Abstract:
An optical system having at least one switched mirror for switching and routing at least one incident beam of electromagnetic radiation. The switched mirrors are controlled by a voltage source and based upon the desired controllability of the switched mirrors and wide range of switching and or routing of an optical signal can be effected. In addition by the selective placement of components with the optical system polarization effects can be controlled with the optical system to more effectively provide for subsequent signal transmission.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. application Ser. No. 09/102,700 filed Jun. 22, 1998 now U.S. Pat. No. 6,072,923, SWITCHING, ROUTING, AND TIME DELAY SYSTEMS USING SWITCHED MIRRORS which is in turn a continuation-in-part of U.S. application Ser. No. 08/734,139 filed Oct. 21, 1996 entitled OPTICAL SWITCHING AND ROUTING SYSTEM now U.S. Pat. No. 5,771,320 which is in turn a continuation-in-part of U.S. application Ser. No. 08/640,187 filed Apr. 30, 1996 entitled OPTICAL TIME SHIFTER AND ROUTING SYSTEM now U.S. Pat. No. 5,692,077 and U.S. application Ser. No. 08/641,195 filed Apr. 30, 1996 entitled ACTIVE NOISE SUPPRESSOR FOR MULTICHANNEL OPTICAL SYSTEMS now U.S. Pat. No. 5,706,383. All of the above being incorporated herein by reference. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with U.S. Government support from the U.S. Air Force under Contract No. F30602-98-C-0079. The U.S. Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to optical systems, and more particularly, to optical switching and routing systems which incorporate controllable, switchable mirrors therein. 
     With the advent of substantial new uses for high bandwidth digital and analog electro-optic systems, there exists a greater need to effectively control the route of electro-optic or optical signals from among many possible paths. This is especially true in digital computing systems where signals must be routed among processors, in analog systems such as phased array radar, and in the switching of high bandwidth optical carriers in communication systems. However, it should be realized that these are just several of numerous electro-optic systems which require the use of an optical switching or routing mechanism. 
     In many current and future systems light beams are modulated in a digital and/or analog fashion and are used as “optical carriers” of information. There are many reasons why light beams or optical carriers are preferred in these applications. For example, as the data rate required of such channels increases, the high optical frequencies provide a tremendous improvement in available bandwidth over conventional electrical channels such as formed by wires and coaxial cables. In addition, the energy required to drive and carry high bandwidth signals can be reduced at optical frequencies. Further, optical channels, even those propagating in free space (without waveguides such as optical fibers) can be packed closely and even intersect in space with greatly reduced crosstalk between channels. Finally, operations that are difficult to perform in the lower (e.g., radio) frequencies such as time shifting for phased array applications can often be performed more efficiently and compactly using optical carriers. 
     A common problem encountered in applications in which high data rate information is modulated on optical carrier beams is the switching of the optical carriers from among an array of channels. These differing optical channels may represent, for example, routes to different processors, receiver locations, or antenna element modules. One approach to accomplish this switching is to extract the information from the optical carrier, use conventional electronic switches, and then re-modulate the optical carrier in the desired channel. However from noise, space, and cost perspectives it is more desirable to directly switch the route of the optical carrier from the input channel to the desired channel. 
     Another common problem arises in applications where there is a need to arbitrarily interconnect any of n electronic input channels to any of n output channels. This “crossbar switch” type of function is difficult to implement electronically. In such a case better performance may be obtained by modulating the electronic information on optical carriers, and switching the optical carriers to the desired channel where they may be reconverted to electronic information if desired. 
     Still another problem that is typical in switching systems is the insertion loss they impose. Some switching systems divide the input signal power into many parts, and block (absorb) the ones that are not desired. Others use switches that are inefficient and absorb or divert a significant part of the input signal. 
     It is therefore an object of this invention to provide an optical system that can independently route the optical carriers from an array of input channels to selected members of an array of output channels. 
     It is another object of this invention to provide an optical system that can independently route the optical carriers from an array of input channels to selected members of an array of output channels including the capability to switch one input channel to more than one output channel if desired. 
     It is also an object of this invention to provide an optical system that provides for a reduced complexity in terms of number of required optical switching elements and control points when compared with many other optical switches. 
     It is a further object of this invention to provide an optical system that relies upon a series of uniquely designed switching components. 
     It is a further object of this invention to provide an optical system that uses switchable mirrors to route or switch multiple wavelength optical signals, such as wavelength division multiplexed digital telecommunication signals, to selected output locations. 
     It is a still further object of this invention to provide optical systems that provide variable optical attenuation of input optical signals. 
     It is a further object of this invention to provide optical systems that provide polarization independent switching and attenuation of optical signals. 
