CA1314741C

CA1314741C - Dynamic coupler using two-mode optical waveguides

Info

Publication number: CA1314741C
Application number: CA000559311A
Authority: CA
Inventors: Byoung Y. Kim; Herbert J. Shaw
Original assignee: Leland Stanford Junior University
Current assignee: Leland Stanford Junior University
Priority date: 1987-02-20
Filing date: 1988-02-19
Publication date: 1993-03-23
Anticipated expiration: 2010-03-23
Also published as: US4895421A; US4741586A

Abstract

DYNAMIC COUPLER USING
TWO-MODE OPTICAL WAVEGUIDES
Abstract of the Disclosure An optical mode coupling apparatus includes an optical waveguide that couples an optical signal from one propagation mode of the waveguide to a second propagation mode of the waveguide. The optical signal propagating in the waveguide has a beat length, and the coupling apparatus includes a source of perturbational light signal that propagates in the waveguide in two spatial propagation modes having different propagation constants so as to have a perturbational signal beat length. The perturbational signal has an intensity distribution in the waveguide that causes periodic perturbations in the refractive indices of the waveguide in accordance with the perturbational signal beat length. The periodic perturbations of the refractive indices of the optical waveguide cause cumulative coupling of the optical signal from one propagation mode to another propagation mode.
The perturbational light signal can be selectively enabled and disabled to selectively enable and disable coupling of the optical signal between the propagation modes.

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Description

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DYNAMIC COUPL~R USING
TWO-~ODE O~TICAL WAVEGUIDES
Field of the Invention The present invention relates generally to optical waveguide devices and, more specific lly, to devices which incorp~rate two-mode optical waveguides to control the propagation of optical energy in the two-mode of waveguide.
Background of the Invention An optical fiber is an optical waveguide having central core surrounded by an ou~er cladding. The refractive indices of the core and cladding are ~elected ~o that optical energy propagating in the optical fiber is ~ell-guided by the fiber.
As is well known in ehe art, a single op~ical fiber may provide one or more propagation paths under certain conditions. These propsga~ion paths are eommonly referred to as the normal modes of a fiber, which may be conceptualized as independenc optical paths through the ~O fiberO Normal modes have unique electric field distribution pattern~ which remain unchanged, except for amplitude as ~he light propagates through the fiber.
Additionally, each normal mode will propagate through the fiber at a unique propagation velocity.
The number of modes which may be ~upported by a particular optical fiber is deeermined by the wavelength of the light propagating therethrough. If the wavelength is greater than a "second order mode cutoff" wavelength (i.e., the freguency of the li~,ht is les~ than a cutoff frequency), the fiber ~ill support only a single mode~ If the wavelengtn is less than cutoff (i.e~, ~he ~Erequency is gre~cer than the cutoff frequency), ~he f1ber will begin ~o support higher order ~odes. For wavelengths less than, but near cutoff, the fiber will ~upport only the fundamental, or ~ir~t order mode, ~nd the nex~, or second order mode. As the wavelength i8 decreased, the fiber 13~7~

will support additional modes, for example, third order, fourth order, etc.
Each of the normal modes (e.g., first order, second order, etc.) are orthogonal, that is, ordinarily, there is no coupling between the light in S these modes. The orientation of the electric field vectors of the modes defines the polari~ation of the light in the mode, for example, linear vertical or linear horizontal. A more complete discussion of these modes, and their corresponding electric field patterns, will be prov;ded below.
A number of devices have been constructed to utilize the orthogonality of the modes of an optical fiber to provide selective coupling between the modes. For example, copending U.S. Patent Application Serial No. 884,871, entitled "Fiber Optic Modal Coupler," assigned to the assignee of this invention, describes a device which couples optical energy from the first order mode to the second order rnode, and vice versa. U.S.
Patent Application Serial Nos. 820,513 and 909,503, both entitled "Fiber Optic Inter-Mode Coupling Single-Sidebarld Frequency Shifter," and both assigned to the assignee of this invention, disclose frequen~y shifters which couple optical energy from one propagation mode to another propagation mode while shifting the frequen~y of the optical energy. U.S. Patent Application Serial No. 820,411, entitled "Fiber Optic Mode Selector,"
assigned to the assignee of the present invention, discloses a device which separates optical energy propagating in one of the first order and second order propagation modes from the other of the first order and second order propagation modes.
Summary of the Invention The present invention is an optical mode coupling apparatus which comprises an optical waveguide that couples an optical signal propagating in the optical 7 ~- ~

waveguid~ between propagation m~des of the waveguide. The optical signal has an op~ical signal beat length for the ~odes, and the coupling appara~us also includes a light source for introducing a perturbational light signal into the waveguide. The perturbational si~nal has an o~tical wavelength ~elected ~uch that the perturbational signal propagates in the waveguide in two spatial modes which have different propagation constants so as to cause the perturbational signal to beat in the waYeguide in ~ccordance wi~h a perturbational signal beae length, and thereby cause the perturbational ~ignal to have an intensity dis~ribution in ~he waveguide which varies along the length of the waveguide. The perturbational signal has an intensity which is selected ~o op~ically per~urb t5 the refractive index of the waveguide, preferably in accordance with the optical Kerr effect, at intervals defined by the perturbational ~ignal beae length. The optical wavelen~th of the per~urbational signal i~ further selected such that the interval~ have a ~pacing related to the bea~ length of the optical ~ignal to cause cumulative coupling of the opt~cal signal from one of the propagation modes to another~
In the preferred embodiment, the op~ical wave~uide has a non-circular cross ~ection having cross-~ectional dimensions selected such that the waveguide guides a portion of the perturbational light signal in a fundamental 6patial mode and another portion in a higher order ~patial mode. The cross-6ectional dimensions of the core are further selected such that the portion of eAe perturb~tional ~ignal guided by the w~veguide in the higher order mode propsgate~ in onlg ~ single, ~table ~ntensity pattern. The preferred embodiment utilizes the fundamental spatial mode of the waveguide snd ~ higher order patial mode, preferably the second order spa~ial mode. The cros~-~ect~onal dimensions of the core may be further selected to cause the polarization modes of ~he ~ ~ 3 ~

ewO spatial modes to be nondegenerate ~uch that they propagate light a~ different velocities.
Although the invention may be utilized in connection wieh various types of waveguides, the waveguide of the pre~erred embodiment comprises an optical fiber which has an 211iptical cross-sec~ion core, such that the fundamental mode is the LPo1 mode of the op~ical fiber and the higher order mode is the LP11 mode of the optical fiber~ The ~ingle intensity pattern i8 the even mode in~ensity pattern of the LP11 moden Advantageously, the present invention may be implemented as a digi~al switch. In this implementation, t~e perturbational ~ignal is ~electively switched on and off to switch the coupling on and off.
The invention also includes a method of coupling an optical signal between propagation modes of a waveguide having a beat length for the modes. The method comprises the step o~ introducing a perturbational optical signal into the waveguide such that the perturbational signal propagates in ewo spatial modes of the waveguide to cause the perturbational 6ignal eo ~eat in accordance with a perturbational signal beat length. The intensity of the perturba~ional ~ignal is ~elected to cau~e optical perturbation of the waveguide at intervals defined by the perturbational signal beat length. The wave length of ~he perturbational signal is selected such that the intervals have a spacing related to the beat leng~h of the optical ~ignal to cause the coupling to be cumula~ive at ~he ~ntervals. Preferably, the waveguide comprises an optical fiber and the per~urbations are induced in accordance with ~he optical Kerr effect. In ~ preferred embodiment, the method ~180 includes the step of switching the perturbational signal between ~ relatively high intensity level and ~ relatively low inten~ity level. Additionally, the waveguide prefer~bly has a core of non-circular cross section, and the method additionally comp~ises the step of ~i _ 5 _ ~ 3~7~

