United States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,414,540
`
`Patel et a].
`[45] Date of Patent: May 9, 1995
`
`
`
`USOOS414540A
`
`[54] FREQUENCY-SELECTIVE OPTICAL
`SWITCH EMPLOYE‘IG A FREQUENCY
`DISPERSIVE ELEMENT, POLARIZATION
`DISPERSIVE ELEMENT AND
`POLARIZATION MODULATING
`ELEMENTS
`
`[75]
`
`Inventors:
`
`[73] Assignee:
`
`Jayantilal S. Patel, Red Bank; Yaron
`Silberberg, West Windsor Township,
`Mercer County, both of NJ.
`
`Bell Communications Research, Inc.,
`Livingston, NJ.
`
`[21] Appl. No.: 125,607
`
`[22] Filed:
`
`Sep. 22, 1993
`
`Related US. Application Data
`
`[63]
`
`Continuation-impart of Ser. No. 70,591, Jun. 1, 1993.
`
`[51]
`
`Int. CLS ........................ G02F 1/137;G02F1/13;
`H04J 14/06; H04J 14/02
`[52] US. Cl. ........................................ 359/39; 354/94;
`354/122; 354/128; 354/124; 385/37; 385/20;
`385/17; 359/93; 359/245; 359/246
`[58] Field of Search ................... 359/94, 39, 122, 128,
`359/124, 130, 245, 246, 494, 496, 615, 123, 139,
`131, 127, 97; 385/17, 37, 20
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3,506,834 4/1970 Bushsbaum et a1.
`................ 359/138
`3,536,375 10/1970 Mansell ................... 359/246
`
`.. 350/162.12
`4,655,474 4/1989 Heritage et a1.
`.
`
`5,002,349
`3/1991 Cheung et a1.
`350/9613
`
`5,005,166 4/1991 Suzuki et a1.
`.. 359/123
`359/92
`5/1992 Patel .............
`5,111,321
`
`..... 359/78
`5,132,824 7/1992 Patel et al.
`
`5,150,236 9/1992 Patel .................. 359/71
`
`......................... 359/ 123
`5,319,484 6/1994 Jacob et al.
`
`FOREIGN PATENT DOCUMENTS
`
`62-305152 12/1987 Japan ................................... 359/122
`
`OTHER PUBLICATIONS
`
`I. Nishi et a1., “Broad—passband—width optical filter for
`multi/demultiplexer using a diffraction grating and a
`retroreflector prism,” Electronics Letters, 1985, vol. 21,
`pp. 423—424.
`M. Shirasaki et-a1., “Broadening of bandwidths in grat-
`ing multiplexer by original dispersion—dividing prism,”
`Electronics Letters, 1986, vol. 22, pp. 764—765.
`Shirosald et al., “Bistable magnetooptic switch for mut-
`work optical fiber”, Applied Optics vol. 21, #11, Jun. 1,
`1982, pp. 1943—1949.
`
`Primary Examiner—William L. Sikes
`Assistant Examiner—Kenneth Parker
`Attorney, Agent, or Firm—Leonard Charles Suchyta;
`James W. Falk
`
`[57]
`
`ABSTRACT
`
`A liquid-crystal optical switch capable of switching
`separate optical signals in a physical input channel to a
`selected output channel. A diffraction grating spatially
`divides the input channel into its frequency compo-
`nents, which pass through different segments of a liq-
`uid-crystal modulator. The liquid—crystal modulator
`segments are separately controlled to rotate the polar-
`ization of the frequency channel passing therethrough
`or to leave it intact. The channels then pass through a
`polarization-dispersive element, such as calcite, which
`spatially separates the beams in the transverse direction
`according to their polarization. A second diffraction
`grating recombines the frequency components of the
`same polarization into multiple output beams.
`
`16 Claims, 9 Drawing Sheets
`
`
`
`FNC 1022
`
`

