(19) United States
`(12) Reissued Patent
`Wilde et al.
`
`(10) Patent Number:
`(45) Date of Reissued Patent:
`
`US RE42,678 E
`Sep. 6, 2011
`
`USO0RE42678E
`
`(56)
`
`References Cited
`
`359/39
`IFOCIZUI:/IIENTS
`5 414 540 {AS
`ate et
`.
`.................... ..
`,
`,
`359/198.1
`5/1997 Neukermans et al.
`5,629,790 A *
`4/1998 Ford et al.
`................... .. 359/130
`5,745,271 A
`5,835,458 A * 11/1998 Bischel et al.
`369/44.12
`2,329,552.: 2: 13/1223
`:32/1:
`6:028:689 A
`2/2000 Michalicek etal.
`.. 359/224
`6,204,946 B1 *
`3/2001 Aksyuk et al.
`398/9
`6,205,269 B1*
`3/2001 Morton ...... ..
`385/24
`6,222,954 B1*
`4/2001 Riza
`385/18
`6,253,135 B1
`7/2001 Wade
`385/37
`6,256,430 B1
`7/2001 Jin etal.
`385/18
`6,263,135 B1*
`7/2001 Wade ..... ..
`.. 385/37
`22321122 11* 13/2331
`////323/13;
`,
`.
`~
`............................ ..
`(Continued)
`
`
`
`or
`
`Primary Examiner — Brian M Healy
`(74) Attorney, Agent, or Firm — Barry N. Young
`
`ABSTRACT
`(57)
`This invention provides a novel wavelength-separating-rout-
`ing (WSR) apparatus that uses a diffraction grating to sepa-
`rate a multi-wavelength optical signal by wavelength into
`multiple spectral characters, which are then focused onto an
`array of corresponding channel micromirrors. The channel
`rnicromirrors 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 channel-by-charmel basis
`and coupling any spectral channel into any one of the 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 effi-
`ciencies of the spectral charmels into the output ports at
`desired values. The WSR apparatus of 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.
`
`67 Claims, 12 Drawing Sheets
`
`(54) RECONFIGURABLE OPTICAL ADD-DROP
`MULTIPLEXERS WITH SERVO CONTROL
`AND DYNAMIC SPECTRAL POWER
`MANAGEMENT CAPABILITIES
`
`(75)
`
`Inventors: Jeffrey P. Wilde, Morgan Hill, CA (US);
`E-Morganc/x<us>
`,
`(73) Assigneez Capella Photonics, Inc., San Jose, CA
`(US)
`
`(21) APPLNOJ 12/815,930
`
`<2»
`
`«mm
`Related U.S. Patent Documents
`
`Re‘ 39’397
`Nov. 14, 2006
`11/027,586
`Dec. 31, 2004
`
`Reissue Of:
`(64) Patent No‘:
`Issued:
`Appl. No.:
`Filed:
`.
`.
`.
`.
`Whlch 15 a Relssue Of’
`(64) Patent N0~3
`Issued:
`App1_ No‘;
`Filed:
`U.S. Applications:
`(60) Provisional application No. 60/277,217, filed on Mar.
`19, 2001.
`
`556255346
`Sep. 23, 2003
`09/933,426
`Aug 23’ 2001
`
`(51)
`
`Int. Cl.
`(2006.01)
`G02B 6/28
`(52) U.S. Cl.
`............... .. 385/24; 385/11; 385/37; 385/34
`(58) Field of Classification Search .................. .. 385/24,
`385/11, 37, 34
`See application file for complete search history.
`
`100
`
`
`
`103
`
`FNC 1001
`
`

`
`US RE42,678 E
`Page 2
`
`10/3004 B011eVitCh ..................... .. 385/24
`5,310,169 B2
`'7
`lgiigiffifilftfl """""""
`E3
`385/24
`R,E39’397 E >x< 11/3006 Wilde etal
`.
`........... N
`71835633 B2
`2/3007 Daneman 6151’ ’ ’
`’ ’ ’ H257/678
`2002/0131691 A1*
`9/3002 Garrettetal
`‘
`385/24
`. N”
`' """""""" "
`
`* cited by examiner
`
`“
`
`
`
`U.S. PATENT DOCUMENTS
`6,418,250 131*
`7/2002 Corbosieroetal.
