Abstract:
A micro-electro-mechanical (MEMs) mirror device for use in an optical switch is disclosed. A “piano”-style MEMs device includes an elongated platform pivotally mounted proximate the middle thereof by a torsional hinge. The middle portion of the platform and the torsional hinge have a combined width less than the width of the rest of the platform, whereby several of these “piano” MEMs devices can be positioned adjacent each other pivotally mounted about the same axis with only a relatively small air gap therebetween. In a preferred embodiment of the present invention specially designed for wavelength switching applications, a greater range of arcuate motion for a mirror mounted thereon is provided by enabling the platform to rotate about two perpendicular axes. The MEMs mirror device according to the preferred embodiment of the present invention enables the mirror to tilt about two perpendicular axes, by the use of an “internal” gimbal ring construction, which ensures that a plurality of adjacent mirror devices have a high fill factor, without having to rely on complicated and costly manufacturing processes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present invention claims priority from U.S. Patent Application No. 60/383,106 filed May 28, 2002. 

   TECHNICAL FIELD 
   The present invention relates to a micro-electro-mechanical (MEMs) mirror device for use in an optical switch, and in particular to a MEMs mirror device with a centrally disposed hinge providing high fill factor mirror spacing. 
   BACKGROUND OF THE INVENTION 
   Conventional MEMs mirrors for use in optical switches, such as the one disclosed in U.S. Pat. No. 6,535,319 issued Mar. 18, 2003 to Buzzetta et al, to redirect beams of light to one of a plurality of output ports include an electro-statically controlled mirror pivotable about a single axis. Tilting MEMs mirrors, such as the ones disclosed in U.S. Pat. No. 6,491,404 issued Dec. 10, 2002 in the name of Edward Hill, and U.S. Patent Publication No. 2003/0052569, published Mar. 20, 2003 in the name of Dhuler et al, which are incorporated herein by reference, comprise a mirror pivotable about a central longitudinal axis. The MEMs mirror device, disclosed in the aforementioned Hill patent, is illustrated in  FIG. 1 , and includes a rectangular planar surface  2  pivotally mounted by torsional hinges  4  and  5  to anchor posts  7  and  8 , respectively, above a substrate  9 . The torsional hinges may take the form of serpentine hinges, which are disclosed in U.S. Pat. No. 6,327,855 issued Dec. 11, 2001 in the name of Hill et al, and in U.S. Patent Publication No. 2002/0126455 published Sep. 12, 2002 in the name of Robert Wood, which are incorporated herein by reference. In order to position conventional MEMs mirror devices in close proximity, i.e. with a high fill factor, fill factor=width/pitch, they must be positioned with their axes of rotation parallel to each other. Unfortunately, this mirror construction restraint greatly restricts other design choices that have to be made in building the overall switch. 
   When using a conventional MEMs arrangement, the mirror  1  positioned on the planar surface  2  can be rotated through positive and negative angles, e.g. ±2°, by attracting one side  11  or the other side  12  of the planar surface  2  to the substrate  9 . Unfortunately, when the device is switched between ports at the extremes of the devices rotational path, the intermediate ports receive light for fractions of a millisecond as the mirror  1  sweeps the optical beam past these ports, thereby causing undesirable optical transient or dynamic cross-talk. 
   One solution to the problem of dynamic cross-talk is to initially or simultaneously rotate the mirror about a second axis, thereby avoiding the intermediate ports. An example of a MEMs mirror device pivotable about two axes is illustrated in  FIG. 2 , and includes a mirror platform  11  pivotably mounted by a first pair of torsion springs  12  and  13  to an external gimbal ring  14 , which is in turn pivotally mounted to a substrate  16  by a second pair of torsion springs  17  and  18 . Examples of external gimbal devices are disclosed in U.S. Pat. No. 6,529,652 issued Mar. 4, 2003 to Brenner, and U.S. Pat. No. 6,454,421 issued Sep. 24, 2002 to Yu et al. Unfortunately, an external gimbal ring greatly limits the number of mirrors that can be arranged in a given area and the relative proximity thereof, i.e. the fill factor. Moreover, the external gimbal ring may cause unwanted reflections from light reflecting off the support frame. 
   Another proposed solution to the problem uses high fill factor mirrors, such as the ones disclosed in U.S. Pat. No. 6,533,947 issued Mar. 18, 2003 to Nasiri et al, which include hinges hidden beneath the mirror platform. Unfortunately, these types of mirror devices require costly multi-step fabrication processes, which increase costs and result in low yields. 
