Abstract:
A two-axis tiltable linear array of MEMS micromirrors is described. The individual micromirrors of the array are flexibly suspended over a common substrate by using two pairs of serpentine hinges coupled by a gimbal ring and are actuated by using tilt and roll electrodes. The tilt actuator regions of the micromirrors are disposed within the gimbal rings, the roll hinges connecting the tilt actuator regions to the micromirrors, which provides for decoupling of the tilt and the roll of the micromirror. The structure allows for considerable decoupling of the tilt and the roll and, or the pistoning effects observed upon micromirror actuation. The structure is suitable for application in a wavelength selective optical switch.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention is a continuation-in-part of U.S. patent application Ser. No. 11/945,307 filed Nov. 27, 2007, which claims priority from U.S. Provisional Patent Application Ser. No. 60/867,841 filed Nov. 30, 2006; and the present application claims priority from U.S. Provisional Patent Application Ser. No. 61/030,678 filed Feb. 22, 2008, all of which are incorporated herein by reference for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a micro-electro-mechanical (MEMS) device including an array of tilting platforms actuated by electrodes, and in particular to a linear array of MEMS micromirrors, wherein each micromirror is tiltable about two orthogonal axes. 
       BACKGROUND OF THE INVENTION 
       [0003]    A micro-electromechanical system (MEMS) is a micro-sized mechanical structure having electrical circuitry fabricated together with the device by using microfabrication processes mostly derived from integrated circuit fabrication processes. The developments in the field of MEMS process engineering enabled batch production of electrostatically tiltable MEMS micromirrors and micromirror arrays that can be used in visual displays, optical attenuators and switches, and other devices. There are at least three general micromachining techniques used to manufacture MEMS micromirror devices. 
         [0004]    One such technique is based on so called bulk micromachining, in which the whole thickness of a silicon wafer is used for building micro-mechanical structures. Silicon is machined using various etching processes. Anodic bonding of glass plates or additional silicon wafers is used for adding features in the third dimension and for hermetic encapsulation. Three-dimensional mechanical micro-structures can be created using bulk micromachining Detrimentally, the bulk micromachining technique is very complex and requires many process steps. 
         [0005]    Another technique is based on so called surface micromachining, in which layers are deposited on the surface of a substrate as the structural materials to be patterned, instead of a three-dimensional processing of the substrate itself, which significantly simplifies the manufacturing processes involved. The original surface micromachining concept was based on thin polycrystalline silicon layers patterned as movable mechanical structures and released by sacrificial etching of an underlying oxide layer. This MEMS paradigm has enabled the manufacturing of low cost MEMS devices. 
         [0006]    New etching technology of deep reactive ion etching (RIE) has made it possible to combine performance and versatility of bulk micromachining with in-plane operation of surface micromachining This combination formed a basis of a third micromachining technique called high aspect ratio (HAR) micromachining While it is common in surface micromachining to have structural layer thickness in the range of 2 microns, in HAR micromachining, the achievable thickness of MEMS devices is from 10 to 100 microns. The materials commonly used in HAR micromachining are thick polycrystalline silicon, known as epi-poly, and bonded silicon-on-insulator (SOI) wafers. Due to its versatility and efficiency, this combined technology is quickly becoming the technology of choice for manufacturing MEMS tiltable micromirror devices. 
         [0007]    MEMS tiltable micromirror devices are often used in optical switch applications. When a MEMS device is actuated, a micromirror supported by the device is tilted about a working axis, which makes an optical beam falling thereupon to steer from one output optical port to another, thereby realizing the switching function. By having a plurality of output ports disposed along a single line, a multiport optical switch can be constructed. In a multiport optical switch, however, it is advantageous to have the micromirror also tiltable about a secondary axis perpendicular to the working axis, so that the micromirror can be tilted about the secondary axis during the switching process, to deflect the optical beam laterally and avoid transient optical signals from briefly appearing in output ports that are on the way of the optical beam being steered towards a desired optical port. Therefore, having a MEMS micromirror device tiltable about a pair of mutually orthogonal axes is highly desirable and advantageous from the standpoint of an optical switch application. 
