Patent Publication Number: US-6909530-B2

Title: Movable microstructure with contactless stops

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/705,390 entitled “BISTABLE MIRROR WITH CONTACTLESS STOPS,” filed Nov. 10, 2003, U.S. Pat. No. 6,778,304, which is a divisional of U.S. patent application Ser. No. 09/899,004, entitled “BISTABLE MIRROR WITH CONTACTLESS STOPS,” filed Jul. 3, 2001, U.S. Pat. No. 6,657,759. This application is also related to the following U.S. Patents.: U.S. Pat. No. 6,701,037, entitled “MEMS-BASED NONCONTACTING FREE-SPACE OPTICAL SWITCH” by Bevan Staple and Richard Roth; U.S. Pat. No. 6,614,581, entitled “METHODS AND APPARATUS FOR PROVIDING A MULTI-STOP MICROMIRROR,” filed Jul. 30, 2001 by David Paul Anderson, and U.S. Pat. No. 6,625,342, entitled “SYSTEMS AND METHODS FOR OVERCOMING STICTION USING A LEVER,” filed Jul. 3, 2001 by Bevan Staple, David Paul Anderson and Lilac Muller; all of which are herein incorporated by reference in its entirety for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     This application relates generally to microelectromechanical systems, and more particularly to MEMS devices and methods configured to avoid stiction. 
     In recent years, increasing emphasis has been made on the development of techniques for producing microscopic systems that may be tailored to have specifically desired electrical and/or mechanical properties. Such systems are generically described as microelectromechanical systems (MEMS) and are desirable because they may be constructed with considerable versatility despite their very small size. In a variety of applications, MEMS component structures may be fabricated to move in such a fashion that there is a risk of stiction between that component structure and some other aspect of the system. One such example of a MEMS component structure is a micromirror, which is generally configured to reflect light from at least two positions. Such micromirrors find numerous applications, including as parts of optical switches, display devices, and signal modulators, among others. 
     In many applications, such as may be used in fiber-optics applications, such MEMS-based devices may include hundreds or even thousands of micromirrors arranged as an array. Within such an array, each of the micromirrors should be accurately aligned with both a target and a source. Such alignment is generally complex and typically involves fixing the location of the MEMS device relative to a number of sources and targets. If any of the micromirrors is not positioned correctly in the alignment process and/or the MEMS device is moved from the aligned position, the MEMS device will not function properly. 
     In part to reduce the complexity of alignment, some MEMS devices provide for individual movement of each of the micromirrors. An example is provided in  FIGS. 1A-1C  illustrating a particular MEMS micromirror structure that may take three positions. Each micromirror includes a reflective surface  116  mounted on a micromirror structural film  112  that is connected by a structural linkage  108  to an underlying substrate  104 . Movement of an individual micromirror is controlled by energizing actuators  124   a  and/or  124   b  disposed underneath the micromirror on opposite sides of the structural linkage  108 . Hard stops  120   a  and  120   b  are provided to stop the action of the micromirror structural film  112 . Energizing the actuator  124   a  on the left side of the structural linkage  108  causes the micromirror to tilt on the structural linkage  108  towards that side until one edge of the micromirror structural film  112  contacts the left hard stop  120   a , as shown in FIG.  1 A. Alternatively, the actuator  124   b  on the right side of the structural linkage  108  may be energized to cause the micromirror to tilt in the opposite direction, as shown in FIG.  1 B. When both actuators are de-energized, as shown in  FIG. 1C , the micromirror returns to a static position horizontal to the structural linkage  108 . In this way, the micromirror may be moved to any of three positions. This ability to move the micromirror provides a degree of flexibility useful in aligning the MEMS device, although the alignment complexity remains significant. Sometimes hard stops  120   a  and  120   b  are not provided so that the micromirror structural film  112  is in direct contact with the substrate  104 . 