   An object of the present invention is to overcome the shortcomings of the prior art by providing a high fill factor MEMs mirror device that can pivot about the same axis as an adjacent mirror. 
   Another object of the present invention is to provide a MEMs mirror device that is relatively easy to fabricate, with an internal gimbal ring, applicable in high fill factor applications. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention relates to a micro-electro-mechanical device for pivotally supporting an optical element on a substrate comprising: 
   a platform for supporting the element, the platform including first and second supporting regions each defined by a width and a length, and brace means extending therebetween defined by a width and a length; 
   a first torsional hinge, rotatable about a first axis perpendicular to said brace means, and extending between at least one anchor post of the substrate and the platform proximate the brace means; 
   wherein the total width of the first torsional hinge and said brace means is less than the width of the first or the second supporting regions, whereby a plurality of platforms pivotable about the first axis are positionable in close proximity with only an air gap between adjacent first supporting regions. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
       FIG. 1  is an isometric view of a conventional tilting MEMs mirror device; 
       FIG. 2  is a plan view of a pair of conventional external gimbal ring MEMs mirror devices; 
       FIG. 3  is an isometric view of a plurality of Piano-MEMs mirror devices according to the present invention; 
       FIG. 4  is an isometric view of a hinge structure of the mirror devices of  FIG. 3 ; 
       FIG. 5  is an isometric view of an electrode structure of the mirror devices of  FIG. 3 ; 
       FIG. 6  is an isometric view of a plurality of Piano-MEMs mirror devices according to an alternative embodiment of the present invention; 
       FIG. 7  is a plan view of a pair of internal gimbal ring MEMs mirror devices according to the present invention; 
       FIG. 8  is an isometric view of an internal gimbal ring MEMs mirror device according to the present invention; 
       FIG. 9  is an isometric view of an alternative embodiment of the internal gimbal ring MEMs mirror devices according to the present invention; 
       FIG. 10  is an isometric view of a hinge structure of the mirror devices of  FIG. 9 ; 
       FIG. 11  is an isometric view of an electrode structure of the mirror devices of  FIGS. 9 and 10 ; 
       FIG. 12  is a graph of Voltage vs Time provided by the electrode structure of  FIG. 11 ; 
       FIG. 13  is a schematic diagram of a wavelength switch utilizing the mirror devices of the present invention; 
       FIG. 14  is a schematic diagram of an input/output assembly for the wavelength switch of  FIG. 13 ; and 
       FIG. 15  is a schematic diagram of an alternative embodiment of an input assembly for the wavelength switch of FIG.  13 . 
   

   DETAILED DESCRIPTION 
   In accordance with the present invention an array of “Piano” MEMs mirror devices  21 ,  22  and  23 , which pivot about a single axis of rotation θ y  above a substrate  25 , is illustrated in  FIGS. 3 ,  4  and  5 . Each mirror device  21 ,  22  and  23  includes a pivoting platform  26  defined by first and second substantially-rectangular planar supporting regions  27  and  28  joined by a relatively-thin substantially-rectangular brace  29  extending therebetween. Typically, each planar surface is coated with a reflective coating, e.g. gold, for simultaneously reflecting a pair of sub-beams of light traveling along parallel paths, as will be hereinafter discussed. Each brace  29  acts like a lever and is pivotally mounted to anchor posts  30  and  31  via first and second torsional hinges  32  and  33 , respectively. The anchor posts  30  and  31  extend upwardly from the substrate  25 . The ends of the first torsional hinge  32  are connected to the anchor post  30  and the brace  29  along the axis θ y . Similarly, the ends of the second torsional hinge  32  are connected to the anchor post  31  and the brace  29  along the axis θ y . Preferably, each of the first and second torsional hinges  32  and  33  comprises a serpentine hinge, which are considerably more robust than conventional torsional beam hinges. The serpentine hinge is effectively longer than a normal torsional hinge, which spans the same distance, thereby providing greater deflection and strength, without requiring the space that would be needed to extend a normal full-length torsional hinge. 
   With particular reference to  FIG. 5 , each platform  26  is rotated by the selective activation of a first electrode  36 , which electrostatically attracts the first planar section  27  thereto or by the selective activation of a second electrode  37 , which electrostatically attracts the second planar section  28  thereto. A gap  38 , illustrated in  FIG. 5 , is provided between the first and second electrodes  36  and  37  for receiving the anchor posts  31 , which extend from the substrate  35  to adjacent the platforms  26 . 
