|||||||||||||||||||||||||||||[||||||||||||||||||||||||||||||||||||||||||I||
`
`USOURE42368E
`
`(19) United States
`(12) Reissued Patent
`US RE42,368 E
`(10) Patent Number:
`
`
`
` Chen et at. (45) Date of Reissued Patent: May 17, 2011
`
`(S4) RECONFIGURABLE OPTICAL ADD-DROP
`
`{55)
`
`REfErenceS Cited
`
`(75)
`
`MULTIPLEXERS WITH SERVO CONTROL
`AND DYNAMIC SPECTRAL POWER
`_
`~
`~_
`-
`1
`~
`MNAGEMLNT LAP‘BILITIE‘S
`Inventors:
`'l‘aiChen.San Jose.CA(US):Jsffrey P-
`Wilde. Morgan Hill. CA (US): Joseph
`«
`.
`.
`-
`1
`l“Davis‘Mmgwlhu‘LAlUS)
`.
`.
`(73) Asstgnee: Capella Phntonlcs, ll'lc.. San JOSE. CA
`(US)
`
`.
`(2]) Appl'NU” 121816.084
`
`(22)
`
`litled:
`
`.Iun.15. 2010
`Related U.S. Patent Documents
`
`6.879.750
`Apr. 12. 2005
`l 01"}45.364
`Dec. 22., 2003
`
`Reissue of:
`(64)
`Patent No.:
`Issued:
`Appl. No .:
`Filed:
`US. Applications:
`(63) Continuation of application No. 101005.714. filed on
`Nov. 7. 2001. now Pat. No. 6.681.431. which is a
`continuation of application No. 091938.426. filed on
`Aug. 23. 2001. now Pat. No. (1.625.346.
`
`(60) Provisional application No. 60121112111“. filed on Mar.
`l9, 200].
`
`(5])
`
`Int. (21.
`G023 6/28
`1104.! 14/02
`(52) U.S.CI.
`
`(2006.01)
`(2006.01)
`3851'24;3851‘10:3851"33; 385187:
`398.383
`
`385124.
`(58) Field of Classification Search
`3851'] l. 10. 3'1". 34. 33'. 398179. 82. 83, 84.
`398188. 87
`Sec application file for complete search history.
`
`..
`_
`‘
`, ,,,,,
`‘
`U-b‘ 112110011 DOC-”Mm 15
`5.414.540 A
`511995 i‘atelela].
`5.029.790 A
`5-1997 Neukermansetal.
`5.745.271 A
`41199::
`F d =1
`1.
`5.835.458 A
`11119951 ailch‘étlaat.
`5.960.133 A
`911999 Tomlinson
`5.974.207 A
`1011999 Aksyuk el al.
`6.028.589 A
`272000 Michalicek etal.
`(204.941:
`111
`372001 Al:
`lit 1.
`6.205.260 B]
`35300] Moi-13:“ L a
`6.222.954 111
`472001 Rim
`0.250.430 111
`712001
`.1111 eta].
`6.263.135 111
`772001 Wade
`0.389.155111
`912001 Wade
`0.307.057 111
`1012001 Ford
`5.418.250 111
`772002 (forbosieroelal.
`6.498.831: 152
`l2120102 IBouchtch ct :11.
`(C onttnued}
`
`Primmjt‘ haamt'ner — Brian M Healy
`(2'4) Attorney. Agent. or Fir-711 — Barry N. Young
`
`ABSTRACT
`[57)
`This invention provides a novel wavelength-separating-roul-
`ing (WSR) apparatus that uses a diffraction grating to sepa—
`rate a multi~wavelength optical signal by wavelength into
`multiple spectral channels. which are then focused onto an
`array of corresponding channel 111icromirrors. The channel
`nticrotnirrors are individually controllable and continuously
`pivotable to reflect the spectral channels into selected output
`ports. As such, the inventive WSR apparatus is capable of
`routing the spectral channels on a cltannel-by-channel basis
`and coupling any spectral channel into any one ofthe output
`ports. The WSR apparatus of the present invention may be
`further equipped with servo-control and spectral power-man-
`agement capabilities. thereby maintaining the coupling. elli-
`cicncies of the spectral channels into the output ports at
`desired values. The WSR apparatus 01‘ the present invention
`can be used to construct a novel class of dynamically recon~
`figurable optical add-drop multiplexers [OADMs} for WDM
`optical networking applications.