`

`US. Patent
`
`May 9,1995
`
`’ Sheet 1 of 9
`
`5,414,540
`
`
`
`FIG. 2
`
`

`

`US. Patent
`
`May 9, 1995
`
`Sheet 2 of 9
`
`5,414,540
`
`
`
`FIG. 3
`
`

`

`US. Patent
`
`May 9, 1995
`
`Sheet 3 of 9
`
`5,414,540
`
`
`
`FIG. 5
`
`

`

`US. Patent
`
`May 9, 1995
`
`Sheet 4 of 9
`
`5,414,540
`
`y
`
`X
`
`26
`
`24
`
`28
`
`48x
`;:x:
`
`I
`I —
`
`f—
`
`FIG. 6
`
`

`

`US. Patent
`
`May 9, 1995
`
`Sheet 5 of 9
`
`5,414,540
`
`((113111)
`
`-70
`
`1.54
`
`(dBm)
`
`-70
`
`1.54
`
`1.55
`
`1.56
`
`WAVELENGTH (mm)
`
`FIG. 7
`
`1.55
`
`1.56
`
`WAVELENGTH (pm)
`
`FIG. 8
`
`

`

`US. Patent
`
`May 9, 1995
`
`Sheet 6 of 9
`
`5,414,540
`
`—20
`
`-45
`
`(dBm)
`
`1.54
`
`-20
`
`-45
`
`(dBm)
`
`1
`1.55
`
`‘
`
`I
`
`1.56
`
`WAVELENGTH (um)
`
`FIG. 9
`
`-70
`
`1.54
`
`-
`
`-
`
`1.55
`
`1.56
`
`WAVELENGTH (11m)
`
`FIG. 10
`
`

`

`US. Patent
`
`May 9, 1995
`
`Sheet 7 of 9
`
`5,414,540
`
`
`
`FIG. 11
`
`

`

`US. Patent
`
`May 9, 1995
`
`Sheet 8 of 9
`
`5,414,540
`
`14
`
`.n ----.\--- x
`
`30
`
`42
`
`.......X
`
`,,
`
`Add—Y—
`
`1e
`
`/
`
`\
`
`18
`
`24
`
`110
`
`112
`
`—T\—Drop
`
`44
`
`FIG. 12
`
`
`
`

`

`US. Patent
`
`May 9, 1995
`
`Sheet 9 of 9
`
`5,414,540
`
`142
`
`140
`
`FIG. 14
`
`

