Patent Publication Number: US-2003223678-A1

Title: Optical switching system

Description:
[0001] This application is a continuation-in-part of co-pending patent application Ser. No. 09/628,536, filed Jul. 31, 2000, entitled OPTICAL SWITCHING SYSTEM. 
    
    
     
       TECHNICAL FIELD  
       [0002] The present invention relates generally to a thermally-activated bimorph optical switching element. More particularly, the invention relates to an optical switching system for fiber optic cross connect and add-drop multiplexers in wavelength division multiplexing systems.  
       BACKGROUND OF THE INVENTION  
       [0003] The growth of the Internet and the World Wide Web has created a demand for large data bandwidths and high-speed channel switching within the telecommunications industry. Traditionally, telecommunications and other data transfer signals have been exchanged electrically via electrically-conductive wires and cables. Increased bandwidth requirements have led to the adoption of optical signal transfer via fiber optic cables. Optical fiber communications were originally developed for large bandwidth applications, such as long distance trunk cables connecting local metropolitan telephone exchanges. As the cost of these fiber optic networks has decreased and the demand for signal bandwidth increased, fiber optic communications networks are being installed in local metropolitan area networks, and as parts of local area networks (LANS) for data exchange between computers in offices and other commercial and government environments.  
       [0004] Crucial elements in these fiber optic communications networks are the switches and routers that direct the optical signals from various sources to any one of a multitude of destinations. Such switches can be used in a variety of applications, including N×N cross-connect switches for switching signals between arrays of input and output optical fibers, add-drop multiplexers in wavelength-division-multiplexing (WDM) and the more recent dense-wavelength-division-multiplexing (DWDM) systems, reconfigurable networks, and hot backups for vulnerable components and systems. These and other similar applications require switches with moderate switching speeds of one millisecond or less, low insertion loss, low noise, low dispersion, and low cross talk. Additional needs are for low cost, small form factor, and easily manufacturable components to facilitate the rapid adoption of these technologies in wider arrays of optical switching applications.  
       [0005] Many switching techniques have been developed to facilitate the generation and transfer of these optical signals in fiber optic networks. Initially, to switch signals between various arrays of optical fibers, the optical signals had to be converted to electrical signals, conditioned, amplified, and routed to laser light sources and modulators before being retransmitted to the output optical fiber. Electrical switching schemes are undesirable in these applications due to the reduced bandwidth, increased noise, and added cost and complexity of the resulting system designs. As a result, several all-optical switching schemes have been developed, or are under development, for use in telecommunications and data transfer networks, but all of these designs have limitations in these applications.  
       [0006] What is needed, therefore, is a reliable, low-cost, high-bandwidth, low-loss optical switching system.  
       SUMMARY OF THE INVENTION  
       [0007] The foregoing and other needs are met by a device for switching optical signals completely within optical media, from an arbitrary number of N input optical fibers to a different set of M output optical fibers. The invention uses a thermally-activated bimorph optical switch to redirect optical radiation emitted by a laser light source, or from the end of an optical fiber, to an input end of another optical fiber. The modulated optical radiation from the input fiber optic bundle is collimated into parallel beams and projected in free space across the tops of an array of microcantilever bimorph optical switches. When selected, a particular switch is activated, thereby causing a first bimorph switch element to pop up, and intercept and reflect the optical radiation at an angle approximately 90° to the original optical path direction. This reflected radiation is intercepted by another essentially identical bimorph switch element, located above or below the first switch element, that directs the radiation in a plane essentially parallel to that of the original beam plane, but above or below the original plane, and at approximately right angles to the original beam direction. The optical radiation is directed by the second switch element to output optical fiber collimating optics. This radiation is then coupled into the selected output optical fiber.  
       [0008] Both the input and output optical fibers and the bimorph switching elements can be operated in an interchangeable fashion. Consequently, the optical radiation can travel in either direction once a connection between two optical fibers has been established. Thus, the invention is a bi-directional optical switching system with complete symmetry between the inputs and outputs of the switch. Since the input and output optical radiation traverses two different and parallel optical planes, the input and output optical beams do not cross or interact when traversing the switch.  
       [0009] The microcantilever (or bimorph) optical switch elements are composed of two or more thin film layers that possess large differences in their thermal expansion coefficients. When heated, the difference in thermal expansion of the layers induces a surface stress on the cantilever structure, causing it to bend to relieve the resultant stress. The bending of the cantilever structures causes the free end of the cantilevers to move to intercept the optical radiation emitted by the input optical fiber.  
       [0010] The bottom layer of the bimorph switch elements is fabricated from a metal that is highly reflective at the wavelength of the optical radiation, and that possesses a large thermal expansion coefficient. A thin electrically-insulating middle layer is preferably included in the structure to electrically isolate the bottom layer from the upper layer. The upper layer is fabricated from a low thermal expansion, doped semiconductor material, which is patterned and doped to create a resistive heater for uniformly heating the bimorph structure.  
       [0011] When a switch is selected, a pulsed electrical current passes through the resistive heaters of the switch elements, thereby heating the bimorph structures. When the bimorph structures are heated, the free end of the cantilevers of both structures rise and intercept the optical beam. The bending angle of the bimorph sections of each of the switch paddles is preferably about 45°. The optical radiation is reflected by the upper metal surface of the lower bimorph switch paddle, or the lower metal surface of the upper switch paddle, in an intermediate direction at an angle approximately 90° to the original path of the optical beam. The optical radiation is then intercepted by the lower surface of the upper switch element, or the upper surface of the lower switch element, and is reflected at an angle that is approximately 90° to both the original beam direction and the intermediate beam direction, and in a beam plane that is parallel to, but not coplanar with, the original beam direction. The reflected optical radiation is directed to the collimating optics of an adjacent output optical fiber and is focused onto the core region of the output fiber.  
       [0012] The optical switching device of the present invention offers the following benefits: (1) high speed switching with thermal time constants of less than one millisecond, (2) N×N switching operation, (3) scalability to large switch arrays where N may be several hundred to several thousand switch array dimensions, (4) one-dimensional switch paddle controls with simple feedback control to precisely align the switch paddles to the input and output optical fibers, (5) in-plane switch construction with resultant ease of fabrication and alignment, (6) low insertion losses and cross talk, (7) low switch activation power on the order of milliwatts per switch element, (8) small physical size, and (9) low cost fabrication due to complete silicon integrated circuit compatibility.  
       [0013] In one aspect, the invention provides an optical switch for directing at least one optical beam from a first propagation plane to a second propagation plane that is parallel with the first propagation plane. The switch includes a first switch portion having a substantially-planar first reflective surface portion. The first reflective surface portion is operable to move in a first pitch plane between a first deflection angle and a second deflection angle. When at the first deflection angle, the first reflective surface portion intercepts the optical beam propagating along a first propagation path in the first propagation plane and reflects the optical beam at a first reflection angle into an intermediate propagation path. When at the second deflection angle, the first reflective surface portion does not intercept the optical beam. The switch also includes a second switch portion having a substantially-planar second reflective surface portion. The second reflective surface portion is operable to move in a second pitch plane between at least the first deflection angle and the second deflection angle, where the second pitch plane is substantially perpendicular to the first pitch plane. When at the first deflection angle, the second reflective surface portion intercepts the optical beam propagating along the intermediate propagation path, and reflects the optical beam at a second reflection angle into a second propagation path in the second propagation plane. When at the second deflection angle, the second reflective surface portion does not intercept the optical beam.  
       [0014] In a preferred embodiment, the first switch portion includes a first paddle disposed adjacent the first propagation path. The first paddle has a fixed portion and a free end, with the first reflective surface portion adjacent the free end. The first paddle is operable to bend upon a change in temperature, thereby moving the first reflective surface portion between the first and second deflection angles. The second switch portion includes a second paddle oriented substantially perpendicular to the first paddle. The second paddle, which is disposed adjacent the second propagation path, has a fixed portion and a free end, with the second reflective surface portion adjacent the free end. The second paddle is operable to bend upon a change in temperature, thereby moving the second reflective surface portion between the first and second deflection angles.  
       [0015] Preferably, the first and second paddles each include an upper layer having a first coefficient of thermal expansion, and a lower layer mechanically coupled to the upper layer. The lower layer has a second coefficient of thermal expansion that is different from the first coefficient of thermal expansion. The change in temperature causes the upper and lower layers to change in size at different rates due to the differences in coefficients of thermal expansion, thereby creating stress in the paddle and causing the paddle to bend.  
       [0016] In another aspect, the invention provides an optical switching device for selectively transferring at least one optical beam between at least one first optical channel and M number of second optical channels. The device comprises a first optical structure for propagating the optical beam along a first propagation path in a first propagation plane. The device further comprises a 1×M-dimensional array of optical switches that includes M number of first switch portions adjacent to and aligned in parallel with the first propagation path. The first switch portions have substantially-planar first reflective surface portions that are operable to move in a first pitch plane between a first deflection angle and a second deflection angle. When at the first deflection angle, one of the first reflective surface portions intercepts the optical beam propagating along the first propagation path in the first propagation plane and reflects the optical beam at a first reflection angle into an intermediate propagation path. When at the second deflection angle, the first reflective surface portions do not intercept the optical beam. The array of optical switches further includes M number of second switch portions adjacent the corresponding first switch portions. The second switch portions have substantially-planar second reflective surface portions that are operable to move in second pitch planes between the first deflection angle and the second deflection angle. The second pitch planes are substantially perpendicular to the first pitch plane. When at the first deflection angle, one of the second reflective surface portions is operable to intercept the optical beam propagating along the intermediate propagation path and reflect the optical beam at a second reflection angle into one of M number of substantially parallel second propagation paths in a second propagation plane. The second propagation paths are substantially perpendicular to the first propagation path, and the second propagation plane is substantially parallel with the first propagation plane. When at the second deflection angle, the second reflective surface portions do not intercept the optical beam. The device also includes M number of second optical structures disposed in the second propagation plane. Each of the second optical structures is optically aligned with a corresponding one of the M number of second switch portions in a corresponding one of the M number of second propagation paths. Each of the second optical structures is operable to receive the optical beam from the corresponding one of the second reflective surface portions and direct the optical beam to a corresponding one of the M number of second optical channels. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017] Further advantages of the invention will become apparent by reference to the detailed description of preferred embodiments when considered in conjunction with the drawings, which are not to scale, wherein like reference characters designate like or similar elements throughout the several drawings as follows:  
