`Smith et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,798,941 B2
`Sep. 28, 2004
`
`US006798941B2
`
`(54) VARIABLE TRANSMISSION MULTI-
`CHANNEL OPTICAL SWITCH
`
`(75) Inventors: David A. Smith, Atlanta, GA (US);
`John E. Golub, Atlanta, GA (US);
`Fariborz Farhan, Alpharetta, GA (US)
`
`(73) Assignee: Movaz Networks, Inc., Norcross, GA
`(Us)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 78 days.
`
`(21) Appl. No.: 09/957,312
`(22) Filed:
`Sep. 20, 2001
`(65)
`Prior Publication Data
`
`US 2002/0071627 A1 Jun. 13, 2002
`
`Related US. Application Data
`(60) Provisional application No. 60/234,683, ?led on Sep. 22,
`2000, and provisional application No. 60/267,285, ?led on
`Feb. 7,2001.
`
`(51) Int. Cl.7 ............................ .. G02B 6/26; G02B 6/42
`
`(52) US. Cl. . . . . .
`
`. . . . . . . . . . . . . .. 385/18; 385/19
`
`(58) Field of Search ............................ .. 385/18, 19, 15,
`385/33; 398/156
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,696,062 A * 9/1987 LaBudde .................. .. 398/156
`4,911,526 A
`.. 350/9624
`5,414,540 A
`..... .. 359/39
`5,621,829 A
`385/22
`5,745,271 A
`359/130
`5,771,320 A
`385/16
`5,796,479 A
`356/326
`5,828,800 A
`385/20
`5,915,063 A
`385/140
`5,960,133 A
`385/18
`
`8/2000 Solgaard 6161. ............ .. 385/17
`6,097,859 A
`8/2000 Laor ......................... .. 385/16
`6,101,299 A
`6,111,686 A * 8/2000 Toyohara ........ ..
`.. 359/337.13
`6,204,946 B1
`3/2001 Aksyuk et al.
`359/131
`6,263,123 B1
`7/2001 Bishop et al. .............. .. 385/15
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`EP
`JP
`WO
`W0
`
`0 984 311
`0 729 044
`55-159402
`W/O 02/25358
`WO 00/20899
`
`3/2000
`1/2003
`12/1980
`3/2000
`4/2000
`
`OTHER PUBLICATIONS
`
`Lin et al., “Micro—electro—mechanical systems (MEMS) for
`WDM optical—crossconnect networks”, Military Communi
`cation Conference Proceedings, 1999. MILCOM 1999.
`IEEE Atlantic City, NJ. USA 31, IEEE, US, Oct. 31, 1999,
`954—957 pp.
`G. Jeong et al., “Comparison of Wavelength—interchanging
`and Wavelength—selective cross—connects in multiWave
`length all—optical networks”, IEEP, 1996, 156—163 pp.
`
`* cited by examiner
`
`Primary Examiner—Chandrika Prasad
`(74) Attorney, Agent, or Firm—Charles S. GuenZer
`(57)
`ABSTRACT
`
`A multi-Wavelength or White-light optical sWitch including
`an array of mirrors tiltable about tWo aXes, both to control
`the switching and to provide variable poWer transmission
`through the sWitch, both for optimization and for poWer
`equalization betWeen Wavelength channels in a multi
`Wavelength signal. The output poWer of a channel is
`monitored, thereby alloWing feedback adjustment of the
`transmitted poWer. The mirrors are preferably formed in a
`micro electrornechanics system array to be tiltable in
`orthogonal directions and having electrostatically controlled
`tilting by tWo pairs of electrodes beneath the mirrors. Input
`poWer of the separate channels may also be monitored.
`
`37 Claims, 11 Drawing Sheets
`
`NETWORK
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`Cisco Systems, Inc.
`Exhibit 1004, Page 1
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`US. Patent
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`Sep. 28,2004
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`Sheet 1 0f 11
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`US 6,798,941 B2
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`‘IO
`
`20
`
`
`
`(PRIOR ART)
`
`FIG.
