US006984917B2
`
`(12) United States Patent
`Greywall et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,984,917 B2
`Jan. 10, 2006
`
`(54)
`
`(75)
`
`OPTICAL ELEMENT HAVING TWO AXES
`OF ROTATION FOR USE IN TIGHTLY
`SPACED MIRROR ARRAYS
`
`Inventors: Dennis S. Greywall, Whitehouse
`Station, NJ (US); Dan M. Marom,
`Howell, NJ (US)
`
`(73)
`
`Assignee: Lucent Technologies Inc., Murray Hill,
`NJ (US)
`
`(*)
`
`Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 169 days.
`
`(21)
`(22)
`(65)
`
`(51)
`
`(52)
`
`(58)
`
`Appl. No.: 10/164,537
`
`Filed:
`
`Jun. 6, 2002
`
`Prior Publication Data
`
`US 2004/0212864 A1
`
`Oct. 28, 2004
`
`Int. Cl.
`H02N 1/00
`G02B 26/08
`G02B 26/10
`
`(2006.01)
`(2006.01)
`(2006.01)
`
`US. Cl. .................... .. 310/310; 359/198; 359/224;
`359/226; 359/385
`
`Field of Classi?cation Search .............. .. 310/309;
`359/223—224, 872, 198, 225; 385/18
`See application ?le for complete search history.
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`6/1993 Jeandeau .................. .. 359/196
`5,223,969 A *
`2/1999 Kiang et al. .... ..
`359/198
`5,867,297 A *
`6/1999 Asada ......... ..
`335/222
`5,912,608 A
`7/1999 Johnson
`359/223
`5,920,417 A
`3/2001 Greywall ........ ..
`359/245
`6,201,631 B1
`5/2002 Solgaard et a1. ..
`385/18
`6,389,190 B2 *
`2/2004 Greywall ........ ..
`385/18
`6,690,850 B1 *
`6,819,822 B2 * 11/2004 Behin et al. ................ .. 385/18
`
`* cited by examiner
`
`Primary Examiner—Karl Tamai
`
`ABSTRACT
`(57)
`Arotatable element includes a plate, a plate support, a cradle
`and a cradle support. The plate is coupled to the cradle via
`the plate support. The cradle is coupled to a surrounding
`frame by the cradle support. The plate and cradle are
`suspended over a cavity so that, in conjunction With the plate
`support and the cradle support, both the plate and cradle are
`capable of freely rotating about different axes of rotation
`When suitably actuated. Since the plate is capable of rotating
`independently of the cradle, yet also rotates When the cradle
`is rotated, the plate is rotatable about tWo axes of rotation.
`In some cases, the axis of rotation of the plate is perpen
`dicular to the axis of rotation of the cradle. Since the cradle
`does not surround the plate, the plates of adjacent rotatable
`elements can be placed very close to one another (i.e., as
`close as about 1 micron) to provide, for example, an array of
`very-closely-spaced mirrors.
`
`22 Claims, 6 Drawing Sheets
`
`w
`312
`
`Capella 2035
`Fujitsu v. Capella
`IPR2015-00727
`
`1
`
`

`
`U.S. Patent
`
`Jan. 10, 2006
`
`Sheet 1 6f 6
`
`US 6,984,917 B2
`
`FIG. 7
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`
`U.S. Patent
`
`Jan. 10, 2006
`
`Sheet 2 6f 6
`
`US 6,984,917 B2
`
`FIG. 2
`
`W
`
`\
`—*||*—
`
`308
`
`PORTION OF
`ROTATABLE
`[ELEMENT
`\‘
`42s
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`426
`|_
`
`424mm\ 422 l
`450
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`PORTION OF
`
`3
`
`

`
`U.S. Patent
`
`Jan. 10, 2006
`
`Sheet 3 of 6
`
`US 6,984,917 B2
`
`..........--/VEV
`
`06
`
`ofimm06mm
`
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`
`4
`
`
`

`
`U.S. Patent
`
`Jan. 10, 2006
`
`Sheet 4 6f 6
`
`US 6,984,917 B2
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`FIG. 5
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`13/700
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`
`U.S. Patent
`
`Jan. 10, 2006
`
`Sheet 5 6f 6
`
`US 6,984,917 B2
`
`844
`
`FIG. 6
`
`6
`
`

`
`U.S. Patent
`
`Jan. 10, 2006
`
`Sheet 6 0f 6
`
`US 6,984,917 B2
`
`FIG. 7
`
`MAWA/
`
`300-1
`
`FIG. 8
`
`1000
`
`~/\ 1002
`
`ROTATABLY COUPLING A PLATE TO A
