(12)
`
`United States Patent
`Smith et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 6,798,941 B2
`Sep. 28, 2004
`
`US()()6798941B2
`
`8f2D00 Solgaard et al.
`6,09?,859 A
`8;'2000 Laor
`6,101,299 A
`arzooo Toyohara
`6,111,686 A *
`3;2oo1 Aksyuketal.
`s,2o4,945 B1
`‘#2001 Bishop etal.
`6,263,123 B1
`FOREIGN PATENT DOCUMENTS
`
`
`
`385f1'?'
`..... .. 385316
`359,637.13
`3591131
`
`385115
`
`(54)
`
`(75)
`
`VARIABLE TRANSMISSION MULTI-
`CHANNEL OPTICAL SWITCH
`
`Inventors: David A. Smith, Atlanta, GA (US);
`John E. Golub, Atlanta, GA (US);
`Fariborz Farhan, Alpharetta, GA (US)
`
`(73)
`
`Assignee: Muvaz Networks, I.nc., 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.
`
`EP
`EP
`JP
`WO
`W0
`
`0 984 311
`0 729 044
`55-159402
`W10 02125358
`WO 00120899
`
`3/2000
`U2003
`1211980
`312000
`4,0000
`
`OTHER PUBLICATIONS
`
`(21)
`
`App]. No.: o9;9s7,312
`
`(22)
`
`Filed:
`
`Sep. 20, 2001
`Prior Publication Data
`
`(65)
`
`(50)
`
`(51)
`(52)
`(53)
`
`(55)
`
`US 2oo2;oo71n27 A1 Jun. 13, 2002
`
`Related U.S. Application Data
`22,
`Provisional application No. 60,034,683, filed on
`2000, and provisional application No. 60;'26’?',285, filed on
`Feb. 7, 2001.
`
`Int. Cl.7 ............................ .. G02B 6,126; G02B 6t'42
`
`385118; 385119
`Field of Search ............................ .. 385118, 19, 15,
`385.33; 3980.56
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,696,062 A "
`4,911,526 A
`5,414,540 A
`5,621,829 A
`5,'?45,2':'l A
`5,771,320 A
`5,796,479 A
`5,828,800 A
`5,915,063 A
`5,950,133 A
`
`..
`
`911987 L.aBudde .................. ..398f156
`31990 Hsu et al.
`. 350196.24
`510995 Patel et al.
`.... .. 359,89
`#199? Ford
`.... .. 385.-’22
`4131998 Ford el al.
`3591130
`610998 Stone ..................... .. 385116
`811998 Dericlrson et al.
`........ .. 3561326
`10.0998 Henry et al. ................ .. 385f20
`611999 Colbourne et al.
`....... .. 385/140
`9;1999 Tomilinson ................ .. 335113
`
`Lin et al., “Micro—elect.ro~mechanical systems (MEMS) for
`WDM optical—crossconnect networ
`”, Military Communi-
`cation Corgference 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—oonnects in multiwave-
`length all—optical networks”, IEEP, 1996, 156-163 pp.
`
`* cited by examiner
`
`Primary Examiner—Chandrika Prasad
`(74) Attorney, Agent‘, or Ft'rm—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 electromechanics 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.
`
`3'} Claims, 11 Drawing Sheets
`
`.
`
`
`
`
`
`
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`I’
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`Capella 2026
`JDS Uniphase v. Capella
`IPR2015-00731
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`

`
`U.S. Patent
`
`Sep. 28,2004
`
`Sheet 1 of 11
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`US 6,798,941 B2
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`"OK.
`
`
`FIG. 2
`
`(PR1OR ART)
`
`22
`
`

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`U.S. Patent
`
`Sep. 28,2004
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`Sheet 2 of 11
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`US 6,798,941 B2
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`

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`U.S. Patent
`
`Sep. 28,2004
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`Sheet 3 of 11
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`US 6,798,941 B2
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`U.S. Patent
`
`Sep. 28,2004
`
`Sheet 4 of 11
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`US 6,798,941 B2
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`NETWORK
`
`,
`,
`IV:
`
` FROM
`
`
`
`

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`U.S. Patent
`
`Sep. 28,2004
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`Sheet 5 of 11
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`US 6,798,941 B2
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`

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`U.S. Patent
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`Sep. 28,2004
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`Sheet 6 of 11
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`US 6,798,941 B2
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`170
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`
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`228
`
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`
`U.S. Patent
`
`Sep. 28,2004
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`Sheet 7 of 11
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`US 6,798,941 B2
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`164156
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`

`
`U.S. Patent
`
`Sep. 28,2004
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`Sheet 8 of 11
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`US 6,798,941 B2
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`

`
`U.S. Patent
`
`Sep. 28, 2004
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`Sheet 9 of 11
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`US 6,798,941 B2
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`260
`
`260
`
`264
`
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`