`
`22 Claims, 12 Drawing Sheets
`
`101
`
`
`
`103
`
`FNC 1001
`
`

`

`US RE42,368 E
`Page 2
`
`U.S. PATENT DOCUMENTS
`5x'2003
`Ma e1 31.
`I 2003
`Aksyuk el al.
`9:20 03
`Wilde et al.
`WHO 03
`Wagcner et al.
`10!20 03
`Ford e1 20.
`32004
`Chen et a1.
`[0.32004
`Bouevitch el al.
`
`6567,5711 31
`6.600851 BI!
`6.625.346 BE
`6.63l.222 Bl
`6.634.8[0 Bl
`6.687.431 32‘
`6.810.169 82
`
`(LSTQJSO BZ"
`6.898.348 BZ
`6.989.532} B2
`?.183.633 B2
`200230131691 Al
`2003:004347] Al
`
`452 005
`532 005
`12006
`20907
`932002
`3-‘2003
`
`Chen at al.
`Morozov el al.
`Bernstein at 20.
`Daneman e1 .1].
`Garrett er al.
`Belser el 81.
`
`385-"24
`
`3851524
`
`“ cited by examiner
`
`

`

`US. Patent
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`May 17, 2011
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`May 17, 2011
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`

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`US. Patent
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`May 17, 2011
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`US. Patent
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`US. Patent
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`May 17, 2011
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`US RE42,368 E
`
`l
`RECON FIGURABLE OPTICAL ADD-DROP
`MULTIPLEXERS WI'I‘I-l SERVO CONTROL
`AND DYNAMIC SPECTRAL POWER
`MANAGEMENT CAPABILITIES
`
`Matter enclosed in heavy brackets [ ] appears in the
`original patent but forms no part of this reissue specifica-
`tion: matter printed in italics indicates the additions
`made by reissue.
`
`to
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`This application is a contintlation of U.S. application Ser.
`No. 10l005.7l4. filed Nov. 7. 2001 now U.S. Pat. No. 6.687.
`43]. which is a continuation of U.S. application Ser. No.
`091938.426. filed Aug. 23. 2001. now U.S. Pat No. 6.625.346
`which claims the benefit of U.S. application Ser. No. 60!277.
`217. filed Mar. 19. 2001.
`
`30
`
`FIELD OF THE INVENTION
`
`This invention relates generally to optical cotmnunication
`systems. More specifically.
`it relates to a novel class of -
`dynamically reconfigurable optical add-drop multiplexers
`[OADMs] t'or wavelength division multiplexed optical net-
`working applications.
`
`30
`
`40
`
`45
`
`50
`
`UI 'Jl
`
`60
`
`BACKGROUND
`
`As fiber-optic communication netWorlts rapidly spread
`into every walk ol'modern life. there is a growing demand for
`optical components and subsystems that enable the fiber-
`optic communications networks to be increasingly scalable.
`versatile. robust. and cost-effective.
`Contemporary fiber-optic conununications networks com-
`monly employ wavelength division multiplexing (WDM). for
`it allows multiple information (or data) channels to be simul-
`taneously transmitted on a single optical fiber by using dill
`ferent wavelengths and thereby significantly enhances the
`information bandwidth of the fiber. The prevalence of WDM
`technology has made optical add—drop multiplexers indis—
`pensable building blocks of ntodern fiber-optic communica-
`tion networks. An optical add-drop multiplexer {OADM}
`serves to selectively remove (or drop) one or more wave
`lengths from a multiplicity of wavelengths on an optical fiber.
