Patent Publication Number: US-6707594-B2

Title: Method and device for switching wavelength division multiplexed optical signals using two-dimensional micro-electromechanical mirrors

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of U.S. patent application Ser. No. 09/826,982, filed on Apr. 4, 2001, now U.S. Pat. No. 6,532,115 incorporated herein by reference, which is a continuation-in-part application of U.S. patent application Ser. No. 09/716,196, filed Nov. 17, 2000, now U.S. Pat. No. 6,313,936. U.S. Pat. No. 6,313,936 is a continuation-in-part application of U.S. patent application Ser. No. 09/666,898 filed on Sep. 20, 2000 now U.S. Pat. No. 6,580,845. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to optical switching and, more particularly, to an optical switching system, device, and method using two-dimensional micro-electromechanical mirrors. 
     2. Description of Related Art 
     Optical communication systems are a substantial and rapidly growing part of communication networks. The expression “optical communication system,” as used herein, relates to any system that uses optical signals to convey information across an optical transmission device, such as an optical fiber. Such optical systems may include, but are not limited to telecommunication systems, cable television systems, and local area networks (LANs). 
     While the need to carry greater amounts of data on optical communication systems has increased, the capacity of existing transmission devices is limited. Although capacity may be expanded, e.g., by laying more fiber optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical transmission devices. 
     Wavelength division multiplexing (WDM) has been adopted as a means to increase the capacity of existing optical communication systems. In a WDM system, plural optical signals are carried over a single transmission device, each channel being assigned a particular wavelength. 
     An essential part of optical communication systems is the ability to switch or route signals from one transmission device to another. For example, micro-electromechanical mirrors (MEMs) have been developed for routing signals between transmission devices. A discussion of MEM devices can be found in K. E. Peterson, “Micromechanical Light Modulator Array Fabricated on Silicon,” Applied Physics Letters, Volume 31, Page 521 (1977). This technique operates by changing the angular orientation of the mirrors, thereby reflecting signals to different locations. 
     Designers have also considered using bubbles that are capable of changing their internal reflection for switching optical signals. A discussion of this can be found in “Compact Optical Cross-connect Switch Based on Total Internal Reflection in a Fluid-containing Planar Lightwave Circuit,” by J. E. Fouquet, in  Trends in Optics and Photonics Series , A. Sawchuk, ed., Vol. 37, (Optical Society of America, Washington, D.C., 2000) pp. 204-206. However, these techniques are unable to switch between multiple wavelengths. Furthermore, both of these devices have limited switching speeds, in the range of 10 kHz for the mirror devices and in the range of 100 Hz for the bubble devices. 
     Zigzag multiplexers are also well known in the art for transmitting signals on multiple transmission devices. For example, U.S. Pat. No. 6,008,920 discloses a multiplexer/demultiplexer device utilizing a filter that is sensitive to the angle of incidence of light. However, such multiplexers have not been used for switching or routing applications in conjunction with arrays of fibers, detectors, and emitters. 
     Other switching approaches, such as the approach disclosed in U.S. Pat. No. 4,769,820, issued to Holmes, can switch data at GHz rates, which is effectively switching at GHz transition rates. However, this approach requires substantial optical switching power, has potential cross talk, and cannot resolve wavelength over-utilization issues. What is needed is a means for switching wavelength division multiplexed signals that is capable of doing so at high speeds with no cross talk and requires low switching power. What is also needed is a switch device that is capable of switching large numbers of signals. 
     SUMMARY OF INVENTION 
     1. Advantages of the Invention 
     One or more embodiments of the present invention may achieve, but do not necessarily achieve, one or more of the following advantages: 
     the ability to switch signals of different wavelengths; 
     the ability to switch signals at high speeds; 
     does not require high power; 
     has low crosstalk; 
     the ability to switch between wavelengths and fibers to avoid transmission device or wavelength over-utilization; 
     the ability to broadcast to multiple transmission devices or couplers simultaneously; and 
     the ability to efficiently switch a large volume of signals. 
     These and other advantages of certain embodiments of the present invention may be realized by reference to the remaining portions of the specification, claims, and abstract. 
     2. Brief Description of One Embodiment of the Present Invention 
     The present invention comprises an optical switch element for use with at least one source and a plurality of targets. The source is adapted to transmit an optical signal to the optical switch device. The targets are adapted to receive the optical signal from the optical switch device. 
     The optical switch device comprises a beam splitter, a first wave plate, a direction altering device, and a second wave plate. The beam splitter is adapted to transmit light in a first predetermined polarization and reflect light in a second predetermined polarization. The first wave plate is positioned between the source and the beam splitter. The first wave plate is adapted to alter the polarization so that it is reflected by the beam splitter, wherein light transmitted by the source passes through the wave plate and is reflected by the beam splitter. 
     The direction altering device is positioned to receive light reflected by the beam splitter and to selectively reflect light to a plurality of paths, the paths corresponding to the positions of the plurality of targets. The second wave plate is positioned between the direction altering device and the beam splitter. The second wave plate is adapted to alter the polarization so that it is transmitted by the beam splitter, wherein light redirected by the direction-altering device passes through the second wave plate and is transmitted by the beam splitter. 
     The above description sets forth, rather broadly, a summary of one embodiment of the present invention so that the detailed description that follows may be better understood and contributions of the present invention to the art may be better appreciated. Some of the embodiments of the present invention may not include all of the features or characteristics listed in the above summary. There are, of course, additional features of the invention that will be described below and will form the subject matter of claims. In this respect, before explaining at least one preferred embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of the construction and to the arrangement of the components set forth in the following description or as illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is substantially a schematic diagram of one embodiment of a switch device of the present invention. 
