(12) United States Patent
`Wagener et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,631,222 B1
`Oct. 7, 2003
`
`US006631222B1
`
`(54) RECONFIGURABLE OPTICAL SWITCH
`
`OTHER PUBLICATIONS
`
`(75) InVeIltOrSI JE?'EI‘SOII L- Wagener, Aberdeen, WA
`(US); ThOmaS AIldI‘eW 8131115891‘,
`Warren’ NJ (Us)
`-
`_
`-
`-
`(73) Asslgnee' Photuns’ Inc" plscataway’ NJ (Us)
`( * ) Notice:
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U_S_C_ 154(k)) by 0 days_
`
`,
`
`.
`
`..
`
`(21) Appl NO _ 09/571 833
`_
`May 16’ 2000
`(22) Flled:
`51
`Int. Cl.7 ................................................ .. G02B 6/35
`(52) US. Cl. ........................... .. 385/16; 385/17; 385/18;
`385/31
`(58) Field of Search ............................ .. 385/16, 17, 18,
`385/24, 31, 33, 47; 359/131, 196, 212,
`127, 124
`
`E. Murphy, OpticalFiber Telecommunications IIIB, Chapter
`10, edited by T. Koch and I. KaminoW, Academic Press.
`C.R. Doerr, Proposed WDM Cross Connect Using A Planar
`Arrangement of Waveguide Grating Routers and Phase
`Shifters, Photonics Technology Letters, vol. 10, No. 4, Apr.
`1998'
`CR. Giles, et al., “LoW—Loss ADD/DROP Multiplexers for
`WDM LightWave Networks,” Tenth International Confer
`ence on Integrated Optics and Optical Fibre Communica
`tion, IOOC, vol. 3, Jun. 29, 1995.
`JDS Uni hase Cor oration, Add—Dro Modules, Product
`P
`P
`P
`Bulletin 2000, Ontario, Canada.
`D.O.Culverhouse et al., LoW—loss all—?ber acousto—o tic
`P
`tunable ?lter, Optical Society OfAmerica, vol- 22, No- 2, Jan
`15, 1997, pp. 96—98.
`Roberto Sabella et al., “Impact of Transmission Performance
`on Path Routing in All—Optical Transport NetWorks,”,Jour
`mil ofLightware Technology, vol. 16, No. 11 (Nov. 1998),pp.
`1965—1971.
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`* cited by examiner
`
`1/1981 Nosu et al. .................. .. 370/3
`4,244,045 A
`4,626,066 A 12/1986 Levinson .... ..
`. 350/9618
`
`5,479,082 A 12/1995 Calvani et al.
`5,504,827 A
`4/1996 Schimpe . . . . . . .
`5,583,683 A 12/1996 Scobey ....... ..
`5,612,805 A
`3/1997 Fevrier et al. .
`5,621,829 A
`4/1997 Ford . . . . . . . . . . . .
`
`359/127
`. . . .. 385/24
`359/127
`.. 359/124
`. . . .. 385/22
`
`*
`
`11311131223:
`2
`5’841’917 A 11/1998 Juigerman et all
`5:915:050 A
`6/1999 Russell et a1_
`5,920,411 A * 7/1999 Duck et a1, _____ __
`5,959,749 A
`9/1999 Danagher et al. .
`5,960,133 A
`9/ 1999 Tomlinson . . . - - - - -
`5,974,207 A * 10/1999 Aksyuk ct a1~
`6,005,993 2 * 13/
`gacgfmald "
`A
`22000
`359224
`6’O75’632 A * 6/2000 Braun ___________
`359/124
`6’289’148 B1 * 9/2001 Lin et a1_ _ _ _ _ _ _ _
`_ _ _ __ 385/24
`6:327:398 B1
`12/2001 Solgaard et al. ............ .. 385/18
`
`"
`
`385/17
`____ __ 385/7
`359/127
`.. 359/124
`- - - -- 385/18
`385/24
`
`'
`
`Primary Examiner_JOhn D_ Lee
`-
`- _ -
`
`A7825?” Examiner Julillla K' Kang F k &W.H.
