(12)
`
`United States Patent
`Bouevitch et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 6,859,573 B2
`Feb. 22, 2005
`
`US0068595?3B2
`
`(54)
`
`(75)
`
`DOUBLE PASS ARRANGEMENT FOR A
`LIQUID CRYSTAL DEVICE
`
`Inventors: Oleg Bouevitch, Gloucester (CA); Paul
`Colbourne, Nepean (CA)
`
`(73)
`
`Assignee: JDS Uniphase Inc., Ottawa (CA)
`
`(‘)
`
`Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.s.c. 154(3) by 104 days.
`
`(21)
`
`(22)
`
`(65)
`
`(63)
`
`(60)
`
`(51)
`
`(52)
`
`(58)
`
`(56)
`
`Appl. No.: 10124'1,4.31
`
`Filed:
`
`Sep. 20, 2002
`Prior Publication Data
`
`US 200310035605 A1 Feb. 20, 2003
`
`Related U.S. Application Data
`
`Continuation-in-part of application No. 091729,2'10, filed on
`Dec. 5, 2000, now Pat. No. 6,498,872.
`Provisional application No. 601183,155, filed on Feb. 17,
`2000, now abandoned.
`
`Int. Cl.’ ................................................ .. (3023 6126
`
`385116; 385124; 385120;
`U.S. Cl.
`385134; 385136; 3591123; 3591130; 3591246;
`3591247; 3591301; 3591302; 3491193; 3491196;
`3491197
`
`Field of Search ............................... .. 3491193, 196,
`3491197; 3591115, 122, 128, 130, 131.
`245, 246, 247, 301, 302; 385116, 13, 24,
`31, 32, 39, 47
`
`References Cited
`
`U.S. PATENT DOCUMENTS
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`711991
`5,089,786 A 1
`211992
`5,233,405 A 1 31993
`
`C010 ......................... .. 356144
`McMahon ................ .. 3591320
`Biltner ...................... .. 385131
`Scliloss ....................... .. 37013
`Jameson ................... .. 3591484
`Tamura .................... .. 3591333
`Wildnauer et al.
`........ .. 3561333
`
`111994 Pan ........................... .. 385134
`5,216,747 A
`511994 Asalrura et al.
`385,133
`5,311,606 A ‘
`511995 Patel et al.
`3491196
`5,414,540 A ‘
`5,477,350 A " 1211995 Rizaetal.
`349124
`5,499,132 A ‘
`311996 Tojo etal.
`3591281
`
`(List continued on next page.)
`FOREIGN PKFENT DOCUMENTS
`
`EP
`EP
`EP
`W0
`W0
`
`0 654 917 A
`0 859 249 A
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`WO 99138348
`W0 2144800 A2
`
`411995
`811999
`1011999
`711999
`1112001
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`OTHER PUBLICATIONS
`
`GOQF111133
`
`U.S. application Publication US200110050738 Al Publica-
`tion Date Dec. 13, 2001, Miller.
`S.W. Knight et 111., “Wavelength dependence of persistent
`photoconductivity in i.nd.ium—doped Pb1_,,Sn,Tc", Semicon-
`ductor Science and Technology, Institute of Physics. Lon-
`don, GB, vol. 5, No. 345, Mar. 1, 1990, pp. 5155-158.
`Joseph F. Ford et 211., “Wavelength Add-Drop Switching
`Using Tilting Micrornirrors”, Journal of Lightwave Tech-
`nology, IEEE, vol. 17, No. S, May 1999, pp. 904-911.
`
`Primary Exam£ner—John R. Lee
`Assistant Examiner—David A. Vanore
`
`(74) Attorney, Agent, or Firm—Teitelbaum & Macbean;
`Neil Teitelbaurn; Doug MacLean
`
`(5 7)
`
`ABSTRACT
`
`An optical device for rerouting and modifying an optical
`signal
`that uses a double pass through a liquid crystal
`modulator is disclosed. The optical device includes a first
`polarizer for providing polarized light, a liquid crystal
`modulator for selectively modifying the polarized light, a
`second polarizer for analyzing the light passed through the
`liquid crystal modulator, and a reflector for reflecting the
`analyzed light back through the second polarizer, the liquid
`crystal modulator, and the firsl polarizer. This arrangement
`provides a double pass through the liquid crystal modulator,
`thus significantly improving the attainable extinction ratio.
`
`30 Claims, 17 Drawing Sheets
`
`110a
`
`120
`
`'1‘10b
`
`'1 50
`
`OA
`
`T026
`
`1 O2!)
`
`1
`
`Ca
`
`JD
`
`IP
`
`Capella 2030
`JDS Uniphase v. Capella
`IPR2015-00739
`
`1
`
`