selecting the wavelength of the optical signal in relation to the cross-sectional dimensions of the core such that (1) the waveguide guides a portion of the optical signal in one spatial mode and another portion in a higher order spatial mode, such as the second order mode, and (2) the portion of the optical signal guided by the waveguide in the higher order mode propagates in only a single, stable, intensity pattern. The method also preferably comprises the step of selecting the wave-length of the perturbational signal in relation to the cross-sectional dimensions of the core of the waveguide such that (1) the waveguide guides a portion of the perturbational signal in one spatial mode and another portion in a higher order spatial mode, and (2) the lS portion o the perturbational signal guided by the waveguide in the higher order mode propagates in only a single, stable intensity pattern.
In accordance with another broad aspect, the invention relates to an optical mode coupling apparatus comprising an optical waveguide which couples an optical signal propagating in the optical waveguide between propagation modes of the waveguide, the optical signal having an optical signal beat length for the modes, the waveguide (a) comprising a guiding structure formed of materials having dissimilar indices of refraction and (b) having perturbations optically induced by a perturbational light signal, the perturbations being spaced at intervals related to the beat length of the optical signal to cause cumulative coupling of said optical signal from one of the propagation modes to another.
Brief Des_ription Qf the Drawings Figure 1 is a cross-sectional view of an exemplary circular core optical fiber.
Figures 2a and 2b ill~strate th0 electric field intensit~ distribution patterns for the vertically - 5a - 13~7~1 polarized and horizontally polarized HE11 (fundamental) propagation modes of the circular core optical fiber of Figure 1.
Figure 2c is a graph of the electric field amplitude distribution corresponding to the intensity distribution patterns of Figures 2a and 2b.
Figures 2d, 2e, 2f, and 2g illustrate the electric field intensity distribution patterns for the TEo1, TMo1, even HE21 and odd HE21 (second order) propagation modes, respectively, of the circular core optical fiber of Figure 1.
Figure 2h is a graph of the electric field amplitude distribution patterns for the 8econd order modes of the optical fiber of Figure 1.

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Figures 3a and 3b illustrate the LPo~ approximations for the first order propagation modes of the optical fiber of Fi~ure 1.
Figures 3c, 3d, 3el and 3f illustrate the LYll S approximations for the second order propagation modes of the optical fiber of Figure 1.
Figure 4 is an unscaled graph of the propagation cons~ant of an optical waveguide versus the ellipticity of the core of the optical wave~uide.
Figure S is a cross-sectional view of an exe~plary elliptical core.
Figures 6a and 6b illustrate the electric field intensity paeterns for the LPo1 (fundamen~al) propagation modes of the elliptical core optical fiber of Figure 5.
9 ~ Fi8ure 6c is a graph of the elcctric f ield amplitude distribution for the LPol propa~ation ~ode of the elliptical core optical fiber of Figure 5.
Figures 6d and 6e $11us~rate the electric field inten~iey patterns for the even LP1 1 propagation modes of the ellipticsl core optical fiber of Figuse 5.
Figure 6f is a graph of the electric f~eld amplitude distribution for the even LP1 1 propagation modes of the elliptical core s)ptical fiber of Figure 5.
Figures 6g and 6h lllustrat~ the electric f ield intenRity pat'cerns for the odd LPl 1 propagation modes of ehe elliptical core optical fiber of Figure 5.
Figure 7 illustrates a dynamic s)ptical coupler constructed in accordance ~ith the present invention in which light f rom a high power la~er light ~ource is propagating in an optical fibes in the same direction as light from aD optical ~ignal source.
Figure 8 illu~trates a por~ion of the optical f iber from the dynamic optical coupler of Figure 7.
Flgures 9a-9i illustrate cro~s 8ections of the electrical field intensity patterns taken at locationg 9a-9a, 9b-9b, etc. in Figure 8.

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Figure 10 is an alternative embodiment o the presene invention in which the light from a high power laser light source is propagating in ~n op~ical fiber in the opposite direction as light from an optical ~ignal source.
Figure 1la illus~rates the LP1~ intensity pattern of the light emitted by the embodiment of Figure 10 when ~he perturbational light source is on.
Figure 1lb illustrates the LPol in~ensity pattern of the light emitted by ~he embodiment ~f Figure lO when the perturbational ligh~ F'ource is off.
etailed Descr ~tion of the Preferred Embo~iment~
The present invention u~ilizes an op~ical waveguide tha~ operates at a wavelength belo~ cutoff such that the waveguite ~uppor~s both fundamental and second order guided modes. The fundamental and 6econd order guided modes provide two orthogonal paths through the optical waveguide ~hich permits the device to be used as a two-channel optical propagation medium. The embodimen~s of the present invention utilize sn opeical waveguide having the ge~metry of the core 6elected so thst only one stable 6patial orientation of the second order ~ode i~ supported in the waveguide.
Before discu6sing the specific embodiments of the present invent~on, a detailed description of the optical waveguide and a brief ~ummary of the applicsble mode theory will be presen~ed to provide a 30re complete understanding of the invention. Although de~cribed below in connection with a silica glas~ optical fiber waveguide, one ~killed in t~e art will understand that the concepts presented are also applicable to other optical waveguides, such a8 a LiNbO3 optical fiber, integra~ed optics, or the like.
Mode T~eory An exemplary cross-~ection of a ~ilic~ glass optical fiber 1~U i8 illustrated in Figure 1. The fiber 10 comprises an inner core 102 and an outer cladding 1 U4 * ~3~7~

The inner core 102 has a radius of r. In the exemplary fibe 10~, th~ core has a refractive index n~0 and the cladding has a refractive index nCl~ As is well known in the art, the core refractive index ncO is g eater than the cladding index nCl fiO that an optical ~ignal propagatin~
in the optical fibe_ 100 is well-guided. The nu~ber of modes guided by the optical fiber 100 depends upon ~he ~iber geome~ry and upon the wavelength of the optical signal propagaeing therethrough~ Typically, the wavelength above which an optical fiber will propagate only the fundamen~al or firs~ order mode i9 referred to as the "second order mode cutoff" wavelength ~, which may be calculated for ~ circular core fiber u~ilizing the ~ollowing equation:

2 ~ 2 2 c Z.4~
If the wavelength of ~he optical signal i5 greater than ~Q the wavelength ~c (i.e., the frequency of the optical ~ignal i~ less than a cutoff frequency), only the first order or fundamental propagation mode of the opeical 8ign81 will be well-guided by the fiber and will be propagated by the fiber. If ~he wavelength of an optical signal is les than ~s (i.e., the frequency of the optical signal i8 greater than the cutoff frequency), higher order modes, such as ~he second order modes, will begin to propagate.
The true fir~ and second ~rder aodes of a circular core optical fiber and their refipective ~lectric field amplitude distributions are illustrated in Figures 2a-2h. The ~wo first order modes are the vertically polari~ed HE11 mode represented by an electric field pattern 110 in Figure 2g, and the hori20ntally polarized H~11 mode, represented by an electric field pattern 112 in Figure 2b. The outer c~rcle in each figure :13~7~

represents the boundary of the core 1~2 of the fiber 10 of Fi~ure 1.
As illustrated in Figure 2c, the L~ol modes have an electric field amplitude distribution 116 that is ~ubstantially symme~rical around the center line of the core 102~ The electric field amplitude distribution 116 is concentrated in the center of the core 1~2 and decreases as the distance from the center of the core 102 increases. A ~mall portion o~ the electric field amplitude distribution 116 often extend~ beyond the boun~aries of the core. This extended electric field is commonly referred to a~ the evanescent field of the guided modes.
The four ~rue ~econd order modes are illustrated in Figures 2d-2g. These four true modes are distinguished by the orienta~ion of the transver~e elec~ric field, denoted by the direc~ions of the arrows in Figures 2d~2g, ~nd are commonly referred to as the TEol mode, represented by an electric ~ield pat~ern 120 in Figure 2d; the TM01 mode, represented by an electric field pa~ern 122 in Figure 2e;
the HE21 even mode, repre~ented by an electric ~ield pattern 124 in Figure 2f; and the ~E21 odd mode, represented by an electric field pattern t26 in Figure 2g.
An electric field amplitude distribution 130 for an exemplary optical signal propagating in the second order modes i8 illustrated in Figure 2h. As illustrated, ehe electric field amplitude distribueion 130 i~ substantially equal to zero at the central line of the core, and has two maximum amplitudes 132 and 134 near the boundary of the core. As further illustrated, the two amplitude maxima 132 and 134 are 180 out of pha~e. Further~ a greater porti~n of the electric field distribution extends beyond the boundary of the core in the second order modes, thus providing a larger evanescent field than ~or the ~E
modes.