`(
`) Immey) gem) 0’ ”'”_Mayer 0“ on
`1 “ms,
`PC; Stuart H- Mayer, Esq
`
`(57)
`
`ABSTRACT
`
`An optical sWitch includes at least one input port for
`receiving a WDM optical signal having a plurality of Wave
`length components, at least three output ports, and a plural
`ity of Wavelength selective elements each selecting one of
`the Wavelength components from among the plurality of
`Wavelength components. Aplurality of optical elements are
`also provided, each of Which are associated With one of the
`Wavelength selective elements. Each of the optical elements
`direct the selected Wavelength component that is selected by
`its associated selected element to a given one of the output
`ports independently of every other Wavelength component.
`The given Output POIt is variably Selectable from among all
`the output ports.
`
`FOREIGN PATENT DOCUMENTS
`
`JP
`
`60-88907
`
`* 5/1985
`
`88 Claims, 3 Drawin Sheets
`8
`
`34013“)2 342340
`$340"
`UNSWITCgED
`1%
`OUTPUTS
`BYPAS
`315
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`FIBER
`301V
`“ARRAY
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`303 I
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`COLLIMATING LENSES
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`343
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`<-——- SILICA BLOCK
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`X
`322 ‘71,7“324
`/322
`310 177g 2 7k 2
`304
`302
`318
`312
`
`314
`
`INPUT
`
`7? TILT MIRROR
`
`v BULK LENS
`
`Cisco Systems, Inc.
`Exhibit 1025, Page 1
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`

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`U.S. Patent
`
`Oct. 7, 2003
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`3f01LI.66_.nS
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`US 6,631,222 B1
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`Cisco Systems, Inc.
`Exhibit 1025, Page 2
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`

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`U.S. Patent
`
`Oct. 7, 2003
`
`3f02LI.66_.nS
`
`US 6,631,222 B1
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`Cisco Systems, Inc.
`Exhibit 1025, Page 3
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`

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`U.S. Patent
`
`0a. 7, 2003
`
`Sheet 3 of3
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`US 6,631,222 B1
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`i 1
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`K I
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`2.22.30 5
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`Fm
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`om
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`Cisco Systems, Inc.
`Exhibit 1025, Page 4
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`

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`US 6,631,222 B1
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`1
`RECONFIGURABLE OPTICAL SWITCH
`
`STATEMENT OF RELATED APPLICATIONS
`
`This application claims the bene?t of priority to US.
`Provisional Patent Application Ser. No. 60/182,289, ?led
`Feb. 14, 2000, entitled “An all Optical Router With Petabyte
`Per Second Switching Capability. ”
`
`FIELD OF THE INVENTION
`
`10
`
`The invention relates generally to an optical communica
`tions system and more particularly to an optical sWitch for
`?exibly routing light in a Wavelength-selective manner.
`
`BACKGROUND OF THE INVENTION
`
`15
`
`Signi?cant interest exists in multi-Wavelength communi
`cation systems, Which are typically referred to as Wave
`length Division Multiplexed (WDM) systems. These sys
`tems use a WDM optical signal having different Wavelength
`components that support different streams of information.
`While WDM systems Were initially investigated to increase
`the information capacity that a ?ber could transmit betWeen
`tWo points, recent improvements in optical ?ltering
`technology, among other things, has led to the development
`of sWitching elements Which alloW a complex netWork of
`paths to be constructed that differ from Wavelength to
`Wavelength. Furthermore, in addition to the availability of
`Wavelength dependent sWitching elements in Which a given
`Wavelength is routed along a given path, recon?gurable
`optical elements have become available. Such recon?g
`urable optical elements can dynamically change the path
`along Which a given Wavelength is routed to effectively
`reconstruct the topology of the netWork as necessary to
`accommodate a change in demand or to restore services
`around a netWork failure.