`
`39871
`Wu et al.
`38571’?
`Solgaard et al.
`385716
`Chang
`430714
`King
`359715
`Nishiet al.
`385716
`Wu el al.
`Keyworlh et al.
`.......... .. 385733
`Borrelli et a1.
`.
`335711
`Braun el. al
`3567327
`
`Pan
`385718
`Sanchez-Rubio et al.
`372792
`Pan ........................... .. 385718
`Liu ei al
`.... .. 39879
`Can ......................... .. 3597484
`Liu et al
`39879
`Xie et al
`3597484
`Patel et al
`3497196
`Wu et al
`385716
`Riza . . . . .
`385722
`Miller
`35972.3‘?
`Cao
`385724
`Xu et 8.1
`35973311
`Wu et al
`398765
`Wooten ....... ..
`385711
`Bouevitch el al
`385724
`Z1130 et al
`385716
`
`
`
`. .. . ..
`
`
`
`.
`
`US 6,859,573 B2
`Page 2
`
`372000
`6,097,513 A *
`372000
`6,097,359 A
`972000
`6,113,910 A *
`5’13°’°'3 A 5 W200”
`6,134,031 A
`1072000
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`1552555
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`534933473 B1 ' ‘mm
`53495572 B2 ' 1252552
`255375113555 A1 '
`572553
`
`* cited by examiner
`
`U.S. PATENT DOCUMENTS
`
`385m
`‘am
`I
`I--------------------- 39%?
`t
`K‘?
`K677596631"
`W3597434
`Winston et a]_ __
`__ 385,146
`Weber et al.
`349796
`wu
`393755
`Pan et al
`.. 3357140
`Pan
`385,11
`Ford elal.
`393737
`Bergmann .................. 3597434
`Tomlinson, 111 et al.
`3567364
`Hinnrichs ................. .. 3567310
`Li
`.. 3357140
`mm
`--
`§;.:.‘,:‘; ,1:":::::::::::::::'3§'3?{'§E
`Bishop et al.
`3597124
`Ford el al.
`.. 3597295
`Wu el al.
`................... .. 393755
`Tomlin-son ................. .. 335713
`Wu etal.
`393749
`Hunter et 41.
`335737
`Wu elal.
`................... .. 393743
`Lundgren eta]. ............. 335733
`Sharp et al.
`.. 3497117
`Chang
`.. 3597495
`Teichmann ............... .. 3567328
`
`
`
`371996
`671996
`1171996
`171997
`1171997
`371998
`371998
`471998
`471998
`671998
`1271993
`271999
`371999
`471999
`671999
`671999
`871999
`8171999
`371999
`971999
`1171999
`1271999
`1271999
`172000
`472000
`49'2C00
`672000
`
`
`
`I!!!Ilfifi!
`
`5,499,307 A
`5,526,155 A
`5,574,595 A
`5,594,330 A
`5,636,979 A
`5,724,165 A
`5,727,109 A
`5,740,233 A
`5,745,271 A
`5,771,120 A
`5,847,831 A
`5,367,264 A
`5,331,199 A
`5,394,233 A -
`5,912,743 A *
`5,917,625 A
`5,936,752 A
`5,943,153 A
`5,946,116 A
`5,960,133 A
`5,973,116 A
`5,999,672 A
`6,005,697 A
`6,013,603 A
`6,049,367 A
`6,055,104 A
`6,031,331 A
`
`II-I-§
`
`2
`
`

`
`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 1 of 17
`
`US 6,859,573 B2
`
`1103
`
`120
`
`110;;
`
`150
`
`0A
`
`102a
`
`1021:
`
`FIG. 1
`
`3
`
`

`
`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 2 of 17
`
`US 6,859,573 B2
`
`105
`
`K
`
`118
`
`107
`
`105b§‘
`
`“6a\
`
`FIG. 2b
`
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`
`16
`
`114
`
`FIG. 2a
`
`107
`
`112
`
`4
`
`

