequency used m the network. This example is tor a two frequency WDM network, but may be generalized to any number of frequencies of communication Three "input" waveguides 1276 are shown m FIG. 39B connected to three ports la, 2a, and 3a, and three "output waveguides 1276 are shown connected to three ports lb, 2b, and 3b. The waveguides form nine intersections. At each mtersection, there are three additional optical paths connectmg each "input" and each "output". The additional paths are identical in this example, and consist of three types. The first type 1266 of optical path consists of a pair of fixed frequency switched reflectors both capable of reflecting the first one of the two signal frequency bands of the WDM system. The reflectors are preferably gratings transverse of the "input" and the "output" waveguides associated with the intersection, and reflect power in the first frequency band between the corresponding waveguide and an additional waveguide segment connecting the two gratmgs The second type 1268 of optical path consists of a second pair of fixed frequency switched reflectors both capable of reflectmg the second one of the two signal frequency bands of the WDM system Again, the reflectors are preferably gratmgs placed transverse of the respective waveguides and reflect power in the second frequency band between the corresponding waveguides and an additional waveguide segment connectmg the second two gratmgs. The third type 1270 of optical path consists of a pair of frequency independent switched reflectors both capable of reflectmg both signal frequency bands of the WDM system. This third type of optical path may be implemented as the pair of TIR reflectors connected by waveguides and fixed mirror (descnbed m reference to FIG. 37). In this case, ports la, 2a, lb, and 2b plus the associated waveguides 1276, 1277 form a 2 x 2 switchmg network capable of switching two frequency channels simultaneously between any "mput" port and any "output" port. System control ports 3a and 3b with associated waveguides 1276, 1277 provide momtormg and system communication functions. If the first frequency of the WDM system is desired to be switched between port 2a and lb, for example, the two switches associated with the optical path of type 1266 at the mtersection of the waveguides connectmg to ports 2a and lb are turned on, routmg optical power at the first frequency between ports 2a and lb through the waveguide connectmg the two switches. If all frequencies associated with a given port are to be routed mto another port, the switches and optical path of type 1270 are turned on at the intersection corresponding to the two ports. The optical paths 1270 are really superfluous in a 2 x 2 network because to switch both WDM frequencies between any two channels, both corresponding paths 1266 and 1268 may be activated. However, m a high order commumcations network with many WDM frequencies, a single all-frequency connection is desirable smce it will have the lowest loss. FIG. 40 shows a two dimensional one-to-manv routing structure. A first row of waveguide routmg switches connects optical power rrom an input waveguide into columns of pixel waveguides. Agam, no deUils of the switches are snovvn; they are snown scnematically only as gratmgs, but may be implemented m several different ways. A two dimensional array of "pixel" switches routes power out of the pixel waveguides at "pixel locations (What happens to this power at the pixel locations depends on the application.) Two levels or switchmg are used to reach ail the pixels This structure may be used for display, to actuate or control processes or devices, or to read certain types of data. In the latter case the direction of the power flow is reversed, and the device operates as a many-to-one routmg structure. An mput optical beam 1342 propagates in an input waveguide 1352 and is coupled mto one of many pixel waveguides 1354 by one of a two dimensional array 1356 of switching elements. The switchmg elements 1364 may be implemented as grating switches as descπbed above in reference to FIGS. 7, 8, 12, 13, and 38, or they may be ΗR switches as descπbed in reference to FIGS. 30-32 and 37, or they may be any other switchable element. The beam 1344 is shown being switched by switch element 1358 mto a pixel waveguide whereupon it is switched for a second time by switch element 1360, forming beam 1346 which propagates into the pixel element 1362. The pixel elements 1366 may be separated from the waveguides 1354 by waveguide segments as shown, or they may abut the waveguides at a short distance so that little of the switched light passes by the pixel elements.

In the case of the display application, the pixel elements may be for producmg emission of the light 1346 out of the plane of the substrate 1348. The pixel elemente may then be roughened patches on the surface of the substrate 1348, or angled micromirrors, or roughened angled micromirrors for light diffusion, or phosphor-filled pits, or other means of producing visible light. In the case of the display, the mput beam 1342 may conUin several colors, in which case the waveguides are capable of guidmg all of the colors and the switches are capable of coupling all of the colors. The waveguide switches are scanned in a sequence to produce the image of the display. A grating switch is implemented as a multiple penod gratmg, but the ΗR switch needs little modification for this puφose. The waveguides, if smgle mode, must effectively guide the shortest wavelength beam. The mput beam 1342 is preferably modulated externally (mcludmg all its color components) so that the switching elements are simple on-off devices. Note that a smgle row electrode may be disposed across the columns of waveguides to actuate a row of pixel switches if the pixel elements are arranged in a more-or-less straight line and are connec ble electπcaily along a row.

In die case of a projection display, a additional lens structure is required to collect the light emitted by all the pixels m the array and refocus them on a screen at a (large) distance from the lens. The lens should preferably have a good off axis performance so that the focal plane is reasonably flat at the screen, and it should have a large enough numencal aperture (NA) to collect most of the light emitted by the pixel array. It would be advanugeous to couple a iens array to the pixel structure to reduce the divergence of the beams produced by the individual lenses, reducing the (costly) NA requirement on the pro_jection lens. Another way to achieve this is again to Uper the waveguides to the largest possible size at the pixel. It is reiativeiy easy to Uper the pixeis to a iarge transverse size, but difficult to ob in a very deep waveguide. Large pixels may be made by coupling a wide waveguide with a long grating coupler.

The light distπbuted in the routing structure may also be used to activate processes, as for example in the case of a DNA reader or an allergy reader, or a protein reader. In each of these specific cases, a separate array of DNA or allergens or proteins is prepared with fluorescent ugs which can be excited by the light. One type of molecule or one preparation of molecules may be arranged for exciution over each pixel. The light is scanned electronically among the different pixels, and the speed and order of the scanning may be determined according to the results. The fluorescence may be collected for detection by an external lens and detector. However, for some applications, it is advanugeous for the pixel (and its lens) and waveguide structure itself to collect and guide the emitted radiation to an optical energy detection means as well as to control the emission of the source light. Depending on the desired light illumination and collection geometry, the lens may be a collimating lens, a refocussine lens, or even, conceivably a lens to produce a diverging beam. A collimating lens is separated from the end of the waveguide by the focal length of the lens so that the transmitted (and collected) beam is essentially parallel. Collimating lenses are most useful if there is a large volume of mateπal to be traversed by the interrogating light beam. A refocussmg lens is separated from the end of the waveguide by the object distance, the inverse of which is related to the difference between the inverse of the image distance and the inverse of the focal length, where the image distance is the distance from the lens to the desired image beam spot. The refocussmg lens is used if it is desired to concentrate the sample into a small spot and to illuminate and/or read it from a waveguide. A divergmg beam is created by a lens separated by less than its focal length from the end of the waveguide. The ouφut beam from a simple lens is not necessarily round if the divergences of the wave approaching the lens are different in the two planes. The simplest way to make a beam round ( for minimum spot area after refocussmg) is to sUrt with a round beam at the end of the waveguide, which may be accomplished by design in the waveguide, or by upering the waveguide. The lens preferably has the appropπate numerical aperture to admit the entire wave from the waveguide and focus it to a diffraction limited spot or collimated beam according to the application.

