(12) United States Patent
`Ranalli et al.
`
`US006285500B1
`(10) Patent N0.2
`US 6,285,500 B1
`(45) Date of Patent:
`Sep. 4, 2001
`
`(54) WAVELENGTH SELECTIVE SWITCH
`
`(75) Inventors: Eliseo R. Ranalli, Irvine; Bradley A.
`Scott, Huntington Beach, both of CA
`(Us)
`
`(73) Assignee: Corning Incorporated, Corning, NY
`
`4,917,452
`5,377,026
`5,414,540
`576067439
`5,694,233
`5,724,165
`5’867’291
`
`4/1990 Liebowitz .
`12/1994 Liu et al..
`5/1995 Patel 61 ‘IL -
`2/1997 Wu -
`12/1997 Wu et al. .
`3/1998 Wu .
`2/1999 Llu et a1‘ '
`
`(Us)
`
`FOREIGN PATENT DOCUMENTS
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`WO9833289
`WO9835251
`
`7/1998 (W0) _
`8/1998 (WO) .
`
`* cited by examiner
`
`(21) Appl. No.: 09/450,142
`(22) Filed;
`NOV_ 29, 1999
`
`Related US. Application Data
`(60) Provisional application No. 60/141,556, ?led on Jun. 29,
`1999'
`7
`Int. Cl. ..................................................... .. G02B 5/30
`51
`(52) us. Cl. ........................ .. 359/497; 359/127; 359/494;
`359/499; 385/11
`(58) Field of Search ................................... .. 359/124, 127,
`359/128, 130, 246, 494, 495, 497, 499;
`385/11
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`Primary Examiner—Darren Schuberg
`(74) Attorney, Agent, or Firm—Daniel P. Malley
`
`ABSTRACT
`(57)
`A recon?gurable bi-directional Wavelength selective sWitch
`'d'ldIh
`p'ly h'y 'b
`15 156 056 .
`t as ano tlca s stemt at lss mmetrlca out
`a polarization modulator The Symmetric Optical System
`consists of an input birefringent optical system and output
`birefringent Optical System disposed around Polarization
`modulator. The optical system delivers the Wavelength chan
`nels that are to be sWitched as a superimposed Wavelength
`channel incident the polarization modulator. As a result,
`crosstalk is reduced beloW —35 dB and greater optical
`performance is achieved.
`
`4,783,851 * 11/1988 Inou et al. ......................... .. 359/495
`
`35 Claims, 9 Drawing Sheets
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`FNC 1043
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`U.S. Patent
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`Sep. 4, 2001
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`FIG. 3
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`/32
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`signal =
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`FIG. 4
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`Sheet 9 0f 9
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`f 10
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`100
`/
`
`Input
`Fiber 1
`(Through channel
`trunking)
`'"Put
`Fiber2
`
`Output
`Fiber 1
`
`WaveIength
`selotctlire
`SW' 0
`(WSS)
`
`Output
`Fiber2
`
`110
`
`WDM
`(Add)
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`WDM
`(Drop)
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`

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`US 6,285,500 B1
`
`1
`WAVELENGTH SELECTIVE SWITCH
`
`2
`SUMMARY OF THE INVENTION
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`This Application claims the bene?t of priority under 35
`U.S.C. §119(e) for Us. Provisional Patent Application Ser.
`No. 60/141,556 ?led on Jun. 29, 1999, the content of Which
`is relied upon and incorporated herein by reference in its
`entirety.
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The present invention relates generally to optical
`sWitches, and particularly to Wavelength selective sWitches
`using a polariZation rotating device.
`2. Technical Background
`In the past tWo-decades ?ber optics have transformed the
`telecommunications market place. Initially, netWork designs
`included relatively loW-speed transceiver electronics at each
`end of the communications link. Light signals Were sWitched
`by being converted into electrical signals, sWitched
`electronically, and reconverted into light signals. The band
`Width of electronic sWitching equipment is limited to about
`10 GHZ. On the other hand, the bandWidth of single mode
`optical ?ber in the 1550 nm region of the electromagnetic
`spectrum is in the TerahertZ range. As the demand for
`bandWidth increases exponentially, netWork designers have
`sought Ways to exploit the available bandWidth in the 1550
`nm region. Thus, a need exists for optically transparent
`cross-connects and sWitches.