     FIG. 2 is substantially a schematic diagram of one embodiment of a switch element of the present invention. 
     FIG. 3 is substantially a schematic diagram of one embodiment of a switch element of the present invention illustrating one possible physical configuration of the switch element. 
     FIG. 4 is substantially a schematic diagram of one embodiment of the present invention in which a central processor is in communication with a plurality of switch elements. 
     FIG. 5 is substantially a schematic diagram of another embodiment of the switch device of the present invention that utilizes a single source emitter. 
     FIG. 6 is substantially a schematic diagram of a prior art switch device that utilizes two micro-electromechanical mirrors. 
     FIG. 7 is substantially a schematic diagram of another embodiment of the switch device of the present invention that utilizes two micro-electromechanical mirrors and two wave plates. 
     FIG. 8 is substantially an alternate configuration of the embodiment illustrated in FIG.  7 . 
     FIG. 9 is substantially an alternate configuration of the embodiment illustrated in FIG.  7 . 
     FIG. 10 is substantially an alternate configuration of the embodiment illustrated in FIG. 7 that utilizes four wave plates. 
     FIG. 11 is substantially schematic diagram of one embodiment of the switching element of the present invention. 
     FIG. 12 is substantially a graphical representation of the transmission loss of the embodiment of FIG.  1 . 
     FIG. 13 is substantially a graphical representation of the wavefront quality impact on transmission of the embodiment of FIG.  1 . 
     FIG. 14 is substantially a schematic diagram of another embodiment of the switching device of the present invention. 
     FIG. 15 is substantially a schematic diagram of one embodiment of a demultiplexing device of the present invention for use with a number of optical switching devices of FIG.  1 . 
     FIG. 16 is substantially a schematic diagram of one embodiment of an optical switching system of the present invention including a plurality of switching devices shown in FIG. 1 used in conjunction with demultiplexing device of FIG.  15 . 
     FIG. 17 is substantially a schematic diagram of one embodiment of an optical switch system of the present invention that utilizes an optical isolation subsystem. 
     FIG. 18 is substantially a schematic diagram of one embodiment of an optical switch system of the present invention including a subsystem to adjust the apparent angular size of the fiber array and the apparent angular spacing of the fibers. 
     FIG. 19 is substantially another configuration of the embodiment illustrated in FIGS. 7-10 that utilizes one micro-electromechanical mirror. 
     FIG. 20 is substantially a detailed schematic diagram of one embodiment of a “two-dimensional” micro-electromechanical mirror that may be used with the present invention. 
     FIG. 21 is substantially a detailed schematic diagram of one embodiment of a mirror element of a micro-electromechanical mirror that may be used with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part of this application. The drawings show, by way of illustration, specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
     Switch Device 
     As seen in FIG. 1, the present invention comprises a switch device generally indicated by reference number  10 . Switch device  10  may be used in almost any optical communication system. Switch device  10  comprises sources and targets  12  and at least one switch element  26 . Sources and targets  12  comprise a source of incoming light signals and targets on to which switch array  20  transmits outgoing signals. Sources and targets  12  may be the same or different devices or objects. In the example shown in FIG. 1, sources and targets  12  are optical fibers  14 . However, many other devices and transmission mediums may be used. Sources and targets  12  may include any number of fibers  14  and may use many different types of fibers. Each optical fiber  14  comprises an end  16 . Ends  16  are preferably arranged in a two dimensional array, wherein the ends are substantially planar. It is recognized that the array may have many different configurations, such as the square array shown in FIG.  1 . 
     As an optical signal enters switch device  10  it is transmitted from fiber end  16  through a collimating lens  24 . Collimating lens  24  collimates the light and transmits the signal to imaging lens  25   a . Imaging lens  25   a , together with other lenses, images the optical signal on to a bandpass filter  30 . In the configuration illustrated in FIG. 1, one or more reflection devices or mirrors  27   a  and  27   b  are provided for directing the optical signal to the switch elements  26 . As will be discussed below, this configuration may allow switch device  10  to be connected to additional switch devices, each switch device being adapted to handle a set or range of wavelengths. However, it is recognized that the fibers  14  may be positioned differently to direct optical signals to switch elements  26  without the need for mirrors  27   a  and  27   b.    
     After the optical signal is reflected by mirrors  27   a  and  27   b , the signal passes through imaging lens  25   b , which, together with imaging lens  25   a , images the signal on a beam splitter or bandpass filter  30   a . Bandpass filter  30   a  is preferably a narrow band filter that is adapted to allow light within a predetermined range of wavelengths to pass through the filter and reflect light outside the range of wavelengths. Such bandpass filters are available from JDS Uniphase in Santa Clara, Calif. The range of wavelengths is selected to correspond to the range of wavelengths in which switch element  26   a  is designed to operate. Since many bandpass filters are sensitive to the angle of incidence, in this embodiment each bandpass filter  30  preferably receives the optical signal at nearly normal incidence. The configuration of the components of switch device  10  allows for each bandpass filter  30  to be placed at near normal incidence. The particular form of imaging, mentioned above, ensures that the phase at each bandpass filter  30  does not have additional focus, which would further degrade throughput because of the filter&#39;s angular sensitivity. This particular form of imaging may include 4-f or telecentric imaging, known to those skilled in the art. 
