WWWMWWWM MWWWMMWW
`USUUGZSSSUUB!
`
`(.2) United States Patent
`US 6,285,500 BI
`(I0) Patent No.:
`Ranalli et al.
`
`(45) Date of Patent: Sep. 4, 2001
`
`(54) WAVELENGTH SELECTIVE SWITCH
`
`(7’5)
`
`Inventors: Eliseo R. Ramllli. Irvine; Bradley A.
`Scull, Huntington Beach. both of (TA
`(U3)
`
`(1’3) Assignee: Corning Incorporated. Corning. NY
`(US)
`
`( ") Notice:
`
`Suhjuel lo an).r disclaimer, the [arm ot‘lhis
`patent Ls extended or adjusted under 35
`U.S.C. 154(1)) by [I days.
`
`(21) Appl. No.'. DQMSELMZ
`(22)
`Filed:
`Nov. 29, 1999
`
`[60)
`
`Related [1.5. Application Data
`Provisional application No. otJr'HLSSU.
`tiled on Jun. 2‘).
`I999.
`
`:02}; 5130
`1m. cr.’ .......................................................
`(5))
`(52) vs. C].
`5593497; 359,927; 3SW494;
`559.-'499; 32:5” 1
`(58) Field of Search .................................... 359.e'124.127,
`359mm, 159. 246, 494, 495, 497. 499;
`585m
`
`(56)
`
`References Cited
`U3. PATEN'I‘ Dt')(.'UML-‘NTS
`
`4:199“ Lichowilz .
`4.917.452
`5,3710% 12.n’l9‘i4 Lin et at. _
`5.414.540
`531995 Patel cl 3!. .
`5,t)l'H’.).43‘J
`2.5!qu Wu _
`5.594.113
`12.9199? Wu et al.
`5.724.165
`3.51998 Wu .
`5.85129]
`2,9100";
`].ill :31 at. .
`
`.
`
`l-UREIGN PA'l’IiN’l‘ l)()('_‘UMLT.N'lS
`““09833239
`W09855251
`
`(W0) .
`m 99::
`an 993 (W0) _
`
`" cited by examiner
`
`Primate Errimincr—Darren Sehuberg
`(T4) Attorney. Agent. or Firm—Daniel P.
`
`l'itla'tlle}r
`
`[57)
`
`rulifzi'l‘R./‘\I‘..."I~
`
`A rcwnligurahle iii—directional wavelength scleclivc switch
`is disclosed. It has an optical system [Ital is symmetric aboul
`a polarization modulator. The symmelric oplical system
`consists of an inpul birefringent oplieal system and outpul
`birefringent optical system disposed around polarization
`rnodu lator. The optical system dclivcrs [he wavelength chan—
`nels thal are to be switched as a superimposed wavelcnglh
`channel
`incitlcnl
`llte polarizalion modulator. As a resull,
`crosstalk is reduced below ~35 dB and greater optical
`performanou is achieved.
`
`4.731.351 - 11,9933 Inouclul.
`
`............................2 599495
`
`35 Claims. 9 Drawing Sheets
`
`
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1027, Page 1
`Exhibit 1027, Page 1
`
`

`

`US. Patent
`
`Scp.4,2001
`
`Sheet 1 of9
`
`US 6,285,500 Bl
`
`Output
`
`‘IIIIII.
`
`fiber2
` 14
`
`E
`.92
`E%
`
`
`E.2 D-
`1 W
`‘IIIFL
`‘
`
`
`
`F-
`Q
`U.
`
`8
`
`8
`
`E
`
`.. %
`it?
`
`COO
`
`
`
`fl
`3‘-
`13.”
`£11-
`En:
`
`fl
`3'—
`I10
`1)“
`El:
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1027, Page 2
`Exhibit 1027, Page 2
`
`

`

`US. Patent
`
`Sup. 4, 2001
`
`Sheet 2 0f 9
`
`US 6,285,500 B1
`
`.-
`
`N;Ema:39:0
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1027, Page 3
`Exhibit 1027, Page 3
`
`

`

`US. Patent
`
`Scp.4,2001
`
`Sheet 3 01'9
`
`US 6,285,500 B1
`
`Input
`signal
`
`signal
`
`Input
`
`FIG. 4
`
`LA n[out)
`
`Amman)
`
`78
`
`
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1027, Page 4
`Exhibit 1027, Page 4
`
`