`

`1
`
`5,414,540
`
`FREQUENCYSELECTIVE OPTICAL SWITCH
`ENIPLOYING A FREQUENCY DISPERSIVE
`ELEMENT, POLARIZATION DISPERSIVE
`ELEMENT AND POLARIZATION MODULATING
`ELEMENTS
`
`RELATED APPLICATIONS
`
`This application is a continuation-in—part of Ser. No.
`08/070,591, filed Jun. 1, 1993.
`FIELD OF THE INVENTION
`
`The invention relates generally to liquid-crystal de-
`vices. In particular, the invention relates to liquid—crys-
`tal and similar devices useful for switching in a multi-
`frequency communication system.
`BACKGROUND ART
`
`Communication networks increasingly rely upon
`optical fiber for high-speed, low-cost transmission. Op-
`tical fibers were originally envisioned as an optical
`replacement for electronic transmission media, such as
`high-speed coaxial cable and lower-speed twisted-pair
`cable. However, even high-speed optical fibers are lim-
`ited by the electronics at the transmitting and receiving
`ends, generally rated at a few gigabits per second, al-
`though 40 Gb/s systems have been prototyped. Such
`high-speed electronic systems are expensive and still do
`not fully exploit the inherent bandwidth of fiber—optic
`systems, measured in many terabits per second.
`All-optical transmission systems offer many intrinsic
`advantages over systems that use electronics within any
`part of the principal transmission path. Wavelength-
`division multiplexing (WDM) electronically impresses
`different data signals upon different carrier frequencies,
`all of which are carried by a single optical fiber. The
`earliest WDM systems did not provide optical switch-
`ing but only point-to-point WDM.
`Recent research and development have suggested
`that an all-optical network can be constructed having
`switching nodes that can switch the separate WDM
`channels (carrier frequencies) in different directions
`without the necessity of converting the optical signals
`to electronic signals. If such optical switching can be
`accomplished with simple optical components, a sophis-
`ticated optical network can be constructed at relatively
`low cost with the high—speed electronics being confined
`to end terminals that require speeds of only the individ-
`ual channels and not of the total throughput of the
`system.
`However, such optical switching needs to effectively
`separate the switched channels. A cross-talk require-
`ment of 20 dB is a minimum, 35 dB would be a reason-
`able design requirement, 40 dB would be better. Also,
`the switching bands should be relatively wide to accom-
`modate significant frequency fluctuations in the optical
`transmitters, particularly due to frequency chirping in
`directly modulated laser sources. That is, the switch
`must have its frequency bands registered with the trans-
`mitter even when the transmitting frequency is varying
`somewhat. The combination of a wide switching band
`and low cross talk requires a flat-top switch spectrum.
`Furthermore, a somewhat minimal WDM switch has a
`size of Z‘X 2", that is, two physical input fibers and two
`output fibers, each bearing four WDM channels freely
`switched from either input to either output.
`Cheung et al. in U.S. Pat. No. 5,002,349 have sug-
`gested that an acousto-optical tunable filter (AOTF) be
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`used in such a WDM network, either at the switching
`node or at the terminal end. However, AOTFs have
`many intrinsic problems, such as cross-talk between
`adjacent-frequency signal. To date,
`these problems
`have prevented ATOFs from being adopted into com-
`munication networks. The physical mechanisms of
`AOTFs seem to preclude a good flat-top response.
`Patel, sometimes in conjunction with co-inventors,
`has suggested that liquid-crystal filters be used in such
`WDM communication networks; see, for example, U.S.
`Pat. Nos. 5,111,321 and 5,150,236. Indeed, Patel has
`suggested in U.S. Pat. No. 5,111,321 that a liquid-crystal
`system could be used as a drop-add circuit. However,
`such a system appears difficult to implement.
`Weiner and collaborators have disclosed how an
`optical signal can have its frequency-divided compo-
`nents
`separately
`phase-modulated
`or
`amplitude—
`modulated by using a diffraction grating to divide the
`input signal into spatially separated frequency compo-
`nents which are separately operated upon by a seg—
`mented modulator. See, for example, U.S. Pat. No.
`4,685,547 to Heritage et a1. Patel et al. have applied this
`concept to a system incorporating liquid-crystal modu-
`lators, as disclosed in U.S. Pat. No. 5,132,824.
`The use of diffraction gratings for multiplexing in a
`WDM system has been described by Nishi et al.
`in
`“Broad-passband-width optical filter for multi/demulti-
`plexer using a diffraction grating and a retroreflector
`prism,” Electronics Letters, vol. 21, 1985, pp. 423—424
`and by Shirasaki et al. in “Broadening of bandwidths in
`grating multiplexer by original dispersion—dividing
`prism,” Electronics Letters, vol. 22, 1986, pp. 764—765.
`Nonetheless, the prior art fails to disclose an effec-
`tive, economical optical switch for a WDM telecommu-
`nication system.
`SUMMARY OF THE INVENTION
`
`The invention may be summarized as an optical
`switch, preferably using a segmented liquid-crystal
`modulator. The switch divides an input signal into mul-
`tiple outputs according to the frequency components of
`the input signal. In particular, the input signal is spa-
`tially divided into its frequency components, which pass
`through different segments of a liquid-crystal polariza-
`tion modulator. The different frequency components,
`depending upon their polarization impressed by the
`polarization modulator, are separated by a polarization
`divider. The frequency-divided components are then
`separately recombined according to their polarization,
`thereby producing two or more output signals that have
`been selectively separated according to optical fre-
`quency.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIGS. 1, 2, and 3 illustrate respective horizontal,
`vertical, and isometric views of a polarization-sensitive
`1x 2 switch of the invention.
`FIGS. 4, 5, and 6 illustrate respective horizontal,
`vertical, and isometric views of a polarization—sensitive
`2X2 switch of the invention.
`FIGS. 7, 8, 9, and 10 are graphs of experimental data
`of an embodiment of the invention.
`FIG. 11 is a vertical view of a polarization-insensitive
`embodiment of the invention.
`FIG. 12 is a vertical view of an alternative polariza-
`tion-sensitive embodiment of the invention using Wol—
`Iaston prisms.
`
`

