(12) United States Patent
`
`Wagener et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,631,222 B1
`Oct. 7, 2003
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`US006631222B1
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`(54)
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`RECONFIGURABLE OPTICAL SWITCH
`
`OTHER PUBLICATIONS
`
`1I1VeI1t0rSZ JBfi'e1‘SOIl L- Wagener, Aberdeen, WA
`(US); Th0m35 Andrew St1‘3SSe1‘,
`Warren’ NJ (US)
`.
`.
`_
`.
`ASS1gnee' Photurls’ Inc" Plscataway’ NJ (US)
`Notice:
`Subject to any disclaimer, the term of this
`patent is cxtcndcd or adjusted under 35
`U.s_C, 154(b) by[) days_
`
`Appl. N0.: 09/571,833
`.
`May 169 2000
`F1199?
`Int. Cl.7 ................................................ .. G02B 6/35
`U.S. Cl.
`........................... .. 385/16; 385/17; 385/18;
`385/31
`Field of Search ............................ .. 385/16, 17, 18,
`385/24, 31, 33, 47; 359/131, 196, 212,
`127, 124
`
`E. Murphy, Optical Fiber 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 Technologv Letters vol 10 No 4 Apr
`1998'
`C.R. GllC.S, 61211., “LOW—I_0SS ADD/DROP IVl11lt1pl6X6I'S fOI'
`WDM Liglitwave Networks,” Tenth International Coiifer-
`ence on Integrated Optics and Optical Fibre Communica-
`tion, IOOC, vol. 3, Jun. 29, 1995.
`JDS Uniphase Corporation, Add—Drop Modules, Product
`Bulletin 2000, Ontario, Canada.
`D.O.Culverhouse et al., Low—loss all—fiber acousto—optic
`tunable filter, Optical Snciety 0fAmerica, 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,”,J0ur—
`mzl 0fLig/irware Technology, vol. 16, No. 11 (Nov. 1998),pp.
`1965-1971.
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`* cited by examiner
`'
`
`Primary Examl'ner__10hH D_ Lee
`.
`- _ -
`‘E‘7S:;S’XI’:;r
`Kaang, Fmkm &Wfl]l.amS
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`ABSTRACT
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`An
`optica signa
`3.V1Ilg 3 p ura liy O WE1VC-
`I'CCC1V1Hg 21
`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
`1
`h
`1
`'
`1
`_E h fh
`'
`1
`1
`33:23:ififtseficfiéfiiieiilieIificofi oi)1ertitfthC:11ftisce8elee:i::1cfl:rl1)tS
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`ports 1ndependently of. every other wavelength component.
`The given output port is variably selectable from among all
`the output ports.
`
`.................. .. 370/3
`1/1981 Nosu et al.
`4,244,045 A
`12/1986 Lcvinson .... ..
`350/96.18
`4,626,066 A
`12/1995 Calvaiii et al.
`359/127
`5,479,082 A
`4/1996 Schimpe ..... ..
`385/24
`5,504,827 A
`12/1996 Scobey ....... ..
`359/127
`5,583,683 A
`3/1997 Fevrier et al.
`359/124
`5,612,805 A
`4/1997 Ford .......... ..
`385/22
`5,621,829 A
`3/133:
`:t;::<,:::.13.;1:3 -
`2:222:21: ::
`11/1998 Jungerman et a1_ N
`518411917 A
`6/1999 Russell 61 a1.
`5,915,050 A
`7/1999 Duck et a1,
`,,,,, ,,
`5,920,411 A *
`Q/1999 Danagher et al.
`.
`5,959,749 A
`9/1999 T0m1iHS0I1 ------ ~-
`5,960,133 A
`§Ik53]/1311‘ eidal
`2 *
`/
`_
`,,,,,
`ac ona
`359/127
`670082920 A * 12/1999 Hendrix -------------~-
`359/224
`6,028,689 A
`2/2000 Michalicek et al.
`.
`359/124
`6,075,632 A *
`6/2000 Braun ............... ..