     It is an even further object of this invention to provide optical systems that can selectively polarize an input unpolarized optical beam or signal under electronic control. 
     SUMMARY OF THE INVENTION 
     The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described hereinbelow. 
     More specifically, the present invention overcomes problems associated with sensitivity to the wavelength of the optical carriers, insertion loss, number of required switching devices and control signals, switch isolation, noise and crosstalk suppression, spurious reflections, data skew, and compactness that are present in other optical switching, routing, interconnection, and time delay systems. The present invention includes devices that use high efficiency switched mirrors to form optical switching, interconnection, routing, variable optical attenuation, polarizing systems, and time delay networks. 
     Furthermore, the switched mirrors can function, for example, by diffraction (diffractive mirrors) or reflection (reflective mirrors) and have the benefits of a lack of dispersion, where the steered direction does not strongly depend on wavelength. This added benefit can be an asset in very high bandwidth and wavelength multiplexed systems. 
     In addition, the present invention overcomes difficulties in obtaining uniform system performance on input optical signals or signal components that are orthogonally polarized. 
    
    
     For a better understanding of the present invention, together with other and further objects, reference is made to the following description taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appended claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a canonical router system of this invention incorporating electrically switchable mirrors therein; 
     FIG. 2 is a schematic representation of one configuration of switchable mirrors useful in this invention; 
     FIG. 3 is a schematic representation of a second configuration of switchable mirrors useful in this invention; 
     FIG. 4 is a schematic representation of a third configuration of switchable mirrors useful in this invention; 
     FIG. 5 is a schematic representation of a second canonical router system of this invention incorporating electrically switchable mirrors therein; 
     FIG. 6 is a schematic representation of a third canonical router system of this invention incorporating electrically switchable mirrors therein; 
     FIG. 7 is a schematic representation of an optical time delay system of this invention incorporating electrically switchable mirrors therein; 
     FIG. 8 is a schematic representation of a second optical time delay loop system of this invention incorporating electrically switchable mirrors therein; 
     FIG. 9 is a schematic representation of a closed loop optical time delay system of this invention incorporating electrically switchable mirrors therein; 
     FIG. 10 is a schematic representation of a fourth canonical router system of this invention incorporating electrically switchable mirrors therein; 
     FIG. 11 is a schematic representation of a variable optical attenuator/switching system of this invention incorporating an electrically switchable mirror therein; 
     FIG. 12 is a schematic representation of a of a second variable optical attenuator/switching system of this invention incorporating an electrically switchable mirror therein; 
     FIG.  13 ( a ) is a schematic representation of a polarization insensitive variable optical attenuator system of this invention incorporating an electrically switchable mirror therein; 
     FIG.  13 ( b ) is a schematic representation of a polarizing system of this invention incorporating an electrically switchable mirror therein; 
     FIG. 14 is a schematic representation of a polarization insensitive variable optical attenuator/switching system of this invention incorporating an electrically switchable mirror therein; 
     FIG. 15 is a schematic representation of an array of variable optical attenuator systems of this invention incorporating electrically switchable mirrors therein; 
     FIG. 16 is a schematic representation of an array of polarization insensitive variable optical attenuator systems of this invention incorporating electrically switchable mirrors therein; and 
     FIG. 17 is a schematic representation of a canonical polarization insensitive optical switching, routing, and time delay system of this invention incorporating electrically switchable mirrors therein. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It should be noted that the terms switched mirrors and switchable mirrors are used interchangeably throughout the application. In addition, the generic term electromagnetic radiation is used to cover all types of optical signals including, but not limited to, light, beams and channels. 
     The switching, routing, and time delay systems of this invention utilize switchable mirrors to steer light from input optical channels to various output spatial locations or output optical channels, or alternatively, from an input channel to among a variety of optical paths with differing path lengths or time delays. 
     Reference is now made to FIG. 1 of the drawings which illustrates the broad concept of a fundamental form of the invention in schematic fashion, thereby presenting a building block of the optical time shifter, routing, and switching systems of the present invention in one of numerous embodiments, the other embodiments being set forth below with respect to the remaining figures. 