`6,498,872 B2
`12/2002 B011eVitChet31~ ~
`615671574 B1
`5/2003 Ma 8‘ 31‘
`~~~~ ~~
`515001851 B2
`7/2003 Aksyuketal‘
`' ' ’ ' ' ' ' ' ' ' ' "
`385/16
`10/2003 Wageneretal.
`10/2003 Ford et al.
`..................... .. 398/88
`
`
`
`...... .. 385/24
`~ 709/318
`385/15
`385/18
`
`6,631,222 B1
`6,634,810 B1
`
`"'
`
`

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`U.S. Patent
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`Sep. 6, 2011
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`Sheet 1 of 12
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`US RE42,678 E
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`Fig. 1A
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`103
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`

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`U.S. Patent
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`Sep. 6, 2011
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`Sheet 2 of 12
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`US RE42,678 E
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`Sep. 6, 2011
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`Sheet 3 of 12
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`Sep. 6, 2011
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`U.S. Patent
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`Sep. 6, 2011
`
`Sheet 5 of 12
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`US RE42,678 E
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`200
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`U.S. Patent
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`Sep. 6, 2011
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`Sheet 6 of 12
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`US RE42,678 E
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`Sep. 6, 2011
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`US RE42,678 E
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`Sep. 6, 2011
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`Sheet 8 of 12
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`US RE42,678 E
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`

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`U.S. Patent
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`Sep. 6, 2011
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`Sheet 9 of 12
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`US RE42,678 E
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`U.S. Patent
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`Sep. 6, 2011
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`Sheet 10 of 12
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`US RE42,678 E
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`Sep. 6, 2011
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`US RE42,678 E
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`Sep. 6, 2011
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`US RE42,678 E
`
`1
`RECONFIGURABLE OPTICAL ADD-DROP
`MULTIPLEXERS WITH SERVO CONTROL
`AND DYNAMIC SPECTRAL POWER
`MANAGEMENT CAPABILITIES
`
`Matter enclosed in heavy brackets [ ] appears in the origi-
`nal patent but forms no part of the first and this reissue
`specification; matter printed in italics indicates the addi-
`tions made by the first reissue. Matter enclosed in double
`heavy brackets
`appears in the first reissue patent
`but forms no part of this reissue specification; matter
`printed in bold face indicates the additions made by this
`reissue.
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`This application claims priority of U.S. Provisional Patent
`Application No. 60/277,217, filed Mar. 19, 2001 which is
`incorporated herein by reference.
`
`FIELD OF THE INVENTION
`
`This invention relates generally to optical communication
`systems. More specifically,
`it relates to a novel class of
`dynamically reconfigurable optical add-drop multiplexers
`(OADMs) for wavelength division multiplexed optical net-
`working applications.
`
`BACKGROUND
`
`As fiber-optic communication networks rapidly spread
`into every walk ofmodern life, there is a growing demand for
`optical components and subsystems that enable the fiber-
`optic con1n1unications networks to be increasingly scalable,
`versatile, robust, and cost-effective.
`Contemporary fiber-optic communications 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 dif-
`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 modern 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 ofwavelengths on an optical fiber,
`hence taking away one or more data charmels from the trafiic
`stream on the fiber. It further adds one or more wavelength
`back onto the fiber, thereby inserting new data charmels in the
`same stream of traffic. As such, an OADM 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 of the 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-
`plexers/demultiplexers (e. g. 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. In the
`parallel architecture, as exemplified i11 U.S. Pat. No. 5,974,
`207, a demultiplexer (e.g., a waveguide grating router) first
`separates a multi-wavelength signal into its constituent spec-
`tral components. A wavelength switching/routing means
`
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`(e.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.e., the pass-
`through) wavelengths into an output multi-wavelength opti-
`cal signal. In the serial architecture, as exemplified in U.S.
`Pat. No. 6,205,269, tunable filters (e.g., Bragg fiber gratings)
`in combination with optical circulators are used to separate
`the drop wavelength from the pass-through wavelengths and
`subsequently launch the add channels into the pass-through
`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 OADMs 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 difiicult to reconfigure them in a
`dynamic fashion.