`

`5,414,540
`
`3
`FIG. 13 is a vertical View of an extension of the em-
`bodiment of FIG. 11 that has been made polarization
`insensitive.
`FIG. 14 is a vertical view of a reflective embodiment
`of the switch of the invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`The invention achieves all-optical switching of the
`frequency-multiplexed multi-channel optical signals by
`frequency-dividing an optical input signal into spatially
`separated channels, selectively changing the polariza—
`tion characteristics of the frequency—separated chan-
`nels, further spatially dividing the channels according
`to polarization characteristics, and then recombining
`the channels of similar polarization characteristics.
`Preferably, a segmented liquid—crystal modulator selec-
`tively changes the polarization of the physically sepa—
`rated channels.
`A first, polarization-sensitive embodiment is shown in
`cross—section in FIG. 1 in which a relatively broad-band
`input beam 10 strikes an entrance frequency-dispersive
`medium, such as a diffraction grating 12. It is assumed
`that the input beam 10 is polarized along the x-direction.
`Other active or passive dispersive media are possible,
`such as prisms. The frequency-dispersive medium 12
`divides the broad-band input beam 10 into multiple
`frequency—separated input beams 14 and 16 which are
`spatially separated in the illustrated x—direction. An
`entrance lens 18 focuses the frequency-divided compo—
`nents upon separate segments 20 and 22 of a segmented
`liquid—crystal polarization modulator 24. An entrance
`polarization-dispersive element 26, such as a birefrin-
`gent crystal, such as calcite, is disposed on the entrance
`side to spatially separate the different polarization com-
`ponents of the input beam, but its effects are not evident
`for the first embodiment from FIG. 1 because the input
`beam 10 is assumed to be linearly polarized along the
`x-axrs.
`
`The number of frequency—divided input beams 14 and
`16 and the number of liquid—crystal segments 20 and 22
`depend on the number of WDM components on the
`optical medium (optical fiber) which require switching.
`Four frequency sub-bands provide a meaningful tele—
`communication system. The segments 20 and 22 of the
`segmented liquid-crystal modulator 24 are separately
`controllable to change the polarization direction or
`other polarization characteristic of the physically sepa-
`rated frequency-divided input beams 14 and 16. In the
`simplest case, each segment 20 or 22 either linearly
`rotates the polarization of the properly polarized fre—
`quency—separated input beam 14 or 16 by 90° or does
`not rotate the polarization. A twisted nematic liquid-
`crystal modulator provides such performance.
`After traversing the liquid—crystal modulator 24, the
`frequency-separated beams 14 and 16 traverse the exit
`polarization—dispersive element 28, which, as addition-
`ally illustrated in FIG. 2, further separates the beams 14
`and 16 into their respective polarization components 32,
`34 and 36, 38. An exit lens 30 recollimates the beams.
`An exit frequency-dispersive medium 40, such as an-
`other grating, acts reciprocally to the entrance frequen—
`cy-dispersive medium 123 and recombines frequency-
`and polarization—separated beams into only polariza-
`tion-separated beams 42, 44, which, as will be shown
`later, are spatially separated as well.
`Turning more completely now to the perpendicular
`illustration of FIG. 2, the two frequency beams 14 and
`
`4
`16 are congruent along the x-direction. It is assumed
`that the two input beams 14 and 16 are polarized along
`the x-direction and thus not affected by the entrance
`polarization-dispersive element 26. This assumption
`manifests that the system of FIGS. 1 and 2 is polariza-
`tion sensitive. As a result, the entrance polarization-dis-
`persive element 26 is not required for this polarization-
`sensitive, single-input embodiment. Referring simulta-
`neously to FIGS. 1 and 2 and to an isometric view,
`illustrated in FIG. 3, of the central portion of these
`figures, when the first segment 20 of the segmented
`liquid—crystal modulator 24 is not actively biased,
`it
`rotates by 90° the polarization of the incident beam 14
`of the first frequency such that, when it traverses the
`output polarization-dispersive element 28, it is displaced
`downwardly along the y—axis
`into displaced output
`beam 34 of the first frequency. On the other hand, when
`the first segment 20 is actively biased, it does not rotate
`the polarization of the entrance beam 14 of the first
`frequency. As a result, it traverses the output polariza-
`tion—dispersive element 28 without spatial displacement
`into undisplaced output beam 32 of the first frequency.
`Similarly, active biasing of the second segment 22 ro-
`tates by 90° the polarization of the entrance beam 16 of
`the second frequency, and thus the output polarization-
`dispersive element 28 converts it into displaced output
`beam 38 of the second frequency; while inactive biasing
`leaves its polarization unaffected, and thus the disper-
`sive element 28 converts it into undisplaced output
`beam 36 of the second frequency. The output frequen-
`cy-dispersive element 40 then recombines the undis—
`placed output beams 32 and 36 of both frequencies into
`a combined undisplaced output beam 42 and the dis-
`placed output beams 34 and 38 of both frequencies into
`a combined displaced output beam 44.
`Therefore, the biasing of both of the segments 20 and
`22 of the liquid-crystal modulator 24 determines into
`which output beam 42 and 44 either or both of the
`entrance beams 14 and 16 are directed. That is, a polari—
`zation—sensitive 1X2 switch has been described.
`Referring now to FIGS. 4, 5, and 6, a second input
`fiber outputs a second entrance beam 46, which strikes
`the entrance frequency-dispersive element 12 at a verti-
`cally oblique angle so as to produce from the second
`input fiber multiple angularly separated,
`frequency—
`separated beams 48 and 50. The second entrance beam is
`assumed to be polarized along the y-axis so that the
`entrance polarization-dispersive element 26 deflects it
`along the y-axis. The angular resolution of the input
`frequency-dispersive element 12 and birefringent length
`of the first polarization-dispersive element 26 are such
`that the components of the same frequency from the
`two input beams 10 and 46 are focused upon the same
`segment 20 or 22 of the segmented liquid—crystal modu—
`lator 24. As a result, the respective segmented polariza-
`tion rotator of the liquid-crystal modulator 24 either
`rotates both the WDM components of the same fre-
`quency by the same polarization angle or does not.
`Preferably, the liquid-crystal modulator 24 rotates the
`polarization by 90° or does not rotate it. That is, either
`the linear polarization directions of either beam pair 14,
`48 or 16, 50 are reversed or left intact (within an angular
`factor of 180°).
`The second polarization—dispersive element 28 is ori-
`ented so as to act conversely to the first polarization-dis-
`persive element 26. The beams 32 and 36 polarized
`along the x—axis remain undetected, while the beams 34
`and 38 polarized along the y—axis are deflected by the
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`60
`
`65
`
`