`hence taking away one or more data channels from the traffic
`stream on the fiber. [1 further adds one or tnore wavelengths
`back onto the fiber. thereby inserting new data channels in the
`same stream ol‘tralllc. As such. an GADM makes it possible
`to launch and retrieve multiple data channels (each charac-
`terized by a distinct wavelength] onto and from an optical
`fiber respectively. without disrupting the overall traffic flow
`along the fiber. Indeed. careful placement ofthe OADMs can
`dramatically improve an optical communication network‘s
`flexibility and robustness. while providing significant cost
`advantages.
`Conventional OADMs in the art typically employ multi-
`plexersldemultiplexers (eg. waveguide grating routers or
`arrayed-waveguide
`gratings).
`tunable
`filters.
`optical
`switches. and optical circulators in a parallel or serial archi-
`tecture to accomplish the add and drop Functions. hi the
`parallel architecture. as exemplified in U.S. Pat. No. 5.974.
`207. a dcmultiplcxcr (c.g.. a waveguide grating router] first
`separates a multi-wavelengtlt signal into its constituent spec-
`tral components. A wavelength switchingfrouting means
`
`2
`
`(c.g.. a combination of optical switches and optical circula-
`tors) then serves to drop selective wavelengths and add others.
`Finally, a multiplexer combines the remaining (i.c._. the pa ss-
`through) wavelengths into an output mulli-wavelength opti-
`cal signal. In the serial architecture. as exemplified in US.
`Pat. No. 6.205.269. tunable filters (cg. Bragg fiber gratings]
`in combination with optical circulators are used to separate
`the drop wavelengths from the pass-through wavelengths and
`subsequently launch the add channels into the pass-tlmmgh
`path. And if multiple wavelengths are to be added and
`dropped. additional multiplexers and demultiplexers are
`required to demultiplex the drop wavelengths and multiplex
`the add wavelengths, respectively. Irrespective of the under—
`lying architecture. the ()Ales currently in the art are char-
`acteristically high in cost. and prone to significant optical loss
`accumulation. Moreover. the designs of these OADMS are
`such that it is inherently dillicult to reconfigure them in a
`dynamic fashion.
`US. Pat. No. (1.204.946 to Askyuk et al. discloses an
`OADM that makes use of free-space optics in a parallel con-
`struction. In this case. a multi—wavelength optical signal
`emerging from an input port is incident onto a ruled diffrac-
`tion grating. The constituent spectral channels thus separated
`are then focused by a focusing lens onto a linear array of
`binary micromachined mirrors. Each micromirror is config—
`ured to operate between two diserctc states. such that it eithcr
`rctrorefiects its corresponding spectral channel back into the
`input port as a pass-through channel, or directs its spectral
`channel to an output pon as a drop channel. As such. the
`pass-through signal (i.e.. the combined pass-through chan-
`nels) shares the same input port as the input signal. An optical
`cirCulator is therefore coupled to the input port. to provide
`necessary routing of these two signals. Likewise, the drop
`channels share the output port with the add channels. An
`additional optical circulator is thereby coupled to the output
`port. from which the drop channels exit and the add channels
`are introduced into the output port. The add channels are
`subsequently combined with the pass-through signal by way
`of the djfl‘raction grating and the binary micromirrors.
`Although the aforementioned OADM disclosed byAskynk
`et a]. has the advantage ot‘perfornting wavelength separating
`and routing in free space and thereby incurring less optical
`loss. it suffers a number of limitations. First. it requires that
`the pass-through signal share the same portfliber as the input
`signal. An optical circulator therefore has to be implemented,
`to provide necessary routing of these two signals. Likewise.
`all the add and drop channels enter and leave the GADM
`through the same output port. hence the need for another
`optical circulator. Moreover. additional means must be pro-
`vided to multiplex the add channels before entering the sys-
`tem and to dcmultiplex the drop channels after exiting the
`system. This additional multiplexingfdemult iplexing require-
`tnent adds more cost and complexity that can restrict the
`versatility of the OADM thus—constntcted. Second. the opti—
`cal circulators implemented in this OADM for various rout-
`ing purposes introduce additional optical losses. which can
`accumulate to a substantial amount. Third. the constituent
`
`optical components must be in a precise alignment. in order
`for the system to achieve its intended purpose. There are.