`U.S. Pat. No. 6,204,946 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 discrete states, such that it either
`retrofits its corresponding spectral charmel back into the input
`port as a pass-through charmel, or directs its spectral channel
`to an output port as a drop channel. As such, the pass-through
`signal (i.e., the combined pass-through channels) shares the
`same input port as the input signal. An optical circulator is
`therefore coupled to the input port, to provide necessary rout-
`ing oftl1ese two signals. Likewise, the drop channels share the
`output port with the add channels. An additional optical cir-
`culator is thereby coupled to the output port, from which the
`drop channels exit and the add channels are introduced into
`the output ports. The add channels are subsequently com-
`bined with the pass-through signal by way of the diffraction
`grating and the binary micromirrors.
`Although the aforementioned OADM disclosed by Askyuk
`et al. has the advantage of performing 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 port/fiber 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 OADM
`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 demultiplex the drop charmels after exiting the
`system. This additional n1ultiplexing/demultiplexing require-
`ment adds more cost and complexity that can restrict the
`versatility of the OADM thus-constructed. 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 environmental effects
`such as thermal and mechanical disturbances over the course
`
`of operation.
`U.S. Pat. No. 5,906,133 to Tomlinson discloses an OADM
`that makes use of a design similar to that of Aksyuk et al.
`There are input, output, drop and add ports implemented in
`
`

`
`US RE42,678 E
`
`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 chan-
`nel (coming from the input port) to the output port, or con-
`comitantly reflects its channel to the drop port and an incident
`add charmel to the output port. As such, this OADM is able to
`perform both the add and drop functions without involving
`additional optical components (such as optical circulators and
`in the system of the Aksyuk et al.). However, because a single
`drop port is designated for all the drop charmels and a single
`add port is designated for all the add charmels, the add chan-
`nels would have to be multiplexed before entering the add
`port and the drop channels likewise need to be demultiplexed
`upon exiting from the drop port. Moreover, as in the case of
`Askyuk et al., there are no provisions provided for maintain-
`ing requisite optical alignment in the system, and no mecha-
`nisms implemented for combating degradation in the align-
`ment due to environmental effects over the course of
`
`operation.
`As such, the prevailing drawbacks suffered by the OADMs
`currently in the art are summarized as follows:
`1) The wavelength routing is intrinsically static, rendering it
`difficult to dynamically reconfigure these OADMs.
`2) Add and/or drop channels often need to be multiplexed
`and/or demultiplexed, thereby imposing additional com-
`plexity and cost.
`3) Stringent fabrication tolerance and painstaking optical
`alignments are required. Moreover, the optical alignment is
`not actively maintained, rendering it susceptible to envi-
`ronmental effects such as thermal and mechanical distur-
`
`bances over the course of operation.
`4) In an optical communication network, OADMs 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 ofthe network, it
`is essential that 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) equaliza-
`tion at each stage. Such a power equalization capability is
`also needed for compensating for nonuniform gain caused
`by optical amplifiers (e.g., erbium doped fiber amplifiers)
`in the network. There lacks, however, a systematic and
`dynamic management of the power levels of various spec-
`tral channels in these OADMs.
`
`5) The inherent high cost and heavy optical loss further
`impede the wide application of these OADMs.
`I11 view of the foregoing, there is an urgent need in the art
`for optical add-drop multiplexers that overcome the afore-
`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-focuser; a11d an
`array of channel micromirrors.
`In operation, a multi-wavelength optical signal emerges
`from the input port. The wavelength-separator separates the
`multi-wavelength optical signal into multiple spectral chan-
`nels, each characterized by a distinct center wavelength and
`associated bandwidth. The bearn-focuser focuses the spectral
`channels into corresponding spectral spots. The channel
`micromirrors are positioned such that each charmel micro-
`mirror receives one of the spectral channels. The channel
`micromirrors are individually controllable and movable, e.g.,
`
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`60
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`65
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`4
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`continuously pivotable (or rotatable), so as to reflect the spec-
`tral charmels into selected ones of the output ports. As such,
`each channel micromirror is assigned to a specific spectral
`channel, hence the name “charmel micromirror”. And each
`output port may receive any number of the 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 continouously adjusted. This enables each chan-
`nel micromirror to scan its corresponding spectral channel
`across all possible output ports and thereby direct the spectral
`channel to any desired output ports.