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1 o Each of ~he four true ~econd order modes has a filightly different propagation velocity from the other of the four ~econd order mcdes. Thus, when one or more o~
ehe ~rue second order modes are eo-propagating in a two-mode fiber, ~he intensity dis~ribution of ~he second order mode varies as a function of the length of the fiber as a result of changes in the phase differences between the four modes as they propagaee. The cross-sectional intensiey distribution of ~he ~econd order mode changes in response to environmental changes that induce differential phase shifts between the almost degenerate four modes.
In order to more easily analyze the charaoteris~ics of optical ~ignals propaga~ing in the second order propa~a~ion modes, the characteris~ics of the modes are analyzed using the LP approximations for the modes defined and described in de~ail in Do Gloge, "Weakly Guiding Fibers," Applied O~tics, Vol. 1~, No. 10, October 1971, pp. 2252-2258.
A bettcr understanding of the mode theory of optical propagation in an optical ~iber or other eircular core waveguide can be obtained by refer~ing to Figures 3a-3f, and wherein the first and second modes are represented in accordance with the LP approximations de~cribed by Gloge in his paper. The outer circles in each of the 2S illustrations again represent the cross-3ection of ehe core 102 of the optical fiber 100 of Figure 1. The outlines within the core circles represent the elec~ric field distribution~. Arrows with the inner outlines represent the direc~ion of polariza~ion.
Figures 3a-3b show the field paetern~ of the two polarization modes in the fundamental LP01 set of ~odes.
A $ield pattern 140 in Figure 3a represents vertically polarized light in the LPo1 fundamental mode, and a field pattern 142 in Figure 3b repre~ents ho izontally polarized light in the fundamental L~ol mode.

1 3 .lL 4 7 ~ ~

~igures 3c-3f illustr~te the LP11 approximations for the ~econd order modes. As illustrated in Figures 3c-3f, there are four LP11 modes, each havin~ ~wo lobes for the electric field diseribution~ Two of the modes~
S repres nted by an LP~1 mode pa~tern 150 in Figure 3c and an LPl1 mode pattern 152 in Figure 3d, are refer.ed to ~erein as the LP11 even modes. The other two LP11 modes, represen~ed by an LPll mode pattern 154 in Figure 3e and an LP11 mode pat~ern 156 in Figure 3f, are referred to as the LP11 odd modes. The four LP11 modes are distinguished by the orient~tion of the lobe patterns and the orientation of the electr~c field vec~or~ (i.e., the polarization vectors) within the l~be pa~terns. For example, the first LPll even mode field pattern 150 (~igure 3c) ha~ two lobes that ~re symme~rically located about a horizontal zero electric field line 160. Within ~he two lobes, the electric field vectors sre parallel ~o and anti~ymmetrie about ~he zero electric field line 160. For ¢onvenience, the LP11 mode represented by the lobe pattern 150 will be referred to as the horizon~ally polsrized LP11 even mode.
The second LP11 even lobe pattern 152 (Figure 3d) is symmetric~lly located about a horizontal zero electric field line 142. Within the two lobes of the field pattern ~5 152, the electric field vectors are perpendicular to and ~ntisymmetric about the zero electric field line 162. The LPl1 mode represented by the electric field pattern 152 will be referred to as the vertically polarized LP~1 even mode.
3Q The first LP~l odd ~ode field pattern 154 ~s ~wo lobes that ~re sy~metr~callg loca~e~ about a vertic~lly oriented zero electri~ f ield line 164. Within the two lobe~, the electric field vector i~ perpendicular to ~nd anti~ymmetric about the zero electric field line 164, and are thus oriented horizon~ally. The LP11 m~de represented ~3~7~

by the ~ield pattern 154 will thus be referred to as the horizontally polariz~d LP11 odd m~de, The electric field pattern 156 of the second LP11 odd m~de has two lobes ~hat are symmetrically located about a vertically oriented zero electric field line 166~ Wi~hin the tw~ lobes, the electric field vectors are parallel to and antisymmetric about rhe zero electric field line 166. Thus, the LP11 mode represented by the electric field pattern 156 will be referred ~o as the vertically polarized LP11 odd mode.
In the LP~mode approximations, each of the ~ix electric field patterns in Figures 3a-3f, namely, the two LPo1 patterns and the four LP11 patterns, are orthogonal to each other. In other words, in the absence of perturbations to the optical waveguide, there is substantially no coupling of optical energy from one of the field pat~erns to arly of the other field patterns.
Th~s, the ~ix electric field patterns may be viewed as lndependent ~ptical paths through the optical waveguide, which ordinarily ds not couple with each other.
If the indices of the core 102 and the ~ladding 104 of the optical fiber 10~ are approximately equal, the two LPo1 ~odes will travel ~hrough the fiber at approximately the ~ame propa~ation velocity, ~nd the four second order LP11 modes will travel through the f iber at appro~imately the same propagation velocity~, llowever, ~he propagation velocity for the ~ndamental LPo1 ~et of modes will be slower than the propsgation velocity for ~he second order LPl 1 se~ of modes. Thus, the two ~ets of modes, LPol and LP11. will ~ove in and out of pha~e with esch other as the light propagate~ through t~e fiber~ The propaga~ion distance requ~red for the two ~et~ of modes to Dlove ou~c of phase by 360 (i.e., Xl~ radians) is commonly referred to as the beat length of the fiber, which may be mathematically expre6~ed as:

-13- 13147~

LB ~n ~ (2) where LB is the beat length, ~ is the optical wavelength in a vacuum, ~n is the difference in ehe effective refractive indices of ~he two ~ets of modes, and ~ is the difference in the propagation constants ~or the ~wo sets of modes.
It has been previously shown that coheren~ pnwer transfer between the two 6ets of the modes, LPo1 and LP11, can be achieved by producing periodic perturba~ion~ in the optical fiber ehat match the beat length of the two modes. A number of optical device~ have been constructed to control the coupling of optical energy between the two modes to provide useful devices for selective coupling, filtering ~nd frequency ~hifting of an opticsl signal.
See, for example, W. V. Sorin, et al., "Highly ~elective evanescent modal filter for two-~ode optical fibers,"
OPTICS LETTERS, Vol. 11, No. 9, September 1986, .
pp. 581-583; R. Cq Youngqui~tD et ~1., "All-fibre components u~ing periodic coupling," ~ ~, Vol. 132, Pt. J, No. 5, October 1985, pp~ 277-286i R. CO
Youngquist, et al., ~'Two-mode fiber modal coupler," OPTICS
LETTERS, Vol. 9, ~o. 5, May 1984, pp. 177-179; J. ~.
Blake, et al., "Fiber-optic modal coupler using periodic microbending," oPr;cs LETI~KS, Vol. 11, No, 3, ~arch 1986, pp. 1~7-179; B.Y. Kim, et ~l., "All-fiber ~cousto-optic frequency ~hifter," OPTICS LETTERS, Vol. 11~ No~ 6, June 1986, pp. 3~9-391; an~ J. N. Blake, et al., I'All-fiber acousto-optic frequency ~hifter using two-mode fiber,"
ProceedinKs_ of the SPIE, Vol. 719, 1986. The presen~
invention provides substsntial improve~ent to many of those devices ~nd provides a number of new devices that utilize coupling between the mode8 to fur~her control an optic~l signal.