     [0018]FIG. 1 depicts an optical switching device according to a preferred embodiment of the invention;  
     [0019]FIG. 2 is an isometric view of a portion of the optical switching device containing four switches according to a preferred embodiment of the invention;  
     [0020]FIG. 3A is an isometric view of one non-activated bimorph switch element according to a preferred embodiment of the invention;  
     [0021]FIG. 3B is an isometric view of one activated bimorph switch element according to a preferred embodiment of the invention;  
     [0022]FIG. 4A is an elevation view of one non-activated bimorph switch element according to a preferred embodiment of the invention;  
     [0023]FIG. 4B is an elevation view of one activated bimorph switch element according to a preferred embodiment of the invention;  
     [0024]FIG. 5A is a top view of one non-activated bimorph switch element according to a preferred embodiment of the invention;  
     [0025]FIG. 5B is a top view of one activated bimorph switch element according to a preferred embodiment of the invention;  
     [0026]FIG. 6 is schematic diagram of a circuit for selectively activating individual switches of the optical switching device according to a preferred embodiment of the invention;  
     [0027]FIG. 7 is an isometric view of an active feedback circuit used for alignment of the reflecting surfaces of the bimorph switch to optimize signal transmission through the switch according to a preferred embodiment of the invention;  
     [0028] FIGS.  8 A-E depict expected locations of the optical beam on a four-quadrant optical detector under various alignment conditions according to a preferred embodiment of the invention;  
     [0029]FIG. 9 is a schematic diagram of an equivalent thermal circuit of a bimorph optical switch according to a preferred embodiment of the invention;  
     [0030]FIG. 10 is a graph depicting modeled response time of a bimorph switch element as a function of the length of a thermal isolation gap for several switch paddle lengths;  
     [0031]FIG. 11 is a graph depicting modeled switch positional responsivity in microns/C as a function of the thickness of the polysilicon resistive film layer for several different bi-material film thicknesses;  
     [0032]FIG. 12 is a graph depicting the maximum temperature required to fully activate a bimorph switch as a function of the polysilicon film thickness for several bi-material film thicknesses; and  
     [0033] FIGS.  13 A-C show three graphs depicting modeled switch paddle tip position as a function of time after application of a switch heating pulse.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0034]FIG. 1 depicts a cross-connect optical switching device  10  for switching optical signals between a first optical structure, comprising N number of first optical fibers F 1N , to a second optical structure, comprising M number of second optical fibers F 2M . The optical signals, such as telephone or data signals in a telecommunication network, generally consist of modulated optical radiation. In the example depicted in FIG. 1, there are five first fibers F 11 -F 15  (N=5) and five second fibers F 21 -F 25  (M=5). However, it should be appreciated that the invention is not limited to any particular number of first or second channels.  
     [0035] In the preferred embodiment of the invention, the switching device  10  is a completely reciprocal device, such that optical fibers F 1N  and F 2M  can act as input fibers or output fibers, depending on the direction of propagation of the optical signals. Thus, it should be appreciated that the operation of the invention is not limited to the direction of propagation of the optical signals. Further, at any particular instant in time, any one or more of the fibers F 1N  could be input fibers, while other of the fibers F 1N  are output fibers. Similarly, at any particular instant in time, any one or more of the fibers F 2M  could be input fibers, while other of the fibers F 2M  are output fibers.  
     [0036] When the first fibers F 1N  act as input fibers, the optical signals radiate from the ends of the first fibers F 1N  and are captured by beam-collimating optics, such as lenses L 1N . The lenses L 1N  collimate the optical signals into N number of parallel beams of light, each of which is projected in free space over M number of microcantilever optical switches S NM . When the second fibers F 2M  act as input fibers, the optical signals radiate from the ends of the second fibers F 2M  and are captured by beam-collimating optics, such as lenses L 2M . The lenses L 2M  collimate the optical signals into M number of parallel beams of light, each of which is projected in free space over N number of the microcantilever optical switches S NM .  
     [0037] In an alternative embodiment, the first or second optical structures comprise modulated light sources, such laser sources or light-emitting diode (LED) sources, for generating modulated optical signals. In this embodiment, the first optical fibers F 1N  or the second optical fibers F 2M  are replaced by the modulated light sources.  
     [0038] As shown in FIG. 1, a preferred embodiment of the device  10  includes optical switches S 11 - S 15  aligned in a column substantially parallel to the beam emitted from the lens L 11 , the switches S 21 - S 25  aligned in a column substantially parallel to the beam emitted from the lens L 12 , and so forth. As described in more detail hereinafter, when a particular one of the switches, such as S 25 , is selected, a first portion S 25a  of the switch S 25  pops up, intercepts the corresponding optical beam, and reflects the optical beam toward a second portion S 25b  of the switch S 25 . The radiation is reflected from the first portion S 25a  in an intermediate direction at an angle of about 90°, travels a short distance in the intermediate direction, and is reflected from the second portion S 25b  that popped up when the switch S 25  was selected. Based on the angle of reflection from the second portion S 25b , which is also preferably about 90°, the radiation is directed into a second lens L 25 . The second lens L 25  directs the optical radiation into the corresponding second fiber F 25 . Preferably, there are M number of second lenses L 2M  corresponding to the M number of second fibers F 2M .  
     [0039] In the example of FIG. 1, switches S 25 , S 31 , S 43 , and S 52  are selected to route optical signals between the first fiber F 12  and second fiber F 25 , between first fiber F 13  and second fiber F 21 , between first fiber F 14  and second fiber F 23 , and between first fiber F 15  and second fiber F 22 , respectively.  
     [0040] Thus, each column of switches, such as S 11 -S 15 , forms a 1×M-dimensional array which is capable of routing optical signals between any one of the N number of first optical fibers F 1N  and any one of the M number of second optical fibers F 2M , and between any one of the M number of second optical fibers F 2M  and any one of the N number of first optical fibers F 1N , completely in optical media. Since there is no need to convert the input signals from an optical format to an electrical format to accomplish the switching, there is little or no loss in bandwidth. Further, the switching system  10  introduces much less insertion loss than would be introduced by a system that requires conversion from optical to electrical and back to optical formats. Other advantages of the invention will be obvious as further details of the system  10  are described below.  
     [0041] With continued reference to FIG. 1, the preferred embodiment of the invention includes N number of output lenses L o1N  disposed on the opposite side of the array of switches from the first lenses L 1N , and M number of output lenses L o2M  disposed on the opposite side of the array of switches from the second lenses L 2M . If no switch S NM  is selected in a given column of the array, then the optical beam from a first lens L 1N  corresponding to that column is incident upon the corresponding output lens L o1N . For example, as shown in FIG. 1, the optical beam from the first fiber F 11  and lens L 11  is not intercepted by any of the switches S 11 - 15 , and is therefore incident upon the output lens L o11 . Also, the optical beam from the second fiber F 24  and lens L 24  is not intercepted by any of the switches S 14 -S 54 , and is therefore incident upon the output lens L o24 . The lenses L o11  and L o24  focus the beams onto corresponding optical light detectors D 11  and D 24  which convert the optical signals into electrical signals. The electrical signals from the detectors D 11  and D 24  are provided to a switch control logic unit  19 .  
     [0042] Generally, the modulated optical signals carried by the optical beams include header information indicating the source and the intended final destination of the information carried by the optical signals. Preferably, before any switch in a column of an array is activated, the switch control logic unit  19  receives and decodes the header information in the optical beam, and determines which switch in the column should be activated to send the signal to the appropriate output fiber to carry the signal to its final destination. Based on the header information, the switch control logic unit  19  generates a switching signal that is used in selecting the appropriate switch in the array. Preferably, after a particular packet of information has been routed through to the appropriate output fiber by selection of one of the switches S NM , the selected switch is deactivated, and the optical beam is again provided to the corresponding one of the detectors D 1N  or D 2M  so that the header information in the next information packet may be processed.  
     [0043] As shown in FIG. 1, the preferred embodiment of the switching device  10  includes a number of spare switches S S11 -S S25  that may be used in the event that any of the switches S 11 -S 55  fail. For example, if any of the switches S 11 -S 15  were to fail, an input signal on fiber F 11  could be rerouted to the spare fiber F s1 , and the spare switches S 11 -S S15  would be used in place of the switches S 11 -S 15 .  
     [0044] In the preferred embodiment, the system  10  of FIG. 1 is operable in a number of switching modes. The optical beams can be redirected from N number of first optical fibers F 1N  to an equivalent number of second optical fibers F 2N (M=N) in an arbitrary fashion to form an N×N switch. Alternatively, the signals from N number of first optical fibers F 1N  can all be directed into a single one of the second fibers F 2M (M=1) to form an N×1 switch. In yet another configuration, the radiation from two or more first optical fibers F 1N  can be combined in an arbitrary fashion to produce an output array with up to N operational second optical fibers F 2N (an N×M switch). Thus, N and M may be any number, and are completely independent.  
     [0045] FIGS.  2 ,  3 A-B,  4 A-B, and  5 A-B depict portions of the switching system  10  in more detail. FIG. 2 is an isometric view of four of the switches, such as S 11 , S 12 , S 21 , and S 22 . Switch S 21 , in a non-activated position, is depicted in even greater detail in FIGS. 3A, 4A, and  5 A. Switch S 22 , in an activated position, is depicted in FIGS. 3B, 4B, and  5 B. Preferably, each switch S NM  includes a cantilevered paddle  20  having a fixed end attached to a paddle support post  35  and a free end extending over a cavity region  22 . The paddle section  20  is preferably 50-500 microns in length, 20-200 microns in width, and composed of three primary layers: a lower layer  24 , a middle layer  26 , and an upper layer  28 . The lower paddle layer  24  is preferably metal that is highly reflective at the wavelength of the optical radiation, and which possesses a large coefficient of thermal expansion, such as Al, Au, Pb, or Zn. The middle paddle layer  26  is a thin electrically-insulating dielectric layer for electrically isolating the lower layer  24  from the upper paddle layer  28 . Preferably, the middle layer  26  is composed of SiO 2 , SiC, Si 3 N 4 , Si x O y N z  or H x Si y C z . In the preferred embodiment, a very thin bonding layer  30 , such as Cr, Ti, TiW, TiN, or nichrome, is used to bond the lower metal layer  24  to the middle insulating layer  26 .  
     [0046] The paddle support post  35 , which mechanically connects the fixed end of the paddle  20  to the substrate  21 , is preferably constructed from silicon carbide, silicon nitride, or hydrogenated/oxygenated versions of these materials. As well as supporting the fixed end of the paddle  20 , the paddle support post  35  provides electrical isolation between the vias  34  and  36 . The substrate  21  is preferably formed from a silicon semiconductor wafer.  
     [0047] The upper paddle layer  28  is preferably fabricated from a semiconductor material having a low coefficient of thermal expansion, such as doped polysilicon. The upper paddle layer  28  is patterned, preferably in a serpentine shape, and doped to somewhat increase its electrical conductivity, thereby forming serpentine resistive heaters  32   a  and  32   b.  Preferably, the heaters  32   a  and  32   b  have a resistivity of a few hundred to a few thousand ohms. As described in more detail below, when an electric current flows through the heater  32   a  or  32   b,  it uniformly heats the paddle section  20  which causes activation of the switch. In the preferred embodiment of the invention, the resistive heaters  32   a  and  32   b  are connected to underlying control and voltage supply lines by via structures  34  and  36 , which are preferably formed from aluminum.  