`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 bearn-focuser
`may be a single lens, an assembly of lenses, or other beam-
`focusing means known in the art. The channel micromirrors
`may be provided by silicon micromachined mirrors, reflec-
`tive ribbons (or membranes), or other types of bearn-deflect-
`ing means known in the art. And each channel micromirror
`may be pivotable 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 channel micromirrors must be pivotable biaxially.
`The WSR apparatus of the present invention may further
`comprise an array of collimator-alignment mirrors, in optical
`COIIIIIIL nication with the wavelength-separator and the fiber
`collimators, for adjusting the alignment of the input multi-
`wavele1gth signal and directing the spectral channels into the
`selected output ports by way of angular control of the colli-
`mated beams. Each collimator-alignment mirror may be
`rotatab e about one or two axes. The collimator-alignment
`mirrors may be arranged in a one-dimensional or two-dimen-
`sional array. First and second arrays of imaging lenses may
`additio 1ally be optically interposed between the collimator-
`alignment mirrors and the fiber collimators in a telecentric
`arrangement,
`thereby “imaging” the collimator-alignment
`mirrors onto the corresponding fiber collimators to ensure an
`optima 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 of the channel micromirrors on an individual basis, so
`as to maintain a predetermined coupling efficiency of each
`spectral charmel in one of the output ports. As such, the
`servo-control assembly provides dynamic control of the cou-
`pling of the spectral charmels into the respective output ports
`and actively manages the power levels of the spectral chan-
`nels coupling into the output ports. (If the WSR apparatus
`includes
`an array of collimator-alignment 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 effectively relaxes the requisite fabrication toler-
`ances and the precision of optical alignment during assembly
`of a 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.
`
`

`
`US RE42,678 E
`
`5
`the WSR-S (or WSR) apparatus of the
`Accordingly,
`present invention may be used to construct a variety of optical
`devices, including a novel class of dynamically reconfig-
`urable optical add-drop multiplexers (OADMs), as exempli-
`fied in the following embodiments.
`One embodiment of an OADM of 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 pass-through 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
`N><l
`(N§2) 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 of an OADM 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 pass-through port
`and one or more drop ports. The second WSR-S (or WSR)
`apparatus includes a plurality of input ports and an exiting
`port. The configuration is such that the pass-through charmels
`from the first WSR-S apparatus and one or more add charmels
`are directed into the input ports of the second WSR-S appa-
`ratus, 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 (e.g., the first one) effectively
`performs a dynamic drop function, whereas the other WSR-R
`apparatus (e.g., 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 communication system.
`Moreover, the underlying OADM 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.
`
`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 OADM 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 charmel-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) The coupling of the spectral channels into the output ports
`is dynamically controlled by a servo-control assembly,
`rendering the OADM less susceptible to environmental
`effects (such as thermal and mechanical disturbances) and
`therefore more robust in performance. 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 of the spectral charmels 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 spectral power-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 OADM architecture allows a multiplicity
`of the OADMs according to the present invention to be
`readily assembled (e.g., cascaded) for WDM optical net-
`working applications.
`The 11ovel 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. 1A-1D show a first embodiment of a wavelength-
`separating-routing (WSR) apparatus according to the present
`invention, and the modeling results demonstrating the perfor-
`mance of the WSR apparatus;
`FIGS. 2A-2C depict second and third embodiments of a
`WSR apparatus according to the present invention;
`FIG. 3 shows a fourth embodiment of a WSR apparatus
`according to the present invention;
`FIGS. 4A-4B show schematic illustration of two embodi-
`
`ments 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 channel may carry a
`unique information signal, as in WDM optical networking
`applications.
`FIG. 1A depicts a first embodiment of a 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 of a wavelength-separat-
`ing-routing (WSR) apparatus of the present invention, the
`WSR apparatus 100 comprises multiple input/output ports
`which may be in the form of an array of fiber collimators 1 10,
`providing an input port 110-1 and a plurality of output ports
`110-2 through 110-N (N§3); a wavelength-separator which
`in one form may be a diffraction grating 101; a beam-focuser
`in the form of a focusing lens 102; and an array of channel
`micromirrors 103.