   A consequence of closely packed micro-mirrors is that the actuation of a single mirror will impart a torque, i.e. an angular rotation, onto adjacent mirrors as a result of fringing electric fields. In an effort to minimize this cross-talk, electrode grounding shields  41  are positioned on the substrate  25  around the first and second electrodes  36  and  37  forming electrode cavities, which are electrically isolated from each other. The grounding shields  41  are kept at ground potential, i.e. the same as the mirrored platforms  26 , while one of the first and second electrodes is held at an activation voltage, e.g. 100 Volts. To further eliminate cross-talk between adjacent electrodes, additional platform shields  42  ( FIG. 6 ) can be added to the underside of the platform  26 , outside or inside of the electrode shields  41 . Typically, in the rest position, the two different sets of shields  41  and  42  do not overlap; however, as the platform  26  tilts the platform shields  42  begin to overlap the grounding shielding  41 . The added protection provided by overlapping shielding is particularly advantageous, when the tilt angle of the platform  26  is proportional to the voltage applied to the electrode  36  (or  37 ), such as in open loop configurations. Accordingly, the greater the tilt angle, the greater the required voltage, and the greater the amount of potential cross-talk, but consequently the greater the amount of shielding provided by the overlapping ground and platform shields  41  and  42 , respectively. 
   With reference to  FIG. 7 , a pair of internal gimbal ring MEMs mirror devices  131  and  132  are illustrated mounted adjacent each other on a substrate  133 . The present invention enables mirrors  134  and  135  to be positioned relatively close together, i.e. with a high fill factor, while still providing the two degrees of motion provided by the more complicated prior art. 
   With further reference to  FIG. 8 , a first torsion hinge  137 , preferably in the form of a rectangular beam, is fixed, proximate the middle thereof, to the substrate  133  via a central anchor post  138 . The supporting structure for the mirror device of the present invention is based on a single anchor post  138 , rather than the dual anchor points required in the aforementioned external gimbal ring devices. The first torsion hinge  137  provides for rotation about a first axis θ y , and may also include a serpentine hinge  140 , as illustrated in mirror device  131 , or any other torsional hinge known in the art. Opposite sides of an internal gimbal ring  139  are connected to opposite ends of the first torsion hinge  137 , whereby the first torsion hinge  137  bisects the internal gimbal ring  139 . The internal gimbal ring  139  is preferably not flexible, but can take various geometric forms, although rectangular or circular frames would be the most convenient to fabricate and use. Spring arms  141  and  142 , which define a second torsion hinge, extend outwardly from opposite sides of the internal gimbal ring  139  perpendicular to the first torsion hinge  137 . Each of the spring arms may also include a serpentine hinge as hereinbefore described. The second torsion hinge provides for rotation about a second axis θ x , which is perpendicular to the first axis θ y , but still substantially in the same plane as the mirrors  134  and  135 . A generally rectangular platform  143 , for supporting one of the mirrors  134  or  135 , is mounted on the ends of the spring arms  141  and  142 . Preferably, the platform  143  is comprised of a pair of rectangular planar surfaces  144  and  145  joined together by a pair of elongated braces  147  and  148 , which extend on either side of the internal gimbal ring  139  parallel with the spring arms  141  and  142 . 
   Fabrication of the preferred embodiment illustrated in  FIGS. 7 and 8 , is simplified by having all of the structural elements, i.e. the first torsional hinge  137 , the gimbal ring  139 , the spring arms  141  and  142 , and the first and second planar surfaces  144  and  145 , in the same upper substrate layer and having coplanar upper surfaces, whereby the same basic process steps are used as are used to fabricate the MEMs device illustrated in FIG.  1 . In particular, a single photolithographic step is used to identify the structural elements, followed by a deep reactive ion etching (DRIE) step used to remove the unwanted portions of the upper substrate. Finally the moveable elements in the upper substrate are released from the lower substrate by removal of a sacrificial layer therebetween. 