     One embodiment of the subject invention is the routing system  10  illustrated in FIG. 1 of the drawings which depicts a plurality (three being illustrated therein) of switchable mirrors (or switchable mirror arrays, used interchangeably here)  12 ,  14 ,  16  which are controlled by any suitable control signals C 1 -C 3  and shown in a cascading fashion. The switchable mirrors (or mirror-arrays)  12 ,  14  and  16  are systematically and some of the possible structures for these mirrors are described hereinbelow. This embodiment enables a beam of electromagnetic radiation, preferably in the form of an optical input carrier  20  to follow 8 possible optical paths leading to 8 differing spatial locations or output channels shown in ouput plane  18 . All of these paths being configured in free-space, without external limitations, although it should be realized the system can be encompassed in a medium of dielectric constant other than unity. In other words, glass or other transparent slabs may be used to separate the planes of switched mirrors to provide for a monolithic and stable device that is not easily misaligned. Thus, “free space” in the context of this invention describes the use of freely propagating electromagnetic waves in comparison to “guided wave” systems in which the optical carriers are confined to waveguides which are typically of fiber or planar form. In related embodiments of the present invention, each optical path can add a preselected increment in time delay to a transmitted signal. 
     By selecting one of the 8 combinations of “on or off” states in the mirrors  12 ,  14 , and  16 , an input optical carrier signal  20  emanating from any suitable source of electromagnetic radiation (not shown) is reflected (or diffracted as in the case of a holographic mirror) to follow one of the series of paths as shown in FIG.  1 . For example, the beam of electromagnetic radiation (or optical carrier)  20  follows either path A or B after passing through mirror  14  follows preselected paths C, D, E, or F. Thereafter further different optical paths may be followed as illustrated in FIG.  1 . 
     Important parameters of the router shown in FIG. 1 include the state separation  21  and deviation angle  23 . The mirrors, which may take the form of holographic elements described below, have a distinct tradeoff relating to the Bragg regime of their operation. Parameters such as thickness and spatial period must be balanced to assure high diffraction efficiency but also to retain maximal angular and spectral bandwidth tolerances. Finally, the use of micro-optics enables very compact routing, switching, and density and diffractive crosstalk effects as will be pointed out hereinbelow. As the number of stages are increased, their separation increases in multiples so it is usually advantageous to keep spacing small, thus keeping the overall shifter compact. 
     The switchable mirrors used in the present invention can be made using many technologies, such as volume holographic mirrors, multilayer mirrors, deformable mirrors and micro electro-mechanical mirrors. But the common feature is that the mirrors exhibit a reflectance that is variable and controllable. When in an “off” state, the mirrors are transparent (or, alternatively, tilted, or displaced out of the path of the incident beam). When in an “on” state, the mirrors are reflective (or, alternatively, displaced into the path of the incident beam). Many technologies, as described below, also allow for intermediate reflectance values which allow for “fanning-out” the incident signal to among more than one of the delay paths or output channels. 
     Reference is now made to FIGS. 2-4 of the drawings which illustrate three possible configurations of switchable mirrors. For each of the switchable mirror configurations and optical carrier  20  is incident on the switched mirror structure for the cases of the mirrors off, on, and at an intermediate value of reflectance. In FIG. 2, the optical carrier  20  is incident on switched mirror  30  and is reflected from the mirror that is “on” transmitted through the mirror that is “off”, and split into both reflected and transmitted beams by the intermediate state mirror. Switchable mirror  30  is the usual geometry for a mirror, in which incident and reflected waves make equal angles,  36  and  37 , with respect to the surface normal,  38 . 
     In FIG. 3 the switchable mirror  31  is shown in a transmission configuration. This reflecting structure can be built, for example, with arrayed conventional switched mirrors, by arrays of deflectable or deformable mirrors, or holographically. 
     In FIG. 4 the switchable mirror  32  is shown in an asymmetric reflecting configuration, in which incident and reflected waves make unequal angles,  36  and  37 , with respect to the surface normal,  38 . This reflecting structure can be fabricated, for example, using standard holographic techniques. In such a process, two plane waves incident on opposite sides of the volume recording material, and at asymmetric angles, interfere and produce a spatial refractive index variation throughout the volume of the recording material. 
     A significant benefit of using the switched mirror elements described here in place of switched transmission gratings is that there is little or no angular dispersion of the optical channel or beam when steered with these mirror elements. Thus multiple wavelengths or broad-spectrum light can be routed, delayed, interconnected, or switched with little or not dispersive angular deviation of the optical carrier. 
     In one of the preferred embodiments of the invention, volume phase holographic switchable mirrors are used to permit switching of the incident energy between the transmitted and reflected directions. Such switchable mirrors may be controlled by electrical switching, optical switching, and polarization switching of the mirrors, as discussed with specific embodiments of the invention. Recently it has been demonstrated that high efficiency volume diffraction gratings which are recorded in permeable media, such as the DMP-128 photopolymer manufactured by Polaroid Corporation, Cambridge, Massachusetts can be made to be rapidly switchable between high and low diffraction efficiency states under electric control by imbibing the structure with liquid crystals. In this technique, the crystals are rotated by the applied electric field and their refractive index switches in the range between ordinary and extraordinary values. By choosing the materials so that one of these switchable values of refractive index matches that of the phase modulation in the grating, the grating modulation is effectively switched “off-and-on” as the liquid crystal “fill” material index matches and mismatches the modulation, respectively. 