`
`1
`
`24
`
`24
`
`24
`
`22
`
`(PRIOR ART)
`
`FIG. 2
`
`26
`
`26
`
`26
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`Cisco Systems, Inc.
`Exhibit 1004, Page 2
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`Cisco Systems, Inc.
`Exhibit 1004, Page 2
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`U.S. Patent
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`Sep. 28,2004
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`Sheet 2 0f 11
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`US 6,798,941 B2
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`FIG. 3
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`Cisco Systems, Inc.
`Exhibit 1004, Page 3
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`U.S. Patent
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`Sep. 28,2004
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`Sheet 3 0f 11
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`US 6,798,941 B2
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`Exhibit 1004, Page 4
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`
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`U.S. Patent
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`Sep. 28,2004
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`Sheet 4 0f 11
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`US 6,798,941 B2
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`Cisco Systems, Inc.
`Exhibit 1004, Page 5
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`
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`U.S. Patent
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`Sep. 28,2004
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`Sheet 5 0f 11
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`US 6,798,941 B2
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`220
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`218
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`FIG. 9
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`TO
`FROM
`NETWORK NETWORK
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`Cisco Systems, Inc.
`Exhibit 1004, Page 6
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`US. Patent
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`Sep. 28, 2004
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`Sheet 6 0f 11
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`US 6,798,941 B2
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`
`
`SWITCH
`COMMANDS
`
`228
`
`NETWORK
`
`FROM
`
`Cisco Systems, Inc.
`Exhibit 1004, Page 7
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`Cisco Systems, Inc.
`Exhibit 1004, Page 7
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`
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`U.S. Patent
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`Sep. 28,2004
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`Sheet 7 0f 11
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`US 6,798,941 B2
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`190
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`FIG. 11
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`170
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`164166
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`Cisco Systems, Inc.
`Exhibit 1004, Page 8
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`U.S. Patent
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`US 6,798,941 B2
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`188
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`180
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`178
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`176
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`168
`166
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`216
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`
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`FROM
`NETWORK
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`Cisco Systems, Inc.
`Exhibit 1004, Page 9
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`U.S. Patent
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`Sep. 28,2004
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`Sheet 9 0f 11
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`US 6,798,941 B2
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`260
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`Cisco Systems, Inc.
`Exhibit 1004, Page 10
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`U.S. Patent
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`Sep. 28,2004
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`Sheet 10 0f 11
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`US 6,798,941 B2
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`r204
`
`FIG. 16
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`Cisco Systems, Inc.
`Exhibit 1004, Page 11
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`
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`US. Patent
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`Sep. 28,2004
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`Sheet 11 0f 11
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`US 6,798,941 B2
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`308
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`302
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`504
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`300
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`306
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`\—312
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`Cisco Systems, Inc.
`Exhibit 1004, Page 12
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`Cisco Systems, Inc.
`Exhibit 1004, Page 12
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`US 6,798,941 B2
`
`1
`VARIABLE TRANSMISSION MULTI
`CHANNEL OPTICAL SWITCH
`
`RELATED APPLICATIONS
`
`This application claims bene?t of US. Provisional Appli
`cations Nos. 60/234,683, ?led Sep. 22, 2000, and 60/267,
`285, ?led Feb. 7, 2001.
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`The invention relates generally to optical sWitches. In
`particular, the invention relates to optical sWitches used in
`multi-channel optical communications netWorks and having
`controlled transmissivity for different channels.
`2. Background Art
`Modern communications netWorks are increasingly based
`on silica optical ?ber, Which offers very Wide bandWidth
`including several transmission bands usable for communi
`cations. In a conventional point-to-point optical communi
`cations link, at the transmitter, an electrical data signal is
`used to modulate the output of a semiconductor laser
`emitting, for example, in the 1550 nm band, and the modu
`lated optical signal is impressed on one end of the silica
`optical ?ber. Other transmission bands at 850 nm and 1310
`nm are also available. On very long links, the optical signal
`may be ampli?ed along the route by one or more optically
`pumped erbium-doped ?ber ampli?ers (EDFAs) or other
`optical ampli?ers. At the receiver, the optical signal from the
`?ber is detected, for example, by an optical p-i-n diode
`detector outputting an electrical signal in correspondence to
`the modulating electrical signal. The transmission band
`Width of such systems is typically limited by the speed of the
`electronics and opto-electronics included in the transmitter
`and receiver. Speeds of 10 gigabits per second (Gbs) are
`available in ?elded systems, and 40 Gbs systems are reach
`ing production. Further increases in the speed of the elec
`tronics Will be dif?cult. These speeds do not match the
`bandWidth inherent in the ?ber, Which is Well in excess of
`one terabit per second. Furthermore, such fast optical trans
`mitters and receivers are expensive and may require special
`environmental controls.