     In certain applications, once the micromirror is moved to the proper position, it may remain in that position for ten years or more. Thus, for example, one side of an individual micromirror structural film may remain in contact with the hard stop or substrate for extended periods. Maintaining such contact increases the incidence of dormancy-related stiction. Such stiction results in the micromirror remaining in a tilted position even after the actuators are de-energized. Some theorize that stiction is a result of molecule and/or charge build up at the junction between the micromirror structural film and the hard stop or substrate. For example, it has been demonstrated that an accumulation of H 2 O molecules at the junction produces capillary forces that increase the incidence of stiction. 
     Thus, one solution to overcome stiction is to package the MEMS device in a hermetic or inert environment. Such an environment reduces the possibility of molecule accumulation at the junction. However, such packaging is costly and prone to failure where seals break or are not properly formed. Further, such packaging is incompatible with many types of MEMS devices. In addition, such packaging does not reduce stiction related to charge build up at the junction. 
     In “Ultrasonic Actuation for MEMS Dormancy-Related Stiction Reduction”, Proceedings of SPIE Vol. 4180 (2000), which is herein incorporated by reference for all purposes, Ville Kaajakari et al. describe a system for overcoming both molecule and charge related stiction. The system operates by periodically vibrating an entire MEMS device to overcome stiction forces. While there is evidence that vibrating the entire MEMS device can overcome stiction at discrete locations within the device, such vibration causes temporary or even permanent misalignment of the device. Thus, freeing an individual micromirror often requires performance of a costly alignment procedure. Even where the device is not permanently misaligned by the vibration, it is temporarily dysfunctional while the vibration is occurring. 
     Thus, there exists a need in the art for systems and methods for overcoming stiction in MEMS devices without causing misalignment. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention are therefore directed to a microstructure for steering light that mitigates stiction. A substrate is provided on which a structural linkage is connected to support a structural film. The structural film includes a reflective coating. A hold electrode is connected with the substrate at a position laterally beyond an orthogonal projection of the structural film on the substrate. It is configured to hold the structural film electrostatically in a tilted position with respect to the substrate upon application of a potential difference between the structural film and the hold electrode. Because of its positioning with respect to the structural film, it is ensured that the structural film is not in contact with the substrate when the structural film is being held by the hold electrode. 
     In some embodiments, a snap-in electrode is also provided. The snap-in electrode is connected with the substrate at a position laterally within the orthogonal projection of the structural film on the substrate. It is configured to tilt an end of the structural film in a direction towards the snap-in electrode upon application of a potential difference between the structural film and the snap-in electrode. 
     The hold electrode may be configured as a comb structure having a plurality of teeth. With such a configuration, a plurality of tilted positions for the structural film may be realized by the application of various potential differences between the structural film and the hold electrode. For example, it may be configured such that an increase in potential difference results in a hold position that deviates more strongly from horizontal. 
     The microstructure may be configured in different embodiments with a cantilever arrangement or with a torsion-beam arrangement. In embodiments that use the torsion-beam arrangement, a second hold electrode and/or second snap-in electrode may be provided on an opposite side of the structural linkage. 
     Embodiments of the invention are also directed to a method for fabricating a microstructure for steering light. A structural linkage is formed on a substrate. A structural film is formed on the structural linkage. A reflective coating is deposited on the structural film. A hold electrode is formed on the substrate at a position laterally beyond an orthogonal projection of the structural film on the substrate and configured to hold the structural film electrostatically in a tilted position with respect to the substrate upon application of a potential difference between the structural film and the hold electrode. A snap-in electrode may additionally be formed to tilt the end of the structural film towards the snap-in electrode upon application of a potential difference between the structural film and the snap-in electrode. The hold electrode may be fabricated as a comb structure to permit the selection of a plurality of tilted positions with variation in the potential difference applied. The microstructure may also be fabricated with cantilever or torsion-beam configurations. For embodiments fabricated according to torsion-beam configurations, additional hold and/or snap-in electrodes may be formed on the substrate opposite the structural linkage. 