The pixel element 1362 may be any of the elements mentioned above in this case, and it may be associated directly with the material to be activated, or indirectly as by alignment with an external plate to which the material has been conjugated. Each pixel element may con in a lens aligned as described above so that a switch array may be coupled with a lens array with the image beam spots in a substantially common plane of focus. (Substantially common, m this case, means within a Rayleigh range or so of the true plane of focus, which may be quite distorted due to aberration. Use of a type of reflector instead of a diffuser in the pixel element 1362 is preferred if the routing structure is also used to detect the fluorescent emission: the reflector couples the emission back into the waveguide whence it came. This coupling is maintained for as long as the switches for a given pixel are activated. If desired, the light source may be switched off prior to switching to another pixel element in order to resolve the decay of the emission. Used as a daU reader, the sense ot the light propagation is reversed from that illustrated m FIG. 40. Light from a device conUining daU is collected at the pixel elements and coupled mto the routmg waveguide structure which guides it back out the mput waveguide 1352. Connected to the waveguide 1352 is a detector to read the daU. The detector may be simultaneously connected to the waveguide via a beamsplitter between the waveguide 1352 and the light source used for illummation of the daU media. The pixel elements 1366 (or simply "pixels") are preferably coupled with the daU spots via lenses to collect the light routed through the structure 1350 and direct it to the daU medium. The lens couplmg also serves for collecting reflected or otherwise emitted light from the daU medium and refocussmg it on the end of the waveguide coupled to the pixel element. The data may be in a target volume, in which case the lens may be configured to collimate the light beam 1346. The dau may be on a target surface, m which case the different pixel elements may correspond to different tracks on the routing disk of a magneto-optical daU storage surface, for example, or of a CD. The lens is configured to refocus the light from the pixel to the dau spot m a diffraction limited way. By associating the different pixels with different tracks, track-to-track switchmg may be accomplished electromcally with essentially no delay time.

The different pixels may also be coupled to different planes on the daU medium. This is useful for readmg daU which have been recorded in multiple planes on the medium, to mcrease total storage capacity. Switchmg between the planes may also be accomplished electronically by switching among pixels coupled to the different planes. In addition, several different pixel elements may be focussed to locations separated by a fraction of the track separation transverse of (preferably normal to) a given track. When the track wanders, positive tracking may be accomplished electronically by switching between pixels, mstead of mechanically. A sensor and electronics is needed to detect track wander, and a controller for switchmg to the desired pixels. The signal strength or the signal to noise ratio (SNR) may be detected in the different channels to determine the preferred (best aligned) channel. If the switches along the waveguide 1352 are configured as 4-way crosses mstead of 3-way, with the fourth leg emergmg at the edge of the substrate, a detector array 1368 may be placed m registration with the fourth legs, with mdividual detectors 1367 individually aligned with the columns for detecting the return power from each column. The optimal reflectivity for the gratmgs which lie along the waveguide 1352 is approximately 50% if the detectors 1367 are used, m order to maximize the return power from the daU media on the detector array 1368. If a smgle beamsplitter is disposed in the waveguide 1352 upstream of the router structure, its optimal reflection is also 50%.

Note that partial exciUtion of the different pixels can be achieved by partial exciUtion of the switches along either the mput waveguide or the pixel waveguides. The switching elements 1364 can be adjusted by means of the applied electπc field to vary their reflection coefficient. Some of the beam may be transmitted through the desired partially-excited switches for use in a second pixel simultaneously. Multiple pixel exciUtion is of particular mterest m the case of track wander correction, smce multiple detectors may also be configured m the router 1350. For example, if three different pixels on three different columns of the routmg structure 1350 are to be simultaneously excited their corresponding pixel column switches will need to be partially excited A compu tion is required of the controller to determine the appropπate exciUtion of the multiple switches Neglecting losses at the switches, to produce equal intensities on their respective detectors for optimal SNR. the first switch corresponding to the first pixel column should be excited to reflect about 3/16 of the incident light, the second switch corresponding to the second pixel column should be excited to reflect about 114 of the remaining light which has passed through the first switch, and the last switch coπesponding to the third pixel column should be excited to reflect about 1/2 of the remaining light which has passed through the previous two switches About 15% of the mcident beam is reflected mto each detector, assuming 100% reflection from the medium and 100% light collection efficiencies. This result is quite good compared with the optimal 25 % of the beam which is received on a smgle detector m the case of a single pixel (optimum switch exciUtion = 50% reflectivity). Indeed, more toUl photons are collected with three beams than with only one Electronic tracking will result m cheaper, faster, and more reliable dau read/write devices.

Any combination of these approaches (electronic track switching, electronic daU plane switchmg, and electronic tracking) may be taken to increase the performance of a daU storage device. A means is also needed to accomplish vanable focussing electronically, potentially removing all mechanical motion (except for roUtion of the media) from the dnve. As descnbed below in reference to FIG. 54, electromcally vanable focussmg may be accomplished with a zone-plate lens by changmg the wavelength of the light beam 1342. As drawn, the routmg stmcture of FIG 40 is a polanzmg structure, with the 90° grating switches reflectmg only the TM mode. As a result, only beamsplitting based on intensity can be used. It would be quite advanugeous to be able to use polaπzing beamsplitters because this would result m a factor of four mcrease m the signal strength for a given light intensity. However, a switching stmcture capable of transporting and then separating the two poiaπzations is required. Although the polanzation dependence of the TIR switches may be made negligible at a sufficiently grazing TIR angle (well below the angle for toUl internal reflection for the TE mode), there is a packing density penalty in usmg very low angle switchmg geometnes.

FIG. 41 shows a lmear array of strongly polanzation dependent switches arranged as a daU reader 1370. The switches are excited with a beam 1342 which is TM poianzed and highly reflected m the activated switch 1372. Waveguides 1376 and 1378 such as titamum indiffused waveguides m lithium mobate are used which guide both poiaπzations. The pixel elements are implemented as micromirrors 1374 combined with integrated lenses 1380 and data spots e.g. 1382 arranged m tracks 1384 on a disk 1386 routmg about the axis 1388. The orthogonally poianzed light which is reflected from birefπngent da spots (or separators) on the daU track is collected by the lens 1380, refocussed back to the waveguide 1378, and reflected by the micromirror back mto the plane of the guides with TE polanzation. Because the

TE mode is both poianzed at Brewster's angle for the grating and has different propagation constant not phase matched for reflection, it propagates through the switch without reflection into the detector 1367 of the detector array 1368. (Alternately, if the switch is a TIR switch, the reflectivity is much less for the TE wave than the TM wave, and a large portion ot the TE wa e transmits through the switch an impmges on the detector.) If another switch 1373 is actuated instead of the switch 1372, the beam will propagate to a different pixel 1375 and be tocussed according to the alignment ot the pixel 1375 and its microlens 1381 either mto another daU track, or to another dau plane, or to the same track but with a transverse deviation of a fraction of a track width (according to whether the pixel 1375 is for track switching, daU plane switchmg, or tracking control).

Many vanations are apparent on the stmctures descnbed in reference to FIGS. 40 and 41, such as that any of the switches in the router may be onented differently to change directions of optical propagation m the plane, that multiple types of switches may be used in a smgle device, and that higher levels of switchmg may be added. Additional vanations are too numerous to mention.

FIG. 42 shows a switchable integrated spectmm analyzer 930. The input beam 921 enters the mput waveguide 923 which stops after a certain distance. The input beam 921 may be propagatmg m another waveguide or it may be a free space beam which is preferably aligned and mode matched to optimize the power mto the waveguide 923. The device 930 is provided with a planar waveguide 835 which constrains propagation withm the plane. The light beam 927 emerging from the end of the mput waveguide diverges in one plane within the planar waveguide until it passes through the integrated lens element 925. The integrated lens has an elevated index of refraction relative to the planar waveguide withm a boundary defining an optical thickness that reduces approximately quadratically from the optical axis. (Or if it has a depressed index, the optical thickness increases approximately quadratically.) The lens may be fabπcated by masked indiffusion or ion exchange, or it may be a reverse poled segment excited by electrodes.

The lens 925 coUimates the light beam which then passes to at least one of three gratmg sections 929, 931, and 933. The gratmgs are formed from mdividual cells, each cell being a domam, the domains being distinguished from the background matenal and separated by varying amounts accordmg to the application. The cells have a permanent or adjustable index of refraction difference from the substrate, and different cells may be of different domam types. The permanent domam types include, for example, lndiffused regions, ion exchanged regions, etched regions, radiation bombarded regions, and m general, regions formed by any type of mdex of refraction modifying process. The gratmg sections may be fabncated by etching, ion exchange, or indiffusion, in which case the gratings are permanent, but they are shown m the preferred embodiment fabπcated from poled domains. Electrodes 932, 934, and 936 are used to individually excite the gratmgs m combination with the common electrode 938. The common electrode 938 may be placed on the opposite side of the substrate as shown for simplicity, or surrounding the electrodes 932, 934, and 936 for low volUge exciUtion.