`
`U.S. Patent
`
`Sep. 28,2004
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`Sheet 10 of 11
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`US 6,798,941 B2
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`

`
`U.S. Patent
`
`Sep. 28,2004
`
`Sheet 11 of 11
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`US 6,798,941 B2
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`FIG 18
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`
`

`
`US 6,798,941 B2
`
`1
`VARIABLE TRANSMISSION MULTI-
`CHANNEL OPTICAL SWITCH
`
`RELATED APPLICATIONS
`
`This application claims benefit of U.S. Provisional Appli-
`cations Nos. 60.n"234,683, filed Sep. 22, 2000, and 60r‘26"r',
`285, Filed Feb. 7, 2001.
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`10
`
`2
`
`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 fiber-
`interconnected routes. To achieve a flexible dynamic inter-
`connection within the network 10, it is desirable that the
`fiber infrastructure be wired dilferently for dilferent wave-
`lengths. As a result, the optical switches 12 should not only
`be dynamically reconfigurable 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 crcss-connects 12 should be wavelength
`selective. Channel paths 20 are illustrated in FIG. 1 in which
`three wavelength channels at wavelengths X1, 7&2, 7»; can
`enter a switching node 12 on a single fiber 16 but be
`switched at its outputs to three different fibers 16. Also note
`that
`this architecture allows the reuse of wavelengths
`between dilferent pairs of terminals 14, for example, the
`wavelength X, as illustrated. Frequency reuse further
`increases the trafic capacity of the network 10. Such switch-
`ing requirements apply as well to a more regularly config-
`ured ring network having switching nodes distributed
`around a fiber 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 fiber 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
`fibers. Thus, a
`wavelength-routing node will generally switch WDM chan-
`
`15
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`20
`
`25
`
`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 fiber, 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 fiber. Other transmission bands at 850 nm and 1310
`nm are also available. On very long links, the optical signal
`may be amplified along the route by one or more optically
`pumped erbium-doped fiber amplifiers (EDFAS) or other
`optical amplifiers. At the receiver, the optical signal from the
`fiber 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 fielded systems, and 40 Gbs systems are reach-
`ing production. Further increases in the speed of the elec-
`tronics will be dimcult. These speeds do not match the
`bandwidth inherent in the fiber, which is well in excess of
`one terabit per second. Furthermore, such fast optical trans- 40
`miners and receivers are expensive and may require special
`environmental controls.
`
`30
`
`35
`
`Transmission capacity of fiber 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 dilferent
`respective wavelengths within the transmission band. The
`1550 nm transmission band has a bandwidth of about 35 nm,
`determined by the available amplification band of an EDFA.
`Other amplifier types and amplification 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 dilferent electrical data signal, and the different
`laser outputs are optically combined (multiplexed) into a
`multi-wavelength optical signal which is impressed on the
`optical fiber and which together can be amplified by an
`EDFA without the need to demultiplex the optical signal. At
`the receiver, an optical demultiplexer, such as one based on
`a difiraction grating, an arrayed waveguide grating, or a
`thin-film filter array, spatially separates the diflerent wave-
`length components, which are separately detected and out-
`put as respective electrical data signals. For an N wavelength
`WDM wavelength grid, the fiber 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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`nels in an all-optical manner unless there is a specific need
`to electrically regenerate a specific 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 5
`optical to electrical and back to optical at each intervening
`node in a fiber 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 fiber 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 WxW 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 23 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 san:te nominal
`wavelength enter a respective one of N white-light cross
`connects 321 to 32”, 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 45
`demultiplexers may be accomplished in a number of ways
`typically including dispersive elements such as Bragg
`gratings, thin-film interference filter 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 2. The illustrated structure of FIG. 3 represents a
`replicative approach using N distinct white-light cross con-
`nects 32,, to 32”. A more integrated design, schematically
`illustrated in FIG. 4, uses the same input and output ports,
`multiplexers 36, and demultiplexers 28, but uses fibers 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
`configured 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 corniption of another signal.
`Solgaard et al.
`in U.S. Pat. No. 6,097,859 (hereinafter
`referred to as Solgaard) disclose a multi-wavelength cross
`
`S0
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`connect switch based on an array of micro electromechani-
`cal systeni
`(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 diifraction grating that reflects
`dilferent wavelength channels at difierent 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 reflected 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 dilferent fibers,
`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. A2-input, 2-output, 7-wavelength switching
`system 50 is schematically illustrated in FIG. 5. Two input
`fiber waveguides 52, 54 and two output fiber 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 dilfraction grating 70.
`Considering the first 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 fiber 52, as well
`as on all the other fibers 54, 56, S8 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 fibers 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 first
`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 first row 75 are associated with the
`
`wavelength channels of the first input fiber 52 while those in
`second row 77 are associated with the second input fiber 54.
`The mirrors 76 are also arranged in a second dimension in
`which each column 78 is associated with one of the wave-
`lengths A, through 7&—, 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 refiects
`that beam to the output mirror 30 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 minors of the third
`row 82 are amociated with respective wavelength channels
`on the first output fiber 56 while those of the fourth row 84
`are associated with the wavelength channels on the second
`output fiber 58. The optics are arranged and controlled such
`that an optical signal from an input mirror 76 is reflected
`only to one of the output mirrors S0 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 configuration, the designation of input and
`
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`US 6,798,941 B2
`
`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 reflect a wavelength-separated input beam
`back to a selected output fiber although this configuration
`presents problems with uniformity of coupling.
`Each output mirror 30 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 63, 70,
`74 used to focus and demultiplex the beams from the input
`fibers 52, 54 are also used to multiplex the wavelength-
`separated output beams onto the two output fibers 56, 58.
`That is, the diflraction 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 fibers
`52, 54 can be switched to the same wavelength channel on
`either of the output fibers 56, 58. It is of course understood
`that the described structure may be generalized to more
`input and output fibers 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 fibers 92 are arranged in a first
`linear array 94 and output fibers 96 are arranged in a second
`linear array 96. The system includes a first two-dimensional
`array 100 of input mirrors 102 and a second two-
`dimensional array 106 of output minors 108. In both arrays
`100, 106, the mirrors 102, 108 are arranged in row directions
`according to fiber 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 minor arrays 100,
`106 and eliminate the need for separate gratings and further
`allow the input and output fibers 92-93 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. A white-light system 120 illustrated in the schematic
`illustration of FIG. 7 includes a substantial number of input
`fibers 122 bundled together in a two-dimensional array 124
`and preferably a like number of output fibers 126 bundled
`together in another two-dimensional array 128. One of the
`input fibers 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 fibers 126. An optional rnirror 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 minors 106. Each
`output mirror 106 is similarly tilted in a complementary
`fashion to direct the beam towards the output fiber 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
`fibers 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 diflerent 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 fiber contains
`wavelength components which generally have traversed
`many diverse paths from dilferent sources and with different
`losses and difierent 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 elfects, such as diffraction or
`waveguide interference, which are very sensitive to absolute
`wavelength, which cannot be precisely controlled.
`EDFAS or other optical amplifiers may be used to amplify
`optical signals to compensate loss, but they amplify the
`entire WDM signal and their gain spectrum is typically not
`flat. 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 significant 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 significant degree of uniformity of power
`levels across the WDM spectrum, so that dynamic range
`considerations at receivers and amplifier, non-linear elfects,
`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-fiber WDM switching
`nodes where many beams from diverse sources are inter-
`changed among the fibers.
`Bishop et al. have disclosed in U.S. 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 reflecting light to an output
`port determines the transmission coefficient
`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 U.S. 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.
`
`