`however. no provisions provided for maintaining the requisite
`alignment; and no mechanisms implemented for overcoming
`degradation in the alignment owing to envirorunental effects
`such as thermal and mechanical disturbances over the course
`
`of operation.
`U.S. Pat. No. 5,906,133 to ‘l‘omlinson discloses an OADM
`that makes use ol'a design similar to that of Aksyuk et all
`There are input. output. drop and add ports implemented in
`
`

`

`US RE42,368 E
`
`[0
`
`3
`this case. By positioning the four ports in a specific arrange-
`ment. each micromirror. notwithstanding switchable between
`two discrete positions. either reflects its corresponding chari-
`nel (coming from the input port) to the output port, or con-
`com itantly reflects its chatme] to the drop port and an incident
`add channel to the output port. As such. this (MUM is able to
`perform both the add and drop functions without involving
`additional optical components [such as optical circulators
`used in the systeln of Aksyuk et at). However, because a
`single drop port is designated for all the drop channels and a
`single add port is designated for all the add channels. the add
`channels would have to be multiplexed before entering the
`add port and the drop channels likewise need to be demutiv
`plexcd ttpon exiting from the drop port. Moreover, as in the
`case of Askyuk et at. there are no provisions provided for
`maintaining requisite optical alignment in the system, and no
`mechanisms implemented for combating degradation in the
`alignment due to environmental effects over the course of
`operation.
`As such, the prevailing drawbacks suffered by the DADMs
`currently in the art are summarized as follows:
`I] The wavelength routing is intrinsically static. rendering it
`dill'icult to dynamically reconfigure these OADMs.
`2) Add andtor drop channels oiten need to be multiplexed
`andfor demultiplexed. thereby imposing additional com- -
`plexity and cost.
`3) Stringent fabrication tolerance and painstaking optical
`alignment are required. Moreover, the optical alignment is
`not actively maintained. rendering it susceptible to envi-
`ronmental effects stlch as thermal and mechanical distur-
`
`30
`
`3t!
`
`bances over the course of operation.
`4) in an optical cortunttnication network. ()ADMs are typi-
`cally in a ring or cascaded configuration. In order to miti-
`gate the interference amongst OADMs. which often
`adversely affects the overall performance of the network. it
`is essential lhat the power levels of spectral channels enter-
`ing and exiting each OADM be managed in a systematic
`way, for instance, by introducing power (or gain] equal ira-
`lion at each stage. Such a power equalization capability is
`also needed forcompensating for non—unjfonn gain caused
`by optical amplifiers (cg. erbium doped fiber amplifiers)
`in the network. There lacks, however, a systematic and
`dynamic management of the power levels of various spec—
`tra] channels in these OADMs.
`5) The inherent high cost and heavy optical loss further
`impede the wide application of these OADMs.
`In view of the foregoing, there is an urgent need in the art
`for optical add-drop multiplexers that overcome the alore-
`mentioned shortcomings in a simple. effective. and economi-
`cal construction.
`
`SUMMARY
`
`The present invention provides a wavelength—separating—
`routing (WSR) apparatus and method which employ an array
`of fiber collimators serving as an input port and a plurality of
`output ports: a wavelength—separator; a beam~focuserz and an
`array of channel mieromirrors.
`In operation. a multi-wavelength optical signal emerges
`from the input port. The wavelength-separator separates the
`mulli-wavelenglh optical signal into multiple spectral chan-
`nels. each characterized by a distinct center wavelength and
`associated bandwidth. The beam-focuser focuses the spectral
`channels into corresponding spectral spots. The channel
`micromirrors are positioned such that each channel micro-
`mirror receives one of the spectral chamtels. The channel
`micromirrors are individually controllable and movable. eg.