`Examples of recon?gurable optical elements include opti
`cal Add/Drop Multiplexers (OADM) and Optical Cross
`Connects (OXC). OADMs are used to separate or drop one
`or more Wavelength components from a WDM signal, Which
`is then directed onto a different path. In some cases the
`dropped Wavelengths are directed onto a common ?ber path
`and in other cases each dropped Wavelength is directed onto
`its oWn ?ber path. OXCs are more ?exible devices than
`OADMs, Which can redistribute in virtually any arrange
`ment the components of multiple WDM input signals onto
`any number of output paths.
`The functionality of the previously mentioned recon?g
`urable optical elements can be achieved With a variety of
`different devices. For example, a common approach
`employs any of a number of different broadband sWitching
`fabrics inserted betWeen a pair of demultiplexers/
`multiplexers. Examples of OADM elements are disclosed in
`US. Pat. Nos. 5,504,827, 5,612,805, and 5,959,749, and
`general OXC sWitching architecture is revieWed by E.
`Murphy in chapter 10 of Optical Fiber Telecommunications
`IIIB, edited by T. Koch and I. KaminoW. As shoWn in these
`references, these approaches sequentially demultiplex the
`Wavelengths, perform the necessary sWitching and then
`remultiplex, Where the OXC can direct a given Wavelength
`onto any output because a conventional OXC uses a rela
`tively complex MxM device for the sWitching fabric, While
`OADMs are less ?exible due to their use of an array of 2x2
`optical sWitches that can only direct betWeen one of tWo
`outputs. TWo alternate approaches to OADMs employ sWit
`chable mirrors effectively inserted betWeen a device that
`simultaneously performs Wavelength demultiplexing and
`
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`35
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`40
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`2
`multiplexing. The ?rst of these approaches uses a thin ?lm
`dielectric demultiplexer/multiplexer that is traversed tWice
`by the Wavelengths (e.g., US. Pat. No. 5,974,207), While the
`second approach uses dispersion from a bulk diffraction
`grating to demultiplex (separate) the Wavelength channels
`before they re?ect off an array of tiltable mirrors (US. Pat.
`No. 5,960,133). Another set of OADM technologies employ
`4-port devices that drop multiple Wavelengths onto a single
`?ber output in a recon?gurable manner, and thus require an
`additional demultiplexer if the channels need to undergo
`broadband optoelectronic conversion at the receiver. One
`realiZation of such functionality uses ?ber optic circulators
`added to a tWo-port version of the previously-described
`diffraction grating demultiplexer and tilt mirror array (Ford
`et al., Postdeadline papers LEOS ’97, IEEE Lasers and
`Electro-Optics Society). Asecond realiZation uses integrated
`silica Waveguide technology (e.g., Doerr, IEEE Phot. Tech.
`Lett ’98) With thermo-optic phase shifters to sWitch betWeen
`the add and drop states for each Wavelength. Another
`four-port OADM employs a ?ber optic circulator and an
`optional tunable ?ber grating re?ector to route the dropped
`channels (e.g., C. R. Giles, IOOC ’95, JDS 2000 catalog)
`All of the aforementioned conventional optical sWitching
`technologies have shortcomings. These devices generally
`fall into tWo classes With respect to their shortcomings: very
`?exible devices With high cost and high optical loss, and
`loWer ?exibility devices, Which are less expensive and have
`loWer optical loss. The most ?exible OXCs can be pro
`grammed to sWitch the path of any of a large number of
`Wavelengths, each onto its oWn ?ber (e.g. demux/mux With
`sWitches), hoWever these devices may have up to 20 dB of
`insertion loss and therefore require an optical ampli?er to
`compensate for the loss. This substantially adds to the cost
`of an already expensive device. Because these devices are so
`costly, less ?exible alternatives such as ?ber gratings and
`thin ?lm ?lters are often used. While these devices have a
`signi?cantly loWer cost and insertion loss (2-5 dB/node),
`they are typically less ?exible because they are implemented
`as ?xed Wavelength OADMs that cannot be recon?gured.
