(12) United States Patent
`Bishop et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 6,798,992 B1
`*Sep. 28, 2004
`
`US006798992B1
`
`(54) METHOD AND DEVICE FOR OPTICALLY
`CROSSCONNECTING OPTICAL SIGNALS
`USING TILTING MIRROR MEMS WITH
`DRIFT MONITORING FEATURE
`
`(75)
`
`.
`Inventors: David John Bishop, Summit, NJ (US);
`Randy Clinton Giles, Whippany, NJ
`(US)
`(73) Assignees: Agere Systems Inc., Allentown, PA
`S[JuSr)r;a§I]JtII{Cfi1I1tI\IT;eEI1}I§¢))108i9S 111°->
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(58) Field of Search ............................ .. 398/45, 46, 49,
`398/48, 50, 52, 55, 56, 12, 19; 356/73.1;
`385/16, 17, 18, 19, 20-24, 33, 119
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`6,198,856 B1 *
`3/2001 Schroeder et al.
`.......... .. 385/17
`
`glifilslaetetalal‘ "" “
`:
`7/2002 Sparks et 61.":..........3: 385/16
`6:424:757 B1 *
`OTHER PUBLICATIONS
`
`U.S. application No. 09/512174 (Aksyuk et al) filed on Feb.
`24, 2000.*
`
`This patent is subject to a terminal dis-
`Clalmen
`
`* Cited by examiner
`Primary Exami/1er—Hanh Phan
`
`(21) Appl. No.: 09/518,070
`
`(22)
`
`Filed:
`
`Mar. 2, 2000
`
`(90)
`
`Related U-S- APPliC3ti0n Data
`PT9Vi5i°na1 aPP1i°ati0I1 N0- 60/164,459, filed 011 N0V- 10;
`1999'
`
`Int. Cl.7 ........................ .. H04J 14/00
`(51)
`(52) U.S. Cl.
`........................... .. 398/45; 398/50; 398/52;
`398/55, 398/56; 398/12, 398/19, 398/47,
`398/48, 398/46, 385/16, 385/17, 385/18,
`385/19, 385/20, 385/21, 385/22, 385/23,
`385/24; 385/33, 385/119, 356/73.1
`
`(57)
`
`ABSTRACT
`
`A device and method for detecting rotational drift of mirror
`elements in a MEMS tilt mirror array used in an optical
`crossconnect. The optical crossconnect directs optical sig-
`nals from an input fiber to an output fiber along an optical
`path by rotatably positioning mirror elements in desired
`positions. A. monitoring device disposed outside of the
`Optlcal Path 15 used to Obtam “Pages of the MEMS array or
`to transmit and receive a test signal through the crossconnect
`for detecting the presence of mirror element drift.
`
`16 Claims, 2 Drawing Sheets
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`FNC 1025
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`

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`U.S. Patent
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`Sep. 28, 2004
`
`Sheet 1 of 2
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`US 6,798,992 B1
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`

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`U.S. Patent
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`Sep. 28, 2004
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`Sheet 2 of 2
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`US 6,798,992 B1
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`FIG. 3
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`US 6,798,992 B1
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`1
`METHOD AND DEVICE FOR OPTICALLY
`CROSSCONNECTING OPTICAL SIGNALS
`USING TILTING MIRROR MEMS WITH
`DRIFT MONITORING FEATURE
`
`This application is based on U.S. Provisional Application
`Ser. No. 60/164,459 filed on Nov. 10, 1999.
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention pertains to fiber optic communica-
`tions systems and, more particularly, to monitoring devices
`and methods for monitoring shifts in optical crossconnect
`configurations utilizing micro electromechanical systems
`(MEMS) tilting mirror arrays.