     [0048] In the preferred embodiment, the switch paddles  20  are surrounded by an inert gas filled environment. The gas is preferably contained within a switch enclosure that covers the switch arrays. As discussed in more detail hereinafter, among other things, the gas surrounding the paddles  20  serves to dampen oscillatory vibrations of the paddles  20  when the switches are activated.  
     [0049] An example of the operation of the bimorph switches S 11 -S 55  is depicted in FIGS. 4A and 4B. As shown in FIG. 4A, which is an elevation view of the switch S 21  in the direction I (FIG. 3A), optical radiation, substantially in the form of an input beam  60   a,  is emitted from the lens L 12  (FIG. 1). Since switch S 21  is not selected, the input beam  60   a  continues uninterrupted past the switch S 21 . Also, optical radiation from the lens L 21  (FIG. 1), in the form of an input beam  62   a,  continues uninterrupted past the switch S 21 . As shown in FIG. 4B, which is an elevation view taken in the direction I (FIG. 3B), when the switch S 21  is selected, an electrical current flows through the resistive heaters  32   a  and  32   b,  and the bimorph structures of the paddles  20  are heated to preferably less than 100° C. When the paddles  20  are so heated, the various layers of the paddles  20  begin to expand. However, since the lower layers  24  possesses a higher coefficient of thermal expansion than do the upper layers  28 , the lower layers  24  expand at a greater rate than do the upper layers  28 , thereby creating differential stress across the bimorph structures. This causes the paddles  20  to bend upward, thereby raising the free ends of the paddles by 10-20 microns or more. A reflective surface portion, also referred to herein as a mirror section  25 , of the lower metal layer  24  of the switch portion S 22a  intercepts the input beam  60   a,  and reflects the input beam  60   a  in the intermediate direction to form an intermediate beam  60   b  propagating toward the paddle  20  of the switch portion S 22b . The mirror section  25  of the switch portion S 22b  intercepts the intermediate beam  60   b,  and reflects the intermediate beam  60   b  to form the output beam  60   c  propagating toward the lens L 22 . As indicated in FIG. 4B, a selected switch, such as S 22  completely intercepts the output beam  60   c  in this embodiment of the invention. Thus, any one input optical beam, such as emitted from any one of the first fibers F 11 -F 15 , is preferably directed to only a single one of the second fibers F 21 -F 25  at any given time.  
     [0050]FIG. 6 depicts a preferred selection circuit  52  for selectively driving the heaters  32   a  and  32   b  to activate the switches S 11 -S 55 . The circuit  52  includes a second select logic circuit  54  and multiple first select logic circuits  56 . Generally, the second select logic circuit  54  selects one or more rows of the switches S 11 -S 55 associated with one or more of the second fibers F 2M , and the first select logic circuits  56  select which of the switches S 11 -S 55  within the selected row or rows are to be activated. The first select logic circuits  56  are connected to the second select logic circuit  54  by second select lines L S21 -L S25 , and the first select logic circuits  56  are connected to semiconductor switching devices D 11a -D 55a  and D 11b -D 55b  by first select lines L S11a -L S55a  and L S11b -L S55 b. In the preferred embodiment, the semiconductor switching devices D 11a -D 55a  and D 11b -D 55b  are CMOS transistor devices. So as not to unnecessarily complicate FIG. 6, only two input select logic circuits  56   a  and  56   b  and two sets of associated switches S 11 -S 51  and S 12 -S 52  are depicted. It should be appreciated that the other input select logic circuits  56  and associated switches S 13 -S 53 , S 14 -S 54 , and S 15 -S 55  are configured according to the pattern depicted in FIG. 6.  
     [0051] As shown in FIG. 6, one side of each of the semiconductor switching devices D 11a -D 55a  and D 11b -D 55b  is preferably connected to the low side of an associated one of the resistive heaters  32   a  and  32   b,  respectively. The other side of each of the semiconductor switching devices D 11a -D 55a  and D 11b -D 55b  is preferably connected to a common return bus  57 . The high side of each of the resistive heaters  32  is preferably connected to a voltage supply  58  for providing the voltage V s  through the supply bus  55 . In the preferred embodiment, the voltage supply  58  provides V s  in pulses of 3-5 volts in amplitude and having pulse widths of 1-10 μs. It is anticipated that during normal switch activation, the pulse duty cycle is approximately 50%. To facilitate rapid initial heating of the switch elements to decrease the switch activation time, the duty cycle may be increased to as much as 100%, if needed.  
     [0052] As an example of the operation of the selection circuit  52 , consider a situation in which it is desired to transfer an optical signal from the first fiber F 13  to the second fiber F 21  (FIG. 1). The output select logic circuit  54  generates an output select signal on the line L S21  having a 3-bit binary value, such as 011. The input select logic circuit  56 A decodes the value 011 and sets the lines L S31a  and L 31b  high. When the lines L S31a  and L S31b  go high, the switching devices D 31a  and D 31b  are turned on, thereby connecting the low side of the associated heaters  32   a  and  32   b  to the ground bus  57 . With the circuit completed to ground, current flows through the heaters  32   a  and  32   b  associated with the switch S 31 , thereby generating heat that activates the associated paddles  20 .  
     [0053] As shown in FIGS.  3 A-B,  4 A-B, and  5 A-B, in the preferred embodiment of the invention, the mirror section  25  that intercepts the optical beam extends beyond the ends of the middle layer  26  and upper layer  28 . As the paddle  20  is heated, only the bi-material section of the paddle  20 , where the upper layer  28  and the lower layer  24  overlap, bends due to the difference in thermal expansion rates. Thus, the mirror section  25  remains relatively planar when the paddle  20  is heated. As FIG. 4B indicates, since the mirror section  25  of the paddle  20  remains substantially planar, the upper edge of the optical beam  60   a  is incident upon the mirror section  25  at about the same angle as the lower edge of the beam  60   a,  such that the beam  60   a  does not substantially diverge when reflected from the mirror section  25 .  
     [0054] One of the advantages of the invention is a somewhat relaxed requirement on the positioning of the free end of the paddle  20 . When a switch is not activated, the free end of the paddle  20  merely needs to be positioned out of the beam path. When activated, the free end of the paddle  20  should get at least high enough to preferably intercept all of the optical radiation. Thus, the paddle  20  can be designed such that variations of several microns in the on or off position of the free end of the paddle  20  will not affect the operation of the switch.  
     [0055] As one skilled in the art will appreciate, if the mirror sections  25  of the two portions S NMa  and S NMb  of a switch S NM  do not deflect to the correct angle θ upon activation, the reflected optical beam will not be correctly aligned with the lenses L 1N  and L 2M . Preferably, as shown in FIGS. 3B and 4B, the deflection angles of the reflecting surfaces are about 45°. If the deflection angles θ are not correct, thereby causing beam misalignment, significant loss of signal power may occur.  
     [0056] One of the advantages of the present invention is that the orthogonal arrangement of the paddles  20  of the two switch portions S NMa  and S NMb  allows for simple and accurate alignment of the reflecting surfaces with respect to the first and second lenses L 1N  and L 2M  and optical fibers F 1N  and F 2M . In a preferred embodiment, as depicted in FIG. 7, beam splitters  70  are interposed between the second lenses L 2M  and the second fibers F 2M . Each beam splitter  70  extracts a small fraction of the optical signal incident thereon, and directs it onto an optical detector  72 , which is preferably a four-quadrant photodetector. The position of the beam on the detector  72  is determined by the deflection angles θ of the two orthogonal mirror sections  25  of the paddles  20 . If the beam is not exactly centered on the detector  72 , with each quadrant of the detector  72  receiving exactly the same amount of signal, then the resultant offset signal is used as a feedback signal to control the voltage supply  58  to apply more or less power to one or both of the resistors  32   a  and  32   b.  By adjusting the voltage V s  applied to the resistors  32   a  and  32   b,  the heat generated by the resistors  32   a  and  32   b  is controlled, thereby controlling the angles of deflection θ of the mirror sections  25 . In this manner, the beam reflected from the mirror section  25  may be properly centered on the lens L 2M  and in the fiber F 2M .  
     [0057] Specifically, when the angle θ of the mirror section  25  of the switch portion S NMa  is too large or too small, a vertical offset signal results in the detector  72 . If the angle θ of the mirror section  25  of the switch portion S NMb  is too large or too small, a horizontal offset signal results. Consequently, two distinct signals can be generated for independently controlling the angles θ of the mirror sections  25 . In contrast to known multi-mirror switching approaches, each mirror section  25  of the device  10  moves in a one-dimensional space, thereby facilitating simple feedback and control of the alignment of the reflecting surfaces.  
     [0058] One skilled in the art will appreciate that the beam splitter  70  and the detector  72  could be positioned in the beam path between the first lenses L 1N  and the first fibers F 1N , or between the second lenses L 2M  and the second fibers F 2M , depending on the direction of propagation of the optical beam used for alignment.  
     [0059] In an alternate alignment approach, alignment radiation from an external light source, operating at a different wavelength from that of the optical signal-carrying beam, can be introduced, such as through a beam combiner, into the same optical path as that followed by the signal-carrying beam. The alignment beam is then intercepted and extracted from the signal-carrying beam, such as by using a dichroic mirror that highly reflects the alignment beam, but which is highly transmissive of the signal-carrying beam. The alignment beam is then directed onto a four quadrant detector as in the above-described method. In this manner, the alignment beam traverses exactly the same optical path through the optical switch S NM  as does the signal-carrying beam. The advantage of this approach is that none of the signal-carrying beam light is used for the alignment detection function, so that the signal-to-noise ratio of the signal-carrying beam is not thereby reduced.  
     [0060] Some of the important physical parameters affecting the operation of the thermally-activated bimorph switch of the present invention include (1) the thermal time constant, τ th , of the bi-material switching element, which determines the fastest switching time for a given set of switch design parameters, and which is dependent on the material properties and dimensions of the thin film structures; (2) the switch stabilization time, t s , which is the time required for the switch paddle  20  to stabilize, and which is dependent on the mass and stiffness of the paddle, and viscous air damping among other parameters; (3) the temperature, T max , required to fully activate the switch to intercept the optical radiation, which depends on the positional responsivity of the bimorph structure along with the maximum displacement of the free end of the paddle  20  and the precise deflection angle required to intercept and redirect the optical radiation beam; (4) the maximum power, P max , required to heat the bimorph structure to cause the mirror section  25  of the paddle  20  to fully intercept the optical beam, which should be minimized to reduce switch thermal management problems; and (5) the supply voltage, V s , and polysilicon conductivity, σ poly , required to provide the maximum power P max .  
     [0061] As discussed in more detail below, there is also a maximum temperature rise ΔT max  within the bimorph structure which constrains the design. The material properties of the thin films of which the switch is comprised limit the maximum temperature to which these materials can be exposed over extended periods of time and number of switching operations before significant performance degradation and switch failure can be expected.  
     [0062] The thermal response of the switch is analyzed using an equivalent thermal circuit, such as depicted in FIG. 9. The thermal time constant τ th  of the switch elements determine at least in part the speed at which the switch can both be activated and deactivated. Generally, the faster the rate of increase and decrease in the switch paddle temperature, the more quickly the paddle  20  moves to intercept or unblock the optical radiation. The thermal time constant τ th  is determined by the thermal capacitance C th  and the thermal impedance R th  of the switch paddle structure according to:  
     τ th   =R   th   ×C   th .   (1)  
     [0063] The thermal capacitance C th  of the switch is determined by the capacities of the individual thin film structures which make up the switch paddle  20  shown in FIGS.  4 A-B and  5 A-B. The thermal capacitance of a specific thin film layer C th (i) is determined according to:  
       C   th ( i )=ρ( i )×λ( i )× t ( i )× l ( i )× w ( i ),   (2)  
     [0064] where ρ(i) is the density, λ(i) is the specific heat, t(i) the thickness, l(i) the length, and w(i) the width of the specific layer. The lumped thermal capacitance of the switch paddle  20  is the sum of all the individual layer capacitances, i.e.:  
               C   th     =       ∑   i              C   th          (   i   )       .               (   3   )                       
 