`These devices are also in?exible because as you scale them
`so that they drop more Wavelengths their loss, cost, siZe
`and/or complexity increase to the point that the more ?exible
`OXC alternatives become more attractive. Recently, as
`shoWn in US. Pat. No. 5,479,082, some ?exibility has been
`added to these loWest cost OADM devices so that they can
`selectively drop or pass a predetermined subset of Wave
`lengths that Was previously designated as ?xed. In addition,
`the previously described recon?gurable OADM devices
`offer someWhat enhanced ?exibility, but typically at the
`expense of higher insertion loss (for Demux/sWitches), lim
`ited Wavelength resolution (for bulk grating approaches),
`and/or higher cost for additional Mux/Demux equipment
`used in connection With four-port devices.
`One particular limitation of the conventional OXC and
`OADM approaches, Which demultiplex the incoming signal
`before optical sWitching is performed, is that each output
`port can only drop a particular ?xed Wavelength that cannot
`be altered. In this con?guration each sWitch is arranged so
`that it only receives a preselected Wavelength component
`from the demultiplexer, and therefore can only output that
`particular Wavelength. Unless subsequent optical sWitching
`is used, the ?exibility of these devices is limited since it is
`not possible to redirect a given Wavelength from one output
`port to another output port or to redirect multiple Wave
`lengths to a given output port, should that become necessary.
`This functionality is desirable When a unique element Within
`the netWork is accessible through a particular port, and it is
`
`Cisco Systems, Inc.
`Exhibit 1025, Page 5
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`US 6,631,222 B1
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`3
`desirable to (a) change the Wavelength channel directed to
`that port, or (b) direct additional Wavelengths over that
`particular ?ber accessed via that port. TWo situations Where
`this functionality proves useful is When a link needs to be
`restored using an alternate Wavelength, or When the infor
`mation capacity directed to a speci?c port needs to be
`increased by adding additional WDM Wavelengths doWn the
`same ?ber.
`In vieW of the important role of optical sWitching to the
`?exibility and thus the value of an optical communications
`network, it Would be advantageous to provide a sWitching
`element that does not have the shortcomings of the previ
`ously mentioned devices.
`Accordingly, there is a need for an optical sWitching
`element that is inexpensive, imparts relatively loW loss to
`optical signals and Which is suf?ciently ?exible to direct
`each and every Wavelength component from any input port
`to any output port independently of one another.
`
`4
`nents includes the steps of directing the ?rst and second
`Wavelength components through a free space region.
`In accordance With yet another aspect of the invention, the
`?rst Wavelength is demultiplexed by a thin ?lm ?lter having
`a passband corresponding to the ?rst Wavelength.
`In accordance With another aspect of the invention, the
`?rst Wavelength component is directed through the free
`space region by a tiltable mirror.
`In accordance With another aspect of the invention, the
`demultiplexing and directing steps are performed by a
`plurality of narroW band free space sWitches. Alternatively,
`the demultiplexing and directing steps are performed by a
`plurality of tunable, Wavelength selective couplers.
`
`15
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`SUMMARY OF THE INVENTION
`The present invention provides an optical sWitch that
`includes at least one input port for receiving a WDM optical
`signal having a plurality of Wavelength components, at least
`three output ports, and a plurality of Wavelength selective
`elements each selecting one of the Wavelength components
`25
`from among the plurality of Wavelength components. A
`plurality of optical elements are also provided, each of
`Which are associated With one of the Wavelength selective
`elements. Each of the optical elements direct the selected
`Wavelength component that is selected by its associated
`selected element to a given one of the output ports indepen
`dently of every other Wavelength component. The given
`output port is variably selectable from among all the output
`ports.
`In accordance With one aspect of the invention, the optical
`sWitch includes a free space region disposed betWeen the
`input port and the Wavelength selective elements.
`In accordance With another embodiment of the invention,
`the Wavelength selective elements are thin ?lm ?lters each
`transmitting therethrough a different one of the Wavelength
`components and re?ecting the remaining Wavelength com
`ponents.
`In accordance With yet another embodiment of the
`invention, the optical elements are mirrors that are selec
`tively tiltable in a plurality of positions such that in each of
`the positions the mirrors re?ect the Wavelength component
`incident thereon to a different one of the output ports. The
`tiltable mirrors may be actuated by a micro
`electromechanical system or a pieZoelectric system, for
`example.