`2. Description of the Related Art
`In fiber optic communication systems, signal routing is
`essential for directing an optical signal carrying data to an
`intended location. Existing routing techniques typically
`experience optical power loss due to inefficient coupling of
`optic signals between input and output fibers. This increases
`the dependence on optical power sources (e.g., pump lasers)
`which are used to compensate for power losses by injecting
`optical power back into the optical system. The need for
`optical power sources increases the overall cost of the
`optical system.
`Another criteria for signal routing is the ability to direct
`a signal received from one of a plurality of input fibers or
`ports to any of a plurality of output fibers or ports without
`regard to the frequency of the optical signal.
`Free-space optical crossconnects allow interconnecting
`among input and output ports in a reconfigurable switch
`fabric. An example of such an optical crossconnect utilizing
`mirco-electromechanical systems (MEMS) tilting mirror
`devices is disclosed in commonly owned and copending
`U.S. patent application Ser. No. 09/410,586, filed Oct. 1,
`1999. By adjusting the tilt angles of the MEMS mirror
`devices, optical signals can be directed to various
`destinations, i.e. to numerous output fibers.
`MEMS devices and, in particular, tilting mirror devices
`are susceptible to unwanted movement or drift due to
`external factors such as temperature changes and mechanical
`fatigue experienced by actuator elements used to deploy and
`control the individual mirror elements. As a result, optical
`signal power may be lost due to misalignment of the
`reflected optical signal with its intended target (e.g. an
`output fiber). Accordingly, a system is desired to monitor
`MEMS optical crossconnect configuration to provide for
`displacement adjustment.
`
`SUMMARY OF THE INVENTION
`
`An optical crossconnect device having a monitoring fea-
`ture for detecting optical signal drift is provided. The device
`provides optical connection of optic signals between input
`fibers and output fibers by using a MEMS tilt mirror array.
`The MEMS array includes a plurality of tiltable mirror
`elements which are positionable in an intended orientation
`for directing optical signals, but which are susceptible to
`drift that causes degradation in the optical coupling of the
`signals to the output fibers. A monitoring device positioned
`outside of the optical path dynamically monitors the position
`of one or more of the mirror elements to detect drift.
`
`In a preferred embodiment, the monitoring device is a
`camera for obtaining an image of one or more mirror
`elements.
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`In another embodiment, the monitoring device comprises
`an optical transmitter and an optical receiver for transmitting
`a test signal through the optical crossconnect to monitor
`mirror position drift.
`In yet another embodiment, a pattern is formed on one or
`more of the mirror elements and an image or reflection of the
`pattern is obtained for determining the presence of mirror
`drift.
`
`A method is also described for monitoring mirror element
`positions of mirror elements in a MEMS tilt mirror array
`used in an optical crossconnect. The method is used with a
`MEMS mirror array having mirror elements disposed at
`desired tilt positions for crossconnecting an optic signal
`between an input fiber and an output fiber along an optical
`path. A monitoring device disposed outside of the optical
`path monitors the positions of the mirror elements to detect
`when position drift occurs. The mirror positions are then
`adjusted by forming control signals based on the detected
`drift and applying the control signals to the drifted mirror
`elements.
`
`Other objects and features of the present invention will
`become apparent from the following detailed description
`considered in conjunction with the accompanying drawings.
`It
`is to be understood, however,
`that
`the drawings are
`designed solely for purposes of illustration and not as a
`definition of the limits of the invention, for which reference
`should be made to the appended claims. It should be further
`understood that the drawings are not necessarily drawn to
`scale and that, unless otherwise indicated, they are merely
`intended to conceptually illustrate and explain the structures
`and procedures described herein.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`In the drawings, wherein like reference numerals denote
`similar elements throughout the several views.