     [0065] Likewise, the thermal impedance of the switch is controlled by the thermal conductivity of the paths to the substrate through the switch structure and the surrounding environment as shown by the thermal impedance pathways represented in FIG. 9. The radiation thermal resistance, R thr , for the paddle structures of the preferred embodiment is found from the following expression:  
       R   −1   thr =4 ×A   d ×(ε b +ε t )×σ×T s   3 ,   (4)  
     [0066] where A d  is the area of the paddle  20 , ε b  and ε t  are the emissivities of the top and bottom surfaces of the switch paddle respectively, σ is the Stefan-Boltzmann constant, and T s  is the temperature of the paddle. For the paddle areas and materials of the preferred embodiment of the invention, R thr  is approximately 10 7  to 10 8  ° K./watt, and contributes negligibly to the switch power dissipation. In a gas-filled switch enclosure, the conductive and convective thermal impedances, R con , (FIG. 9) are difficult to calculate analytically. An approximate estimate of the contribution to thermal impedance losses can be found in “Advances in Amorphous Silicon Uncooled IR Systems” by J. Brady et al., SPIE Conference on Infrared Technology and Applications XXV, Orlando, Fla., April 1999, SPIE Vol. 3698. The Brady reference documents the thermal impedance of similar structures measured as a function of the fill gas pressure. For the switch paddle areas of the preferred embodiment, R con  is approximately 10 6  to 10 7 ° K./watt. This mechanism also contributes only a small thermal loss in comparison with the thermal impedance (i.e., R th =10 4  to 10 5 ° K./watt) and hence much higher power losses due to thermal conduction through the paddle structure to the support posts and underlying substrate. Thus, the R con  loss mechanisms are neglected in the following analysis.  
     [0067] The remaining impedance pathways shown in FIG. 9 are conduction pathways through the various layers making up the bimorph switch structure. The thermal impedance for a path (i) is determined by:  
                   R   th          (   i   )       =       l        (   i   )         2   ×     κ        (   i   )       ×     t        (   i   )       ×     w        (   i   )             ,           (   5   )                       
 