`One approach that has been considered involves a
`frequency-selective optical sWitch employing a polariZation
`beam splitter, Wollaston prism and a liquid crystal sWitch
`element. HoWever, this design has a major draWback. The
`polariZing beam splitter, Which is used to recombine the
`beams, is alWays located betWeen the focusing lens and the
`spatial light modulator. One effect of this is that the polar
`iZing beamsplitter must be able to accept a large acceptance
`angle, Which leads to poorly superimposed beams if bire
`fringent crystals are used. If beamsplitting cubes are used
`contrast ratio is reduced and crosstalk is increased. This Was
`addressed by using a Wollaston Prism. Wollaston Prisms are
`designed to convert a collimated beam of mixed polariZation
`into tWo de?ected collimated beams, Which are separated by
`an angle that is roughly bisected by the optical axis of the
`original mixed polariZation beam. This solves many of the
`problems associated With placing the polariZing beam sepa
`rator betWeen the focusing lens and the LC sWitch element,
`but there are substantial problems associated With using
`Wollaston Prisms. The most signi?cant of these lies is the
`fact Wollaston Prisms cannot produce beams that are exactly
`symmetrically de?ected. Because the effect of the Wollaston
`Prism is not symmetrical, the beams cannot be superim
`posed at the LC sWitch element. Thus, the positions of the
`beams at the LC sWitch element must be balanced With the
`differing angles of incidence at the LC sWitch element to
`minimiZe crosstalk and insertion loss variation for the dif
`ferent sWitched states. Due to this asymmetry, the optical
`system must groW to unattractively long lengths in order to
`achieve acceptable crosstalk With an acceptable channel
`bandWidth.
`Thus, What is needed is a Wavelength selective sWitch
`having an optical system that is symmetric about a polar
`iZation modulator and capable of delivering superimposed
`beams at the polariZation modulator in order to reduce
`crosstalk, reduce insertion loss, and improve spectral reso
`lution.
`
`10
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`25
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`30
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`35
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`40
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`A Wavelength selective sWitch is disclosed that includes
`an optical system that is symmetric about a polariZation
`modulator and capable of delivering superimposed beams at
`the polariZation modulator in order to reduce crosstalk,
`reduce insertion loss, and improve spectral resolution.
`One aspect of the present invention is an optical device for
`selectively directing a ?rst signal and a second signal to a
`selected output. The optical device includes: a birefringent
`optical system having a system input that receives the ?rst
`signal and the second signal, and a system output to Which
`the birefringent optical system transmits a superimposed
`signal formed by superimposing a ?rst polariZed signal and
`a second polariZed signal, Wherein the ?rst polariZed signal
`and the second polariZed signal are polariZed versions of the
`?rst signal and the second signal, respectively; and a polar
`iZation modulator coupled to the system output, Whereby the
`polariZation modulator selectively rotates a polariZation
`state of the superimposed signal.
`In another aspect, the present invention includes an opti
`cal device for selectively directing a ?rst signal and a second
`signal to a selected output. The optical device includes a ?rst
`polariZation beam splitter for separating the ?rst signal and
`second signal into ?rst signal polariZation components and
`second signal polariZation components, respectively. A ?rst
`half-Wave retarder is coupled to the polariZation beam
`splitter, the ?rst half-Wave retarder causes all of the ?rst
`signal polariZation components and the second signal polar
`iZation components to be uniformly polariZed in a ?rst
`polariZation state. A ?rst grating is coupled to the ?rst
`half-Wave retarder, for producing a plurality of ?rst signal
`Wavelength channels and a plurality of second signal Wave
`length channels. A second half-Wave retarder is coupled to
`the ?rst grating, for causing the plurality of second signal
`Wavelength channels to be uniformly polariZed in a second
`polariZation state. A ?rst optical compensator is coupled to
`the ?rst grating, for causing an optical distance of the
`plurality of ?rst signal Wavelength channels to be substan
`tially equal to an optical distance of the plurality of second
`signal Wavelength channels. A ?rst polariZation beam com
`biner is coupled to the optical compensator and the second
`half-Wave retarder, for combining the plurality of ?rst signal
`Wavelength channels and the plurality of second signal
`Wavelength channels into a plurality of superimposed Wave
`length channels. A focusing lens is coupled to the polariZa
`tion beam combiner; and an array of polariZation modulators
`coupled to the focusing lens, each of the modulators has a
`sWitch state, Wherein each superimposed Wavelength chan
`nel is focused onto a predetermined modulator.