`

`US. Patent
`
`Sup. 4, 2001
`
`Sheet 4 of 9
`
`US 6,285,500 B1
`
`m.OE
`
`3
`
`\\
`
`N.m~
`
`
`2.9mlizIEI‘3.2Al-IEIIAI
`hasswuwwhmulgllaadillmr/Aa
`LI-EEIIIIVEa:AIIIiIEI-EV
`mmmmmm.35
`Ja...EA_m._..m_m
`om.\\8\
`
`9..ESE.3.22%.
`
`2%8:235
`
`mm
`
`
`\li-I_!\.ia7mmmm’IN...Ecgmmm8cmmmD.35as.rE.I»!I
`
`2.8mAQnE-E-glni‘flflE-IZWuI—MH-fi‘
`
`
`a.a:Ala—E-EII“WM.AIIIVi-I_!I
`
`3mNmm6.?mm"_mca_m
`
`A.NV.E_I......_‘3:___1|-__-_
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1027, Page 5
`Exhibit 1027, Page 5
`
`
`

`

`US. Patent
`
`Sup. 4, 2001
`
`Sheet 5 0f 9
`
`US 6,285,500 B1
`
`32:
`
`m.O_n_
`
`EEO
`
`
`
`$0me.235an0
`
`we
`
`
`
`8—.
`
`6ch
`
`mm
`
`EEmESmm
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1027, Page 6
`Exhibit 1027, Page 6
`
`
`
`

`

`US. Patent
`
`Scp.4,2001
`
`Sheet 6 01'9
`
`US 6,285,500 B1
`
`LO
`LO
`CLO
`O1—
`.9
`g
`I.-
`ND”
`2I:
`(.98
`u_E""
`EU
`a.)
`D.
`(D
`U!
`
`8_l
`
`1556
`
`1552
`
`1553
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1027, Page 7
`Exhibit 1027, Page 7
`
`

`

`US. Patent
`
`Scp.4,2001
`
`Sheet 1' of9
`
`US 6,285,500 Bl
`
`1565
`
`co
`m‘_
`
`A
`E
`I:
`‘—'
`5
`2*
`a:
`>
`to
`3
`
`I 9
`
`?0
`fig,_
`
`a9;‘_
`
`gloom
`‘T‘T‘T
`
`cmcmom
`wwwwwv
`
`(an) uouenuanv
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1027, Page 8
`Exhibit 1027, Page 8
`
` :2
`
`C O T
`
`.)
`.25
`0.
`km
`om
`Em
`~*‘
`O
`b
`a.)
`as
`
`c:
`§m
`In
`3"
`:
`(U
`
`CI
`
`00
`I
`(D
`
`LL
`
`

`

`US. Patent
`
`Scp.4,2001
`
`Sheet 8 01'9
`
`US 6,285,500 B1
`
`I‘D")
`h—
`
`2
`
`CO
`CD
`
`2
`
`3
`:2
`
`0")
`.55
`
`CD
`.
`<9
`E
`
`8
`5
`D-
`E
`5
`g
`3
`co
`
`A
`E
`I;
`5
`E
`Q
`cu
`E
`
`0')
`0")
`
`fl :2)
`
`E.
`
`a.
`
`a
`
`a
`
`(map) finsuezw
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1027, Page 9
`Exhibit 1027, Page 9
`
`

`

`US. Patent
`
`Scp.4,2001
`
`Sheet 9 01'9
`
`US 6,285,500 B1
`
`FIG. 10
`
`100
`
`10
`
`/
`
`Input
`Fiber 1
`
`(Through channel
`trunking)
`
`Input
`Fiber 2
`
`
`Wave[ength
`
`selcctive
`sw1tch
`
`
`(W55)
`
`
`Output
`
`Output
`Fiber 1
`
`
`
`"1 A2
`
`"3 ‘“ P‘N
`
`"1
`
`l2 "3 °” "N
`
`Local
`Ports
`
`Local
`Ports
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1027, Page 10
`Exhibit 1027, Page 10
`
`