1 3 ~

Althou~h the four LP11 modes provide four orthogonal channels for the propagation of optical energy through an optical fiber or other waveguide, it has often been found ~o be difficult to fully utilize the four channels independently. As set forth above, the LP11 modes are approximations of real mode~ and are nearly degenerate in a circular core fiber 100. This makes the LP11 modes very sensitive ~o couplin~ caused by perturbations in the optical fiber, 6uch as bending, ewi~ting ~nd la~eral ~tressing. Furthermore, since the LP11 modes are only an approxima~ion of the real modes, there will be a ~light amount of coupling even in the absence of perturba~ions of ~he fiber 100. The net result iB that the propagation of an LP11 mode ele~tric field pattern in a given ~ode is not ~table. In like manner, the electric field patterns of the two LPo1 polarization modes are likewlse unstable.
It h~s been previously ~hown ehat the 1~se of an elliptical sore cross-section in an optical fiber or other waveguide can i~roduce birefringence and ~eparate ~he 2~ propagation con~tant~ for the two polarizations of the LPo1 first order mode. The separation of the propagation constants locks the polarization of the signal to a principle axis of the core cross-section. It has also been shown that ~n elliptical core ~l~o ~ncreases the separation between the propagation constants of the LP11 mode patterns. This tends to enhance modal stability.
This i~ illustrated in Figure 4 w~ich is an unscaled representation of the pr~pagation constant ~ ver~us the ellipticity of the core of an optic~l wave~uide. As illustrated, the LPo1 propagation mode has a larger propsgation con~ant than t~e LP11 propagatlon mode. From Equation (2), this difference in the propaga~ion constants i~ related to the beat length LB between ~he LPo1 and LP
propagation ~ode~ a~ follows:
~01 ~

~ 3 ~

~here ~01 is the d~fference in the propagat~Gn constants between the LPo1 mode and the LP11 mode and Lgo1 is the beat length between the LPg1 ~nd LP11 modes.
As illustra~ed in ~he left-hand porti~n of Figure 4, when the core of the optical waveguide i~ ~ubstantialiy circular, ~he LP11 odd and even modes ha~e ~ubstantially the ~ame propag~tion cons~ant. ~owever, when the core of the optical waveguide is elliptic~l, the propagation constant~ of the odd and even LP11 mode~ are different~
This is illustrated by the propagation constant difference ~11 in the right half of Figure 4. As illustrated, the difference in the propsgation constants of ehe odd and even LP11 modes (~11) increases as the ellipticity increases. The use of an elliptical core optical fiber has been suggested as a ~eans of ~voiding the degeneracy of the orthogonal l~be orien,tation~ of the LP11 modesO
See, for example, J. N. Blake, et al., "All fiber acou~to-optic frequency shifeer using two-mode fiber," Proceedin~s f the SPIE, VQ1. 719, 1986.
The foregoing differences in the propagation cons~ants between ehe LPo1 ~ode and the odd and even LP~1 modes when the core of the opticsl fiber is elliptical, al80 results in a change in ~he cutoff wavelength and the corresponding cutoff frequency. For example, for a circular eore optical fiber, the cutoff wavelength i~ related ~o the radius of the fiber core, a~ se~ forth in Equation (1) above. Thus, optical signals h~ving wavelengths above the second order ~ode cutoff wsvelength ~c (l~e., frequencies below ~he second order mode cutoff ~requency) will not propagate in the ~econd order or higher modes in the optical fiber. Optical signals having wavelengths less than he cu~off ~avelength ~c will propagate in the second order modes. If the wavelength is further reduced to a wavelength ~c2~ third order and higher modes will be ~3~7~

supported by the optical waveguide. For a circular core optical waveguide, ~c2 can be found by the following equation:
-7 ~ -~c2 where r, nCO and nCl are ac set forth above for Equation (1~. One skilled in the art will understand that the foregoing can also be represented by cutoff frequencies.
For example, the irst cutoff wavelen~th hc corre~pcnds to a fir~t cutoff frequency fc~ and the cecond cutoff wavelength ~c2 corresp~nds tO a 8econd eutoff frequency fc2 ~hat i8 grea~er than the fir~t cutoff frequency fc~
Specifically, fQr the circular core optical waveguide, if the first cutoff frequency fc i~ normalized eo 2.405, ~he second cutoff frequency fc2 will be normalized to 3~832.
In other words, the second cutoff frequency will be 1.59 times greater than ~he first cutoff frequency (e~g., fc2/fc ~ 3.832/2.405 - 1059). Thus, an optical ~ignal having a normalized frequenc~ le~s than 2.405 will propaga~e in the optical waveguide only in ~he L~l ~ode. An optical ~ignal having a normalized frequency in the range of 2.405 to 3.832 will also propagate in the second order LP11 mode. An optical ~ignal having a normalized frequency greater ~han 30832 will propagate ln higher order modes.
The foregoin~ relationships al80 apply when the core of he optical waveguide i8 ell$ptical or has some other noncircular geometry. For example, Allan W. Snyder and Xue-Heng Zheng, i~ "Opeical Fibers of ~rbitrary Cross-Sect1on6," Journal of the Optical Society of America A, VolO 3, No. 5, May 1986, pp. 600-609, set forth the normalization factor~ for a number of different waveguide cro3s-eections. For example, an elliptical rore waveguide having a ma~or ~Xi8 that i8 twice the length of the minor axis, w$11 have a normalized ~utoff frequency fc of 1.88 131~

when t~e minor axis has the same leng~h a6 the diameter of a corresponding circular core optic~l fiber of the same material construction. In o~her words, below the normalized frequency of 1.889, only ~he first order LP~l modes will propagate. Similarly, Snyder and Zheng ~uggest that the LP11 even mode will have ~ normalized cutoff frequency of 2.505, and the LP11 odd mode will have a normalized cutoff frequency of 30426.
Snyder and Zheng generalize the foregoin~ concep~ for an elliptical sore optical wavegu~de with varying ratios between the length of the min~r axis ~nd ~he length vf the major ~xis as follows:

fc G 1.700 (1+~b/a)2)1/2 (5a) fc2even ' 1-916 (1+3~b/a)2)1/2 (5b) fc2odd ~ lr916 (3+(bla)2)1~2 (5c) where fc i8 the normalized cutoff frequency for the LPo1 mode, below which optical energy will propagate only in the LPo~ mode in the elliptical core optical fiber; where fc2even i~ the normalized cutoff frequency for optical energy propagating in the LP11 even mode, below which op~i~al energy will propagate only in the LP11 even m~de bu~ not in t~e LP11 odd mode; and where fc2odd is the normalized cutoff frequency for the LPll odd mode, below which optical energy will propagate in the LP11 odd mode as ~el 1 a~ the L~l t even ~ode, ~Ut DO~ in ~ny of the hi~her order modes; b i8 o~e-half the ~en~h of the minor axi~ of the elliptica~ core; and ~ i8 one-half the length of ~he ~ajo~ ~xi~ of the elliptical core. Equations (5a), (5b~ ~nd (~c) can be evaluated for an elliptical core fi~er ~aving a ma~or a~i~ length 2a of twice the minor axis length 2b to obtain ehe normalize~ frequencies 1.889, 2.505 and 3.426, Bet forth above. Equation~ (5a)l (5b) ~ ~3~7~