The cells m an mdividual grating may be arranged in alternate ways to form the desired penodicity m the desired direction to supply virtual photons with the required momenU. They may be arranged in rows to define certain planes with a virtual photon momentum normal to the planes with momentum defined by the spacing of the rows. In this case, there will also be virtual photons with momentum along the planes with momentum defined by the spacing of the cells m the rows. To phasematch retroreflection. the momentum or the virtual photon is exactly twice the momentum of the incident photons, and is directed in the opposite direction. Any other reflection process has a smaller momentum and is directed transverse of the incident axis. The penod Λ of the row spacing is therefore fractionally related to the incident wavelength λ in that Λ is a fraction of the quantity λ/2n__ff. In a general case, the cells may be separated by a distribution of distances which vanes with position through the gratmg so that the virtual photon momentum aiong any axis of incidence is determined by the spatial frequency spectmm (determined through the Fouπer transform) of the cell distribution along that axis.

At least one of the gratings 929, 931, or 933 is turned on by adjusting the potential sUte of the corresponding electrode. In FIG. 42, grating 929 is shown activated. The activated grating contributes virtual photons to the incident photons, phase matching the scattering process into an output direction forming a plurality of output beams 935 and 937 with different wavelengths, the output beam being separated in angle according to their wavelength. The output beams from the activated grating 929 pass through the lens 939 which retocusses the output beams onto a detector array 941. The detector array is a group of sensors disposed to receive a portion of the output beams for detection, and are preferably bonded to an edge of the device 930 as shown. However, if it is desired to integrate the device 930 onto a larger substrate, it may not be desirable to have an edge of the substrate in this location. In this case, other beam extraction methods (such as vertical deflecting mirrors) can be used to deflect a portion of the beams 935 and 937 into the detector array. The sensing means is placed approximately within about one Rayleigh range of the focal plane of the output lens 939. In this position, the input beam angles are mapped into output beam positions. Since the gratings map input wavelength into output beam angles, a collimated input beam results in different input wavelengths being mapped into different positions in the focal plane, with spatial resolution of the wavelength spectmm depending on the characteristics of the grating. The detected power as a function of the location of the detector in the array 941 is related to the frequency power spectmm of the input beam 921. The device 930 is therefore a spectmm analyzer. It is also a multichannel detector if the input beam is divided into channels occupying several displaced frequency channels, and the device is configured to disperse the channels into predetermined detectors or groups of detectors.

By switching on different gratings, the device can be reconfigured to function in different frequency ranges. For example, if grating 931 or 933 is activated, the dispersed light is focussed by lens 939 onto either a different detector array 943 or a different portion of an extended detector array 941. The frequency range of the gratmgs is determined by the angle of the grating to the beam, and the periodicities of the grating. Grating 931 is shown to have a shallower angle to the beam so that a higher optical frequency range is selected when it is activated. Grating 933 has multiple periodicities transverse to each other so that multiple overlapping frequency ranges can be selected. Multiple frequencies may be mapped into poled region boundaries as described above m reference to FIG. 18. The poled elements of the grating 933 may be arranged generally in planes oriented normal to the two principle virtual photon momentum directions. The phasing of the planes is determined by the process for transcribing the component frequencies of the desired grating into domain boundaries. However, the general grating may have momentum components in all directions, in which case the resulting domain boundaπes may not organize mto planes except possibly in a pnnicpal direction.

A transmitted beam 913 is refocussed by integrated lens 907 into an output waveguide segment 909 to form the ouφut beam 911 which contains at least a portion of the out of band portions of the mput beam 921 which did not interact with the gratings.

A useful vanation of the switched range spectmm analyzer combines elements of FIGS. 42 and 30-35. The basic idea stems from the fact that the spectral range of a gratmg can be shifted by changmg its angle, or equivalently the source pomt. In this vanation, a waveguide routmg stmcture is used to allow the source pomt to be switched. Waveguide switches are placed on the mput waveguide 923 (and possibly on the emanating waveguides) at one or more locations, producmg an array of parallel source waveguides among which the input light beam 921 is switchable. The waveguides all end m the same plane, preferably the focal plane of the input lens 925. The remainder of the spectmm analyzer remains the same, although with multiple inputs it may not be necessary to have the additional gratmgs 931 and 933. The separation of the multiple switched mput waveguides is adjusted according to the application to achieve the desired switchable spectral ranges for the analyzer 930.

FIG. 43 shows a poled acoustic multilayer mterferometπc stmcture 953. The mcident acoustic wave 972 may be a bulk or a surface acoustic wave. A poled stmcture is fabπcated m the region 955 of a piezoelectπc substrate 965, containing two types of domams 963 and 964. It is known (e.g. U.S. Patent 4,410.823 Miller et al.) that polanty reversals result in partial acoustic wave reflection. The reflection mto beam 973 and the transmission mto beam 961 is affected by the spacing of the interfaces between the poled regions. If high reflection and low transmission is desired, adjacent interfaces should be spaced by a distance equal to an integral multiple of half an acoustic wavelength. If high transmission is required through a stmcture, with low reflection, the spacing should be equal to a quarter of an acoustic wavelength plus any integral multiple of half a wavelength. By applying an appropnate number of poled regions near an mterface where the acoustic impedance changes, an antireflection (AR) stmcture can be fabncated provided that the phases of the reflected waves are chosen to be out of phase with and the same amplitude as the reflected wave from the interface.

FIG. 44 shows a poled bulk acoustic transducer 971. An mput acoustic beam 972 is mcident on a poled region of a piezoelectnc substrate 965 containing a pair of electrodes 974 and 975. The poled region conUins two types of domams 963 and 964 which are optimally reversed domains. The electnc field mduced by the acoustic wave m each of the poled regions can be selected to be identical by reversing the polmg direction every half acoustic wavelength. In this case, a smgle electrode may be used to pick up the mduced volUge mstead of the pnor art lnterdigitated electrodes. The electrodes 974 and 975 are used to detect the presence of the mput wave 972. The output volUge, Upped by conductors 979 and seen in the electromc controller 978, vanes smusoidally (for a narrowband mput) as a function of time with an amplitude related to the amplitude of the acoustic wave. As discussed above, if the poled mterface spacing is a half wavelength, the stmcture also acts as a high reflector, which may not be desirable a given implemenUtion. This characteπstic may be eliminated by spacmg the interfaces alternately at one quarter wavelength ano three quarters or a wavelength as snown in FIG. 44. In this case, the stmcture is an antireflection coatmg, eliminating the unoesired reflection Since almost the entire acoustic wave penetrates mto the poled stmcture. where its energy can oe almost toullv absorbed into the detection electronics, this stmcture 971 is an efficient tuned detector ot acoustic energy The bandwidth of the stmcture is inversely related to the number or acoustic peπods that fit within the poled stmcture covered by the electrodes. The efficiency is related to the acoustic path length under the electrodes The bandwidth and the efficiency of the detector are therefore related, and can be adjusted by changing the size of the detection region.

The stmcture 971 can also be used as an acoustic generator, essentially by running the process m reverse. A time dependent electncal signal is applied between the two electrodes at the frequency of the acoustic wave it is desired to excite. The piezoelectπc coefficient of the substrate produces a peπodic strain at the frequency of the acoustic wave, and a pair of waves are generated, one 961 propagatmg in the forward direction and one 973 in the reverse direction. A high efficiency unidirectional generator can be made if it is desired to generate only a single wave, by combining the devices 953 and 971. with 953 being configured as a total reflector tor the undesired wave If the total reflector is onented at 90° to the undesired wave and the phase ot the reflected wave is chosen to be in phase with the desired wave, the two waves will emerge in a single direction as essentially a smgle wave. A vanation of the stmcture of FIG 44 is a strain-actuated optical interaction device. In this device, the poled regions 964 and 963 are actuated by a strain field, producing a change m the mdex of refraction through the photoelastic effect. Now the stmcture 975 is a strain-inducing pad which may be deposited onto the substrate mateπal 965 at an elevated temperature so that the different coefficients of thermal expansion of the film and the substrate create a strain field at room temperature The mechanical stram field, working through the photoelastic tensor, produces index changes in the substrate which change from domam to domam. again producmg a substrate with patterned index of refraction which can be used as descπbed elsewhere herem. Electπc fields using the electro-optic effect can be combined with the photoelastic effect provided that the deposition process of the electrodes do not undesirably affect the desired stram field.