`
`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 3 of 17
`
`US 6,859,573 B2
`
`
`130
`/wf 144
`
`142
`
`FIG. 3a
`
`FIG. 3b
`
`
`102a
`
`130
`
`102b
`
`FIG. 3c
`
`150 J
`
`K140
`
`5
`
`

`
`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 4 of 17
`
`US 6,859,573 B2
`
`102a
`
`102b
`
`FIG. 3d
`
`130
`
`150 —*”
`
`RK-—14o
`
`6
`
`

`
`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 5 of 17
`
`US 6,859,573 B2
`
`152
`
`130 142
`
`
`
`FIG. 4a
`
`‘N150
`
`
`
`153
`
`%
`
`FIG. 4b
`
`‘N750
`
`155
`
`7
`
`

`
`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 6 of 17
`
`US 6,859,573 B2
`
`510
`
`605
`
`620
`
`650
`
`FIG. 6a
`
`8
`
`

`
`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 7 of 17
`
`US 6,859,573 B2
`
`620
`
`605
`
`650
`
`610
`
`FIG. 7
`
`9
`
`

`
`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 8 of 17
`
`US 6,859,573 B2
`
`10
`
`

`
`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 9 of 17
`
`US 6,859,573 B2
`
`990
`
`916
`
`9 4 9 2
`
`996
`
`998,999
`
`19 8
`
`996
`
`FIG. 9c
`
`11
`
`

`
`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 10 of 17
`
`US 6,859,573 B2
`
`914
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`990
`
`913
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`9 2
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`999
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`
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`
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`
`

`
`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 11 of 17
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`US 6,859,573 B2
`
`
`
`FIG.10
`
`13
`
`

`
`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 12 of 17
`
`US 6,859,573 B2
`
`mwmmmxmm.50
`
`§\
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`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 13 of 17
`
`US 6,859,573 B2
`
`15
`
`

`
`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 14 of 17
`
`US 6,859,573 B2
`
`16
`
`

`
`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 15 of 17
`
`US 6,859,573 B2
`
`70""'*-1
`
`72 \
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`
`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 16 of 17
`
`US 6,859,573 B2
`
`18
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`

`
`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 17 of 17
`
`US 6,859,573 B2
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`
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`