`The present invention also provides a method for direct
`ing at least ?rst and second Wavelength components of a
`WDM signal, Which includes a plurality of Wavelength
`components, from an input port to selected ones of a
`plurality of output ports. The method begins by demulti
`55
`plexing the ?rst Wavelength component from the WDM
`signal. The ?rst Wavelength component is then directed to a
`given output port. The second Wavelength component is also
`demultiplexed from the WDM signal and directed to one of
`the output ports selected independently from the given
`output port.
`In accordance With one aspect of the invention, the step of
`demultiplexing and directing the second Wavelength com
`ponent is performed after the step of demultiplexing and
`directing the ?rst Wavelength component.
`In accordance With another aspect of the invention, the
`steps of directing the ?rst and second Wavelength compo
`
`45
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`65
`
`FIG. 1 shoWs the functionality to be achieved by an
`optical sWitching fabric constructed in accordance With the
`present invention.
`FIG. 2 illustrates one embodiment of the optical sWitching
`element according to the present invention.
`FIG. 3 shoWs an alternative embodiment of the invention
`that employs Wavelength dependent acoustic null couplers.
`FIG. 4 shoWs another alternative embodiment of the
`invention that employs multiplexers/demultiplexers.
`
`DETAILED DESCRIPTION
`
`FIG. 1 shoWs the functionality to be achieved by an
`optical sWitching fabric constructed in accordance With the
`present invention. A Wavelength division multiplexed
`(WDM) signal is received on input port 10. Additional input
`ports may also be provided to accept additional WDM
`signals. Optical sWitching fabric 12 is designed to direct the
`individual Wavelength components of the WDM signal to
`select ones of the output ports 141, 142, .
`.
`. 14”. That is,
`sWitching fabric 12 can selectively direct any Wavelength
`component from any input port to any output port, indepen
`dent of the routing of the other Wavelengths.
`It should be noted that sWitching fabric 12 operates in a
`symmetric manner so that any Wavelength components
`directed to any of the output ports can be alternatively
`directed to any of the input ports. Accordingly, one of
`ordinary skill in the art Will recogniZe that the sWitching
`paths are reciprocal, and thus the terms input and output as
`used herein are not limited to elements that transmit a WDM
`signal or Wavelength component in a single direction rela
`tive to the sWitching fabric. In other Words, When light enters
`the device from any so-called output port, this output port
`serves as an input port, and similarly, any so-called input
`port can equally serve as an output port.
`As explained beloW, the present invention can achieve the
`functionality depicted in FIG. 1 in a variety of different
`Ways. The different arrangements can be broadly divided
`into tWo categories. In the ?rst category, ?lters having ?xed
`transmission and re?ection bands may be employed Which
`enable independent direction of the Wavelength components
`onto different optical paths. Alternatively, in the second
`category, tunable ?lters may be employed Which direct the
`Wavelength components along ?xed paths.
`FIG. 2 illustrates a ?rst embodiment of the optical sWitch
`ing element constructed in accordance With the present
`invention. In FIG. 2, the optical sWitching element 300
`comprises an optically transparent substrate 308, a plurality
`of dielectric thin ?lm ?lters 301, 302, 303, and 304, a
`plurality of collimating lens pairs 3211 and 3212, 3221 and
`3222, 3231 and 3232, 3241 and 3242, a plurality of tiltable
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`mirrors 315, 316, 317, and 318 and a plurality of output ports
`3401, 3402, .
`.
`. 340”. Substrate 308 has parallel planar
`surfaces 309 and 310 on Which ?rst and second ?lter arrays
`are respectively arranged. The ?rst ?lter array is composed
`of thin ?lm ?lters 301 and 303 and the second ?lter array is
`composed of thin ?lm ?lters 302 and 304. Individual ones of
`the collimating lens pairs 321—324 and tiltable mirrors
`315—318 are associated With each of the thin ?lm ?lters. As
`described beloW, each thin ?lm ?lter, along With its associ
`ated collimating lens pair and tiltable mirror effectively
`forms a narroW band, free space sWitch, ie a sWitch that
`routes individual Wavelength components along different
`paths. The overall physical dimensions of sWitching element
`300 Will be determined by the beam diameter of the WDM
`signal.