`

`US 6,285,500 Bl
`
`1
`WAVELENG’l‘l-I SELECTIVE SWITCH
`CROSS-REFERENCE TO RELNl'El)
`APPLICA’I'IUNS
`
`This Application claims the benefit of priority under 35
`U.S.C. §119(e) for U.S. Provisional Patent Application Ser.
`No. 60:141556 tiled 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 fiber optics have transformed the
`telecommunications market place. initially, network designs
`included relativer low-speed transceiver electronics at each
`end of the communications link. Light signals were switched
`by being converted into electrical signals, switched
`electronically, and reconvened 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 liber in the 1550 nm region of the electromagnetic
`spectrum is in the ‘l‘eraltertz 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. Wollaslon 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 it' bire-
`fringent crystals are used. If hearnsplitting cubes are used
`contrast ratio is reduced and crosstalk is increased. This was
`addressed by using a Wollaston Prism. Wollaston Prisms are
`designed to cottven a collimated beam of mixed polarization
`into two deflected 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
`Woliaston Prisms. The most significant of these lies is the
`fact Wollaston Prisms cannot produce beams that are exactly
`symmetrically deflected. Because the elfect of the Wollaston
`Prism is not symmetrical,
`the beams cannot be superint-
`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.
`
`1t]
`
`15
`
`3t}
`
`40
`
`45
`
`St}
`
`55
`
`at]
`
`65
`
`2
`SUMMARY OF THE lNVENTlON
`
`A wavelength selective switch is disclosed that includes
`an optical system that
`is symmetric about a polarization
`modulator and capable ofdelivering 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 first signal and a second signal to a
`selected output. The optical device includes: a birefringent
`optical system having a system input that receives the Iirst
`signal and the second signal. and a system output to which
`the bireEringent optical system transmits a superimposed
`signal formed by superimposing a first polarized signal and
`a second polarized signal, wherein the first polarized signal
`and the second polarized signal are polarized versions of the
`first 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 first signal and a second
`signal to a selected output. The optical device includes a Iirst
`polarization beam splitter for separating the first signal and
`second signal into first signal polarization components and
`second signal polarization components, respectively. A first
`half—wave retarder is coupled to the polarization beam
`splitter, the Iirst halfnwavc retarder causes all ol. the Iirst
`signal polarization components and the second signal polar-
`ization components to be uniformly polarized in a
`first
`polarization state. A first grating is coupled to the first
`half-wave retarder, for producing a plurality of first signal
`wavelength channels and a plurality of second signal wave-
`length channels. A second half-wave retarder is coupled to
`the first grating, for causing the plurality of second signal
`wavelength channels to be uniformly polarized in a second
`polarization state. A first optical compensator is coupled to
`the Iirst grating,
`for causing an optical distance of the
`plurality of first signal wavelength channels to be substan-
`tially equal to an optical distance of the plurality of second
`signal wavelength channels. A Iirst polarization beam com~
`biner is coupled to the optical compensator and the second
`half—wave retarder, for combining the plurality or first 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 ofpolarization 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 first 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 Iirst signal into at least one first polarized
`component and the second signal into at least one second
`polarized component. Superimposing the at least one first
`polarized component with the at least one second polarized
`component to form a superimposed signal, wherein the at
`least one Iirst 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 first 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
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1027, Page 11
`Exhibit 1027, Page 11
`
`

`

`US 6,285,500 Bl
`
`3
`steps. Providing an array of liquid crystal pixels, wherein
`each of the liquid crystal pixels includes a switch state.
`Demultiplexing the first signal and the seotmd signal
`to
`thereby form a plurality of first signal wavelength channels
`and a plurality of second signal wavelength channels.
`respectively. Superimposing each first 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.
`[1 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 ofthis specification. 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
`
`is a block diagram showing on overview of the
`I
`1-16.
`Wavelength Selective Switch (W595) according to a first
`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 beamsplittcr
`in accordance with the present invention;
`FIG. 4 is a diagram showing an a thermalized grating in
`accordance with the present invention;
`[310.5 is a diagram showing the polarization management
`architecture of the WSS depicted in FIGS. 1 and 2;
`Fifi. IS is a perspective View of the mechanical design of
`the W553 in accordance with a wcond embodiment of the
`present invention;
`[-16.7 is a plot showing the channel profiles of the W83
`of the present invention;
`FIG. 8 is a plot showing the broadband ripple of a
`40—chanoel WSS of the present invention;
`FIG. 9 is a plot showing the broadband ripple of a
`8(l—channel WSS of the present invention; and
`FIG. [0 is a block diagram of a WADM that incorporates
`the WSS in accordance with a third embodiment of the
`present invention.
`DETAILED DESCRIPTION (ll-"11“}.
`PREFERRED EMBODIMENTS
`
`to the present
`Reference will now be made in detail
`preferred embodimean of the invention, examples of which
`are illustrated in the accompanying drawings. Wherever
`possible, the saute 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 III.
`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
`
`5
`
`llil
`
`[5
`
`3t]
`
`40
`
`45
`
`St]
`
`55
`
`till
`
`65
`
`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
`waveIength 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 first
`input fiber with an orthogonal polarized signal from a
`second input fiber.
`As embodied herein and depicted in FIG. 1, W33 Ill
`according to the [lost embodiment of the present invention is
`disclosed. [nput fiber 1 and input fiber 2 are connected to
`input port 12. Input port IE is connected to inputbirefringent
`optical system 3|]. 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 5|} which routes the output light beams to output port
`14. Output port 14 is connected to output fiber 1 and output
`fiber 2.
`Output birefringent optical system 5tl is the mirror image
`of input birefringent optical system 30. Thus, W35 10 is a
`reconfigurable Iii-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. W88 10 as depicted in FIG. 1 is a 2x2
`Wavelength Selective Switch.
`Input fiber 1 and input fiber 2 provide WSS ll] with
`randomly polarized light signals having multiple wave-
`length channels. In a first embodiment. W55 [0 accommo-
`dates 40 wavelength channels at ttltl (it [2 spacing between
`channels. In an alternate embodiment, W55 10 accommo-
`dates 80 wavelength channels at 50 G] [2 spacing between
`channels. Any individual channel may be selectively
`switched between input fiber 1 and input fiber 2. W88 It]
`operates by converling the wavelength channels from input
`fiber
`1
`into s—polarized (perpendicular) signals and the
`second fiber 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—polarizcd signals are then superimposed and focused on
`polarization modulator.
`'l‘hus.
`traffic carried by the input
`fibers is identified by its polarization state. Polarization
`modulator 20 rotates the polarization slate of the superim-
`posed signa} by 90° when switching channels between fibers
`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-ntultiplexes the
`wavelength channels according to their polarization state
`and maps s—polarimd output channels {as polarized after
`leaving polarization modulator 20) lo the output fiber 1 and
`maps p-polarired output channels (as polarized after leaving
`polarization modulator 2.0} to the output fiber 2. Because of
`the symmetrical design, this convention can be reversed.
`The operation of WSS II} will be discussed in more detail
`below.
`As embodied herein and depicted in FIG. 2, a schematic
`of WSS ll] according to a first embodiment of the present
`invention is disclosed. Input fiber 1 and input fiber 2 are
`connected to W53 10 at input 12. The light signal from fiber
`1 and fiber 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
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1027, Page 12
`Exhibit 1027, Page 12
`
`