`
`US 6,859,573 B2
`
`1
`DOUBLE PASS ARRANGEMENT FOR A
`LIQUID CRYSTAL DEVICE
`
`CROSS-REFERENCE TO RELKFED
`APPLICATIONS
`
`This application is a continuation-in-part of U.S. patent
`application Ser. No. 09fl29,270 filed on Dec. 5, 2000 now
`U.S. Pat. No. 6,498,872 and claiming priority from Provi-
`sional Appl. No. 60,083,155 filed on Feb. 17, 2000 now
`abandoned.
`
`FIELD OF THE INVENTION
`
`The present invention relates to an optical device for
`rerouting and modifying an optical signal, or more
`specifically, a liquid crystal device having a double pass
`arrangement.
`BACKGROUND OF THE INVENTION
`
`com-
`In optical wavelength division multiplexed
`munication systems, an optical waveguide simultaneously
`carries many different communication channels in light of
`difierent wavelengths. In WDM systems it is desirable to
`ensure that all channels have nearly equivalent power. To
`help achieve this, gain equalizers are disposed at various
`points throughout the system to control the relative power
`levels in respective channels.
`Dense WDM systems require special addfdrop multiplex-
`ers (ADM) to add and drop particular channels (i.e.,
`wavelengths). For example, at predetermined nodes in the
`system, optical signals of predetermined wavelength are
`dropped from the optical waveguide and others are added.
`Typically, gain equalizing and addfdrop multiplexer
`devices involve some form of multiplexing and demulti-
`plexing to modify each individual channel of the telecom-
`munication signal. In particular, it is common to provide a
`first dilfraction grating for demultiplexing the optical signal
`and a second spatially separated diffraction grating for
`multiplexing the optical signal after it has been modified. An
`example of the latter is disclosed in U.S. Pat. No. 5,414,540,
`incorporated herein by reference. However,
`in such
`instances it is necessary to provide and accurately align two
`matching dilfraction gratings and at
`least
`two matching
`lenses. This is a significant limitation of prior art devices.
`To overcome this limitation, other prior art devices have
`opted to provide a single diffraction grating that is used to
`dernultiplex an optical single in a first pass through the
`optics and multiplex the optical signal in a second pass
`through the optics. For example, U.S. Pat. Nos. 5,233,405,
`5,525,155, 5,745,271, 5,936,752 and 5,960,133, which are
`incorporated herein by reference, disclose such devices.
`However, none of these prior art devices disclose an
`optical arrangement suitable for both dynamic gain equalizer
`(DGE) and configurable optical addfdrop multiplexer
`(COADM) applications. In particular, none of these prior art
`devices recognize the advantages of providing a simple,
`symmetrical optical arrangement suitable for use with vari-
`ous switchingfattenuating meats.
`Moreover, none of the prior art devices disclose a
`multiplexingidemultiplexing optical arrangement
`that
`is
`compact and compatible with a plurality of parallel input!
`output optical waveguides.
`For example, U.S. Pat. No. 5,414,540 to Patel et al.
`discloses a liquid crystal optical switch for switching an
`input optical signal to selected output channels. The switch
`includes a dilfraction grating, a liquid crystal modulator, and
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`S0
`
`55
`
`60
`
`65
`
`2
`
`a polarization dispersive element. In one embodiment, Patel
`et al. suggest extending the 1x2 switch to a 2x2 drop-add
`circuit and using a reflector. However, the disclosed device
`is limited in that the addfdrop beams of light are angularly
`displaced relative to the inputfoutput beams of light. This
`angular displacement
`is disadvantageous with respect to
`coupling the addldrop andfor inputfoutput beams of light
`into parallel optical waveguides, in addition to the additional
`angular alignment required for the input beam of light.
`With respect to compactness, prior art devices have been
`limited to an excessively long and linear configurations,
`wherein the input beam of light passes through each optical
`component sequentially before being reflected in a substan-
`tially backwards direction.
`US. Pat. No. 6,081,331 discloses an optical device that
`uses a concave mirror for multiple reflections as an alter-
`native to using two lenses or a double pass through one lens.
`However, the device disclosed therein only accommodates a
`single pass through the dilfraction grating and does not
`realize the advantages of the instant invention.
`It is an object of this invention to provide an optical
`system inclttding a diffraction grating that
`is relatively
`compact.
`It is a further object of the instant invention to provide an
`optical configuration for rerouting and modifying an optical
`signal that can be used as a dynamic gain equalizer andlor
`configurable addfdrop multiplexer.
`It is yet a further object of the instant invention to provide
`an optical device that uses a liquid crystal array in a double
`pass arrangement.
`
`SUMMARY OF THE INVENTION
`
`In accordance with the invention there is provided an
`optical device comprising a first optical fibre for providing
`an input beam of light, first polarizing means for producing
`light having a predetermined polarization state from the
`input beam of light, a liquid crystal modulator positioned to
`receive the light having a predetermined polarization state
`and for selectively altering its polarization, second polariz-
`ing means positioned to receive the light transmitted from
`the liquid crystal modulator, said second polarization means
`designed for passing light having one of the predetermined
`polarization and a polarization perpendicular to the prede-
`termined polarization and for blocking or diverting the other,
`a reflective surface positioned to receive the light passed
`through the second polarizing means and refiect it for a
`second pass through the liquid crystal modulator, a control-
`ler coupled to the liquid crystal modulator to direct the
`selective altering of polarization in dependence upon a
`desired attenuation setting for light exiting the optical device
`through one of the first optical fibre and a second optical
`fibre.
`
`In accordance with the invention there is provided an
`optical device comprising an optical fibre for launching an
`input optical signal, a first polarizer disposed for receiving
`the input optical signal and for producing polarized light
`therefrom, a liquid crystal modulator disposed for selec-
`tively altering the polarization of the polarized light, at least
`one region of the liquid crystal modulator operable between
`a first state where the polarization of light
`transmitted
`therethrough is not rotated and a second state where the
`polarization of light transmitted therethrough is rotated by
`about 90 degrees, a second polarizer disposed for passing
`light transmitted from the liquid crystal modulator in depen-
`dence upon its polarization state, and a reflective element
`disposed for reflecting the light transmitted from the second
`
`20
`
`20
`
`