`Thin ?lm ?lters 301—304 are Well-knoWn components (for
`example, see US. Pat. No. 5,583,683), Which have a dielec
`tric multilayer con?guration. The thin ?lm ?lters 301—304
`have a Wavelength dependent characteristic, that is, their
`re?ectivity and transmissivity depends on the Wavelength of
`light. In particular, among the Wavelength components of the
`WDM optical signal received by thin ?lm ?lter 301, only the
`component With Wavelength K1 is transmitted therethrough.
`The remaining Wavelength components are all re?ected by
`thin ?lm ?lter 301. Likewise, thin ?lm ?lter 302 transmits
`only the component With wavelength )»2 and re?ects all other
`Wavelengths. In the same manner, the thin ?lm ?lters 303
`and 304 transmit components With Wavelengths k3, and M,
`respectively, and re?ect all other Wavelengths. Thus, the
`present invention demultiplexes Wavelengths through a plu
`rality of thin ?lm ?lters With different pass bands.
`The tiltable mirrors 315—318 are any mirrors that can be
`precisely tilted on 2 axes and are preferably small and very
`reliable. The exemplary mirrors discussed here are sup
`ported by one or more ?exure arms that employ a micro
`electromechanical system (MEMS). Actuation of the ?exure
`arms tilts the mirror surface to alter the direction of propa
`gation of an incident beam of light. Examples of such
`micro-electromechanical mirrors are disclosed in US. Pat.
`No. 6,028,689 and the references cited therein. Of course,
`other mechanisms may be alternatively employed to control
`the position of the mirrors, such as pieZoelectric actuators,
`for example.
`In operation, a WDM optical signal composed of different
`Wavelengths K1, K2, k3 and k4 is directed from the optical
`input port 312 to a collimator lens 314. The WDM signal
`traverses substrate 308 and is received by thin ?lm ?lter 301.
`According to the characteristics of the thin ?lm ?lter 301,
`the optical component With Wavelength K1 is transmitted
`through the thin ?lm ?lter 301, While the other Wavelength
`components are re?ected and directed to thin ?lm ?lter 302
`via substrate 308. The Wavelength component M, which is
`transmitted through the thin ?lm ?lter 301, is converged by
`the collimating lens 3211 onto the tiltable mirror 315.
`Tiltable mirror 315 is positioned so that Wavelength com
`ponent K1 is re?ected from the mirror to a selected one of the
`output ports 3401-340” via thin ?lm ?lters 302—304, Which
`all re?ect Wavelength component M. The particular output
`port that is selected to receive the Wavelength component
`Will determine the particular orientation of the mirror 315.
`As mentioned, the remaining Wavelength components k2,
`k3, and k4 are re?ected by thin ?lm ?lter 301 back into
`substrate 308 and directed to thin ?lm 302. Wavelength
`component )»2 is transmitted through thin ?lm ?lter 302 and
`lens 3221 and directed to a selected output port by tiltable
`mirror 316 via thin ?lm ?lters 303—304, Which all re?ect
`Wavelength component k2. Similarly, all other Wavelength
`
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`6
`components are separated in sequence by the thin ?lm ?lters
`303—304 and subsequently directed by tiltable mirrors
`317—318 to selected output ports. By appropriate actuation
`of the tiltable mirrors, each Wavelength component can be
`directed to an output port that is selected independently of all
`other Wavelength components. Any Wavelengths that have
`not been redirected by any of the tiltable mirrors may be
`received by an optional bypass port or ?ber 343. Although
`the embodiment of FIG. 2 is con?gured to selectively sWitch
`four Wavelengths, it Will be recogniZed that the invention
`more generally may selectively sWitch any number of Wave
`lengths by employing a corresponding number of narroW
`band, free space sWitches.