`. . . . N 385/24
`6,289,148 B1 4
`9/2001 Lin 61 a1.
`. . . . . . . . . _
`6,327,393 31
`12/2001 solgaard et a1,
`____________ N 335/18
`FOREIGN PATENT DOCUMENTS
`
`
`
`.
`
`
`
`N 385/'17
`385/7
`359/127
`359/124
`- 385/18
`~~~~ ~~
`
`*
`
`JP
`
`60-88907
`
`*
`
`5/1985
`
`88 Claims, 3 Drawing Sheets
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`315
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`

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`U.S. Patent
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`Oct. 7, 2003
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`Sheet 1 of 3
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`US 6,631,222 B1
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`U.S. Patent
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`Oct. 7 2003
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`Sheet 3 of3
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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 benefit of priority to U.S.
`Provisional Patent Application Ser. No. 60/182,289, filed
`Feb. 14, 2000, entitled “An all Optical Router With Petabyte
`Per Second Switching Capability. ”
`
`FIELD OF THE INVENTION
`
`'lhe invention relates generally to an optical communica-
`tions system and more particularly to an optical switch for
`flexibly routing light in a wavelength-selective manner.
`
`BACKGROUND OF THE INVENTION
`
`Significant 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 fiber could transmit between
`two points,
`recent
`improvements in optical
`filtering
`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, reconfigurable
`optical elements have become available. Such reconfig-
`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 reconfigurable 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 fiber path
`and in other cases each dropped wavelength is directed onto
`its own fiber path. OXCs are more flexible devices than
`OADMs, which can redistribute in virtually any arrange-
`ment the components of multiple VVDM input signals onto
`any number of output paths.
`The functionality of the previously mentioned reconfig-
`urable optical elements can be achieved with a variety of
`different devices. For example, a common approach
`cmploys any of a number of different broadband switching
`fabrics inserted between a pair of demultiplexers/'
`multiplexers. Examples of OADM elements are disclosed in
`U.S. 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 Telecommzmications
`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 flexible 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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`multiplexing. The first of these approaches uses a thin film
`dielectric demultiplexer/multiplexer that is traversed twice
`by the wavelengths (e .g., U.S. Pat. No. 5,974,207), while the
`second approach uses dispersion from a bulk diffraction
`grating to demultiplex (separate) the wavelength channels
`before they reflect off an array of tiltable mirrors (U.S. Pat.
`No. 5,960,133). Another set of OADM technologies employ
`4-port devices that drop multiple wavelengths onto a single
`fiber output in a reconfigurable manner, and thus require an
`additional demultiplexer if the channels need to undergo
`broadband optoclcctronic conversion at thc rcccivcr. Onc
`realization of such functionality uses fiber 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 fiber optic circulator and an
`optional tunable fiber grating reflector 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
`flexible devices with high cost and high optical loss, and
`lower flexibility devices, which are less expensive and have
`lower optical loss. The most flexible OXCs can be pro-
`grammed to switch the path of any of a large number of
`wavelengths, each onto its own fiber (e.g. demux/mux with
`switches), however these devices may have up to 20 dB of
`insertion loss and therefore require an optical amplifier to
`compensate for the loss. This substantially adds to the cost
`of an already expensive device. Because these devices are so
`costly, less flexible alternatives such as fiber gratings and
`thin film filters are often used. While these devices have a
`significantly lower cost and insertion loss (2-5 dB/node),
`they are typically less flexible because they are implemented
`as fixed wavelength OADMs that cannot be reconfigured.
`These devices are also inflexible because as you scale them
`so that they drop more wavelengths their loss, cost, size
`and/or complcxity increase to thc point that the more flcxiblc
`OXC alternatives become more attractive. Recently, as
`shown in U.S. Pat. No. 5,479,082, some flexibility 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 fixed. In addition,
`the previously described reconfigurable OADM devices
`offer somewhat enhanced flexibility, 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 fixed wavelength that cannot
`be altered. In this configuration 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 flexibility 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
`
`

`
`US 6,631,222 B1
`
`3
`desirable to (a) change the wavelength channel directed to
`that port, or (b) direct additional wavelengths over that
`particular fiber 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 specific port needs to be
`increased by adding additional WDM wavelengths down the
`same fiber.
