(12) Ulllted States Patent
`Bouevitch et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,498,872 B2
`Dec. 24, 2002
`
`US006498872B2
`
`4/1998 Pan ........................... .. 385/11
`l;ord1etal.
`..... ..1..... ..
`2/1999
`ct ai
`3/1999 Li
`
`8/1999 Bishop et al.
`
`8/1999 Ford et a1,
`9/1999 Tomlinson .... ..
`Flfiigren 9131-
`eic mann ....... ..
`385/33
`10/2000 Keyworth et al.
`..
`7/2002 Sappey etal.
`............ .. 359/127
`
`
`
`..
`
`356/310
`385/140
`359/124
`359/295
`385/18
`
`5,740,288 A
`5,745,271 :
`512073232411 A
`5:881:199 A
`5,936,752 A
`5,943,153 A
`5,960,133 A
`1':
`,
`,
`6,134,359 A
`6,415,080 B1 *
`
`(54) OPTICAL CONFIGURATION FORA
`DYNAMIC GAIN EQUALIZER AND A
`CONFIGURABLE ADD/DROP
`MULTIPLEXER
`
`(75)
`
`_
`Inventors: Oleg Bouevitch; Gloucester (CA);
`Thomas Ducellier, Ottawa (CA); W.
`John Tomlinson, Princeton, NJ (US);
`,
`II)£i‘:'luE‘;ll1;‘;;'nr1':l':ilN(§It’tea“(€83
`'
`q
`=
`awa
`
`(73) Assignee: JDS Uniphase Inc., Ottawa (CA)
`
`* cited by examiner
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 17 days.
`
`Primary Examiner—Hemang Sanghavi
`.
`Assistant Examiner—Omar Rojas
`(74) Attorney, Agent, or Firm—Lacasse &Associates, LLC
`
`(57)
`
`ABSTRACT
`
`An optical device for rerouting and modifying an optical
`signal
`that
`is capable of operating as a dynamic gain
`equalizer (DGE) and/or.a configurable optical add/drop
`multiplexer (COADM) 1S disclosed. The optical design
`includes a front-end unit for providing a collimated beam of
`light, an element having optical power for providing
`collimating’focusing effects, a diffraction element for pro-
`Viding spatial dispersion, and modifying means which in a
`preferred embodiment includes one of a MEMS array and a
`hquid Crista; my ffcir ffltfimg 451% xnodlfymg éftltlleast a
`P°‘“°“ ° *1 Cam 0
`lg "
`C ‘"0 1 ymg means
`“°“°“S
`as an attenuator when the optical device operates as a DGE
`and as a switching array when the optical device operates as
`a COADM_ Advamageouslyy this invention provides a 4_f
`system wherein a preferred embodiment the element having
`optical power is a concave reflector for providing a single
`means for receiving light from the front-end unit, reflecting
`the received light to the dispersive element, receiving light
`ftrlom the dispersive element, andproviding dispersed light to
`t e modifying means. Conveniently and advantageously,
`this same concave reflector is utilized on a return path,
`obviating the requirement of matching elements. In one
`embodiment a single focussing/collimating lens is provided
`substantially at a focal plane of the element having optical
`powen
`
`41 Claims, 12 Drawing Sheets
`
`610
`
`FNC 1002
`
`(21) Appl. N0.: 09/729,270
`(22)
`Filed:
`Dec‘ 5, 2000
`
`(65)
`
`prior publication Data
`
`Us 2002/0009257 A1 -laI1- 24, 2002
`
`(60)
`
`(56)
`
`Related U-S- APPlie3ti011 Data
`Provisional application No. 60/183,155, filed on Feb. 17,
`2000
`1m.c1.7 ............................. G02B 6/28; H04J 14/02
`(51)
`(52) U.s. Cl.
`......................... .. 385/24; 385/37; 359/130;
`359/246; 359/247; 359/301; 359/302; 359/128
`(58) Field Of Search ............................... .. 349/193-, 196;
`359/115» 122» 128» 124» 130» 1-31> 24-“247=
`301-302; 385/16, 18, 24, 31, 37, 39; 47
`_
`References Clted
`US‘ PATENT DOCUMENTS
`4,367,040 A
`1/1983 Goto ......................... .. 356/44
`4,707,056 A
`11/1987 Bittner
`. 350/96.12
`
`4,839,884 A
`6/1989 SCh10SS ~ ~ ~ ~ - ~ ~ - ~ ~ ~ ~
`~ ~ ~ ~ ~ ~- 370/3
`8/1993 Wfldnau“ 9‘ a1~ ~~~~~~~~ ~~ 356/333
`5,233,405 A
`5,311,606 A *
`5/1994 Asakura etal. ...... .. 359/337.21
`5,414,540 A *
`5/1995 Patel et al.
`. . . .
`. . . . . .. 349/196
`5,477,350 A
`12/1995 Riza et al.
`. . . . .
`. . . . .. 359/39
`5,526,155 A
`6/1996 Knox et al.
`359/130
`
`
`
`605
`
`620
`
`650
`
`