and (5c) can be further evaluated for b ~ ~ (i.e., for a circular core) to obtain the LPo1 cutoff frequency of 2.4~5 and the LP11 cutoff frequency of 3~832 for bo~h the odd and even modes, as set forth above.
The foregoing properties ~f the elliptic~l eore opticsl waveguide are ~dvantageously utilf7ed in the present invention to improve the operating characteristics of the optical waveguide by elimin~ting the LP11 odd propagation ~ode and thus provide only one spatial orienta~ion for ~he electric field pattern of ~he ~econd order mode. This ~8 illustrated in Figu~es 5 and 6a-Sg.
Figure S illustrates an exemplary optical fiber 200 having an elliptical core 202 and a surrounding eladding 204. The dimensions of ~he ellipt}cal core 20~ are selected 80 that the cutoff wavelength~ and frequencies ~or the ~wo orthogonal lobe patterns of ~he second order mode are well Geparated. An op~ical signal is applied to the fiber 20~ that is within a frequency range seleeted to be above ~he cuto~f frequency fc2even ~nd to be below the requ~ncy fc2oddo For example, ~n an exemp~ary optical fiber, having a fir~t cutoff frequency fc that is normalized to 1.889, and ~ ~econd frequenoy fc2ev~n of 2.505, the freqency of the i~put optical ~ignsl is selected to have a normalized frequency in the range of 1.8~9 to 2.505. Thus, a light ~Guree is 8elected 80 ~hat substantially all of ~he light produced by ~he light source has a normalized frequenc~ t~at is substantially less than the ~econd cu off requency f~2even~ and that has a subs~antial poreion of the light ~hat has a n~rmalized frequency that ~8 greater than the first cutoff frequency fc~ In terms of wavelength, ~ubstantially all of the light produced by the light source has one or ~ore wsveleng~hs that are greater than the second cutoff lengch 1~C2eVen . and wherein a substancial portion of 3S the light ~as at least one wavelength t~at is less than ~he f~rst cutoff wavelength ~c Thus, ~he light entering 7 ~ ~

~ 1 g-the optical fiber is caused to propagate only in either the firs~ order LPo1 ~ode or the LP11 even mode. Since the frequency of the optical 6ignal i~ ~elected to be less than ~he cutoff wavelen~th for the LP11 odd m~de, substantially no light propagates in the LP11 odd mode.
Tne foregoin~ i6 illustrated in Figures 6a-6g. In Figures 6a and 6b, the two polarization modes for the LPo1, fir~t order mode are illustrated. An electric field pattern 210 in Figure 6a represen~ the electric field for the vertically polarized LPo1 m~de, and an electric field pattern 212 in Figure 6b represents ~he electric field for the horizontally polarized LPo~ mode. One skilled $n the art will under6tand that the op~ical fiber 200 (Figure 5) i8 birefringent for the fir~ order LPo1 mode, and that the horizontally polarized LPo1 mode will propagate a~ a greater velocity than ~he vertically polarized LPo1 mode. An electric field amplitude distribution 214 for the LPo1 propagation modes is illu3trated in Figure 6c.
As illustrated, the electric field ampli~ude di~ribution 214 is ~imilar to the electric ~ield amplitude distribution 116 in Figure 2b, ~or ~ circular core fiber ~nd ha~ a peak amplitude 216 proximate to he center line of the core 2~3.
Figures 6d and 6e ~llu~trates the LP11 even modes for ~5 the elliptical core fiber 2Q0.` As illus~rated in Figure 6d and Figure 6 , respectively, ~ vertically polarized even mode elec~ric field pateern 220 and a horizonthlly polarized even mode electric field pattern 222 are both well-guided by the opt$cal fiber 2000 As illustrated ~n ~igure 6f, the LP11 even ~odes have an electric field ~plitude distribution, represented by a curve 224, that has a fir~t maxima 226 proximate to one boundary of the core, and that has a ~econd maxima 228 proximate to an oppo~ite boundary of the core, ~nd wnerein the ~ t ~axima 226 and the second maxi~a 228 ~re 180 out of phase.

The ~Pl~ odd Yertical polarization mode, repre~ented by an electric ~ield pattern 230 (Figure 6~), and the hPll odd horizontal polarization mode, repre~ented by an electric field pattern 232 (Figure Çg), are not guided by the optical fiber 200 when the optical wavelength is selected to be above the ~econd cutoff wavelength ~c2even- Thus, the optical energy in the LPll odd modes, represented by the field patterns 230 and 232, will not propagate. Thus, rather than providing four degenerate optical communication channals, ~uch as provided by a circular core waveguide or a ~lightly elliptical core wavegl-id~, the hiyhly elliptical core 202 of the optical fiber 200 provides only two LPol mode propagation channels and two LPl1 even mode propagation channels~ ~urthermore, the co~munication channels are well-defined and stable and, in the absence of a perturbation in the optical fiber 200, t~ere is no coupling batween any of the four channels. Therefore; an optical signal can be launched in the ~econd order LPll ~ode and it will propagate only in the LPll even mode~ It is no~ necessary to avoid exciting the odd lo~e patterns of th~ ~econd order LPll mode because optical energy in those lobe patterns will not propagate. Futhermore, optical energy will not be coupled to the odd lobe patternsO
Be~ause of th~ ~tability of the electric ~i~ld intensity patterns of the LPol mode and the LPll even modas, the performances of fiber optic devices previously developed to utilize the Eecond order LPll mode will be increa ed. Specific examples o~ devices utilizing the ~ighly elliptical core waveguide will be set ~orth hereinafter.
Descrie~ io~ of th~ ~ynamic 0ptica~ er The optical ~iber 200 of Figure 5, or ~nother optical waveguide having a non-circular cross ~ection, can be 3~ advantageously used in a dynamic optical coupler 300 illustrated in Figure 7. The dynamic optical coupler 300 ~3~ ~7~

of Figure 7 compri6es an optical fiber 302. The optical fiber 302 has a first ~nd portion 304 and A ~econd end portion 306. An intermediate portion of the optiral fib~r 302 is formed into a tightly wound coil 310 to provide an 5 LPll ~ode stripper, that will be explained more fully below. A high power laser light sQurces 320 i6 provided ~hat yenerates a laser output ~ignal having a wavelength ~1 The laser output signal can be switched on and off by selectively enabling and di6abling the electricAl input to the high power laser light eource 320 with an electrical switch, or the like, by modul~ting the laser output signal, or by other conventional ~eans. The laser output ~ignal generated by the source 320 is a perturbational signal, as will be explained below. The laser output signal from the high power laser light ~ource 320 is directed to a beam splitter 322. Approximately 50% of thP
optical energy in the laser output ignal passes through the beam splitter 322 and is input in to the ~irst end portion 304 o~ the optical ~iber 302. The first end portion of the optical fiber 302 is positioned with respect to the beam splitter 322 ~o that the laser output signal ~rom the high power laser light 60urce 320 provides approximately equal excitation in the ~undamental and second order ~odes of the optical fiber 302.
A signal source 330, which is advantageously a laser signal source is also provided. The xignal source 330 generates a relatively low power output signal having a wavelength ~2 that is preferably close to but not egual to the wavelength Of ~1 The lower power output cignal from the signal source 330 is directed to the beam ~plitter 322 which directs approximately 50% o~ the optical energy of the lower power output signal to the ~irst end portion of the optical ~iber 302. ~he ~ignal source 330 is positioned with respect to the first end portion 304 of the optical ~iber 302 ~o that Gubstantially all of the optical energy ~ncident upon the first end portion 304 is 7 ~ ~