`Transmission capacity of ?ber systems can be greatly
`increased by Wavelength division multiplexing (WDM) in
`Which the optical signal is generated in a transmitter includ
`ing multiple semiconductor lasers emitting at different
`respective Wavelengths Within the transmission band. The
`1550 nm transmission band has a bandWidth of about 35 nm,
`determined by the available ampli?cation band of an EDFA.
`Other ampli?er types and ampli?cation bands are being
`commercialiZed so that the available WDM spectrum is
`groWing each year. In a WDM system, each laser is modu
`lated by a different electrical data signal, and the different
`laser outputs are optically combined (multiplexed) into a
`multi-Wavelength optical signal Which is impressed on the
`optical ?ber and Which together can be ampli?ed by an
`EDFA Without the need to demultiplex the optical signal. At
`the receiver, an optical demultiplexer, such as one based on
`a diffraction grating, an arrayed Waveguide grating, or a
`thin-?lm ?lter array, spatially separates the different Wave
`length components, Which are separately detected and out
`put as respective electrical data signals. For an N Wavelength
`WDM Wavelength grid, the ?ber capacity is increased by a
`factor of N using electronics of the same speed. Dense
`WDM (DWDM) systems are being designed in Which the
`WDM comb includes 40, 80 or more Wavelengths With
`Wavelength spacings of under 1 nm. Current designs have
`
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`Wavelength spacings of betWeen 0.4 and 0.8 nm, that is,
`frequency spacings of 50 to 100 GHZ. Spectral packing
`schemes alloW for higher or loWer spacings, dictated by
`economics, bandWidth, and other factors.
`Point-to-point WDM transmission systems as described
`above enable very high transmission capacity in netWorks
`having simple connectivity. HoWever, a modem communi
`cations netWork 10, such as that illustrated in FIG. 1, tends
`to be more complex and requires that the WDM concept be
`expanded to cover not only transport but also sWitching in a
`complex communication netWork. This netWork 10 has
`multiple sWitching nodes 12 sWitching signals betWeen
`multiple terminals 14 at the edges of the netWork 10. Fiber
`optic links 16 interconnect the sWitching nodes 12 and
`terminals 14. The sWitching nodes 12 should be capable of
`sWitching single-Wavelength WDM channels in different
`directions With the directions being changeable over some
`time period. The netWork diagram of FIG. 1 is highly
`conceptual but emphasiZes the sWitching requirement of the
`illustrated complexly connected netWork. Such a WDM
`netWork 10 achieves high capacity through multi
`Wavelength channel transport along complexly ?ber
`interconnected routes. To achieve a ?exible dynamic inter
`connection Within the netWork 10, it is desirable that the
`?ber infrastructure be Wired differently for different Wave
`lengths. As a result, the optical sWitches 12 should not only
`be dynamically recon?gurable betWeen multiple ports but
`also the sWitch state should be able to be Wired
`simultaneously, yet independently in each Wavelength chan
`nel. That is, the cross-connects 12 should be Wavelength
`selective. Channel paths 20 are illustrated in FIG. 1 in Which
`three Wavelength channels at Wavelengths k1, k2, k3 can
`enter a sWitching node 12 on a single ?ber 16 but be
`sWitched at its outputs to three different ?bers 16. Also note
`that this architecture alloWs the reuse of Wavelengths
`betWeen different pairs of terminals 14, for example, the
`Wavelength L1 as illustrated. Frequency reuse further
`increases the traffic capacity of the netWork 10. Such sWitch
`ing requirements apply as Well to a more regularly con?g
`ured ring netWork having sWitching nodes distributed
`around a ?ber ring With a terminal associated With each
`node.
