Patent Publication Number: US-6335782-B1

Title: Method and device for switching wavelength division multiplexed optical signals using modulated emitter arrays

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 09/716,196, filed on Nov. 17, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a device and method for switching wavelength division multiplexed light signals using modulated emitter arrays. 
     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. Designers have considered using bubbles that are capable of changing their internal reflection for switching optical signals. However, this technique is unable to switch multiple wavelengths individually. 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. 
     Micro-electromechanical mirrors are capable of switching optical signals. However, these mirrors have not been utilized in a way that would allow them to be used in a WDM system. 
     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. 
     SUMMARY OF INVENTION 
     1. Advantages of the Invention 
     One advantage of the present invention is that it is able to switch signals of different wavelengths. 
     Another advantage of the present invention is that it is able to switch at high speeds. 
     A further advantage of the present invention is that it does not require high power. 
     Another advantage of the present invention is that it does not suffer from crosstalk. 
     Another advantage of the present invention is that it is able to switch between wavelengths and fibers to avoid transmission device or wavelength over-utilization. 
     Another advantage of the present invention is that it is able to broadcast to multiple transmission devices or couplers simultaneously. 
     A further advantage of the present invention is that it is able to regenerate and restore signals. 
     An additional advantage of the present invention is that it can transmit through air or other intervening media to a receiver without a costly or slow electrical interface. 
     These and other advantages of the present invention may be realized by reference to the remaining portions of the specification, claims, and abstract. 
     2. Brief Description of the 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 element and the targets are adapted to receive the optical signal from the optical switch element. 
     The optical switch element comprises a beam splitter, first and second wave plates, and first and second micro-mechanical mirrors. 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 and it is adapted to transmit light in the polarization that 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 first micro-electromechanical mirror is positioned to receive light reflected by the beam splitter and it is adapted to selectively reflect light in a plurality of paths, the paths corresponding to the positions of the plurality of targets. The second micro-electromechanical mirror is positioned to receive light reflected by the first micro-electromechanical mirror and it is adapted to reflect light in a path, the path being a predetermined orientation relative to at least one of the targets. 
     The second wave plate is positioned between the second micro-electromechanical mirror and the beam splitter and it is adapted to transmit light in the polarization that is transmitted by the beam splitter, wherein light reflected by the second micro-electromechanical mirror passes through the second wave plate and the beam splitter and is transmitted to a target. 
     The above description sets forth, rather broadly, the more important features of the present invention so that the detailed description of the preferred embodiment that follows may be better understood and contributions of the present invention to the art may be better appreciated. 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 perspective schematic diagram of one switch device of the present invention. 
     FIG. 2 is substantially a front schematic diagram of one embodiment of the switch array of the present invention. 
     FIG. 3 is substantially a side schematic diagram of the linear array of switch elements of the present invention. 
     FIG. 4 is substantially a schematic diagram of the switch element of the present invention. 
     FIG. 5 is substantially a schematic diagram of the switch array and central processor of the present invention. 
     FIG. 6 is substantially a flow chart of operation of the switch controller of the present invention, with regard to the transmission of signals. 
     FIG. 7 is substantially a flow chart of operation of the central controller of the present invention, with regard to the transmission of signals. 
     FIG. 8 is substantially a schematic diagram of the preferred embodiment of the switch device of the present invention. 
     FIG. 9 is substantially a schematic diagram of the switch element of the preferred embodiment of the present invention. 
     FIG. 10 is substantially a schematic diagram of another embodiment of the switch device of the present invention. 
     FIG. 11 is substantially a schematic diagram of another embodiment of the switch device of the present invention that utilizes a single source emitter. 
     FIG. 12 is a schematic diagram of a prior art switch device that utilizes two micro-electromechanical mirrors. 
     FIG. 13 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. 14 is an alternate configuration of the embodiment illustrated in FIG.  12 . 
     FIG. 15 is an alternate configuration of the embodiment illustrated in FIG.  12 . 
     FIG. 16 is an alternate configuration of the embodiment illustrated in FIG. 12 that utilizes four wave plates. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Single Detector Switch Element 
     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 a switch array  20 . Sources and targets  12  comprise a source of incoming light signals and targets on to which switch array  20  transmits outgoing signals. The sources and targets 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 array  18  may have many different configurations, such as the square array shown in FIG. 1 or rectangular arrays. 
     Turning to FIG. 2 and 3, switch array  20  comprises a plurality of linear arrays  22 . In this embodiment, there is a linear array  22  for each optical fiber  14  in the sources and targets  12 . This allows switch array  20  to receive signals from each optical fiber  14  in sources and targets  12 . As will be discussed below, however, switch array  20  may comprise a different number of linear arrays  22 . 
     As seen in FIGS. 1,  2 , and  3 , each linear array  22  is provided with a lens  24 . As will be discussed below, lenses  24  focuses light passing between array  18  and linear arrays  22 . The focal length of lens  24  should equal the distance from the end  16  to the front of the switch array  20 . 
     Referring to FIG. 3, each linear array  22  comprises at least one switch element  26 . Any number of switch elements may be provided. 
     Turning to FIG. 4, each switch element  26  is arranged to receive incoming light  28  from an optical fiber  14  (not shown in FIG.  4 ). As incoming light  28  enters switch element  26 , it intersects beam splitter  30 . Beam splitter  30  is a dichroic beam splitter that is adapted to reflect a predetermined wavelength or range of wavelengths of light  32 . The beam splitter may be a beam splitter, such as model number 03BSC 23 or 03BDL 005 available from Melles Griot, having an office in Irvine Calif. 
     If incoming light  28  contains the predetermined wavelength that may be reflected by beam splitter  30 , the beam splitter reflects that portion  32  of the light. Light that is not the predetermined wavelength will pass through beam splitter  30 . This non-reflected light  34  may be transmitted to a second switch element (not shown in FIG. 4) where it would it is subjected to another beam splitter (not shown). However, the beam splitter in the second switch element would be adapted to reflect light in another range of wavelengths and transmit light not in that range to another switch element. In this way, linear array  22  separates wavelength division multiplexed light signals into its individual signals. 
     As will be discussed below, each switch element may be capable of producing light signals. Light that is produced by other switch elements, outgoing light  35 , is transmitted back along the path of incoming light  28 . Since the outgoing light does not contain light in the range of wavelengths that is reflected by beam splitter  30 , this light passes through the beam splitter and is transmitted out to the front of the linear array. 
     Reflected light  32  is directed through an optional focusing lens  36 . In one embodiment, light  32  then falls on beam splitter  38 . Beam splitter  38  allows light  32  to pass to detector  42 . Detector  42  is adapted to detect signals in reflected light  32 . Detector  42  may generate electrical signals based on the light signals. Detector  42  may be 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. 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. However, in this embodiment, in order to prevent simultaneous transmissions in the same wavelength on the same optical fiber, which would result in interference when the signals are received, it is necessary for each switch controller  44  to coordinate with other switch controllers. In this embodiment, 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. 5, central controller  54  is in communication with each bus  52  of each linear array  22 . 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. 
     Referring now to FIG. 6, when switch element  26  receives a signal, the header of the signal is transmitted to switch controller  44 , as seen in step  80 . Switch controller  44  then determines the destination of the signal  82  and transmits the destination and other information to central controller  54 . Other information may include the size of the signal, the wavelength of the signal, wavelengths in which the switch element capable of transmitting, etc. 
     Turning now to FIG. 7, central controller  54  receives the destination and other information from the requesting switch element  86 . Central controller  54  then determines the preferred fiber for the particular destination  88 . This may be performed by referring to a transmission registry that contains destinations and a number of different fibers that are capable of transmitting the signal to the destination. 
     The registry may be represented by the following table (Table 1): 
     