`CRADLE IN A FIRST SUBSTRATE
`1
`ROTATABLY COUPLING THE CRADLE
`TO THE FIRST SUBSTRATE w 1004
`1
`FORMING AT LEAST TWO ELECTRODES
`AT A FIRST REGION IN A SECOND
`SUBSTRATE
`1
`FORMING AT LEAST TWO ELECTRODES
`AT A SECOND REGION IN THE
`SECOND SUBSTRATE
`I
`ALIGNING THE FIRST SUBSTRATE
`WITH THE SECOND SUBSTRATE
`I
`ATTACHING THE FIRST SUBSTRATE TO
`THE SECOND SUBSTRATE
`
`~/\ 1006
`
`w 1008
`
`$1012
`
`7
`
`

`
`US 6,984,917 B2
`
`1
`OPTICAL ELEMENT HAVING TWO AXES
`OF ROTATION FOR USE IN TIGHTLY
`SPACED MIRROR ARRAYS
`
`FIELD OF THE INVENTION
`
`The present invention relates generally to micro-electro
`mechanical systems. More particularly, the present inven
`tion relates to an optical element that is movable about tWo
`perpendicular aXes.
`
`10
`
`BACKGROUND OF THE INVENTION
`
`An array of individually-addressable, movable, micro
`machined mirrors can be used in optical communications
`netWorks to route or sWitch optical signals, e. g., optical cross
`connect, etc. Each mirror in the array is supported over a
`group of electrodes in such a Way that the mirrors are free
`to move, e.g., rotate about an aXis, etc., When actuated, such
`as by applying a voltage across a mirror and one or more of
`the underlying electrodes. By varying the amount that a
`mirror tilts, or the direction in Which it tilts, or both, an
`optical signal that is incident on the mirror can be directed
`to a desired location, such as a particular optical ?ber.
`Some neWer mirror arrays have mirrors that are rotatable
`about tWo perpendicular aXes of rotation, e.g., as is
`described in US. Pat. No. 6,201,631, Which is incorporated
`by reference herein.
`It is desirable to provide a high density of optical transfer
`for communications applications. In particular, in some
`applications, e.g., de-multipleXing, etc., the mirrors must be
`very tightly spaced (about 1 to 2 microns) to enable ?at pass
`bands With high spectral ef?ciency. Gimbaled mirrors, as
`exempli?ed by those described in US. Pat. No. 6,201,631,
`are not suitable for such applications because the gimbals
`present a limitation as to hoW close adjacent mirrors can be
`to one another. In particular, there must be a gap betWeen
`adjacent mirrors that is at least tWice the Width of a gimbal.
`In fact, the minimum gap is someWhat larger than this, since
`the minimum gap must also take into account the gap
`betWeen the mirror and the gimbal and the gap betWeen the
`gimbal and the support. Furthermore, some minimum sepa
`ration distance must be provided betWeen adjacent gimbals
`to maintain the integrity of the substrate layer to Which the
`gimbals are attached.
`It is possible to fabricate gimbaled mirrors that are
`someWhat smaller than the exemplary structure disclosed in
`the ’631 patent. Nevertheless, With the structure of prior-art
`gimbaled mirrors, it is not currently possible to achieve a
`mirror spacing of less than about 15 to 20 microns betWeen
`prior-art gimbaled mirrors. Consequently, prior-art gim
`baled-mirror arrays are not suitable for use in applications
`that require very close perimeter-to-perimeter spacing, e.g.,
`about 15 microns or less betWeen adjacent mirrors in a
`mirror array.
`
`15
`
`25
`
`35
`
`40
`
`45
`
`55
`
`SUMMARY OF THE INVENTION
`
`An array of rotatable elements, e.g., mirrors, etc., that
`avoids some of draWbacks of the prior art is disclosed. In
`particular, although the rotatable elements in the array are
`movable about tWo aXes of rotation that have different
`orientations, e.g., are perpendicular to one another, etc., they
`are nevertheless capable of being positioned very closely to
`one another.