`

`5
`second polarization-dispersive element 28 back toward
`normal propagation path. The exit lens 30, however,
`angulme separates the resultant output beam 44 from
`the output beam 42.
`In the parlance of a drop-add circuit, the input beam
`10 is the IN channel, the input beam 46 is the ADD
`channel, the output beam 42 is the OUT channel, and
`the output beam 44 is the DROP channel.
`By the means of the illustrated circuitry, the frequen-
`cy-dedicated segment 20 or 22 of the liquid-crystal
`modulator 24 determines whether a pair of channels of
`the same frequency on the two multi-frequency input
`fibers are to be switched to different output fibers. Of
`course, the two segments 20 and 22 can be separately
`controlled for the two frequency channels.
`Although only two frequency channels have been
`described, it is understood that more frequency chan-
`nels can be accommodated by a liquid-crystal modula-
`tor 20 having additional separately controlled segments
`along the x-direction.
`The above embodiments are sensitive to polarization
`of their input signals. But, in many cases, the input light
`polarization cannot be controlled. Merely using an
`input polarizer is unsatisfactory because possibly all the
`light may be lost and because the polarization state
`tends to be randomly vary in time, therefore leading to
`polarization-caused intensity fluctuations. However,
`the invention can be made to be polarization insensitive.
`As illustrated in FIG. 11, a first polarization-disper-
`sive element 60, such as a calcite crystal, divides an
`input beam 62 into two polarization—separated beams 64
`and 66, one the ordinary beam 64 and the other the
`extraordinary beam 66. One of the beams, in the illus~
`trated case, the extraordinary beam 66, passes through a
`polarization converter 68, such as a half-wave plate
`which rotates the polarization by 90°, so that both
`beams 64 and 66 have the same well-defined polariza-
`tion characteristic, here a linear polarization along the
`x—axis. The entrance lens 18 focuses both beams 64 and
`66 upon the same segment 20 or 22 of the liquid—crystal
`modulator 24, which simultaneously acts on both beams
`64 and 66, either leaving their polarization intact or
`rotating them or producing a combination between
`beams. The exit polarization-dispersive element 28 then
`spatially separates them according to polarization; if
`unrotated, into beams 80 and 82; if rotated, into beams
`84 and 86. Two more polarization rotators 88 and 90 are
`disposed in two of the beams 82 and 84. The exit lens 30
`recollimates the beams 80—86, and a second polariza—
`tion-dispersive element 92 acts conversely to the first
`one 60 to recombine the beams 80 and 82 into a com-
`bined OUT beam 44 and to recombine the beams 84 and
`86 into a combined DROP beam 96.
`The frequency-dispersed beams are not illustrated but
`are arranged similarly to those of FIG. 4. The embodi-
`ment can be easily extended to a 2X2 drop-add circuit
`having an additional ADD input beam 98 by including
`a polarization rotator 100 for the added input on the
`entrance side.
`The above embodiments have been described in
`somewhat theoretical terms. The following discussion
`involves some of the design considerations. Let f repre-
`sent the focal lengths of the two lenses 18 and 30; d1, the
`lateral shift of the inner polarization-dispersive elements
`26 and 28; d2, the lateral shift of the outer polarization-
`dispersive elements 60 and 92; and L the distance be—
`tween the input polarization—dispersive element 60 and
`its associated lens 18. The switched (extraordinary