     Further embodiments provide a method for operating an optical switch. A first end of a micromirror assembly is tilted towards a substrate by applying a first electrostatic force. Thereafter, the micromirror assembly is held in a first tilted position with respect to the substrate with a second electrostatic force originating from a point laterally beyond an orthogonal projection of the micromirror assembly on the substrate. In one embodiment, the micromirror assembly is released from the first tilted position. Thereafter, a second end of the micromirror assembly is tilted towards the substrate by applying a third electrostatic force. Thereafter, the micromirror assembly is held in a second tilted position with respect to the substrate with a fourth electrostatic force that originates from a point laterally beyond the orthogonal projection of the micromirror assembly on the substrate. In a certain embodiment, the first tilted position is selected from a plurality of possible first tilted positions by establishing a potential difference between the micromirror assembly and a first electrode used to establish the second electrostatic force, and the second tilted position is selected from a plurality of possible second tilted positions by establishing a potential difference between the micromirror assembly and a second electrode used to establish the fourth electrostatic force. 
     In still other embodiments, a wavelength router is provided that incorporates a microstructure for steering light. The wavelength router is configured for receiving light having a plurality of spectral bands at an input port and for directing subsets of the spectral bands to a plurality of output ports. A free-space optical train is disposed between the input port and the output ports providing optical paths for routing the spectral bands. The optical train also includes a dispersive element disposed to intercept light traveling from the input port. A routing mechanism is provided having at least one dynamically configurable routing element to direct a given spectral band to different output ports. The dynamically configurable routing element includes a micromirror assembly connected with a substrate by a structural linkage. A hold electrode connected with the substrate at a position laterally beyond an orthogonal projection of the micromirror assembly on the substrate is configured to hold the micromirror assembly electrostatically in a first tilted position with respect to the substrate upon application of a potential difference between the micromirror assembly and the hold electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings wherein like reference numerals are used throughout the several drawings to refer to similar components. In some instances, a sublabel is associated with a reference numeral and is enclosed in parentheses to denote one of multiple similar components. When reference is made to a reference numeral without specification to an existing sublabel, it is intended to refer to all such multiple similar components. 
         FIGS. 1A ,  1 B, and  1 C are cross-sectional drawings of a tilting micromirror in three positions effected by actuation of different actuators; 
         FIGS. 2A ,  2 B,  2 C,  2 D, and  2 E are cross-sectional drawings of a torsion-beam micromirror configuration in accordance with the invention; 
         FIG. 3  is a schematic drawing defining a geometry of an electromechanical system defined by the torsion-beam micromirror assembly; 
         FIG. 4  is a graph illustrating the behavior of capacitive energy stored in one micromirror configuration in accordance with the invention; 
         FIGS. 5A ,  5 B, and  5 C are cross-sectional drawings of a cantilever micromirror configuration in accordance with the invention; 
         FIGS. 6A ,  6 B,  6 C, and  6 D are cross-sectional drawings of a multistable micromirror configuration in accordance with the invention; 
         FIG. 6E  is a graph illustrating the behavior of capacitive energy stored in a multistable micromirror configuration; 
         FIGS. 7A ,  7 B, and  7 C are schematic top, side, and end views, respectively, of one embodiment of a wavelength router that uses spherical focusing elements; 
         FIGS. 8A and 8B  are schematic top and side views, respectively, of a second embodiment of a wavelength router that uses spherical focusing elements; and 
         FIG. 9  is a schematic, top view of a third embodiment of a wavelength router that uses spherical focusing elements; and 
         FIGS. 10A and 10B  are side and top views of an implementation of a micromirror retroreflector array. 
     
    
    
     DESCRIPTION OF THE SPECIFIC EMBODIMENTS 
     1. Introduction 
     Embodiments of the invention are directed to MEMS methods and devices in which a microstructure is held in one of at least two possible stable positions without contacting either a substrate or hard stop. In certain embodiments, the microstructure is a micromirror that may be rotated to at least two such positions. Because of the ready applicability of such a rotating micromirror to optical-switch applications, some of the embodiments are directed to a wavelength router that uses optical switching. The stability of the microstructure positions is achieved without contact by employing electrostatic fields to hold the microstructure. Since there is no direct contact with the microstructure, stiction is thereby avoided. As will be clear to those of skill in the art from the following description, the invention may be adapted to different types of micromirror configurations, including cantilever micromirrors and torsion-beam micromirrors. 
