(12)
`
`United States Patent
`Bouevitch et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,859,573 B2
`Feb. 22, 2005
`
`US006859573B2
`
`(54) DOUBLE PASS ARRANGEMENT FORA
`LIQUID CRYSTAL DEVICE
`
`1/1994 Pan ........................... .. 385/34
`5,276,747 A
`5,311,606 A * 5/1994 Asakura et a1.
`385/33
`5,414,540 A * 5/1995 Patel et al. ..... ..
`349/196
`
`(75) Inventors: Oleg Bouevitch, Gloucester (CA); Paul
`Colbourne, Nepean (CA)
`
`~ ~ ~ -- 349/24
`57477350 A * 12/1995 RiZf‘ et a1- ~ ~ ~ ~ ~
`5,499,132 A * 3/1996 T010 et al. ................ .. 359/281
`
`(73) Assignee: J DS Uniphase Inc., Ottawa (CA)
`
`(Llst Connnued on next page)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 104 days.
`
`(21) Appl_ No_; 10/247,431
`(22) Filed:
`Sep. 20, 2002
`(65)
`Prior Publication Data
`
`US 2003/0035605 A1 Feb. 20, 2003
`Related US, Application Data
`
`(63) Continuation-in-part of application No. 09/729,270, ?led 0n
`DeC-_5_, 2000, I10_W Pat NO- 6,498,872
`(60) ggggslrforiilaiggggitégn NO‘ 6O/183’155’ ?led on Feb‘ 17’
`’
`'
`(51) Int. c1.7 ................................................ .. G02B 6/26
`
`(52) US. Cl. ........................... .. 385/16; 385/24; 385/20;
`385/34; 385/36; 359/123; 359/130; 359/246;
`359/247; 359/301; 359/302; 349/193; 349/196;
`349/197
`
`EP
`EP
`EP
`W0
`W0
`
`FOREIGN PATENT DOCUMENTS
`0 654 917 A
`4/1995
`0 859 249 A
`8/1999
`0 947 865 A 10/1999
`WO 99/38348
`7/1999
`W0 2/44800 A2 11/2001
`
`......... .. G02F/1/133
`
`OTHER PUBLICATIONS
`
`US. application Publication US2001/0050738 A1 Publica
`tion Date Dec‘ 13, 2001, Miller‘
`S.W. Knight et al., “Wavelength dependence of persistent
`photoconductivity in indiurn—doped PblixSnxTe”, Semicon
`ductor Science and Technology, Institute of Physics. Lon
`don, GB, vol. 5, No. 3—S, Mar. 1, 1990, pp. S155—158.
`Joseph F. Ford et al., “Wavelength Add—Drop Switching
`Using Tilting Micromirrors”, Journal of LightWave Tech
`nology’ IEEE’ vol‘ 17’ NO‘ 5’ May 1999’ pp‘ 904_911'
`Primary Examiner—John R. Lee
`Assistant Examiner_David A_ Vanore
`(74) Attorney, Agent, or Firm—Teitelbaum & MacLean;
`Neil Teitelbaum; Doug MaCLean
`
`(57)
`
`ABSTRACT
`
`_
`
`_
`
`_
`
`_
`
`_
`
`_
`
`optical device for reroutmg and modifying an optical
`signal that uses a double pass through a liquid crystal
`modulator is disclosed. The optical device includes a ?rst
`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 re?ector for re?ecting the
`analyzed light back through the second polarizer, the liquid
`crystal modulator, and the ?rst polarizer. This arrangement
`provides a double pass through the liquid crystal modulator,
`thus signi?cantly improving the attainable extinction ratio.
`
`30 Claims, 17 Drawing Sheets
`
`1 10b
`
`}50
`
`(58) Field of Search ............................... .. 349/193, 196,
`
`349/197; 359/115, 122, 128, 130, 131,
`245, 246, 247, 301, 302; 385/16, 18, 24,
`31 32 39 47
`’
`’
`’
`
`(56)
`
`References Cited
`Us PATENT DOCUMENTS
`
`4,367,040 A * 1/1983 GOtO ------------------------- -- 356/44
`47461543 A : 7/1984
`2 1;;
`5’O33’83O A * 7/1991
`
`5,089,786 A * 2/1992 Tamura .................... .. 359/333
`5,233,405 A * 8/1993 Wildnauer et a1. ........ .. 356/333
`
`1 10a
`
`120
`
`/
`U41;
`l
`
`0A
`1'O2a—-—<->——
`
`1021)
`
`Capella 2037
`Fujitsu v. Capella
`IPR2015-00727
`
`1
`
`