`The practice in the recent past has been to form each link
`16 in the netWork 10 as a separate point-to-point WDM
`system so that each sWitching node 12 includes an optical
`receiver and an optical transmitter for each Wavelength
`channel. The electrical data signals derived from the optical
`receiver are spatially sWitched by conventional electronic
`sWitches and then converted back to optical form for trans
`mission on the next link. HoWever, a point-to-point design
`does not integrate Well into a complex netWork such as in
`FIG. 1. The required number of optical receivers and trans
`mitters become very expensive. Also, such a system requires
`the electronics and opto-electronics at each sWitching node
`to be operating at the highest data rate supported by the
`netWork. Such a system is unduly costly When some end
`to-end links require only modest data rates, and the system
`is difficult to upgrade since all the nodes must be upgraded
`at the same time.
`For these and other reasons, there is much interest in
`all-optical communication netWorks in Which each sWitch
`ing node demultiplexes the multi-Wavelength WDM signal
`from an input ?ber into its Wavelength components, spatially
`sWitches the separate single-Wavelength beams in different
`directions, and multiplexes the sWitched optical signals for
`retransmission on one or more output ?bers. Thus, a
`Wavelength-routing node Will generally sWitch WDM chan
`
`Cisco Systems, Inc.
`Exhibit 1004, Page 13
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`nels in an all-optical manner unless there is a speci?c need
`to electrically regenerate a speci?c subset of channels, for
`example, to remove accumulated optical noise through
`electrical regeneration or to perform Wavelength conversion.
`Indeed, the goal is to reduce the number of conversions from
`optical to electrical and back to optical at each intervening
`node in a ?ber optic end-to-end link.
`An all-optical Wavelength-selective sWitching node 12
`may be implemented by a Wavelength cross connect (WXC)
`such as a 3x3 WXC 22, represented in the simple schematic
`diagram of FIG. 2, coupling three input ports 24 to three
`output ports 26, each port being typically equated With a
`transmission ?ber in the netWork. The WXC 22 has the
`capability of sWitching any Wavelength channel on any input
`port 24 to the corresponding Wavelength channel on any
`output port 26. The design is not limited to a 3x3 cross
`connect With three input and three output ports and may be
`generaliZed to a W><W cross connect With W input ports and
`W output ports, Where W is greater than 1. Systems under
`development have values of W up to 12, and further repli
`cation is possible. An unequal number of input and output
`ports is possible but Will not be further discussed here.
`Although other designs are possible, the WXC 22 is
`typically accomplished, as illustrated in the schematic dia
`gram of FIG. 3, by splitting the WDM channels into their
`Wavelength components and sWitching those Wavelength
`components by optical elements generally insensitive to the
`Wavelength values, often referred to as White-light elements.
`The multi-Wavelength signal on each optical input port 24
`enters a respective one of 3 (more generally W) optical
`demultiplexers 28 Which separate the multi-Wavelength sig
`nal into its N Wavelength channels, here illustrated as N=4,
`and outputs single-Wavelength switching beams 30. The
`single-Wavelength sWitching beams 30 of the same nominal
`Wavelength enter a respective one of N White-light cross
`connects 321 to 32N, Which can sWitch any input sWitching
`beam 30 to any of its output sWitching beams 34 Without
`regards to Wavelength except that the sWitching beams 30,
`34 in any plane have the same Wavelength or color. W
`optical multiplexers 36 each receive N single-Wavelength
`output sWitching beams of N different Wavelengths and
`combines them into a multi-Wavelength signal impressed on
`one of the W output ports 26. By such an arrangement, the
`N Wavelength channels on each of the W input ports 24 are
`simultaneously and independently routed to selected ones of
`the W output ports 26. The Wavelength multiplexers and
`demultiplexers may be accomplished in a number of Ways
`typically including dispersive elements such as Bragg
`gratings, thin-?lm interference ?lter arrays, and arrayed
`Waveguide gratings (AWGs) that spatially separate Wave
`length components.