     It is noted that throughout herein micromirror configurations are shown schematically in the figures for illustrative purposes. As will be understood by those of skill in the art, the point of rotation of the micromirror structural film should be selected so that in the desired static micromirror configurations both the forces on the structural film and the torques about the point of rotation cancel. 
     2. Torsion-beam Micromirror 
     One embodiment of the invention as applied to a torsion-beam micromirror configuration is illustrated in  FIGS. 2A-2E . Each micromirror includes a reflective surface  216  mounted on a micromirror structural film  212  that is connected by at least one structural linkage  208  to an underlying substrate  204 . In some embodiments, multiple structural linkages  208  are provided in the plane orthogonal to the page, the axis of rotation of the micromirror structural film  212  being defined by the alignment of the structural linkages. In one such embodiment, two structural linkages  208  are provided approximately on opposite sides of the micromirror along the axis of rotation. Two snap-in electrodes  224   a  and  224   b  and two hold electrodes  220   a  and  220   b  are provided on the substrate  204 , with one of each type of electrode provided on either side of the structural linkage  208 . The electrodes  220  and  224  and structural film  212  may be fabricated using standard MEMS techniques. Such MEMS techniques typically involve a combination of depositing structural material, such as polycrystalline silicon, depositing sacrificial material, such as silicon oxide, and dissolving the sacrificial material during a release step, for example with hydrofluoric acid (HF). It is thus sometimes convenient to identify the different structural layers in a MEMS microstructure as “poly-N” layers, where N denotes that a particular such layer was the Nth polysilicon layer deposited in a process that included multiple depositions. Often the first such layer is described as the “poly-0” layer. 
     The hold electrodes  220   a  and  220   b  are connected with the substrate  204  at a position laterally beyond an orthogonal projection of the structural film  212  onto the substrate  204 . With such a configuration, the hold electrodes  220   a  and  220   b  are outside the region underneath the micromirror structural film  212 . This geometry ensures that when the micromirror is in the hold positions shown in  FIGS. 2C and 2E , the micromirror structural film  212  is not in contact with the substrate  204 . In certain embodiments, the hold electrodes  220   a  and  220   b  have a greater height above the substrate  204  than the snap-in electrodes  224   a  and  224   b . The electrodes may thus be fabricated with MEMS techniques in which a poly-0 layer is deposited to form the structure of the snap-in electrodes  224   a  and  224   b  and the lower part of the structure of the hold electrodes  222   a  and  222   b . The remainder of the structure of the hold electrodes  221   a  and  221   b  may be fabricated with a subsequently deposited poly-1 layer. The micromirror structural film  212  is formed with a poly-3 layer. The reflective surface  216  is formed by depositing a layer of reflective metal, such as gold. 
       FIG. 2A  shows the static horizontal configuration of the micromirror when all four of the electrodes  220   a ,  220   b ,  224   a , and  224   b  are commonly grounded with the structural linkage  208 . According to embodiments of the invention, the micromirror may be deflected to a position tilted to the right, as shown in  FIG. 2C , or to a position tilted to the left, as shown in FIG.  2 E. Either of these tilted positions is maintained through activation of the right or left hold electrode  220  as appropriate, such that the micromirror and structural film  212  have no contact with the substrate  204  or with a hard stop. 
     In order to achieve the right-tilted position, for example, the right snap-in electrode  224   b  is activated, as shown in  FIG. 2B , by applying a voltage V to that electrode with respect to the common ground. The potential difference between the structural film  212  and the right snap-in electrode  224   b  thus creates an electric field with dotted field lines  228  shown. That right side of the structural film is thus deflected downwards such that the structural film  216  may come into contact with the substrate  204 . Subsequently, the right hold electrode  220   b  is activated and the right snap-in electrode  224   b  is deactivated, creating an electric field between the structural film  212  and the right hold electrode  220   b , shown by dotted electric field lines  232 . This electric field thus maintains the micromirror in its tilted position without any contact with the substrate  204  or a hard stop, thereby avoiding stiction problems. 