`This is achieved, in accordance With the principles of the
`invention, by a rotatable element that includes a plate, a plate
`
`65
`
`2
`support, a cradle and a cradle support. The plate is rotatably
`coupled to the cradle via the plate support. Likewise, the
`cradle is rotatably coupled to a surrounding frame, e.g.,
`substrate, etc., by the cradle support. The rotatable element
`is suspended over a cavity so that, in conjunction With the
`plate support and the cradle support, both the plate and
`cradle are capable of freely rotating. In some embodiments,
`the aXis of rotation of the plate is perpendicular to the aXis
`of rotation of the cradle.
`Electrodes are disposed in the cavity beneath each rotat
`able element. In one embodiment, tWo electrodes are dis
`posed in the cavity under the rotatably-coupled portion of
`the cradle, on opposite sides of its aXis of rotation. Similarly,
`tWo electrodes are disposed in the cavity beneath the plate,
`on opposite sides of its aXis of rotation.
`When an electrical potential is applied across the plate
`and one of its underlying electrodes, the plate rotates out
`of-plane, i.e., out of the plane de?ned by the cradle, Which
`is the plate in Which the plate lies When it is in its quiescent
`or unactuated position, about its aXis of rotation toWard the
`electri?ed electrode. This provides one aXis of rotation for
`the plate. When an electrical potential is applied across the
`cradle and one of its underlying electrodes, the cradle rotates
`out-of-plane, i.e., of the substrate or frame, about its aXis of
`rotation toWard the electri?ed electrode. As the cradle
`rotates, the plate rotates With it. Furthermore, the plate can
`be rotated independently of the cradle, providing it With a
`second aXis of rotation.
`The plate is advantageously capable of providing an
`optical function. For eXample, in some embodiments, the
`plate functions as a mirror. Unlike prior-art gimbaled mir
`rors, in Which the gimbal completely surrounds the mirror,
`in a rotatable element in accordance With the principles of
`the invention, the cradle does not completely surround or
`encircle the plate, e.g., mirror. Consequently, adjacent mir
`rors in an may of rotatable elements can, advantageously, be
`very closely spaced. This makes them suitable for use in
`some optical applications in Which the prior-art gimbaled
`mirrors cannot be used.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 depicts a rotatable element having a cradle and a
`plate, in accordance With the principles of invention.
`FIG. 2 depicts a torsional support that rotatably couples
`rotatable elements to other rotatable or non-rotatable ele
`ments.
`FIG. 3 depicts a cross-sectional vieW of the rotatable
`element of FIG. 1 along the line A—A and in the direction
`indicated, but With the cradle partially rotated.
`FIG. 4 depicts a cross sectional vieW of the rotatable
`element of FIG. 1 along the line B—B and in the direction
`indicated, but With the plate partially rotated.
`FIG. 5 depicts an array of rotatable elements in accor
`dance With the principles of the invention.
`FIG. 6 depicts a de-multipleXer in accordance With the
`principles of the invention.
`FIG. 7 depicts an illustrative optical energy distribution of
`a spatially resolved WDM signal as a function of position at
`the front focal plane of a collimating/focusing lens Where an
`array of rotatable mirrors is positioned.
`FIG. 8 depicts a method for making a rotatable element or
`an array of rotatable elements in accordance With the prin
`ciples of the invention.
`
`8
`
`

`
`US 6,984,917 B2
`
`3
`DETAILED DESCRIPTION
`
`The terms listed below are given the following de?nitions
`for the purposes of this speci?cation.
`“Coupled” means that (coupled) elements interact With
`one another, e.g., by a direct physical connection, by an
`indirect mechanical linkage, through electrostatic, magnetic
`or optical interaction, etc. The coupled elements can, but do
`not have to be, physically attached to one another. For
`eXample, in some instances, tWo coupled elements Will be
`indirectly linked, such as through a third element, etc. When
`tWo elements that are indirectly linked are referred to as
`“coupled,” it means that movement of one of the coupled
`elements in?uences, e.g., imparts motion to, etc., the other
`coupled element. This ability to in?uence is not necessarily
`reciprocal as betWeen the tWo coupled elements.