`There are a number of Ways of achieving the Wavelength
`routing functionality represented by the Wavelength cross
`connect 22. The illustrated structure of FIG. 3 represents a
`replicative approach using N distinct White-light cross con
`nects 321, to 32N. A more integrated design, schematically
`illustrated in FIG. 4, uses the same input and output ports,
`multiplexers 36, and demultiplexers 28, but uses ?bers 42,
`44 on the sWitch side of the demultiplexers 28 and multi
`plexers 36 to route the single-Wavelength signals to a large
`White-light optical cross connect 46 having WN inputs and
`WN outputs. The optical cross connect 46 needs to be
`con?gured so that it connects inputs to outputs of the same
`nominal Wavelength. Any cross-Wavelength connection
`results in loss of that signal at the output multiplexers 36 and
`perhaps corruption of another signal.
`Solgaard et al. in Us. Pat. No. 6,097,859 (hereinafter
`referred to as Solgaard) disclose a multi-Wavelength cross
`
`40
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`45
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`55
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`4
`connect sWitch based on an array of micro electromechani
`cal system (MEMS) mirrors. In this device, multi
`Wavelength WDM signals are received on typically tWo or
`more input ports. An input lens systems collimates these
`beams and directs them to a diffraction grating that re?ects
`different Wavelength channels at different angles. The result
`ing tWo-dimensional array of beams, in Which input beams
`are separated in one dimension and the Wavelength channels
`are separated in the other dimension, is imaged onto an array
`of electronically actuated MEMS micro-mirrors. Each beam
`is re?ected by its micro-mirror at a selected angle that
`depends upon the voltage applied to the mirror actuator.
`Because sWitching is performed betWeen corresponding
`Wavelength channels of different ?bers, in the simplest
`design the mirrors need to tilt only in a single dimension.
`A more integrated design folloWing the Solgaard design
`includes the Wavelength multiplexing and MEMS sWitching
`in a single unit. A 2-input, 2-output, 7-Wavelength sWitching
`system 50 is schematically illustrated in FIG. 5. TWo input
`?ber Waveguides 52, 54 and tWo output ?ber Waveguides 56,
`58 are aligned linearly parallel to each other to couple into
`tWo free-space input beams 60, 62 and tWo free-space output
`beams 64, 66. A lens 68 collimates the input beams 60, 62
`to both strike a diffraction grating 70.
`Considering the ?rst input beam 60, the diffraction grating
`70 angularly disperses it into a fan-shaped collection 72 of
`beams angularly separated according to Wavelength, as is
`Well knoWn in the art and taught by Solgaard. That is, the
`grating 70 acts as a Wavelength-dispersive element. The
`Wavelengths of the signals on the one input ?ber 52, as Well
`as on all the other ?bers 54, 56, 58 correspond to the WDM
`Wavelengths of one of the standardized grids, for example,
`the ITU grid, and each optical carrier signal of the different
`Wavelengths on the separate ?bers is modulated according to
`its oWn data signal. Each of the beams in the collection 72
`of beams corresponds to one of the Wavelength channels of
`the ITU grid. A lens 74 focuses these beams toWard a ?rst
`roW 75 of tiltable input mirrors 76, typically formed as a
`tWo-dimensional array in the plane of a MEMS structure.
`The mirrors 76 of the ?rst roW 75 are associated With the
`Wavelength channels of the ?rst input ?ber 52 While those in
`second roW 77 are associated With the second input ?ber 54.
`The mirrors 76 are also arranged in a second dimension in
`Which each column 78 is associated With one of the Wave
`lengths k1 through A7 for the illustrated 7-Wavelength sys
`tem. The mirrors 76 described to this point are input mirrors.