`
`US 6,859,573 B2
`
`3
`polarizer back towards the optical fibre via the second
`polarizer, the liquid crystal modulator, and the first polarizer
`to provide an improved extinction ratio for one of the first
`and second states.
`
`In accordance with the invention there is provided a
`variable optical attenuator comprising a birefringent element
`positioned to separate the optical signal into two spatially
`separated, orthogonally polarized beams, a liquid crystal
`modulator positioned to receive the polarized beams of light
`and to selectively alter their polarizations, a reflective ele-
`ment positioned to reflect the polarized beams back through
`the liquid crystal modulator and the birefringent element,
`wherein the birefringent element recombines orthogonally
`polarized components of the refiected beams to produce an
`output optical signal, and a polarizer optically disposed
`between the liquid crystal array and the reflective element,
`wherein the polarizer is positioned to Contact the beams
`during at least one of a first pass from the liquid crystal
`modulator to the reflective element and a second pass from
`the reflective element back to the modulator.
`
`In accordance with the invention there is further provided
`a liquid crystal modulator comprising a first substrate, a
`second substrate disposed a fixed distance from the first
`substrate, a layer of liquid crystal disposed between the first
`and second substrates, a polarizer coupled to the second
`substrate, and a reflective surface coupled to the polarizer for
`reflecting light transmitted through the first substrate, liquid
`crystal, second substrate, and polarizer in a backwards
`direction for a second pass therethrough, wherein the reflec-
`tive surface is disposed at an angle relative to the second
`substrate.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Exemplary embodiments of the invention will now be
`described in conjunction with the drawings in which:
`FIG. 1 is a schematic diagram illustrating an embodiment
`of an optical configuration that can be used as a dynamic
`gain equalizer andfor add-drop multiplexer (DGEJCOADM)
`in accordance with the invention;
`FIG. 2a is a detailed side view of a front-end module for
`use with the DGEJCOADM shown in FIG. 1 having means
`for compensating for polarization mode dispersion (PMD);
`FIG. 2b is a detailed side view of an alternative front-end
`
`module having means for reducing or substantially elimi-
`nating PMD;
`FIG. 3a is a top view of one embodiment of modifying
`means comprising a liquid crystal array for use with the
`DGEICOADM shown in FIG. 1, wherein a liquid crystal
`element is switched to an ON state;
`FIG. 3b is a top view of the modifying means shown in
`FIG. 3a, wherein the liquid crystal element is switched to an
`OFF state;
`FIG. 3c is a top view of another embodiment of the
`modifying means for use with the DGEICOADM shown in
`FIG. 1, wherein the liquid crystal element is switched to an
`ON state;
`FIG. 3d is a top view of the modifying means shown in
`FIG. 3c, wherein the liquid crystal element is switched to an
`OFF state;
`FIG. 4a is a top view of another embodiment of the
`modifying means for use with the DGEICOADM shown in
`FIG. 1 having a birefringent crystal positioned before the
`liquid crystal array, wherein the liquid crystal element is
`switched to an OFF state;
`FIG. 4b is a top view of the modifying means shown in
`FIG. 4a, wherein the liquid crystal element is switched to an
`ON state;
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`FIG. 5 is a top view of yet another embodiment of the
`modifying means for use with the DGE shown in FIG. 1
`utilizing a MEMS device;
`FIGS. Ga and 6b are schematic diagrams of an embodi-
`ment of the invention that is preferred over the one shown
`in FIG. 1, wherein the focal plane of a single concave
`reflector is used to locate the inputfoutput ports, diflfraction
`grating, and modifying means;
`FIG. 7 is a schematic diagram of an embodiment of the
`invention that is similar to that shown in FIGS. 6a and 6b,
`wherein the input/output ports are disposed between the
`modifying means and dispersive element;
`FIG. 8 is a schematic diagram of a DGE having a
`configuration similar to that shown in FIGS. 6:: and 6b
`including an optical circulator; and
`FIG. 9 is a schematic diagram of a DGEICOADM in
`accordance with the instant invention including a lens hav-
`ing a single port for launching and receiving light from the
`concave reflector;
`FIG. 9a is a top view showing a lenslet array coupling
`inputfoulput optical waveguides to the lens in accordance
`with the instant invention;