     The micromirror may similarly be tilted to the left position shown in FIG.  2 E. Activation of the left snap-in electrode  224   a  deflects the structural film  212  to the left, perhaps in contact with the substrate  204 , with the electric field shown by dotted electric field lines  236 . Subsequent deactivation of the left snap-in electrode  224   a  and activation of the left hold electrode  220   a  creates an electric field shown by dotted field lines  238  that acts to hold the micromirror in its left tilted position without contact with the substrate  204  or with a hard stop. 
     The electromechanical behavior of the system may be better understood with reference to  FIGS. 3 and 4 . In  FIG. 3 , the geometry is shown for an arrangement in which a micromirror structural film  312  is held in a left-tilted position on a structural linkage  308  supported by a substrate  304 . The structural linkage point O may be defined as an origin for the system with vectors r defining spatial positions, with angle θ defining the tilt. The electric field E that acts to hold the micromirror structural film  312  in position may be approximately represented with image charges P and Q. The potential difference created by activation of the hold electrode creates a capacitive arrangement defined by the micromirror structural film  312 , the active electrode, and the gap between them. This capacitive arrangement has a capacitance 
         C   =       2   ⁢           ⁢   U       V   2         ,       
 
where U is the capacitive energy stored and V is the potential difference applied to the electrode. The capacitive energy may be defined in terms of the displacement and electric fields as 
       U   =       1   2     ⁢     ∫       ⅆ   r     ⁢           ⁢       D   ⁡     (   r   )       ·       E   ⁡     (   r   )       .                 
 
The displacement field D(r) is related to the electric field E(r) according to the permittivity ∈(r) of the air in the gap,
 
 D ( r )=∈ E ( r )
 
       FIG. 4  illustrates the approximate dependence of the capacitive energy U as a function of the tilt angle θ. In orienting the micromirror structural film relative to the active holding electrode, the system will seek to minimize the energy U by selecting angle θ 0 . The fact that the system has a preferred tilt angle θ 0  may alternatively be understood from the fact that the attractive electrostatic force is inversely proportional to the square of the separation between the electrode and the micromirror structural film; the system thus seeks to minimize that separation. In some embodiments, it is preferable to activate the hold electrode when the system is already oriented near θ 0 . This is achieved in such embodiments, as illustrated in  FIGS. 2B and 2D , by using one of the snap-in electrodes to move the micromirror structural film such that θ≅θ 0  before activation of the hold electrode. 
     3. Cantilever Micromirror 
     Embodiments of the invention may also be used with cantilever micromirror arrangements. Cantilever arrangements are similar to torsion-beam arrangements, but use a flexure positioned at one side of the micromirror. An example of a cantilever micromirror arrangement in accordance with the invention is illustrated in  FIGS. 5A-5C . The cantilever arrangement generally permits a static horizontal position, as shown in  FIG. 5A , and a tilted position, as shown in FIG.  5 C. Like the torsion-beam arrangement, the tilted position of the cantilever arrangement is maintained without contacting either the substrate  504  or a hard stop. 
     Each micromirror includes a reflective surface  516  mounted on a micromirror structural film  512  that is connected by at least one flexure  508  to an underlying substrate  504 . A snap-in electrode  524  and a hold electrode  520  are provided. The hold electrode  520  may be composed of a poly-0 layer  522  and a poly-1 layer  521 . When the snap-in electrode  520  and hold electrode  525  are both commonly grounded with the flexure  508 , as shown in  FIG. 5A , the micromirror is in the horizontal position. The tilted position may be reached by activating the snap-in electrode  524  to produce the electric field shown with electric field lines  528  in  FIG. 5B , and thereby move the micromirror structural film  512  downwards, such that it may come in contact with the substrate. Subsequent deactivation of the snap-in electrode  524  and activation of the hold electrode  520  causes the electric field shown with electric field lines  532  in  FIG. 5C  to hold the micromirror structural film  512  in its tilted position in a contactless fashion. As for the torsion-beam configuration, the hold electrode  520  is connected with the substrate  504  at a position laterally beyond an orthogonal projection of the structural film  12  onto the substrate  504 . With such a configuration, the hold electrode  520  is outside the region underneath the micromirror structural film  512 . This geometry ensures that when the micromirror is in the hold position shown in  FIGS. 5C , the micromirror structural film  512  is not in contact with the substrate  504 . 