     If the incoming optical signal is not within the range of wavelengths bandpass filter  30   a  is adapted to transmit, the signal is reflected to a second bandpass filter  30   b  and corresponding switch element  26   b . In the configuration shown in FIG. 1, additional imaging lenses  25   c  and  25   d  and a mirror  27   c  are provided for facilitating the transmission of the signal to the subsequent switch element  26   b . Together, imaging lenses  25   c  and  25   d , which preferably incorporate a 4-f lens design or a suitable generalization that permits magnification, image the optical signal on to bandpass filter  30   b . This process of transmission or reflection is then repeated for each subsequent switch element  26   c-e . In this way, each switch element  26  receives signals in a range of wavelengths that it is intended to receive and signals outside of that range are reflected to other switch elements. 
     This configuration allows switch elements  26  to be placed side by side in a vertical configuration. This achieves several advantages. First, because bandpass filters  30  are transmissive in this design, dispersion, and attenuation are minimized for the reflected light that is incident on many filters in sequence. Second, the approach shown in FIG. 1 uses filters that are ostensibly at shallow angles, which decreases undesirable angular sensitivity. Third, the approach shown in FIG. 1 reduces cross talk from off-axis light. 
     The components of the present invention may be grouped into imaging units that, for example, comprise a switch element  26   a , bandpass filter  30   a , and imaging lenses  25   b  and  25   c . Imaging units may be packaged individually and installed and maintained separately. 
     Switch Element 
     Turning to FIGS. 2 and 3, each switch element  26  is arranged to receive incoming light  32  from bandpass filter  30 . As incoming light  32  enters switch element  26 , it is directed through an focussing lens  36  that focuses light signals on various components. In one embodiment, light  32  then falls on beam splitter  38 . Beam splitter  38  allows light  32  to pass to detector array  42 . Detector array  42  is adapted to detect signals in light  32  and generate electrical signals based on the light signals. Detector array  42  may comprise many different well known devices, such as 2609C Broadband Photodiode Module for both 1310 and 1550 nm detection available from Lucent Technologies or InGaAs p-i-n photodiodes for 1000-1700 nm detection, Part C30641E, available from EG&amp;G. The electrical signals are transmitted to switch controller  44 . 
     Switch controller  44  comprises a microprocessor  46  and memory  48 . Microprocessor  46  is adapted to determine the intended destination of the light signal and route the signal to an appropriate fiber. Microprocessor  46  may be any of a number of devices that are well known in the art. For example, microprocessor  46  may be an Intel Pentium III or other similar processor. Memory  48  is preferably random access memory that also may be any of a number of devices that are well known in the art. Switch controller  44  may also comprise non-volatile memory  50  that may contain programming instructions for microprocessor  46 . 
     Each light signal preferably carries a header that contains information that either identifies the signal or indicates its intended destination. This header information may be carried on a separate “control channel,” which may be a particular predetermined fiber or wavelength. Switch controller  44  is adapted to read the header. Switch controller  44  may be adapted, either alone or in coordination with other devices, to determine the destination of the light signal. In order to avoid contention for the same wavelength on the same optical fiber, which would result in interference when the signals are received, it may be necessary for each switch controller  44  to coordinate with other switch controllers. This may be facilitated by bus  52 . Bus  52  is connected to each switch element  26  and it allows each switch element to communicate with a central controller  54  (not shown in FIG.  4 ). 
     As seen in FIG. 4, central controller  54  is in communication with each bus  52  of each set  22  of switch elements  26 . This allows central controller  54  to receive signals from each switch element  26 . Central controller  54  may comprise a processor  60  that is adapted to perform computer operations. Processor  60  is in communication with memory device  62 , which may be random access memory (RAM), and non-volatile memory  64 , which is adapted to store data when power to controller  54  is interrupted. Non-volatile memory  64  may be many different kinds of memory devices, such as a hard disk drive, flash memory, or erasable programmable read only memory (EPROM). Central controller  54  may be in communication with a display device  66 , such as a monitor or printer, and input device  68 , such as a keyboard. Display device  66  and input device  68  are adapted to allow an operator or user to communicate with switch device  10  (see FIG.  1 ). 
     Central controller  54  may also comprise a communication device  70 , which may be external or internal. Communication device  70  is adapted to allow central controller  54  to communicate with other devices, such as other central processors or a computer that controls the optical system. Communication device  70  may be many different types of devices that are well known in the art, such as a modem, a network card, or a wireless communication device. 
     Central controller  54  may utilize a number of different techniques for resolving conflicts between switch elements. These may include the methods discussed in co-pending patent application Ser. No. 09/666,898, filed Sep. 20, 2000. Alternatively, each switch element  26  may be adapted to resolve conflicts or interferences internally. Switch controller  44  may have its own destination table and transmission table, and it can be programmed to manage signals entering switch element  26 . If each switch element  26  is assigned to handle a specific wavelength or range of wavelengths, coordination between switch elements may not be necessary. However, some degree of coordination may be desirable. Therefore, a relatively low-bandwidth data connection to central processor  54  (see FIG. 3) may be provided. 
     It is also recognized that it may be desirable to provide some form of communication device, such as bus  52 , to update switch controller  44 . For example, if a fiber has been disconnected from the network, switch controller  44  would need to be informed that this fiber is no longer available for transmission. In addition, switch element  26  may also be a node from which data is downloaded. This would necessitate transmission of data from each switch element  26  to another device to make use of the information. 
     In switch mode, once controller  44  has determined the destination of the optical signal, the controller causes at least one emitter in emitter array  56  to transmit an outgoing signal  28 . The position of the emitter corresponds the position of the target of the signal. Outgoing optical signal back along the general path of the incoming signal. Returning to FIG. 1, in the case of switch element  26   a , outgoing signal  28  passes through bandpass filter  30   a , imaging lens  25   b , mirrors  27   a  and  27   b , imaging lens  25  and, collimating lens  24  and is received by at least one target  12 . 