`FIG. 9b is a top view showing a prior art polarization
`diversity arrangement coupling inputloutput optical
`waveguides to the lens in accordance with the instant
`invention;
`FIG. 9c is a side view of the prior art polarization diversity
`arrangement shown in FIG. 9b;
`FIG. 9:1 is a top view showing an alternative arrangement
`to the optical components shown in FIG. 92;;
`FIG. 9c is a side view of the alternate arrangement shown
`in FIG. 9d;
`FIG. 9f is a top view showing an asymmetric offset of the
`inputfoutput optical waveguides with respect to the optical
`axis of the lens, in accordance with the instant invention;
`FIG. 10 is a schematic diagram of another embodiment of
`a DGE/COADM in accordance with the invention;
`FIG. 11 is a schematic diagram of the preferred embodi-
`ment of a COADM in accordance with the instant invention;
`FIG. 12 is a schematic diagram of a COADM in accor-
`dance with the instant invention, wherein an asymmetric
`arrangement of the inputtoutput optical waveguides comple-
`ments the angular displacement provided by a MEMS
`element;
`FIG. 13 is a schematic diagram of a prior art attenuator;
`FIG. 14 is a schematic diagram of a variable optical
`attenuator including two liquid crystal stages;
`FIG. 15 is a schematic diagram of a reflective variable
`optical attenuator in accordance with an embodiment of the
`invention exhibiting an increase extinction ratio;
`FIG. 16a is a schematic diagram of a reflective variable
`optical attenuator in accordance with an embodiment of the
`invention having a wedged backrefiector;
`FIG. 16b is a schematic diagram of a reflective variable
`optical attenuator in accordance with another embodiment of
`the invention having a wedged backreflector;
`FIG. 17 is a schematic diagram of a reflective variable
`optical attenuator in accordance with another embodiment of
`the invention including a wedged polarizer;
`FIG. 18a is a schematic diagram of a reflective variable
`optical attenuator in accordance with another embodiment of
`the invention including a birefringent wedge;
`FIG. 18!) is a schematic diagram of the birefringent wedge
`depicted in FIG. 18:: showing the beam defiection;
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`US 6,859,573 B2
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`FIG. 19:: is a schematic diagram of a reflective variable
`optical attenuator in accordance with another embodiment of
`the invention including a birefringent wedge, wherein the
`device is in an “ON" state;
`FIG. 19b is a schematic diagram of a reflective variable
`optical attenuator in FIG. 19a, wherein the device is in an
`“OFF” state;
`FIG. 20 is a schematic diagram of a reflective variable
`optical attenuator in accordance with yet another embodi-
`ment of the invention; and
`FIG. 21 is a schematic diagram of a reflective variable
`optical attenuator in accordance with another embodiment of
`the invention including a 4-)‘ imaging system.
`DETAILED DESCRIPTION
`
`Referring now to FIG. 1, an optical device for rerouting
`and modifying an optical signal
`in accordance with the
`instant invention is shown that is capable of operating as a
`Dynamic Gain/“Channel Equalizer (DGE) andfor a Config-
`urable Optical Add.t’Drop Multiplexer (COADM).
`The optical design includes a diffraction element 120
`disposed between and at a focal plane of identical elements
`110.9 and 1101) having optical power, respectively. Two ports
`102a and 102!) are shown at an inputfoutput end with
`bi-directional arrows indicating that light launched into port
`102a can be transmitted through the optical device and can
`be reflected backward to the input port from which it was
`launched 102:1, or alternatively, can be switched to port 102!)
`or vice versa in a controlled manner. The inputfoutput ports
`102:: and 102!) are also disposed about one focal plane away
`from the element having optical power 110:: to which they
`are optically coupled. Although only two inputfoutput ports
`are shown to facilitate an understanding of this device. a
`plurality of such pairs of ports is optionally provided. At the
`other end of the device, modifying means 150 for modifying
`at least a portion of the light incident thereon is provided
`about the focal plane of the element having optical power
`1105.
`
`Since the modifying means andfor dispersive element are
`generally dependent upon polarization of the incident light
`beam, light having a known polarization state is provided to
`obtain the selected switching andfor attenuation. FIGS. 2::
`and 2b illustrate two different embodiments of polarization
`diversity arrangements for providing light having a known