`
`40
`
`45
`
`so
`
`UI 'Jl
`
`60
`
`4
`
`continuously pivotable (or rotatable), so as to reflect the spec-
`tral channels into selected ones of the output ports. As such.
`each channel micromirror is assigned to a specific spectral
`channel, hence the name “channel micromirror". And each
`output port may receive any number ofthe reflected spectral
`channels.
`A distinct
`
`feature of the channel micromirrors in the
`
`present invention, in contrast to those used in the prior art. is
`that the motion. e.g.. pivoting (or rotation]. of each channel
`micromirror is under analog control such that its pivoting
`angle can be continuously adjusted. This enables each chan-
`nel micromirror to scan its corresponding spectra] channel
`across all possible output ports and thereby direct the spectral
`channel to any desired output port.
`In the WSR apparatus of the present invention. the wave-
`length-separator may be provided by a ruled diffraction grat-
`ing. a holographic diffraction grating. an echelle grating. a
`curved diffraction grating. a dispersing prism. or other wave-
`length-separating means known in the art. The beam-focuser
`may be a single lens. an assembly of lenses. or other beatn—
`focusing means known in the art. The channel micromirrors
`may be provided by silicon micromachined minors. reflec-
`tive ribbons {or membranes], or other types of beam~defiect-
`ing means known in the art. And each channel micromirror
`may be pivolable about one or two axes. The fiber collimators
`serving as the input and output ports may be arranged in a
`one-dimensional or two-dimensional array. In the latter case,
`the chrome] micromin-ors must be pivotable biaxially.
`The WSR apparatus of the present invention may further
`comprise an array of coll imator-al ignment mirrors. in optical
`communication with the wavelength-separator and the fiber
`collimators. for adjusting the alignment of the input multi-
`wavelength signal and directing the spectral channels into the
`selected output ports by way of angular control of the colli-
`matcd beams.
`Iiach collimator-alignment mirror may be
`rotatable about one or two axes. The collimator-alignment
`mirrors may be arranged in a one-dimensional or two-dimen-
`sional array. 11" irst and second arrays of imaging lenses may
`additionally be optically interposed between the collimator~
`alignment mirrors and the fiber collimators in a telecentric
`arrangement.
`thereby “imaging" the colliniator-aligmnent
`mirrors onto the corresponding fiber collimators to ensure an
`optimal alignment.
`The WSR apparatus of the present invention may further
`include a servo-control assembly. in communication with the
`channel micromirrors and the output ports. The servo-control
`assembly serves to monitor the power levels of the spectral
`channels coupled into the output ports and further provide
`control ofthe channel micromimirs on an individual basis. so
`as to maintain a predetermined coupling efficiency of each
`spectral channel in one of the output ports. As such. the
`servo-control assembly provides dynamic control o f the cou-
`pling of the spectral channels into the respective output ports
`and actively manages the power levels of the spectral chan-
`nels coupled into the output ports. (If the WSR apparatus
`includes an array of collimator~aligmnent mirrors
`as
`described above, the servo—control assembly may addition—
`ally provide dynamic control of the collimator-alignment
`mirrors.) Moreover. the utilization of such a servo-control
`assembly elfectively relaxes the requisite fabrication toler-
`ances and the precision of optical alignment during assembly
`ofa WSR apparatus of the present invention, and further
`enables the system to correct for shift in optical alignment
`over the course of Operation. A WSR apparatus incorporating
`a servo-control assembly thus described is termed a WSR-S
`apparatus. thereinafter in the present invention.
`
`

`

`the WSR-S (or WSR] apparatus of the
`Accordingly,
`present invention may be used to construct a variety ofoptical
`devices, including a novel class of dynamically reconfig-
`urable optical add-drop multiplexers (OADMs). as exempli-
`fied in the following embodiments.
