CA2123310A1 - Chiral smectic liquid crystal optical modulators - Google Patents

Abstract

The present invention provides optical modulators which comprise aligned chiral smectic liquid crystal cells within an optical resonance cavity. The cavity configurations include symmetric and asymmetric Fabry-Perot etalons. The liquid crystal cells can be planar- or homeotropically-aligned and can be discrete state or analogue cells. The device configurations of the present invention provide discrete or continuous optical modualtion of the phase, intensity, and wavelength of elliptically polarized light, without requiring polarization analyzers. The modulators are optically or electronically addressable in single pixels or arrays of multiple pixels. Certain homeotropically-aligned cells are provided as an aspect of this invention, as are certain variable retarders comprised of planar-aligned cells in combination with birefringent elements. For example, the figure T shown includes a photo sensitive layer (110), reflective surfaces (113-114), a wave plate (112) and a chiral smetic liquid crystal (111).

Description

~, WO93/10477 PCTJUS92/09707 x 2123310 ,,, 1 ,,~., ~ .
,,, .' CHIRAL SMECTIC LIQUID CRYSTAL
OPTICAL MODULATORS
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,~ Field of the_Invention The present invention relates to tunable electro-optical modulators having a folded optical path structure using chiral smectic li~uid crystal materials as tuning ~ elements and, in particular, relates to Fabry~Perot j~ intererometer and etalon modulators.

~0 Backaround o~ the Invention Liquid crystal devices operate on the basic principle . that due to the dielectric a~isotropy vf nematic, cholesteric and smectic:~ liquid ~cry~itals, the average molecular axis, ~r director, can be oriented in the presence of an~ applied electric field. The coupling of non-ferroelectrlc ~i~uid crystals to the applied field is a weak, sscond~order interactionO In general, slow : r~sponse~ti~es::are~charac~eristic oP non-ferroelectric, non~chiral,~liqu~d~crys~al optical devices.
~ ~eyer et al~ 'Ferroele tric ~iquid Crystal~9', in Le Journai de Physique, V~ 36, March, 1975~ pp. L69 L71) howed that chiral:C or H~, smectic liquid crystals, could be~ferroelectric~, tha~ is, possess a permanent electric dîpoIe: density, P. ~This permanent polarization, P, is ~:;25: ~ perpendicul2r to the average orien~ation of the long axis of the molecules ~denoted by the molecular director, n,) and generalIy contained within a plane parallel to th~
smectic layers. In these chiral smectic liquid crystals ~(CSLCs), the molecular director makes a temperature ~'~

dependent angle, ~, with respect to the layer normal, z as shown in Figures 1 and 2. In general, ~ ranges from 0 to 45. The presence of the eiectric dipole provides a much stronger coupling to the applied electric field as compared to non-ferroelectric liquid crystals. Furthermore, the coupling, and hence aligning torque is ab~ut linear with applied field. The significance of this is that changing the sign of the applied electric field will change the direction of P in smectic C , H~, A~ and other chiral smectic phase liq~id crystals.
N. A. Clark et al. in U.S. Patent No. 4,367,924, realized a ferroelectric liquid crystal switching device ~y sandwiching a thin layer o~ a smectic C~ ~SmC~) liquid crystal between two glass plates coated with transparent electrodes. In this patent, they describe the surface-stabilized ferroelectric liquid crystal (SSFLC) device, ~;1 which employs SmC~ or SmH~ liquid crystal phases in the so-called bookshelf geometry, otherwise designated the planar alignment, where the smectic layers are perpendicular to and the liquid crystal molecules are parallsl to the glass ,l plates which also contain the electrodes, as illustrated in ~.l Figure 1 (see also N. A. Clark et al. U.S. Patent 4,563,059 `l and N.A. Clark and S.T. Lagerwall in Applied Phys. Letts.
(1980) 36:899 and S.T. Lagerwall and I.Dahl Mol. Cryst.
1 25 Liq. Cryst. ~1984) 114:151-187). 55FLC SmC materials have ¦ been shown to be useful in a number of electro-optic device applications including switches, shutters, displays and spatial light modulators (SLM's). The advantagec of planar aligned ~hiral sme~tic C,F,G,H, and I l~iquid crys~al devices i~ their ne~rly three orders of magnitude increase .i in swit~hing speeds over non chiral liquid crystal devices . and their intrinsic bistability, which has applications for , optical me.mory~units, Tristable switching of a planar-aligned CSLC cell has been reported (I. Nishiyama et al. (1989) Jpn. J. App. Phy.
~:~ 28:L2248; and A.D.I. Chandani et al. (1988) Jpn. J. App.
Phy. 27:L729). The third state of such tristable cells has . I .

~ ~ .
WOg3/1~77 PCT/US92/~7~7 2 1~310 been linked with the presence of an antiferroelectric phase, designated SmCA;. This type of CSLC cell has been designated an antiferroelectric LC cell. CSLC materials which aan exhibit this antiferroelectric effect have been report~d by K. Furukawa et al. (1988) Ferroelectrics B5:63;
M. Johno et al. (1989) Jpn. J. App. Phy. 28:~119 and Y.
Suzu~i et al. (1989) Liq. Cryst. 6:167~
Lagerwall et al. in U.S. Patent No. 4,838,663, de~cribe a non-tilted, non-ferroelectric, chiral smectic A~
(SmA ) liquid crystal electro-optic switch. With planar-aligned, surface-stabilized SmA material between substrate walls with no electric field applied (zero field state), n I is parallel to z (i.e., ~ = 0). The molecular director of the SmA material exhibits rotation in a plane relative to z (~ ~ 0) in response to an applied electric field due to the electroclinic e~fect tfirst described by S. Garaff and R. B. Meyer (1977) Phys. Rev. Letts. 38:84B)~ These cells display an analog dependence of ~ with applied field to a maximum tilt angle ~x~ which angle is an intrinsic property of the SmA material~ Materials having ~ ranging from about 6 to 22.5 have be@n observed (see also, Sharp, G.
D. et al., (Opt. Lett:. 15) (199~) pp. 523-525). The advantage of these planar-aligned SmA~ cells is submicrosecond switchi~g cpeeds and analog rotation of the 2S optic axis. ~ ~
~' L.A: Beresne~et al. t European Patent Application No.

3 309774, published 1989, has recently described a new type i of chiral smectic~ferroelectric liquid crystal cell called the distorted helix~ferroelectric (DHF), li~uid crystal c~ This type of device is similar to the planar-aligned '1 chiral SmA device of Lagerwall et al., except that it is j not strongly surface-stabilized, so that the helix along ~! the direction of the layer normal, z is not suppressed.
ll Application of an applied electric field to the DHF cell perpendicular to z, partially orients the molecular directors by an angle ~ to z. The angle ~ is dependent on i the size and magnitude of the field so the DHF device " , WOg3/1~77 PCT/US92/0~707 operates in an analog mode similar to a Sm~ device. In a DHF device there is a change in the birefringence o~ the material as the molecules align, which does not occur in Z~ either the SSFLC Sm~ or planar-aligned SmA device. The ~ 5 DHF materials, such as Hoffmann-La Roche DHF 6300, having ', ~x as large as ~37 have been described. The advantage of DHF switching de~ices over other FLC switching devices , described above i~ the variable birefringence with applied ~l~ voltage. This is similar to the operation of nematic 'i 10 li~uid crystals, which also yield a variable birefri~gence ~¦ with applied voltage. In contrast to nematic liquid crystals, the DHF molecular directors rotate by their full tilt angle within 40 ~sec, a significant advantage.
' Furthermore, the voItages required to rotate the optic axis ~; 15 are generally much lower than those required for SmA~ and SmC cells. An interesting feature of DHF devices is the coupling of the change in birefringence with the rotation ~,, of the optic axis as a function of applied voltage.
Z.M. Brodzeli et al.~(l990) Technical Digest on SLM's and ~heir Applications 14:128 have reported fast electro-optic response (20 ~:sec) in a homeotropically-aligned Sm~
liquid crystal. In homeotropic alignment, the smectic layers of the liquid crystal are parallel to the surfa~res of the substrate~ ~alls (se~e Figure 2) and as in planar-~, 25 aligned CSLCs, the molecular director makes an angle, ~, with the layer~normal. In the optical modulator described ,~
by Brodzeli~;et~ al./ th homeotropically-aligned SmC~
material is~ positioned between substrate walls having deposited electrodes: (the width of the cell was given as 17 ~m.) Polarized non-monochromatic light entering the device, propagati~g~along the axis normal to the layers, was~reported to be modulated in intensity by application of a voltage a~ross the~electrodes.
Phase modulation of optical signals is often ;35~ accomplished by means of an electro-optic effect in which a change in index~of refra~tion of a suitable material is achieved with the application of an electric field, for .~`( ' .

WO93/10477 PCTlUS92J09707 example, by the Pockels or Kerr effect (see, e.g., Yariv, A. and Yeh, P. Optical Waves in Crystals (1984) Wiley and Sons, NY~. While the Pockel's and Kerr effects are high speed effects, they require large voltages for bulk S implementations in order to achieve very small electro-optic effects~ A techni~ue that has been used to improve the characteristics of electro-optic Pockel's and Kerr effect phase modulators, is to fold the optical path l~ngth using a Fabry-Perot etalon or resonator, which transt`orms the low amplitude input signal ~o an output optical intensity with high contrast.
A Fabry-Perot device consists of two plane parallel, highly reflecting surfaces, or mirrors, separa~ed ~y a distance, L. When the mirrors are fixed at distance L, the device is called an etalon. When L can be varied the device is called an interferometer~ ~ Fabry-Perot etalon operates on the principle of multiple interference of the waves reflected or transmitted by the mirrors. If L is a ', multiple of ~, then the transmitted waves destructively , 20 interfere and the light incident upon the device is ideally l totaliy reflected by thP etalon. If L is a multiple of 2~
j all the light is: ideally transmitted by the etalon ! (assuming no absorption losses)~ If the etalon thickness is som~where in between ~ and 2~, then partial tran mission 1 25 or reflection occurs. If the optical thickness of the I etalon can be changed, the etalon operates as a variable modulator.
~' Miller:et al., U.S. Patent No. 4,790,643, disclose an 5, .optically bîstable device comprising a Fabry-Perot etalon containing an intracavity, optically non-linear, nematic uid crystal material. The device provides an eleckr~-optic bistable switch tha~ is designed to modulate a mosnochrom5atic or coherent light source. Since the liquid crystal of this device is neither chiral or ferroPlectric, the switching spseed of this particular optical modulator is relatively slow.

5~ .