`In another aspect, the present invention includes a method
`for selectively directing a ?rst signal and a second signal to
`a selected output in an optical device. The method includes
`the folloWing steps. Providing a polariZation modulator.
`Converting the ?rst signal into at least one ?rst polariZed
`component and the second signal into at least one second
`polariZed component. Superimposing the at least one ?rst
`polariZed component With the at least one second polariZed
`component to form a superimposed signal, Wherein the at
`least one ?rst polariZed component and the at least one
`second polariZed component are co-linear in at least one axis
`direction; and focusing the superimposed signal onto the
`polariZation modulator.
`In another aspect, the present invention includes a method
`for selectively directing a ?rst signal and a second signal to
`a selected output in an optical device that includes a bire
`fringent optical system. The method includes the folloWing
`
`

`
`US 6,285,500 B1
`
`3
`steps. Providing an array of liquid crystal pixels, wherein
`each of the liquid crystal pixels includes a sWitch state.
`Demultiplexing the ?rst signal and the second signal to
`thereby form a plurality of ?rst signal Wavelength channels
`and a plurality of second signal Wavelength channels,
`respectively. Superimposing each ?rst signal Wavelength
`channel over its corresponding second signal Wavelength
`channel to thereby form a plurality of superimposed Wave
`length channels; and focusing each superimposed Wave
`length channel onto a predetermined liquid crystal pixel.
`The features and advantages of the invention Will be set
`forth in the detailed description Which folloWs, and in part
`Will be readily apparent to those skilled in the art from that
`description or recogniZed by practicing the invention as
`described herein, including the detailed description Which
`folloWs, the claims, as Well as the appended draWings.
`It is to be understood that the folloWing detailed descrip
`tion is merely exemplary of the invention, and are intended
`to provide an overvieW or framework for understanding the
`nature and character of the invention as it is claimed. The
`accompanying draWings are included to provide a further
`understanding of the invention, and are incorporated in and
`constitute a part of this speci?cation. The draWings illustrate
`various embodiments of the invention, and together With the
`description serve to explain the principles and operation of
`the invention.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a block diagram shoWing on overvieW of the
`Wavelength Selective SWitch (WSS) according to a ?rst
`embodiment of the present invention;
`FIG. 2 is a schematic of the WSS depicted in FIG. 1;
`FIG. 3 is a diagram shoWing a parallel plate beamsplitter
`in accordance With the present invention;
`FIG. 4 is a diagram shoWing an a thermaliZed grating in
`accordance With the present invention;
`FIG. 5 is a diagram shoWing the polariZation management
`architecture of the WSS depicted in FIGS. 1 and 2;
`FIG. 6 is a perspective vieW of the mechanical design of
`the WSS in accordance With a second embodiment of the
`present invention;
`FIG. 7 is a plot shoWing the channel pro?les of the WSS
`of the present invention;
`FIG. 8 is a plot shoWing the broadband ripple of a
`40-channel WSS of the present invention;
`FIG. 9 is a plot shoWing the broadband ripple of a
`80-channel WSS of the present invention; and
`FIG. 10 is a block diagram of a WADM that incorporates
`the WSS in accordance With a third embodiment of the
`present invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`Reference Will noW be made in detail to the present
`preferred embodiments of the invention, examples of Which
`are illustrated in the accompanying draWings. Wherever
`possible, the same reference numbers Will be used through
`out the draWings to refer to the same or like parts. An
`exemplary embodiment of the Wavelength selective sWitch
`(WSS) of the present invention is shoWn in FIG. 1, and is
`designated generally throughout by reference numeral 10.
`In accordance With the invention, a Wavelength selective
`cross-connect sWitch is provided having an optical system
`that is symmetric about a polariZation modulator, and
`
`10
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`20
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`25
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`30
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`4
`capable of delivering superimposed beams at the polariZa
`tion modulator 20 in order to reduce crosstalk, reduce
`insertion loss, and improve spectral resolution to thereby
`achieve high optical throughput. The present invention for a
`Wavelength selective sWitch (WSS) includes a birefringent
`optical system that transmits a superimposed signal to the
`polariZation modulator. The superimposed signal is formed
`by superimposing a parallel polariZed signal from a ?rst
`input ?ber With an orthogonal polariZed signal from a
`second input ?ber.