`One embodiment ofan OADM ol' the present invention
`comprises an aforementioned WSR—S (or WSR) apparatus
`and an optical combiner. The output ports of the WSR-S
`apparatus include a pass-through port and one or more drop
`ports. each carrying any number of the spectral channels. The
`optical combiner is coupled to the passehrough port. serving
`to combine the pass-through channels with one or more add
`spectral channels. The combined optical signal constitutes an
`output signal of the system. The optical combiner may be an
`le (NEZ) broadband fiber-optic coupler,
`for instance,
`which also serves the purpose of multiplexing a multiplicity
`of add spectral channels to be coupled into the system.
`In another embodiment ofan DADM of the present inven-
`tion. a first WSR-S (or WSR) apparatus is cascaded with a
`second WSR-S (or WSR) apparatus. The output ports of the
`first WSR—S [or WSR) apparatus include a passthrough port
`and one or tnore drop ports. The second WSRAS (or WSR)
`apparatus includes a plurality of input ports and an exiting
`port. The configuration is such that the passwthrough channels
`from the first WSR—S apparatus and one or more add channels -
`are directed into the input ports of the second WSR-S appa-
`rattls. and consequently multiplexed into an output multi-
`wavelength optical signal directed into the exiting port of the
`second WSR-S apparatus. That is to say that in this embodi-
`ment. one WSR-S apparatus (cg, the first one) effectively
`performs a dynamic drop function. whereas the other WSR-S
`apparatus (cg, the second one) carries out a dynamic add
`function. And there are essentially no fundamental restric»
`tions on the wavelengths that can be added or dropped. other
`than those imposed by the overall conuuunication system.
`Moreover. the underlying 0A] )M architecture thus presented
`is intrinsically scalable and can be readily extended to any
`number of the WSR-S (or WSR) systems. if so desired for
`performing intricate add and drop functions in a network
`environment.
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`Those skilled in the art will recognize that the aforemen-
`tioned embodiments provide only two of many embodiments
`of a dynamically reconfigurable OADM according to the
`present invention. Various changes. substitutions, and alter-
`nations can be made herein. without departing from the prin-
`ciples and the scope of the invention. Accordingly, a skilled
`artisan can design an DADM in accordance with the present
`invention. to best suit a given application.
`All in all. the OADMs of the present invention provide
`many advantages over the prior art devices, notably:
`1) By advantageously employing an array of channel micro-
`mirrors that are individually and continuously control-
`lable, an OADM of the present invention is capable of
`routing the spectral channels on a channel-by—channel
`basis and directing any spectral charmel into any one ofthe
`output ports. As such, its underlying operation is dynami-
`cally reconfigurable. and its underlying architecture is
`intrinsically scalable to a large number of channel counts.
`2) The add and drop spectral channels need not be multi-
`plexed and demultiplexed before entering and after leaving
`the OADM respectively. And there are not fundamental
`restrictions on the wavelengths to be added or dropped.
`3) Ilie coupling of the spectral channels into the output pons
`is dynamically controlled by a servo-control assembly.
`rendering the GADM less susceptible to environmental
`effects [such as thermal and mechanical disturbances) and
`therefore more robust in perihrmance. By maintaining an
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`optimal optical alignment. the optical losses incurred by
`the spectral channels are also significantly reduced.
`4) The power levels ofthe spectral channels coupled into the
`output ports can be dynamically managed according to
`demand, or maintained at desired values (e.g.. equalized at
`a predetermined value) by way of the servo-control assem-
`bly. This spectra] poorer-management capability as an inte-
`gral part of the OADM will be particularly desirable in
`WDM optical networking applications.
`5) The use of free—space optics provides a simple, low loss.
`and cost—effective construction. Moreover. the utilization
`of the servo-control assembly effectively relaxes the req-
`uisite fabrication tolerances and the precision of optical
`alignment during initial assembly, enabling the OADM to
`be simpler and more adaptable in structure, lower in cost
`and optical loss.
`6} The underlying DADM architecture allows a multiplicity
`of the DADMs according to the present invention to be
`readily assembled (e.g.. cascaded) liar WDM optical net-
`working applications.