`,~ WO g3/10477 , PCr/USg2/09707 ~; 6 Summar~ of the Invention The present invention provides optical modulators which comprise folded optical path structures, etalons and interferometers containing chiral smectic liquid crystal materials within the optical cavity which function for optical modulation by an application of an electric field.
~ The intensity, phase and wavelength modulators provided t'' herein combine modest voltage requirements and low power consumption with rapid tuning . The device configurations of the present invention permit discrete or continuous optical modulation useful in a wide variety of applications including, among others, spectrometry, remote sensing, discrete modulation for differential absorption or transmission in optical filters, rapid wavelength ;1 15 modulation useful in color displays, intensity modulation for binary or gray-scale generation for shutters and SLMs, phase modulation~for 2-D and 3-D holographic displays, SLMs, beam steering, refractive and diffractive optical elements. Folded optical~path structures include optical modulators having~Fabry-Perot (FP) interferometer and etalon structures and~ asymmetric Fabry-Perot cavities i.e., Gires-Tournois etalons). The modulator ~ configurations~of~;the present invention are useful in i~ single and multiple pixel elements which are electronically or optically~addressable by a variety of means.
Most~ generally, this invention involves the positioning~of~an~aligned Iayer of a chiral smectic liquid ,. .
crystal material within an optical resonance cavity. As is ~i conventional, ~an optical cavity is formed by opposed ~j 30 reflective surfaces. Preferably the reflective surfaces are such that;a~substantial portion of the light entering the cavity makes more than one pass through the cavity, i.e., having at ~least one reflection, before exiting the cavity. In a FP~etalon or interferometer configuration, it ~35 is preferred ;that~interference between at least two phase retarded waves of light occur before light exits the cavity. In an asymmetric FP configuration, folding the ~1 WO93/10477 2 1 2 3 3 1 0 PC~/US92/09707 optical path increases the interaction length of light within the modulator. Typically, the reflective surfaces are plane parallel with respect to one another and at least one of the reflective surfaces is transmissive to allow entrance of light into the cavity. FP cavities can be operated in transmission/reflection- or reflection-only mode with the choice dependent on the modulation application and/or design requirements- The chiral smectic liquid crystal material is aligned between substrate walls.
The ahiral smectic liquid crystal material can be planar-or homeotropically-aligned. Means for achieving the decired alignment, such as appropriate alignment layers, as is known in the art, can be provided on the inside surfaces ..
of the substrate walls in contact with the chiral smectic ¦ 15 liquid crystal material. Means for applying an electric field across the aligned material are provided such that molecular director of the material, i.e., optic axis, is rotatable on application of the electric field across the material. A dc or ac electric field, or both, may be applied to tbe cell to rotate the optic axis. In certain device configurations with E applied parallel to the smectic layers (i.e.~, E~is perpendicular to z), the optic j axis is rotatable in a plane perpendicular to the direction 1 of the applied~field. The aligned chiral smectic liquid crystal material,~substrate walls and means for application of an el~ctric field~across the liquid crystal comprise a ~hiral smectic liquid crystal cell. Light traversing the FP cavity containing~a chiral smectic liquid crystal cell is modulated by~rotation of the optic axis of the cell by `1 30 application of an electric field. The modulators of the ¦~ present invention optionally include i~otropic spacer elements, lenses~,~ birefringent elements and wave plates within the etalon cavity.;
This invention ~specifically provides tunable optical ~35 modulators of ;elliptically polarized light. Certain optical modula~tor configurations herein are particularly useful with linearly polarized light. The optical .

.~ .

WOg3/l0477 PCTtUS92/09707 212~310 modulators of these specific embodiments do not require exit polarizer means or polarization analy~ers to obtain the desired modulation. It may, however, be desired, in a particular application, to employ a polarization analyzer to ~elect a certain polarization state of modulated light.
FP-type interferometer and etalon configurations of the present invention, include binary and analog intensity, phase and wavelength modulation. The modulators of the present invention are optiaally or electronically addres~able in single pixels or arrays of multiple pixels.
The i~v~ntion also specifica}ly provides tunable optical modulators of elliptically polarized light which comprise asymmetric FP cavities wherein the optical cavity contains an intracavity modulator element positioned , 15 between means for reflecting lighk entering the cavity.
~he light reflecting means in these modulators comprise one reflective surface ha~ing si~nificantly higher re~lectivity tha~ the other reflecti~e surface, ideally 1, such that the device operates in the reflection-only mode. Both planar-i 20 and hom~otropically-aligned materials can be employed as 1 the modulator element and appropriate alignment means are optionally c~mprised within the CS~C cellO These specific embodim~nts do not require the use of a polarizer to detect ~, optical modulation. The asymmetric FP etalon modulatoxs of ¦ 25 the present~ invention inalude binary and analog phase modulatore.
~ Chiral smectic liquid crystal materials useful in the :~ rapidly tunable~ or: switchable modulators of the pres~nt invention include ferroelectric liquid crystal materials, electFoclinic li~uid crystal materials, distor ed helix ferroelectric materials and antiferroelectric materials lj within cell c~nfiguration which allow rapid rotation of the i optic axis of the li~uid crystal material by application of an electric field. Chiral smectic liquid crystal cells , ~
incIude discrete state cells and analog cells. Within ~I cells the chiral smectic liquid crystal mate~ial may be ¦ planar-aligned or homeotropically-aligned.
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, .
s W093/1~77 PCTtUS92/~707 212~310 .," ' 9 1,' ,j Certain homeotropically-aligned cells are provided as an aspect of this invention and are useful in this ~, invention~ These cells can have transparent or semi-transparent substrate walls and light can traverse the cell through the substrate walls (traversing the smectic layers) or through one of the ~ides of the cell (bisecting the smectic layers).
This invention also provides methods of modulating the phase, intensity or wavelength of light, particularly elliptically polarized light and more particularly linearly polarized light, employing the resonance cavity modulators ; and non-resonance cavity modulators described herein.
, Brief Description of the Fiqures Figure 1 is an illustration of planar alignment of chiral smectic liquid crystal cells.
~I Figure 2 is an illustration of homeotropic alignment of chiral smectic liquid crystal cells.
Figure 3 is a two-dimensional, cross-sectional schematic view of an exemplary device configuration for a planar-aligned, chiral~smectic LC switchable Fabry-Perot modulator.
Figure 4 is~a schematic representation of an exemplary homeotropically-aligned, lateral-electrode, smectic LC
modulator. Figure 4A is a three-dimensional view of a side of the devioe~ and Figure 4B is a two-dimensional, cross-se tional view f~rom~the top of the device.
FigNre ~5~are schematic representations of planar-aligned smectic LC analog optical modulator employing quarter-wave plates. Figure 5A is a transmission-mode modulator. Figure 5B is a reflection mode modulator.
Figure 6 is a cross-sectional view of an exemplary ~; device configuration for an analog Fabry-Perot intenæity or wavelength modulator employing a planar-aligned chiral smectic liquid crystal layer.
Figure 7 is a graph showing the relationship ~etween optical transmission of a typical device of Figure 6 and . -.

7~ PCT/USg2/09707 molecular rotation corresponding to different values of mirror reflection.
Figure 8 i6 a cross-sectional view of an exemplary device configuration for an analog chiral smectic liquid crystal phase modulator operating in reflection-mode and employing a planar-aligned chiral smectic liquid crystal ~ layer.
¦ Figure 9 is a graph of the relationship between resultant phase shift through a device of Figure 8 corresponding to different values of reflectivity of the front mirror.
Figure 10 is a cross-sectional view of an exemplary application of the modulator of Figure 5A of the present invention in an optically addressable transmission mode single pass spatial light modulator.
Figure 11 is cross-sectional view of an exemplary application of phase modulators of the present invention in an optically addressable reflection mode etalon spatial light modulator.
I 20 Figure 12 ~i5 ~a cross-sectional view of an exemplary I pixel of a VLSI binary phase, intensity or wavelength modulator having a planar-aligned SmC~ LC cell and two quartér-wave plates.
~l Figure 13~ is a cross-sectional view of an exemplary ¦ 25 pixel of a VLSI analog intensity or wavelength modulator - having a-planar-aligned CSLC cell.
Figure 14 is~a cross-sectional view of an exemplary 1 application of a l~homeotropically-aligned modulator of `1 Figure 4 of the~present i mention as a pixel in a multi~
1 30 pixel array.
Figure 15 is a exe~plary application configuration for an analog transmission-reflection mode modulator of the ~present invention.

¦ Detailed Descri~tion of the Preferred Embodiments The term chiral smectic liquid crystal (CSLC) cell~ is used generally herein to refer to transparent or semi-i~
'`i ~

, WO93/1~77 ?~12 3 310 PCT/US92/~707 transparent cells or light switches containing a chiral smectic liquid crystal material which functions on application of an electric field to cell electrodes to rotate the polarization of light passing through the cell.
Cells are typically formed of uniformly-spaced transparent or semi-transparent retaining walls of an inert substraté, such as glass or quartz. A conducting material is typically coated on the inside surface of the substrate walls to provide tran~parent or semi-transparent electrodes. ~ chiral nonracemic smectic liquid crystal composition, often a mixture of components, is inserted between the uniformly-space transparent electrodes. A
method of alignment of *he ferroelectric liquid cry~tal molecules within the cell is typically employed. One ' 15 preferred alignment is the "bookshel~" alignment which has ¦ been described by Clark and Lagerwall, supra. A schematic ¦ diagram of a planar-aligned CSLC is provided in Figure l.
Smectic layers are aligned perpendîcular to the substrate walls which bound the LC layer. The molecular director n makes an angle ~ to the~ smectic layer normal ( z). The molecular director is the average direction of the long axis of the molecule in the layer. Surface stabilization suppresses the formation of a helix within the material so that the optic axis is confined to rotate in-a plane (yz in Figure l). Surface~stabilization is raquired for SmC~ and SmA pla~ar-aligned cells.~ In a discrete, multi-state FLC
cell, for exa~ple~a~bistable ~LC cell like a SmC SSFLC
cell, a~plication~of an~ appropriate electric field to the cell ~lectrodes can allow selection between states. The , discrete states of the cell are associated with ~ orientations~of~the~chiral~smectic ~C molecules within the j cell on applioation of~the electric field. For example, ~ ~ application of a voltage, above a certain threshold i ~ voltage, to the cell electrodes result in switching of the ~1 ~35 orientation of the chiral smectic LC molecules. Bistable cells have two such orientations. Tristable cells have ~ three such orientations. ~ith a multistable state LC cell, t ~ ~ a voltage need not be applied to maintain the orientation , WO~3/10477 PCT/US92/09707 ; 2123310 12 of the CSLC molecules which defines the state of the cell.
In a CSLC cell that does not have stable states, it may be necessary to apply a voltage to maintain the cell in the de~ired switching state. The optic axis of a planar-aligned CSLC cell is in the plane of the substrate walls of ~, the cell which form the aperture of the cell through which light enters ~he cell~
Analog CSLC materials for example SmA electroclinic j materials and DHF materials, when incorporated into FLC
;l 10 cells and aligned in a planar or bookshelf geometry display an analog rotation of the cell optic axis with applied electric field. The maximum rotation angle that can be , obtained is twice the maximum tilt angle (~x) of the electroclinic or D~F material employed in the cell. Analog FLC cells can be operated in a multi-state mode by ¦ appropriate application of an electric ~ield to the cell electrodes. DHF materials in addition to the field i dependent rotation of the optic axis, display a voltage dependent change in birefringence (~n).
Homeotropic alignment as employed herein refers to alignment of CSLC materials as described in Figure 2.
Homeotropic a}ignment refers to CSLC alignment in which the smectic layers~are parallel to the substrate walls (which in this case may or may not be the aperture of the cell).
The electric field is applied to such a cell across the smectic -l~ayers, i.e., parallel to the layers (e.g., in the xy plane as indicated in Figure 2) by electrodes that are lateral to the substrate waIls. The layer normal is z.
'~l The liquid crystal molecules are aligned with respect to 'l 30 each other within the smectic layers by application of an aligning electric fi~eld during cell preparation as is well-known in the art. The molecular director (n) makes an ~ll angle ~ with respect to z, as indicated in Figure 2.
p Application of an electric field along y as indicated, ~ 35 rotates n in the xz plane. In this case, light entering i~ through the substrate walls, particularly linearly polarized light propagating along 2 (with k along z) with .` ~ .

, .