`As embodied herein and depicted in FIG. 1, WSS 10
`according to the ?rst embodiment of the present invention is
`disclosed. Input ?ber 1 and input ?ber 2 are connected to
`input port 12. Input port 12 is connected to input birefringent
`optical system 30. Input birefringent optical system 30 is
`optically coupled to polariZation modulator, Which sWitches
`the incident light beam in accordance to the sWitch state as
`determined by netWork command (not shoWn). PolariZation
`modulator 20 is connected to output birefringent optical
`system 50 Which routes the output light beams to output port
`14. Output port 14 is connected to output ?ber 1 and output
`?ber 2.
`Output birefringent optical system 50 is the mirror image
`of input birefringent optical system 30. Thus, WSS 10 is a
`recon?gurable bi-directional Wavelength selective sWitch.
`The birefringent optical system, Which consists of input
`birefringent optical system 30 and output birefringent opti
`cal system 50, is exactly symmetric about polariZation
`modulator 20. WSS 10 as depicted in FIG. 1 is a 2x2
`Wavelength Selective SWitch.
`Input ?ber 1 and input ?ber 2 provide WSS 10 With
`randomly polariZed light signals having multiple Wave
`length channels. In a ?rst embodiment, WSS 10 accommo
`dates 40 Wavelength channels at 100 GHZ spacing betWeen
`channels. In an alternate embodiment, WSS 10 accommo
`dates 80 Wavelength channels at 50 GHZ spacing betWeen
`channels. Any individual channel may be selectively
`sWitched betWeen input ?ber 1 and input ?ber 2. WSS 10
`operates by converting the Wavelength channels from input
`?ber 1 into s-polariZed (perpendicular) signals and the
`second ?ber Wavelength channels into p-polariZed (parallel)
`signals. One of ordinary skill in the art Will recogniZe that
`the p-polariZed signals and the s-polariZed signals are
`orthogonal one to the other. The p-polariZed signals and the
`s-polariZed signals are then superimposed and focused on
`polariZation modulator. Thus, traf?c carried by the input
`?bers is identi?ed by its polariZation state. PolariZation
`modulator 20 rotates the polariZation state of the superim
`posed signal by 90° When sWitching channels betWeen ?bers
`and doesn’t rotate the polariZation state When a given
`channel is passed through the sWitch. After sWitching, the
`output birefringent optical system 50 re-multiplexes the
`Wavelength channels according to their polariZation state
`and maps s-polariZed output channels (as polariZed after
`leaving polariZation modulator 20) to the output ?ber 1 and
`maps p-polariZed output channels (as polariZed after leaving
`polariZation modulator 20) to the output ?ber 2. Because of
`the symmetrical design, this convention can be reversed.
`The operation of WSS 10 Will be discussed in more detail
`beloW.
`As embodied herein and depicted in FIG. 2, a schematic
`of WSS 10 according to a ?rst embodiment of the present
`invention is disclosed. Input ?ber 1 and input ?ber 2 are
`connected to WSS 10 at input 12. The light signal from ?ber
`1 and ?ber 2 are collimated by collimator 120. Collimator
`120 is connected to polariZing beam splitter 32. PolariZing
`beam splitter 32 is connected to half-Wave plate 34. A
`
`

`
`US 6,285,500 B1
`
`5
`fold-mirror 36 is optically coupled to polarization beam
`splitter 32 and half-Wave plate 34 causing the light signals
`to be re?ected toward grating 38. As depicted in subsequent
`embodiments, fold-mirror 36 can be eliminated and the
`optical signal is directed from half-Wave plate 34 to grating
`38. Grating 38 demultiplexes the ?rst ?ber light signal and
`the second ?ber light signal into its constituent Wavelength
`channels. Half-Wave plate 40 and optical compensator 42 are
`coupled to the grating. Half-Wave plate 40 provides an
`optical path for the second ?ber Wavelength channels. Opti
`cal compensator 42 provides an optical path for the ?rst ?ber
`Wavelength channels. The function of these elements Will be
`discussed in more detail beloW. Half-Wave plate 40 and
`optical compensator 42 are optically coupled to polarization
`beam combiner 44. Polarization beam combiner 44 super
`imposes the ?rst ?ber Wavelength channels coming from
`half-Wave Wave plate 40 and the second ?ber Wavelength
`channels coming from optical compensator 42. Focusing
`lens 46 is optically coupled to polarization beam combiner
`44 and is used to focus each superimposed Wavelength
`channels exiting polarization beam combiner 44 onto its
`respective polarization modulating cell 22 of polarization
`modulator 20.