`
`U.S. Patent
`
`Dec. 24, 2002
`
`Sheet 1 of 12
`
`US 6,498,872 B2
`
`110a
`
`120
`
`110b
`
`150
`
`()4
`
`102a
`
`102b
`
`FIG. 1
`
`

`
`U.S. Patent
`
`Dec. 24, 2002
`
`Sheet 2 of 12
`
`Us 6,498,872 B2
`
`(Ho
`
`Figure 2a.
`
`
`
`Figure 2b.
`
`

`
`U.S. Patent
`
`Dec. 24, 2002
`
`Sheet 3 of 12
`
`US 6,498,872 B2
`
`146
`
`FIG. 3a
`
`150/‘
`
`K 140
`
`142
`
`146
`
`FIG. 3b
`
`150 /J’
`
`‘\\
`
`140
`
`142
`
`730
`
`102a
`
`102b
`
`'
`
`FIG. 3c
`
`,50J
`
`740
`
`130
`
`102a
`
`102b
`
`FIG. 3d
`
`,5.)
`
`‘V40
`
`

`
`U.S. Patent
`
`Dec. 24, 2002
`
`Sheet 4 of 12
`
`US 6,498,872 B2
`
`152
`
`130 142
`
`
`
`FIG. 4a
`
`‘W750
`
`
`
`153
`
`FIG. 4b
`
`‘W150
`
`156
`
`157
`
`155
`
`102a
`
`102b
`
`158
`
`FIG. 5
`
`M
`
`“O
`
`

`
`

`
`U.S. Patent
`
`Dec. 24, 2002
`
`Sheet 6 of 12
`
`US 6,498,872 B2
`
`620
`
`605
`
`650
`
`610
`
`FIG. 7
`
`

`
`U.S. Patent
`
`Dec. 24, 2002
`
`Sheet 7 of 12
`
`US 6,498,872 B2
`
`

`
`U.S. Patent
`
`Dec. 24, 2002
`
`Sheet 8 of 12
`
`Us 6,498,872 B2
`
`998
`
`
`
`990
`
`976
`
`914 912
`
`996
`
`g
`
`5
`
`%
`
`;
`
`918
`
`FIG. 9c
`
`/998,999
`
`996
`
`

`
`U.S. Patent
`
`Dec. 24, 2002
`
`Sheet 9 of 12
`
`US 6,498,872 B2
`
`914
`
`916
`
`916
`
`
`
`994
`
`996
`
`998
`
`999
`
`916
`
`914 990
`
`913 912
`
`996
`
`998,999
`
`918
`
`990
`
`OA2 ___
`
`20¢
`
`

`
`U.S. Patent
`
`Dec. 24, 2002
`
`Sheet 10 of 12
`
`US 6,498,872 B2
`
`FIG.10
`
`\\\\\‘
`
`

`
`U.S. Patent
`
`Dec. 24, 2002
`
`Sheet 11 of 12
`
`US 6,498,872 B2
`
`8mm»mm8
`
`Q»
`
`
`
`

`
`U.S. Patent
`
`Dec. 24, 2002
`
`Sheet 12 of 12
`
`US 6,498,872 B2
`
`FIG12
`
`I
`
`\:’/50 51'
`
`r I I
`
`I
`
`x
`
`I
`
`-.—- E 0
`
`N
`
`12a
`
`Q0
`
`3
`
`Q7 —N
`
`O)
`
`0‘s
`
`