     It is also recognized that a plurality of detector and emitter arrays may be used in one switch element to detect and emit a plurality of wavelengths. This would allow one switch element to perform the same function of an array of switch elements. Thus, the switch device of the present invention may comprise only a single switch element. The same result could be obtained by using single detector and emitter arrays that are adapted to detect and emit a plurality of wavelengths. 
     Single Source Emitter Switch Element 
     As seen in FIG. 5, the present invention comprises an alternative embodiment generally indicated by reference number  426 . As incoming light  432  enters switch element  426 , it is directed through focusing lens  436 . In this embodiment, light  432  then falls on beam splitter  438 . Beam splitter  438  reflects light  432  to detector array  442 . Detector array  442  is adapted to detect signals in reflected light  432  and, as mentioned above, detector array  442  is capable of distinguishing different signals that are being transmitted by different sources. Detector  442  may generate electrical signals based on the light signals. The electrical signals are transmitted to switch controller  444 . 
     Switch controller  444  may be similar to switch controller  44  with a microprocessor and memory (not shown). The microprocessor is adapted to determine the intended destination of light signals and route the signals to an appropriate fiber. As in the previous embodiment, conflicts or interferences between signals can be handled within switch element  426 . 
     Switch element also comprises an emitter  456  that is adapted to constantly transmit light  458  over a period of time. The light is produced in a desired range of wavelengths. Light  458  is transmitted to lens  460 , which is adapted to collimate the light. Light  458  may then pass through optional lenslet array  462 , which is adapted to concentrate the light on individual modulators in modulator array  464 . The individual modulators in modulator array  464  may be modulators that are well known in the art, such as lithium niobate modulators available from Ortel in Azusa, Calif. Modulator array  464  is in communication with controller  444 , which may drive individual modulators to allow light to pass through the array. The position of the individual modulators corresponds to the position of targets for the light  458 . 
     By driving an individual modulator to allow light to pass through the modulator at selected times, the modulator can produce an outgoing optical signal  428 . The signal passes through beam splitter  438  and lens  436  and is transmitted to a predetermined target. 
     Micro-Electromechanical Mirrors Switch Element 
     The present invention also comprises an embodiment that utilizes micro-electromechanical mirrors (MEMs). MEMs are well known in the art, an example of which has been produced by Lucent Technologies in Murray Hill, N.J. MEMs are mirrors that may be selectively positioned in a plurality of positions. This allows the MEMs to reflect light transmitted from a source to a plurality of locations or targets. A plurality of MEMs may be placed in an array to switch light from a plurality of sources. 
     As seen in FIG. 6, MEMs can be used to switch light spatially using what is called a “3D” or “beamsteering” approach. In this approach, a first MEMs array  300  is positioned to receive a plurality of incoming parallel light beams  300 , sometimes called “pencil beams,” from a source or sources  304 . Before light falls on a particular MEM, the MEM is positioned or aimed to reflect light along a selected path. The path of the light corresponds to a location of a particular target  306  among a plurality of targets. 
     For some targets, such as an optical fiber, it is desirable that light being transmitted to the target be substantially parallel to the normal axis of the target. If first MEM array  300  were to reflect light directly to a target, it may cause the light to be non-parallel to the normal axis of the target. This is so because each MEM on array  300  may not be aligned with the intended target and it is necessary to reflect light at an angle relative to the path of the incoming light. To address this problem, a second MEM array  308  is provided. First MEM array reflects light  310  to a MEM on second MEM array  308 . The particular MEM on second MEM array  308  is aligned with the axis of the desired target  306  and the MEM is positioned so that light reflected by it is parallel to the preferred axis of the target. 
     A lenslet array  314 , which may comprise an array of lenses, may be provided between second MEM array  308  and target  306  to focus the light on the respective target. A controller may also be provided (not shown) for controlling the position of the individual MEMs in the MEM arrays. 
     The present invention comprises embodiments that utilize MEMs to switch optical signals. These embodiments utilize polarization of light signals to selectively reflect and transmit light. Polarization is a well-known property of light. There are two polarization states, typically denoted x and y, in which the electric field of the light oscillates in the x or y direction, respectively, as it propagates in the z direction. Such light is called linearly polarized x or y light, respectively. 
     Light of different polarizations can be superposed, i.e., added, so that states of polarization ax+by are possible. Furthermore, a and b can be complex; a complex part denotes a phase lag or lead between the two possible states. In particular, a polarization state x+iy, i=(−1) ½ , corresponds to a polarization state that rotates in the positive angle sense as it propagates and therefore is called right-circularly polarized. The state x−iy corresponds to rotations of the electric field that rotates in the negative angle sense, and is called left-circularly polarized. 
     Light can be switched from one polarization state to another using half-wave and quarter-wave wave plates, which are well known to those skilled in the art. A quarter-wave plate applies an additional factor of i (one-quarter of a full wave) to the y state, converting x+y to x+iy, or converting x+iy to x−y. Similarly, a half-wave plate applies a factor of −1 (one half of a full wave) to the y component, converting x+y to x−y. These facts are used in the embodiments described below. 
     Additionally, it is well known to those skilled in the art that polarizing beam splitters can reflect one linear polarization, for example, x, and transmit the second linear polarization state, y. These devices may be used to reflect or transmit light depending on the polarization of the light. 