`“Stress” means tensile stress or compressive stress.
`“Torsional” refers to a tWisting motion (of a connector,
`etc.) such as results from tWo opposing turning forces acting
`at right angles to the rotational aXis (of the connector, etc.).
`“Cradle,” Which is used as a noun, refers to a movable
`support element that supports (cradles) an element, e.g., a
`plate, etc., that is free to move. Furthermore, the cradled
`element is free to move independently of the cradle. The
`cradle itself is movably supported by another element, e.g.,
`a substrate, etc. Movement of the cradle causes the cradled
`element to move. That is, the orientation in space of the
`cradled element changes as the cradle moves. The term
`“cradle,” as used herein, is not intended to imply any
`particular structure and none is to be inferred.
`“Frame,” Which is used as a noun, refers to a stationary
`support element that supports an element that is free to
`move. The frame can be, for eXample, a substrate layer that
`surrounds the mechanical (movable) elements.
`“Optical Functionality” or “Optical Function” means an
`ability of affecting an optical signal in some predictable Way.
`EXample of optical functionalities include, Without limita
`tion, the ability to re?ect, diffract, ?lter, modulate, polariZe,
`focus, or collimate an optical signal. In other Words, an
`element that is characteriZed by such functionality is capable
`of functioning as a ?xed-re?ectivity mirror, a diffraction
`grating, an optical ?lter, an optical modulator, a polariZer or
`a lens, respectively. An additional optical functionality is the
`ability to function as a Wavelength-selective sWitch. In some
`variations, an element Will intrinsically possess an optical
`functionality, e.g., due to its composition, etc. In some other
`variations, an element can be modi?ed or processed in some
`Way, such as by depositing a re?ective material, or by
`depositing layers of material have particular refractive indi
`ces, or by depositing and patterning layers to create an
`optical device (a modulator), etc., so that it is capable of
`performing an optical function.
`
`I.A. Structure of a Rotatable Element in Accordance With the
`Principles of the Invention
`FIG. 1 depicts rotatable element 300. Rotatable element
`300 includes plate 302, cradle 304, plate support 306 and
`cradle support 310, inter-related as shoWn. Rotatable ele
`ment 300 is coupled to stationary frame 312. More particu
`larly, cradle support 310 couples cradle 304 to frame 312.
`Plate 302 is advantageously, but not necessarily, capable of
`performing an optical function.
`For the illustrative embodiment, portion 303 of plate 302,
`i.e., the portion of the plate that is “above” ads 3—3 in FIG.
`1, has a re?ective surface such that it functions as a
`?xed-re?ectivity mirror. It is Will be understood, hoWever,
`that in some variations of the illustrative embodiment, plate
`
`10
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`15
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`25
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`35
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`40
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`45
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`55
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`65
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`4
`302 has a different optical functionality, such as one or more
`of the other functionalities listed above. Those skilled in the
`art Will knoW hoW to use standard techniques to modify plate
`302, e.g., via metalliZation, via thin-?lm optics techniques,
`via lithography, etc., so it provides an optical function.
`Plate 302 is rotatably coupled to cradle 304 via plate
`support 306. That is, plate support 306 enables plate 302 to
`rotate about rotational aXis 3—3 When the plate is suitably
`actuated. In similar fashion, cradle 304 is rotatably coupled
`to frame 312 via cradle support 310. The cradle support
`enables cradle 304 to rotate about rotational aXis 4—4 When
`the cradle is suitably actuated. As depicted in FIG. 1,
`rotational aXis 3—3 is aligned With plate support 306 and
`rotational aXis 4—4 is aligned With cradle support 310.
`Furthermore, rotational aXis 3—3 is perpendicular to rota
`tional aXis 4—4.
`In the illustrative embodiment, plate support 306 and
`cradle support 310 are each implemented as paired torsional
`members 308, individually identi?ed as torsional members
`308A and 308B (for plate support 306) and torsional mem
`bers 308C and 308D (for cradle support 310). Members 308
`are referred to as “torsional” members because they tWist to
`enable an attached element, e.g., plate 302, cradle 304, etc.,
`to rotate (see, De?nitions, above).