`Similarly arranged output mirrors 80 in roWs 82, 84 are
`output mirrors. The mirrors 76, 80 are tiltable about respec
`tive axes lying generally horiZontally in the illustration so
`that the input mirrors 76 direct each input beam 60, 62 beam
`toWard a folding mirror 86. Depending upon the tilt angle of
`the respective input mirror 76, the folding mirror 84 re?ects
`that beam to the output mirror 80 in a selected one of the
`output roWs 82, 84. The tWo illustrated connections shoW
`coupling to output mirrors 80 located alternatively in the
`third and fourth roWs 82, 84. The output mirrors of the third
`roW 82 are associated With respective Wavelength channels
`on the ?rst output ?ber 56 While those of the fourth roW 84
`are associated With the Wavelength channels on the second
`output ?ber 58. The optics are arranged and controlled such
`that an optical signal from an input mirror 76 is re?ected
`only to one of the output mirrors 80 in the same column 78,
`that is, associated With the same WDM Wavelength. The
`input and output mirrors 76, 80 typically have the same
`construction and differ only by their placement in a tWo
`dimensional array in a single MEMS structure. Practically
`speaking, in this con?guration, the designation of input and
`
`Cisco Systems, Inc.
`Exhibit 1004, Page 14
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`US 6,798,941 B2
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`5
`output mirrors is arbitraxy and the input and output roWs
`may be interleaved.
`In some applications, it is possible to dispense With the
`folding mirror 86 and to use only a single set of micromir
`rors to directly re?ect a Wavelength-separated input beam
`back to a selected output ?ber although this con?guration
`presents problems With uniformity of coupling.
`Each output mirror 80 is also tiltable in correspondence to
`the tilt angle of the input mirror 76 to Which it is coupled
`through the folding mirror 86 so that the same optics 68, 70,
`74 used to focus and demultiplex the beams from the input
`?bers 52, 54 are also used to multiplex the Wavelength
`separated output beams onto the tWo output ?bers 56, 58.
`That is, the diffraction grating 70 acts as both a demulti
`plexer on the input and a multiplexer on the output.
`By means of the illustrated optics and MEMS micromir
`ror array, a Wavelength channel on either of the input ?bers
`52, 54 can be sWitched to the same Wavelength channel on
`either of the output ?bers 56, 58. It is of course understood
`that the described structure may be generaliZed to more
`input and output ?bers and to more WDM Wavelengths.
`Another system 90, as illustrated in the schematic dia
`gram of FIG. 6, provides much of the functionality of the
`system 50 of FIG. 5. Input ?bers 92 are arranged in a ?rst
`linear array 94 and output ?bers 96 are arranged in a second
`linear array 96. The system includes a ?rst tWo-dimensional
`array 100 of input mirrors 102 and a second tWo
`dimensional array 106 of output mirrors 108. In both arrays
`100, 106, the mirrors 102, 108 are arranged in roW directions
`according to ?ber and in column directions according to
`Wavelength. The beams are directly coupled betWeen the
`input and output mirrors 102, 108 Without the use of a
`folding mirror. HoWever, such a coupling mirror may be
`advantageously applied betWeen the tWo mirror arrays 100,
`106 and eliminate the need for separate gratings and further
`alloW the input and output ?bers 92—98 to be placed in a
`single linear array. Advantageously, separate demultiplexing
`and multiplexing gratings 110, 112 are provided on the input
`and output sides respectively, and birefringent Wave plates
`are inserted so as to substantially eliminate polariZation
`dependence Within the sWitch, as is Well understood by those
`in art.
`A large White-light cross connect may have a structure
`similar to that illustrated in FIG. 6 but Without the diffraction
`grating. AWhite-light system 120 illustrated in the schematic
`illustration of FIG. 7 includes a substantial number of input
`?bers 122 bundled together in a tWo-dimensional array 124
`and preferably a like number of output ?bers 126 bundled
`together in another tWo-dimensional array 128. One of the
`input ?bers 122 concentrates its beam at one of the input
`mirrors 102 of the input mirror array 100. Similarly, the
`output mirror array 106 has its output mirrors 108 near the
`focus of the output ?bers 126. An optional mirror 130
`couples the input and output mirrors 102, 104, Each input
`mirror 102 is tiltable about tWo axes to alloW it to direct its
`input beams to any ones of the output mirrors 106. Each
`output mirror 106 is similarly tilted in a complementary
`fashion to direct the beam toWards the output ?ber 126
`associated With that output mirror 108.