`
`US 6,859,573 B2
`Page 2
`
`US. PATENT DOCUMENTS
`
`6,097,518 A * 8/2000 Wu 6161. .................... .. 398/1
`
`,
`
`,
`
`385/11
`t k
`3/1996 I
`5499 307 A
`5,526,155 A * 6/1996 Knox 6161. ................. .. 398/87
`
`Wa su a ................... ..
`
`8/2000 Solgaard et a1. ............ .. 385/17
`6,097,859 A
`$331932? : “9x888 gig """""""""""" " Z2312
`
`5,574,595 A * 11/1996 Ku1616 6161.
`
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`
`’
`
`’
`
`.
`
`. """"""""""""" "
`
`5,594,830 A * 1/1997 W11161611 6161. ........... .. 385/146
`
`g’gj’gzé 2 : 135888 ‘biiljhéteglal' """"""""" " 322%
`
`5,686,979 A * 11/1997 W6b616161. ............... .. 349/96
`5724165 A * 3/1998
`" 398/55
`5’727’1O9 A
`3/1998 Pan et a1
`385/14O
`537403288 A * 4/1998 Pan
`' """"""""" " 385m
`5745271 A ,,
`4/1998 Ford et a1‘ "
`398/87
`5771 120 A * 6/1998 B61g11161111 ................ .. 359/484
`3
`3
`.
`5,847,831 A * 12/1998 TOIIl11IlSOIl,IH 6161.
`_
`5867 264 A
`2/1999 Hinnrichs """"" ~~
`5881199 A
`3/1999 L1 ............................ .. 385/140
`538943233 A ,,
`4/1999 Yoon
`327/55
`539123748 A * @1999 Wu et'i'i'l """"""""""" "
`539173625 A
`@1999 ogusu et'ai"
`"559/130
`3
`3
`.
`' """"""" "
`5,936,752 A
`8/1999 BlShOp 6161. ............ .. 359/124
`5943158 A * 8/1999 Ford 6161.
`539463116 A ,,
`8/1999 Wu et al
`
`398/55
`
`356/364
`
`3
`3
`.
`' """""""""" "
`5,960,133 A * 9/1999 TOIIlllIlSOIl ................. .. 385/18
`5978116 A ,, 11/1999 Wu et al
`398/49
`’
`3
`' "" "
`5,999,672 A 12/1999 Hu11161 6161. ............... .. 385/37
`6005 697 A * 12/1999 Wu 6161. ................... .. 398/48
`6’O18’6O3 A
`H2000 Lundgren et al
`385/33
`630493367 A * 40000 Sharp etaL
`________ :549/117
`
`6,055,104 A
`6,081,331 A
`
`4/2000 c11611g ...................... .. 359/495
`6/2000 Teichmann ............... .. 356/328
`
`3
`3
`*
`' """ "
`"
`6,134,359 A 10/2000 Keyworth et a1. .......... .. 385/33
`6,175,668 B1 * 1/2001 Borrelli et a1. ............. .. 385/11
`6,177,992 B1 * 1/2001 B16u116161..
`356/327
`6,181,846 B1: 1/2001 P611 ..........
`385/18
`631923062 B1
`2/2001 sanchez'Rubl‘’ 6‘ al'
`372/92
`6195 479 B1 * 2/2001
`.. 385/18
`.
`3
`3
`*
`3/2001 L1u 6161. ..................... .. 398/9
`6,208,442 B1
`632363506 B1 * 5/2001 Ca" """"""""""""" " 359/484
`6,285,478 B1 * 9/2001 Liu 6161
`398/9
`6,285,499 B1 * 9/2001 X16 6161
`359/484
`6,327,019 B1 * 12/2001 P61616161
`349/196
`6337 934 B1
`1/2002 Wu 6161
`.. 385/16
`3
`3
`.
`633603037 B1
`3/2002 R1.”
`6,373,614 B1 * 4/2002 M11161
`
`" 385/22
`.359/237
`
`6421480 B2 * 7/2002 C66 . . . . .
`3
`3
`*
`8/2002 Xu 6161
`6,429,962 B1
`6452 702 B1 * 9/2002 Wu 6161
`3
`3
`*
`" 385m
`12/2002 Wooten """" "
`634933473 B1
`.. 385/24
`6,498,872 B2 * 12/2002 Bouevitch et a1
`2003/0113055 A1 * 6/2003 Z1166 6161. ................. .. 385/16
`
`359/3371
`.. 398/65
`
`. . . .. 385/24
`
`* cited by examiner
`
`2
`
`