The stmcture 890 of FIG. 45 is a tuned coherent detector of pairs of light waves. It is tuned m the sense that it will only sense frequency differences between light waves within a certam bandwidth about a desired central "resonant" frequency difference. In the simplest case, the device is configured with equal separations between lnterdigitated electrodes 885 and 886 which form a peπodic stmcture with penod Λ. At a given instant, the two input frequencies present in the input beam 887 produce an interference pattern of electnc fields withm the waveguide 888 with a spatial penod which depends on the optical frequency difference and the index of refraction of the substrate 889 at the optical frequency. At a frequency difference where the spatial penod of the interference pattern equals the penod

Λ, the electrode stmcmre is on resonance, and the electrodes will be excited to a potential difference due to the mduced displacement charge at the top of the waveguide. The frequency response characteπstic is related to a sιnc^: function with resonant frequency determined by the optical frequency difference at which two optical waves slip phase by 2π m a poled gratmg penod. The buffer layer 891 is required to minimize the loss to the propagatmg optical waves when the electrode stmcture is laid down. It does not substantially reduce the strength of the mduced potential if its thickness is much smaller than the penod Λ The interference pattern has a low frequency component which oscillates at the frequency difference between the two light waves. The electromc signal which is picked up by the electronic controller 978 via leads 979 therefore also oscillates at the difference frequency The amplitude of the electronic signal is large at the resonance difference frequency, and falls off at other difference frequencies according to the bandwidth of the device, which is related to the mverse of the number of beat penods contained withm the lnterdigitated electrode stmcture.

The lnterdigiUted electrodes may alternately be configured with multiple frequency components so that there are several resonant frequencies, or so that the bandwidth of the response is modified. Note also that the device may be sensitive to multiple orders If the electrodes are narrow compared to a half penod, there will be a significant response at the odd harmonics of the resonant difference frequency. By shifting the fingers relative to each other so that there is asymmetry along the axis of the waveguide, a responsivity can be created to the even harmonics. This higher order response can only be improved at the expense of loweπng the first order response. It can be minimized by centermg the electrodes relative to each other, and by increasing their width. Finally, the waveguide 888 is not stπctly necessary. It may be omitted, but the detected waves should be brought very close to the electrodes to optimize the signal pickup.

FIG. 46 shows a low loss switchable waveguide splitter 780. This device has a permanent wye waveguide splitter 774 consisting of an input waveguide segment widening mto a wye junction and branching mto two output waveguide segments 775 and 776 which are both optical path possibilities for light incident the input segment. The widths and mdex profiles of the mput and output segments are preferably equal. The splitter 780 also has a poled stmcture 778 which has an electro-optic coefficient withm the region of the wye splitter 774. The poled region 778 may be a thm layer near the top of the substrate, which may have multiple layers, or it may extend throughout the substrate. The remamder of the substrate may be poled or unpoled A pair of planar electrodes 777 and 779 are disposed adjacent to each other over the waveguides, with one electrode 777 covering a portion of one output waveguide 775, and the other electrode 779 covering a portion of the other output waveguide 776. The electrodes are planar only to the extent that this optimizes fabπcation convenience and function: if the surface they are applied to is flat or curved, they conform. The edge 781 of the electrode 777 crosses the waveguide 775 at a very shallow angle, and forms a smooth continuation of the mside edge of the waveguide 776 at the wye junction. Likewise, the edge 783 of the electrode 779 crosses the waveguide 776 at a very shallow angle, and forms a smooth continuation of the inside edge of the waveguide 775 at the wye junction. When the electrodes are excited relative to each other with one polaπty, the mdex of refraction under the electrode 777 is depressed and the index under the electrode 779 is increased. As a result, an excited region under the electrode edge 781 forms a waveguide boundary, steering the mput beam 789 almost entirely mto the output beam 784 with erv little power leakage into the alternate output beam 782. The increased mdex under the electrode 779 aids in steeππg the optical energy away from the boundary 781. When the opposite polaπty is applied between the electrodes, the input beam is steered almost entirely mto the other output beam 782. If no voluge is applied, the input power is evenly divided into the two output ports if the stmcture is made symmetπc. This stmcture is theretore a 3 dB splitter which can be eiectπcally switched as a beam director into one of two directions with low loss.

The electrodes 777 and 779 are pered away from the wye stmcture 774 at the mput to the stmcture forming a gradual approach of the lower index region towards the waveguide to minimize optical losses. The smoothing effect of the electros tic field distπbution produces a very smooth mdex of refraction transition under both electrodes. The edge of the electrodes which crosses the output waveguides far from the wye branching region is preferably arranged at 90° to the waveguide to minimize losses. The wye splitter may be arranged in an asymmetπc way to produce a splitting ratio different from 3 dB with the fields off. This can be done by increasing the deviation angle for one of the waveguides and/or decreasing the angle for the other. The switching function operates almost as well with an asymmetnc stmcture as with a symmetπc stmcture, provided that a sufficiently large electnc field is applied with the electrodes The extinction ratio the ratio between the power in the switched-on waveguide and the power in the switched-off waveguide) can remain very large despite a large asymmetry. However, the optical losses will be somewhat different in the two legs of an asymmetnc switchable waveguide splitter. The device 780 may, therefore, be configured as a splitter with any desired splitting ratio, and still be switched with good efficiency and high extinction ratio.

This device may be cascaded to allow switching among more than two ouφut waveguides. If, for instance, the output waveguide 775 is connected to the input of a second device similar to 780, its power may be passively or actively switched into an additional pair of waveguides. Sixteen switched output lmes may be accomplished with four sets of one. two, four, and eight switches similar to 780. The power division ratio among these lmes may be configured to be equal in the unswitched sUte, or any other power division ratio. When the switches are activated, a smgle output waveguide may be tumed on, a smgle output waveguide may be tumed off, or any combination of output waveguides may be tumed on and off.

The direction of propagation of the light in the device may be reversed. In this case, an mput on either one of the output ports 775 and 776 can be switched to emerge from the input port. In the absence of an applied volUge, the power at each output port is coupled into the input port with a given attenuation (3 dB in the case of a symmetπc device). When the field is switched on, power m the "on" waveguide is connected mto the mput port with very low loss, while the power in the "ofr waveguide is very effectively diffracted away from the mput waveguide. The "off" waveguide is essentially isolated from the mput port.

Alternatively, a mirror image device may be connected back-to-back with the switch 780 so that the mput waveguides join together, forming a 2 x 2 switch or router. An input on either pair of waveguide ports may be switched into either waveguide ot the other port pair. Aga . cascading is possible, producmg an n x n switch/ router.

FIG. 47 shows an alternative realization 790 of a switchable waveguide splitter usmg multiple poled regions. In this configuration, the switched index difference along the boundaπes of the waveguides in the wye region is enhanced, thereby confining better the optical mode into a naπower region, and reducmg the residual coupling mto the switched-off output waveguide. Two poled regions 785 and 786 are disposed on each side of the input waveguide 774 along the wye splitting region. The poled regions have boundaπes 787 and 788 which cross the output waveguides 775 and 776 at a very shallow angle, and which form a smooth continuation of the mside edges of the waveguides 776 and 775 at the wye junction. The boundanes of the poled regions uper slowly away from the mput waveguide to allow a slow onset of the electπcaily excited mdex change, and they cross the output waveguides at a large distance from the wye junction where the electnc field is substantially reduced, in order to reduce the optical loss. Electrodes 791 and 792 are disposed substantially over the poled regions 785 and 786.