`
`US 6,498,872 B2
`
`1
`OPTICAL CONFIGURATION FOR A
`DYNAMIC GAIN EQUALIZER AND A
`CONFIGURABLE ADD/DROP
`MULTIPLEXER
`
`This application claims the benefit of Ser. No. 60/183,
`155, filed Feb. 17, 2000.
`
`FIELD OF THE INVENTION
`
`The present invention relates to an optical device for
`rerouting and modifying an optical signal, or more
`specifically, to an optical configuration including a diffrac-
`tion grating that can be used for a dynamic gain equalizer
`and/or a configurable add/drop multiplexer.
`
`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
`dilferent 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 multiplexers (ADM)
`to add and drop
`particular channels (i.e., wavelengths). For example, at
`predetermined nodes in the system, optical signals of pre-
`determined 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
`first diffraction 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 diffraction 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
`demultiplex an optical signal
`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,
`5,526,155, 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 configurable 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.
`Morcovcr, 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, 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 difiraction grating, a liquid crystal modulator, and
`a polarization dispersive element. In one embodiment, Patel
`et al. suggest extending the 1x2 switch to a 2x2 drop-add
`
`10
`
`15
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`circuit and using a reflector. 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 alignmcnt 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. U.S. Pat. No. 6,081,331 discloses
`an optical device that uses a concave mirror for multiple
`reflections as an alternative 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 configuration for rerouting and modifying an optical
`signal that can be used as a dynamic gain cqualizcr and/or
`configurable add/drop multiplexer.
`SUMMARY OF THE INVENTION
`
`The instant invention provides a 4-f optical system com-
`prising a dispersive element for spatially separating an input
`optical signal i11to different spectral channels and a modi-
`fying array for selectively modifying each of the different
`spectral channels. At least one element having optical power,
`such as a lens or a spherical mirror, provides optical com-
`munication between the dispersive element and the modi-
`fying array.
`Conveniently and advantageously, the dispersive element
`and the modifying array are disposed substantially at a focal
`plane of the at least one element having optical power.
`Moreover, the dispersive element and element having opti-
`cal power are used in a first and a second pass through the
`optics, thus obviating the requirement of providing matching
`elements.
`
`In accordance with the instant invention there is provided
`an optical device comprising: a first port for launching a
`beam of light; first redirecting means disposed substantially
`one focal length away from the first port for receiving the
`beam of light,
`the first redirecting means having optical
`power; a dispersive element disposed substantially one focal
`length away from the first redirecting means for dispersing
`the beam of light into a plurality of sub-beams of light;
`second redirecting means disposed substantially one focal
`length away from the dispersive element for receiving the
`dispersed beam of light, the second redirecting means hav-
`ing optical power, and, modifying means optically disposed
`substantially one focal length away from the second redi-
`recting means for selectively modifying each sub-beam of
`light and for reflecting each of the modified sub-beams back
`to the second redirecting means, wherein each sub-beam of
`light is incident on and reflected from the modifying means
`along substantially parallel optical paths.
`In accordance with the instant invention there is provided
`an optical device for rerouting and modifying an optical
`signal comprising: a first port for launching a beam of light;
`a concave reflector having a focal plane for receiving a beam
`of light launched from the first port; a dispersive element
`disposed substantially at the focal plane for spatially dis-
`
`

`
`US 6,498,872 B2
`
`3
`persing a beam of light reflected by the concave reflector and
`for redirecting a spatially dispersed beam of light back to the
`concave reflector; and modifying means disposed substan-
`tially at the focal plane for modifying the spatially dispersed
`beam of light reflected by the concave reflector and for
`reflecting the modified spatially dispersed beam of light
`back to one of thc first port and a sccond port via the concave
`reflector and the dispersive element.
`In accordance with the instant invention there is further
`
`provided a method of rerouting and modifying an optical
`signal comprising the steps of: launching a beam of light
`towards an element having optical power off an optical axis
`thereof; redirecting the beam of light incident on the element
`having optical power to a dispersive element disposed
`substantially one focal length away from the element having
`optical power; spatially dispersing the redirected beam of
`light into a plurality of difierent sub-beams of light corre-
`sponding to a plurality of different spectral channels with a
`dispersive element disposed substantially one focal length
`away from the element having optical power; redirecting the
`plurality of different sub-beams of light
`to a modifying
`means optically disposed substantially two focal lengths
`away from the dispersive element; selectively modifying the
`plurality of different sub-beams of light and reflecting them
`in a substantially backwards dircction; and rcdirccting thc
`selectively modified plurality of different sub-beams to the
`dispersive element and combining them to form a single
`output beam of light, wherein the plurality of different
`sub-beams of light and the selectively modified plurality of
`diiferent sub-beams follow substantially parallel optical
`paths to and from the modifying means, respectively.
`In accordance with the instant invention there is provided
`an optical device for rerouting and modifying an optical
`signal comprising: a lens including a first end having a single
`port coincident with an optical axis thereof and a second end
`having two ports disposed off the optical axis; an element
`having optical power disposed about one focal length away
`from the lens for receiving a beam of light launched from the
`single port; a dispersive element disposed about one focal
`length away from the element having optical power for
`spatially dispersing a beam of light received therefrom; and
`modifying means optically disposed about two focal lengths
`away from the dispersive element for modifying and reflect-
`ing a beam of light spatially dispersed by the dispersive
`element, wherein said one focal length is a focal length of
`the element having optical power.
`In accordance with the instant invention there is provided
`a method of modifying and rerouting a beam of light
`comprising the steps of: launching the beam of light through
`a first port disposed about a first end of a lens oif the optical
`axis of the lens, the beam of light launched in a direction
`parallel to the optical axis; allowing the beam of light to pass
`through the lens to a single port disposed about an opposite
`side of the lens coincident with the optical axis, and allowing
`the beam of light to exit the single port at a first predeter-
`mined angle to the optical axis; modifying the beam of light
`and reflecting the modified beam of light back to the single
`port at a sccond prcdctcrmincd angle to the optical axis; and,
`allowing the modified beam of light to pass through the lens
`to a second port disposed about the first end of the lens, the
`second port disposed off the optical axis and spatially
`separated from the first port.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Exemplary embodiments of the invention will now be
`described in conjunction with the drawings in which:
`
`4
`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. 211 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. 3:1 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 statc;
`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;
`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. 96 is a side view of the alternate arrangement shown
`in FIG. 9a’;
`
`10
`
`15
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`