`FIG. 1 is a planar view of an example of a MEMS mirror
`array used in connection with the present invention;
`FIG. 2 is a schematic representation of an optical cross-
`connect monitoring device in accordance with one embodi-
`ment of the present invention; and
`FIG. 3 is a schematic representation of a monitoring
`device for a “folded” optical crossconnect in accordance
`with another embodiment of the present invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`Arrays of two-axis tilt mirrors implemented using micro-
`electromechanical systems (MEMS) technology in accor-
`dance with the invention allow for the construction of large
`scale optical crossconnects for use in optical systems. Opti-
`cal crossconnects are commonly employed to connect a
`number of input optical paths to a number of output optical
`paths. Atypical requirements of optical crossconnects is that
`any input be capable of being connected to any output. One
`example of a MEMS mirror array 10 is depicted in FIG. 1.
`The mirror array 10 includes a plurality of tilt mirrors 12
`formed on a substrate 11, mounted to actuation members or
`springs 14 and controlled by electrodes (not shown). Each
`mirror 12 is approximately 100-500 Microns across, may be
`shaped as square, circular or elliptical, and is capable of
`operatively rotating or tilting about orthogonal X-Y axes,
`with the tilt angle being selectively determined by the
`amount of voltage applied to the control electrodes. Further
`details of the operation of the MEMS mirror array 10 are
`found in copending U.S. patent application Ser. No. 09/415,
`
`

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`US 6,798,992 B1
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`3
`178, filed Oct. 8, 1999, the entire contents of which are
`incorporated herein by reference. The general concept of
`utilizing two or more such tilt mirror arrays 10 to form an
`optical crossconnect is disclosed in copending U.S. patent
`application Ser. No. 09/410,586, filed Oct. 1, 1999, the entire
`contents of which are also incorporated herein by reference.
`The use of one or more MEMS tilt mirror arrays in
`conjunction with a lens array is disclosed in co-pending U.S.
`patent application Ser. No. 09/512,174, filed Feb. 24, 2000,
`the entire content of which is also incorporated herein by
`reference. As disclosed in that application, various optical
`crossconnect configurations of compact size (i.e. minimal
`spacing between crossconnect components) and exhibiting
`minimal optical power loss can be realized. One such optical
`crossconnect 100 discussed in the aforementioned applica-
`tion is depicted in FIG. 2. Crossconnect 100 receives input
`optic signals 108 through a plurality of optic fibers 112,
`preferably formed in an array as is well known in the art. For
`ease of illustration fiber array 110 is shown as a one-
`dimensional array having four fibers 112a, 112b, 1126, 112d.
`It is in any event to be understood that fiber array 112 as well
`as other fiber arrays discussed herein are preferably two-
`dimensional arrays such as, for example, N><N arrays.
`Fiber array 112 transmits the optical signals 108 to an
`array of lenses 114 that function as collimating lenses. The
`lens array 114 is positioned relative to fiber array 112 so that
`each lens communicates with a corresponding fiber for
`producing pencil beams 116 from the optic signals 118.
`Thus, beam 116a is produced from a signal carried by fiber
`112a, beam 116d is produced from a signal carried by fiber
`112d, etc.
`Afirst MEMS tilt mirror array 118, also referred to as the
`input array, is positioned in alignment with lens array 114 so
`that each mirror element 12 will receive a corresponding
`beam 116. The mirror elements are operatively tilted, in a
`manner discussed in application Ser. No. 09/415,178,
`to
`refiect
`the respective beams 116 to a second or output
`MEMS mirror array 122 positioned in optical communica-
`tion with MEMS array 118. Depending on the tilt angle of
`each mirror element in input MEMS array 118, the refiected
`signals can be selectively directed to specific mirror ele-
`ments in output MEMS array 122. To illustrate this
`principle, beam 116a is shown in FIG. 2 generating reflec-
`tion beams 120a and 120a‘ and beam 116d is shown in the
`
`figure generating reflection beams 120a’ and 120d‘. These
`beams are received by mirror elements in the output MEMS
`array 122 and are directed as beams 124 to an output lens
`array 126. An output fiber array 128 is aligned with lens
`array 126 to receive and output optical signals 129. Thus,
`lens array 126 couples beams 124 into the output fiber array
`128.