`A number of important advantages are achieved by the
`embodiment of the invention shoWn in FIG. 2. For example,
`because free space sWitching is employed, the number of
`optical connections is kept to a minimum, reducing the
`insertion loss, complexity and cost of the device. This
`advantage Will be more clearly demonstrated beloW When
`the number of connections required in FIG. 2 is compared to
`the number of connections required by the embodiment of
`the invention shoWn in FIG. 4.
`The folloWing description sets forth for illustrative pur
`poses only one particular example of the embodiment of the
`invention shoWn in FIG. 2. In this example, the substrate 308
`is a rectangular silica block having a thickness of 10 mm, a
`Width of 50 mm and a length of 90 mm. Asingle collimating
`lens that directed light to the input ?ber is ?xed relative to
`the block at a 57° angle With respect to the normal to the
`block. The focal length of the lens is chosen such that light
`exiting a Corning SMF-28TM ?ber and passing thru a lens
`results in a collimated optical beam With a Width of 1 mm.
`At the output, an array of collimating lenses is provided,
`each of Which couples light to one ?ber in the output array.
`The ?ber ends are polished ?at and have an anti-re?ective
`coating. An optional bypass port or ?ber may also be
`provided, Which collects any Wavelengths received at the
`input ?ber that has not been transmitted through any of the
`thin ?lm ?lters. The bypass ?ber provides an output for
`future upgrades that use additional Wavelengths not resonant
`in the original device. Alternately, this port might also be
`used if cost or loss restrictions make it preferable to sWitch
`a subset of the total incident Wavelengths, Where the remain
`ing (unsWitched) Wavelengths bypass the sWitching fabric.
`The ?rst and second array of narroW band free-space
`sWitches each include eight thin ?lm ?lters. The thin ?lm
`?lters are each a three-cavity resonant thin ?lm ?lter With a
`surface dimension of 10 mm by 10 mm. In the ?rst array, the
`?rst thin ?lm ?lter, Which is located 10 mm from the edge
`of the substrate, is bonded With optical-quality, index match
`ing epoxy to the substrate and has a passband centered at
`194.0 THZ (1545.32 nm). The optical pass band is nominally
`0.4 nm Wide at —0.5 dB doWn from the peak, With an
`isolation of better than —22 dB starting 100 GHZ from the
`center Wavelength. A 5 mm focal length collimating lens is
`bonded to the thin ?lm ?lter. A commercially available,
`micro-electro-mechanical (MEMS) tiltable mirror is then
`positioned at the focal point of the lens. Voltages can be
`applied to the tiltable mirror to vary its angular orientation
`along tWo axes. Typical angles over Which the mirror is
`adjusted do not exceed 30°.
`The ?rst array also includes a second narroW band free
`space sWitch located 10 mm from the ?rst free-space sWitch.
`The thin ?lm ?lter employed in this sWitch has a center
`optical Wavelength of 193.8 THZ (1546.92 nm). Six addi
`tional narroW band free-space sWitches are located along the
`substrate having center Wavelengths of 1548.52 nm, 1550.12
`
`Cisco Systems, Inc.
`Exhibit 1025, Page 7
`
`

`
`US 6,631,222 B1
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`nm, 1551.72 nm, 1553.32 nm, 1554.92 nm, and 1556.52 nm,
`respectively. The center-to-center distance between each
`switch is 10 mm.
`The second array of narrow band free space switches is
`located on the substrate surface opposing the substrate
`surface on which the ?rst array of switches is located. The
`second array of switches, which are also located 10 mm
`apart from one another, are laterally oriented half way
`between the ?rst array of switches. The eight thin ?lm ?lters
`employed in the second array of switches have center pass
`band wavelengths of 1544.52 nm, 1546.12 nm, 1547.72 nm,
`1549.32 nm, 1550.92 nm, 1552.52 nm, 1554.12 nm, and
`1555.72 nm, respectively.
`Each individual tiltable mirror has an electronics circuit to
`which a voltage is applied to steer the mirror. The voltage
`necessary to steer the mirror so that the wavelength it re?ects
`is directed to a particular output ?ber will differ from mirror
`to mirror. The operating voltages (-20 to +20 volt range) for
`steering the mirror are chosen to maximize the optical power
`coupled into the desired output ?ber.