     Turning now to FIG. 7, the present invention also comprises an alternative switch element generally indicated by reference number  350 . Circularly polarized light  352  is transmitted by source  353 . In the example calculations that follow, incoming light  352  is assumed to be right polarized light. Light  352  passes through lens  354 , which focuses the light onto image plane  356 . The light is allowed to diverge from the image plane until the light from the individual sources is of a size that matches the size of the individual micro-mirrors on MEMs array  366 . Light  352  then passes through a lenslet array  358  that is adapted to collimate the light, i.e., make it into a “pencil beam” that neither diverges nor converges. 
     A beam splitter  357  may be provided in the path of incoming light  352  to reflect a portion of the incoming light to a detector array  388 . Detector array  388  is adapted to convert the light signal to electrical signals and transmit the signals to controller  382 . Controller  382 , similar to controllers in the embodiments discussed above, is adapted to determine the destination of the incoming signal and drive MEM arrays  366  and  370  to route the signal to the appropriate target  386 . As described above, each optical signal may be provided with a header that allows controller  382  to determine the destination of the signal. A time gap may be provided between the header and the rest of the signal to provide sufficient time for controller  382  to determine the destination and drive particular MEMs in MEM arrays  366  and  370  to their desired angular positions. 
     After passing through lenslet array  358 , light  352  passes through λ/4 plate  360 . This converts the right-circularly polarized light from a state x+iy to x−y. The state x−y is a purely linearly polarized state of light in a 45 degree direction, and will be denoted by x′. A properly oriented polarizing beam splitter  362  will then reflect the x′-polarized light to MEM array  366 . 
     Reflected light  364  is transmitted to a particular MEM  367  that is aligned with the particular source  353  that emitted incoming light  352 . MEM  367  is angularly positioned by controller  382  to reflect the light to a particular MEM  371  on MEM array  370 . MEM  371  is aligned with a particular target  386  in a plurality of targets  384 . It is recognized that targets  384  may be the same devices as sources  351 . MEM  371  is angularly positioned by controller  382  to reflect incoming light  368  to target  386 . The angular position of MEM  371  depends on the position of MEM  367  on MEM array  366 . MEM arrays  366  and  370  are oriented so that the light passes through free space in this embodiment. 
     Reflected light  372  then passes through a λ/2 plate, which converts the polarization of the incident light from x′=x−y to y′=x+y, which is an orthogonal to x′. The light is then reflected by mirror  376 . Reflected light  380  passes through lens  378 , which acts to image the input lenslet array to the output lenslet array. Light  380  then passes through, if necessary, polarizing beam splitter  362 . After passing through polarizing beam splitter  362  by virtue of its y′ polarization, it then returns to the original λ/4 plate, which converts the y′=x+y polarized light to a polarization state x+iy, i.e., identical to the original input polarization state. Light  380  then exits the switching element the same way it came in, and proceeds to target  386 . 
     FIGS. 8 and 9 illustrate embodiments that operate in substantially the same way as the embodiment illustrated in FIG.  7 . In the embodiment shown in FIG. 8, MEM array  370  is on the same side of switch element  349  as MEM array  366 . In switch element  348  in FIG. 9, MEM array  370  is positioned in line with polarizing beam splitter  362  and targets  384 . Thus, mirror  376  (seen in FIGS. 7 and 8) is not required. 
     FIG. 10 illustrates an embodiment that utilizes four λ/4 plates  360 ,  392 ,  394 , and  396 . Light  352  is focused, converted, reflected as described above. However, a λ/4 plate  392  between beam splitter  362  and MEM array  366  is used to convert the polarization state from x′=x−y to x−iy. Light  364  impinges on MEM array  366  as before and then propagates back through quarter-wave plate  392 , which then converts the polarization from x−iy to x+y=y′. Thus, light  398  becomes orthogonally polarized and passes through polarizing beam splitter  362  to MEM array  370 . 
     Individual beams are directing the light in many different directions after being reflected by MEM array  366 , and if these directions are larger than about 10 degrees from normal incidence at quarter-wave plate  392  and at polarizing beam splitter  362 , significant errors in the polarization state of the light may occur. Thus, reflection angles are limited in this embodiment to less than about 10 degrees from normal incidence. 
     After light  398  passes through polarizing beam splitter  362 , the light passes through a third quarter-wave plate  394  that converts the polarization state from y′=x+y to x+iy. The light  398  then proceeds to MEM array  370 , which performs the same functions as in the previous embodiments. Reflected light  399  passes through the third quarter-wave plate  394  where its polarization state is changed from x+iy to x′=x−y. 
     By virtue of this new polarization state, the light is now reflected by the polarizing beam splitter upwards towards a fourth quarter-wave plate  396  that converts the polarization state from x′=x−y to x−iy. Light  397  then passes through lens  378 , reflects from mirror  376  back through the lens. Lens  378  focal length is chosen so that the double transmission of the light results in imaging lenslet array  358  onto it self, similar to what was done in the embodiment shown in FIG.  7 . 
     Light is again incident on fourth λ/4 plate  396 , which now converts the polarization state from x−iy to y′=x+y. By virtue of this new polarization state, light  380  transmits through polarizing beam splitter  362  and then passes out switching element  390  in the same manner as described in the previous embodiment. 
     Two-dimensional MEM Switch Element 
     FIG. 19 discloses an alternative preferred embodiment that utilizes a “two-dimensional” direction altering device. As in previous embodiments, source  353  transmits incoming light signal  352 . A portion of incoming light  352  is transmitted by beam splitter  357  through lens  354 , which focuses the light onto image plane  356 . The light is allowed to diverge from the image plane until the light from the individual sources is of a desired size. Light  352  then passes through a lenslet array  358  that is adapted to collimate the light, i.e., make it into a “pencil beam” that neither diverges nor converges. After passing through lenslet array  358 , the light is reflected by beam splitter  362  to a direction altering device  902 . Direction altering device  902  may be a number of devices that are known in the art for redirecting light, such as MEMs, bubbles of gas, or acousto-optic devices. In the description of the embodiment that follows, the direction altering device  902  shall be referred to as a MEM array. 