`With continuing reference to the illustrative embodiment
`depicted in FIG. 1, one end of each of the paired torsional
`members depends from opposed regions, e.g., sides, por
`tions, etc., of an element that moves, e.g., plate 302, etc. The
`other end of each of the paired torsional members depends
`from opposed regions of an element that functions as a
`support for the movable element.
`Thus, in the illustrative embodiment, one end of each of
`torsional members 308A and 308B depend from respective
`opposed sides 314 and 316 of plate 302, i.e., the element that
`moves. The other end of torsional members 308A and 308B
`depend from opposed portions of cradle 304, i.e., the ele
`ment that supports plate 302. LikeWise, one end of each of
`torsional members 308C and 308D depends from opposed
`portions 318 and 320 of cradle 304, i.e., an element that
`moves, While the other end depends from opposed portions
`of frame 312, i.e., the element that supports cradle 304.
`As depicted in FIG. 1, torsional members 308A and 308B
`are substantially recessed Within plate 302 and torsional
`members 308C and 308D are substantially recessed Within
`cradle 304. Recessing torsional members 308 in this fashion
`decreases What Would otherWise be a larger gap betWeen the
`rotatable element, e.g., plate 302, etc., and the structure to
`Which it’s coupled, e.g., cradle 304, etc.
`FIG. 2 provides further detail of torsional members 308.
`As depicted in FIG. 2, torsional member 308 includes
`connector 422 and cross-piece 428, Which are joined in a
`“T” con?guration. Connector 422 couples tWo elements: (1)
`an element that moves and (2) its support structure. For
`eXample, With regard to torsional members 308A and 308B,
`connector 422 couples plate 302 to cradle 304. As to
`torsional members 308C and 308D, connector 422 couples
`cradle 304 to frame 312. The aXis of rotation (of the element
`that moves) is aligned With the paired torsional members
`308 that couple the element to its support structure.
`In the illustrative embodiment depicted in FIG. 2, end 424
`of connector 422 is attached to the support structure, e.g.,
`cradle 304, etc., While the other end, end 426, couples to the
`element that moves via cross-piece 428. Cross-piece 428
`functions as a “shock absorber” for connector 422. In
`particular, cross-piece 428 is capable of ?exing, as neces
`sary, to absorb any stresses on connector 422, as commonly
`arise during fabrication procedures. Connector 422 and
`
`9
`
`

`
`US 6,984,917 B2
`
`5
`cross-piece 428 each include widened region 430 near points
`of attachment. This Widened region decreases stress con
`centration at the points of attachment.
`It Will be understood that other types, e. g., con?gurations,
`of torsional members, as are knoWn in the art, can be used.
`Furthermore, other types of members, i.e., non-torsional
`members, that are suitable for rotatably coupling tWo ele
`ments can suitably be used as Well.
`FIGS. 3 and 4 depict cross-sectional vieWs of rotatable
`element 300 depicted in FIG. 1. FIG. 3 is cross section along
`the line A—A, vieWed in the direction shoWn, and FIG. 4 is
`a cross section along the line B—B, vieWed in the direction
`shoWn. As depicted in those Figures, plate 302 and cradle
`304 are suspended over cavity 532 so that they are free to
`rotate. Electrodes 534A, 534B, 534C, and 534D are dis
`posed in cavity 532. More particularly, electrodes 534A and
`534B underlie plate 302, With one electrode on each side of
`aXis-of-rotation 3—3. Electrodes 534C and 534D underlie a
`portion of cradle 304, With one electrode on each side of
`aXis-of-rotation 4—4.
`When an electrical potential is applied across an element
`that moves, e.g., plate 302, etc., and one of the underlying
`electrodes, the element rotates out-of-plane, i.e., out of the
`plane de?ned by the support structure, about its aXis of
`rotation toWard the electri?ed electrode.
`For example, With reference to FIG. 3 (Which shoWs a
`portion of cradle 304), assume that an electric potential is
`applied across cradle 304 and electrode 534D. As a conse
`quence, cradle 304 rotates out-of-plane of frame 312 about
`aXis 4—4 such that the portion of cradle 304 that overlies
`electrode 534D moves doWnWard toWard that electrode (see
`FIG. 3). Since plate 302 is coupled to cradle 304, plate 302
`also rotates about aXis 4—4, i.e., the cradle’s aXis of
`rotation, although, for clarity, rotation of plate 302 is not
`depicted in FIG. 3.