         [0008]    MEMS devices can be actuated using a variety of actuators. One frequently used actuator for a MEMS device is an electrostatic actuator having a static electrode called a “stator”, and a movable, for example rotatable or tiltable, electrode called a “rotor”. An electrostatic attraction force between the stator and the rotor, applied against a returning force of a spring and, or a hinge element on which the rotor is suspended, tilts or rotates the rotor supporting a micromirror, whereby the micromirror is tilted in a controllable, predictable way. A special care is taken not to exceed the elastic limit of the spring and, or the hinge element. When the elastic limit is not exceeded, millions or even billions of tilting cycles are achievable over a lifetime of a single MEMS device. 
         [0009]    Perhaps the simplest electrostatic actuator is a pair of planar plates, one being the stator and the other being the rotor. As the plates attract, the rotor plate tilts and is brought closer to the stator. By making the rotor plate tiltable about two orthogonal axes, e.g. X and Y axes, and by providing two stator plates, one for each axis of rotation, a micromirror attached to the rotor plate can be made electrostatically tiltable in two orthogonal axes of rotation. For example, U.S. Pat. No. 6,934,439 in the name of Mala et al., assigned to JDS Uniphase Corporation and incorporated herein by reference, teaches a linear array of tightly-spaced “piano” MEMS micromirror devices for use in a wavelength-selective optical switch application. Each micromirror of the MEMS micromirror array of Mala et al. is tiltable about two perpendicular axes X and Y, by the use of two stator plates, one for each axis of tilt, and by the use of two pairs of torsional hinges connected to an “internal” gimbal ring structure at the center of the micromirror. The flexible torsional hinges of Mala et al. provide for a pivotal mounting of the micromirrors, wherein each micromirror is independently tiltable. 
         [0010]    Referring to  FIG. 1 , a top view of a prior-art tiltable MEMS device  100  of Mala et al. is shown having a platform  102  for supporting a micromirror, not shown, an anchor post  104  for supporting the platform  102 , a Y-hinge  106  rotatable about a Y axis, a gimbal ring  108 , an X-hinge  110  rotatable about an X axis, two Y-electrodes  112  for tilting the platform  102  about the Y-axis, and an X-electrode  114  for tilting the platform  102  about the X-axis. The electrodes  112  and  114 , as well as the anchor post  104 , are disposed on a substrate  116 . The hinges  106  and  110 , although shown by straight lines for simplicity, are serpentine spring hinges the platform  102  is suspended upon. The platform  102  is suspended over the substrate  116 . In operation, a voltage is applied between the platform  102  and one of the electrodes  112  to tilt the platform  102  about the Y axis, and a voltage is applied between the platform  102  and the electrode  114  to tilt the platform  102  about the X axis. Tilting the platform  102  about the two orthogonal axes X and Y allows for a two-dimensional steering of an optical beam reflected from a mirror coating, not shown, of the platform  102 . 
         [0011]    Limitations of the MEMS device  100  of the prior art and, correspondingly, many advantages offered by a MEMS device of the present invention, are better understood upon considering a typical task of steering of an optical beam by a MEMS micromirror for a wavelength selective optical switch application. Turning now to  FIG. 2 , an orthographic projection view of a MEMS micromirror  200  is presented, consisting of a plan View A and orthogonal side Views B and C. The micromirror  200  is tiltable about a Y axis and an X axis. An incoming optical beam  202  has an elliptical cross-section  204  seen in View A. The elliptical cross-section  204  of the beam  202  is preferable over a circular cross-section because, for a typical application of a tiltable MEMS micromirror device in a wavelength selective optical switch, many optical beams at different wavelength are positioned so as to have their cross-sections disposed along a common axis, in this case, the Y axis. Correspondingly, decreasing the cross-section of the optical beam  200  in a Y-direction is advantageous, since it allows one to accommodate more individual beams  200  and more mirrors  200  along the Y axis, thereby increasing the wavelength resolution of the wavelength selective switch device. However, decreasing the beam size in Y-direction increases the beam divergence in that direction. For example, by comparing projections of a reflected beam  206  in the Views B and C of  FIG. 2 , one can see that the beam  206  diverges more in the projection of View C than it does in the projection of View B. Increased divergence requires one to increase the tilt angle for a switching application since the beam must be steered by an angle exceeding the beam divergence angle. Thus, a minimum tilt angle θ x  for switching the optical beam by tilting about the X axis is larger than a minimum tilt angle θ y  for switching the optical beam by tilting about the Y axis. 