W~93/10477 PCT/US92/09707 212~310 E along x will be modulated. Light entering the exemplified cell laterally through the zy plane, particularly linearly polarized light propagating along x (with k along x) and E along z will be modulated.
Application of an electric field along x results in rotation of n in the zy plane, whlch results in modulation of linearly polarized light with k along y and E along z or k along z and E along y.
The terms "transmission mode" and "reflection mode,"
as applied to single-pass and multi-pass modulators, and transmission-reflection mode and reflection-only mode, as applied to multi-pass modulators, refer generally to the light path through the modulator. In a transmission-mode, single-pass modulator, light exits the device after a 1 15 single pass through a CSLC cell without being reflected back. In a reflection-mode, single-pass modulator, light exits the device after two passes through a CSLC cell, light is ideally fully back reflected by a reflective surface with R ideally equal to 1. When the terms transmission mode or reflection mode are used in reference to a multi-pass FP cavity device, they refer to the light path in the transmission/reflecti~n mode modulator where the reflectivities of both reflective surfaces, which form the optical cavity,~are less than 1. In either case, light exits the device after multiple passes through the CSLC
` cell. In a transmission/reflection-mode, multi-pass modulator, the reflection mode output is related to the transmission output by the well-known relationship T = l-R
where R is reflectance. In a reflection-only mode ~ 30 modulator, ideally all of the incident light is back ¦ re~lected after multiple passes through a CSLC cell so that there is ideally no intensity loss. The back reflective ; surface of a multi-pass, reflection-only mode modulator ideally has reflectivity RB = l. No reflective surface will, however, have an ideal reflectivity of 1Ø For reflection-only mode devices, the back reflective surfaces preferably have a reflectivity (~) of approximately 1. In ;

S!'.' ~ WO93/1~77 PCT/US92tO97Q7 ~1~33 l~ 14 reflection-only mode devices the re~lectivity of the back reflective surface must be higher than that of the fxont ~ reflective surface. The choice of reflective sur~aces in ,' transmission/reflection-mQde device depends on the desired .. 5 finesse of the modulator. For purposes of this application, high reflectivity refers to reflectivities of about 0.85 or higher. The choice of relative reflectivities of the reflective surfaces of ~n optica}
~,l cavity for obtaining transmission/reflection or reflection-only operation and for obtaining a desired finesse is '7i~ ~nderstood by those of ordinary skill in the art.
The terms optical cavity and resonance cavity are used intercha~geable in this application. The FP etalon (and interferometers) and asymmetric FP etalons (and interf~rometers) are, in most general terms, called folded optical path devices. A reflection mode, single-pass device is al~o a folded optical path ~evice. The ~erm FP
cavity is used herein to refer to symmetric and asymmetric etalons and interferometers.
In the present invention the term polarizer is used to refer to any device or device element which separates incident . light into orthogonal polarizations and can include among others: polarizing beam splitters, Wollaston prisms, etc. An entrance polarizer defines the polarization of light enterlng a light modulator or switch. An exit polarizer:or polarization analyzer is any device or device , element that can be employed to analyze the polarization of light exiting a light modulator or switch.
Table 1 provides a summary of exemplary multi-pass . 30 optical modulator configurations of the present invention.Exemplified configurations include one or more CSLC cells in series within optical cavities. In some configurations birefringent ~elements such as ~uarter-wave plates are included within the resonance cavity. When CSLC cells are combined in series, they can be configured by choice of application of electric field or by choice of CSLC material ~.
~., so that their optic axes rotate in ~he same or opposi~e directions.
Detailed descriptions are given below for exemplary device configurations arising out of the present invention.

I. Planar ~liqned Smect c Liquid Crvstal Fabry~Perot Modulators , A. Binary Fabr~Perot Modulators Figure 3 is a schematic cros~-sectional diagram of an exemplary Fabry-Perot etalon which incorporates a planar-~0 or bookshelf-aligne~ chiral smectic LC material (see Figure l) and which selects between two transmission outputs corresponding to the two extremes of an electric field applied across the smectic layer. The device can employ a discrete state chiral smectic LC material, such as a SmC
material, or an analog smectic LC material,.such as a SmA
material. The modulator can be employed in the trans~issionJref~ection ~ode. An analogous folded optical path structure, also called an asymmetric Fabry-Perot cavity can be operated in the reflection-only mode O In either case, the de~ice can select between two spectral, i.e. waYelength, and/or intensity outputs, or modulate . phase, dependent on the ~ight entering the etalon and whether the devi~e is operated in the tra~smi~sion/reflection mode or reflection-only mode. The ~ 25 FP etalon device, operated in the transmissionJraflection i mode with linearly polarized, monochromatic or coher~nt :. : light, modulates intensity. With linearly polarized, non-monochromatic, incoherent light , e.g., white light, the etalon modulates wavelength. The reflection-only mode device with coher~nt light modulates phase. These devices can, thus, function as either a binary wavelength filter and a binary intensity modulator, i.e., an on/off light switch, or a binary phase modulator. Wavelength, intensity and phase modulation by these devices do not require the 3~ use of an output or analyzer polarizer.

W093/10477 PCT/US92/Og707 212331~
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In a planar-aligned, surface-stabilized chiral smectic LC, the molecular director of the material rotates in the plane of the electrodes, the yz plane, on application of an electric field across the electrodes, as shown in Figure 1.
5The direction of rotation of the optic axis depends on the sign of the applied field. In a two state FLC material, a SmC~ material, two orientation of the optic axis are I possible. In such a material the optic axis is rotated by an angle 2~, where ~ is the întrinsic tilt angle of the FLC
;~ 10material, by application of a threshold electric field. In la material having a tilt angle of ~50, the optic axis can ibe rotated by a total of 90. During operation of the :,planar-aligned CSLC cell, rotation of the optic axis does not effect a change in birefringence of the CSLC material.
~ ~5The device of Figure 3 comprises a c~iral smectic LC
:~cell containing a planar-aligned layer of a smectic LC
1material, 31, between inert substrate walls 32 and 33.

When SmC* or SmA* FLC layers are employed, the layers are ¦ also surface-stabilized to disrupt helix formation. The device view provided in Figure 3 is a cross-sectional view in the xz planef as indicated, where the z direction is the direction of~ the normal to the planar-aligned smectic layers ( z). The:~inside surfaces of the ~ubstrate walls 1. (yz plane) are provided with apposed internal reflective ; 25 surfaces 34 and 35. The intérnal surface of the substrate j walls are~also~provided with electrodes, 36 and 37, which mAy be the reflective surfaces or separated transparent ~ conducting el~ctrodes, e.g., IT0 electrodes. The internal `i sur~aces of the reflective surface may be provided with appropriate alignmen`t layers (38, 39) to assist in ob~aining planar-alignment of the chiral smectic LC. The substrate walls ~orm a uniformly spaced resonance cavity of . . .
length, L, between the internal reflective surfaces. In Figure 3, L is~substantially equal to the thickness of the CSLC layer (~d). The device can, optionally, include an isotropic spacer such that L is greater than d. The device can, again optionally include means for spacing the ~, . . .

'' !

~' WO93/10477 PCT/~S92/097~7 i''Z 2123~L0 ~"
~:j 17 substrate walls (not shown in Figure 3). A means, for applying an electric ~ield or voltage across the electrodes, 36 and 37, i5 also provided.
i The device of Figure 3 can be operated in transmission/reflection mode or reflection-only mode. When operated in the transmission/reflection mode, the substrate walls, any electrodes, and alignment layers are preferably transparent or semi-transparent to light entering the modulator~ At least one of the reflective surfaces must be transmissive to light entering the resonance cavity. The ~ubstrate walls can be constructed, for example, ~rom glass plates. A ref lective surface can, for example, be depo~ited by conventional means on the glass substrate to obtain a surface of the desired re~lectivity. For example, aluminum mirrors can be deposited. Alternatively, dielectric mirrors can be employed. If the reflective surfaces are ~ormed by metal deposition, the mirrors can also function as the electrodes. Alternatively, a transparent electrode layer, for example of tin oxide or indium tin oxide, can be provided. An alignment layer of an FLC alignment promoting material, for example PBT, can optionally be provided as the final layer on the inside sur~aces of the~ substrate walls (38 and 39). Alignment layers which promote: the desired bookshelf or planar geometry are well-know in the art. After the substrate walls aré prepared, the cell can be ~apped using spacers, the chiral smectic LC material is introduced b~tween th~
spaced walls and aligned within the resultant resonance cavity. ~ , I
~: 30 When operated in the reflection-only mode, the device has an asymmetric FP cavity in which the re~lectivity of on~ of the reflective surfaces (i.e., the back reflective surface) is approximately 1, and only one of the substrate walls with electrode need be transparent or semi-~:~ 35 transparent.
The operation of a typical binary intensity modulator ~: of Figure 3, incorporating a planar-aligned, surface-i, .
..
,;:
' .

WO9~/1~77 PCT/US92/09707 stabilized SmC with a tilt angle of ~5 is described.
commercially available material, designated Chisso 2004, (Chisso, Japan), is an example of a SmC FLC mixture with a tilt àngle of 45. Such a material is positioned and aligned in the cavity (31). Other such high tilt ~aterials are known in the art. Reversing the polarity of the applied electric field across the cell rotates the optic axis of the device by 90. The SmC~ material, on such a configuration, displays either of two molecular orientations which coincide with the two possible orientations of the optic axis at + ~ in the yz plane. The material has two states dependent on the sign of the electric field. Linearly polarized monochromatic or coherent light enters the etalon. The entering light is propagated along the x axis and preferably polarized parallel to the optic axis of the smectic LC in one of its switching states.
Entering light with polarization parallel, for example, to the extraordinary axis of the device excites the extraordinary eigenmode of propagation. Transmission I through the device, neglecting absorption losses from the mirrors and scattering losses in the FLC material, is given by: ;

T(~ eR2)2 + ~ sina~/2] (1) (see Yariv et al.~(~1984) Optical Waves in CrYstals, Chapter 8, John ~iley; and Sons, NY) where R1 and R~ are the reflectivities of the reflective surfaces of the etalon, is the phase~change due to a single round-trip of light of wavelength~ in a cavity of index of refraction n and thickne~s L. L is the separation between the mirrors in the etalon. Assuning normal incident light, ~ is given by 4~nL (2) 3~ A
, ~

' W093/1~477 2 1 2 3 3 ~ O PCT/US92/09707 ,, 19 ;j When monochromatic or coherent light is incident on the etalon of Figure 3 in transmission/re~lection mode and the tilt angle of the FLC mixture is 45, the device functions as an intensity modulator, an on/off switch, ~ 5 since little or no light is transmitted through the etalon i~ when ~he optic axis of the SmC LC is rotated perpendicular to the polarization of incident light. Depending nn the values of R1 and R~ and any device losses, the device will ~unction as a high or low contraæt intensity modulator. An output polarizer is not required to detect the intensity modulation produced by this device.
'j The operation of a typical binary wavelength modulator :, of Figure 3, incorporating a planar-aligned, surface-., stabilized SmC material having tilt angle of 45 is also ,j 15 described. This device is identical to structure to the binary phase modulator abo~e. Light entering the device .` is, however, non-monochromatic, linearly polarized light.
According to Equation 1, in an etalon like that of Figure 3, a transmission maximum occurs for ~/2 - m~ , . ~
where m is the order of the resonance of the cavity.
Because the tilt angle of the molecular director of the SmC~
LC material is 45, the input light can be linearly polarized either along the ordinary or extraordinary axis, , ~
2.5 depending upon the sign of the applied electric field.
Switching the FLC cell by, for example, reversing the 'I , .
applied electric field, switches the index of refraction ; s~en by the Incident lightifrom ne t~ nO. This changes t~e ~: effective sptical path length of the cavity, thexeby ;,` 30 shifting the resonance frequencies of the ca~ity. When incident light~ is polarized along the ordinary axis, a ; transmission maximum occurs for wavelengths 2n"L ( 4 ) )'~ ~ m i~

,~, ~, .