A potential difference is applied to the electrodes, exciting an electnc field in an electrosutic pattern throughout the volume between and around them. The electnc field penetrates the poled regions and the surrounding regions, inducing a corresponding pattern ol optical mdex changes. The local optical mdex change is related to the product of the local electnc field direction and the local electro- optic coefficient. The poled regions are preferably surrounded by regions of opposite polanty, m which case their electro-optic coefficient is of opposite sign to that of the surrounding regions. At the interfaces 787 and 788 there is a shaφ change in the index of retraction. On one side of the waveguide, the mdex is reduced at the mterface, producmg a guiding tendency away from the low index region. The opposite is tme of the other side. If the applied electπc field is large enough, the interface with the reduced mdex forms a waveguide boundary. Since the guiding mterface connects smoothly as an extension of the mside boundary of the output waveguide across from the poled region, the input light beam 789 is guided mto that output waveguide. The light leak is low into the switched-off waveguide if the curvature of the guidmg boundary is gradual. There is low loss at the mput, because the poled regions approach the waveguide slowly. There is low loss at the wye junction, because the portions of the poled regions which extend beyond the junction depress the guidmg effect of the switched-off output waveguide, and enhance the guidmg of the switched-on output waveguide. As an alternative, the poled regions could be surrounded by unpoled mateπal. There is still an abrupt change in the mdex at the interfaces 787 and 788 so the device still functions, but the mdex change is only half the value obtained when the poled regions are surrounded with reverse poled matenal, so the applied field must be higher. The alternatives descnbed before also apply to this device.

FIG. 48 shows the key design elements of a 1 x 3 switch. The design elements illustrated here show how to transform the device 780 of FIG. 46 into a 1 x 3 switch with a smgle poled region and patterned electrodes. The device contains a permanent branching waveguide with the desired number n (n = three) of output branches. The waveguide passes through a poled region which extends deeper than the waveguides (for good extmction ratio) and significantly beyond the junction region where the waveguides have become separated by a large amount (sucn as three times their width). Several zones are defined by the waveguide boundanes, by their smooth extensions back into the boundanes of the mput waveguide, and by normal boundanes across the output waveguides at a distance significantly beyond the _junction region. There are (n^: + 2n - 2)/2 zones so defined. It is useful to extend the outermost zone beyond the outeide of the outermost waveguide as shown to Uper the mput A separate electrode is placed over each of the regions with a small gap between all electrodes, but sufficient gap to avoid electncal breakdown when excited.

To operate the device, electπc fields are independently applied to the zones with polaπty determined by whether or not the coπesponding zone is confined within the desired waveguide. For example, the five zones of FIG. 48 are excited according to Table I. As before, the magnitude of the electnc field is adjusted to produce a good guiding boundary along the edges of adjacent zones excited at different polaπties.

Electrode Number Top Middle Bottom

1 + - -

2 + + -

3 - + -

4 - + +

5 - - +

Table I

Alternatively, the design elements of FIG. 48 also show how to transform the device 790 of FIG. 47 mto a 1 x 3 switch with multiple poled regions. The device agam conUins a permanent branchmg waveguide with the desired number n (n = three) of output branches. Agam, several zones are defined by the waveguide boundanes, by their smooth extensions back mto the boundaπes of the mput waveguide, and by boundaπes which cross the output waveguides at a distance significantly beyond the junction region. Agam, it is useful to extend the outermost zone beyond the outside of the outermost waveguide as shown, m order to Uper the input. Each zone is poled in the opposite direction to neighboring zones with a common zone boundary Zones w ith the same poling direction may share at most a vertex. Preferably, the mput waveguide region is poied oppositely to the innermost zones (i.e. the zones closest to the mput waveguide). In FIG. 48 the innermost zones are labelled zones 2 ano 4 This zone- based polanty selection procedure results in only zones 2 and 4 being reverse poled, while zones 1, 3, and 5, which are the output waveguide zones, are poled positive (in the same direction as the surrounding region, if the surrounding region is poled). If four output waveguides are used, there are n ne zones, six of which are reverse poled, mcluding all of the output waveguide zones. The splitter implemenUtions which have an even number of output waveguides, therefore, have some advantage because only the even splitters have their output waveguide zones poled opposite to a potential substrate poling, with the attendant advanuge of mcreased confinement at the final division point and higher transmission for the "on" sUtes and better reverse isolation m the "ofr sUtes. A separate electrode is placed over each of the regions.

To operate the device, electπc fields are independently applied to the zones, but now the mle for the polanty is different. The polaπty is determined by two factors: whether or not the corresponding zone is conumed within the desired waveguide, and the polaπty of the poled region undemeath. For example, if a positive polaπty applied to a positively poled region produces an mcrease m the mdex of refraction, the following selection mles are followed: if a zone is poled positive, the electncal exciUtion polanty is selected to be positive if the zone is mside the desired waveguide and negative if the zone is outeide; if a zone is reverse poled (negative), the polanty is selected to be negative if the zone is inside the desired waveguide, and positive if the zone is outeide. In Table II are shown the optimal polmg direction of the zones for the n = 3 case with three output ports as shown FIG. 48. The design of 1 x n and n x n switches is denved by induction from the descπptions of the FIGS. 46, 47 and 48.

Zone Poling Direction Top Middle Bottom

1

- - + +

2 + + + -

3 - + - +

4 + - + +

5 - + + -

Table II

The planar componente described herein may be s cked into multiple layer three dimensional stmctures conUining electro-optically controlled devices and waveguide components. Stacks or three-dimensional constmctions of planar waveguides and switches are fabricated by alternately layering or depositing electro-optically active, polable thin films, preferably polymers, and buffer isolation layers, which may be either insulating or electrically conducting. Advantages of sucked stmctures include better crossulk isolation due to more widely spaced waveguide elements. Higher power handling capability is also achieved because more optical power can be distributed among the layers. Individual layers can be used if desired to distribute individual wavelengths in a display device.

Once deposited on a suiuble substrate, poling of the active optical waveguide/switching layer is done using the techniques heretofore described. A buffer layer of lower index is necessary to isolate one active layer from adjacent layers, and is designed to esUblish the desired guiding in the dimension normal to the plane. Buffer layers of Si0₂ , for example, may be used. Next comes a ground plane which can be fabricated from a meUllic layer since it is isolated from the optically active layers, followed by a thick buffer layer. The buffer layers must also be capable of withstanding the applied volUges between the different layers of electrodes and ground planes. In polymers, a large area may be poled, and desired regions selectively de-poled by UV irradiation techniques as previously described in order to create waveguide features, even after a transparent buffer layer, such as Si0₂ has been applied. Or, poling can be performed electrically. With polymers, de-poling one layer by UV irradiation will not affect the layer behind it because of the shielding provided by the underlying meUllic ground plane. MeUl electrodes and conductive paths may then be laid down by standard masking and coating techniques, followed by another insulating buffer layer, and the next active layer. The buffer layer should be planaπzed to minimize the loss in the subsequent active optical waveguide/switching layer. This process of add g layers may be repeated as often as desired for a given device. A vanation in fabπcation technique for making activation paths and electrodes for the poled device stacks is to coat the electro-optic layer with an insulating layer that is subsequently doped or infused to produce electπcaily conductive patterns within the buffer layer using standard lithographic masking techniques. Incoφorating the electrodes into the buffer layer would serve to minimize the thickness of the sucked device. Hybπdized devices consisting of different electro-optically active mateπals could be used to ameliorate fabπcation complexities. For example, the first electro-optically active layer conUining waveguide devices could be fabπcated in a LiNbO, substrate, which would also serve as the support substrate. Next a buffer layer and a layer of electrodes for the lithium niobate devices are deposited. Two insulating buffer layers sandwiching a conducting plane would then be coated onto the device pnor to depositmg the next active layer which could be a polable polymer. Subsequent layers are built up, poled and patterned as descπbed earlier. The conducting planes in between buffer layers may serve both as electrodes to permit area polmg of each polymer layer and to shield previous layers from the polmg process.

Sucked waveguide arrays may be used, for example, as steeπng devices for free space beam manipulation. Electπcaily activated and individually addressable waveguide elements sucked closely together, and aligned with a source array form a controllable phased array for emitting optical radiation. The relative phases of the beams can be adjusted by varying the volUges on the poled zones as descπbed previously. By adjustmg these phases m a linear ramp, the emitted light from an array of waveguides can be swept m direction rapidly withm the plane of the array. A linear array of devices on a plane can therefore sweep withm the plane only. However, when poled waveguide aπay planes are vertically integrated mto a three dimensional bulk device, optical beams emanating from the device may be directed m two dimensions.