         [0012]    A requirement for a comparatively large tilt angle about the X axis has important implications for a tiltable MEMS micromirror device. Referring back to  FIG. 1 , an ellipse  101  denotes the elliptical cross-section of an impinging optical beam. To steer said optical beam about the X axis, a voltage is applied to the electrode  114 . Due to the electrode  114  being located closer to the X axis than the electrode  112  is to the Y axis, the created X-torque is smaller than the Y-torque created by applying a voltage to any one of the electrodes  112 . To ensure a larger tilt angle as has been explained above, at a smaller torque, the X-hinges  110  are typically made much more “weak”, or flexible, than the Y-hinges  106 . 
         [0013]    The flexible X-hinges  110  of the MEMS device  100  of the prior art are the weakest structures of the entire MEMS structure shown in  FIG. 1 . The flexibility of the X-hinges  110 , although required for proper functioning of the device  100 , leads to serious drawbacks inherent to the device  100 . First, pistoning effects are significant due to the weaker X-hinges  110 . When a voltage is applied between the platform  102  and one of the electrodes  112 , and, or between the platform  102  and the electrode  114 , the platform  102  shifts towards the electrodes  112  and  114 , which changes the gap between the platform  102  and the electrodes  112  and  114 , resulting in a change of sensitivity of the angle of tilt about the X and the Y axis to the voltage applied. This change of sensitivity leads to cross-coupling between the X and the Y tilts. Herein, the term “cross-coupling” is understood as mutual influence of X and Y actuation, that is, the actuation of tilt of the platform  102  about the X and the Y axes. Second, the X-hinges, being the weakest mechanical link in the entire MEMS device  100 , lower the overall device reliability by making the device  100  more susceptible to shock and vibration. Third, manufacturing process related misalignments between the electrodes  112  and the X axis defined by the X-hinges  110  cause tilting the platform  102  about the X axis, or so called “roll”, upon application of a voltage to one of the electrodes  112  to tilt the platform  102  about the Y axis. The weaker hinges  110  make this “rolling” effect more pronounced. 
         [0014]    Yet another drawback of the MEMS device  100  of  FIG. 1  is that, upon tilting the platform  102  about the X axis, the gap between the electrodes  112  and the platform  102  changes, not only due to pistoning, but also due to tilting of the platform  102  itself about the X axis. This results in a further increase of cross-coupling of the X and Y tilts of the platform  102  and changing of the actuator sensitivities upon applying a voltage to the electrode  114 . 
         [0015]    In addition to complicating calibration and control, the pistoning effect lowers reliability of optical switch devices based on MEMS micromirror devices, since micromirror perturbations caused by vibration and shock can impact the transfer characteristic of the MEMS devices, leading to degradation of optical characteristics of the optical switch devices, such as insertion loss and isolation. 
         [0016]    It is therefore a goal of the present invention to provide a tiltable MEMS micromirror device, in which the cross-coupling and pistoning effects are reduced by an order of magnitude, as compared to prior-art devices. 
         [0017]    The tiltable MEMS micromirror device of the present invention meets this goal. Advantageously, the area of the rotor and the stator electrodes of the MEMS device of the present invention may be further increased by at least 50%, without increasing micromirror size or spacing in a micromirror array. This results in a further improvement of reliability, since the increased actuator area results in an increase of the electrostatic force; therefore, stronger hinges may be used to support the micromirror, and stronger micromirror hinges enhance the overall reliability of the tiltable MEMS micromirror device. 
       SUMMARY OF THE INVENTION 
       [0018]    In a two-axis tiltable MEMS device of the present invention, the hinges and the rotor electrodes are disposed so that the rotor electrode for tilting the platform about the first axis is not responsive to a tilt of the platform about the second axis, which greatly reduces the cross-coupling effects mentioned above. Further, a mechanical load due to actuation of the tilt about the first axis is decoupled from the hinge for tilting about the second axis. Decoupling of the loads of the two hinges considerably reduces the pistoning effect as well. 
         [0019]    In accordance with the invention there is provided a micro-electro-mechanical (MEMS) device for pivotally supporting an optical element, comprising:
   a substrate including an anchor post;   a first torsional hinge attached to the anchor post, rotatable about a first axis;   a gimbal structure surrounding the first torsional hinge, wherein opposite ends of the first torsional hinge are attached to opposite sides of the gimbal structure;   first and second actuator regions disposed on opposite sides of the first axis on opposite sides of the gimbal structure;   a second torsional hinge rotatable about a second axis perpendicular to the first axis, the second torsional hinge comprising first and second arms extending from the first and the second actuator regions, respectively;   a platform for supporting a reflective surface, connected to the second torsional hinge and tiltable about the first and the second axes, the platform having a third actuator region;   a first stator electrode positioned on the substrate beneath the first or the second actuator region, for tilting the gimbal structure and the platform about the first axis; and   a second stator electrode positioned on the substrate beneath the third actuator region, for tilting the platform relative to the gimbal structure about the second axis.   