`
`In operation, a multi-wavelength optical signal emerges
`from the input port 110-1. The diffraction grating 101 angu-
`larly separates the multi-wavelength optical signal into mul-
`tiple spectral channels, which are in mm focused by the
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`US RE42,678 E
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`7
`focusing lens 102 into a spatial array of distinct spectral spots
`(not shown in FIG. 1A) in a one-to-one correspondence. The
`channel micromirrors 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 charmel micromirrors 103 are individually controllable
`and movable, e.g., pivotable (or rotatable) under analog (or
`continuous) control, such that, upon reflection, the spectral
`channels are directed into selected ones of the output ports
`1 10-2 through 1 10-N by way ofthe focusing lens 102 and the
`diffraction grating 101. As such, each charmel micromirror is
`assigned to a specific spectral channel, hence the name “chan-
`nel micromirror”. Each output port may receive any number
`of the reflected spectral channels.
`For purposes of illustration and clarity, only a selective few
`(e.g., three) of the spectral charmels, along with the input
`multi-wavelength optical signal, are graphically illustrated in
`FIG. 1A and the following figures. It should be noted, how-
`ever, that there can be any number of the spectral charmels in
`a WSR apparatus of the present invention (so long as the
`number of spectral charmels does not exceed the number of
`channel mirrors employed in the system). It should also be
`noted that the optical beams representing the spectral chan-
`nels shown in FIG. 1A and the following figures are provided
`for illustrative purpose only. That is, their sizes and shapes
`may not be drawn according to scale. For instance, the 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.
`In the embodiment of FIG. 1A, it is preferable that the
`diffracting grating 101 and the channel micromirrors 103 are
`placed respectively at the first and second (i.e., the front and
`back) focal points (on the opposing sides) ofthe 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 charmels to
`be efiiciently coupled into the respective output ports, thereby
`minimizing various translational walk—off effects that may
`otherwise arise. Moreover, the input multi-wavelength opti-
`cal signal is preferably collimated and circular in cross-sec-
`tion. The corresponding spectral charmels diffracted from the
`diffraction grating 101 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 polarization-dependent. That is, the dif-
`fraction efiiciency of a grating in a standard mounting con-
`figuration may 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-polarization, especially
`as the number of groove lines (per unit length) increases. To
`mitigate such polarization-sensitive effects, a quarter-wave
`plate 104 may be optically interposed between the diffraction
`grating 101 and the channel micromirrors 103, and preferably
`placed between the diffraction grating 101 and the focusing
`lens 102 as is shown in 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, if a beam of light has P-polarization with
`first encountering the diffraction grating, it would have pre-
`dominantly (if not all) S-polarization upon the second
`encountering, and vice versa.) This ensures that all the spec-
`tral channels incur 11early the same amount of round-trip
`polarization dependent loss.
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`In the WSR apparatus 100 of FIG. 1A, the diffraction
`grating 101, by way of example, is oriented such that the
`focused spots of the spectral charmels fall onto the channel
`micromirrors 103 in a horizontal array, as illustrated in FIG.
`1B.
`
`Depicted in FIG. 1B is a close-up view of the channel
`micromirrors 103 shown in the embodiment of FIG. 1A. By
`way of example, the charmel micromirrors 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 of each channel micromirror
`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. Each spectral channel, upon reflec-
`tion, is deflected in the y-direction (e.g., downward) relative
`to its incident direction, so to be directed into one ofthe output
`ports 110-2 through 110-N shown in FIG. 1A.
`As described above, a unique feature of the present inven-
`tion is that the motion of each charmel micromirror is indi-
`vidually and continuously controllable, such that its position,
`e.g., pivoting angle, can be continuously adjusted. This
`enables each channel micromirror to scan its corresponding
`spectral channel across all possible output ports and thereby
`direct the spectral channel to any desired output port. To
`illustrate this capability, FIG. 1C shows a plot of coupling
`efiiciency as a function of a channel micromirror’s pivoting
`angle 0, provided by a ray-tracing model of a WSR apparatus
`in the embodiment of FIG. 1A. As used herein,