     [0068] where κ(i) is the thermal conductivity of the material of which path (i) is composed. The total conductive thermal impedance R th  to the interconnects and metal vias, and thus to the silicon substrate, is given by the following equation for the structure shown in FIGS.  4 A-B and  5 A-B:  
               R   th     =         (       1     R   poly       +     1     R   insul         )       -   1       +       (       1     R   poly       +     1     R   insul       +     1     R   bond       +     1     R   metal         )       -   1                 (   6   )                       
 
     [0069] where, as shown in FIG. 9, the thermal impedance R poly  of the polysilicon upper layer  28  is in parallel with the thermal impedance R insul  of the insulating middle layer  26  for the section closest to the switch substrate, and is in series with the section containing the parallel thermal impedances R poly , R insul , the bonding metal layer R bond , and the thermal impedance R metal  of the metal lower layer  24 . In this analysis, the thermal impedances of the lower layer  24  and bonding layer  30  of the paddle  20  are assumed to be negligible in comparison with the other layers, as layers  24  and  30  are composed of high thermal conductivity metals. The overlapping insulation layer  26  is physically small such that it also makes a negligible contribution to the switch thermal impedance. In practice, the thermal response time of the switch is preferably controlled primarily by the length of the thermal isolation gap, L gap , (FIGS.  4 A-B) as shown by the modeled results in FIG. 10. The switch can be designed to have a thermal time constant τ th  in the range from several tens of microseconds to seconds, depending on the switching application (e.g. switch size, required paddle movement, materials, design, and power dissipation) as shown in FIG. 10 and in Table II below.  
     [0070] The thermal properties of several materials that are preferred for use in the invention are shown in Table I. The material properties shown in Table I are not necessarily accurate values for the thin films used in the fabrication of the switch, as they are, in most cases, bulk material properties, which can be significantly different from the thin film values, depending the film thickness and fabrication technique. Nevertheless, they can be used in the present calculations to estimate the dependence of the switch parameters on these materials. The electrical impedances of the insulator materials are not included in the tables as they, being very large, have no bearing on the calculation results presented in Table II below.  
                                           TABLE I                                   Thermal                               Expansion   Thermal   Specific   Young&#39;s       Intrinsic           Coefficient   Conductivity   Heat   Modulus   Density   Resistivity           (10 −6  K −1 )   (W/m-K)   (J/g-K)   (10 11  N/m 2 )   (g/cm 3 )   (Ω-cm)                                                                Semi-                               conductor       Polysilicon   4.7   33.2   0.702   1.9   2.33     1 × 10 5         Insulators                                         SiO 2     0.55   1.4   0.17   0.73   2.65   —       SiC   3.5   2.1   0.17   7.0   3.21   —       A-H x Si y C z     5   0.34   0.17   0.7   3.2   —       Si 3 N 4     0.8   19   0.17   3.85   3.17   —       Bimetals                                         Al   25.0   237   0.897   0.70   2.70   2.65 × 10 −8         Au   14.2   317   0.129   0.8   19.3   2.27 × 10 −8         Zn   35.0   116   0.388   0.79   7.13    5.5 × 10 −8         Pb   28.9   35   0.129   0.16   11.3     20 × 10 −8         Bonding       Metals       Cr   4.9   93.7   0.449   1.52   7.15   12.7 × 10 −8         Ti   8.6   21.9   0.523   1.2   4.51     39 × 10 −8         W   4.5   174   0.116   4.1   19.3    5.4 × 10 −8         TiW   6   14.4   0.21   3.5   16.3     10 × 10 −8         Ni   13.4   90.7   0.444   2.1   8.9    6.9 × 10 −8         nichrome   10   12   0.445   2.1   8.5     11 × 10 −8                    
 