     4. Multistable Micromirror Configurations 
       FIGS. 6A-6D  illustrates an embodiment of the invention in which multistable micromirror configurations are realized. The illustration in  FIGS. 6A-6D  is shown for a cantilever-type micromirror assembly, although it may be adapted to other micromirror configurations, including torsion-beam configurations. In  FIGS. 6A-6D , a reflective surface  616  is mounted on a micromirror structural film  612  connected to underlying substrate  604  by at least one flexure  608 . A snap-in electrode  624  may be provided such that the creation of a potential difference between it and the micromirror structural film  612  may be used to tilt the micromirror, perhaps in contact with the substrate  604  as described with respect to FIG.  5 B. The hold electrode  620  is configured as a comb structure with multiple teeth  621 ,  622 , and  623 . The teeth are configured at different heights above the substrate and may be used to achieve different tilt orientations of the micromirror. As in the previous embodiments, the hold electrode  620  is connected with the substrate  604  at a position laterally beyond an orthogonal projection of the structural film  612  onto the substrate  604 . With such a configuration, the hold electrode  620  is outside the region underneath the micromirror structural film  612 . Thus, as illustrated in  FIGS. 6B-6D , the application of a potential difference between the hold electrode  620  and the micromirror structural film  612  results in an electric field that holds the micromirror in a tilted position without contact with the substrate  604  or a hard stop. 
     The degree of tilt is dependent on the size of the potential difference, as may be understood with further reference to FIG.  6 E. For example, when a potential difference V 1  is applied, as shown in  FIG. 6B , a capacitive system is formed between the micromirror structural film  612  and the hold electrode  620 . The resulting electric field is shown with field lines  628  and the energy behavior U as a function of tilt angle θ is shown in FIG.  6 E. The general behavior of U as a function of θ is similar to that described with respect to  FIG. 4 , with the system seeking the energy minimum, and thereby being held at tilt angle θ=θ 1 . For a smaller potential difference V 2 &lt;V 1 , the energy behavior U has the same qualitative behavior, but has a shallower minimum located at a higher tilt angle, as shown in  FIGS. 6C and 6E . Thus, upon application of potential difference V 2 , the system seeks a hold position at tilt angle θ=θ 2 , with the electric field shown by field lines  630 . Similarly, a still smaller potential difference V 3 &lt;V 2  results in an energy curve in  FIG. 6E  having a still shallower minimum at a still higher angle. Accordingly, the system seeks a hold position at tilt angle θ =θ 3  with electric field lines  632 , as shown in FIG.  6 D. The snap-in electrode  624  is useful for achieving an initial tilt for the micromirror structural film  612 . 
     5. Fiber-Optics Applications 
     a. Wavelength Router 
     Tilting micromirrors according to the embodiments described above, and their equivalents, may be used in numerous applications as parts of optical switches, display devices, or signal modulators, among others. One particular application of such tilting micromirrors is as optical switches in a wavelength router such as may be used in fiber-optic telecommunications systems. One such wavelength router is described in detail in U.S. Pat. No. 6,501,877, entitled “Wavelength Router,” which is herein incorporated by reference in its entirety, including the Appendix, for all purposes. The various micromirror embodiments may be used in that wavelength router or may be incorporated into other wavelength routers as optical switches where it is desirable to avoid stiction problems. 