W093/10477 - . PCr/US92/09707 212~310 20 , where nO is the ordinary index of refraction and Ao is the ,~ wavelength corresponding to the mth resonance of the cavity in one switched state of the FLC. Upon switching the device, the incident light is polarized along the molecular director giving the following condition for transmission maxima Ae_ 2n L (5) where n~ is the extraordinary index of refraction and Ao is the wavelength corresponding to the 1th resonance of the l lO cavity in the second switched state of the FLC.
.l In the ordinary transmîssion, (~
x 2n L A 2D L (6) is the ordinary FSR between those wavelengths and can be written as~

~A l20 / [2nrL] (7 ) The difference in~:wavelength between two maxima in the ` ordinary transmission~and the extraordinary transmission at :~' the same resonance value, i.e., m=l (~ ) is:

A - ~n Ao , where; ~n is ~the~difference in extraordinary and ordinary indices of refraction, i.e~,~the birefringence (nO-nO) of the ~`, material at wavelength ~
Wavelength tuning:of the device between two adjacent i.i : ~
maxima of~the;~;~ordinary~and~extraordinary transmissions is :: accomplished~;~by~reversing the polarity of the electric ~r ~ ~ field applied ~aàross~the FLC layer. T~ning of the device ~, results in shi~ts~betweien the ordinary and extraordinary transmissions within a~the Free Spectral Range (FSR) of the W093/10477 212 3 3 1 0 PCT/US92/~707 device, and the tuning range can be written as a fraction of the ordinary FSR as ~ ~n 12nOL I (9) Note that L and the FLC material (actually ne and nO/ of the material) are chosen so that at the design wavelength the FSR between adjacent etalon resonance maxima i5 larger than the ~A over which tuning is desired.
If for example, it is desired to tune the device by one half of an FSR from a design wavelength of 630 nm, with ' 10 an FLC ha~ing ~n = 0.~5 and nO = 1 . 5, th~ resonance cavity width L should be set at 1~26 ~m. Under thes~ conditions, with incident white light and appropriate blocking filters, ; the device will transmit a series of maxima in the visible wavelength region (ordinary) at approximately 756, 630, 540, 473, and 420 nm and on switching of the applied electric ~ield will transmit a second series (extraordinary) of maxima in the visible at approximately 832, 693, 594, 520, 462, and 416 nm. The etalon can, thus, functio~ as a binary filter to select between adjacent wavelengths in the ordinary and extraordinary transmission series~ One or more blocking filters external to the etalon can be employed t~ block higher or lower order maxima .
As ~a second example, if it is desired to select between bands centered at about 600 and about 700 nm, from incid~nt w~ite light, an FLC etalon can be designed with a high tilt 45 SmC FLC material ~aving n~ = 1.5 and n =
0.25 and spacing the etaion cavity at 1.0 ~m. In one state such an etalon will transmit maxima ~in the visible) at . 30 approximately 750, 600, 500 and 428 nm and in the othPr state maxima at approximately 875, 700, 583, 500, and 437 ~ nm. Note that this etalon can also select between i wavelength pairs: 875 and 750 nm, 750 and 700 nm, and 600 ~ and 583 nm. The etalon cannot se~ect 500 nm light, since , 3S a 500 nm band is transmitted in both states of the device.

.

WO93/10~77 PCT/US92/09707 With the use of appropriate blocking filters external to i the etalon, reversing the polarity of the electric field in such an etalon will allow selection between the desired pairs of wavelengths.

- 5 The contrast of intensity and wavelength modulation o~
~ the binary etalon of Figure 3 will decrease if a SmC~
`, material ha~ing a tilt angle ~ 45 is employed. This is illustrated by the following example. In a binary etalon wavelength modulator of Figure 3, in which a SmC material having a tilt angle of 22.5 is employed, the optic axis of the etalon will rotate by 45 when the electric field applied to the device is rever~ed. If in one state of the device, corresponding to one polarity of applied field, the ; incident non-monochromatic light is polarized along the extraordinary axis, the device will transmit a series of ., maxima at wavelengths, ~e =2nOL/m~ When the polarity of the ', electric field is reversed, the optic axi~ of the FLC is rotated by 45~ In this case, tWQ eigenmodes of ~, propagation ar~ excited within the cavity: one oriented ~! 20 a}ong the optic axis and one oriented perpendicular to the optic axis. ~In~this case, two series of wavelength maxima Ao and ~e~ of comparable intensity, where Ao - 2nOL/m and Ae ~! ~ = 2neL/m, will~b~ transmitted. The relative intensities of the two series~of bands transmitted will depend on the tilt angle For ~the ~xtraordinary transmission neglecting losses,--~he~intensity~Ee = Ej~ cos2~ and for the ordinary ' ~ transmission, the intensity Eo = Ej~ sin2~, where Ej~ is the incident light intensity. At 2~ = 45 Eo = Ee. At 2 less than 45, Eo > Ee. At 2~ greater than 45, Eo < Ee.
~: :
Bo naloa Fabrv-Perot Modulators ~ Analog chiral~ smectic LC materials, such as Sm~
h~ ~ materials, display a voltage dependent analog rotation of tilt angle from ~the~field free state where ~ - 0 to a maximum voltage dependent ~x The use of such an analog material (planar-aligned and surface-stabilized) in the device of Figure 3 will result in analog intensity ~...................................................... .

F,~.
.
j -WOg3/10q77 212 3 ~1 ~ PCTJUS92/Og707 modulation of monochromatic or coherent light when operated in transmission/reflection mode. Howe~er, non-monochromatic or coherent light will undergo binary/wavelength modulation with the etalon between two transmission series of maxima dependent on L and the re~ractive indices of the material. Most known SmA~
m~terials have maximum tilt angles of 22.5 or less. Thus, a typical SmA~ etalon of Figure 3 will select between the extraordinary transmission maxima (~e) and the transmission of both the ordinary and extraordinary transmissions (~e and Ao). A Sm~*-based etalon functions similarly to the etalon employing a SmC~ material where ~ = 45 in that the relative intensiti~s of the ordinary and extraordinary transmissions will vary as indicated above as a function of 2~.
When monochromatic spatial coherent light is incident on the device of Figure 3 employing a SmA FLC, the device operates like an analog amplitude, i.e., intensity, modulator. With this material the tilt angle rotates in linear proportion to the applied electric field. The light available to be resonantly reflected or transmitted is given by, - E1 - E1nccos~2~(~ ) (10) w~ere V is the app}ied voltage and 2~ is the rotation of the optic axis, referenced to the direction of incident j 25 polarization. Varying the applied voltage V, var~es ~, and ', thus varies the;~amplitude El. When ~ is oriented parallel ¦ to ~he incident polarization, ~ = 0 at V0, incident light will be transmitted. When ~ is rotated to ~V) by changing the applied field, a phase shift is induced between the = 0 state and any of the ~(V) states. Hence for monochromatic or coherent light, the device in transmission/reflection mode operates like an analog amplitude , binary phase modulator.
In certain applications in which a detector which averages the wavelength output of the device, such as the human eye, is employed, the etalons of Figure 3 which , WO93/10477 ~ ~ PCT/US92/09707 , 212331~ 24 employ materials with tilt angle less than 45, will allow sele~tion between a pure spectral output at ~e t at some m) and combined ~utputs ~ ~ ~q. These two outputs will be perceived by the averaging detector as two distinct wavelengths. For example, for an etalon functioning in the visible region, the human eye would perceive two different colors, the second being a linear combination of ~0 and Ae~
The relative intensities of the two wavelengths in the transmission will be a function of the tilt angle, so that some wavelength variation will be perceived by the averaging detector as a function of tilt ang~e. In addition, a CSLC cell can he operated in a multi-discrete state mode and can be temporally multiplexed for applications employing slow response detectors, such as the `I 15 human eye.
1 Antiferroelectric liquid crystal materials can be ¦ employed in the devices of Figure 3 in place of SmC~
materials and function analogously to the SmC materials.
~ If a rotation~of the optical~axis of 90 can be achieved by 1, 20 switching between ~ any of the states of the antiferroelectric material, then a high contrast binary .j ~
~ wavelength modulator which modulates inaident linearly polarized non-monochromatic light between two pure wavelengths will~result. Similarly, a binary phase modulator will result when such a material is included in sl a reflec~ion-on;~y;~mode~device of Figure 3. If the optic axis of the mater~ial~can~rotate only by less than (or more than) 90, then~the wavelength modulator will display ,., contrast that is dependent on the tilt angle of the material and will modulate between a pure wavelength and a Iinear combination of two wavelengths. Again analogous to the amplitude ~modulating -etalons implemented with SmC~
materials, those implemented wi~h discretely switching multiple state materials can function as on/off light 35~ switches or multiple-level amplitude modulators.
` Certain FLC materials, such as distorted helix ferroelectric (DHF3 effect FLCs tDHF), when placed in a ,j ' ! ~

~ ,.

, : 25 , planar aligned cell, not only display a linear rotation of the optic axis as a function of the voltage applied to the ~ electrodes, they also display a voltage dependent change in ,~ the birefringence ~ n)). These materials are also of 't 5 interest because they operate at much lower voltage (the ,t, voltage saturates at ~3 V/~m compared to + 15-50 V/~m) than " SmC , SmA or antiferroelectric materials and the tilt angles are relatively }arge + 34. A device of Figure 3, operated in transmission-reflection mode which incorporates a planar-aligned DHF material operates as a binary modulator of intensity (monochromatic or coherent light) or a binary modulator of wavelength (non-monochromatic light and tilt angle = 45) and a coupled wavelength/intensity modulator of non-monochromatic light as described above for ~;, 15 SmA* materials angle ~ 45.
In the device illustrated in Figure 3, the length L of the resonance~cavity of the etalon is approximately equal to the width of smectic LC layer, typically designated d.
Functional etalons can al60 have ~ ~ d. For example, etalons with L>d:can be constructed by introducing one or more isotropic~spacers along:~the light propagation axis between the reflective~surfaces and the smectic LC layer.
Functional planar-aligned, ~urface-stabilized FLC cells can be made with :cell:wldths,~ d, ranging from a~out 0.5 to ~5 about.. 15 micron.~ The~upper limit is presently determined by: the -limits~of surfwe: stabilization. The thinness of : ~ the cell is typiaally determined as the minimal thickness required to~avoid shorting of the electrodes. Resonance .cavity lengths ~qreater than about 15 microns can be .~ 30 obtained by using isotropic spacers.

II. HomeotropicallY Aliqned Smectic Liquid Crystals Fabry-Perot Modulators~
Continuous~ly~ tunable Fabry-Perot etalon optical modulators are~also provided herein. In one aspect ' a continuously tuna~le modulation results from positioning of ~ a homeotropically-aligned chiral smectic LC between etalon : ::
~ :

~ , WO~3/10477 PCT/US92/~9707 2123~10 26 r~flectors. Homeotropic alignment is illustr~ted in Figure 2.
The homeotropic CSLC cell alignment of Figure 2 is believed to be distinguishPd from prior art homeotropically-aligned CSLC con~igurations in that the electric field is applied across the smectic layer, perpendicular to the layer normal. This cell configuration provides very rapid binary or analog variation of the bire~ringence of the LC materia~. Light entering the cell through the substrate walls propagating along the z axis, referring to Figure 2, sees ~his change in birefringence.
The optical path through the cell is effectively changed when the birefringence of the material is changed. In an alternate homeotropic cell c~nfiguration, polarized light enter~ the cell, propagating along the y axis, bisecting the plane of the smectic layers.
, CSLC cells having homeotropic alignment as displayed in Figure 2 and relative positioning of electrodes, I substrate walls and smectic layers as described in relation 'J 20 to ~igure 2 can function as light modulators in simple transmission-mode or reflection-mode. A h~meotropic~lly-aligned SmC cell can function as a~binary pha~e modulator or a ~inary wavel~ngth modulator. A homeotropically ~f aligned antiferroelectric cell can function as a ternary phase and wa~elength modulator. A homeotropically ali~ned S~A or 'DHF cell can function as an analog phase or wa~elength modulator. Homeotropically-aligned CSLC cells can al~o function as intensity modulators. These homeotropic cell configurations can be positioned with etalon and asymmetric etalon cavities to provide optical modulators of the present invention.
, ~ Figure 4 pro~ides a schematic illustration of a homeotropically-aligned smectic LC modulator. Figure 4A is i a three-dimensional side view of such an etalon modulator.
The elements of this modulator are similar to those of Figure 3 except for the relative positioning of electrodes in the devices. Figure 4B is a two-dimensional cross-:~ :

i ., .

; 27 section of the device given to illustrate the position of the electrodes with respect to the substrate walls and ~mectic LC layer. In Figure 4, a homeotropically-aligned LC layer (40) is positioned between substrate walls (47 and 48) and reflective surfaces (41 and 42). Lateral electrodes are positioned at the side of the cell (43 and 44). Optional alignment layers are provided (45 and 46).
The positioning of these electrodes is more clearly seen by reference to Figure 4B. Homeotropic alignment is defined with respect to the substrate walls through which light enters the device. Thus, the smectic layers of the homeotropic layer of the illustrated device are in the yx plane. The layer normal z is along the z axis.
Application of an electric field to the electrodes across the LC layer results in rotation of the optic axis of the ' liquid crystal in the yz plane.
An attractive feature of the device configuration of ~ Figure 4 is that light propagating through the device need ;~ not interact with the electrodes, precluding cavity !.1 absorption thereby permitting high-resolution transmission.
1 The homeotropic cell configuration of the invention is an `JI improvement over the conventional book-shelf geometry I alignment in that a~small~ percentage of incident light is !
il absorbed by even the most transparent electrodes such as tin oxide or~indium~tin~oxide commonly used with book-shelf type alignment.~ herefore, the electrodes employed with ,, homeotropic alignment can be opaque. They can also serve as spacers to create a bias phase inside the resonance '~ cavity for providing greater filter selectivity.
The operation of the modulator of Figure 4 is described for modulation of linearly polarized light as follows. Polarized light traverses the cell as indicated.
An electric field of suitable strength applied to the cell eIectrodes triggers a rotation of the smectic LC optic axis. This rotation results in a change in birefringence of the material along the direction of light propagation 1~ and results in modulation of the light. The reflective , ..