     Most of the optical time shifter and routing systems described herein can utilize electrically switched gratings. In this approach the volume phase diffraction grating, which is typically on the order of tens of microns thick, is bounded by transparent conducting electrodes. 
     Referring once again to FIG. 1, it should be further understood that although three such gratings are illustrated in the figure, the number of grating can vary in accordance with the utilization of this invention. For example, with the diffraction efficiency of all three gratings off, the shortest, straight through path (that is, following along optical path A, C, etc.) is selected. If the first grating  12  is “on”and all others “off,” the top path is selected following optical path B, F, etc. Consequently, by a combination of “on”“off” signals applied to the gratings, it is possible in free space to not only route the optical signal to a desired output area, but also (shown in later time-delay configurations) to do so with a predetermined time delay. 
     Reference is now made to FIG. 5 of the drawings which illustrates another embodiment of the invention in schematic fashion, thereby presenting another building block of the optical time shifter, routing, and switching systems of the present invention. 
     In this embodiment of the subject invention the routing system  50  illustrated in FIG. 5 of the drawings includes a plurality of switchable mirrors in three groups,  40 ,  42 ,  44  which are controlled by any suitable control signals C 1 -C 3  and shown in a cascading fashion. The switchable mirrors  40 ,  42 , and  44  are shown schematically and may consist of volume holographic switchable mirrors. This embodiment enables a beam of electromagnetic radiation, preferably in the form of an optical input carrier  20  to follow 8 possible optical paths leading to 8 differing spatial locations or output channels shown in output plane  18 . All of these paths being configured in free-space, without external limitations, although it should be realized the system can be encompassed in a medium of dielectric constant other than unity. 
     Each of the mirrors grouped at a given cascade depth, e.g., group  40 ,  42 , or  44 , may be switched in common. Accordingly, all 4 gratings  44  can be set “on” or “off” with a single control line. In such a fashion n control lines can route a given optical carrier among any of 2**n output channels or different delay paths. 
     Reference is now made to FIG. 6 of the drawings which illustrates another embodiment of the invention in schematic fashion, thereby presenting another building block of the optical time shifter, routing, and switching systems of the present invention. 
     In this embodiment of the subject invention the routing system  60  illustrated in FIG. 6 of the drawings includes a plurality of switchable mirrors in three groups,  62 ,  64 ,  66  which are controlled by any suitable control signals C 1 -C 3  and shown in a cascading fashion. The switchable mirrors  62 ,  64 ,  66  are shown schematically and may consist of volume holographic switchable mirrors. This embodiment enables a beam of electromagnetic radiation, preferably in the form of an optical input carrier  20  to follow 8 possible optical paths leading to 8 differing spatial locations or output channels shown in output plane  18 . Here the spacing of the three grating groups or stages  62 ,  64 ,  66  has been arranged such that the gratings of group  64  line up with two of the gratings in group  66 . This allows for a practical simplification since a single switched grating with two pixels (shown in the dashed oval) can be located in place of those two grating elements. In such a configuration, one pixel is switched with group  64  and the other with group  66 , as shown in FIG.  6 . 
     Using these principles and the switched mirror elements of FIGS. 2,  3 , and  4 , the canonical switched mirror router shown in FIGS. 1,  5 , and  6  can be applied in a straightforward manner to extend the time delay, routing, interconnection, and switching systems of the three cross-referenced applications cited at the beginning of the present application to the use of switched mirror elements. In so doing, the benefits of wavelength insensitivity and lack of angular dispersion can be imparted to these systems. Further, since the discrete selectable paths are isolated as in these earlier systems, the saturable absorber, active, and passive crosstalk or noise suppressors described in these earlier applications also may be incorporated in the switched mirror systems in a straightforward fashion. To this end, the 4 figures of U.S. Pat. No. 5,771,320 issued Jun. 23, 1998 entitled OPTICAL SWITCHING AND ROUTING SYSTEM and their descriptions are incorporated herein by reference. Similarly, the 16 figures of U.S. Pat. No. 5,692,077 issued Nov. 27, 1997 entitled OPTICAL TIME SHIFTER AND ROUTING SYSTEM and the 5 figures of U.S. Pat. No. 5,706,383 issued Jan. 6, 1998 entitled ACTIVE NOISE SUPPRESSOR FOR MULTICHANEL OPTICAL SYSTEMS and their respective descriptions are also incorporated herein by reference. In each of the above cases, the switched gratings are replaced in this invention with the switched mirrors to form the systems of the present invention. 