`
`The rotatable positions or orientations of the individual
`mirror elements 12 of arrays 118 and 122 are, however,
`affected by environmental conditions such as temperature
`changes. As a result, once the positions of the mirror
`elements 12 are set, those intended positions may drift or
`change due (for example) to temperature variations, thereby
`adversely causing inefficient or unintended signal routing
`and associated power losses. A similar problem may be
`caused by mechanical fatigue and stress on the actuators
`used to control mirror position, and by electric charging
`effects on the actuators. These variations can result
`in
`conditions referred to as macro-drift, wherein all of the
`mirror elements in an array drift by an equal amount, and
`micro-drift,
`in which only some of the mirror element
`positions unintendedly change.
`To detect such unwanted mirror drift in optical crosscon-
`nects in accordance with the present invention to compen-
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`sate for actual mirror positions, one or more monitoring
`devices 130, 132 are included in the crossconnect system
`100 shown in FIG. 2. The monitoring devices may be used
`to detect both macro-drift and micro-drift conditions of the
`MEMS mirror arrays 118, 122. For example, each monitor-
`ing device may be a camera or other imaging devices which
`operates independently of other cameras. Each camera is
`shown in FIG. 2 positioned outside of the optical path of the
`crossconnect (i.e. the path in which optical signal 116 travels
`through the crosssconnect to fiber array 128) and obtains an
`image of its respective MEMS array. Thus, camera 130 is
`focussed on MEMS array 118 and camera 132 is focussed on
`MEMS array 122. The resulting images are then compared
`to reference images of mirror array positions stored, for
`example, in a controller block 500 containing a processor
`and a database (not shown) in a manner well-known to those
`having ordinary skill in the art. In the event that an unac-
`ceptable amount of drift is detected for the entire mirror
`array, feedback control signals can be generated by the
`control block 500 for adjusting the tilt angles to compensate
`for drift by applying appropriate voltages to the mirror
`actuators. If on the other hand only certain mirror elements
`need to be adjusted, these mirrors can be identified, through
`the aforementioned image comparison with a reference
`image, and then re-positioned by applying appropriate volt-
`ages to the desired actuators.
`The monitoring system of FIG. 2 can also be employed in
`connection with a folded crossconnect configuration, as for
`example shown in FIG. 3, wherein a single input/output fiber
`array 312, single MEMS mirror array 318, and reflective
`surface element 330 comprise the folded configuration. A
`camera 340 positioned outside of the optical path 316
`obtains an image 342 of the mirror elements in the array 318
`for use in calculating and compensating for detected drift.
`As an alternative or in addition to the use of cameras,
`device 130 (FIG. 2) may comprise one or more illuminators
`(not shown) for producing, for example, one or more infra-
`red beams 131, 133 directed at mirror arrays 118, 122 and
`devices 130, 132 may comprise an infra-red detector for
`detecting the refiected infra-red beams. The illumination
`source may produce a test signal having a different wave-
`length from the signal wavelength or can be modulated to
`discriminate and distinguish it from the signal wavelength.
`The infrared beams 131, 133 may be pencil beams for
`illuminating a single mirror element which may be desig-
`nated as a reference element, such as element 16 in FIG. 1.
`The refiected infra-red signal will pass through the optical
`crossconnect for receipt by its respective infra-red detector.
`For example, for an infra-red test beam directed at a mirror
`element in array 118, the test beam will be refiected and
`directed to detector 130, and for an infrared beam directed
`at a mirror element in array 122,
`the test beam will be
`received by detector 132. Depending on the characteristics
`of the refiected and received infra-red beams—such as a
`
`reduction in beam power or intensity and/or a change of
`position on the detector at which the beam is received,
`etc.—macrodrift can be dynamically detected. For example,
`and as a result of a temperature change, drift may occur
`among all mirror elements in mirror arrays 118, 122. By
`measuring and detecting drift from a reference mirror ele-
`ment (e.g. mirror 16), the mirror arrays can be adjusted to
`compensate for drift by generating appropriate feedback
`signals from control blocks 500 to be applied to mirror
`control actuators.