     Fiber optic telecommunications systems are currently deploying a relatively new technology called dense wavelength division multiplexing (DWDM) to expand the capacity of new and existing optical fiber systems to help satisfy the steadily increasing global demand for bandwidth. In DWDM, multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data. The carrying capacity or a fiber is multiplied by the number of DWDM channels used. Today DWDM systems employing up to 80 channels are available from multiple manufacturers, with more promised in the future. 
     In all telecommunication networks, there is the need to connect individual channels (or circuits) to individual destination points, such as an end customer or to another network. Systems that perform these functions are called cross-connects. Additionally, there is the need to add or drop particular channels at an intermediate point. Systems that perform these functions are called add-drop multiplexers (ADMs). All of these networking functions are performed with a wavelength router used with the current invention by an all-optical network. Optical networks designed to operate at the wavelength level are commonly called “wavelength routing networks” or “optical transport networks” (OTN). In a wavelength routing network, the individual wavelengths in a DWDM fiber must be manageable. New types of photonic network elements operating at the wavelength level are required to perform the cross-connect, ADM and other network switching functions. Two of the primary functions are optical add-drop multiplexers (OADM) and wavelength-selective cross-connects (WSXC). 
     Wavelength routing functions may be performed optically with a free-space optical train disposed between the input ports and the output ports, and a routing mechanism. The free-space optical train can include air-spaced elements or can be of generally monolithic construction. The optical train includes a dispersive element such as a diffraction grating, and is configured so that the light from the input port encounters the dispersive element twice before reaching any of the output ports. The routing mechanism includes one or more routing elements and cooperates with the other elements in the optical train to provide optical paths that couple desired subsets of the spectral bands to desired output ports. The routing elements are disposed to intercept the different spectral bands after they have been spatially separated by their first encounter with the dispersive element. 
       FIGS. 7A ,  7 B, and  7 C are schematic top, side, and end views, respectively, of one embodiment of a wavelength router  10 . Its general functionality is to accept tight having a plurality N of spectral bands at an input port  12 , and to direct subsets of the spectral bands to desired ones of a plurality M of output ports, designated  15 (l) . . .  15 (M). The output ports are shown in the end view of  FIG. 7C  as disposed along a line  17  that extends generally perpendicular to the top view of FIG.  7 A. Light entering the wavelength router  10  from input port  12  forms a diverting beam  18 , which includes the different spectral bands. Beam  18  encounters a lens  20  that collimates the light and directs it to a reflective diffraction grating  25 . The grating  25  disperses the light so that collimated beams at different wavelengths are directed at different angles back towards the lens  20 . 
     Two such beams are shown explicitly and denoted  26  and  26 ′, the latter drawn in dashed lines. Since these collimated beams encounter the lens  20  at different angles, they are focused towards different points along a line  27  in a transverse plane extending in the plane of the top view of FIG.  7 A. The focused beams encounter respective ones of a plurality of retroreflectors that may be configured according as contactless micromirror optical switches as described above, designated  30 (l) . . .  30 (N), located near the transverse plane. The beams are directed back, as diverging beams, to the lens  20  where they are collimated, and directed again to the grating  25 . On the second encounter with the grating  25 , the angular separation between the different beams is removed and they are directed back to the lens  20 , which focuses them. The retroreflectors  30  may be configured to send their intercepted beams along a reverse path displaced along respective lines  35 (l) . . .  35 (N) that extend generally parallel to line  17  in the plane of the side view of FIG.  7 B and the end view of  FIG. 2C , thereby directing each beam to one or another of output ports  15 . 
     Another embodiment of a wavelength router, designated  10 ′, is illustrated with schematic top and side views in  FIGS. 8A and 8B , respectively. This embodiment may be considered an unfolded version of the embodiment of  FIGS. 7A-7C . Light entering the wavelength router  10 ′ from input port  12  forms diverging beam  18 , which includes the different spectral bands. Beam  18  encounters a first lens  20   a , which collimates the light and directs it to a transmissive grating  25 ′. The grating  25 ′ disperses the light so that collimated beams at different wavelengths encounter a second lens  20   b , which focuses the beams. The focused beams are reflected by respective ones of plurality of retroreflectors  30 , which may also be configured as contactless micromirror optical switches, as diverging beams, back to lens  20   b , which collimates them and directs them to grating  25 ′. On the second encounter, the grating  25 ′ removes the angular separation between the different beams, which are then focused in the plane of output ports  15  by lens  20   a.    