`'l'he novel features ofthis invention, as well as the invention
`itself. will be best understood from the following drawings
`and detailed description.
`
`BRIEF DESCRIPTION OF THE FIGURES
`
`FIGS. lA-lD show a first embodiment ofa wavelength-
`separating-routing (WSR) apparatus according to the present
`invention. and the modeling results demonstrating the perfor-
`mance of the WSR apparatus;
`FIGS. ZA-ZC depict second and third embodiments of a
`WSR apparatus according to the present invention:
`FIG. 3 shows a fourth embodiment ofa WSR apparatus
`according to the present invention:
`FIGS. 4A~4B show schematic illustrations ot'two embo di—
`merits of a WSR-S apparatus comprising a WSR apparatus
`and a servo-control assembly. according to the present inven-
`tion:
`
`FIG. 5 depicts an exemplary embodiment of an optical
`add«drop multiplexer (OADM) according to the present
`invention: and
`FIG. 6 shows an alternative embodiment of an OADM
`according to the present invention.
`
`DETAILED DESCRIPTION
`
`In this specification and appending claims, a “spectral
`channel" is characterized by a distinct center wavelength and
`associated bandwidth. Each spectral chamtel may carry a
`unique information signal. as in WDM optical networking
`applications.
`FIG. IA depicts a first embodiment ofa wavelength-sepa-
`rating-routing (WSR) apparatus according to the present
`invention. By way of example to illustrate the general prin—
`ciples and the topological structure ofa wavelength-separat-
`ing-routing (WSR) apparatus of the present invention. the
`WSR apparatus 100 comprises multiple inputloutput pons
`which may be in the form ofan array of fiber collimators 11 0.
`providing an input port 110-1 and a plurality of output ports
`110-2 through llfl-N (N23): a wavelength-separator which
`in one form may be a diffraction grating 10]: a beam-focuser
`in the form ofa focusing lens 102: and an array of channel
`micromirrors 103.
`
`In operation. a mold-wavelength optical signal emerges
`from the input port 110-]. The diffraction grating lll] angu-
`larly separates the multi-wavelength optical signal into mul-
`tiple spectral channels. which are in turn focused by the
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`focusing lens 102 into a spatial array ofdistinct spectral spots
`(not shown in FIG. 1A] in a one-to-one correspondence. The
`channel microniirrors 103 are positioned in accordance with
`the spatial array formed by the spectral spots, such that each
`channel micromirror receives one of the spectral channels.
`The channel micromirrors 103 are individually controllable
`and movable. e.g.. pivotable (or rotatable] under analog (or
`continuous) control. such that. upoti reflection. the spectral
`channels are directed into selected ones of the output pons
`110-2 through 110-N by Way ofthe focusing lens 102 and the
`diffraction gratuig 101. As such. each channel micromirror is
`assigned to a specific spec! ral channel. hence the name “citati-
`iiel niicroiiiirror“. ['iach output port may receive any iituiiber
`of the reflected spectral channels.
`For purposes of illustration and clarity. only a selective few
`(cg. three) of the spectral channels. along with the input
`mum-wavelength optical signal. are graphically illustrated in
`FIG. 1A and the following figures. lt should be noted. how-
`ever. that there can be any number oftlie spectral channels in
`a WSR apparatus of the present invention (so long as the
`nutiiber of spectral channels does not exceed the number of
`channel mirrors employed in the system). It should also be
`noted that the optical beams representing the spectral chanv
`nefs shown in FIG. 1A and the following figures are provided -
`for illustrative purpose only. ‘l'hat is. their sizes and shapes
`may not be drawn according to scale. For instance, tfie input
`beam and the corresponding diffracted beams generally have
`different cross-sectional shapes, so long as the angle of inci-
`dence upon the diffraction grating is not equal to the angle of
`diffraction. as is known to those skilled in the art.