W093/1~?7 PCT/US92/09707 surfaces of the device serve to fold and lengthen the optical path.
In a homeotropically aligned cell, the phase and retardation of the device càn be modulated in an analog manner with an applied electric field. For a uniaxial , anisotropic material, the index ellipsoid is used to give the two indexes of refraction seen by an incident optical , field. For a positive uniaxial material, the semi-major axis of the ellipsoid corresponds to the extraordinary index of refraction, ne,and the semi-minor axis gives the ordinary index of refraction nO. Here, the slice of the i index ellipsoid containing the incident optical ~!j polarization determines the birefringence. For propagation along the major axis of the ellipsoid (the directors), the slice is a circle. Therefore, the material appears isotropic ~n = 0. Propagation normal to the major axis yields the maximum birefringence, given by the anisotropy of the material ~n - (ne ~ nO). For propagation at an angle to the major axis, which is intermediate to these two extremes, the;~two refractive indices seen by the optical field are nO and,~ ~
.1 : :: : .

r2Cos2o~n~sin2~] l/2 (11) Modulation of phase~or retardation, depending upon the mode in which~ the~device is used, is achieved by 1 ~25 electron~ically;~varying the orientation of the director, ~.
Phase modulation~is;achieved by linearly polarizing the input field along the variable projection of the extraordinary index, net~). This phase modulation can be used in an optical cavity to change in an analog manner the ~30 resonance conditi~on. The wavelength transmitted by a Fabry-Perot cavity of length ~, corresponding to the mth resonance is given by~
2ne(~)L (12) - m ~' ~l :
:
i, W093/10477 2 ~ 2 3 31 0 P~r/US92/09~07 The wavelength shift in changing the tilt angle ~rom ~1 to angle ~2 is th~refore gi~en by ~A -2 L ln~ (~2 ) - n~(al)] ~l3) For a full ~t2 rotation of the director orientation, the maximum wavelength shift of ~x = 2~n L/m is achieved.
When an analog SmA or DHF material is homeotropically~
aligned in the modulator of Figure 4, and operated in the transmission/reflection mode, an analog intensity or wavelength modulator result~. When configured as an asymmetric FP operated in the reflection-only mode, an analog phase modulator results. Inclusion of discrete homeotropically aligned chiral smectic LC materials in the device of Figure 4, results in discrete state intensity or wavel~ngth modulators. When a discrete state CSLC material is configured as an asymmetric FP operated in reflection-only mode, a discrete phase ~odulator results.
Means for obtaining ho~eotropic alig~ment are well-known in ;the art. For example, cetyltrimethyl ammonium bromide can be used a~ a homeotropic alignment agent.
Homeotropically-aligned cells have been fabricated with } thickn~ss ranging between 0.25 to 250 ym or greater. Very thin cells, down to about 0.} ~m, can be produced to accoDmodate decigns requiring small cavity length for . a~c~mplishing wider free spectral ranges ~FSR's) due to the 25 : u~e o~ lateral electrodes. The device FSR is given by the square of the wa~elength, propagating through the cavity divided by twice the index of refraction, n~of the FLC
times the thickness, l, of the FLC cell. The thickness, d, of the FLC cell, in the~absence of a spacer, dete~mines the device FSR. For a typically smectic LC cell n = }.5, and ; d = lO ~m, an FSR of 80 nm can be obtained for ~ ~wavelength of interest for communications applicatîons~ = ~.5 ~m. A
dec~ease in the cell :thickness d to l ~m results in an increase in FSR to 800 nm. The finesse, or the number of . ~
i ~ WO93/10477 PCT/USg2/09707 2123~1~ 30 independent full width, hal~ m~ximum peaks capable of being ~ stored in the cavity, is a function of the mirror ; reflectivity. Hence, given a FSR and cavity finesse, the number of independent communication channels that can be f'~ 5 demultiplexed, for example, is determined.
" .
III. Co~tinuously Tunablv Planar-Aliq~_d Chiral Smectic LC
Qptical Modulators In fabricating high diffraction efficiency programmable optical elements, it is desirable to have a high r~solution spa~ial light modulator (<l~m) in which the ;, phase of a particular pixel can be changed continuously between 0 and 2~. A substantial phase change resulting from a relatively small induced phase shift can be obtained, through phase interference, by means of a reso~ance cavity. However, for certain applications, it is important that there be no change in the state of ;1 polarization of the light beam as it is being reflected off ,, the cavity mirrors.
f', As has been discussed above, electro-optically tunable ~i 20 binary and analog smecti~ liquid crystal materials exhibit ,~ little or ~o birefringence change wi~h an applied electric field, when they are aligned in the pla~ar geometry. An electric field applied across the substrate walls effects only a ro~ation,: within the plane of those walls, of the molecular director about the axis normal to the smectic ;~ layers. In other words, in a planar aligned smectic FLC, the phase change resulting from passage of a linearly polarized optical~beam through a smectic LC layer is necessarily accompanied by a change in the state of polarization of that beam.
Therefore, a method o~ inducing a phase change of an optical bea~ through a smectic LC layer, without a change in the state of polarization, would have application in el~ctro-optic modulation. The "decoupling" of phase change and rotation of the polarization can be accomplished with the smectic LC phase modulator schematically described in Figure s. This device configuration, represented in the ~' .. . . . .

W~93/10477 2 1 2 3 3 1 0 PCr/USg2/~g707 transmission mode in Fig. 5A, positions an analog planar-aligned chiral smectic LC cell half-wave plate (50), e.g., a SmA* or DHF cell, between two quarter-wave plates ~51 and 52). ~he smectic LC cell (50) comprises transparent or semi-transparent substrate walls, transparent electrodes and optional alignment layers (not represented). The width of the LC layer in the cell is chosen so that the cell is a h~lf-wave plate for the wavelength(s) to be modulated.
A basic principle for optical modulation with this device is that conversion of incident linearly polarized light to circular states of polarization via the quarter-wave plate allows the induction of an absolute phase retardation through a smectic LC half-wave plate, without a change in the state of polarization. Passage through ~he second quarter-wave plate reconverts the circularly polarized light to linearly polarized light~ The resulting phase shift is ;a function of the orientation, e, of the ha}f-wave plate with respect to the direction of polarization of incident light and is, thus, a function of j 20 the voltage (or e}ectric field) dependent tilt angle of the 1~ smeatic LC material. Voltage dependent rotation of the optic axis of the~smectic LC produces an analog change in phase which in~ transmission mode results in analog modulation of intensity of incident coherent monochromatic light or in anaIog modulation of the wavelength of incident non-monochroma~ic light. The quarter-wave and half-wave ~ plates are preferably achromatic over the wavelength region I of interest. As~ indicated in Figure 5, the optic axes of quarter-wave plate(s~ of the phase modulator are parallel or perpendicular to each other and are oriented at ~45O to the direction of polarization of incident light.
1~ ~ The device~of Figure 5B is configured for reflection-J~ mode operation by adding an approximately lO0% reflective surface (53). ~Reflective surface (53) may replace one of the transparent substrate walls of the CSLC cell (50).
This reflection ~mode device requires only one quarter-wave plate (51) and the smectic LC cell (50) is designed to be a quarter-wave plate for the wavelength of monochromatic or 1~ :
Y~

~ W093/l0477 PCT/~92/~g707 : 32 coh~rent light to be modulated. The reflection only device operates as a pure phase modulator.
The Jones matrix describing the composite structure of Figure 5A is given by (~.D. Sharp, Ph.~. Thesis, Univ. of Colorado, 1992) ~(~) ella~A~ e o I (14~

where ~ /4), and ~A and ~u are the common phase fackors due to the quarter-wave and half-wave plates, re~pectively. This can be reognized as a single retarder , 10 orien~ed at angle 0 (with respect to the z axis~ with retardation 4~.
The phase nodulator of Figure 5 is a specific example of a variable retardex implemented with a planar-aligned, ~ analog CSLC cell. In this example, the input light is `~l 15 linearly polarized and oriented at oo. In the generalized ., case, a variable retarder comprises the same elements as the modulator of Fisure 5. However, the polarization of l the input light can be elliptical as well as linear and can ;7 ha~a any orientation. Equation 14 describes the co~posite 1 20 structure of the general ~ase.
Fig~re 6 illustrates an optical modulator of this invention comprising the analog phas~ ~odulatcr of Figure 5 .within a resonance cavity operating in transmis~ionlreflection mode. The numbering of elements in Figure 6 is the same as in Figure 5. The resona~ce caYity of the device of Figure 6 is formed by refl~ctive surfaces 61 and 62, which are parallel and opposed su~h that light ent~ring the resonance cavity is reflected at least once.
f The resonanc~ cavity has length, L, and extends between the reflective surfaces, at least one of which is transmissive to allow light to enter the resonance ca~ity. The wa~e ~: plates are po~itioned parallel to and between the reflective surface~. The optic axes of the two quarter-wave plates 51 and 52 are oriented at an angle of ~45 with respect to the direction of polarization of linearly polarized incident light (i.e., at 45 to the plane of vibration of incident light). The cavity may optionally ,~, , ,= .. , .. .~ . ..

1 WO93/1~77 212 3 31~ PCT/US92/09707 include one or more isotropic layers (e.g., 63) aligned parallel to the wave plates ser~ing to alter the length of the resonance cavity if so desired.
The operation of the etalon of Figure 6 in transmi~sion/reflection mode is as follows: Linearly polarized, monochromatic or coherent light of the design wavelength is illuminated into the etalon along a propagation axis through the reflective surfa~es and wave plates. In transmission mode, modulated light is analyzed or detected on the opposite side of the device from which it ~nters the etalon. A variable voltage sr electric field is applied to the electrodes of the LC cell to rotate the optic axis of the cell in the plane of the substrate walls ~-, which form the aperture of the smectic LC cell. The optical axis of the material can be rotated from ~~x to +~ (a total of 2~x) by application of a maximal voltage -V~ or ~V~, respectively, where ~x is the maximum voltage ~ dependent tilt angle intrinsic to smectic LC material.
bl Rotation of the optic axis of the smectic LC modulates the intensity of monochromatic, coherent light. An exit polarizer is not required to detect intensity modulation.
Analogously, the wavelength of non-monochromatic linearly polarized light is modu1ated by the device of Figure 6 and polarization analysis is~ not required to detect the wavelength modulation.~
Assuming that the etalon mirrors have the same reflecti~ity~(R) and that there is no mirror absorption, the intensity ;transmission function for the device of Figure 6, can be expressed as ~Sharp, Ph.D. Thesiæ, p. 180) ~] 30 ~ : T(A) - ll R)2+4Rsinz(~2~) (15) ~` where ~ is the~sum of all absolute phases accumulated in the cavity in ~a single pass. Using reasonably high reflecti~ity mirrors (e.g., R>85%), this function is 3S characterized ~y a series of narrow spectral peaks (with a ~;~ theoretical unity transmission) separated by broad bands ~:

,. .