-~2-caused to propagate in one or the o~her of ~he fundamental or the ~econd ~rder spati~l mode~
A ditfraction grating 34~ is positioned proximate IO
~he second end portion 306 of the optical fiber 302. The S diffraction grating 340 i5 orien~ed with respect to the second end portion 306 of the fiber 3U2 ~o that optical signals output from the ~eeond end portion ~06 having a wavelength ~1 are refracted to a first location and optical signals ou~put form the second end portion 3U6 having a wavelength ~2 are refracted to a ~econd loeation different from the first location. The refracted optical signals can ~e viewed with a screen (not shown), or, alterne~ively, ~he in~ensities of the refraceed ~ignals can be de~ected ~y fir~t detector 350 po6itioned to detect the refracted optical ~ignal having the wavelength ~1 and a ~econd detector 352 positioned to detect ehe refracted optical signal having the wavelength ~2~
When the high power laser si~nal is applied eo the firs~ end portion 304 of the optical fiber 302, the approximately equal excit~tion of the first order LPUl and the ~econd `LP11 m~des ~n the elliptical core of the two-mode fiber 302 creates a periodic pattern in the cross-sec~ional ~ntensity distribution ~long the length of the optical fiber 302 as the two spatial modes propagate in ~5 the optical fiber 302 wi~h differen~ phase veloci~ies.
This i6 ~llustrated in Figure 8 and in Figures 9a~
where Figure 8 represents a portion o~ the optical fiber 302 and Figures 9a-gi represent cros~-sections of ~he op~ical intensity di~eribution patterns ~t the locations 9a-9a, 9b-9b, etc., in Figure 8. In Figure~ 9a-9i, the presence of optical energy in the inten3ity distribution is represented by the dark portions of the in~ensity patterns, and the ~bsence of optical energy is illustra~ed by the light portions oP the patterns. Figures 9a, 9c, 9e, 9~ and 9i illu~trate ~he hi~hly asymmetric intensity dis~ributions that occur st locations where the phase diference between the two modes is Nn nd most of ~he op~ical power is concentrated in. one half of the elliptical core. For example, Figure 9a illu~trates the intensity distribution when the pha~e diference is zero (i~e., U~); Figure 9c illustrates the intensity distribu~ion when the phase difference i~ ~; and Figure 9e illustrates the intensity distribu~ion when the phase difference is 2~. ~hen ~he phase difference is (N~
the intensity distribution is symmetric. Figure 9b illustrates the symmetric intensity di ~ribution when the phase difference i8 ~l2; and Figure 9d illus~rates ~he symmetric ~ntens~ty distribution when tne phase difference is 3~/2. As illustrsted in Figure 8 and in ~igures 9a-9i, the mode in~ensity pattern~ are periodic and repeae every beat length LB along the length of the optical fiber 302.
The exi~tence of optic~l power in an optical waveguide, such as the optical fiber 302, alters the refraction index of the glass medium through ~he optical Kerr effect. ThiR effect i8 due to the third order non-linear polarization of the glass medium and occurs even when the cp~ical power i~ small. When the high power laser light from the high power la~es light ~ource 320 is launched into the optical fiber 302 with approximately equal intensity for ~he fundamental LPo1 and the ~econd 2~ order LP11 modes, this non-linear interac~ion of the light energy with the glass Dedium of the optical fiber 3U?
causes a periodic ~symmetric perturbation of the refractive ~nduces of the optical fiber 302. It has been shown that periodic per~urbations in ~n op~ical fiber can 3~ cause coupling between two ~patial ~ropagation modes of an optical fiber when the periodiclty of ~he perturbations are matched to the beat length of the two ~odesO Example~
of ~ode coupling cau~ed by periodic s~resses are illustrated in B.Y. Kim, et al. "All-fiber ~cousto-optic frequency ~hifter>~ OPTICS LETTERS, Vol. 11, No. 6, June 1986, pp. 3~9-391; J.N. Bl~ke, et ~l~, "Fiber-op~ic modal ~L3~ ~7~

coupler using periodic microbending, "OPTICS LETTERS, Vol.
11, No. 3, ~arch 1986, pp. 177-179; and J.N. Bl~ke,e~ al., "All-fiber acou to-optic ~requency shifter u~ing two-mode fiber," Proceedinqs ~f the ~P~, Vol. 719, 1986.
As set orth above, the wavelength ~2 Of the light generated by the signal ~ource 330 i8 close to the wavelength of the light generated by the high p~wer laser source 320. It has been ~hown that the beat length between the two spatial propagation modes o~ ~n optical ~ignal does not vary ~ignificantly over a relatively wide range of wavelengths. Thus, the periodic changes in the re~ractive induces o~ optical fiber 302 caused by the hi~h power laser li~ht having the wavelength ~1 are substantially well matched with the wavel~ngth ~2~ As ~
result, the perturbation of the refractive indices cause coupling of optical energy between the fundamental and second order modes c~ the co-propagating optical energy from the signal source 330 ~Figure 7) in a manner similar to that provided by externally applied periodic perturbations.
The total amount of coupling from one propagation mode to the other propagation mode will vary in accorda~ce with the power applied from the high power laser li~ht ~ource 320 and in accordance with the length ~f the optical fiber 302 in which the two optical signals interact. By varying the amount o~ power of the high pow~r laser light source 320 and thus controlling the magnitude of the perturbations caused by the optical Kerr effect, ~he amount of coupling between the propagation modes of the optical ~ignal generated by the ~ignal ~ource 320 can be controlled.
As an example of the operation o~ the present invention, the apparatus illustrated ~n Fi~ure 7 can be used as a dynamic optical switch in an optical signal processing syst~m, an optical communications ~ystem, and the like. As ~et ~orth above, the optical energy ~3~
-2~-generated by the signal source 330 is advantageously input into the first end portion 304 of the optical fiber 302 at a wavelength ~2 wi~h substantially of the op~ical energy in one or ~he other of the fundamental LPo1 mode or the second order LP1 1 mode. Fvr example, the optical energy from the signal 60urce 330 can be advantageously input only in the LPl 1 mode. When the high power laser light 60urce 320 is off, the optical energy from the signal source 330 will propagate through the optical fiber 302 with 8ubstantially no coupling of optical energy from the LP11 mode to ~he LPo1 mode. When the op~ical energy in the LP11 mode reaches the ~ode ~ripper 310 D the optical energy will be radia~ed from the optical fiber 302 and substantially no optical energy ~ill be emit~ed from the t5 second end portion 306 of the op~ical ~iber 302. Thus, the A2 detector 352 will detect 3ubstantially no optical energy. Conversely, when the high power laser light ~ource 320 iB activated, the perturbations of the refractive induces of the optical fiber 302 ~ill cause coupling of the optical energy from the opt$cal fiber 302 ~ill cause coupling of the optical energy from the LP11 propagation mode of the ~2 optical ~ignal to ~he LPo1 propagation mode. The power of the high power laser light source 320 i~ advantageously ad~usted 80 that ~ubstantially 100%
coupling ~o the LPo1 mode occurs. Any residual optical energy in the LP11 propagation mode will be radiated from the optical fiber 302 by the ~ode ~ripper 310. The optical energy in the LP~1 propagation ~ode at the wavelength ~2 will be emitted from the $econd end por~ion 306 of the optical fiber 302 and will be dir~cee~ to ~he ~2 detector 352 by the diffraction grating 3~2. Thus, the signal output of the ~2 detector 352 will be responsive to the on/off control provided by the activation/deactivation of the high power laser light source 320. In an op~ical signal pro~essing system or optical communicR~ions system, the ~2 8ignal output from the diffraction grating 340 ~an -2~-be advantageously provided as an input to additional optical components ~or further processing.
In like manner, the optical energy fro~ the signal source 330 can be introduced into the first end porti~n 302 of the optical fiber 3~4 in the LPo1 propagation mode. When the high power laser light sourre 320 is off, the optical energy ~n the LPo1 propagation mode will propagate through the op~ical fiber 302 substantially unehanged and will be emitted from the second end portion 306 and detec~ed by the ~2 detector 352. Activation of the high power laser light source 32~ will cause coupling of the optical energy from the LPo1 propagation mode to the LP11 propagation mode of the ~2 optical ~ignal. The energy of the LP~1 propagation mode will be radiated from the optical fiber 3~2 at the ~ode ~tripper ~10~ Thus, if the high power laser light source 320 is ad~usted to provide 100~ coupling of the optical energy to the LP11 propagation mode, ~ubstantially no ~2 optical energy will be emitted from the second end portion 306 and detected by 20, the ~2 detector 352.
As 8e~ foreh above, the optical ener~y from. ~he high power laser light source 320 is preferably introduced into the first end por~ion 304 of the optical fiber 302 with approximately equal intens~ties in each of ~he fundamental LPo1 and ~econd order LP11 propagation modes. This is advantageously accomplished by adju~ting the position of ~he high p~wer laser light source 320 with respect to the first end portion 304 while observing intensity patterns of the optical output from the 6econd end portion 306 of the opeical fiber 3~2. The intensity patterns can be observed by directing the optical output onto a screen (not showm) or the like. There will be a position wherein ~he centerline of the beam of optical energy from the high power laser light Bource ~8 offset from the centerline of ~5 the $nput end portion 304 of the optical fiber 3V2 such that the funda~ental LPo1 made and the seconB order LP11 ~ 3 ~