`As discussed above, output birefringent optical system 50
`is a mirror image of input birefringent optical system 50.
`Polarization modulator 20 is connected to focusing lens 66.
`Focusing lens 66 is coupled to polarization beam splitter 64.
`Polarization beam splitter separates the superimposed output
`channels into an output ?ber 1 Wavelength channel and an
`output ?ber 2 Wavelength channel. Polarization beam split
`ter is coupled to half-Wave plate 60 and optical compensator
`62. Optical compensator 62 adjusts the optical path length of
`output ?ber 1 Wavelength channels. Output ?ber 2 Wave
`length channels propagate through half-Wave plate 60. Out
`put ?ber 1 Wavelength channels and output ?ber 2 Wave
`length channels are multiplexed by grating 58. Grating 58 is
`coupled to fold-mirror 56 Which directs a portion of the
`output signals through half-Wave plate 54. Half-Wave plate
`54 is coupled to polarization beam combiner 52 Which forms
`output signal 1 and output signal 2. Output signal 1 and
`output signal 2 are collimated by collimator 140 and directed
`into the ?rst output ?ber and the second output ?ber,
`respectively.
`Polarizing beam splitters 32 and 64, and polarizing beam
`combiners 44 and 52, may be of any suitable type, but there
`is shoWn by Way of example, in FIG. 3 beamsplitter 32
`having a single plate 320 of light-transmitting material. Plate
`320 has parallel sides. An antire?ection coating 326 is
`disposed on the light incident side of input signals 1 and 2.
`Beamsplitting coating 322 is disposed on the light exiting
`side of plate 320. Coating 320 alloWs s-polarized light to
`pass through While internally re?ecting p-polarized light.
`The p-polarized light is re?ected by re?ective coating 324.
`Subsequently, the p-polarized light exits the slab in a beam
`that is parallel to the s-polarized light. This approach pro
`vides arcsecond tolerances, is inexpensive and can be imple
`mented in one part. Beam splitters 32 and 64, and polarizing
`beam combiners 44 and 52, have been arranged so that all
`separation and recombination functions occur orthogonal to
`the color dispersion axis (tilted axis of the grating), Which
`simpli?es the optical distance compensation required to
`minimize insertion loss and insertion loss variation due to
`sWitch state. This arrangement improves optical perfor
`mance because the optical path distance differences betWeen
`the grating and focusing lens is made identical for all
`con?gurations. In addition, beam combiner 44 and splitter
`64 are disposed betWeen the grating and the focusing lens.
`
`6
`This innovation provides improved optical performance and
`eliminates asymmetries associated With the Wollaston
`Prism, typically found in other designs. Examples of such
`beamspliiter/combiner devices are disclosed in Provisional
`Patent Application No. 60/153,913 Which is herein incorpo
`rated by reference.
`One of ordinary skill in the art Will recognize that beam
`splitting cubes, birefringent plates, and prisms, in addition to
`thin ?lm ?lters, can also be used depending on the desired
`tolerances, package size, expense, and mounting require
`ments. Although the cube approach is more expensive, these
`devices can be mounted on optical surfaces and have a
`smaller package size.
`Gratings 38 and 58 may be of any suitable type, but there
`is disclosed, by Way of example in FIG. 4, an athermalized
`grism 78 that includes input grating 38 and output grating 58
`in one package as depicted in FIG. 6. In this embodiment,
`input grating 38 is replicated onto substrate 386 and mated
`to prism 382 by epoxy 384. The CTE of the grating spacing
`is intermediate betWeen the CTE of the prism material and
`the CTE of the substrate material. By varying the thickness
`of the substrate material the CTE of the grating spacing can
`be controlled. The glass used for the prism should have a loW
`dn/dt. For example, prism 382 can be implemented using
`Ohara glass type S-TIL6 and Corning ULE glass for the
`substrate 386 (586). The angle of the light entrance face is
`90° and the exit face angle is 50.42°. One advantage of using
`this approach is that all components are physically linked,
`making alignment signi?cantly easier, ensuring that the
`angular relationships Will not signi?cantly change With
`temperature. Examples of such athermalized devices are
`disclosed in Provisional Patent Application No. 60/ 153,913
`Which is herein incorporated by reference. One of ordinary
`skill in the art Will recognize that any standard diffraction
`grating system or grisms can be used depending on the level
`of athermalized performance required by the system.