An extension of this concept is the mode control of multimode laser bar arrays usmg a stack of waveguide gratmg reflectors. The waveguide suck is dimensionally matched to butt-couple to a laser diode array. By controlling the phase of the individual elemente, the emission mode pattern of a multi element laser bar can be controlled. In devices where smgle mode waveguide confinement is not necessary, multimode or bulk arrays may also be sucked, for example, to increase the power handling capacity of a switched poled device.

FIG. 49 illustrates an embodiment of the phased array waveguide sUck section 1630 with only a smgle column of waveguides illustrated for clanty. Optical radiation 1640 enters the sUck 1630 through waveguides 1638 which have been fabncated in an electro-optically active thm film 1650, such as a polable polymer. Here the input beams 1640 are shown staggered to represent beams of identical wavelength, but with different phases. Light travels along the waveguides 1638 in which they encounter poled regions 1634 withm which the index or rerractioπ mav oe modified electronically usmg the techniques descπbed herein. Beams 1642 represent the output of the pnased array after each light wave has been individually phase adjusted to produce output component beams that are aligned in phase.

Many other mput and output wave scenaπos are possible. For instance, a smgle mode laser beam with a flat phase wavefront could illuminate an area or waveguide elements, which would then impose arbitrary phase delays across the spatial mode of the beam, thereby allowing the beam to be electromcally steered m free space. Directional beam control devices using this method would be much faster and more compact than current mechanical or A-O devices. Using optical-to-electncal pickup devices descπbed herein or known in the art, pnase differences or the presence of multiple frequency componente may be sensed withm or external to the sucked device in order to provide instantaneous information for a feedback loop.

The device segment 1630 represented here is constmcted on a substrate 1632, such as SιO_:, by alternately depositing electrodes, buffer layers, and polable mateπal in the following manner. A broad area planar electrode 1654, composed or an opaque metallic film or transparent conductive mateπal such as indium-tin-oxide, is deposited, and followed by an electπcally insulating buffer layer 1652, such as

SιO_;, which also serves as the lower boundary layer tor the waveguide 1638 fabπcated in the next layer of polable matenal 1650. On top of the polable layer 1650, another buffer layer 1652 is added to form an upper waveguide bound before depositing the patterned electrode 1636 used to activate the poled stmctures. Another buffer layer 1652 is then added, this time to electπcaily insulate the patterned electrode from the next layer, another broad area planar electrode 1654. The patterned electrode 1636 is separated from one planar electrode only by a thick buffer layer, and from the other by buffer layers and the polable matenal. Smce it is desired to apply fields across the polable mateπal, the electncal separation across the polable matenal should be less than the separation across the buffer layer only. The layenng sequence between broad area electrodes is repeated until the last layer of polable mateπal 1650. after which only a buffer layer 1652, patterned electrode 1636, and optional final insulating layer 1652 need be added to complete the suck. Electncal leads 1646 and 1648 are brought into contact with electrodes 1636 and 1654, respectively, through integration and bonding techniques known to the art, and connected to volUge distnbution control unit 1644.

The volUge control unit 1644 serves a dual puφose: to activate the poled devices individually, and to isolate each from the electπc field used to control neighboring layers of active elements. The unit 1644 would be m essence a collection of coupled floating power supplies m which the voluges between electrodes 1636 and 1654 sandwichmg an active layer may be controlled without changmg the volUge differences across any other active layer.

Region 1634 depicts a poled region with one or more domains, and electrode 1636 depicts an unbroken or a segmented or patterned region with one or more isolated elements. Waveguide sUck

1630 is descnbed as a device for phase control, but sucks of waveguide stmctures may include any number of combmations of poled devices descπbed herein, in seπes optically, or otherwise configured.

FIG. 50 shows a pπor art adjusuble attenuator 1400. An mput waveguide 1402 traverses an electro-optically active region of a substrate 1404 An input optical beam 1406 propagates along the mput waveguide mto an output waveguide 1408, forming the output optical beam 1410. Electrodes 1412, 1414, and 1416 are disposed over the waveguide so that when electrode 1414 is excited at a given polaπty (positive or negative) with respect to the two electrodes 1412 and 1416, there is an induced change in the mdex of refraction withm the segment 1418 region of the waveguide under and adjacent to the electrodes due to the electro-optic effect. The electrode configuration is somewhat arbitrary and may be different and more complex than shown m the pπor art represented by FIG. 50, but the common factor which all the patterns have m common is that overall, they reduce the index of refraction in the core when excited to a volUge, and mcrease the mdex of the surrounding regions. In the absence of applied electπc field, the loss of the waveguide segments is low, determined pπmanly by scatteπng on roughness along the waveguide walls. However, when the electπc field is applied, the loss can be increased to a very large value. The three electrode pattern allows a negative mdex change withm the waveguide at the same time as a positive mdex change occurs outside the waveguide, substantially flattening and broadening the index profile. When the field is applied, the modified section of the waveguide 1418 under the electrodes has a much wider lowest order mode profile from the mput 1402 and output 1408 sections of the waveguide. As a result, mode couplmg loss occurs both when the mput beam 1416 transitions mto the section 1418 and when the light m section 1418 couples back mto the output waveguide 1408. If the index changes are large enough, the lowest order mode goes below cutoff, and the light emerging from the end of the waveguide 1402 diffracts almost freely mto the substrate, resultmg in a large coupling loss at the beginning of the waveguide 1408.

When a given mode enters the modified section 1418 of the waveguide, the overlap between its intensity profile and any mode profile of the modified section 1418 is reduced by the change m the mdex profile of the modified segment. If the segment 1418 is multimode, several propagatmg modes and radiation modes will be excited. If it is smgle mode, many radiation modes will be excited. The combination of these modes then propagates to the far end of the segment 1418 and couples mto the output waveguide section 1408, where only a fraction of the light couples back mto a mode of the waveguide to form the output beam 1410. By controlling the volUge applied to the electrodes, the loss m the device 1400 can be adjusted from very low to very high.

The maximum loss which can be obtained depends on the magnitude of the mdex change, the size of the excited regions, their length, and on whether the mput and output waveguides are smgle mode or multimode. In a vanation of the geometry, only two electrodes might be disposed over the waveguide segment 1418, decreasing the mdex within the waveguide segment and mcreasmg the mdex to one side mstead of on both sides. The function is again as an attenuator, but the rejected radiation fields will tend to leave the device towards the side of the increased index. This ability to direct the lost radiation might be of advanUge m some systems where control of the rejected light is desired. An absorber may also be placed downstream of the segment 1418, on one or both sides, to prevent the rejected light from interfering with other functions elsewhere m the system.

FIG. 51 shows a poled switched attenuator 1420. This device is an improvement on the pπor art device of FIG. 50 in at poled regions are used to increase the defimtion of the mdex change and mcrease the mdex discontinuity, thereby mcreasing the amount of attenuation which can be obtained m a smgle suge. Regions 1422 and 1424 are electro-optically poled in a reverse direction from the surroundmg matenal. (As an alternative, the surrounding mateπal may be unpoled, or have no electro-optic coefficient, or it may simply be poled differently from the regions 1422 and 1424 ) The central electrode 1426 covers both poled regions and surroundmg mateπal. It is excited relative to the electrodes 1428 and 1430 to produce a change in mdex of refraction in the poled regions 1422, 1424, and the surroundmg matenal. The device 1420 operates m a similar way as descπbed above in reference to the device 1400. The applied volUge reduces and broadens the index profile of the waveguide segment 1418, reducing the couplmg between the mode of the output waveguide 1408 and the modes excited in the segment 1418 by the mput beam 1406. In this configuration, the change in the mdex profile is abmpt at the beginmng of the modified waveguide region 1418. and therefore the loss is larger. The number and shape of the poled segments 1422 and 1424 can be vaned so long as the mode coupling with the excited waveguide segment 1418 is different from the mode couplmg with the unexcited segment. The device may be configured with high loss m the electπcaily unexcited condition, adjusting to low loss in the electncally excited condition. In this case the electπcaily excited regions and/or the poled regions form a portion of the stmcture of the waveguide segment 1418. The waveguide segment 1418 may itself may be configured m many different ways, most noubly if it is absent entirely without exciUtion, in which case the device is similar to the switched waveguide modulator of FIG. 29 A. As descnbed above, these devices may be cascaded, in this case to mcrease the maximum attenuation.