     A third embodiment of a wavelength router, designated  10 ″, is illustrated with the schematic top view shown in FIG.  9 . This embodiment is a further folded version of the embodiment of  FIGS. 7A-7C , shown as a solid glass embodiment that uses a concave reflector  40  in place of lens  20  of  FIGS. 7A-7C  or lenses  20   a  and  20   b  of  FIGS. 8A-8B . Light entering the wavelength router  10 ″ from input port  12  forms diverging beam  13 , which includes the different spectral bands. Beam  18  encounters concave reflector  40 , which collimates the light and directs it to reflective diffraction grating  25 , where it is dispersed so that collimated beams at different wavelengths are directed at different angles back towards concave reflector  40 . Two such beams are shown explicitly, one in solid lines and one in dashed lines. The beams then encounter retroreflectors  30  and proceed on a return path, encountering concave reflector  40 , reflective grating  25 ′, and concave reflector  40 , the final encounter with which focuses the beams to the desired output ports. Again, the retroreflectors  30  may be configured as contactless micromirror optical switches. 
     b. Contactless-Micromirror Optical-Switch Retroreflector Implementations 
       FIG. 10A  shows schematically the operation of a retroreflector, designated  30   a , that uses contactless-micromirror optical switches.  FIG. 10B  is a top view. A pair of micromirror arrays  62  and  63  is mounted to the sloped faces of a V-block  64 . A single micromirror  65  in micromirror array  62  and a row of micromirrors  66 (l . . . M) in micromirror array  63  define a single retroreflector. Micromirror arrays may conveniently be referred to as the input and output micromirror arrays, with the understanding that light paths are reversible. The left portion of the figure shows micromirror  65  in a first orientation so as to direct the incoming beam to micromirror  66 (l), which is oriented 90° with respect to micromirror  65 &#39;s first orientation to direct the beam back in a direction opposite to the incident direction. The right half of the figure shows micromirror  65  in a second orientation so as to direct the incident beam to micromirror  66 (M). Thus, micromirror  65  is moved to select the output position of the beam, while micromirrors  66 (l . . . M) are fixed during normal operation. Micromirror  65  and the row of micromirrors  66 (l . . . M) can be replicated and displaced in a direction perpendicular to the plane of the figure. While micromirror array  62  need only be one-dimensional, it may be convenient to provide additional micromirrors to provide additional flexibility. 
     In one embodiment, the micromirror arrays are planar and the V-groove has a dihedral angle of approximately 90° so that the two micromirror arrays face each other at 90°. This angle may be varied for a variety of purposes by a considerable amount, but an angle of 90° facilitates routing the incident beam with relatively small angular displacements of the micromirrors. In certain embodiments, the input micromirror array has at least as many rows of micromirrors as there are input ports (if there are more than one), and as many columns of mirrors as there are wavelengths that are to be selectably directed toward the output micromirror array. Similarly, in some embodiments, the output micromirror array has at least as many rows of micromirrors as there are output ports, and as many columns of mirrors as there are wavelengths that are to be selectably directed to the output ports. 
     In a system with a magnification factor of one-to-one, the rows of micromirrors in the input array are parallel to each other and the component of the spacing from each other along an axis transverse to the incident beam corresponds to the spacing of the input ports. Similarly, the rows of micromirrors in the output array are parallel to each other and spaced from each other (transversely) by a spacing corresponding to that between the output ports. In a system with a different magnification, the spacing between the rows of mirrors would be adjusted accordingly. 
     Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Accordingly, the above description should not be taken as limiting the scope of the invention, which is defined in the following claims.