`

`US 6,285,500 Bl
`
`5
`fold-mirror 36 is optically coupled to polarization beam
`splitter 32 and half-wave plate 34 causing the light signals
`to be reflected toward grating 38. As depicted in subsequent
`embodimean, fold—mirror 36 can be eliminated and the
`optical signal is directed from half—wave plate 34 to grating
`33. Grating 38 demultiplexes the llrst fiber light signal and
`the second fiber light signal into its constituent wavelength
`channels. Half-wave plate 40 and optical compensator 42 are
`coupled to the grating. Ilalf~wave plate 49 provides an
`optical path for the second fiber wavelength channels. Opti—
`cal compensator 42 provides an optical path for the first fiber
`wavelength channels. The function of these elements will be
`discussed in more detail below. Half—wave plate 4|! and
`optical compensator 42 are optically coupled to polarization
`beam combiner 44. Polarization beam combiner 44 super—
`imposes the first fiber wavelength channels coming from
`half-wave wave plate 40 and the second fiber 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 2|}.
`Asdiscussed above, output birefringent optical system 50
`is a mirror image of input birefringent optical system 5|].
`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 fiber 1 wavelength channel and an
`output fiber 2 wavelength channel. Polarization beam split—
`ter is coupled to halfmwave plate 60 and optical compensator
`62. Optical compensator 62 adjusts the optical path length of
`output fiber 1 wavelength channels. Output fiber 2 wave-
`length channels propagate through half-wave plate 6t}. Out-
`put fiber 1 wavelength channels and output fiber 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 first output
`fiber and the second output
`fiber,
`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 antirefiection 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 fight
`to
`pass through white internally reflecting p—polal'iznd light-
`'l'he p—polarized light is reflected by reflective 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 arcsccond 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
`simplifies 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 dilferences between
`the grating and focusing lens is made identical
`for all
`configurations.
`in addition, beam combiner 44 and splitter
`64 are dispowd between the grating and the focusing lens.
`
`Ill
`
`[5
`
`3t]
`
`40
`
`45
`
`St]
`
`55
`
`ill]
`
`65
`
`6
`This innovation provides improved optical performance and
`eliminates asymmetries associated with the Wollaston
`Prism, typically found in other designs. Examples of such
`beamspliiten’combiner devices are disclosed in Provisional
`Patent Application No. 60f153,913 which is herein incorpo-
`rated by reference.
`(Jne of ordinary skill in the art will recognize that beam
`splitting cubes, birefringent plates, and prisms, in addition to
`thin film filters, can also be used depending on the desired
`tolerances, package sine, 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
`grisrn 78 that includes input grating 38 and output grating 53
`in one package as depicted in FIG. 6. In this embodiment,
`input grating 38 is replicated onto substrate 336 and mated
`to prism 382 by epoxy 384. The CTE of the grating spacing
`is intermediate between the (TIL: of Ihc prism material and
`the (.‘fli of the substrate material. By varying the thickness
`of the substrate material the (TE of the grating spacing can
`be controller]. The glass used for the prism should have a low
`dnidt. For example, pn'sm 382 can be implemented using
`()hara glass type Sw‘l‘llfi and Coming UIJE. 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 significantly easier, ensuring that
`the
`angular relationships will not significantly change with
`temperature. Examples of such athermalimd devices are
`disclosed in Provisional Patent Application No. 60i153,913
`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 athermalimed 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 first fiber signal and the second fiber signal
`to be very nearly equal. For any beam combiner and
`half-wave retardcr, optical compensators 42 and 62 are
`designed such that the wavelength channels from input fiber
`1 and input fiber 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:
`
`
`.
`. "r - "u
`Hans
`lip”
`"at,
`Lora—rm = :1 _
`
`Where To is the thickness of optical compensator 42 (62), n0
`is the optical index of compensator 42 (62), n" is the index
`ofair, H is the difi‘erence in the distance traveler] by the light
`from fiber input 1 as compared to input fiber 2 within the
`beam combiner, n,“ is the index of the beam combiner
`material. T, is the thickness of the rctarder, and n, 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 4ft 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
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1027, Page 13
`Exhibit 1027, Page 13
`
`