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`6O
`
`65
`
`5,414,540
`
`6
`beams) have a virtual focus shifted by d1 from the ordi-
`nary focus. The extraordinary and ordinary beams
`therefore form an angle of d1/f with respect to the input
`and output ordinary beams. If f: 100 mm and d: 100
`mm, the angle is 0.02 rad or about 1°. The main ordinary
`input beam is assumed to define x=0 for each fre-
`quency. The ordinary beam is then at x=~d2. The
`ordinary and extraordinary beams of the ADD (or
`DROP) channel at the lens 18 or 30 are located at
`x=d1 and x=d2—-d1, respectively. At the external crys-
`tals,
`these
`beams
`are
`at
`x=ld1/f—d1
`and
`x: ld1/f—d1—s.
`For the beams to overlie at that point, it is required
`that L=f.
`
`The preceding embodiments have used a calcite crys-
`tal or similar uniaxial medium for the polarization-dis-
`persive element. Wollaston prisms offer an advanta-
`geous alternative design. Such prisms have two prisms
`of calcite, for example, separated by a thin layer of
`material having a refractive index intermediate between
`the refractive indices of the ordinary and extraordinary
`refractive indices of the calcite. The two component
`prisms are oriented such that one of the rays is totally
`internally reflected by the intermediate thin layer. The
`result is that the ordinary and extraordinary rays are
`angularly separated.
`A polarization-sensitive embodiment utilizing Wol-
`laston prisms is illustrated in FIG. 12. The perpendicu-
`lar construction is very similar to that of FIG. 4. The
`entrance and exit calcite crystals 26 and 28 of FIGS. 1,
`2, and 3 are replaced by entrance and exit Wollaston
`prisms 110 and 112. Their birefringent thicknesses and
`the focal lengths of the two lenses 18 and 30 are ar-
`ranged such that the two optical input beams 14 and 16,
`the IN and ADD beams, are focused to the interface of
`the entrance Wollaston prism 110 having such a length
`that both beams 14 and 16 (of differing polarizations)
`then are congruent as they pass the liquid-crystal modu-
`lator 24. Preferably, the input beams 14 and 16 can be
`made parallel. Similar design factors on the output side
`allow the two output beams 42 and 44, the OUT and
`DROP beams, to be parallel.
`EXAMPLE 1
`
`We have constructed and tested a switch according
`to the above embodiment. It was designed to switch one
`or more of six channels having 4 nm spacing between
`the channels and to have a wavelength resolution of 2
`nm. The liquid-crystal modulator was filled with com-
`mercially available E7 nematic liquid crystal and was
`twisted by 90°. The polarization-dispersive element was
`a Wollaston prism. Many of the details of fabrication are
`found in the parent patent application and the various
`cited patents to Patel. The design of the switch was
`optimized for 1.5 pm. In an experimental prototype, we
`have shown an extinction ratio of at least 35 dB between
`the switched and unswitched states of the polarizers. In
`FIGS. 7 and 8 are shown the optical power spectra on
`the unswitched output channel and the switched output
`channel respectively when no switching is performed.
`That is, FIG. 8 shows the residual power in the four
`unswitched channels. The power levels indicated on the
`vertical scale are somewhat arbitrary and reflect an 8
`dB system loss. In FIGS. 9 and 10 are shown the optical
`spectra of the unswitched and switched outputs respec-
`tively when the first and third channels are switched. It
`is thus seen that
`the inventive system effectively
`switches the WDM channels.
`
`