WO93/10477 . PCT/US92/09707 2123~10 34 with strong rejection. Transmission maxima occur when the sinusoidal term in the denominator of Equation 15 vanishes, , or ~+2~-m~ , (16) , ' .
5where m is the order of the cavity resonance. ~ecause each of the absolute phase factors is completely arbitrary, the ~ total phase factor c~n be expressed in the following manner .' without sacrificing the generality of transmission function lO~_ 2AL , (17) .~ ~here L represents the distance of a single optical pass ;l throu~h the device (or actual cavity length thereof].
Substituting the above expression into Equation l6, the wavelengths of peak transmission can be calculated.
15In order to electronically tune or shift the spectral p~k transmitted by the davice of Figure 5, the molecular director of the smectic LC half-wave plate is rotated so that:the angle ~ is varied to a ~x~ When the optic axis of the FLC is aligned with the polarization of incident 20light, i.e., ~ - 0, maxima of order m, and (m l~ occur for ~ m12L and ~1 _ (m-l) , (18) ?1 respectively. Th difference in these expressions gives the Free-Spectral-Range (FSR) (~ ), which is the 25wavelength separation between transmission maxima ~1- 1 1/~ . (19 By changing ~, the resonance of order m shifts from Ao~ 2L ~O la~ 1 (2 //~m~ (20) , . .

WO93/10477 PCT/U~2/09707 ~` 21233~L0 The difference of the two expressions in the above equations, ~ , represents the spectral shift due to the electro optic tuning of the analog CSLC

~ aA2/~L (21) " ~7~;
,, Allowing ~ A, the rotation of smectic LC molecular director required to tune the analog device of Figure 6 through a FSR is ~ = ~/2. Currently, analog SmA* materials are commercially a~ailable which are capable of yielding tilt angles (~) greater than ~/8, h~nce, total director ~0 rotations exceeding ~/4, thereby permitting analog tuning over a range greater than one-half of a FSR with a single SmA /liquid rystal cell.
The device of Figure 6 also functions as an analog I intensity modulator of monochromatic or coherent light 1 15 with, for example, a planar-aligned surface stabilized SmA
LC. Assuming a monochromatic or coherent input, the device ~' optical thickness can be selected such ~hat the IJ . tran~mission, with a large negative bias ~10: V/~m~ applied to the modulator, is unity. A sli~ht change in voltage pro~uces a small phase: shift in the c~vity adequate to ~` ~hift ~ the wav~length of the device resonance by a t3l signi~i~ant perce~nta~e of its ~pectral w}dth. This, in turn, GaUSeS a ~harp~change in the intensity transmitted by the device. ~For a phase change of ~/2, the transmission at , 25 he w~velength of incident light is midway between the mth and (m+l ) ~h resonance corresponding to a minimum in transmission. This approach provides analog optical ii~ intensity modulation without resor~ to the large Y~ltages ~,i which are required:to:produce a very small change in the index of refrac$ion of the electro-optic materials in prior i:~
'i art Fabry-Perot electro-optic modulators.

.` ~

The achievable contrast ratio, TMAx/TMIN~ is given by C ~ 4R 2sin2(~) , (22) where the maximum contrast ratio occurs for ~x = ~/8. The design of ~he resonance cavity depends upon the application requirements. A particular design, for example, might require a high contrast ratio, while another might require linearity in the transmission versus voltage characteristic. From the above expression, it can be seen :l 10 that high contrast ratio is achieved a~ the expense of il linearity. ~he contrast ratio, while varying with tilt , angles, is much more sensitive to the reflectivity of the .l mirrors. This is because the change in transmission occurs '~ rapidly near resonance, but flattens out quickly~
Consider a modulator containing intracavity a single SmA* FLC cell positioned between parallel polarizers, with an SmA* cell being designed to be a half-wave plate at the wavelength of incid~nt radiation. For a 20 to~al range of rotation in the FLC molecular director, the contrast ratio is c = 1.7. This contrast ratio is comparable to the output of a typical Fabry-Perot de~ice containing a material with the same range of tilt. ~ mirror reflectivity of R = 0.7, 0.8 and 0.9 results in a contrast ratio of c = 14, 34 and 150, respectively. Figure 7 shows the xelationship of transmission versus mo~ecular rotation, ~, for- three dif~erent mirror reflectiviti~s~ R = 0.7, R - 0.8 and R - 0.9. Figure 7 demonstrates that the slope near ~l resonance increases; as: the contrast ratio increases, -~ thereby reducing the resolution and requiring accuracy in tuning of de~ice. In:other words, for a device condition near resonance, a very small change in voltage appli~d to the cell can produce a large change in the transmission.
Figure 7 shows the slope corresponding to R = O.7, or the lowest contrast ratio, is:most linear. On the other hand, .~, 35 the slope representing R - 0.9 is highly non-linear, wherein a 5 change in ~ effects a substantial transmission ~., , . . .

WO93/10477 212 3 31~ PCTtUS92/09707 change from unity to 8%, while a subsequent angular change of 15 produces only a minimal transmission change from 8%
to 0.7%. Thus, a high contrast is exhibited with a highly nonlinear transmission function.
The modulator o f Figure 6 can be operated in the reflection-only mode to give pure phase modulation of incident linearly polarized, coherent, monochromatic light as is illustrated in the device configuration of Figure 8.
Operation in reflection only mode requires that one o~ the reflective surfaces, i.e., 62 be approximately 100%
reflective. The resonance cavity thereof operates to fold the optical path numerous times thereby effecting a long effective interaction length. The increased interaction length in turn produces a large phase change with only a small rotation of the molecular director.
I Figure 8 shows an exemplary configuration of an analog ¦ tuning asymmetric Fabry-Perot~phase modulator operating in reflection-only mode comprising a resonance cavity formed by a front reflective surface (81) having Rl < 1, a quarter-wave plate (83) for the design wavelength to be modulated, an analog chiral smectic LC cell (80), preferably planar-aligned, which is a quarter-wave plate for that design~ wavelength and a back reflective surface (82) having R~ The optic axis of the quarter-wave platé l83) is~oriented at 45 with respect to the direction of polarization of incident light. The electrodes (not illustrated in ~Pigure 8) of the smectic LC quarter-wave plate are connécted to a variable voltage source in order ~ to tune the phase modulator. The coherent, monochromatic 1 30 ~ linearly polarized light beam of design wavelength is ; illuminated, along the axis normal to the mirrors, through the device.
The follow~ing mathematical description of the ¦ ~ reflection-only mode phase modulator is analogous to that ~35 provided above for the transmission mode device and is i based on the assumptions that the amplitude of the light is unaffected by the reflective surfaces, the back reflective .

~ WO93/10477 PCT/US92/09707 ~12331l) 38 sur~ace is an ideal, no-loss, reflector and incident radiation is propagated along the axis normal to the confining ~ubstra~e plates of the device.
The Jones matrix for the round-trip of light through ~, 5 the device of Figure 8, due only to the two wave plates, is the same as for the transmission mode devi ce of Fig. 6 .
The total filed reflected by the device is ii ,, il r t2e~ 2~ (23) ~ 1 - r e~ 2~)J o ,~ .
where t and r are the complex f ield transmission and reflection coefficients of the front mirror. Assuming no mirror absorption, the reflected intensity is equal~to the i, incident intensity, and the relationship between the complex reflection and transmission coeffici~nts can be determined. Since the device is assumed to have no energy lS losses, the reflection coefficient for the phase modulator can be written as r - eix (~4) where X is the induced phase shift. Assuming that the phase is zero for ~ = 0, or ~/2 = m~, where m iS an integer representing the order of the device, the phase expression is X - 2tan~ +~)tan~l . (25) (1-~ O J
~ . :
The induced phase shift SX) is a direct consequence of the rotation of the smectic LC molecular director~ This ph~se 25 ~ shift is enhanced by the gain term due to the reflectiYity of the f~ont mirror.: ~
Figure 9 show6 a series of resultant phase shift X(~) for different Rl values of the front mirror. A SmA*
: material which has a maximum tilt of 10 is assumed in this ~, 30 analysis. Clearly, for Rl = O, the gain factor is unity, and the phase change is limited to a maximum of ~2~. This condition represents the linear solid line shown in Figure 9. For Rl = 0. 7, a dramatic change in modulation depth is observed over the previous case as ~hown by the dotted line of Figure 9. Under this condition, the phase shift through the device can be ~uned continuously from -126 to 126~
As Rl increases, the maximum achievable X saturates toward ~ and the phase function becomes more nonlinear. For Rl = 0.8, the phase shift can be tuned from -145 to 145.
For Rl = 0. 9, the phase shift can be tuned from ~163 to 163. Finally, for Rl = 0. 97, th2 phase shift can be tuned from -175 to 175. Since the maximum phase change ever required is 2~, the analog phase modulator of Figure 8, permits a broad ~nalog tuning range over 97% of the maximum range.
In other exemplary modulator embodiments, the reflective surfaces whi~h form the optical cavity are ~l external to the CSLC cell and the optical cavity can optionally include more than one of such cells. Two or more of the same type:of CSLC cell can be combined or two or more of diferent types of cells can ~e combined.
Particularl~ useful co~binations of CSLC cells within an ¦ optical cavity include~ cascaded planar-~ligned, surface-1 stabilized Sm~; cells (where ~x < 45) to increase the ¦ total rotation o~ polarization of incident light preferably to 90 and combinations:of DHF cells with similarly aligned SmA~ or SmC cells in which the SmA or SmC cell functions to compensate for the induced polarizati~n of the DHF.
Also useful are: combinations of two (or more) DHF cells i whose optical axes rotate in the same direction to compensate for ~he rotation of polarized light or that rotate in opposite directions to compensate for the change ;~ in birefringence~ These cavities can also optionally be provid~d with an isotropic spacer between the reflective surfaces external to the chiral smectic liquid crystal cell( ~ to allow the cavity length to be changed. These modulators can optionally be provided with intracavity len es which function to focus light reflected within the cavity to minimize 105s. Analog phase and wavelength .~ , W093/10477 PCT/US92/~9~07 , .

modulators can be provided by incorporation of quarter-wave plates as described aboveO

'IV. Exem~lary Applications of_the OPtical Modulators_of ~b~
i 5The optical modulators of the present invention can be electronically or optically addressed. Exemplary embodiments of optically addressed modulators include configurations having photosensors, for example, using crystalline silicon or GaAs, or a thin film of an amorphous ~ 10silicon, CdS, CdSe, GaAs, or functional equivalent ¦photodiode~ or photoconductors.
A single-pass transmission modulator is illustrated in Figure 10. Surface-stabilized CSLC material is employe~ in half-wave retarder 100, which is flanked by quarter-wave 15plates 106 and 107 to make a variable retarder. The device further includes substrate walls (101 and 102), transparent electrodes (103 and 104), and a photosensor (105).
A specific embodiment of an optically addressed etalon modulator of the pr~sent invention îs a modification of 1 20spatial light modulators as described, for example, by ~.
Moddel et al. (1989) Appl. Phys. Le ff . 55(6):537 and I.
Abdulhalim et al~(l989) Appl. Phys. Lett. 55 ~16):1603.
., .One ~pecific reflection-only-mode phase modulator configuration: is; provided in Figure 11. The exemplary 2~device: of. Fiqure~ll consists of transparent or semi-,~transparent substrate walls (117 and 118), e.g., made from glass, and contains a~pho~osen or layer (110) which can be a photodiode, for example, a hydrogenated amorphous silicon photodiode or a photoconductor. One of the substrate walls 30is provided with a transparent electrode (115), e.g., a metal oxide film. ~he~:optical or asymmetric FP cavity is .~formed by reflective surfaces (113 and 114). The other -ubstrate wall is provided with a transparent electrode 116 or the reflective surface I14 may serve as the second 3Sele~trode. For a reflection-only mode modulator, the ` reflectivity of 113 is ideally 1, and the reflectivity of `' .