`Referring to FIG. 4 (Which shoWs portions of both cradle
`304 and plate 302), assume that a potential is applied across
`plate 302 and electrode 534B. In response, the portion of
`plate 302 that overlies electrode 534B is draWn toWard that
`electrode, rotating out-of-plane of cradle 304 about aXis
`3—3. Since plate 302 rotates (about aXis 4—4) When cradle
`304 rotates, plate 302 is capable of rotating about tWo
`perpendicular axes: aXis 3—3 and ads 4—4.
`It is understood that for rotatable element 300 to move as
`has been described, electrodes 534, plate 302, and cradle 304
`must be electrically coupled to a controlled voltage source.
`The controlled voltage source and the various electrical
`connections are not depicted in the Figures for the sake of
`clarity and to aid in focusing the reader on elements that are
`germane to an understanding of the principles of the inven
`tion.
`It is notable that in prior-art gimbaled mirrors, the gimbal
`completely surrounds the mirror. In contrast, in rotatable
`element 300, cradle 304 does not completely surround plate
`302. In fact, if region 303 (see FIG. 1) of plate 302 is
`considered to be the “mirror,” then cradle 304 does not
`surround any part of the “mirror.” Stated differently, in
`rotatable element 300, the segment of rotational aXis 3—3
`that is de?ned by the location of torsional members 308A
`and 308B does not overlap or intersect the segment of
`rotational aXis 4—4 that is de?ned by the location of
`torsional members 308C and 308D. This is in contrast to the
`corresponding “segments” of the tWo rotational aXes of the
`prior-art gimbaled mirrors, Wherein the segments do over
`lap, i.e., in the center of the mirror.
`The difference in structure betWeen prior-art gimbaled
`mirrors and rotatable element 300 can be described in yet
`
`6
`another Way. In particular, in prior-art gimbaled mirrors, the
`center of mass of all the electrodes for a given gimbaled
`mirror aligns With the center of mass of the mirror. In
`rotatable element 300, hoWever, the center of mass of all the
`electrodes for a given rotatable element does not align With
`the center of mass of plate 302.
`As described later in this speci?cation, these differences
`in structure enable rotatable element 300 to be used in a
`variety of applications, notably optical communications, for
`Which the prior-art gimbaled mirrors are unsuitable.
`
`EXAMPLE
`
`An illustrative design for rotatable element 300 in accor
`dance With the principles of the invention is presented in this
`EXample.
`Tables I and II, beloW, provide performance parameters
`for rotatable element 300. The parameters are given as a
`function of:
`(1) Length, L, of connector 422 of torsional members 308
`(see, FIG. 2).
`(2) Width, W, of connector 422 of torsional members 308
`(see, FIG. 2).
`(3) Gap, To, betWeen plate 302 (or cradle 304) and the
`underlying electrodes, see, e.g., FIG. 3.
`
`The dimensions of rotatable element 300 (see, FIGS. 1 and
`2) are as folloWs:
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`length, lp, of plate 302:
`Width, WP, of plate 302:
`Width, Wine, of cradle 304:
`Width, WIC, of cradle 304:
`
`35
`
`length, lIC, of cradle 304:
`
`length, 10, of cradle 304:
`gap, gpc:
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`40
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`45
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`thickness of plate 302:
`thickness of cradle 304:
`length of cross piece 428:
`length of Widened region
`430:
`Width of Widened region
`430:
`
`150 microns
`microns
`microns (at Widest point)
`140
`79 microns (rectangular portion above
`electrodes)
`150 microns (rectangular portion above
`electrodes)
`331 microns (length of full cradle 304)
`microns (gap between plate 302
`3
`and cradle 304)
`microns (gap between cradle 304
`and frame 312)
`
`micron
`micron
`microns
`micron
`
`micron
`
`The angle of rotation, (I), of cradle 304 (see, FIG. 3) is
`limited by certain dimensions of rotatable mirror 300. This
`limitation results from one of tWo different constraints. One
`constraint on rotation is that continued rotation of cradle 304
`Will result in the cradle making contact With underlying
`electrode 534. The angle of rotation at contact, (mom, is
`dependent upon the Width, W’C, of cradle 304 (i.e., the Width
`of the portion of the cradle that is above the electrodes) and
`the gap, To, betWeen the cradle and an underlying electrode.