`One of ordinary skill in the art will recogniZe that each of
`the narrow band free space switches shown in FIG. 2 do not
`necessarily require two lenses and a single mirror. Rather,
`other combinations of optical elements may be used to
`properly redirect the wavelength components. For example,
`two tiltable mirrors may be arranged to achieve the same
`result without the use of a lens. Alternatively, a single mirror
`may be used if in addition to being tiltable along two axes
`its position can also undergo a spatial translation.
`It is often important to monitor the presence and intensity
`of each individual wavelength component received by the
`switch shown in FIG. 2. This can become particularly
`dif?cult using conventional ?ber monitoring taps when the
`WDM signal includes a large number of wavelength com
`ponents. In the present invention, this problem may be
`readily overcome since only a single wavelength component
`is received by each of the tiltable mirrors. Accordingly,
`individual wavelength components may be monitored by
`placing a detector behind the mirror so that it receives the
`small portion of the power of the wavelength component
`that passes through the mirror. This information combined
`with conventional tap monitoring can provide network con
`trol and administration a more complete monitoring picture
`of light routed through the switch.
`It is also important to maintain accurate alignment
`between the tiltable mirrors in their various positions and the
`input and output ?bers to optimiZe the power they receive
`from the mirrors. This can be accomplished by slow adjust
`ment of the mirrors while monitoring the power coupled to
`the ?ber via conventional ?ber monitoring taps. However
`this approach becomes complicated if many other wave
`lengths are present on the ?ber, in which case it may be
`useful to improve the detection of each wavelength compo
`nent by encoding a small amplitude modulation with a
`unique RF frequency that is detected at the respective output
`?bers while adjusting the positions of the tiltable mirrors.
`This RF tone can be encoded at the transmitter with a unique
`tone for every wavelength, or alternately the RF amplitude
`modulation can be temporarily encoded during mirror
`adjustment by providing a small oscillation of the mirror tilt
`that slightly changes the coupling efficiency to the ?ber. The
`latter approach is bene?cial in tones that are encoded where
`they are measured, eliminating the need to track them
`throughout the network, and additionally, the tones are only
`encoded when they are needed for adjustments.
`FIG. 3 shows an alternative embodiment of the invention
`that employs wavelength dependent acoustic null couplers to
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`achieve tunable wavelength ?ltering. Such a coupler only
`cross-couples selected wavelengths from a ?rst to a second
`optical ?ber upon application of an appropriate acoustic
`vibration to the coupling region. If the appropriate acoustic
`vibration is not applied, the selected wavelengths continue
`to propagate along the ?rst optical ?ber. Examples of an
`acoustic null coupler are disclosed in D. O. Culverhouse et
`al., Opt. Lett. 22, 96, 1997 and US. Pat. No. 5,915,050.
`As shown in FIG. 3, an input ?ber 50 receiving the WDM
`signal is connected to an input port of a ?rst null coupler 521.
`One output port of the ?rst null coupler 521 is connected to
`an output ?ber 541 on which one or more individual wave
`length components are to be directed. The other output port
`is connected to an input port of a second null coupler 522.
`Similar to the output ports of the ?rst null coupler 521, the
`output ports of the second null coupler 522 are respectively
`connected to a second output ?ber 542 and the input port of
`a third null coupler 523. As indicated in FIG. 3, additional
`null couplers may be cascaded in this manner to provide
`additional output ports on which selected wavelength com
`ponents may be directed.
`In operation, one or more wavelength components
`directed along the input ?ber 50 can be directed to any
`selected output port 541, 542, .
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`. 54m by applying the
`appropriate acoustic wave for those components to the null
`couplers 521, 522, .
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`. 54m preceding those connected to the
`selected output port. For example, if any of the given n
`wavelength components are to be directed to output port 543,
`then the acoustic waves should be applied to null coupler
`523. Although this embodiment of the invention requires the
`wavelength components to tra