-~7-of the optical energy propagating in the optical fiber 304 are substantislly equally excitedO As set forth above, ~he opeical fiber 3~2 has a beat length at the optical wavelength ~1. Because of the beat lengths, the intensity S pa~terns of optical energy in the op~ical fiber 3~2 a e periodic as ill~strated in Figure 8 ~nd Figures 9a-9i. In the adjus~ment method described herein, the position of the high power laser light ~ource 320 i~ preferably sdjusted prior to the formation of the mode stripper 310 (i.e., before the fiber 302 i8 tightly wound ~o cause radi~tion of the optical energy propagating in ~he second order LP11 propagation mode). Thus, optieal ene~gy in bo~n the fundamental LPo1 and the second order LP11 propagation mode will be emitted from the second end portion 306 of the optical fiber 302. While holding the first end portion 304 in a fi~ed location, ~he optical fiber 302 is gen~ly ~tretched to adju~t the len~th of the op~ical fiber 302 between the fir~t end portion 304 and the second end portion 306 until the observed intensity pattern exhibits one of the intensity patterns corresponding to a phase difference between the fundamental LPo1 and the second order LP1~ propagation modes that ~ an integer multiple of ~ (i.e~, one of the mode patterns illustrated in Figures 9a, 9c, 9e, etc.).
~5 After one of the desired intensity patterns is obtained, ~he first end portion 304 and the second end portion 306 are held in their respective po~itions to ~aintain the inten~ity mode pattern. While holding the first end portion 304 and the second end portion 3~6 fixed, the position of the high power laser light ~ource 320 is adjuste~ with respect to the first end port~on 3~4 uncil the maximum contrast i8 obtained between the lighted portions of the intensity pattern and the unlighted por~ion of the inten~ity pa~tern. The ad~ustment of the position of the high power laser light source 320 to obtain ma~imum contrasc correspondc generally to the ~ 3 ~

adjustment of the high power laser light ~ource 320 to obeain ~ubstantially equal intensity in the fundamen~al LPo1 ~nd the ~econd order LP11 propagation modes although the two intensities may n~t be precisely equal.
In a similar manner, the position of the 6ignal source 330 with respect to ~he f$rst end portion 304 of the optical fiber 302 is sdjusted until substantially all of the ~2 optical energy i8 introduced into the first end portion 304 in one or the other of the propagation modes. This adjustmen~ is al8o performed prior to forming ~he mode stripper 310 and while holding the first end portion 304 fixed with respect to the high power laser ligh~ source 320 after the previou~ adjustmen~. However, rsther than attempting to obtain maximum contrast between the light and dark portions of the intensity pat~erns, the position of the signal source 330 i8 adjusted ~o provide minimum variation ~n the intensity pattern as the op~ical fiber 302 is ~tretched. In other words, if the optical energy introduced into the fir~t end portion 304 from the signal source 330 is in one propagation mode only, ~here will be no beating between the two modes and the intensity pattern will not vary BS the length of the optical fiber 302 is increased or decreaxed. Thus, ehe optical fiber 302 is 8tretched and released repeatedly as the position of the ~ignal source 330 is gradua}ly adjus~ed wieh respect to the centerline of the first end portion 304 of the optical fiber 302. When the optical fiber can be stretched and released with no perceptible change in ~he observed intensity pattern, ~ubætantially all of the ~2 optical energy i~ propagating in one or the other of the two propagation ~des. The mode in which the optical energy is propaga~ing can be readily determined by observing the inten i~y pa~tern. A8 illustrated in Figures 6a and 6b, the fundamen~al LPo1 propAgation mode has an intensity pattern that is concencrated in ~nd 8ubs~antially symmetricsl ~bout the center of the optical ~ 3 ~

fiber 302, while the 6econd order LPll propagation m~de has an intensity pattern with two lobes dl~placed equally from the center of she optical fiber 3~2, a~ illustrated in figures 6d and 6e. One ean ~ee that the use of an opticAl fiber or other waveguide have a geometry such as the highly elliptical core 1~ advantageous in enabling the posieions of the two optical ~ignal ~ources to be readily adjusted with respect to the centerline of the core.
After the position of ~he high power laser lighe ~ource 320 and ~he position of the signal ~ource 330 are adjusted with respect to the fir~t end portion 304 of the optical fiber 302, a portion of the optical fiber 302 is formed into the mode stripper 310 ~nd the second end portion 306 is directed at the diffraction gra~ing 322.
lS The apparatus is then operable as described above.
A second embodimen~ of the present i~vention is illustrated in Figure 10 wherein like numbers designate the same elements as were described above in connec~ion with Figure 7. The elements of Figure 10 2re positioned ~0 as in Figure 7 except there i8 no diffraction grating in Figure 10 and the ~ignal source 33U is po~itioned proximate to the second end portion 306 of the optical fiber 302. The perturbational signal from the high power laser light source 320 propagates in ~ first direction in ~5 ehe optical fiber 302 from the first end portion 304 to the econd end portion 306, a~ be~ore. However, the optical signal from the signal source 330 is introduced lnto the seoond end portion 306 and propagates in a second opposite direction from the second end portion 306 to the fir~t end portion 304. The opt~cal energy from the Rignal ~ource 330 is emitted from the first end por~ion 304 and i8 directed by the beam ~plitter 322 towards a ~2 detector 360. In the apparatus in Figure 1~, the position of the high power la er light source 320 with re~pect to the fir~ end portion 304 of the optical fiber 302 i~ adju~ted ~ before prior to forming the mode stripper 310. The ~3~7~