`
`It will be appreciated that both devices 130, 132 can
`operate as combined or dual-function source/receiver
`devices wherein each device produces a signal for receipt by
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`US 6,798,992 B1
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`5
`the other and receives a signal produced by the other.
`Likewise, and in connection with the folded configuration of
`FIG. 3, device 340 can be implemented by or supplemented
`with a detector/receiver for receiving reflected test signals
`342, 343 generated by a source such as an infrared source
`350 for illuminating one or more mirror elements 12.
`For micro-drift compensation, the devices 130, 132 in the
`system 100 of FIG. 2 and the device 340 in the system 300
`of FIG. 3 can be connected to a scanning device which may
`be found in controller block 500 for changing the position of
`the test beam (beam 130 in FIG. 2 and beam 342 in FIG. 3)
`to illuminate multiple mirror elements. For example,
`the
`scanner can adjust the test beam position to illuminate one
`mirror element 12 at any given time for determining the tilt
`angle of each illuminated mirror.
`As another alternative, the reference mirror element 16
`may be formed with an imaging pattern 14, as for example
`by surface etching. This modification allows for the use of
`pattern recognition techniques wherein a generated pattern is
`received or monitored by a detector or camera. Detected
`movement of the pattern indicates mirror drift. Pattern 14
`may be specifically oriented to generate a unique pattern that
`is observable in scattered light so as to provide an enhanced
`signature when a light beam is centered on mirror 16. A
`single unique pattern may be used for all mirrors, or each
`mirror can be coated with its own unique pattern. Entire
`pathways through the mirror array may be defined by unique
`patterning, thus helping to guide light beams through the
`array during switching.
`Thus, while there have shown and described and pointed
`out fundamental novel features of the invention as applied to
`preferred embodiments thereof, it will be understood that
`various omissions and substitutions and changes in the form
`and details of the methods disclosed and devices illustrated,
`and in their operation, may be made by those skilled in the
`art without departing from the spirit of the invention. For
`example, it is expressly intended that all combinations of
`those elements and method steps which perform substan-
`tially the same function in substantially the same way to
`achieve the same results are within the scope of the inven-
`tion. Moreover,
`it should be recognized that structures
`and/or elements and/or method steps shown and/or
`described in connection with any disclosed form or embodi-
`ment of the invention may be incorporated in any other
`disclosed or described or suggested form or embodiment as
`a general matter of design choice.
`It
`is the intention,
`therefore, to be limited only as indicated by the scope of the
`claims appended hereto.
`What is claimed is:
`
`1. An optical crossconnect monitoring device for directing
`optical signals received from a plurality of input optic fibers
`along an optical path to a plurality of output optic fibers, and
`for detecting spatial shifts of the optical signals, comprising:
`a moveable mirror formed on a substrate and positioned
`within the optical path for receiving optical signals
`from one of the plurality of input optic fibers and
`directing said received signals along the optical path
`specific ones of the plurality of output optic fibers, said
`mirror being operatively tiltable about a rotational axis
`to an intended angular orientation relative to said
`substrate for providing desired directional refiection of
`a one of the optical signals received by said mirror; and
`an optical monitoring device positioned outside of the
`optical path and in optical communication with said
`mirror for optically detecting rotational drift of said
`mirror relative to said intended angular orientation, said
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`detected rotational drift being indicative of optical
`signal spatial shifts.
`2. The device of claim 1, wherein said moveable mirror
`comprises a MEMS mirror array having a plurality of
`moveable mirror elements.
`3. The device of claim 2, wherein said monitoring device
`comprises a camera oriented for obtaining an image of said
`mirror array.
`4. The device of claim 3, wherein one of said mirror
`elements is formed with a pattern for receipt of an image of
`said pattern by said camera.