     FIGS. 9 and 10  illustrate an array of internal gimbal ring MEMs mirror devices  201  utilizing a first pair of serpentine torsional hinges  202  for pivoting a rectangular platform  203  about a first axis of rotation θ x , and a second pair of serpentine torsional hinges  204  for rotating the platform  203  about a second axis of rotation θ y  above a base substrate  205 . The first pair of serpentine torsional hinges  202  extend from a single anchor post  206 , which extends upwardly from the base substrate  205  through the center of the platform  203 , i.e. at the intersection of the minor and major axes thereof. Outer ends of the first pair of torsional serpentine torsional hinges  202  are connected to a rectangular gimbal ring  208 , which surrounds the first pair of serpentine hinges  202 , at points along the minor axes (θ y ) of the platform  203 . The second pair of serpentine torsional hinges  204  extend from opposite sides of the gimbal ring  208  into contact with the platform  203 , at points along the major axis (θ x ) of the platform  203 . 
   To provide a full range of motion for the platform  143  or  203 , a set of four electrodes  211 ,  212 ,  213  and  214  are provided (See FIG.  11 ); however, for the present invention only the first, second and third electrodes  211 ,  212  and  213  are required to roll the mirrors out of alignment with any intermediate output ports and then back into alignment with a designated output port. Accordingly, first, second and third voltages can be established between the platform  143  or  203  and the first electrode  211 , the second electrode  212  and the third electrode  213 , respectively. Initially, the first and second electrodes  211  and  212  are activated to rotate the platform  143  or  203  about θ x . Subsequently, the first voltage is gradually lowered to zero, while the third voltage is gradually increased until it is equivalent to the second voltage (See FIG.  12 ). To minimize unwanted effected caused by ringing, i.e. vibration of the mirrors caused by an abrupt start or stop, the first, second and third voltages are increased gradually, as evidenced in  FIG. 12 , which illustrates the voltages curves for the various electrodes (first, second and third) over the actuation time of the mirror device. Various mirror tilting patterns can be designed based on the desired characteristics, e.g. attenuation, of the light. 
   The “piano” MEMs mirror devices according to the present invention are particularly useful in a wavelength switch  301  illustrated in  FIGS. 13 ,  14  and  15 . In operation, a beam of light with a plurality of different wavelength channels is launched via an input/output assembly  302 , which comprises a plurality of input/output ports, e.g. first, second, third and fourth input/output ports  303 ,  304 ,  305  and  306 , respectively. The beam is directed to an element having optical power, such as concave mirror  309 , which redirects the beam to a dispersive element  311 , e.g. a Bragg grating. The dispersive element separates the beam into the distinct wavelength channels (λ 1 , λ 2 , λ 3 ), which are again directed to an element having optical power, e.g. the concave mirror  309 . The concave mirror  309  redirects the various wavelength channels to an array of “piano” MEMs mirror devices  312  according to the present invention, which are independently controlled to direct the various wavelength channels back to whichever input/output port is desired. Wavelength channels designated for the same port are reflected back off the concave mirror  309  to the dispersive element  311  for recombination and redirection off the concave mirror  309  to the desired input/output port. The concave mirror  309  can be replaced by a single lens with other elements of the switch on either side thereof or by a pair of lenses with the dispersive element  311  therebetween. 
   With particular reference to  FIG. 14 , the input/output assembly  302  includes a plurality of input/output fibers  313   a  to  313   d  with a corresponding collimating lens  314   a  to  314   d . A single lens  316  is used to convert a spatial offset between the input/output ports into an angular offset.  FIG. 15  illustrates a preferred embodiment of the input/output assembly, in which the unwanted effects of polarization diversity are eliminated by the use of a birefringent crystal  317  and a waveplate  318 . For incoming beams, the lens  316  directs each beam through the birefringent crystal  317 , which separates the beam into two orthogonally polarized sub-beams (o and e). The half waveplate  318  is positioned in the path of one of the sub-beams for rotating the polarization thereof by 90°, so that both of the sub-beams have the same polarization for transmission into the remainder of the switch. Alternatively, the waveplate  318  is a quarter waveplate and rotates one of the sub-beams by 45° in one direction, while another quarter waveplate  319  rotates the other sub-beam by 45° in the opposite direction, whereby both sub-beams have the same polarization. For outgoing light, the polarization of one (or both) of the similarly polarized sub-beams are rotated by the waveplate(s)  318  (and  319 ), so that the sub-beams become orthogonally polarized. The orthogonally polarized sub-beams are then recombined by the birefringent crystal  317  and output the appropriate input/output port. The micro-electro-mechanical devices according to the present invention are particularly well suited for use in switching devices with polarization diversity front ends, since they provide a pair of reflecting surfaces, i.e. one for each sub-beam.