`

`7
`The embodiment of Wollaston prisms can be made
`insensitive to polarization, as illustrated in FIG. 13, by
`including the first and second polarization-dispersive
`elements 60 and 92, preferably calcite crystals or similar
`material, on the input and output ends. Half-wave plates
`120, 122, and 124 are placed in the path of the laterally
`displaced beams and in the path of both of the input
`ADD beams. The wide half—wave plate 124 causes the
`IN and ADD beams to have differing polarizations as
`they congruently pass through a segment of the liquid-
`crystal modulator 24. Similarly, half-wave plates 126,
`128, and 130 are placed in the to-be—displaced output
`beams and both of the DROP beams.
`The number of pans can be significantly reduced by
`using a reflector and operating in the retro-reflector
`mode. As illustrated in FIG. 14, the input beam 14, after
`diffracting from the grating (not shown), strikes the lens
`18 off-center and is refracted obliquely to the principal
`optical axis. Because it is polarized along the x-direc-
`tion, it passes undetected through the polarization—dis—
`persive element 26, which may be calcite or a Wollaston
`prism. It then passes through one segment of the seg-
`mented liquid—crystal polarization modulator system
`140, which differs from the previously described liquid-
`crystal polarization modulators in that it selectively
`rotates the light polarization by 90° only after a double,
`back-and-forth pass. The light is then reflected from a
`mirror 142 and again traverses the polarization modula-
`tor 140. The polarization of light traversing actively
`biased segments of the modulator 140 is not rotated
`while that of light traversing inactively biased segments
`is rotated by a total of 90°. The light with rotated polar-
`ization is displaced by the polarization-dispersive ele-
`ment 26 and, after diffraction, is output as a first output
`beam 144 while the light with unrotated polarization is
`output as a second output beam 146. The two output
`beams 144 and 146 are angularly displaced so as to be
`easily separated physically.
`The second input beam 46, assumed to be polarized
`along the y-direction strikes the lens 18 obliquely with
`respect to the first input beam 14 but in the same general
`off-axis location. Because of their assumed different
`polarizations,
`the polarization-dispersive element 26
`affects them conversely, but the segmented polarization
`modulator 140 simultaneously rotates (or does not ro-
`tate) both of their polarization states. In the backward
`propagation,
`the diffraction grating recombines the
`optical frequency carriers into the desired ADD and
`DROP channels, as determined by the segmented polar-
`ization modulator 140.
`The optical switch of FIG. 14 can be made frequency
`insensitive using techniques described for the other
`embodiments.
`The frequency dispersion at the liquid-crystal modu-
`lator of the invention allows the modulator to simulta-
`neously change the phase and/or amplitude of the dif-
`ferent frequency components of the signals. Such ad-
`justment is particularly advantageous to additionally
`compensate for the frequency dispersion of the optical
`fiber or to equalize amplitudes between different chan-
`nels.
`Although the described embodiments have placed
`the frequency-dispersive elements on the outside of the
`polarization-dispersive elements, it is recognized that
`the two dispersions can be performed in the opposite
`order and even simultaneously.
`The invention can thus be used in a number of related
`configurations, all of which are useful for providing an
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`60
`
`65
`
`5,414,540
`
`8
`economical, all-optical multi-frequency switch. When
`the polarization modulator is a segmented liquid-crystal
`modulator, the system is both easy to construct, and the
`modulator has transfer characteristics consistent with a
`relaxed system design.
`What is claimed is:
`1. An optical switch, comprising:
`a frequency—dispersive element receiving an input
`beam and dispersing it into a plurality of first beams
`according to frequency;
`a polarization-dispersive element receiving said first
`beams
`and outputting corresponding second
`beams;
`a segmented liquid—crystal polarization modulator
`receiving said second beams on respective seg-
`ments thereof and selectively rotating polarizations
`thereof to form third beams; and
`a reflector reflecting said third beams back through
`said polarization modulator, said polarization-dis-
`persive element, and said frequency-dispersive ele-
`ment.
`
`2. An optical switch comprising
`an entrance frequency dispersive element for receiv—
`ing first and second input optical signals and dis—
`persing them into dispersed beams according to the
`frequencies thereof,
`a focusing lens receiving said dispersed optical beams,
`a segmented polarization modulator having multiple
`individually controlled segments and positioned
`essentially at the focal length of said focusing lens
`for selectively controlling polarization characteris-
`tics of individual elements of said dispersed optical
`beams,
`a first polarization—dispersive element positioned be-
`tween said focusing lens and said segmented polar-
`ization modulator,
`a second polarization-dispersive element positioned
`to the other side of said segmented polarization
`modulator than said first polarization-dispersive
`element for receiving the dispersed outputs of said
`segmented polarization modulator and for spatially
`displacing individual elements of said dispersed
`outputs dependent on the polarization thereof,
`an exit lens for receiving the outputs of said second
`polarization dispersive element, said exit lens being
`essentially its focal distance away from said seg-
`mented polarization modulator, and
`an exit frequency dispersive clement receiving the
`dispersed outputs from the second focusing lens
`and combining frequency components thereof into
`separate output optical signals.
`3. An optical switch in accordance with claim 2
`wherein said segmented polarization modulator is a
`liquid-crystal modulator.
`4. An optical switch in accordance with claim 3
`wherein said entrance and exit dispersive elements are
`gratings.
`5. An optical switch in accordance with claim 2
`wherein said first and second polarization-dispersive
`elements are birefringent crystals.
`6. An optical switch in accordance with claim 2
`wherein said first and second polarization-dispersive
`elements are Wollaston prisms, the focal length of said
`focusing lens being at the interface of said first polariza-
`tion-dispersive Wollaston prism.
`7. An optical switch in accordance with claim 6 fur-
`ther comprising a third polarization-dispersive element
`in front of said entrance frequency dispersive element
`
`