`polarization state, for use with the DGEICOADM devices
`described herein. The polarization diversity arrangement,
`which is optionally an array, is optically coupled to the input
`and output ports.
`Referring to FIG. 2a an embodiment of a front-end
`micro-optical component 105 for providing light having a
`known polarization is shown having a fibre tube 107, a
`microlens 112, and a birefringent element 114 for separating
`an input beam into two orthogonal polarized sub-beams. At
`an output end, a half waveplate 116 is provided to rotate the
`polarization of one of the beams by 90° so as to ensure both
`beams have a same polarization state e.g., horizontal. A glass
`plate or a second waveplate 118 is added to the fast axis path
`of the crystal 114 to lessen the etfects of Polarization Mode
`Dispersion (PMD) induced by the difference in optical path
`length along the two diverging paths of crystal 114.
`FIG. 2b illustrates an alternative embodiment to that of
`
`FIG. 2a, wherein two birefringent elements 114a, 1141!: have
`a half waveplate 116a disposed therebetween; here an alter-
`nate scheme is used to make the path lengths through the
`birefringent materials substantially similar. Optionally, a
`third waveplate 119 is provided for further rotating the
`polarization state.
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`Although, FIGS. 2:: and 2b both illustrate a single input
`beam of light for ease of understanding, the front end unit
`105 is capable of carrying many more beams of light
`therethrough, in accordance with the instant invention (i.e.,
`can be designed as an array as described above).
`FIGS. 3a—3b, 3c—3d, 4, and 5, each illustrate a different
`embodiment of the modifying means for use with the
`DGEICOADM devices described herein. Each of these
`embodiments is described in more detail below. Note that
`
`the modifying means are generally discussed with reference
`to FIG. 1. However, although reference is made to the
`dispersive element 120 and elements having optical power
`110a and 110b, these optical components have been omitted
`from FIGS. 3a—3b, 3c—3d, 4, and 5 for clarity.
`Referring to FIGS. 3:: and 3b a schematic diagram of the
`modifying means 150 is shown including a liquid crystal
`array 130 and a reflector 140. The reflector includes first and
`second polarization beam splitters 144 and 146, and reflec-
`tive surface 142.
`
`When the device operates as a COADM, each pixel of the
`liquid crystal array 130 is switchable between a first state
`e.g., an “ON” state shown in FIG. 31:, wherein the polar-
`ization of a beam of light passing therethrough is unchanged
`(e.g., remains vertical), and a second state e.g., an “OFF"
`state shown in FIG. 3b, wherein the liquid crystal cell rotates
`the polarization of a beam of light passing therethrough 90°
`(e.g.,
`is switched to horizontal). The reflector 140 is
`designed to pass light having a first polarization (e.g.,
`vertical) such that beam of light launched from port 102a is
`reflected back to the same port, and reflect light having
`another polarization (e.g., horizontal) such that a beam of
`light launched from port 102a is switched to port l|l2b.
`When the device operates as a DGE, each liquid crystal
`cell is adjusted to provide phaw retardations between 0 to
`180°. For a beam of light launched and received from port
`102a, 0% attenuation is achieved when liquid crystal cell
`provides no phase retardation and 100% attenuation is
`achieved when the liquid crystal cell provides 180° phase
`retardation. Intermediate attenuation is achieved when the
`
`liquid crystal cells provide a phase retardation greater than
`0 and less than 180°. In some DGE applications, the reflector
`140 includes only a reflective surface 142 (i.e., no beam
`splitter).
`Preferably, the liquid crystal array 130 has at least one
`row of liquid crystal cells or pixels. For example, arrays
`comprising 64 or 128 independently controlled pixels have
`been found particularly practical, but more or fewer pixels
`are also possible. Preferably, the liquid crystal cells are of
`the twisted nematic type cells, since they typically have a
`very small residual birefringent in the “ON” state, and
`consequently allow a very high contrast ratio (>35 dB) to be
`obtained and maintained over the wavelength and tempera-
`ture range of interest. Alternatively, the liquid crystal cells
`are other than the twisted nematic type. Optionally, the
`inter-pixel areas of the liquid crystal array 130 are covered
`by a black grid.
`FIGS. 3c and 3d are schematic diagrams analogous to
`FIGS. 3:! and 3}: illustrating an alternate form of the modi-