The devices of FIG. 50 and FIG 51 can also be operated as a vanable intensity localized ("pomt") light source. The light propagating in waveguide 1402 is confined to follow the path of the waveguide until a voluge is applied the electrode stmcture. When the waveguidmg effect is reduced or destroyed by changmg the mdex of refraction, part or all of the previously confined light beam will now propagate accordmg to free-space diffraction theory. The diffracting beam will contmue to propagate m the forward direction while the beam area expands two dimensions to be much larger than the core of the waveguide 1408. At an appropπate distance away from the electrode stmcture, the beam area can fill a large fraction of the substrate aperture and appear to a viewer as a point source of light emanating from a spatial location near the electrode stmcture.

If desired, a one-dimensional localized source can also be constructed with this techmque. The waveguide segment 1418 m FIGS. 50 and 51 can be embedded in a planar waveguide stmcture fabπcated usmg techniques known to the art, such that when an appropπate volUge level is applied to the electrode stmcture, the transverse confinement of the mode is destroyed while the vertical confinement m the planar waveguide is not. Thus the beam area would expand in one dimension, confining the light to a narrow plane.

FIG. 52 shows a poled device 1500 with an angle broadened poled grating. The method shown for broadening the bandwidth is an alternative to the bandwidth modifying approaches descπbed in reference to FIG. 18 and elsewhere herein. A peπodic stmcture 1500 is shown with poled regions 1502 which are preferably reverse poled mto a poled region of the substrate 1504. Other stmctures such as waveguides and electrodes and additional gratings are incoφorated as desired. The domams 1502 cross the central axis of propagation of the mput beam 1508 with a pattern which may be stπctly peπodic with a 50% duty cycle. The sides of the top surfaces of the poled regions all align along lines drawn from an alignment pomt 1506. The poled regions approximately reproduce their surface shape some distance mto the mateπal. The result is a poled stmcture with peπodicity which changes linearly with the transverse position in the poled substrate. An mput beam 1508 which traverses the poled region may be a freely propagatmg Gaussian beam (if the domams are deeply poled) or it may be confined in a waveguide 1512. Accordmg to the function of the gratmg, the input beam may be coupled into a filtered or frequency converted output beam 1510, or mto a retroreflected beam 1514. The range of peπodicities m the gratmg stmcture (and hence its bandwidth) depends on the width of the beam and separation of the pomt 1506 from the axis of the beam. By adjusting these quantities, the bandwidth of the poled stmcture may be increased substantially over the minimum value determined by the number of first order penods which fit m the gratmg. There is a limit on the maximum desirable angle for the poled boundaπes, and therefore the stmcture shown in FIG. 52 cannot be extended without limit. However, a long interaction region can be obU ed by cascading several segments together. To maximize the coherence between the segments, the peπodicity of the domains along the central axis of the beam should be unmodified at the joms between segments. There will be at least one wedge shaped domain between segments. Although mcreasmg the bandwidth of the grating decreases the interaction strength, it makes a device usmg that gratmg significantly less sensitive to small frequency dπfts. For example, a frequency doubler device usmg an angle broadened grating is more tolerant of temperature dπfts. Another example application is the channel dropping filter which tends to have narrow bandwidth because of the strong gratmgs which must be used. Use of an angle broadened gratmg enables a widened pass band to accept high bandwidth commumcations signals. The angle broadened grating can also be applied m the other gratmg configurations discussed above.

There are alternatives for implementing the angle broadened gratmg which do not follow the exact pattern descnbed above. For example, the relationship between the angle of the grating peπods and the distance along the propagation axis might be more complex than lmear. A quadratic or exponential vanation might be more appropπate for some applications where the majonty of the interacting power exists at one end of the gratmg. The angle broademng technique is also applicable to pπor art types of gratmgs such as indif fused, ion exchanged, and etched gratmgs.

An alternative angle broadened device 1520 using a curved waveguide is shown m FIG. 53. In this case, the poled regions 1522 have parallel faces, and the angle of the faces are inclined only relative to the local direction of propagation within the guide. Again, the bandwidth is broadened by the different components of the wave expeπencing different Fouπer components of the grating. The curved waveguide has a higher loss than the straight waveguide, but large curvatures are not required. Several sections as shown m FIG. 53 may be concatenated, forming for example a sinuous waveguide stmcture that waves back and forth around an essentially straight line.

FIG. 54 shows a controllable poled lens 1530. Concentπcally arranged domains 1532, 1534, 1536, and 1538 are poled into an electro-optic substrate 1540 with a reverse polaπty from that of the substrate. Transparent electrodes 1542 and 1544 are applied to the two opposing surfaces of the device above and below the poled regions. When an electnc field is applied between the two electrodes, the poled regions have their index of refraction either increased or decreased according to the polaπty. The geometry of the poled regions is determmed by the diffractive requirements of focussmg an optical wave of a given color. The separations between the boundanes vanes roughly quadratically with radius. If the application requires focussmg a plane wave to a round spot, for example, the poled regions will be round (for equal focussmg in both planes), and separated by decreasing amounts as the diameter of the poled region increases. The boundaries of the poled regions are determined by the phase of a the desired outgomg wave relative to the incoming wave at the surface of the lens stmcture. A poled region boundary occurs every time the relative phase of the waves changes by x. For example, if the incoming wave is a plane wave its phase is constant along the surface. If the outgoing wave is a converging wave which will focus at a spot far from the surrace, it is essentially a spheπcal wave and the phase changes in that spheπcal wave determine the boundaπes. The lens 1530 is a phase plate with aujusuble phase delay accordmg to the applied volUge, and the domains occupy the Fresnel zones of the object.

To focus a plane wave of a given color, a voltage is applied which is sufficient to phase reUrd (or advance) the plane wave by tr. Each different frequency has a different focal length defined by the Fresnel zone stmcture of the poled lens 1530. Higher frequencies have longer focal lengths. If it were not for dispersion, every wavelength would optimally focus at the same volUge. The volUge can be adjusted to compensate for the dispersion in the substrate mateπal 1540. If the volUge is adjusted away from the optimal value, the amount of light which is focussed to the spot is reduced because the phase of the light from the different zones no longer add optimally. They will interfere partially destmctively, reducmg the net intensity.

The invention has now been explained with reference to specific embodiments. Other embodiments will be apparent to those of ordinary skill in the art. Therefore, it is not intended that the invention be limited, except as indicated by the appended claims, which form a part of this invention descπption.

Claims

WHAT IS CLAIMED IS:

1. An electrically-controlled frequency-selective beam coupler comprising: a solid material, said material having a pattern of differing domains, at least a first type of said domains being a poled stmcture and forming at least two elements alternating with a second type of said domains; at least a first electrically-conductive material forming a first electrode, said first electrode confronting said solid material and bridging at least two of said elemente of said first type of poled stmcture; a first waveguide segment traversing said solid material; a second waveguide segment traversing said solid material and intersecting with said first waveguide segment at an intersection with said at least two elements, said at least two elemente being disposed transverse of said first waveguide segment and said second waveguide segment, said at least two elemente defining a grating.

2. The device according to claim 1 wherein said grating is disposed at an angle to said first waveguide segment to reflect optical energy from said first waveguide segment into said second waveguide segment.