`Another unillustrated White-light system resembles the
`folded White-light system 120 of FIG. 7 but With the input
`and output ?bers arranged in a same one- or tWo
`dimensional array and With the mirror arrays 100, 106
`integrated into a single one- or tWo-dimensional array.
`A complex WDM or White-light netWork is subject to
`many problems. The different optical signals Which are
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`propagating on a particular link or being optically processed
`may have originated from different sources across the net
`Work. In a WDM system, the WDM Wavelength output
`poWer may vary from transmitter to transmitter because of
`environmental changes, aging, or differences in poWer
`injected into the WDM stream. Different optical sources for
`either a WDM or White-light system are additionally subject
`to different amounts of attenuation over the extended net
`Work. Particularly, for a Wavelength-routed transparent
`netWork, the WDM spectrum on a given ?ber contains
`Wavelength components Which generally have traversed
`many diverse paths from different sources and With different
`losses and different impairment accumulation such as deg
`radation of the optical signal-to-noise ratio or dispersion
`broadening. Further, Wavelength multiplexing and demulti
`plexing usually rely on optical effects, such as diffraction or
`Waveguide interference, Which are very sensitive to absolute
`Wavelength, Which cannot be precisely controlled.
`EDFAs or other optical ampli?ers may be used to amplify
`optical signals to compensate loss, but they amplify the
`entire WDM signal and their gain spectrum is typically not
`?at. Therefore, measures are needed to maintain the poWer
`levels of different signals to be the same or at least in
`predetermined ratios.
`In a complex WDM or White-light netWork, a signal may
`be sWitched multiple times. Each sWitching event needs to
`maximiZe transmission of the optical signal and minimiZe
`cross-talk betWeen channels. A maximum of 10 dB attenu
`ation through the sWitch and a minimum of 30 dB channel
`isolation are typical requirements. HoWever, MEMS cross
`connects and their associated optics are subject to internal
`variations of optical characteristics and misalignments, both
`integral to the device and as a result of both manufacturing
`and environmental variation and non-uniformity and of
`mechanical stress, all of Which result in sWitch states having
`a signi?cant variation and instability of insertion loss When
`aligned according to their nominal settings.
`As described above, it is Well knoWn that WDM systems
`must maintain a signi?cant degree of uniformity of poWer
`levels across the WDM spectrum, so that dynamic range
`considerations at receivers and ampli?er, non-linear effects,
`and cross talk impairments can be minimiZed. As a result,
`serious attention must be payed to equalization of poWer
`levels across the spectrum. This equalization should be
`dynamic and under feedback control since the various
`Wavelength components vary in intensity With time and due
`to changes in optical channel routing history among the
`components. One object of this invention is to provide
`means of equaliZation at multiple-?ber WDM sWitching
`nodes Where many beams from diverse sources are inter
`changed among the ?bers.
`Bishop et al. have disclosed in US. Pat. No. 6,263,123 a
`pixellated WDM cross connect using a tWo-dimensional
`array of micromirrors in Which a signal beam of a particular
`carrier Wavelength is distributed to a plurality of the micro
`mirrors. The number of mirrors re?ecting light to an output
`port determines the transmission coef?cient through the
`sWitch. When such a pixellated intensity control is used in a
`WDM cross-connect, the mirrors are arranged in a tWo
`dimensional array and are tiltable about tWo axes. The
`described system is used primarily for characteriZing the
`optical signal, not for controlling it. Similarly, Derickson et
`al. have disclosed in Us. Pat. No. 4,796,479 a system for
`monitoring the intensities of the WDM channels in a WDM
`cross connect With the main emphasis on determining the
`ratio of signal to noise. It Would be desirable to integrate the
`capability of such systems into an optical netWork Without
`unduly increasing the cost and complexity.
`
`Cisco Systems, Inc.