       
         
           
               
             
               
                                                               TABLE 1 
               
             
            
               
                   
               
               
                 1. DESTINATION REGISTRY 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Destination 
                 Preferred Fiber 
                 Next Preferred Fiber 
                 . . . 
               
               
                   
                   
               
               
                   
                 1 
                 A 
                 E 
                 . 
               
               
                   
                 2 
                 F 
                 B 
                 . 
               
               
                   
                 3 
                 C 
                 D 
                 . 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                   
               
            
           
         
       
     
     After central controller  54  determines the preferred fiber, it then determines if the preferred fiber is unavailable for the specified wavelength  90 . This check may be accomplished in different ways. In one method, central controller  54  keeps a registry of signals being transmitted in each wavelength on each optical fiber  14 . This registry may be represented by the table shown below (Table 2): 
     
       
         
           
               
             
               
                                                          TABLE 2 
               
             
            
               
                   
               
               
                 2. TRANSMISSION REGISTRY 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Fiber 
                 Wavelength 1 
                 Wavelength 2 
                 . 
               
               
                   
                   
               
               
                   
                 A 
                 0 
                 1 
                 . 
               
               
                   
                 B 
                 1 
                 1 
                 . 
               
               
                   
                 C 
                 0 
                 0 
                 . 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                   
               
            
           
         
       