     [0071]                                           TABLE II                                   Symbol   Design 1   Design 2   Design 3   Design 4   Units                                        Parameter                                         Tip excursion   Z max     26   26   26   35   μm       Total Paddle length   L paddle     250   250   225   275   μm       Paddle width   W paddle     20   20   20   20   μm       Length polysilicon   L poly     150   150   125   175   μm       Length bi-material region   L metal     150   150   150   200   μm       Length thermal gap   L gap     50   50   25   25   μm       Length of straight mirror   L mirror     50   50   50   50   μm       Thickness of bi-material   t metal     0.2   0.3   0.2   0.2   μm       layer       Thickness of Bonding   T bond     0.01   0.01   0.01   0.01   μm       metal       Thickness of polysilicon   t poly     0.1   0.15   0.1   0.1   μm       Thickness of insulator   t insul     0.02   0.01   0.01   0.01   μm       Supply voltage   V R     5   5   5   5   V       Switch base temperature   T 0     300   300   300   300   K       Calculated Results       Switch thermal impedance   R th     8.3 × 10 4      5.6 × 10 4      4.2 × 10 4      4.2 × 10 4      K/watt       Switch thermal capacitance   C th     2.8 × 10 −9     4.1 × 10 −9     2.7 × 10 −9     3.4 × 10 −9     J/K       Thermal time constant   τ th     0.23   0.23   0.113   0.142   msec       Responsivity   R T     0.58   0.39   0.58   1.04   μm/K       Change in temperature   ΔT   44   66.5   44   33   K       Thermal power   P th     0.53   1.2   1.07   0.8   Watt       Electrical Impedance   R Si     5.3 × 10 4      2.9 × 10 4      4.7 × 10 4      4.7 × 10 4      Ω       Polysilicon resistivity   σ Si     113   121   99   128   (Ω · cm) 2                      
     [0072] The results of several example calculations of the thermal response time for a given switch size and paddle thickness are shown in Table II. It should be appreciated that there are many possible combinations of materials and switch physical dimensions, with the optimum combination dependent on the particular switching application.  
     [0073] In the preferred embodiment of the invention, the switch is fully activated when the two following conditions are satisfied. First, the vertical displacement Δz of the free end of the switch paddles  20 , as shown in FIG. 4A, must be great enough that the paddles  20  fully intercept the full extent of the input optical beams  60   a  and  62   a  projected across the top of the switch structure. Secondly, the deflection angle θ of the paddles  20  is most preferably about 45° with respect to the direction of propagation of the beams  60   a  and  62   a  across the switch structure. This ensures that the intermediate beam  60   b  reflected from the mirror section  25  of the switch portion S 22a  will travel in essentially a perpendicular direction with respect to the direction of the input beam  60   a  across the switch structure, to be intercepted by the mirror section  25  of the switch portion S 22b . The relationship between these two requirements is preferably expressed by:  
     Δ z=Δz   bi   +L   mirror ×sinθ,   (7)  
     [0074] where L mirror  is the length of the mirror section (FIGS.  4 A-B), and Δz bi  is the vertical displacement of the bi-material section of the switch paddle  20  where the upper layer  28  and the lower layer  24  overlap, as given by:  
                 Δ                   z   bi       =       180   θ     ×       L   bi     π     ×       sin                 θ       tan        (     90   -     θ   /   2       )             ,           (   8   )                       
 