 
         [0028]    In accordance with another aspect of the invention there is further provided a MEMS device for pivotally supporting an optical element, such as a micromirror, over a substrate that includes an anchor post, comprising:
   a tiltable platform for supporting the optical element;   a first rotor electrode defined by a width and a length, having first and second regions and a brace extending therebetween;   a first torsional hinge defined by a width and a length, rotatable about a first axis, and extending between the anchor post and the brace;   a second torsional hinge rotatable about a second axis that is perpendicular to said first axis, and extending between the first rotor electrode and the tiltable platform;   a first stator electrode disposed on the substrate beneath the first or the second region of the first rotor electrode, for selectively controlling the tilt of the rotor electrode and of the platform about the first axis; and   a second stator electrode positioned on the substrate beneath the tiltable platform, for selectively controlling the tilt of the platform about the second axis by using the tiltable platform as a second rotor electrode;   wherein the total width of the first and the second stator electrodes is less than the width of the platform;   whereby a plurality of the platforms pivotable about the first axis and the second axis are positionable in close proximity with only an air gap between adjacent platforms.   
 
         [0037]    In accordance with another aspect of the invention there is further provided a linear array of said MEMS devices on a common substrate for supporting the anchor posts and the first and the second stator electrodes of the individual MEMS devices;
   wherein the platforms of the individual MEMS devices are substantially coplanar and are spaced apart along an array axis,   wherein the first axes of the individual MEMS devices are parallel to each other and to the array axis, and   wherein the second axes of the individual MEMS devices are parallel to each other and perpendicular to the array axis.   
 
         [0041]    In accordance with yet another aspect of the invention there is further provided a wavelength selective switch (WSS) module for wavelength selective switching of individual wavelength channels between an input port thereof and a plurality of output ports thereof, the WSS module comprising:
   a wavelength dispersive element for spatially separating individual wavelength channels along a line of dispersion;   a linear array of MEMS devices having micromirrors attached to the tiltable platforms of the MEMS devices, wherein the micromirrors are disposed along the line of dispersion, for redirecting the individual wavelength channels in dependence upon angles of tilt of said micromirrors; and   a coupler for optically coupling the input port to the wavelength dispersive element; the wavelength dispersive element to the micromirrors; the micromirrors back to the wavelength dispersive element; and the wavelength dispersive element to the plurality of output ports, so as to couple the individual wavelength channels to any one of the plurality of the output ports, in dependence upon angles of tilt of individual micromirrors.   
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]    Exemplary embodiments will now be described in conjunction with the drawings in which: 
           [0046]      FIG. 1  is a plan view of a prior-art tiltable MEMS device; 
           [0047]      FIG. 2  is an orthographic projection view of a prior-art two-axis tiltable MEMS micromirror; 
           [0048]      FIGS. 3A to 3L  are views of various embodiments of a two-axis tiltable MEMS device according to the present invention; 
           [0049]      FIG. 4  is a plan view of an interlaced array of the two-axis tiltable MEMS devices of the present invention; and 
           [0050]      FIG. 5  is a plan view of a wavelength selective switch (WSS) module using the MEMS device array of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0051]    While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. 
         [0052]    Referring to  FIG. 3A , a plan view of a two-axis tiltable MEMS device  300 A according to an exemplary embodiment of the present invention, alleviating the drawbacks of the prior art device  100 , is shown. The MEMS device  300 A has a substrate  302  including an anchor post  304  extending from the substrate  302 . A first torsional serpentine hinge  306  is attached to the anchor post  304  and is rotatable about a Y axis. A gimbal structure  308  surrounds the first torsional hinge  306 , opposite ends of the first torsional hinge  306  being attached to opposite sides of the gimbal structure  308 . Y-actuator regions  310  are disposed on opposite sides of the Y axis on opposite sides of the gimbal structure  308  as shown. A second torsional serpentine hinge  312 , rotatable about an X axis perpendicular to the Y axis, extends from the Y-actuator regions  310 . A platform  314  is connected to the second torsional hinge  312  and is tiltable about the Y and the X axes. The platform  314  has an X-actuator region  316 , for tilting the platform  314  about the X axis. Y-electrodes  318  are positioned on the substrate  302  beneath the Y-actuator regions  310 . Their function is to tilt the gimbal structure  308  and the platform  314  about the Y-axis in both directions. An X-electrode  320  is positioned on the substrate  302  beneath the X-actuator region  316 . Its function is to tilt the platform  314  relative to the gimbal structure  308  about the X axis. The Y-actuator regions  310  may be viewed as Y-rotor electrodes for tilting the gimbal structure  308  and the platform  314  about the Y axis, wherein the two regions  310  are connected by a brace in form of two beams  309 A and  309 B. Similarly, the region  316  of the platform  314  may be viewed as an X-rotor electrode, the electrode  320  being the X-stator electrode. 