`
`US 6,798,941 B2
`
`7
`SUMMARY OF THE INVENTION
`
`An optical switching system includes a plurality of optical
`switching elements controllable in two difierent scales or
`dimensions depending upon the switch architecture and
`designed to be able to effect both switching and control of
`the transmission coefficient. 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 fiber.
`An example of such a switching element is a mirror
`tiltable about two orthogonal directions or tiltable in one
`dimension according to a fine 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 fiber 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 fibers 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 fine
`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
`fibers for that wavelength service.
`to white-light
`The feedback control applies as well
`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 oif state and
`may further be used to turn the switch to a hard 01]? 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.
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`S0
`
`55
`
`60
`
`65
`
`8
`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 (VVDM) cross connect using an exter-
`nal optical power monitor.
`FIG. I0 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 VVDM 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 and 18 are cross-sectional views illustrating two
`kinds of mismatch in optically coupling two beams.
`FIG. 19 is an elevational view of a concentrator of FIG.
`10 illustrating a method of switching between two output
`beams.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`The invention includes an optical switching system in
`which the output power on particular channels is monitored
`to enable power adjustments in the optical switch,
`for
`example, adjustment of the transmission coefficient for an
`optical channel transmitted through the switch.
`A block diagram of one embodiment of a controlled
`transmissivity optical switching system 140 of the invention
`is illustrated in FIG. 8. A 2><2 optical cross connect (OXC)
`142 links two input fibers 144, 146 connected to input ports
`IN, IN, to two output fibers 148, 150 connected to output
`ports OUT1, OUT2. However,
`the invention is directly
`applicable to a larger number of input and output fibers.
`In the wavelength division multiplexing (WDM) embodi-
`ments of the invention, each fiber 144, 146, 148, 150 is
`capable of carrying a multi-wavelength WDM optical sig-
`nals at wavelengths 25.1-AN. The OXC 142 is capable of
`switching the separate wavelength components on each
`input fiber 144, 146 to either of the output fiber