`fying means 150 discussed above, wherein the reflector 140
`includes a double Glan prism. The arrangement shown in
`FIGS. 3c and M is preferred over that illustrated in FIGS. 3a
`and 3b, since the respective position of the two-sub beams
`emerging from the polarization diversity arrangement (not
`shown) does not change upon switching.
`Note that in FIGS. 3a—3d,
`the dispersion direction is
`perpendicular to the plane of the paper. For exemplary
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`US 6,859,573 B2
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`7
`purposes a single ray of light is shown passing through the
`modifying means 150.
`FIGS. 40 and 4b are schematic diagrams showing another
`embodiment of the modifying means 150, wherein a bire-
`fringent crystal 152 is disposed before the liquid crystal
`array 130. A beam of light having a predetermined polar-
`ization state launched from port 102::
`is dispersed into
`sub-beams, which are passed through the birefringent crystal
`152. The sub-beams of light passing through the birefringent
`crystal 152 remain unchanged with respect to polarization.
`The sub-beams of light are transmitted through the liquid
`crystal array 130, where they are selectively modified, and
`reflected back to the birefringent crystal 152 via reflective
`surface 142. If a particular sub—beam of light passes through
`a liquid crystal cell in an “OFF” state, as shown in FIG. 413,
`then the polarization thereof will be rotated by 90° and the
`sub—beam of light will be refracted as it propagates through
`the birefringent crystal 152 before being transmitted to port
`102b. If the sub-beam of light passes through a liquid crystal
`cell
`in an “ON” state, as shown in FIG. 4b,
`then the
`polarization thereof will not be rotated and the sub-beam of
`light will be transmitted directly back to port 10211. A half
`wave plate 153 is provided to rotate the polarization of the
`refracted sub-beams of light by 90° to ensure that both
`reflected beams of light have a same polarization state.
`FIG. 5 is a schematic diagram of another embodiment of
`the modifying means 150 including a micro electromechani-
`cal switch (MEMS) 155, which is particularly useful when
`the device is used as a DGE. A beam of light having a
`predetermined polarization state launched from port 1020 is
`dispersed into sub-beams and is passed through a birefrin-
`gent element 156 and quarter waveplate 157. The birefrin-
`gent element 156 is arranged not to alfect the polarization of
`the sub—beam of light. After passing through the quarter
`waveplate 157, the beam of light becomes circularly polar-
`ized and is incident on a predetermined reflector of the
`MEMS array 155. The reflector reflects the sub—beam of
`light incident thereon back to the quarter waveplate. The
`degree of attenuation is based on the degree of deflection
`provided by the reflector (i.e., the angle of reflection). After
`passing through the quarter waveplate 157 for a second time,
`the attenuated sub—beam of light will have a polarization
`state that has been rotated 90° from the original polarization
`state. As a result the attenuated sub—beam is refracted in the
`
`birefringent element 156 and is directed out of the device to
`port 102$). A half wave plate 158 is provided to rotate the
`polarization of the refracted sub-beams of light by 90°.
`Of course, other modifying means 150 including at least
`one optical element capable of modifying a property of at
`least a portion of a beam of light and reflecting the modified
`beam of light back in substantially the same direction from
`which it originated are possible.
`Advantageously, each of the modifying means discussed
`above utilizes an arrangement wherein each spatially dis-
`persed beam of light
`is incident
`thereon and reflected
`therefrom at a 90° angle. The 90° angle is measured with
`respect to a plane encompassing the array of modifying
`elements (e.g.,
`liquid crystal cells, MEMS reflectors).
`Accordingly, each sub-beam of light follows a first optical
`path to the modifying means where it is selectively switched
`such that it is reflected back along the same optical path, or
`alternatively, along a second optical path parallel to the first.
`The lateral displacement of the input and modified output
`beams of light (i.e., as opposed to angular displacement)
`allows for highly efficient coupling between a plurality of
`inputfoulput waveguides. For example, the instant invention
`is particular useful when the input and output ports are
`located on a same multiple bore tube, ribbon, or block.
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