     An example of this straightforward application of the principles herein is given here in FIG.  7 . This time delay system  90  is identical to that of FIG. 15 of U.S. Pat. No. 5,692,077 issued Nov. 25, 1997 entitled OPTICAL TIME SHIFTER AND ROUTING SYSTEM. However, now switched mirrors  72 ,  74 ,  76 ,  78 ,  80 ,  82  are used (of the type in FIGS. 1 and 3) in place of switched gratings. The noise suppressor stage  70  is also directly analogous to that of the earlier patented system. The noise suppressor may consist of a pixellated switchable mirror or grating. In such a case, the desired channel is left to pass, and the grating or mirror pixels in the other (non-selected) channels are set on to deflect the crosstalk signals they contain out of the system. The “steering gratings” of the prior inventions can also be replaced by static mirror arrays, or switched mirrors,  84 , which can also be segmented or pixellated and used to add further active crosstalk suppression as described above. 
     In similar fashion, FIG. 8 contains a related system,  100 , to that of FIG. 7, but where switched mirrors of the type shown in FIG. 2, and the canonical router of FIGS. 5 or  6  are used. Two of the canonical router systems  60  of FIG. 6 are used in FIG. 8 to effect a switched mirror time delay system. 
     A related “closed loop” time shifter system  110  is illustrated in FIG.  9 . Here the same canonical switching system  60  is used in double pass in a closed loop configuration. Setting the n control lines steers the beam through any of the 2**n selectable paths, each with a distinct time delay. The same grating states, on the second pass, steer the optical carrier to the output  112 . This configuration is useful with either switched gratings or mirrors. 
     Two of the preferred embodiments described earlier in FIGS. 5 and 6 make use of switchable holographic multilayer mirrors to electro-optically route or steer the incident light beams. By setting the gratings  40 ,  42 ,  44  or  62 ,  64 ,  66  of FIGS. 5 and 6 respectively to intermediate values of reflectance (i.e., neither fully reflective or transparent) fanout (fan-out) or multicast functions can be obtained. For example, if each of the switchable gratings is set to a reflectivity of nearly 50%, then the incident light beam is steered equally among all output channels. This implements a full broadcast of the input channel to all output channels. Many switched grating technologies possess a gray-level response that makes these fan-out functions possible. For example, liquid crystal imbibed DMP-128 and holographic polymer dispersed liquid crystal gratings can readily be used in this type of device with fanout and/or broadcast capability. 
     Reference is now made to FIG.  10 . When arbitrary fanout among subsets of output channels is desired (for example, but not limited thereto, from the input to a specific subset of three outputs, but none others), it is necessary to provide individual control of some or all of the individual switchable mirror elements or mirrors. For the most general fanout capability, each of the switchable mirror elements must be individually controllable to a desired efficiency or reflectivity. It should also be noted that individually controllable mirrors may also be considered or referred to as reflecting pixels. FIG. 10 shows a switched mirror router  130  with individually controllable reflecting pixels  131 - 137  that is capable of arbitrary fanouts. Individual electrical control of the seven switchable reflecting pixels  131 - 137  is provided by controls C 1 -C 7 , respectively. 
     Operation of optical system  130 , for the case of partial fanout, is described in FIG. 10, for example, for equal fanout of input signal  20  among the three output channels  146 ,  147 , and  148 . For this case, the reflecting pixels  131  and  135  are driven to full reflectivity, mirror pixel  136  is driven to ⅓ reflectivity, and mirror pixel  137  is driven to ½ reflectivity. A look-up table can be incorporated in the driving control circuitry to store drive voltages required for all required fanout functions. 
     The individually controlled reflecting pixels  131 - 137  are arranged in three groups in a binary tree as shown earlier in FIGS. 1,  5 , and  6 . These groups are typically separated by distances which are factors or multiples of factors of two. The first group or switched mirror or switched mirror array (terms used interchangeably) is made of reflecting pixel  131 . The second group is made of reflecting pixels  132  and  136 . And the third group is made of reflecting pixels  133 ,  134 ,  135 , and  137 . The resulting system  130  provides for routing or switching among the eight output locations or channels  141 - 148  as shown. Fewer or greater numbers of cascaded groups may be used to decrease or increase the number of routable output locations (ports) or channels. For example, if only the first and second groups are used, then routing or switching among four output locations is obtained. If an additional group is cascaded, then routing or switching among  16  output channels is provided. In general, cascading n groups of switchable elemtns in this fashion will provide switching among 2**n output locations, ports, or channels. 