' WOg3/1~477 PCT/US9~/09707 114 is < 1. The reflectivity of 113 should be sufficiently higher than that.of 114 so that a significant portion of ~;' the incident light exits the cavity through 114 and 118.
, The deYice is provided with a planar-aligned layer of a , 5 chiral smectic liquid crystal material (111), for example, ,~ a surface-stabilized~ planar-aligned SmA or SmC material, '~l the optic axis of which is rotatable by application of an electric field across the layer. The device of Figure 11 '', may also include alignment layers adjacent to the chiral ii, 10 smectic liquid crystal layer to assist alignment of the i material. A square-wave clock voltage with an optional dc .1 offset i5 ~pplied between the electrodes 116 and 115 and an . electric field is generated across the CSLC layer when an `3 optical signal, i.e., the write beam (119), interacts with 5;~ 15 the photosensor layer ~110).
'' ~ binary or ternary phase modulator results when a ,l discrete planar-aligned, surface-stabilized chiral smectic ' liquid crystal layer is employed in layer 1~1. The orientation of the optic axis of the layer is switched by ~;i 20 the optically activated, electri~ field placed across the ~ CSL~ during the negative cycle of the square-wave clock il voltage. The:de~ice optic axis is switched back during the ~l positiv~ cycle of the square-wave clock voltage. Switching '~i the orientation of the optic axis of the CSLC layer ~l 25 modulates linea~ly polarized light entering (the read beam) the modulator through substrate 118. ~odulated light exits t~e device through substrate 118. An exit polarizer or polarization analyzer is not required to detect light ! modulation.
An analog phase modulator results when an analog . planar-aligned/:surface-stabilized chiral smectic liquid `~, crystal materiaI is employed in layer 111 and when a ~` q~arter-wave plate (112~ is included in the modulator. In .,~ reflsction-only-mode, the CSLC layer is preferably a ., 35 quarter-wave plate for the wavelengths of light to be , modulated.
`: ~
'~' `
~,';1j .~

WOg3/10477 PCT/US92/09707 ,, Optical addressed intensity and wavelength modulators operated in transmission/reflection mode can be implemented .~ by configurations similar to that of Figure 11, uæing the descriptions of the preæent invention and well-known techniques of optical addressing.
In some embodi~ents, the reflective function of the , reflective surface tll3) can be performed by the interface between the photosensor layer (110) and the liquid crystal layer (1113. If the difference in refractive indices between the materials employed for 110 and 111 is large enough, that interface will function to reflect light and, ~i thus, function to form th2 etalon cavity.
An analog, planar-aligned optically addxessed intensity and wavelength modulator operated in transmisæion/reflection-mode, a modification of the device of Figure 11, r~quires two quarter-wave plates in the resonance cavity on either side of the analog CSLC half-wave layer. In a wavelength modulator, the quarter-wave plates are preferably~achromatic.
The optical modulators of the present invention can be .l fabricated as multiple pixel devicas. Several exemplary `I embodiments of multi-pixel modulators are provided (in Figures 12-14).:~
~. ~LSI tVery Large Scal~ Integration) integrated circuit i 25 backplane re~r~sents a means for electrically addressing a ~, ~
multi-p~el chixal smectic liquid crystal FP or folded : optical path device~. Such multiple~ pixel devices can be operated in transmission/reflection- or reflection-only mode. m e desired chiral smectic liquid crystal material ;:. . . . .
is positioned and appropriately a~igned between a substrate . ; overlayer and~a ~VLSI backplane which comprises pixelated ~, ref~ective surfaces. The substrate overlayer is provided - with~ a reflective surface, such that multiple pixel resonance cavi~ies are created between the opposing ,:1 `' 35 reflective surfaces of the substrate overlayer and the VLSI
backplane. One or two birefringent elements, in particular quarter-wave plates can optionally be included in such a i :

W093/10477 PCT/US92/~707 - 212~31~

multi-pixel device between the CSLC material and ~ither of the reflective sur~aces. The individual resonance cavities of a multi-pixel device produced in this manner function as the individual CSLC modulators of this in~ention as described herein-above. The type of modula~ion is dependent on the mode of operation of the ca~ity, the type of alignment employed, the type of CSLC material employed and the tilt angle and birefringence of that material.
~ A multi-pixel, binary phase, intensity or wavelength ;~ 10 modulator, as in Figure 12, results from the inclusion of a planar-aligned layer of a SmC~ material (120) having a 45 tilt angle between a VSLI backplane (125) comprising pixelated reflective surfaces (121) and a substrate overlayer (126) having a reflective surface that is less than 100% reflective (122). When the pixelated reflective surfaces tl21) are also Iess than 100% refl~ctive and the VLSI device is operated in the transmission/reflection mode, binary intensity or wavelength modulation results.
, When the back reflective surfaces (121), i.e., those l 20 of the VSLI backplane, are of significantly higher `, reflectivity than the front reflective surface (122), the device operates in reflection-only-mode, and a binary phase modulator results. The intracavity CSLC layer (120~ is a Sm~ 45 tilt angle material (planar-aligned and surface-: . ~
~ 25 stabilized). In the device of Figure 12, the reflective 3 ~ surfaces, e.g., deposited~ metal mirrors, also serve as ~' electrodes. Polarized~ coherent light incident on the modulator is phase~ modulated by passage through the SmC
layer. Polarized monochromatic incident light and polarized non-monochromatic incident light are intensity and wavelength-modulated, respectively, by the device of Figure 12. ~ ~
Another exemplary pixel of a VSLI configures ~ ~ reflection/transmission mode modulator is given `~35 schematically in~ Figure~13. This modulator can function , either to modulate the intensity of monochromatic or i ~ coherent light, or to modulate wavelengt~ of non-.
~' ",:
. ~

,~ W093/10477 PCT/US92/Og707 2 L2~31~ ~4 monochromatic light. A pixelated VSLI backplane (135) provides a substrate with one reflective surface ~131). A
substrate overlayer (136) is provided with a second reflective surface (132). Within the reso~ance cavity formed by the reflective surfaces, . there are two birefringe~t elements, specifically two quarter-wave plates (133 and 134). A layer of a planar-aligned, surface-stabilized SmA~ liquid is provided (130). The reflecti~e ~¦ surfaces 131 and 132 also serve as electrodes in this ~j 10 configuration. The thickness of the layer is chosen such that the SmA liquid crystal cell (130 between 131 and 132) is a half-wave plate for the light to be modulated. The quarter-waveplates used in the wavelength modul~tor are .l preferably achromatic.
~i 15 Yet another exemplary pixelated modulator is provided ~:~ in Figure 14. In this example, a homeotropically-aligned CSLC layer (140) is employed within a resonance cavity formed by a back reflective surface (141) and a front reflective sur~ace (142) on substrate walls ~14S and 146).
Lateral electrodes are provided (143 and 144) with optional ¦ insulators between the electrodes and reflective ~urfaces.
.~l These electrodes can be patterned, for example, to create ~ a multi-pixel ~array. The figure illustrates a . transmission/refl~ection-mode device in which the ;l 25 reflectivities~ of; 141 and 142 are both less than 1.
Intensit~ and:~wavelength modulation of incident light can ; be performed by~:this :device configuration. Any CSLC
:~ material can ~e~ employed in the homeotropically-aligned . cell of Fi~ure 14. Discrete state CSLC materials give discrete modulation. Analog CSLC materials give analog modulation. : ~
Examples of optically addressable, multi pixel, binary and analog modulators are analogous to the VLSI
; configurations de~cribed above, except that the V~SI
35` integrated circui~ :is replaced with means for optically addressing the multi-pixel array, including a photosensor.
One type of multi-pixel FLC spatial light modulator is ~`
r~"

W093/tO477 2 1 2 3 3 1 0 PCT/US92/~707 described in G. Moddel et al. US Patent 4,941,735. Optical addressing of CSLC cells has been described in Takahashi et al. (1987) Appl. Phys. Letts 51:19; Moddel et al. (1987~
"The Proceedings of SPIE - The International Society for Optical Engineering 759:207-213 and Ashley et al. (1987) Applied Optics 26:241-246. Teachings of these references regarding optical addressing can be readily applied by those of ordinary skill in the art to the devices of the present invention~
The multi-pixel modulation herein can be electronically addressed or optically addressed as in known I in the art. The individual pixels of these multi-pixel ;, devices can be individually addressable, simultaneously add~essable or certain combinations of pixels may be lS simultaneously addressable in desired patterns. Multi-pixel chiral smectic liquid crystal FP optical cavities modulators can be employed for a variety of display applications, including diffractive optical elements and holographic displays and~can in addition be employed by ~20 appropriate choice of addressing schemes to create ,1 ~ patterned modulators,~for; example to create a pattern of ~ lines to generate a~diffraction grating for beamsteering 3 applications. ;~ The operation of ~such beamsteering diffractive grating~is understood in the art.
Another ~ exemplary application of a transmission/re1ection mode CS1C etalon modulator of the , ` present invention~is the so-called "tunable tap" of Figure 15. This ~device provides a ~eans for "tapping" or ~, selecting a desired wavelength of incident non-' 30 monochromatic light while minimizing overall loss of total ~l light intensity.~ ~The device of Figure 15 comprises two polarizing beam splitters (153 and 154), a Faraday retarder ' (155) and two reflective surfaces (157 and 158) forming a `~ resonance cavity on either side of a planar-aligned CSLC
analog wavelength modulator (150-152) and two total internal reflectance mirrors (159 and 160) with elements positioned relative to each other as indicated in Figure . .

~ WOg3/10477 PCT/USg~/09707 .....

15. The figure also indicates an input (A) and two outputs (B and C) for light. The planar-aligned CS~C analog - wavelength modulator is exemplified as a planar-aligned CSLC half-wave plate ~150) with two quarter-wave plates (151 and 152). '~he CSLC material employed is any planar-aligned analog material displaying a voltage-dependent ;~ rotation of its optic axis. Preferably, the CSLC material is a planar~aligned, surface-stahilized SmA~ ~aterial. ~he ~uarter-wave plates of the analog modulator are oriented at 0 or 90 with respect to light entering the resonance cavity.
: Unpolarized light enters the device at input A and PBSl tl53) splits the incident light of the beams i~to two orthogonally polarized beams. A Faraday retarder (155) . rotates the polarization of both beams by 45O. The operation of ~araday retarders is well known and understood in the art. Light enters the resonance cavity containing the modulator. The output PBS2 (154) is oriented at 45 to the input PBSl (153). The rotated light exiting the ~0 resonance cavity is then split by PBS2 so that a selected wavelength is transmitted to output C while the remaining light is reflected~from I58 back through the resonance cavity. (The~ resonance cavity is formed between the . reflective surfaces (157) and (158) with reflectivities R1 ;! 25 and R~ An electric ~ield is applied to the modulator ~across -'the: CSLC layer of the modulator) via cell ~i electrodes (not shown) to select a wavelength for transmission to output port C. The wavelength that satisfies the condition " 2 Jc~nL ( ~ 6 j ~; j h !'l where ~ is the selected ~, ~n is he birefringence of the cavity, L is the cavity length and m is an integer, is ~ transmitted to the output C. All oth~r wavelengths of "5~ ~ ~ incident light are reflected back through the cavity and are transmitted through output port B.
~: :

~` .

.

WO93/1~77 2 12 3 31 0 PCT/US92/09707 While certain illustrative device configurations and : application of the light modulators of the present invention have been described in detail in the specification, it should be understood that there is no , 5 intention to limit the invention to the specific forms and '. embodiments disclosed. On the contrary, the invention is : intended to cover all modifications, alternatives, equivalents and uses fully within the spirit and scope of the invention as expressed in the appended claims.

, I ~ .

'l :
~ ' ;
J -.
l , ~ ~ :

~ :

j '~ :

,~1 ~ !
., , TABLE l: EXEMPL~RY oPTICAL CAVITY CSLC MODULATORS
Type of Modulation2 SType of CS~C Mode of1 Material Operation Phase IntensitY Wavelenqth Planar Ali~nment SmC T/R +(B) ~3 tilt ~ = 45 R +(B)4 SmC T/R +(B) ~(B) tilt ~ = 45 R +tB) SmA T/R ~(A) ~5 lS R +(B)4 DHF TjR +6 ~6 , R +(A) i :
Antiferro- T/R : +(T) ~7 electric : :
: 20 R +(B)4 ~' SmA ~T/R : +(~) ~(A) quarter wave(s) '~ R : : ~+(A) ;~ SmC T/:R ~ ~(B) +~B) ~ quarter-. ~ wave(s) - R ~ +(B) DHF T/R ~(A) +(A) quarter- : :
1 ~ wave(s) :R +(A) !~:
30SmA /SmA8 T/R +(A) ~8 ~: oppOsit~4 : R +(B)8 ( .