`Exhibit 1004, Page 15
`
`
`
`US 6,798,941 B2
`
`7
`SUMMARY OF THE INVENTION
`An optical switching system includes a plurality of optical
`switching elements controllable in tWo different scales or
`dimensions depending upon the sWitch architecture and
`designed to be able to effect both sWitching and control of
`the transmission coef?cient. The poWer adjustment is par
`ticularly useful in a Wavelength-division multiplexing opti
`cal sWitch in Which the poWer is adjusted betWeen different
`Wavelength channels combined in an output path such as an
`optical ?ber.
`An example of such a sWitching element is a mirror
`tiltable about tWo orthogonal directions or tiltable in one
`dimension according to a ?ne and a coarse resolution. In one
`embodiment, tilting about a major axis is used to control
`sWitching betWeen optical ports, and tilting about the minor
`axis is used to control the amount of optical poWer passing
`through the sWitch.
`Such mirrors are advantageously formed in a micro
`electromechanical system (MEMS) array. A mirror may be
`tilted by tWo pairs of electrostatic actuator electrodes posi
`tioned beneath the mirror on opposed sides of tWo torsion
`beams supporting the mirror.
`A portion of the sWitched output signal may be diverted
`to an optical poWer monitor to enable feedback control of the
`poWer adjustment. Dynamic poWer equalization advanta
`geously involves monitoring of poWer levels for the indi
`vidual WDM Wavelength channels on each ?ber in the
`system. This information provides feedback for the poWer
`equalization mechanism. Input poWer may be advanta
`geously also monitored for each Wavelength channel.
`The minor axis tilt may also be used for optimizing
`transmission through the optical sWitch. After the position of
`maximum transmission is established, the transmission may
`be detuned. An example of tWo-axis sWitching is the use of
`a major axis optimized for sWitching betWeen ?bers and a
`minor axis for adjusting transmission, especially insertion
`loss, of the same optical channel. Another example is the use
`of a coarse control of one or more major axes for establish
`ing a sWitch state connection in combination With a ?ne
`control along one or more minor axes (Which may or may
`not be the same as the major axes) to moderate the degree
`of coupling of a Wavelength channel betWeen the chosen
`?bers for that Wavelength service.
`The feedback control applies as Well to White-light
`sWitching systems in Which the same mirrors are used for
`sWitching and for transmission optimization and poWer
`equalization.
`The minor axis tilt may be used to increase the high
`insertion loss for any optical connection in the off state and
`may further be used to turn the sWitch to a hard off during
`sWitching betWeen discrete optical paths.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a netWork diagram of a complexly connected
`optical communications netWork.
`FIG. 2 is a general block diagram of a 3x3 Wavelength
`division multiplexing optical cross connect (WOXC).
`FIG. 3 is a more detailed block diagram of the WOXC of
`FIG. 2
`FIG. 4 is a block diagram of a more integrated version of
`the WOXC of FIG. 2.
`FIG. 5 is a schematic diagram of a WOXC using one array
`of micromirrors and a folding mirror.
`FIG. 6 is a schematic diagram of a WOXC using tWo
`arrays of micromirrors and no folding mirror.
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`FIG. 7 is a schematic diagram of a White-light optical
`cross connect
`FIG. 8 is a block diagram of an optical sWitching system
`including an optical poWer monitor and feedback control.
`FIG. 9 is a schematic plan vieW of a 2x2 Wavelength
`division multiplexing (WDM) cross connect using an exter
`nal optical poWer monitor.
`FIG. 10 is a sectioned orthographic vieW of a concentrator
`included in the cross connect of FIG. 9.
`FIG. 11 is a schematic plan vieW of a 2x2 WDM cross
`connect using an internal external optical poWer monitor.
`FIG. 12 is a block diagram block diagram of the optical
`sWitching system of FIG. 8 additionally including an input
`optical poWer monitor.
`FIG. 13 is a schematic plan vieW of an improved of the
`WDM cross connect of FIG. 9 additionally including moni
`toring the input poWer.
`FIG. 14 is plan vieW of tWo-axis tiltable mirror usable
`With the invention.
`FIG. 15 is a cross sectional vieW taken along vieW line
`15—15 of FIG. 12.
`FIG. 16 is a schematic orthographic vieW of a mirror array
`and its fold mirror.
`FIGS. 17 an