`
`U.S. Patent
`
`Feb. 22,2005
`
`Sheet 1 0f 17
`
`US 6,859,573 B2
`
`110a
`
`120
`
`11Gb
`
`OA
`
`1 02b
`
`’
`
`FIG. 1
`
`3
`
`

`
`U.S. Patent
`
`Feb. 22,2005
`
`Sheet 2 0f 17
`
`US 6,859,573 B2
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`705 k
`
`107
`
`114
`
`FIG. 2a
`
`“wk 116 a\
`
`116
`
`119
`
`112/
`
`114a
`
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`
`114D
`
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`
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`
`4
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`

`
`U.S. Patent
`
`Feb. 22,2005
`
`Sheet 3 0f 17
`
`US 6,859,573 B2
`
`130
`
`142
`
`FIG. 3a
`
`150 J
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`130
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`102b
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`5
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`

`
`U.S. Patent
`
`Feb. 22,2005
`
`Sheet 4 0f 17
`
`US 6,859,573 B2
`
`FIG. 3d
`
`\140
`
`6
`
`

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`U.S. Patent
`
`Feb. 22, 2005
`
`Sheet 5 0f 17
`
`US 6,859,573 B2
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`102a
`
`102b
`
`102a
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`
`FIG. 5
`
`7
`
`

`
`U.S. Patent
`
`Feb. 22,2005
`
`Sheet 6 0f 17
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`US 6,859,573 B2
`
`610
`
`605 W
`
`620
`
`650 \
`
`FIG. 6a
`
`FIG. 6b
`
`8
`
`

`
`U.S. Patent
`
`Feb. 22,2005
`
`Sheet 7 0f 17
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`US 6,859,573 B2
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`9
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`

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`U.S. Patent
`
`Feb. 22,2005
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`Sheet 8 of 17
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`US 6,859,573 B2
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`840
`
`825
`
`10
`
`10
`
`
`

`
`U.S. Patent
`
`Feb. 22,2005
`
`Sheet 9 0f 17
`
`US 6,859,573 B2
`
`998
`
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`
`0A2
`
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`
`FIG. 9a
`
`990
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`
`U.S. Patent
`
`Feb. 22,2005
`
`Sheet 10 0f 17
`
`US 6,859,573 B2
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`916
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`

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`U.S. Patent
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`Feb. 22,2005
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`Sheet 11 0f 17
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`US 6,859,573 B2
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`FIG. 10
`
`13
`
`

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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 12 of 17
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`US 6,859,573 B2
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`mmmmmxmmNSoCV§\
`
`.23
`
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`
`14
`
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`

`
`U.S. Patent
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`Feb. 22,2005
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`Sheet 13 0f 17
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`US 6,859,573 B2
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`O
`
`15
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`

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`U.S. Patent
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`Feb. 22,2005
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`Sheet 14 0f 17
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`US 6,859,573 B2
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`22\
`
`24 f 26
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`FIG. 13
`Prior Art
`
`22\ 24 25
`
`T W
`U
`
`FIG. 15
`
`FIG. 163
`
`A
`
`FIG. 16b
`
`16
`
`

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`U.S. Patent
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`Feb. 22,2005
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`Sheet 15 0f 17
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`US 6,859,573 B2
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`74
`
`76
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`72\ I If"
`
`A
`
`FIG. 17
`
`FIG. 18a
`
`FIG. 18b
`
`17
`
`

`
`U.S. Patent
`
`Feb. 22, 2005
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`Sheet 16 of 17
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`US 6,859,573 B2
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`18
`
`18
`
`

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`U.S. Patent
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`Feb. 22,2005
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`Sheet 17 0f 17
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`US 6,859,573 B2
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`FIG. 21
`
`19
`
`