3. An electrically-controlled frequency-selective beam coupler comprising: a solid material, said material having a pattern of differing domains, at least a first type of said domains being a poled stmcture and forming at least two elements alternating with a second type of said domains; at least a first electrically-conductive material forming a first electrode, said first electrode confronting said solid material and bridging at least two of said elements of said first type of poled structure; a first waveguide segment traversing said solid material; and a second waveguide segment traversing said solid element and adjacent and nonintersecting with said first waveguide segment, said at least two elemente being disposed transverse of at least one of said first waveguide segment and said second waveguide segment, said at least two elemente defining a grating. 8 7

4. An electrically-controlled frequency-selective beam coupler comprising: a solid material, said material having a pattern of differing domams. at least a first type of said domains being a poled stmcture and forming at least two elemente alternating with a second type of said domains and defining a grating; at least a first electrically-conductive material forming a first electrode, said first electrode confronting said solid material and bridging at least two of said elements of said first type of poled stmcture; a waveguide segment traversing said solid material; a waveguide wye junction having a first arm and a second arm, said wye junction being coupled to said waveguide segment, said device for controlling coupling of optical energy in said waveguide segment with said first arm and said second a i through said grating, wherein the electric field controls power in the first optical mode and the second optical mode such that superposition of said first optical mode and said second optical mode result in selective spatial concentration of power in said waveguide segment.

5. The device according to claim 4 wherein said first optical mode is the lowest order transverse mode of said first waveguide segment, and wherein said electric field controls output power splitting between said first arm and said second arm.

6. The device according to claim 4 wherein said first arm is an input for said first optical mode, and wherein the electric field is used for control optimization of output coupling into the lowest order transverse mode of said waveguide segment.

7. An optical wave distributor comprising: a first optical waveguide segment; a second optical waveguide segment; at least a third optical waveguide segment; a switching region at a junction of said first optical waveguide segment, said second optical waveguide segment and said third optical waveguide segment, said switching region comprising: an electro-optic material; and a plurality of spatially-distributed planar electrodes disposed adjacent said electro-optic material, said planar electrodes covering selected sectors of said switching region in a pattern, said pattern defining a plurality of optical path possibilities in said electro-optic material among said first optical waveguide segment, said second optical waveguide segment and said third optical waveguide segment, said electrodes being selectively activauble to change index of refraction along selected optical path possibilities to define an optical path across said switching region selected from said optical path possibilities.

8. The optical wave distnbutor according to claim 7 further including a common electrode disposed opposmg said planar electrodes across said electro-optic mateπal.

9. The optical wave distnbutor according to claim 7 wherein said first optical waveguide segment, said second optical waveguide segment and said third optical waveguide segment, and said switchmg region compnse a single solid mateπal.

10. The optical wave distnbutor according to claim 7 wherein said electrodes are electncally poianzed to cause mdex of refraction to increase within sectors defining a selected optical path.

11. The optical wave distnbutor accordmg to claim 7 wherem said electrodes are electncally poianzed to cause mdex of refraction to decrease withm sectors defining boundanes adjacent to a selected optical path.

12. The optical wave distnbutor according to claim 7 wherein said sectors are formed of poled mateπal, adjacent sectors being formed of domains of alternating polanty.

13. The optical wave distnbutor accordmg to claim 12 wherem said electrodes are electncally poianzed to cause index of refraction to increase within sectors defining a selected optical path.

14. The optical wave distnbutor accordmg to claim 12 wherein said electrodes are electncally poianzed to cause index of refraction to decrease within sectors defining boundaπes adjacent to a selected optical path.

15. An optical wave distnbutor comprising: a first optical waveguide segment; a second optical waveguide segment; at least a third optical waveguide segment; a switchmg region at a junction of said first optical waveguide segment, said second optical waveguide segment and said third optical waveguide segment, said switchmg region compπsmg: an electro-optic matenal; and a plurality of spatially-distπbuted planar electrodes disposed adjacent said electro-optic mateπal. said planar electrodes coveπng selected sectors of said switchmg region m a pattern, said pattern defining a plurality of optical path possibilities m said electro-optic mateπal among said first optical waveguide segment, said second optical waveguide segment and said third optical waveguide segment, said electrodes bemg selectively activauble to change index of refraction along selected optical path possibilities to define a plurality of optical paths simultaneously across said switchmg region selected from said optical path possibilities.

16. An optical wave attenuator comprising: a first optical waveguide segment; a second optical waveguide segment: a switching region at a junction of said first optical waveguide segment and said second optical waveguide segment, said switching region comprising: an electro-optic material; and a plurality of spatially-distributed electrodes disposed adjacent said electro-optic material, said electrodes covering selected sectors of said switching region in a pattern, said pattern defining at least one energy leakage outlet between said first waveguide segment and said second waveguide segment in said electro-optic material, said electrodes being selectively activauble to change index of refraction along said optical path across said switching region in order to allow incremental coupling between said first waveguide segment and said second waveguide segment.

17. The device according to claim 12 wherein said at least two elements form a grating.

18. The device according to claim 17 further including a second grating disposed along one of said first and second waveguide segments transverse to and overlapping at least evanescent electromagnetic fields of optical energy of said one of said waveguide segments.

19. The device according to claim 1 for separating optical energy at different wavelengths, further including: a first waveguide segment traversing said solid dielectric material; at least a second waveguide segment traversing said solid dielectric material and being disposed in close proximity and substantially parallel to said first waveguide segment at at least a first location and at a second location along the length of said first waveguide segment to permit optical coupling between said first waveguide segment and said second waveguide segment; wherein said at least two elements form at least a first grating of a first period and a second grating of a second period, said first grating being disposed transverse of said first waveguide segment and said second waveguide segment and overlapping at least evanescent electromagnetic fields of optical energy in at least one of said first waveguide segment and said second waveguide segment at said first location, said second grating being disposed transverse of said first waveguide segment and said second waveguide segment and overlapping at least evanescent electromagnetic fields of optical energy in at least one of said first waveguide segment and said second waveguide segment at said second location; electric field creating means comprising a first electrode means for controlling said optical coupling between said first waveguide segment and said second waveguide segment at said first location and a second electrode means for controlling said optical coupling between said first waveguide segment and said second waveguide segment at said second location; and control means for controlling selective application of electπc fields through said first electrode means and said second electrode means.

20. The device according to claim 1 further including: a first waveguide segment traversing said solid dielectric material; a second waveguide segment traversing said solid dielectric material and intersecting with said first waveguide segment at an intersection with said at least two elemente, said at least two elemente being disposed transverse of said first waveguide segment and said second waveguide segment, said at least two elements forming a grating thereby forming an electrically-controlled frequency-selective beam coupler, said grating being disposed at an angle to said first waveguide segment to reflect optical energy from said first waveguide segment into said second waveguide segment; a first Upered segment in said first waveguide segment forming an adiabatic expansion in an input path to said grating; and a second Upered segment in said second waveguide segment forming an adiabatic contraction in an output path from said grating.

21. The device according to claim 1 wherein said at least two elements form a first grating and a second grating, and further including: a first waveguide segment along an optical axis; a second waveguide segment disposed adjacent said first waveguide segment in sufficiently close proximity to permit coupling between said first waveguide segment and said second waveguide segment; said first grating being disposed adjacent to and transverse of said first waveguide segment; said first electrode being disposed adjacent said first grating; said second grating being disposed adjacent to and transverse of said first waveguide segment; a second electrode disposed adjacent said second grating; electrode means opposing said first electrode and said second electrode capable of esUblisbing control electric fields through said first grating and said second grating; said first grating having at least a first period along said optical axis substantially equal to a second period of said second grating; said first grating and said second grating being separated by a predetermined optical distance.

22. The device according to claim 1 wherein said at least two elements form a first grating and a second grating; and further including: a first waveguide segment along an optical axis; a second waveguide segment disposed adjacent said first waveguide segment in sufficiently close proximity to permit coupling between said first waveguide segment and said second waveguide segment; said first grating being disposed adjacent to and transverse of said first waveguide segment; said first electrode being disposed adjacent said first grating; said second grating being disposed adjacent to and transverse of said second waveguide segment; a second electrode disposed adjacent said second grating; electrode means opposing said first electrode and said second electrode for esUblisbing control electric fields through said first grating and said second grating; said first grating having at least a first period along said optical axis substantially equal to a second period of said second grating; said first grating and said second grating being separated by a predetermined optical distance; and wherein said control electric fields are applied through said first grating and said second grating.