     A portion of incoming light  352  may be reflected by beam splitter  357  to detector array  388 . Detector array  388  is adapted to convert the light signal to electrical signals and transmit the signals to controller  382 . Controller  382 , similar to controllers in the embodiments discussed above, is adapted to determine the destination of the incoming signal and drive MEM arrays  902  to route the signal to the appropriate target  386 . As described above, each optical signal may be provided with a header that allows controller  382  to determine the destination of the signal. A time gap may be provided between the header and the rest of the signal to provide sufficient time for controller  382  to determine the destination and drive a particular mirror in MEM array  902  to a reflective position. 
     In this embodiment, MEM array  902  is a so called “two-dimensional” array. Two dimensional MEM arrays are well known in the art, a discussion of which can be found in IEEE Communications Magazine, March 2002, by Dobbelaere, Falta, Fan, Gloeckner, and Patra. As seen in FIGS. 20 and 21, incident light  364  travels in a plane that is parallel to the substrate of MEM  902 . All except one of the individual mirrors  904  in a row are in a down or lowered position, thereby allowing light  364  to pass over them. However, controller  382  has selected one of the mirrors  906  to reflect the light. Selected mirror  906  corresponds with the position of one or more targets  386 . 
     As seen in FIG. 21, controller  382  causes selected mirror  906  to move from a down position to an up or active position where it can reflect incident light  364 . Selected mirror may be actuated using a number of different methods and devices, such as comb drives, thermal expansion actuators, electrostatic scratch drive actuators, and gap-closing electrostatic actuators. 
     In the example shown in FIGS. 19 and 20, light is reflected in a 90 degree angle. However, MEM array  902  may be designed to reflect light in different angles and the architecture of switch element  900  may be modified accordingly. 
     Additionally, if all the mirrors in a row of two-dimensional MEMs  902  of FIG. 20 are in the lowered position, the incident light will pass over the entire array and can be detected by a detector  903  with suitable optics at the far side of the substrate. This particular operation is referred to as “dropping a channel,” and may be of benefit in some cases. If all the mirrors in a column of two-dimensional MEMs  902  of FIG. 20 are in the lowered position, light can be injected from a source into the outgoing path at that column by an emitter array  905  with suitable optics at the lower side of the substrate. This particular operation is referred to as “adding a channel,” and may be of benefit in some cases. Both detector  903  and emitter  905  are in communication with controller  382 , which is adapted to read the detected signals and cause emitter  905  to emit appropriate signals. 
     Reflected light  368  is transmitted to mirror  908 . The light is then reflected by mirror  376 . Reflected light  380  passes through lens  378 , which acts to image the input lenslet array to the output lenslet array. Light  380  then passes through beam splitter  362 , lenslet array  358 , lens  354  and beam splitter  357 . Light  380  then exits the switching element to target  386 . 
     As in disclosed in previous embodiments, λ/4 plates  360  and  374  may be provided for allowing bi-directional communication. If λ/4 plates  360  and  374  are used, beam splitter  362  would be a polarizing beam splitter that reflects light in one polarization and transmits light in another polarization. 
     Similar to the embodiment disclosed in FIGS. 7-10, switch element  900  may be utilized in an array of switch elements (not shown in FIG.  19 ). A dichroic beam splitter may be provided between the switch element  900  and sources  353  to reflect light of a predetermined wavelength to the switch element and to transmit light not in the predetermined wavelength to other switch elements. 
     Positioning and Alignment of Switch Device 
     Returning to FIG. 1, switch elements  26  and fibers  14  are preferably arranged substantially vertically so that switching elements  26  and fiber bundle  12  can be inserted vertically. The vertical configuration is advantageous for ease of access to switching elements  26  and for ease of alignment. Referring to FIG. 11, this is accomplished by providing each switch element  26  with a 3-point kinematic alignment unit  514 . Alignment unit  514  may comprise a prism  516 , positioned between the focusing lens  512  and an associated switch element  26 . Prism  516  is used to redirect the incident light to vertical for the embodiment shown in FIG. 1, and will not cause significant chromatic dispersion because the light has a very narrow spectral bandwidth as it enters each switching element  26 . In this embodiment, a power and data cable  518  is attached to switch element  26  at a point that is directly above the center of gravity of the switch element. This helps reduce the effect of forces imparted by power cable  518 . Wireless data ports  520  may also be provided, thereby eliminating the need for a physical data connection. 
     In terms of structural positioning, some specific numbers determine the configuration of switch device  10 . First, the required nominal angle of incidence of an incoming signal is approximately 3.0 degrees to maintain a 0.4 nm of spectral shift or less for a bandpass filter designed for normal incidence at 1500 nm mean wavelength. Within this 3 degrees, contributions come from the nominal incidence angle as well as from off-axis propagation of the light from the various optical carriers  14 . Consider, for example, a situation where a circular fiber bundle  14  is 64 fibers across and each fiber is separated by 100 microns. The resulting radius (r) of the bundle  12  is 3.2 millimeters. The light from this bundle  12  is collimated to about 1 centimeter beam radius (w) because the fiber numerical aperture (NA) is about 0.1. The required focal length (f) of the collimating lens  24  is about w/NA=10 cm. The greatest off-axis angle at collimating lens  24  is, therefore, about r/f=0.032 radians, i.e., about 1.9 degrees. This then leaves an angle θ of about 1.1 degrees, worst case, for the nominal pointing angle. Using 1.1 degrees as the angle of incidence on the bandpass filters  30  results in a 2.2-degree full angle (θ) between incident and reflected light. At this angle and with the imaging lenses  25  fitting with a 20% margin, results in a length of 1.2*w/(θ*π/180)=32.1 cm from imaging lens  25  to each bandpass filter  30 . Hence, the total length is about double, i.e., 64.2 cm, between mirrors  27  and bandpass filter  30 . This length can be reduced if additional signal loss can be tolerated. Focusing lens  25  can conveniently be selected to have a focal length equal to the separation from the lens  25  to corresponding bandpass filter  30 . Each switching element  26 , including detector, emitter, and beam splitter, preferably occupies a region that is about 100 cubic cm or less. 