     [0075] where L bi  is the length of the bi-material section, as shown in FIGS.  4 A-B.  
     [0076] Generally, the vertical movement of the paddle  20  is dependent on the change in temperature ΔT of the bimorph structure, and the switch responsivity R p  in units of microns/°C. The responsivity R p  determines the sensitivity, and for the switch structure shown in FIGS.  4 A-B, is expressed as:  
                 R   p     =         Δ                 z       Δ                 T       =       3   ×     L   bi   2     ×     (       α   bi     -     α   Si       )     ×     (     1   +   x     )           t   bi     ×   K           ,           (   9   )                       
 
     [0077] where t bi  is the thickness of the metal layer  24 , α b1  and α s1  are the thermal expansion coefficients of the metal layer  24  and the polysilicon layer  28 , respectively, and x is the ratio of the thicknesses of the polysilicon layer  28  and the metal layer  24 , i.e.  
             x   =         t   Si       t   bi       .             (   10   )                       
 
     [0078] The term K is a correction factor necessitated by the difference in Young&#39;s moduli of the two layers  24  and  28 , i.e.:  
             K   =     4   +     6      x     +     4        x   2       +     n                   x   3       +       1     n                 x       .               (   11   )                       
 
     [0079] The term n in equation (11) is the ratio of the Young&#39;s moduli, i.e.:  
               n   =       E   Si       E   bi         ,           (   12   )                       
 
     [0080] where E si  and E bi  are the Young&#39;s moduli of the polysilicon layer  28  and the metal layer  24 , respectively.  
     [0081] The responsivity of the switch paddles is modeled in FIG. 11 as a function of the thickness of the polysilicon thin film layer  28  for several thicknesses of the metal film layer  24 . These results show that, in general, the thinner the metal film  24 , the greater the responsivity. These results also indicate that for each given thickness of the metal film layer  24 , there is an optimum thickness of the polysilicon film layer  28  which maximizes the responsivity. Films that are thicker or thinner than the optimum tend to lower the responsivity of the bimorph structure, and increase the temperature and hence the power required to fully activate the switch.  
     [0082] Once the maximum tip displacement z max  required to fully intercept the optical radiation is known, the maximum change in temperature ΔT max  of the bimorph structure required to achieve this displacement can be found according to:  
               Δ                   T   max       =         z   max       R   p       .             (   13   )                       
 
     [0083] The maximum switch operating temperature is critically dependent on the dimensions of the paddle structures. The dependence of ΔT max  on the thicknesses of the polysilicon and metal film layers  28  and  24  is shown in FIG. 12, and has a complex functional dependence on these and the other material properties and film dimensions. For reliable long term switch operation, it is desired that ΔT max  be kept below approximately 100° C. Higher operating temperatures can lead to creep and stress-related fatigue in the thin film structures. The modeled results shown in FIG. 12 indicate that it is possible to keep ΔT max  in the 50 to 70° C. range over a wide range of switch paddle dimensions.  
     [0084] After the switch is activated by application of heat to the switch paddle  20 , there is a time lag before the switch paddle  20  reaches its final optical beam intercept position, as shown in FIG. 4B. This is due to the finite time required to heat the structure to its final operating temperature, which is a function of the thermal time constant τ th  (Eq. 1) and the mechanical responsivity of the paddle structure. The time dependence of the temperature rise when the switch is activated, and the temperature fall after the switch has been turned off, T(t), is determined according to:  
       T ( t )= T   max ×(1 −e   −t/τ   th ).   (14)  
     [0085] The maximum operating switch temperature T max  should be low enough such that the switch does not experience long term performance degradation due to, for example, heat induced stress gradients in the paddle structure, or fatigue and ion mobility issues in the thin film structures. These issues may become problems for standard CMOS IC devices at temperatures much in excess of 100° C. However, this is well above the expected operating temperature of the preferred embodiment of the present invention. (See Table II.)  
     [0086] An additional consideration in determining the time required to activate the switch is the fact that the switch paddles  20  are free standing mechanical structures, fixed at one end, but free to move at the other. These structures vibrate at given resonant frequencies when excited by the thermal activation pulse or other external mechanical forces. A further time constant, referred to herein as the settling time t s , can be defined which indicates the time required for the switching transients and induced vibrations to be dampened sufficiently for the switch to operate without excessive modulation of the transmitted optical radiation.  
     [0087] The switch can be modeled as a vibrating spring by the following expression:  
                   m   switch     ×            2                         t   2              z        (   t   )         +     b   ×                         t            z        (   t   )         +       k   switch     ×     z        (   t   )           =       F   thermal          (   t   )               (   15   )                       
 