`With a Width, W Q, of 79/2=39.5 microns, and a gap, T0, of
`10 microns, ¢mch=82 degrees. This is one limitation on
`angle of rotation, (I), of cradle 304.
`The second constraint on rotation arises due to the use of
`an electrostatic force (in the illustrative embodiment) as the
`actuating force. In particular, due to the nature of electro
`statics, an instability occurs When the displacement of an
`element equals or eXceeds 1/3 of the gap betWeen the
`attracting elements. This instability causes the movable
`element to “snap-doWn” and contact the ?Xed element.
`Consequently, the displacement of the edge of cradle 304,
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`US 6,984,917 B2
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`7
`for example, in a “vertical” direction (as it rotates) is
`restricted to a distance that is less than 1/3 of the distance
`betWeen cradle 304 and underlying electrode 534 (see, FIG.
`3). In other Words:
`[4]
`Displacement<V3To
`This distance de?nes critical angle of rotation, (1)6, of
`cradle 304. The cradle cannot be rotated beyond this point.
`This behavior is Well knoWn to those skilled in the art. For
`the con?guration and dimensions provided above, the criti
`cal angle of rotation for cradle 304, (1)6, is 12.6 degrees.
`For the Example, the critical angle of rotation, (1)6, is
`greater than the angle of rotation at contact, q>mhz 12.6>8.2.
`Consequently, rotation of cradle 304 is limited by contact
`(not instability) to 8.2 degrees.
`The same considerations apply to plate 302. For plate 302,
`the angle of rotation at contact, Grouch, is 7.7 degrees. The
`critical angle of rotation, 06, is 9.2 degrees. Like cradle 304,
`the rotation of plate 302 is limited by contact, Which, for this
`example, is 7.7 degrees.
`
`TABLE I
`
`Performance of Rotatable Element for T = 10 microns
`
`10
`10
`10
`10
`
`0.30
`0.35
`0.35
`0.40
`
`8
`10
`12
`12
`
`112
`125
`114
`135
`
`124
`138
`126
`149
`
`Table I shoWs the voltage requirement at the critical angle
`of rotation for both cradle 304, Which is Vq’m-n-ml, and for
`plate 302, Which is Veal-rim]. The voltage that is required to
`obtain the maximum (for this illustration) cradle rotation of
`8.2 degrees and the maximum (for this illustration) plate
`rotation of 7.7 degrees Will be less than the critical voltages
`shoWn. (Again, this is because, in the Example, the maxi
`mum angle of rotation for both plate 302 and cradle 304 is
`less than the critical angle of rotation.)
`Table II, beloW, provides the same type of information as
`Table I, but for a con?guration Wherein the gap, To, betWeen
`plate 302 or cradle 304 and electrodes 534 is increased to 12
`microns. For this illustration, (preach, :99 degrees, ¢C=15.2
`degrees and 0 much, is 9.2 degrees and 06:11.1 degrees. As
`before, the voltage that is required to obtain the maximum
`(for this illustration) cradle rotation of 9.9 degrees and the
`maximum (for this illustration) plate rotation of 9.2 degrees
`Will be less than the critical voltages shoWn.
`
`TABLE II
`
`Performance of Rotatable Element for T = 12 microns
`
`12
`12
`12
`12
`
`0.30
`0.35
`0.35
`0.40
`
`8
`10
`12
`12
`
`148
`165
`151
`179
`
`164
`183
`167
`198
`
`As Tables I and II and the accompanying description
`indicate, for the illustrative embodiment and illustrative
`dimensions, potential differences in the range of about 100
`volts to about 200 volts Will rotate plate 302 and cradle 304
`up to about 15 degrees. Smaller voltages result in less
`rotation. And, generally, as the gap, To, betWeen plate 302 or
`cradle 304 and underlying the electrodes increases, the
`maximum alloWable rotation increases (both the angle for
`
`8
`contact and the critical angle), but so do the voltage require
`ments. Relatively small rotations (i.e., a feW degrees) are all
`that is required for many applications of rotatable element
`300.