     
     In this table “0” may represent that the designated fiber is not occupied by the designated wavelength and “1” may represent that the fiber is occupied by the designated wavelength. This registry may also be used to store other information about the fibers. When a switch element has completed sending a signal, it may send a signal to central controller  54  that it has completed transmission. Central controller  54  would then clear the registry of the transmission. Alternatively, the registry may be cleared after an appropriate amount of time has passed. The amount of time may be obtained from the original request. In another method, central controller  54  polls each switch element  26  to determine whether it is currently sending a signal. 
     If the preferred fiber is available for the specified wavelength, central controller  54  then authorizes transmission by the requesting switching element  26 , step  98 . In an alternative embodiment, Switch element  26  may be designed to transmit in a plurality of wavelengths. Emitter array  56  may be capable of transmitting in a plurality of wavelengths or additional emitter arrays, adapted to transmit in a different wavelength that the first emitter array, and beam splitters may be provided. The inquiry in step  92  may be performed by referring to a switch element registry (not shown). The switch element registry may contain a listing of all switch elements and the wavelengths in which they are adapted to transmit. If the requesting switch element is capable of transmitting in the specified wavelength, central controller  54  then transmits a message to the requesting switch element to transmit on the selected fiber  98 . If the requesting switch element is not capable of transmitting in the specified wavelength, central controller  54  determines an appropriate switch clement to transmit the signal  94 . Central controller  54  then transmits a message to the requesting switch element to transmit the signal to the appropriate switch element  96  for transmission. 
     Returning to step  90 , if the preferred fiber is not available for the specified wavelength, central controller  54  would then determine the next preferred fiber for the destination  100 . Central controller  54  would then determine if the next preferred fiber is available for the specified wavelength  87 . If the next preferred fiber is available for the specified wavelength, central controller  54  would go to step  92  and repeat until a fiber is found or no fiber is available at the specified wavelength  89 . If no fiber is available for the specified wavelength, central controller  54  would return to step  100 . If all fibers are unavailable for the specified wavelength, central controller  54  would determine that all appropriate fibers are unavailable for all appropriate wavelengths  91 . If all appropriate fibers are not unavailable for all appropriate wavelengths, central controller  54  would select an alternate wavelength  93  and return to step  90 . If all appropriate fibers are unavailable for all appropriate wavelengths, central controller  54  would transmit a “busy” signal to the requesting switch controller  95 . Central controller  54  would then return to step  88 . 
     Returning to FIG. 6, switch controller  44  waits for a message from central controller  54 . When switch element  44  receives a message from the central controller  81 , it determines whether the message is a “busy” signal  83 . If the message is a busy signal, switch controller  44  may store the message  85  and wait for another message from central controller  54 . If the message is not a busy signal, switch controller  44  determines whether the message requires transmission to another switch element  87 . If the message requires transmission to another switch element, switch controller  44  transmits the signal to the indicated switch element  89 . This may be performed by transmitting the signal over bus  52 . If the message does not require transmission to another switch element, switch controller  44  transmits the signal on the indicated fiber  101 . 
     Returning now to FIG. 4, when switch controller  44  sends a signal, it drives emitter array  56  to generate the signal. Emitter array  56  comprises a plurality of different areas or emitters arranged in a two-dimensional array, each area being adapted to independently transmit a light signal. Each individual emitter may be many different kinds of emitters that are suitable for the particular optical fiber system. For example, an individual emitter in the 1310 nm range may be a Daytona laser, model 1861A, available from Lucent Technologies. Emitter array  56  is adapted to produce light in the predetermined range of wavelengths that beam splitter  30  is intended to reflect. Array  56  is also adapted to generate signals in specific areas of the array so that the signal can be mapped on to the appropriate optical fiber or target. As the signal is generated, it is reflected by beam splitter  38  and passes through lens  36 . The signal is then reflected by beam splitter  30  back along the path of the incoming light  28 . When the signal reaches the front of the array, it is imaged by lens  24  on to array  18 . The signal produced by a portion of emitter array  56  is then received by the corresponding optical fiber end  18  or other target. The focal length of lens  36  should be approximately equal to the optical path length from the center of emitter array  56  to the location of the imaging lens. In this way, each switch element can transmit a signal to any or all optical fibers  14  in sources and targets  12 . 
     Detector Array Switch Element 