     [0088] where m switch  is the total mass of each of the switch paddles  20 , b is a damping constant, which for a gas-filled switch is mainly due to the viscous drag of the air in the enclosure, k switch  is the spring constant of the paddle  20 , and F thermal (t) is the thermal force applied to the paddle  20  as the paddle  20  is heated. Without the damping force due to the gas within the switch enclosure (i.e. b≅0), the paddle  20  will oscillate indefinitely, unless the paddle resonant time period (τ switch =(m switch /k switch ) 1/2 ) is significantly shorter than the initial response time (τ th ) to the thermal heating force. Under these conditions, there is generally no phase lag between the applied force and the switch mechanical response, and thus little if any mechanical overshoot and vibration of the switch paddles  20 . These conditions are shown graphically in FIGS. 13A and 13B. With the addition of air damping, this condition is relaxed, as long as there is sufficient gas pressure and the resulting viscous drag to dampen any induced mechanical motion in the paddle  20  within the required switching time. At atmospheric gas pressures, air damping occurs well within the one millisecond switch time required of the present switch. This condition is modeled in FIG. 13C for expected switch conditions.  
     [0089] Similar arguments can be made for externally induced mechanical vibration of the switch paddle  20 . The fundamental switch paddle resonant frequency f res  can be found from the following expression:  
               f   res     =       1     2   ×   π       ×         (       m   switch       k   switch       )       1   /   2       .               (   16   )                       
 
     [0090] Typically, f res  for the switch paddle  20  will be in the range of 5×10 3  to 5×10 4  Hz, depending on the switch dimensions and materials. For mechanical noise with a frequency well below the resonant frequency of the switch paddle  20 , the paddle  20  will move in phase with the external applied force and will not vibrate when the force is removed. At higher frequencies, air viscous drag will tend to dampen any vibrations with periods well under one millisecond, as shown by the modeled results in FIG. 13C. Since the mass of the switch paddles  20  is extremely small (on the order of 10 −11  kg), the resultant inertial and gravitational forces on the paddle structure are correspondingly minute.  
     [0091] As mentioned previously, an important design consideration is thermal power management, both from a power usage requirement, and switch temperature stability and heat dissipation requirements. The heating power P max  required to raise the switch to its operating temperature T max  is given by the following expression:  
                 P   max     =       T   max       R   th         ,           (   17   )                       
 
     [0092] where R th  is the thermal impedance of the structure of the switch paddle  20 . Knowing the switch supply voltage V s , it is possible to estimate the electrical impedance RE poly  of the doped polysilicon resistive heating layer  32  required to achieve the switch operating temperature. Thus, RE poly  may be determined according to:  
               RE   poly     =       V   S       P   max               (   18   )                       
 
     [0093] The required electrical conductivity σ poly  of the polysilicon resistive heater  32  can be found according to:  
               σ   poly     =       2   ×     L   poly           RE   poly     ×     t   poly     ×     w   poly                 (   19   )                       
 
     [0094] Patterning the polysilicon thin film layer  28  as shown in FIGS.  5 A-B, and doping the patterned layer  28  using ion implantation techniques, allows the resistivity of the polysilicon layer  28  to be tailored to the desired value.  
     [0095] Results of several calculations of the switch performance, as a function of the specific switch structures and the total length of the switch paddle  20 , are shown in Table II. These calculations are designed to show the sensitivity of the switch parameters (i.e. switch response time, temperature change, and power requirements) to changes in the switch dimensions. Designs 1 and 2 show the effect of increasing the thickness of the metal and polysilicon layers  24  and  28  on the switch parameters. Thicker structures have a higher resonant frequency, and are potentially more robust, but require a higher thermal power and resultant higher switch activation temperature. This effect is also shown graphically in FIG. 12. Changes in film thicknesses typically have little effect on the thermal response time of the switch. Generally, the response time is highly dependent on the length of the thermal isolation gap L gap , as shown by a comparison between Designs 1 and 3. This effect is also shown graphically in FIG. 10. Smaller values of L gap  tend to lead to smaller thermal time constants and faster switching. However, the power required to activate the switch is generally increased correspondingly. Thus, there appears to be a clear trade-off between speed of response and switch power consumption.  
     [0096] The switch Design 4 shown in Table II indicates the effect on the switch parameters of changes in the length of the bimaterial section L bi  of the switch paddle  20 . In a comparison between Designs 3 and 4, increasing the length of the bimaterial section reduces switch activation temperature and thus the thermal power, but increases the response time and the size of the individual switch elements. The switching time and power requirements are also significantly dependent on the other input parameters listed in Table II, along with the thermal and mechanical properties (shown in Table I) of the of the materials of which the switch is composed.  
     [0097] The manufacturing techniques used to fabricate the optical switching device  10  includes MEMS surface micro-machining steps as well as standard silicon CMOS IC fabrication steps. Both the MEMS and CMOS fabrication processes have been designed to be compatible with, and transferable to, standard silicon foundries. The switch structures described herein may be manufactured in accordance with the fabrication processes described in copending U.S. patent application Ser. No. 09/628,536, filed on Jul. 31, 2000 by the present inventor, or with slight variations to those processes which are within the capabilities of one of ordinary skill in the art. The entire contents of application Ser. No. 09/628,536 are hereby incorporated by reference.  
     [0098] It is contemplated, and will be apparent to those skilled in the art from the preceding description and the accompanying drawings that modifications and/or changes may be made in the embodiments of the invention. Accordingly, it is expressly intended that the foregoing description and the accompanying drawings are illustrative of preferred embodiments only, not limiting thereto, and that the true spirit and scope of the present invention be determined by reference to the appended claims.