`
`US 6,859,573 B2
`
`1
`DOUBLE PASS ARRANGEMENT FOR A
`LIQUID CRYSTAL DEVICE
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`This application is a continuation-in-part of US. patent
`application Ser. No. 09/729,270 ?led on Dec. 5, 2000 now
`US. Pat. No. 6,498,872 and claiming priority from Provi
`sional Appl. No. 60/183,155 ?led on Feb. 17, 2000 noW
`abandoned.
`
`10
`
`FIELD OF THE INVENTION
`
`The present invention relates to an optical device for
`rerouting and modifying an optical signal, or more
`speci?cally, a liquid crystal device having a double pass
`arrangement.
`
`15
`
`BACKGROUND OF THE INVENTION
`In optical Wavelength division multiplexed (WDM) com
`munication systems, an optical Waveguide simultaneously
`carries many different communication channels in light of
`different 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 add/drop 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 add/drop 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
`?rst diffraction grating for demultiplexing the optical signal
`and a second spatially separated diffraction grating for
`multiplexing the optical signal after it has been modi?ed. An
`example of the latter is disclosed in US. Pat. No. 5,414,540,
`incorporated herein by reference. HoWever, in such
`instances it is necessary to provide and accurately align tWo
`matching diffraction gratings and at least tWo matching
`lenses. This is a signi?cant 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
`demultiplex an optical single in a ?rst pass through the
`optics and multiplex the optical signal in a second pass
`through the optics. For example, US. Pat. Nos. 5,233,405,
`5,526,155, 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 con?gurable optical add/drop 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 sWitching/attenuating means.
`Moreover, none of the prior art devices disclose a
`multiplexing/demultiplexing optical arrangement that is
`compact and compatible With a plurality of parallel input/
`output optical Waveguides.
`For example, US. 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 diffraction grating, a liquid crystal modulator, and
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`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 re?ector. HoWever, the disclosed device
`is limited in that the add/drop beams of light are angularly
`displaced relative to the input/output beams of light. This
`angular displacement is disadvantageous With respect to
`coupling the add/drop and/or input/output 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 con?gurations,
`Wherein the input beam of light passes through each optical
`component sequentially before being re?ected in a substan
`tially backWards direction.
`US. Pat. No. 6,081,331 discloses an optical device that
`uses a concave mirror for multiple re?ections 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 diffraction grating and does not
`realiZe the advantages of the instant invention.
`It is an object of this invention to provide an optical
`system including a diffraction grating that is relatively
`compact.
`It is a further object of the instant invention to provide an
`optical con?guration for rerouting and modifying an optical
`signal that can be used as a dynamic gain equaliZer and/or
`con?gurable add/drop 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 ?rst optical ?bre for providing
`an input beam of light, ?rst 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 re?ective surface positioned to receive the light passed
`through the second polariZing means and re?ect 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 ?rst optical ?bre and a second optical
`?bre.
`In accordance With the invention there is provided an
`optical device comprising an optical ?bre for launching an
`input optical signal, a ?rst 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 ?rst 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 re?ective element
`disposed for re?ecting the light transmitted from the second
`
`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 reflected 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 and/or add-drop multiplexer (DGE/COADM)
`in accordance with the invention;
`FIG. 2a is a detailed side view of a front-end module for
`
`use with the DGE/COADM 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
`DGE/COADM 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 DGE/COADM 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 DGE/COADM 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. 6a 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 input/output ports, diffraction
`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. 6a and 6b
`including an optical circulator; and
`FIG. 9 is a schematic diagram of a DGE/COADM 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
`input/output 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 input/output 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. 9a’ is a top view showing an alternative arrangement
`to the optical components shown in FIG. 9b;
`FIG. 9e is a side view of the alternate arrangement shown
`in FIG. 9d;
`FIG. 9f is a top view showing an asymmetric offset of the
`input/output 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 input/output 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 backrefiector;
`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. 18b is a schematic diagram of the birefringent wedge
`depicted in FIG. 18a showing the beam deflection;
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`US 6,859,573 B2
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`5
`FIG. 19a 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-f 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) and/or a Config-
`urable Optical Add/Drop Multiplexer (COADM).
`The optical design includes a diffraction element 120
`disposed between and at a focal plane of identical elements
`110a and 110b having optical power, respectively. Two ports
`102a and 102b are shown at an input/output 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 102a, or alternatively, can be switched to port 102b
`or vice versa in a controlled manner. The input/output ports
`102a and 102b are also disposed about one focal plane away
`from the element having optical power 110a to which they
`are optically coupled. Although only two input/output 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
`110b.
`
`Since the modifying means and/or 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 and/or attenuation. FIGS. 2a
`and 2b illustrate two different embodiments of polarization
`diversity arrangements for providing light having a known
`polarization state, for use with the DGE/COADM 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 effects 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, 114b 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. 2a 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—3a', 4, and 5, each illustrate a different
`embodiment of the modifying means for use with the
`DGE/COADM 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. 3a 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. 3a, 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 102b.
`When the device operates as a DGE, each liquid crystal
`cell is adjusted to provide phase 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. 3a and 3b 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 3a’ 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. 4a 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 102a 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. 4a,
`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 102a. 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 102a 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 affect 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 102b. 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