`
`US 6,498,872 B2
`
`5
`FIG. 9f is a top view showing an asymmetric offset of tlie
`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;
`and,
`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.
`
`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 elen1ent 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.
`
`6
`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—3d, 4, and 5, each illustrate a different
`embodiment of the modifying means for use with the
`DGE/COADM dcviccs dcscribcd hcrcin. Each of thcsc
`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 clcmcnt 120 and clcmcnts 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 diagam of the
`modifying means 150 is shown including a liquid crystal
`array 130 and a reflector 140. The reflector includes first and
`second polarizing beam splitters 144 and 146, and reflective
`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 horizontal), 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 vertical). The reflector 140 is designed
`to pass light having a first polarization (e.g., horizontal) such
`that beam of light launched from port 102a is reflected back
`to the same port, and reflect light having another polarization
`(e.g., vertical) 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 birefringence 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. It is also preferred that 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 3d 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
`i11 FIGS. 311-301’,
`the dispersion direction is
`perpendicular to the plane of the paper. For exemplary
`purposes a single ray of light is shown passing through the
`modifying means 150.
`
`5
`
`10
`
`15
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`

`
`US 6,498,872 B2
`
`7
`FIGS. 4a and 4b are schematic diagrams showing ar1otl1er
`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 fron1 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 ar1 “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-
`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
`input/output 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.
`In order to maintain the desired simplicity and symmetry,
`is preferred that the element having optical power be
`
`it
`
`8
`rotationally symmetric, for example a rotationally symmet-
`ric lens or spherical
`reflector. Preferably,
`the spherical
`reflector is a concave mirror. Moreover, it is preferred that
`the diffraction element 120 be a high efficiency, high dis-
`persion diffraction grating. Optionally, a circulator (not
`shown) is optically coupled to each of ports 102a and 102b
`for separating input/output and/or add/drop signals.
`Referring again to FIG. 1, the operation of the optical
`device operating as a COADM is described by way of the
`following example. A collimated beam of light having a
`predetermined polarization and carrying wavelengths X1, X2,
`.
`.
`. X8 is launched through port 102a to a lower region of lens
`110a and is redirected to the diffraction grating 120. The
`beam of light is spatially dispersed (i.e., demultiplexed)
`according to wavelength in a direction perpendicular to the
`plane of the paper. The spatially dispersed beam of light is
`transmitted as 8 sub-beams of light corresponding to 8
`different spectral channels having central wavelengths X1,
`X2,
`.
`.
`. X8 through lens 110b, where it is collimated and
`incident on the modifying means 150, which for exemplary
`purposes, is shown in FIG. 3a—b. Each sub—beam of light is
`passed through an independently controlled pixel in the
`liquid crystal array 130. In particular, the sub—beam of light
`having central wavelength X3 passes through a liquid crystal
`cell in an “OFF” state, and each of the other 7 channels
`having central wavelengths 7t1—?t2 and 7»4—?t8 pass through
`liquid crystal cells in an “ON” state. As the sub—beam of light
`having central wavelength X3 passes through the liquid
`crystal in the “OFF” state, the polarization thereof is rotated
`90°,
`it is reflected by the polarization beam splitter 144
`towards a second beam splitter 146, and is reflected back to
`port 102b, as shown in FIG. 3b. As the other 7 channels
`having central wavelengths )t1—7t2 and )»4—)t8 pass through
`liquid crystal cells is in an “ON” state,
`the polarizations
`thereof remain unchanged, and they are transmitted through
`the polarization beam splitter 144 and are reflected off
`reflective surface 142 back to port 102a. In summary, the
`beam of light originally launched from port 10211 will return
`thereto having dropped a channel (i.e., having central wave-
`length K3) and the sub—beam of light corresponding to the
`channel having central wavelength 3 will be switched to port
`102b.
`
`Simultaneously, a second beam of light having a prede-
`termined polarization and carrying another optical signal
`having a central wavelength X3 is launched from port 102b
`to a lower region of lens 110a. It