' WO93/10477 1 2 3 ~ 1 0 P~T/US92/~9707 i 2 ~g TABLE l (continued) Type of Modulation2 Type of CSLC Mode o~' ~; 5 _MaterialOperation Phase Intensitv Wave1enq~h DHF/SmA10 T/R +(A) +(A) .. Same ; R ~(A) . .
' DHF/SmA12 T/R +(A) ~12 5,, l0 Opposite i R ~(B)1 ., DHF/DHF13 T/R +(A) ~13 ' opposite : ~ R +(B)3 . ~

DHF/DHF14 ~/R ~(A) ~(A) 5~ Same ~, R ~(A) ~'1 Sl~ Homeotro~ic ~liqnment , . SmC ~/R +(B) ~B~
`' 20 tilt ~ $ 45 or = 45 ~ +(B) , SmA /homeo T/R +(A~ +(A) ~,~
R +(A) .'- , ~ D~IF T/~ ~ (A) + (A) ,; ~
~ 25 R +~A) ,j, , ; . , , Antî~erro- T/R +(T) +(T) ~. electric c R ~(T) ~'^'j ~!

' ,, , WO93/10477 - PCT/US92/0~707 ~123310 50 TABLE 1 (continued~

Footnotes to Table 1.

"TIR" represents transmission/reflection~mode and "R"
represents reflection-only~mode.
2 Many of the modulators of Table 1 effect a phase change in tran~mis~ion/reflection-mode which is detectible as inkensity or wavelength modulation. A
"~i' indicates detectible modulation, "B" represents binary, "A" represents analog and 'tT" represents ternary. Incident coherent light is phase modulated, incident monochromatic and/or coherent light is intensity modulated and incident non-monochromatic : light is wavelength modulated.
I 3 This modulator selects between an output transmitting 1 15 one wavelength and an output transmitting two wavelengths.
~', 4 This modulator selects between two phases of input light, but output of a selected phase will not l necessarily have same direction of polarization.
I 20 5 For non-monochroma ic incident light, this device ., provides analog intensity modulation of two selected wa~elengths.
" 6 In planar-aligned ~HF devices there is a change in , birefringence of the CSLC layer and a rotation of . 25 polarization of incident light on application of an electric field to the device.
7 This modulator selects between one output transmitting one wavelength and two other outputs transmitting two wavelengths at different intensities. Similar to SmC~
~l 30 planar-aligned tilt ~ ~ ~5O.
`1~ 8 optic axes of SmA cells within a series in resonance cavity are rotated in opposite directions to leverage .l rotation of polarization of incident light. Intensity modulation is analog. Wavelength modulation is similar to (footno~e 5, above) except that when both ~`

W~93/10477 ~ PCT/US92/09707 . 2l233la TABLE 1 (continued) SmA~ cells have ~ = 22.5, wavelength ~odulation is binary. Phase modulation is similar to that in footnote 4, above, except that when ~x f both cells is 22.5 phase modulated light exits with the s~me direction of polarization.
9 "Opposite" indicat~s that the optic axes of cells rotate in opposite directions.
10 DHF a~d SmA cells in series with optic axes rotating in the same direction and to the same extent. This cell combination effects a change in birefringence.
"Same" indicates that the optic axes of cells rotate . in same direction.
', ~2 DHF and SmA cells in series with optic axes rotating ~ 15 in opposite directions to the same extent exhibit no ¦ cha~ge in birefringence. This cell combination behaves like two planar-aligned SmA~ cells in series.
13 Two DHF ~cells:in series whose optic axe~ rotate in opposite directicns and to the same extent. The device:exhibits no birefringence changa. Similar to combinations described in footnote6 8 and 12.
14 Two DHF~cells in series whose optic axes rotate in the i! same direction and to th~ same extent. The device .1 ; ;exhibits no rotation of polarization of incident light ;' 25 but-:exhibits ~an increase in the change in birefringence. ~The modulator functio~ to that of footnote 10.

'i !

~1:

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! ;

~! .

Claims (32)

1. A multiple element etalon modulator for light comprising:

a multiple element intracavity modulator, which comprises:

a smectic liquid crystal cell, the optic axis of which is rotatable by application of an electric field parallel to the smectic layers of said cell;

means for applying an electric field to said smectic liquid crystal cell whereby said optic axis of said liquid crystal cell is rotated; and a birefringent element; and a front and a back reflective surface positioned on either side of said multiple element intracavity modulator to form an etalon.

2. The multiple element etalon modulator of claim 1 wherein said liquid crystal cell is selected from the group consisting of a planar aligned SmA? cell, a planar aligned SmC? cell, a planar aligned DHF cell, a homeotropically aligned SmA? cell, a homeotropically aligned SmC? cell and a homeotropically aligned DHF cell.

3. The multiple element etalon modulator of claim 1 wherein said birefringent element is a passive birefringent element.

4. The multiple element etalon modulator of claim 1 wherein said birefringent element comprises a second smectic liquid crystal cell, the optic axis of which is rotatable by application of an electric field parallel to the smectic layers of said second cell, whereby said multiple element modulator comprises two liquid crystal cells.

5. The multiple element etalon modulator of claim 4 wherein said two liquid crystal cells are selected from the group consisting of:

two planar-aligned, surface-stabilized SmA* liquid crystal cells in series within the optical cavity, the optic axes of which cells are configured to rotate in opposite directions;

two planar-aligned DHF liquid crystal cells in series within the optical cavity, the optic axes of which cells are configured to rotate in the same direction;

two planar-aligned DHF liquid crystal cells in series within the optical cavity, the optic axes of which cells are configured to rotate in opposite directions; and a planar-aligned DHF liquid crystal cell and a planar-aligned, surface-stabilized SmA* liquid crystal cell in series within the optical cavity, the optic axes of which cells are configured to rotate in the same direction.

6. A liquid crystal etalon modulator for light comprising:

an intracavity modulator element, which comprises a smectic liquid crystal cell, the optic axis of which is rotatable by application of an electric field parallel to the smectic layers of said cell, means for applying an electric field to said smectic liquid crystal cell whereby said optic axis of said liquid crystal cell is rotated; and a front and a back reflective surface positioned on either side of said intracavity modulator element to form an etalon;

wherein said liquid crystal cell is selected from the group consisting of:

an analog smectic liquid crystal cell the optic axis of which is rotatable in an analog or discrete manner by application of an electric field parallel to the smectic layers of said cell;

discrete smectic liquid crystal cell, the optic axis of which is rotatable discretely among multiple states wherein two of said multiple states are separated by approximately 45° or approximately 90°;
and a homeotropically aligned smectic liquid crystal cell.

7. The liquid crystal etalon modulator of claim 6 wherein said liquid crystal cell is an analog smectic liquid crystal cell, the optic axis of which is rotatable in an analog or discrete manner by application of an electric field parallel to the smectic layers of said cell.

8. The liquid crystal etalon modulator of claim 7 wherein said liquid crystal cell is selected Prom the group consisting of a surface-stabilized SmA? smectic liquid crystal cell and a distorted helix ferroelectric liquid crystal cell.

9. The liquid crystal etalon modulator of claim 6 wherein said liquid crystal cell is a discrete smectic liquid crystal cell, the optic axis of which is rotatable discretely among multiple states by application of an electric field parallel to the smectic layers of said cell, wherein two of said multiple states are separated by approximately 45°.

10. The liquid crystal etalon modulator of claim 6 wherein said liquid crystal cell is a discrete smectic liquid crystal cell, the optic axis of which is rotatable discretely among multiple states by application of an electric field parallel to the smectic layers of said cell, wherein two of said multiple states are separated by approximately 90°.

11. The liquid crystal etalon modulator of claim 6 wherein said liquid crystal cell is a homeotropically aligned smectic liquid crystal cell.

12. The liquid crystal etalon modulator of claim 11 wherein said means for applying an electric field comprises lateral electrodes for applying an electric field parallel to the smectic layers of said smectic liquid crystal cell.

13. The liquid crystal etalon modulator of claim 12 wherein said homeotropically aligned smectic liquid crystal cell is selected from the group consisting of a homeotropically aligned SmA? cell, a homeotropically aligned SmC? cell, and a homeotropically aligned distorted helix ferroelectric cell.

14. The liquid crystal etalon modulator of claim 6 wherein said intracavity modulator element further comprises first and second quarter-wave plates positioned in series with and on either side of said liquid crystal cell, the optic axes of said quarter-wave plates oriented parallel or perpendicular to each other.

15. The liquid crystal etalon modulator of claim 14 wherein said liquid crystal cell is a half-wave plate for the incident light.

16. The liquid crystal etalon modulator of claim 15 wherein said liquid crystal cell is a SmA? liquid crystal cell.

17. The liquid crystal etalon modulator of claim 6 wherein light enters said etalon through said front reflective surface and the reflectivities of said front and back reflective surfaces are such that light exits said etalon through said front reflective surface.

18. The liquid crystal etalon modulator of claim 6 further including photosensor means electrically connected to said electric field applying means whereby said modulator is optically addressable.

19. The liquid crystal etalon modulator of claim 6 which is a pixel in a multiple pixel device.

20. The liquid crystal etalon modulator of claim 6 further including an isotropic spacer.

21. A reflection-mode liquid crystal etalon modulator for light comprising:

an intracavity modulator element, which comprises a smectic liquid crystal cell, the optic axis of which is rotatable by application of an electric field parallel to the smectic layers of said cell;

means for applying an electric field to said smectic liquid crystal cell whereby said optic axis of said liquid crystal cell is rotated; and a front and a back reflective surface positioned on either side of said intracavity modulator element to form an etalon, wherein light enters said etalon through said front reflective surface and the reflectivities of said front and back reflective surfaces are such that light exits said etalon through said front reflective surface.

22. The reflection-mode liquid crystal etalon modulator of claim 21 wherein said intracavity modulator element further includes a birefringent element.

23. The reflection-mode liquid crystal etalon modulator of claim 22 wherein said birefringent element is a passive quarter-wave plate positioned between said front reflective surface and said liquid crystal cell.

24. The reflection-mode liquid crystal etalon modulator of claim 23 wherein said liquid crystal cell is a quarter-wave plate for the incident light.

25. The reflection-mode liquid crystal etalon modulator of claim 24 wherein said liquid crystal cell is a SmA? liquid crystal cell.

26. A variable retarder for light comprising:

a half-wave plate for the incident light, which comprises an analog smectic liquid crystal cell, the optic axis of which is rotatable in an analog manner. by application of an electric field parallel to the smectic layers of said cell;

means for applying an electric field to said smectic liquid crystal cell whereby said optic axis of said liquid crystal cell is rotated; and first and second quarter-wave plates positioned in series with and on either side of said half-wave plate, the optic axes of said quarter-wave plates oriented parallel or perpendicular to each other, wherein the polarization of said incident light may be oriented at any angle with respect to the optic axes of said first quarter-wave plate except for linearly polarized light oriented at ?45°.

27. The variable retarder of claim 26 wherein said liquid crystal cell is a SmA? liquid crystal cell.

28. A reflection-mode variable retarder for light comprising:

a quarter-wave plate for the incident light, which comprises a smectic liquid crystal cell, the optic axis of which is rotatable by application of an electric field parallel to the smectic layers of said cell;

means for applying an electric field to said smectic liquid crystal cell whereby said optic axis of said liquid crystal cell is rotated;

a passive quarter-wave plate positioned in series with and on a first side of said liquid crystal cell; and reflective means positioned in series with and on a second side of said liquid crystal cell.

29. The reflection-mode variable retarder of claim 28 wherein said liquid crystal cell is a SmA* cell.

30. A modulator which comprises a homeotropically aligned smectic liquid crystal cell, the optic axis of which is rotatable by application of an electric field parallel to the smectic layers of said cell, and lateral electrodes for applying an electric field to said liquid crystal cell parallel to the smectic liquid crystal layers of said cell.

31. The modulator of claim 30 wherein said liquid crystal cell is a distorted helix ferroelectric cell.

32. A method for rapid, electro-optical phase modulation comprising the steps of:

introducing coherent light into a homeotropically aligned smectic liquid crystal cell, the optic axis of which is rotatable by application of an electric field parallel to the smectic layers of said cell; and applying an electric field parallel to the homeotropically aligned layers of said liquid crystal cell whereby the phase of light exiting said liquid crystal cell is modulated.