     Reference is now made to FIG.  11 . Here the switchable mirror  152  is used to divert controllable amounts of input beam  154  through reflection into output beam  158 . The input beam  154  is equivalently described as an input channel, input optical signal(s), or input port. The transmitted output beam  156  is thereby an attenuated form of input beam  154  due to the amounts of input beam  154  diverted into output beam  158 . Similarly the reflected output beam  158  is an attenuated form of input beam  154  according to the amount of input beam  154  that is selectively reflected by switchable mirror  152 . The output beams  156  and  158  are equicalently referred to as output signals, output ports, or output channels. The sum of the optical signal powers in outputs  156  and  158  roughly equal the optical input power in input channel  154  minus the usually small optical powers lost to scatter and absorption in the device. This is a versatile system that can be used as a variable optical attenuator or as an optical switch, or both. 
     For example, first consider the use of system  150  as a variable optical attenuator of input signal  154 . Input signal  154  is a free-space input beam that is either directly input or formed by completely or nearly collimating the light (electromagnetic radiation) from input optical fiber or waveguide  160  with lens  162 . Switchable grating  152  is driven by driver C 1  to a reflectance state that provides the desired transmitted signal intensity level in output signal  156 . This output channel  156  may be directly output or coupled into an optical fiber or waveguide  164  using lens  166 . For example, for very little attenuation from input  154  to output  156 , the switchable mirror is set to low reflectivity by C 1  (i.e., is nearly transparent). For increasing values of attenuation, the reflectivity of mirror  152  is increased using C 1 . Maximum signal attenuation is obtained by setting the reflectivity of switchable mirror  152  to its maximum value (e.g., nearly 100%). 
     The optical system  150  can also be used as a variable optical attenuator in reflection rather than in transmission as above. In this reflective case, reflected signal  158  is used as the output signal, and can either be directly output or coupled into optical fiber or waveguide  168  using lens  162 . This is the same lens that was used to form the input beam. Alternatively, another lens can be used for this purpose, as illustrated in FIG.  12 . 
     Finally, optical system  150  can be used as a switch among input channel  154  and two output channels  156  and  158 . When used as a switch, the reflectivity can be set to direct nearly all the incident light  154  into channel  156  (reflectivity of  152  near zero); or can be set to direct nearly all the incident light  154  into channel  158  (reflectivity nearly unity); or can be set to fanout the input light  154  among output channels  156  and  158 . 
     In optical system  150  of FIG. 11 the switched mirror  152  reflects light either normal (perpendicular to) the input beam or displaced at a small angle to avoid coupling the reflected light into the input channel if such feedback is undesirable. Larger angular displacements are also useful as shown in FIG.  12 . In optical system  170  of FIG. 12, switchable mirror  172  is set at desired values of reflectivity by control C 1  as in System  150  to provide variable optical attenuation between input channel  174  and output channel  176  or  178 , or alternatively to provide switching between input channel  174  and output channels  176  and  178 . Free space input channel  174  may be directly input or formed from optical fiber or waveguide  180  and lens  182 . Similarly, the free space output channels  176  and  178 , if used, may be directly output or coupled into guided waves in optical fibers or waveguides  184  and  188  respectively using lenses  186  and  187  respectively. 
     When these systems are used only for optical attenuation rather than switching, the switched mirror components  152  and  172  can be replaced by switchable scatterers or diffusers. With switchable scatterer based variable optical attenuators, the input channels  154  and  174  are typically used with transmitted output channels  156  and  176 , respectively. 
     It is often a requirement (for example in many optical telecommunications applications) that optical switching or variable optical attenuation be accomplished identically without respect to the polarization state of the input optical signal. Yet many of the switchable optical mirror technologies exhibit different characteristics for orthogonal optical input polarization components. When more than one polarization component must be identically switched and/or attenuated (e.g., when using unpolarized input and output channels) a high degree of polarization insensitivity can be obtained using the systems illustrated in FIGS. 13 and 14. 
     An embodiment of this invention that provides for variable optical attenuation that is polarization insensitive is shown in system  190  of FIG. 13 a . Free space optical input beam  194  is incident on polarizing beamsplitter  202  that separates the input beam  194  into two ‘twin’ beams  210  and  211  which are orthogonally polarized. Patterned or pixellated retarder  198  then rotates the polarization of one of the beams relative to the other so that the two resulting beams  212  and  213  then have identical polarization states. These twin beams are then incident on switchable mirror  192  that is controlled by electronic control C 1 . The transmitted twin beams  214  and  215  are then incident on a second patterned or pixellated retarder  200  that restores the twin beams to orthogonal polarization states resulting in beams  216  and  217 . Finally, a second polarizing beam splitter  204  is used to combine the orthogonally polarized twin beams  216  and  217  into a single exiting output beam, channel, or signal  196 . 