`III the embodiment of FIG. 1A. it is preferable that the
`diffraction grating 101 and the channel microniirrors 103 are
`placed respectively at the first and second (i.e.. the front and
`back) focal points (on the opposing sides) ol‘the focusing lens
`102. Such a telecentric arrangement alloWS the chief rays of
`the focused beams to be parallel to each other and generally
`parallel to the optical axis. In this application. the telecentric
`configuration further allows the reflected spectral channels to
`be efficiently coupled into the respective output ports. thereby
`minimizing various translational walk-off effects that may
`otherwise arise. Moreover. the input niulti-waveletigtli opti~
`cal signal is preferably collitriated and circular in cross—sec—
`tion. The corresponding spectral channels diffracted from the
`diffraction grating 10] are generally elliptical in cross-sec-
`tion; they may be of the same size as the input beam in one
`dimension and elongated in the other dimension.
`It is known that the diffraction efficiency of a diffraction
`grating is generally polariralion-dependent. That is. the dif-
`fraction efficiency ofa grating in a standard mounting coli-
`figuration niay be considerably higher for P-polarization that
`is perpendicular to the groove lines on the grating than for
`S—polarization that is orthogonal to P-polarizalion. especially
`as the number of groove lines (per unit length) increases. To
`mitigate such polarization-sensitive effects. a quarter-wave
`plate 1 04 may be optically interposed between the diffraction
`grating 101 and the channelmicromirrors 103. and preferably
`placed between the diffraction grating 10] and the focusing
`lens 102 as is shown iti FIG. 1A. In this way. each spectral
`channel experiences a total of approximately 90-degree rota-
`tion in polarization upon traversing the quarter-wave plate
`104 twice. (That is. ifa beam of light has P—polarizalion when
`first encountering the diffraction grating. it would have pre-
`dominantly [if not all) S-polariration upon the second
`encountering. and vice versa.) This ensures that all the spec-
`tral channels incur nearly the same amount of round-trip
`polarization dependent loss.
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`In the WSR apparatus 100 of FIG. 1A, the diffraction
`grating 10]. by way of example. is oriented such that the
`focused spots of the spectral channels fall onto the channel
`micromirrors 103 in a horizontal array. as illustrated in FIG.
`18.
`Depicted in FIG. 113 is a close-up view of the channel
`iiiicroniirrors 103 shown in the embodiment of FIG. 1A. By
`way of example. the channel microiiiirrors 103 are arranged
`in a one-dimensional array along the x-axis (i.e.. the horizon-
`tal direction in the figure}. so as to receive the focused spots of
`the spatially separated spectral channels in a one»to-one cor-
`respondence. (As in the case of FIG. 1A. only three spectral
`channels are illustrated. each represented by a converging
`beam.) Let the reflective surface ofcacli channel niicromirror
`lie in the x-y plane as defined in the figure and be movable.
`e.g.. pivotable (or deflectable) about the x-axis in an analog
`(or continuous) manner. IIach spectral channel. upon reflec-
`tion. is deflected iii the y-direction (e.g.. downward) relative
`to its incident direction. so to be directed into one ofthe output
`ports 110-2 through llO-N shown in FIG. 1A.
`As described above. a unique feature of the present inven—
`tion is that the motion of each channel microinirror is indi-
`vidually and continuously controllable. such that its position,
`e.g., pivoting angle. can be continuously adjusted. This
`enables each channel inicromirror to scan its corresponding
`spectral channel across all possible output ports mid thereby
`direct the spectral cfianiiel
`to any desired output port. To
`illustrate this capability. FIG. 1C shows a plot of coupling
`efficiency as a function of a channel micromirror‘s pivoting
`angle 0. provided by a ray-tracing model ofa WSR apparatus
`in the embodiment ofFIG. 1A. As used herein. the coupling
`efficiency for a spectral channel is defined as the ratio of the
`amount of optical power coupled into the fiber core in an
`output port to the total amount ofoptical power incident upon
`the entrance surface of lhe fiber (associated with the fiber
`collimator serving as the output port). In