`
`In view of the important role of optical switching to the
`flexibility 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 sufliciently flexible to direct
`each and every wavelength component from any input port
`to any output port independently of one another.
`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
`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 film filters each
`transmitting therethrough a different one of the wavelength
`components and reflecting 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 reflect 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 first 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 dcmulti-
`plexing the first wavelength component from the WDM
`signal. The first 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 first wavelength component.
`In accordance with another aspect of the invention, the
`steps of directing the first and second wavelength compo-
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`nents includes the steps of directing the first and second
`wavelength components through a free space region.
`In accordance with yet another aspect of the invention, the
`first wavelength is demultiplexed by a thin film filter having
`a passband corresponding to the first wavelength.
`In accordance with another aspect of the invention, the
`first 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.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`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 first category, filters having fixed
`transmission and reflection bands may be employed which
`enable independent direction of the wavelength components
`onto different optical paths. Alternatively, in the second
`category, tunable filters may be employed which direct the
`wavelength components along fixed paths.
`FIG. 2 illustrates a first 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 film filters 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
`
`

`
`US 6,631,222 B1
`
`5
`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 first and second filter arrays
`are respectively arranged. The first filter array is composed
`of thin film filters 301 and 303 and the second filter array is
`composed of thin film filters 302 and 304. Individual ones of
`the collimating lens pairs 321-324 and tiltable mirrors
`315-318 are associated with each of the thin film filters. As
`described below, each thin film filter, along with its associ-
`ated collimating lens pair and tiltable mirror elfectively
`forms a narrow band, free space switch, i.e. 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 film filters 301-304 are well-known components (for
`example, see U.S. Pat. No. 5,583,683), which have a dielec-
`tric multilayer configuration. The thin film filters 301-304
`have a wavelength dependent characteristic, that is, their
`reflectivity and transmissivity depends on the wavelength of
`light. In particular, among the wavelength components of the
`WDM optical signal received by thin film filter 301, only the
`component with wavelength X1 is transmitted therethrough.
`The remaining wavelength components are all reflected by
`thin film filter 301. Likewise, thin film filter 302 transmits
`only the component with wavelength X2 and refiects all other
`wavelengths. In the same manner, the thin film filters 303
`and 304 transmit components with wavelengths A3, and A4,
`respectively, and reflect all other wavelengths. Thus,
`the
`present invention demultiplexes wavelengths through a plu-
`rality of thin film filters with different pass bands.
`The tiltable mirrors 315-318 are any mirrors that can be
`precisely tiltcd on 2 axes and are preferably small and very
`reliable. The exemplary mirrors discussed here are sup-
`ported by one or more flexure arms that employ a micro-
`electromechanical system (MEMS). Actuation of the flexure
`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 U.S. 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, X2, X3 and X4 is directed from the optical
`input port 312 to a collimator lens 314. The WDM signal
`traverses substrate 308 and is received by thin film filter 301.
`According to the characteristics of the thin film filter 301,
`the optical component with wavelength K1 is transmitted
`through the thin film filter 301, while the other wavelength
`components are reflected and directed to thin film filter 302
`via substrate 308. The wavelength component K1, which is
`transmitted through the thin film filter 301, is converged by
`the collimating lens 3211 onto the tiltable mirror 315.
`Tiltable mirror 315 is positioned so that wavelength coni-
`ponent K1 is reflected from the mirror to a selected one of the
`output ports 3401-340,, via thin film filters 302-304, which
`all reflect wavelength component kl. 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 7x2,
`X3, and K4 are reflected by thin film filter 301 back into
`substrate 308 and directed to thin film 302. Wavelength
`component X2 is transmitted through thin film filter 302 and
`lens 3221 and directed to a selected output port by tiltable
`mirror 316 via thin film filters 303-304, which all reflect
`wavelength component X2. Similarly, all other wavelength
`
`6
`components are separated in sequence by the thin film filters
`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 fiber 343. Although
`the embodiment of FIG. 2 is configured 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. A single collimating
`lens that directed light to the input fiber is fixed relative to
`the block at a 5 .7° angle with respect to the normal to the
`block. The focal length of the lens is chosen such that light
`exiting a Corning SMF-28T-"1 fiber 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 fiber in the output array.