`Optical compensators 42 and 62 may be of any suitable
`type, but there is disclosed, by Way of example, a polished
`plate of glass having a precise thickness. HoWever, any
`optical design or material that causes the optical path lengths
`traveled by the ?rst ?ber signal and the second ?ber signal
`to be very nearly equal. For any beam combiner and
`half-Wave retarder, optical compensators 42 and 62 are
`designed such that the Wavelength channels from input ?ber
`1 and input ?ber 2 may be exactly superimposed in angle
`and in space. This is achieved by choosing the thickness and
`material of the optical compensator, to satisfy the folloWing
`equation:
`
`10
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`15
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`20
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`25
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`30
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`35
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`40
`
`45
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`o
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`55
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`60
`
`65
`
`Where To is the thickness of optical compensator 42 (62), no
`is the optical index of compensator 42 (62), na is the index
`of air, H is the difference in the distance traveled by the light
`from ?ber input 1 as compared to input ?ber 2 Within the
`beam combiner, nbs is the index of the beam combiner
`material, Tr is the thickness of the retarder, and nr is the
`optical index of the retarder material.
`Polarization modulator 20 may be of any suitable type,
`but there is shoWn by Way of example a linear liquid crystal
`device consisting of an array of pixels represented by
`reference numerals 22, 24, 26, and 28. In a 40 Wavelength
`channel system, array 20 Will consist of 40 sWitch cells 22.
`As depicted, each sWitch cell 22 is a tWisted nematic liquid
`crystal device having liquid crystal molecules aligned in a
`
`

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`US 6,285,500 B1
`
`7
`twisted helix arrangement. One of ordinary skill in the art
`Will recognize that the amount of rotation is dependent on
`the design of the liquid crystal helix arrangement and the
`temperature. As designed, the tWisted helix con?guration
`causes the polarization state of an incident light signal to
`rotate 90° by adiabatic folloWing When no voltage or a
`relatively loW voltage is applied to the device. For example,
`a relatively loW voltage may be applied to compensate for
`temperature. The amount of rotation can be varied incre
`mentally by applying a variable voltage to the liquid crystal
`pixel. In this scenario, W55 10 Would function as a variable
`optical attenuator. As is Well knoWn in the art, When a
`sufficient voltage (approximately 10V or greater) is applied,
`the helical arrangement formed by the liquid crystal mol
`ecules is disrupted and the polarization state of an incident
`light signal is passed through substantially unchanged. Thus,
`in an off-voltage sWitch state, or relatively loW-voltage state,
`the polarization state of an incident light signal is rotated by
`1/2 Wave and p-polarized signals become s-polarized signals,
`and vice-versa In an on-voltage state, the polarization state
`is not rotated.
`One of ordinary skill in the art Will recognize that other
`polarization modulating devices can be used such as bire
`fringent dependent crystals that have a variable birefrin
`gence dependent on the applied voltage. These crystals
`employ the same effect that is used by the liquid crystal
`device. One of ordinary skill in the art Will also recognize
`that ferroelectric liquid crystal rotators, magneto-optical
`Farady rotators, acousto-optic rotators, and electro-optical
`rotators may also be employed as polarization modulator 20.
`FIG. 5 illustrates the operation of W55 10 from a polar
`ization management perspective. Polarizing beamsplitter 32
`separates input signals from the ?rst ?ber and second ?ber
`into their parallel and orthogonal signal components. Thus,
`four beamlets (1s, 1p, 2s, 2p) exit beamsplitter 32. One of
`ordinary skill in the art Will recognize that the convention
`used to number input ?bers 1 and 2 is arbitrary and thus, can
`be reversed. As depicted, the p-polarized components from
`the ?rst ?ber signal and the second ?ber signal (1p, 2p) pass
`through half-Wave plate 34. One could reverse this conven
`tion and pass the orthogonal component through half-Wave
`plate 34. Either Way, after passing through half-Wave plate
`34, all four beamlets (1s, 1s, 2s, 2s) have the same polar
`ization state. Grating 38 throughput is dependent on the
`polarization state of the incident beams. Thus, the uniform
`polarization is implemented to maximize grating 38
`throughput and eliminate polarization dependent loss (PDL).