`

`US 6,285,500 Bl
`
`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 configuration
`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, W88 10 would function as a variable
`optical attenuator. As is well known in the art, when a
`sulIicient 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. "l‘hus,
`in an oil—voltage switch state, or relatively low-voltage state,
`the polarization state of an incident light signal is rotated by
`'fz 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 hirefrin~
`gence dependent on the applied voltage. These crystals
`employ the same ell’ect
`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
`li'arady rotators, acousto—optic rotators, and electro—optical
`rotators may also be employed as polarization modulator 20.
`I’lG. 5 illustrates the operation of W88 10 from a polar-
`ization management perspective. Polarizing beamspiitter 32
`separates input signals from the first fiber and second fiber
`into their parallel and orthogonal signal components. Thus,
`[our beamlcts (ls, 1p, 23, 2;?) exit beamsplitter 32. One of
`ordinary skill in the art will recognize that the convention
`used to number input fibers 1 and 2 is arbitrary and thus, can
`be reversed. As depicted, the p-polarized components from
`the first fiber signal and the second fiber signal (1;), 2p) pass
`through half-wave plate 3-4. 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 {15, 15, 2s, 2.5) 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 (Pl)[.)_
`Grating 38 demultiplexes the wavelengths being carried by
`the four beamleLs,
`to create wavelength diversity. One
`skilled in the art will recognize that each wavelength carried
`by the beamlets is a separate communications channel
`carrying iLs own information payload. For each wavelength
`channel defined for fiber 1, there is a corresponding wave—
`length channel in fiber 2. The corresponding wavelength
`channeLs in fiber 1 and fiber 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 fiber 1 and fiber 2,
`their
`respective information payloads are also switched between
`fiber 1 and fiber 2.
`The two polarized bcamlets derived from the second liber
`signal passes through half-wave plate 40 creating polariza-
`tion diversity. Thus,
`the first fiber wavelength channels,
`which do not pass through half-wave plate 40 remain
`Smpolarized {15, 15), whereas the second liber wavelength
`channeLs are polarized (2p, 2p).
`
`10
`
`[5
`
`-
`
`3t]
`
`40
`
`45
`
`SE]
`
`55
`
`(all
`
`65
`
`8
`One salient feature of the invention is that, absent optical
`compensator 42. the first fiber wavelength channels would
`travel a shorter physical distance. First fiber wavelength
`channels are passed through optical compensator 42 to
`equalize the optical distances of the first fiber wavelength
`channels and the second fiber wavelength channels. Optical
`distance is defined 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 defined 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
`sante 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 seLs of
`s-polarized wavelength channels experience substantially
`the same total dispersion as experienced by the two sets of
`p-potarized wavele ngth channels. Beam combiner 44 creates
`two identical sets of superimposed wavelength channels (ls,
`2p) incident focusing lens 46. By superimposing each of the
`s-potarized wavelength channels with its corresponding
`p-polarized wavelength channel, each superimposed wave-
`length channel includes the information payload from the
`first fiber wavelength channel (ls) and the second fibcr
`wavelength channel (2p). [lens 46 focuses each superim—
`posed wavelcngth channel onto its resp