`The fiber ends are polished flat and have an anti-reflective
`coating. A11 optional bypass port or fiber may also be
`provided, which collects any wavelengths received at the
`input fiber that has not been transmitted through any of the
`thin film filters. The bypass fiber 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 rcstrictions make it preferable to switch
`a subset of the total incident wavelengths, where the remain-
`ing (unswitched) wavelengths bypass the switching fabric.
`The first and second array of narrow band free-space
`switches each include eight thin film filters. The thin film
`filters are each a three-cavity resonant thin film filter with a
`surface dimension of 10 mm by 10 mm. In the first array, the
`first thin film filter, 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 THL (154532 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 film filter. 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 first array also includes a second narrow band free-
`space switch located 10 mm from the first free-space switch.
`The thin film filter employed i11 this switch has a center
`optical wavelength of 193.8 THZ (154692 nm). Six addi-
`tional narrow band free-space switches are located along the
`substrate having center wavelengths of 1548.52 nm, 1550.12
`
`10
`
`15
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`

`
`US 6,631,222 B1
`
`7
`11111, 1551.72 111n, 1553.3211111, 1554.9211n1, and 1556.52 11111,
`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 first array of switches is located. The
`second array of switches, which are also located 10 mm
`apart from one another, are laterally oricntcd half way
`between the first array of switches. The eight thin film filters
`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 reflects
`is directed to a particular output fiber 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 fiber.
`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
`diflicult using conventional fibcr 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 fibers 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 fiber via conventional fiber monitoring taps. However
`this approach becomes complicated if many other wave-
`lengths are present on the fiber, 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
`fibers 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 fiber. The
`latter approach is beneficial 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
`
`8
`achieve tunable wavele11gtl1 filtering. Such a coupler only
`cross-couples selected wavelengths from a first to a second
`optical fiber upon application of an appropriate acoustic
`vibration to the coupling region. If the appropriate acoustic
`vibration is 11ot applied, the selected wavelengths continue
`to propagate along the first optical fiber. Examples of an
`acoustic null coupler are disclosed in D. O. Culverhouse et
`al., Opt. Lett. 22, 96, 1997 and U.S. Pat. No. 5,915,050.
`As shown in FIG. 3, an input fiber 50 receiving the WDM
`signal is connected to an input port of a first null coupler 521.
`One output port of the first null coupler 521 is connected to
`an output fiber 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 first null coupler 521, the
`output ports of the second null coupler 522 are respectively
`connected to a second output fiber 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 fiber 50 can be directed to any
`selected output port 541, 542,
`.
`.
`. 54,” by applying the
`appropriate acoustic wave for those components to the null
`couplers 521, 522, .
`.
`. 54,” preceding those connected to the
`selected output port. For example, if any of the given 11
`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 traverse the null couplers in
`serial fashion,
`the resulting insertion loss need not be
`unacccptably large because the insertion of loss of each
`individual coupler can be quite small (e.g., less than 0.5 dB).
`The serial tunable filtering process used in the embodiment
`of the invention shown in FIG. 4 can also achieve the
`
`switching functionality of the present invention by using the
`previously described four-port tunable OADM device tech-
`nologies.
`FIG. 4 shows another alternative embodiment of the
`
`invention that employs conventional m11ltiplexers/
`dcmultiplcxcrs and conventional 1 xm switchcs, where n
`denotes the number of output ports of the switch. The
`multiplexers/demultiplexers may be employ thin film filters
`or waveguide gratings, for example. As shown, an input fiber
`60 supporting a WDM signal having 11 (where n is not
`necessarily equal to m) wavelength components is