         [0053]    One Y-stator electrode  318  may be used with the present invention; however, two Y-stator electrodes  318  are preferably used as shown in  FIG. 3A  because two Y-stator electrodes  318  can be used for tilting the platform  314  in both directions, which effectively doubles the Y-tilting range of the platform  314 . 
         [0054]    Preferably, the upper surfaces of the torsional hinges  306  and  312 , the gimbal structure  308 , the actuator regions  310  and  316 , and the platform  314  are all coplanar, thereby facilitating manufacture, because all these structures can be formed from a single silicon layer using, for example, the above mentioned technique of deep reactive ion etching of a single silicon layer. Further, preferably, the actuator regions  310  and  316  are all electrically coupled, thereby comprising a ground electrode of the MEMS device  300 A, so that in operation, the platform  314  is tilted about the Y axis upon applying a voltage between one of the Y-stator electrodes  318  and the ground electrode, and the platform  314  is tilted about the X axis upon applying a voltage between the X-stator electrode  320  and the ground electrode. To achieve a high reflectivity, a region  324  of the platform  314 , or the entire platform  314  for that matter, is coated with a mirror coating. When thereby formed micromirror is tilted, an optical beam having a cross-section  322  is steered about the X and Y axes. 
         [0055]    In the MEMS device  300 A, the Y-actuator regions  310  are mechanically decoupled from a tilt of the platform  314  about the X-axis. This arrangement provides for a much higher mechanical stability as compared to the MEMS device  100  of  FIG. 1  because in the device  300 A of  FIG. 3A , only the stronger Y-hinge determines the magnitude of shift of the Y-actuator regions  310 , or pistoning of said regions  310 , upon applying a voltage to the Y-stator electrodes  318 . Preferably, a spring constant of the Y-hinge  306  is larger than a spring constant of the X-hinge  312  by approximately an order of magnitude, whereby a magnitude of a shift of the platform  314  towards the Y-stator electrode  318  upon application of a voltage to the Y-stator electrode  318  is at least 10 times less than a magnitude of a shift of the platform  314  towards the X-stator electrode  320  upon application of the same voltage to the X-stator electrode  320 . The shift of the platform  314  is typically less than 0.05 microns. 
         [0056]    The tilt cross-coupling due to misalignments of positions of the electrodes  318  and  320  relative to the X and the Y axes is also reduced, because in the MEMS device  300 A, it is mostly the stronger Y-hinge  306  that determines a magnitude of such cross-coupling. Preferably, the spring constants of the X-hinge  312  and of the Y-hinge  306  are selected so that a magnitude of tilt of the platform  314  about the X axis upon application of a voltage to the Y-stator electrode is at least 10 times less than a magnitude of the platform  314  tilt about the Y-axis upon application of the same voltage to the Y-stator electrode. The X-tilt upon application of a voltage to one of the Y-stator electrodes  318  also depends on a distance between an axis of symmetry  326  of the Y-stator electrodes  318  and the X-axis. At a typical misalignment of 2 microns between the axis  326  and the X-axis, the X-tilt is less than 0.5% of the Y-tilt. It should be noted that the axis  326  and the X-axis can be deliberately offset with respect to each other, so as to balance the weight of the platform  314 , having the mirror region  324 , about the X axis and, or optimize geometry of the electrodes  318  and  320 . For example, in  FIG. 3A , the axis  326  and the X axis are offset so that the distance between the axis  326  and the X axis is more than one tenth of the width of the Y-electrode  318  measured along the Y axis. Even at an offset between the axis  326  and the X axis of between 10 microns and 50 microns, the undesired X-tilt occurring upon application of a voltage to the Y-stator electrode  318  is still 10 times less than the Y-tilt, due to increased strength of the Y-hinge  306 . 