`Grating 38 demultiplexes the Wavelengths being carried by
`the four beamlets, to create Wavelength diversity. One
`skilled in the art Will recognize that each Wavelength carried
`by the beamlets is a separate communications channel
`carrying its oWn information payload. For each Wavelength
`channel de?ned for ?ber 1, there is a corresponding Wave
`length channel in ?ber 2. The corresponding Wavelength
`channels in ?ber 1 and ?ber 2 are occupied by substantially
`the same set of Wavelengths. HoWever, it is recognized that
`the information payload carried by the corresponding Wave
`length channels is different. By sWitching corresponding
`Wavelength channels betWeen ?ber 1 and ?ber 2, their
`respective information payloads are also sWitched betWeen
`?ber 1 and ?ber 2.
`The tWo polarized beamlets derived from the second ?ber
`signal passes through half-Wave plate 40 creating polariza
`tion diversity. Thus, the ?rst ?ber Wavelength channels,
`Which do not pass through half-Wave plate 40 remain
`s-polarized (1s, 1s), Whereas the second ?ber Wavelength
`channels are polarized (2p, 2p).
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`10
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`15
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`20
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`25
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`30
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`35
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`40
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`45
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`55
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`60
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`65
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`8
`One salient feature of the invention is that, absent optical
`compensator 42, the ?rst ?ber Wavelength channels Would
`travel a shorter physical distance. First ?ber Wavelength
`channels are passed through optical compensator 42 to
`equalize the optical distances of the ?rst ?ber Wavelength
`channels and the second ?ber Wavelength channels. Optical
`distance is de?ned as the distance traveled by the light
`signal, divided by the refractive index of the propagation
`medium. This differs from the term “optical path length,
`Which is de?ned as the distance traveled by the light signal,
`multiplied by the refractive index of the propagation
`medium. Signals that are corrected to have the same optical
`path length behave the same temporally, Whereas signals
`corrected to have the same “optical distance” behave the
`same optically.
`Optical compensator 42 also reduces dispersion created
`by grating 38. The dispersion of the Wavelength channels
`created by the grating is smaller Within optical compensator
`42 as compared to the dispersion in air. Thus tWo sets of
`s-polarized Wavelength channels that propagate through
`optical compensator 42 travel a longer physical distance
`from grating 38 to beam combiner 44 than do the tWo sets
`of p-polarized Wavelength channels that do not propagate
`through optical compensator 42. HoWever, the tWo sets of
`s-polarized Wavelength channels experience substantially
`the same total dispersion as experienced by the tWo sets of
`p-polarized Wavelength channels. Beam combiner 44 creates
`tWo identical sets of superimposed Wavelength channels (1s,
`2p) incident focusing lens 46. By superimposing each of the
`s-polarized Wavelength channels With its corresponding
`p-polarized Wavelength channel, each superimposed Wave
`length channel includes the information payload from the
`?rst ?ber Wavelength channel (1s) and the second ?ber
`Wavelength channel (2p). Lens 46 focuses each superim
`posed Wavelength channel onto its respective liquid crystal
`sWitch cell 22 to thereby combine the tWo identical sets of
`information into one superimposed Wavelength channel
`incident on sWitch cell 22.
`In the high-voltage state, the polarization state of a
`superimposed Wavelength channel at the output of sWitch
`cell 22 is unchanged relative to the polarization state of the
`same superimposed Wavelength channel at the input of
`sWitch cell 22. In the off-voltage state, sWitch cell 22
`converts (1s, 2p) into (1p, 2s) by the polarization rotation
`technique described above and the polarization state of a
`superimposed Wavelength channel at the output of sWitch
`cell 22 is rotated 90° relative to the polarization state of the
`same superimposed Wavelength channel at the input of
`sWitch cell 22.
`As noted previously, the output birefringent optical sys
`tem 50 is exactly symmetrical to the input birefringent
`optical system 30, described in the paragraph above. In the
`high voltage state, channel (1s,1p) is included in the ?rst
`?ber output and channel (2s,2p) in the second ?ber output.
`This is the Wavelength channel pass-through state. In the
`loW-voltage state, channel (2s, 2p) is inserted in the ?rst ?ber
`output and channel (1s,1p) into the second ?ber output.