`
`I.B. Structure of an Array of Rotatable Elements In Accor
`dance With the Principles of the Invention
`FIG. 5 depicts an array 700 of rotatable elements 300.
`Each rotatable element 300 in the array includes plate 302
`and cradle 304, as previously described, see, e. g., FIG. 1 and
`the accompanying description.
`Rotatable elements 300 are surrounded by frame 312 and
`are suspended over cavity 532, see, e.g., FIGS. 3 and 4. Pairs
`of electrodes 534 (not depicted in FIG. 5) underlie plate 302
`and a portion of cradle 304. Each rotatable element 300
`Within array 700 is individually addressable. Furthermore,
`plate 302 and cradle 304 of each rotatable element 300 can
`be individually actuated. In other Words, plate 302 can be
`made to rotate about either one axis, i.e., one of either the
`rotational axis of plate 302 or the rotational axis of cradle
`304, or about tWo axes.
`Since cradle 304 does not completely surround plate 302
`(in contrast to the manner in Which the gimbal surrounds the
`mirror in prior-art gimbaled mirrors), plates 302 of adjacent
`rotatable elements 300 in array 700 can be placed in near
`abutting relation. More particularly, in some embodiments,
`adjacent plates 302 are placed Within 15 microns of one
`another. In some other embodiments, adjacent plates 302 are
`placed Within 10 microns of one another. In some additional
`embodiments, adjacent plates 302 are placed Within 5
`microns of one another. In some other embodiments, adja
`cent plates 302 are advantageously placed as close as about
`1 micron from one another. The spacing betWeen adjacent
`plates 302 Will, in some instances, be dictated by application
`speci?cs.
`Array 700 of rotatable elements 300 has a variety of uses,
`many of Which pertain to optical telecommunications. One
`such use is described beloW.
`
`I.C. Demultiplexer Incorporating an Array of Rotatable
`Mirrors
`The transmission capacity of optical netWorks is signi?
`cantly increased using Wavelength division multiplexing
`(“WDM”). In a WDM communications netWork, many
`optical signals are superimposed on a single optical ?ber.
`Each signal has a different Wavelength, Which de?nes a
`WDM “channel.”
`Typically, the channels in a WDM communications sys
`tem are routed selectively along different paths as a function
`of Wavelength (“Wavelength routing”). To accomplish this,
`optical netWork nodes, Which provide sWitching and routing
`functions in an optical netWork, must be capable of “recog
`niZing” each channel independent of other channels.
`One device that is capable of providing this “recognition”
`to perform Wavelength routing is a de-multiplexer. The
`de-multiplexer spatially resolves the plural WDM channels
`and delivers each channel or spectral component to a desired
`output ?ber.
`In accordance With the principles of the invention, an
`array of rotatable elements, as has been described herein, is
`optically coupled to lenses, a diffraction grating and an input
`and output ports to provide a de-multiplexing capability.
`FIG. 6 depicts illustrative de-multiplexer 800, Which is
`based on a de-multiplexer that is described in US. patent
`application Ser. No. 09/944,800, Which is incorporated by
`reference herein.
`As depicted in FIG. 6, de-multiplexer 800 includes array
`700 of rotatable elements 300-i, i=1,m, array 836 of input/
`output ports 838-j, j=1, n, array 840 of collimating/focusing
`lenses 842-k, k=1, p, diffraction grating 844, and collimat
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`US 6,984,917 B2
`
`ing/focusing lens 846, inter-related as shown. For this appli
`cation, rotatable elements 300-i are rotatable rnirrors.
`Since array 700 provides rotatable mirrors that have tWo
`perpendicular rotation aXes, input/output port array 836 is
`advantageously, but not necessarily, con?gured as a tWo
`dirnensional array of ports 838-j. (If the mirrors in the array
`had only a single rotational axis, then the input/output ports
`Would have to be arranged linearly.) Since input/output ports
`838-j are con?gured as a two-dimensional array, collirnat
`ing/focusing lenses 842-k should be con?gured as a tWo
`dirnensional array as Well. For simplicity and clarity, input/
`output port array 836 and array 840 of lenses are depicted in
`FIG. 6 as linear arrays.