         [0057]    The positioning of the Y-actuator regions  310  within the gimbal structure  308 , with the platform  314  being suspended to the gimbal structure  308  by the weaker X-hinge  312 , greatly reduces the influence of X-actuation on Y-tilt of the platform  314 . When the platform  314  is tilted about the Y-axis upon application of a voltage to one of the Y-electrodes  318 , the further tilt of the platform  314  about the Y-axis upon further application of the same voltage to the X-electrode  320  is much less than the original Y-tilt due to application of the voltage to the Y-electrodes  318 . Furthermore, the area of the X-stator electrode  320  can be increased by up to 50% as compared to the area of the prior-art MEMS device  100 , since the MEMS device structure of  FIG. 3A  provides more room for the X-stator electrode  320 . Thus, the X-hinge  312  can be further strengthened, which, unexpectedly and advantageously, improves overall reliability of the MEMS device  300 A. 
         [0058]    Referring now to  FIGS. 3B and 3C , a plan view of a two-axis tiltable MEMS device  300 B according to another preferred embodiment of the present invention is shown. The MEMS device  300 B has the same elements as the device  300 A with the exception that the platform  314  is tilted about the Y axis by a rotor comb electrode  310 B, which is attracted to a stator comb electrode  318 B upon applying a voltage therebetween. The rotor comb electrodes  310 B and the stator comb electrodes  318 B are planar parallel plates that are interdigitated as shown in View B-B of  FIG. 3B . These planar parallel plates are parallel to the Y axis and spaced apart along the X axis. Such an orientation of the rotor and stator plates  310 B and  318 B is beneficial, because the lateral attraction between the rotor and the stator plates, resulting from a misalignment therebetween, creates a negligible momentum about a Z axis shown in View B-B of  FIG. 3B . The negligible Z-momentum facilitates prevention of rotation of the platform  314  in its own plane, that is, about the Z-axis, whereby a plurality of platforms  314  can be placed close to each other, without running a risk of a collision between neighboring platforms  314 . 
         [0059]    Referring now to  FIGS. 3D and 3E , a MEMS device  300 C differs from the MEMS device  300 B by position and orientation of plates of rotor and stator comb electrodes  310 C and  318 C, respectively. In  FIGS. 3D and 3E , the rotor comb electrodes  310 C and the stator comb electrodes  310 C are planar parallel plates parallel to the X axis, spaced apart along the Y axis, and interdigitated as shown in View C-C of  FIG. 3E . The advantage of this orientation is that during tilting about the Y axis, the plates  310 C and  318 C remain parallel to each other, allowing for a denser comb teeth spacing and, therefore, stronger actuation forces. 
         [0060]    Turning now to  FIGS. 3F ,  3 G and  FIGS. 3H ,  3 I, the corresponding MEMS devices  300 D and  300 E have comb rotor electrodes  316 D and  316 E interdigitated with comb stator electrodes  320 D and  320 E, for X-tilting of the platform  314  upon applying a voltage between the corresponding rotor and the stator comb electrodes. Similarly to MEMS devices  300 B and  300 C, each of the embodiments  320 D and  320 E has its own advantage: while the MEMS device  300 D has an advantage of a negligible Z-torque upon X-actuation, in the MEMS device  300 E, the rotor and the stator electrode plates  316 E and  320 E stay parallel to each other during the X-actuation. 
         [0061]    Referring now to  FIGS. 3J ,  3 K, and  3 L, an embodiment  300 F of a MEMS tiltable device is shown wherein both X- and Y-actuators have pairs of corresponding comb electrodes: the rotor and the stator electrodes  310 B and  318 B for Y-tilt actuation, seen in View G-G of  FIG. 3J , and the rotor and the stator electrodes  316 D and  320 D for X-tilt actuation, seen in View F-F of  FIG. 3L . 
         [0062]    Possible modifications of the MEMS device  300 A to  300 F include modifications of shape of the platform  314 , of the X- and Y-actuator regions  310  and  316 , replacement of a mirror coating of the platform  314  with another optical element, such as a micro-prism, as well as replacement of serpentine hinges  306  and  312  with other types of hinges. It is to be understood, however, that such modifications fall within the scope of the present invention. 