     Input signal  194  is a free-space input beam that is either directly input or formed by focusing the light from input optical fiber or waveguide  206  with lens  207 . Output signal  196  is a free-space beam that is either directly output or coupled into an output optical fiber or waveguide  208  with lens  209 . The polarizing beamsplitters  202  and  205  can be fabricated from birefringent crystals such as calcite, prisms with multilayer coatings, or other usual means in the known in the art. The pixellated retarders  198  and  200  can be fabricated using patterned or discrete birefringent crystals or media. The dashed and solid regions of pixellated retarders  198  and  200  show spatial regions of different pixellation and function. For example, the dashed regions or pixels can be simple blank pixels with no retardance while the solid pixels of pixellated retarders  198  and  200  can be half wave plates that rotate the polarization of incident beams ( 211  and  214  for the case shown in FIG. 13) by 90 degrees. More simply, the dashed pixels of pixellated retarders  198  and  200  can be eliminated and small retarders used only where the solid lines are shown in pixellated retarders  198  and  200 . Finally, several additional variations are possible in the retarders  198  and  200 . The orientation of the pixellated retarders  198  and  200  and polarization beamsplitting and combining components  202  and  204  have been shown symmetrically in FIG. 13 a  so that the optical path length and losses of the twin beams are balanced through the system  190 . This can be important to minimize dispersion and time- and phase- delay effects between polarization components in high bandwidth optical signal attenuation and switching applications. The system can also be used with non-symmetric orientation of the patterned waveplates  198  and  200  and polarizing beamsplitters  202  and  204 , but it is more difficult to balance the losses and optical path lengths in such configurations. 
     Optical system  190  can be used as a variable optical attenuator with the advantage of very uniform attenuation of input beam  194  without regard to the state of polarization of input signal or beam  194 . All polarization components in input  194  can be equally attenuated in output beam  196 . This is especially true since the orthogonal polarization components actually propagate through the device between retarders  198  and  200  as the same polarization. Thus any polarization dependencies in reflectivity of switched mirror  192  have no deleterious effect, since polarization of in the ‘twin’ beams are temporarily identical. As a further benefit, other small sources of attenuation, in addition to that provided by the switched mirror, are now identical in each of the twin beams. 
     For example, surface reflection and scattering losses, which are often polarization dependent, will now affect the different polarization components of the incident beam identically. Finally, an added benefit of the polarization insensitive system  190  is that a switched mirror technology can be used (for switchable mirror  192 ) that is only effective at switching a single polarization component. The polarizing beamsplitters  202  and  204  and retarders  198  and  200  are selected to provide that single polarization state in the region surrounding switched mirror  192 . 
     In optical system  190  as shown in FIG. 13 a , the attenuated output channels are illustrated as those transmitted through he switchable mirror  192 . It is also possible to utilize the reflected beams from the switchable mirror in place of the transmitted beams as shown. This may be advantageous for various switched mirror technologies where, for example, it may be harder to turn the mirrors completely off (transparent) than completely on (reflecting), or visa versa. The use of the reflected twin beams for optical attenuation is analogous to the use of the transmitted beams, and is shown in the system  220  that is described below with respect to FIG. 14 as a polarization insensitive switch. 
     Reference is now made to FIG. 13 b  where optical system  191  is shown that is capable of electronically controlling the polarization of an incident unpolarized optical input signal, beam, or channel. This polarizing optical system  191  is identical to attenuating system  190  except that the switched mirror  192  of system  190  is replaced with switched mirror  191  which has separately controllable reflecting pixels  193  and  195 . These separately controllable pixels  193  and  195  are controlled by drivers C 1  and C 2 , respectively. Reflecting pixels  193  and  195  are used to selectively attenuate the orthogonal polarization components of incident signal, beam, or channel  194  prior to recombining these orthogonal components in output beam  196 . As the reflectivity of reflecting pixels  193  and/or  195  is increased, a greater attenuation is provided for the respective orthogonal polarization component of incident beam  194 . If both reflective pixels  193  and  195  are off, i.e., transparent, the exiting light  196  will be unpolarized. If one reflective pixel is on, i.e., reflective, and the other is off, then the exiting light  196  will be polarized with the polarization of the off pixel. The other orthogonal polarization is available in the reflected beam from the on pixel it desired. In the latter case, this reflected, orthogonally polarized beam, is available using the techniques shown in system  220 . 