positi~n of ~he ~i~nal s~urce 330 i~ adjusted 60 that a l~rge portion of the op~ical energy introduced into ~he second ~nd por~ion 306 is in~roduced in the fundamental LPo1 propagation mode. ~owever, it is not necessary to S accurately adjust the position of the si~nal source 330 with respect to the ~econd end portion 306 æo tha~ the optical energy int~oduced into the second end portion is only in the LPol propagation modeO Rather, substantially flll of the optical energy introduced in~o the 8econd end portion 306 in ~he 8econd order LP11 propagation mode is radiated from ~he optical fiber 302 in the mode stripper 310 so that 6~bstantially all of the optical energy propagating towards the first end por~ion 304 i initially in the fundamental LPol propagation mode. Thus, the position of the signal source 330 with respect to the second end portion 306 can be adjusted after the mode s~ripper 310 is formed in the optical fiber 302. The proper positioning of the signal source 330 can be obtained by monitoring the ou~put ~ignal emitted from the first end portion 304 and direc~ed onto the ~2 detector 360 while adjusting the position of the 8ignal ~ource 330 for maximum detected inten~ity.
The app~ra~us of Figure 10 operates in a 8 imilar manner to the apparaeus of Figure 7. When the high power laser light source 320 i8 not activaeed, the optical energy from the signal 80urce 330 propagates through the optical fiber 3~2 subst~ntially unchanged and is emitted from ehe firs~ end portion 304 in ~he fundamental LP
propagation mode. The emitted optical energy i~
represented by an intensity pattern 402 in Figure 11b. In contrast, when the high power laser ligh~ source 320 is activated, the perturbation3 in the optical f iber 302 caused by the optical Kerr effect CaUse coupling of optical energy from ~he fundamental LPo1 mode to ~he second order LP11 mode. The optical energy in the LP11 ~ode is emitted from the f1r~t end portion of the optical fiber 302 ~nd produces an intensity pattern 400 as illuserated in ~igure 1la. Thus, by selectively ac~ivating and deactivating the nigh power la~er ligh~
source 32~, the light in~roduced into the ~econd end portion 306 ~rom the signal source 330 can be 6eleclively emitted from ~he firs~ end portion 304 of She optical fiber 302 in ei~her the funda~ental LP~1 propagation mode or the second order LYll propagation mode~ The output from the first end portion 304 can be adv~ntageously provided as an input to ~dditional optical eomponentB for further processing.
Since the optical energy from the hi2h power laser light ~ource 320 and the ~ignal source 330 are counter-propagating, it is not necessary that the wavelength ~ of lS the signal source 330 be different from the wavelength ~1 of the high power laser light s4urce 320. Thus, ~1 can be cqual to ~, and ~he beae length of the controlling light signal is precisely matched to the bea~ length of the controlled light signal.
Although described above with reference to the preferred embodiment~, modifica~ions ~ithin the ~cope of the invention may be apparent to those Rkilled in ~he art, and all such modifications are intended to be within ~he scope of the appended claims.

Claims

1 . An optical mode coupling apparatus comprising an optical waveguide which couple an optical signal propagating in the optical waveguide between propagation modes of the waveguide, the optical signal having an optical signal beat length for the modes, the coupling apparatus further comprising a light source for introducing a perturbational light signal into the waveguide, the perturbational signal having an optical wavelength selected such that the perturbational signal propagates in the waveguide in two spatial modes which have different propagation constants so as to cause the perturbational signal to beat in the waveguide in accordance with a perturbational signal beat length and thereby cause the perturbational signal to have an intensity distribution in the waveguide which varies along the length of the waveguide, the perturbational signal having an intensity which is selected to optically perturb the refractive index of the waveguide at intervals defined by the perturbational signal beat length, the optical wavelength of the perturbational signal further selected such that the intervals have a spacing related to the beat length of the optical signal to cause cumulative coupling of said optical signal from one of the propagation modes to another.

2. The device defined by Claim 1, wherein the optical waveguide has a noncircular cross section having cross-sectional dimensions selected such that the waveguide guides a portion of the perturbational signal in a fundamental spatial mode and another portion in a higher order spatial mode, the cross-sectional dimensions of the core further selected such that the portion of the perturbational signal guided by the waveguide in the higher order mode propagates in only a single, stable intensity pattern.

3. The apparatus defined by Claim 2, wherein the fundamental spatial mode includes two polarization modes, the cross-sectional dimensions of the core further selected to cause the polarization modes of the fundamental mode to be non-degenerate.

4. The apparatus defined by Claim 3, wherein the single intensity pattern of the higher order spatial mode includes two polarization modes, the cross-sectional dimensions of the core further selected to cause these polarization modes to be non-degenerate.

5. The apparatus defined by Claim 2, wherein the waveguide comprises an optical fiber, the fundamental mode being the LP01 mode of the optical fiber and the higher order mode being the LP11 mode of the optical fiber, the single intensity pattern being the even mode intensity pattern of the LP11 mode.

6. The apparatus defined by Claim 5, wherein the core of the optical fiber has an elliptical cross section.

7. The device defined by Claim 1, wherein the refractive index perturbations of said waveguide are produced by the optical Kerr effect.

8. The device defined by Claim 1, wherein said propagation modes are the first and second order modes of the waveguide.

9. The device defined by Claim 8, wherein the two spatial modes are the first and second order modes of the waveguide.

10. The apparatus defined by Claim 1, wherein said perturbational signal varies in intensity to cause said coupling to vary.

11. The apparatus defined by Claim 1, wherein said light source includes a switch for digitally varying said perturbational signal.

12. A method of coupling an optical signal between propagation modes of a waveguide, said waveguide having a beat length for said modes, said method comprising:

introducing a perturbational optical signal into said waveguide such that said perturbational signal propagates in two spatial modes of the waveguide to cause the perturbational signal to beat in accordance with a perturbational signal beat length;
selecting the intensity of the perturbational signal to cause optical perturbation of the waveguide at intervals defined by the perturbational signal beat length; and selecting the wavelength of the perturbational signal such that the intervals have a spacing related to the beat length of the optical signal to cause said coupling to be cumulative at said intervals.

13. The method of Claim 12, wherein the intensity of said perturbational signal induces the perturbations in accordance with the optical Kerr effect.

14. The method of Claim 13, wherein the waveguide comprises an optical fiber.

15. The method of Claim 12, additionally comprising the seep of switching the perturbational signal between a relatively high intensity level and a relatively low intensity level.

16. The method of Claim 12, wherein the waveguide has a core of noncircular cross section, said method additionally comprising the step of selecting the wavelength of the optical signal in relation to the cross-sectional dimensions of the core such that (1) the waveguide guides a portion of the optical signal in one spatial mode and another portion in a higher order spatial mode, and (2) the portion of the optical signal guided by the waveguide in the higher order mode propagates in only a single, stable intensity pattern.

17. The method of Claim 16, additionally comprising the step of selecting the wavelength of the perturbational signal in relation to the cross-sectional dimensions of the core of the waveguide such that (1) the waveguide guides a portion of the perturbational signal in one spatial mode and another portion in a higher order spatial mode, and (2) the portion of the perturbational signal guided by the waveguide in the higher order mode propagates in only a single, stable intensity pattern.

18. An optical mode coupling apparatus comprising an optical waveguide which couples an optical signal propagating in the optical waveguide between propagation modes of the waveguide, the optical signal having an optical signal beat length for the modes, the waveguide (a) comprising a guiding structure formed of materials having dissimilar indices of refraction and (b) having perturbations optically induced by a perturbational light signal, the perturbations being spaced at intervals related to the beat length of the optical signal to cause cumulative coupling of said optical signal from one of the propagation modes to another.

19. The device defined by Claim 18, wherein the optical waveguide has a non-circular cross section having cross-sectional dimensions selected such that the waveguide guides a portion of the perturbational signal in a fundamental spatial mode and another portion in a higher order spatial mode, the cross-sectional dimensions of the waveguide further selected such that the perturbational signal guided by the waveguide in the higher order mode propagates in only a single, stable intensity pattern.

20. The apparatus defined by Claim 19, wherein the fundamental spatial mode includes two polarization modes, the cross-sectional dimensions of the core further selected to cause the polarization modes of the fundamental mode to be non-degenerate.

21. The apparatus defined by Claim 20, wherein the single intensity pattern of the higher order spatial mode includes two polarization modes, the cross-sectional dimensions of the core further selected to cause these polarization modes to be non-degenerate.

22. The apparatus defined by Claim 18, wherein the core of the waveguide has an elliptical cross section.

23. The device defined by Claim 18, wherein the refractive index perturbations of said waveguide are produced by the optical Kerr effect.

24. The device defined by Claim 18, wherein said propagation modes are first and second order spatial modes of the waveguide.

25. The device defined by Claim 18, wherein the waveguide has a non-circular cross section.