         [0063]    Referring now to  FIG. 4 , an array  400  of two-axis tiltable MEMS devices  300  according to the present invention is shown. Any MEMS device  300 A to  300 F is usable as the device  300  for forming the array  400 . The individual MEMS devices  300  are disposed over a common substrate  402  supporting the anchor posts  304  and the X- and the Y-electrodes  318  and  320 , respectively, of the individual MEMS devices  300 . The platforms  314  of the individual MEMS devices  300  are substantially coplanar and are spaced apart along an array axis  401 , the Y-axes of every second MEMS device  300  in the array  400  being coaxial and parallel to the array axis  401 . The X-axes of the individual MEMS devices  300  are parallel to each other and perpendicular to the array axis  401 . In  FIG. 4 , the gimbal structures  308  of adjacent MEMS devices  300  are disposed on opposite sides of the array axis  401 , to provide more room for the X- and Y-electrodes  318  and  320  and to reduce electric coupling therebetween. The mirror regions  324  of the platforms  314  are preferably rectangular, and the width of individual regions, measured along the Y axis, is more than one half of the sum of widths of individual stator electrodes  318  and  320 , whereby a plurality of MEMS devices are positionable in close proximity with only an air gap between adjacent mirror supporting regions  324  of said devices, as is seen in  FIG. 4 . One advantage of the interlaced disposition of individual MEMS devices as is shown in  FIG. 4  is that the X-stator electrodes  320  are separated from the Y-stator electrodes  318  of a neighboring MEMS devices  300  by a distance that is sufficient to overcome an electrical cross-talk between a stator electrode of one MEMS device  300  and a rotor electrode of a neighboring MEMS device  300 . The mirror regions  324  of each individual MEMS device  300  form an optical area  403  of the array  400 , the optical area  403  being symmetrical about the array axis  401  and the mirror regions  324  being disposed in close proximity one to another, with only air gap therebetween. The array  400  is preferably hermetically sealed and purged with nitrogen to increase reliability. Therefore, the term “air gap” is understood herein as any gaseous gap or even a vacuum gap. 
         [0064]    One preferred area of application of the MEMS array  400  of  FIG. 4  is optical fiber communications. An information carrying optical signal typically has many “wavelength channels”, or optical signals at individual wavelengths, that are modulated at a very high clock frequency, for example at 10 GHz or 40 GHz. By providing for switching of some of these wavelength channels between various optical paths of an optical network while letting some other wavelength channels propagate along their original paths, significant cost savings, related to both the deployment and the exploitation of the network, can be achieved. This wavelength switching function can be advantageously provided by a MEMS array of the present invention, as follows. 
         [0065]    Turning now to  FIG. 5 , a wavelength selective switch (WSS) module  500  for wavelength selective switching of individual wavelength channels of an input optical signal is presented. The WSS module  500  uses the MEMS array  400  of the present invention, to switch the wavelength channels between an input port and a plurality of output ports. The WSS module  500  has a front end  501  having integrated therein the input port and the plurality of the output ports. A diffraction grating  502  is used for spatially separating individual wavelength channels along a line of dispersion  504 . The optical area  403  of the MEMS array  400  is disposed along the line of dispersion  504 . The MEMS array  400  is used for redirecting the individual wavelength channels at wavelengths λ i , λ 2 , and λ 3  in dependence upon angles of tilt of the reflective surfaces  324  of individual MEMS devices  300  comprising the MEMS array  400 . A concave mirror  506  is used for optically coupling the input port of the front end  501  to the diffraction grating  502 ; the diffraction grating  502  to the MEMS array  400 ; the MEMS array  400  back to the diffraction grating  502 ; and the diffraction grating  502  to the plurality of output ports disposed in the front end  501 . The individual wavelength channels are coupled to any one of the plurality of the output ports, in dependence upon angles of tilt of the reflective surfaces  324  of individual MEMS devices  300  comprising the MEMS array  400 . 
         [0066]    It is recognized by those of skill in the art that the diffraction grating  502  can be replaced by another wavelength dispersive element such as a grism, for example; further, the concave mirror  506  can be replaced by another suitable optical coupler, such as a lens and a flat mirror. All such modification are within the scope of the present invention. A more detailed description of construction and function of a WSS such as the WSS  500  of  FIG. 5  can be found in U.S. Pat. No. 6,498,872 by Bouevitch et al., U.S. Pat. No. 6,707,959 by Ducellier et al., and U.S. Pat. No. 7,014,326 by Danagher et al., all of which are assigned to JDS Uniphase Corporation and are incorporated herein by reference.