`5. The device of claim 2, wherein said monitoring device
`comprises an illumination device for illuminating a selected
`one of said plural mirror elements with a test optical signal
`and a receiver for receiving the test signal after refiection of
`the test optical signal from said selected mirror element.
`6. The device of claim 5, wherein one of said mirror
`elements comprises a pattern for producing a refiection of
`said pattern for receipt by said receiver when said selected
`mirror element is illuminated by the test signal.
`7. The device of claim 5, wherein the plurality of input
`optic fibers and the plurality of output optic fibers form an
`array of optic fibers, said device further comprising a
`reflector element disposed in optical communication with
`said MEMS mirror array for receiving optical signals from
`said MEMS mirror array and for refiecting the received
`optical signals back to said MEMS mirror array, said
`reflected optical signals being redirected by said MEMS
`mirror array back to said array of optic fibers for receipt by
`the output optic fibers.
`8. The device of claim 7, wherein said reflector element
`receives the test signal from said illumination device and
`reflects the test signal to said receiver.
`9. The device of claim 7, wherein said illuminating device
`and said receiver are integrally formed.
`10. The device of claim 2, wherein the plurality of input
`optic fibers and the plurality of output optic fibers form an
`array of optic fibers, said device further comprising a
`reflector element disposed in optical communication with
`said MEMS mirror array for receiving optical signals from
`said MEMS mirror array and for refiecting the received
`optical signals back to said MEMS mirror array, said
`reflected optical signals being redirected by said MEMS
`mirror array back to said array of optic fibers for receipt by
`the output optic fibers.
`11. The device of claim 2, further comprising a controller
`connected to said monitoring device and operable for gen-
`erating a control signal in response to the detected rotational
`drift.
`
`12. A method of monitoring a spatial shift of optical
`signals in an optical crossconnect device which directs
`optical signals received from a plurality of input optic fibers
`along an optical path to a plurality of output optic fibers,
`comprising the steps of:
`placing a mirror formed on a substrate within the optical
`path for receiving optical signals from one of the
`plurality of input optic fibers and directing said
`received signals along the optical path to specific ones
`of the plurality of output optic fibers, said mirror being
`operatively tiltable about a rotational axis to an
`intended angular orientation relative to said substrate
`for providing desired directional refiection of one of the
`optical signals received by said mirror;
`positioning an optical monitoring device outside of the
`optical path and in optical communication with said
`mirror; and
`optically detecting rotational drift of said mirror relative
`to said intended angular orientation using the optical
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`monitoring device, said detected rotational drift being
`indicative of optical signal spatial shifts.
`13. The method of claim 12, wherein said moveable
`mirror comprises a MEMS mirror array having a plurality of
`moveable mirror elements.
`
`14. The method of claim 13, further comprising the steps
`of determining which of said plural mirror elements have
`experienced rotational drift, generating control signals from
`said optically detecting step, and using said control signals
`to operatively adjust rotatable positions of said rotationally
`drifted mirror elements.
`
`15. The method of claim 13, wherein said positioning step
`further comprises the step of positioning an optical signal
`transmitter outside of the optical path for generating an
`optical test signal directed at said MEMS array for reflection
`by said MEMS array, and positioning an optical receiver
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`outside of said optical path for detecting said reflected test
`signal after reflection of the test signal from the MEMS
`array, and wherein said monitoring step further comprises
`monitoring a power level of said test signal received by said
`receiver.
`16. The method of claim 13, wherein at least one of said
`mirror elements is a pattern mirror element, and wherein
`said positioning step further comprises positioning an opti-
`cal signal transmitter outside of the optical path for gener-
`ating an optical test signal directed at said pattern mirror
`element for reflection by said pattern mirror element to
`thereby generate an image of the pattern, and positioning an
`optical receiver outside of said optical path for receiving and
`detecting said pattern image.
`*
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`*
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`*