     Using the above-described approach, a large number of wavelength bands can be sequentially demultiplexed. The limiting effects in this embodiment are beam quality and transmission losses. As shown in FIG. 12, for each switching element  26 , six (6) surfaces are encountered. Each of the surfaces may have 0.1% transmission loss or less, based on modern manufacturing capabilities, except for bandpass filters  30 , which typically have about 15% loss in transmission, and about 1% loss in reflection. Hence, the loss versus number of switching elements M in sequence is 0.85×(0.9995 5 ×0.99) M . A plot of the signal transmission versus number of switch elements M is given in FIG. 12, as well as a case in which the filters have a much worse loss of 15% per element upon reflection. From FIG. 12, it can be seen that even as many as 35 switching elements can be sequenced with less than 3 dB loss for all elements, for the nominal 1% per filter. On the other hand, if each filter loses 15%, only about 14 switching elements can be sequenced with less than 10 dB loss for all elements. 
     Beam quality is also an important issue in determining the number of switching elements  26  that can be sequenced. For the low-spatial-frequency aberrations expected for the 1 cm optics commonly in use, the formula for power loss is plotted in FIG.  13 . For this plot, the following is assumed: 1) 0.1, 0.05, and 0.025 waves rms error per optical element, and 2) these errors combine in root mean square. It is also assumed that switch element detectors  42  are 30 microns in size, that the wavelength is 1.5 microns, and that the F/# of lens  36  is 10, in accord with the assumptions above. The results for 0.05 rms waves per optic, or better, will provide 3 dB of loss on average for the 28 th  switching element. Less loss occurs for earlier switching elements in the sequence. 
     Overall, the combined effect of transmission and wavefront quality with the assumed values (0.05 waves or less, 1% or less for filters, 0.1% or less for other optics) leads to a 6 dB loss, or better, with 30 switching elements in sequence. The overall result is therefore that up to 30 switching elements may be sequenced with reasonable losses. 
     FIG. 14 illustrates an alternative configuration of switch device  10  in which each bandpass filter  30  is angled to receive the optical signal at a substantially non-normal incidence. In this case, each switching element  26  receives the optical signal from a substantially vertical direction, and, therefore, does not require prism  516  (see FIG. 11) for each switching element  26 . However, this embodiment suffers from unsymmetrical paths between imaging lenses, which requires relocation of the imaging lenses in an arrangement that may be more difficult to align. 
     Referring to FIGS. 15 and 16, the present invention includes system  600  that is adapted to perform wide band demultiplexing. System  600  directs optical signals having a predetermined range of wavelengths to an appropriate bank  500 . Each bank  500  (similar to switch device  10  of FIG. 1) comprises a plurality of switching devices  26 . System  600  includes a plurality of directing units  602 . Each directing unit  602  includes a first imaging lens  604  and a second imaging lens  606 , a mirror  608  for reflecting an optical signal from the first imaging lens  604  to the second imaging lens  606 , and a bandpass filter  610 . Each bandpass filter  610  is preferably a wide-band type filter configured to receive an optical signal from second imaging lens  606  and allow an optical signal within a predetermined range of wavelengths to pass through the bandpass filter to bank  500  of optical switches (see FIG.  16 ). An optical signal outside of the predetermined range of wavelengths is reflected to another, subsequent directing unit  602 . Element  609  may be a bandpass filter or a fold mirror depending on the angular alignment tolerances of bandpass filter  610   a.    
     Each bandpass filter  610  may divert, for example, groups of up to thirty (30) wavelengths. For instance, if the wavelength spacing between bands is 0.8 nanometers, then the total wavelength range for one group is 24 nanometers, and this entire group of thirty (30) wavelength bands is sent to one bank  500  of switch elements. 
     Clearly, this alternative embodiment will introduce additional losses, but because the filter  610  bandwidths are rather wide, the losses for sequencing in this case are less than that for the individual switching devices  500 . In this way, many different wavelengths can be simultaneously switched. For example, as noted in FIG. 16, it is not unreasonable to use 25 different switching banks  500 , with an associated additional 3 dB of loss for the furthest bank  500  (with signal regeneration, the signal strength can be arranged to be nearly equal for all the bands upon exiting the system). The combination of twenty-five (25) banks  500  and thirty (30) wavelengths per bank  500  leads to 750 wavelength bands. If 0.4 nanometers is used per band, the total bandpass used is 300 nm, which is essentially the entire telecommunications band from 1360 n to 1560 nm. 
     One layer of the resulting optical switching system  700  is shown schematically in FIG.  16 . In this case, demultiplexing system  600  is oriented perpendicular to the individual switching devices  500 . As indicated, a number of banks  500  may be located adjacent to one another (into the page). In this case, where twenty-five (25) banks  500  are provided, the overall system  700  occupies about 40 cm wide×85 cm high×125 cm long. The latter length assumes 5 cm of thickness per bank  500 . Note that the system shown in FIG. 16 has a total of π*32 2 =3216 input carriers, and 750 input wavelengths. If one assumes 10 Gigabits/sec input per wavelength, the resulting throughput is then 24 Petabits/sec. Accordingly, this embodiment has a tremendous capacity for switching or routing data. 