`

`9
`and a fourth polarization-dispersive element behind said
`exit frequency dispersive element, the input to said third
`polarization-dispersive element being said first and sec-
`ond input signals and the output from said third polari-
`zation-dispersive element being a pair of beams for each
`of said first and second input signals, one beam of each
`pair being laterally displaced dependent on polarization,
`and a half-wave plate positioned in the path of only one
`of each pair of beams.
`8. An optical switch in accordance with claim 7
`wherein said third and fourth polarization-dispersive
`elements are birefringent crystals.
`9. An optical switch in accordance with claim 7
`wherein said half-wave plates are positioned in the path
`of each of said laterally displaced beams only.
`10. An optical switch in accordance with claim 7
`further comprising a further half-wave plate adjacent
`the side of said focusing lens receiving said dispersed
`optical beams but in the path of only one of said pair of
`beams.
`11. An optical switch in accordance with claim 10
`further comprising a half-wave plate adjacent the input
`side of said fourth polarization dispersive element in the
`path of each of said laterally displaced beams only.
`12. An optical switch comprising in sequence
`a frequency dispersive element for receiving an input
`optical signal and dispersing it into a dispersed
`optical beam according to the frequencies thereof,
`a focusing lens receiving said dispersed optical beam,
`a polarization-dispersive element,
`a segmented pol