     FIG. 14 illustrates an optical switching or routing system  220  that is another variation of the variable optical attenuator system  190  described above. In optical system  220 , switchable mirror  222  replaces switchable mirror  192  and provides additionally for twin reflected beams  224  and  225  which are reflected through a large angle allowing them to propagate through patterned waveplate  228  and beam splitter/combiner  234  in the same way as described for transmitted beams  214  and  215  above. These twin reflected beams are combined by polarizing beamsplitter/combiner  234  to form optical output signal, beam, or channel  240 . Output beam  240  is a free-space beam that is either directly output or coupled into output optical fiber or waveguide  236  with lens  238 . The operation of switchable mirror  222  is controlled by C 1  to switch variable amounts of input beam  194  to output beams  196  and  240  which are transmitted and reflected by mirror  222 , respectively. Switching and routing optical system  220  is analogous to switching and routing System  170  but is now polarization insensitive with the polarization insensitive advantages described above for system  190 . 
     The devices in this specification can also be used in arrays to switch, attenuate, or polarize many parallel channels. In many of the switched mirror technologies, such as with holographically fabricated switched mirrors, it is relatively easy to fabricate a larger single grating and then pixellate the electrodes when compared with fabricating individual switchable mirror elements on separate substrates and then aligning them. The systems shown in FIGS. 15 and 16 illustrate such array devices that make use of the ability to pixellate individual switchable mirrors. 
     Optical system  250  of FIG. 15 shows a pixellated switched mirror or mirror element  252  that includes individually controlled pixels  267 ,  268 ,  269  which are controlled by drivers C 1 , C 2 , and C 3 , etc. A plurality of incident beams including  254 ,  256 , and  258 , etc., are incident on separate switchable reflective (mirror) pixels. The setting of drivers C 1 -C 3  etc. Provide independent variable optical attenuation from the parallel array of input channels  254 ,  256 ,  258  to the respective output channels  262 ,  264 ,  266 , etc. 
     Similarly, system  270  is shown in FIG.  16  and is formed by adding polarizing beamsplitters/combiners  280  and  282 , respectively, and pixellated retarders  284  and  286  to system  250 . The resulting system  270  provides polarization insensitive variable optical attenuation independently for an array of input channels. In similar fashion, each of the systems covered in this specification can be stacked to form 1-dimensional and in some cases, 2-dimensional arrays of devices with relatively little added overhead. 
     The technique of a) separating orthogonal polarization components of an incident beam into two spatially separated ‘twin’ beams, b) modifying the twin beams so they have the same polarization state, c)performing a switching, routing, and/or attenuation operation on the twin beams as if they were a single beam, and d) restoring the twin beams to orthogonal states of polarization, and finally e) combining the orthogonal beam components was used in systems  190  and  220 . This procedure results in polarization insensitive performance in the switching, routing, and/or attenuation stages. Further this technique permits polarization independent performance using switched mirror technologies that inherently only function over one polarization state. The same benefit can be obtained in the switched mirror switching and routing systems shown in FIGS.  1  and  5 - 9  of this specification. 
     For example, FIG. 17 illustrates polarization independent switching system  300  where polarization independent switching performance is obtained by adding a front-end to the basic switching system of FIG.  5 . The front-end of optical system  300  of FIG. 17 includes a polarizing beamsplitter  202  and pixellated retarder  198  as described earlier. Incident optical beam  194  is separated into two beams  210  and  211  containing orthogonal polarization components of input beam  194 . The pixellated retarder  198  then modifies these two beams  210  and  211  so that they are polarized in the same polarization state (typically chosen to be the polarization state in which the subsequent switching stages perform with highest efficiency, throughput, contrast, speed, etc.). These ‘twin’ beams  212  and  213  then propagate through the same optical switching and routing system  50  of FIG.  5 . No added complexity is required of the switching system  50 , except for possibly requiring slightly larger reflective pixels, since the ‘twin’ beams, although spatially separated, propagate through the switching system as a single beam. Each is reflected or transmitted by the same reflective pixels and no additional driver complexity is needed. In this fashion the ‘twin’ beams are switched to one or multiple output locations. 
     If desired, the process can be completed as described earlier by including the rear-end described in FIGS. 13 a  and  14 . This rear-end consists of pixellated retarder  200  followed by polarizing combiner  204 . All the beams exit system  300  parallel to one of two different directions. A fixed segmented mirror array can be used to align all beams in a single direction, or alternatively a propagation distance on the order of the size of the switch can be traversed to allow the two groups of exit beams to separate. In either of these two cases, the pixellated retarder array and polarizing combiner can readily be applied to form single output beams exiting each of the ports to which the input beam was steered or routed to. 
     Although the invention has been described with respect to various embodiments, it should be realized this invention is also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims. It should be further noted that each of the systems described above may be reversed or used bi-directionally.