     Faraday Rotator Embodiment 
     In an alternative embodiment, polarization is more carefully controlled and used for added redundancy of processing. This is performed by placing a polarizing beamsplitter  702 , a Faraday rotator  704 , and one additional quarter-wave plate  706  between the input collimating lens and the rest of the system, as shown in FIG.  17 . Polarizing beamsplitter  702  reflects one linear component, say x, and transmits the orthogonal linear component, y, into the switching system. The reflected light from the polarizer may be sent to a second, redundant switch array that is similar to the first array (the array  500  of FIG. 16, for example), and this second array is used to switch those signals for which the reflected light is stronger than the transmitted light, or to switch light in case of failures in the first switching array. 
     Faraday rotator  704  rotates the polarization of the light in polarization y to y′=x+y, and this light enters the first quarter-wave plate  706 . λ/4 plate  706  then converts the light to right-circularly polarized light x+iy, which is then propagated to the relevant switch elements  26 . In some embodiments, a second quarter-wave plate  708  is provided in front of each switch element  26 . Second quarter-wave plate  708  will convert light to linear polarization x′=x−y. The linear polarization x′ is then reflected or transmitted to a detector array or to a MEMs array. If an embodiment is used with detectors, light backscattered from the detectors will be predominately light of the same polarization and will therefore pass back through and out of the optical system. This is evident to those skilled in the art because of the combination of the polarizing beamsplitter and the Faraday rotator at the fiber array. Faraday rotator  704  and two quarter-wave plates  706  and  708  along the optical path are equivalent to a standard optical isolator comprising a polarizing beamsplitter, a Faraday rotator, and a half-wave plate. 
     Light emitted from the emitters, modulators, or exiting the MEMs arrays will pass back through the polarizer in the orthogonal polarization, y′=x+y. This light will then be converted by the quarter wave plate  708  at switch element  26  to right-circularly-polarized light x+iy. This light will then be transmitted back through the system to first quarter-wave plate  706  where it will be converted to polarization state x′=x−y. This light then passes through Faraday rotator  704  a second time and the light is converted to state −y by the rotator. This state of light is transmitted through the polarizer and then passes through the collimator on to the fiber array, as desired. Note that if the range of wavelengths are significant, then dispersion in first quarter-wave plate  706  may be an issue. Dispersion compensation can be added as needed by various means known to those skilled in the art. 
     In an embodiment where the intervening optics&#39; properties vary significantly (&gt;10%) with linear state of polarization, additional consideration is required. Typically, one polarization state is preferred over another. In such cases, the light transmitted through the optical system from fiber array  14  to switching elements  26  is put into the preferred linear polarization state. In this case, first quarter-wave plate  706  near Faraday rotator  704  is moved to the switching elements and combined with the quarter-wave plates  708  at the respective switching elements. The combination of the quarter-wave plates form half-wave plates at the respective switching elements for equivalent functionality. 
     Beam Contractor 
     In another embodiment of the present invention, it is desired to put the fiber array into as small a region as possible to enable the system to achieve better imaging performance over the entire array. Because imaging performance is a function of field angle, it is desired to put the array into as small an angular region θ ar  as possible, as seen from the collimating lens. Some reduction of the extent of the array can be achieved by fiber packaging means. These packaging means are well known to those skilled in the art, and are produced by companies such as Haleos (web site www.haleos.com). On the other hand, much more flexibility in reduction can be achieved using optical system  800  shown in FIG.  18 . In this Figure, a beam contractor  802  is used to form a small image of the fiber array, followed by a lenslet array  804  to adjust the numerical aperture of the light entering or exiting each fiber. Beam contractor  802  reduces the apparent width w of the array by a magnification factor M from the true width W, so that w=W/M. 
     Lenslet array  804  then adjusts the numerical aperture NA to NA 1 . The resulting focal length changes from L=r/(NA×M) to L 1 =r/NA 1 , where r is the clear radius of the lens, NA is the fiber numerical aperture, and NA 1  is the adjusted numerical aperture. The resulting full angular extent of fiber array  14  is reduced from (W×NA)/r to (w×NA 1 )/r. This approach is used to reduce the apparent angular extent of fiber array  14  until the apparent angular separation of neighboring fibers is no less than about 3 times the diffraction limit of the collimating lens, in order to limit-fiber-to-fiber crosstalk. The diffraction limit is given by 1.22 λ/r, where λ is the longest wavelength of the light from the fiber array. As an example of the application of these techniques, assume that r=0.5 cm, and that λ=1.55 microns. The resulting needed angular separation is about 0.380 milliradians. Choose the full angular extent of the image of the fiber array, (w×NA 1 )/r, to be less than or equal to 2 degrees for best imaging performance. The resulting number of fibers across the imaged region is then (2 degrees)×(17 milliradians per degree)/0.38 milliradians=90 fibers across the diameter. The corresponding number of fibers that fit within a circular aperture with a diameter of 90 fibers is equal to 6360. Assuming switching between 6360 fibers and 750 wavelength and 10 Gbps per wavelength, one finds that routing of 47.7 Petabits per second of data can be supported with this optical architecture. 
     CONCLUSION 
     Although the description above contains many specifications, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of presently preferred embodiments of this invention. Thus, the scope of the invention should be determined by the appended claims and their legal equivalents rather than by the examples given.