     Turning now to FIG. 8, the preferred embodiment of the present invention is similar to the previously discussed embodiment. However, switch array  20  is replaced with a single linear array  120 . Linear array  120  comprises a lens  124  and a plurality of switch elements  126 . Lens  124  performs a similar function to lens  24  (see FIGS.  1  and  3 ), however, switch elements  126  differ from switch elements  26  in that each switch element comprises a detector array  142  that is capable of detecting signals from each of the optical fibers  14 . 
     Turning to FIG. 9, each switch element  126  is arranged to receive incoming light  128  from an optical fiber  14  (not shown in FIG.  9 ). As incoming light  128  enters switch element  126 , it intersects beam splitter  130 . Similar to beam splitter  30 , beam splitter  130  is a dichroic beam splitter that is adapted to reflect a predetermined wavelength or range of wavelengths of light  32 . 
     If incoming light  128  contains the predetermined wavelength that may be reflected by beam splitter  130 , the beam splitter reflects that portion  132  of the light. Light that is not the predetermined wavelength will pass through beam splitter  130 . This non-reflected light  134  may be transmitted to a second switch element (not shown in FIG. 4) where it would it is subjected to another beam splitter (not shown). Similar to the first embodiment, the beam splitter in the second switch element would be adapted to reflect light in another range of wavelengths and transmit light not in that range to another switch element. 
     Light that is produced by other switch elements, outgoing light  135 , is transmitted back along the path of incoming light  128 . 
     Reflected light  132  is directed through an optional focusing lens  136 . In this embodiment, light  32  then falls on beam splitter  138 . Beam splitter  138  allows light  132  to pass to detector array  142 . Detector array  142  is adapted to detect signals in reflected light  132  and, as mentioned above, detector array  142  is capable of distinguishing different signals that are being transmitted by different fibers  14  or sources. Detector  142  may generate electrical signals based on the light signals. The electrical signals are transmitted to switch controller  144 . 
     Switch controller  144  may be similar to switch controller  44  with a microprocessor and memory (not shown). Microprocessor  46  is adapted to determine the intended destination of light signals and route the signals to an appropriate fiber. 
     In this embodiment, since each switch element  126  is capable of receiving light signals from each fiber  14  in a predetermined range of wavelengths, conflicts, or interferences between signals can be handled within the switch clement. Switch controller  144  may have its own destination registry (see Table 1) and transmission registry (see Table 2) and it can be programmed to manage signals using the methods described above. 
     Controller  144  drives emitter array  156  to transmit an out going signal. This signal passes through lens  136  and is reflected by beam splitter  130  back along the path of incoming light  128  to a target (not shown). 
     This embodiment has several advantages of the previous embodiment. This embodiment only requires one linear array  122  and it may not be necessary to provide a bus and a central controller. Thus, the complexity and cost of the device may be less. Furthermore, since transmission need not be coordinated through a central controller, signals can be retransmitted more quickly and conflicts can be resolved more quickly. 
     However, it is recognized that it may be desirable to provide some form of communication device, such as bus  52 , and an outside controller, such as central controller  54 , to update switch controller  144 . For example, if a fiber has been disconnected from the network, switch controller  144  would need to be informed that this fiber is no longer available for transmission. In addition, device  10  may also be a node from which data is downloaded. In this application, it would be necessary for each switch element  126  to transmit data to another device to make use of the information. 
     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 a linear 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. 
     The embodiment disclosed in FIG. 10 utilizes a linear array  222  that is similar to linear array  122 . However, each switch element  226  comprises a multi-focal lens that is adapted to focus light differently depending upon the target of the light. This embodiment also includes mirrors  252  that can be used to direct the light to a target  254  without an optical waveguide. This embodiment is useful for applications where light is transmitted to targets over a short distance. For example, instead of installing optical fibers throughout an existing building, this embodiment of the present invention can be used to transmit signals to specific locations on the exterior of the building where a detector can receive the signal. An emitter associated with the detector can transmit signals to the device  10 . 
     Single Source Emitter Switch Element 
     As seen in FIG. 11, the present invention comprises an alternative embodiment generally indicated by reference number  426 . Each switch element  426  is arranged to receive incoming light  428  from a source (not shown in FIG.  11 ). As incoming light  428  enters switch element  426 , it intersects beam splitter  430 . Similar to beam splitter  30 , beam splitter  430  is a dichroic beam splitter that is adapted to reflect a predetermined wavelength or range of wavelengths of light. 
     If incoming light  428  contains the predetermined wavelength that may be reflected by beam splitter  430 , the beam splitter reflects that portion  432  of the light. Light that is not the predetermined wavelength will pass through beam splitter  130 . This non-reflected light  134  may be transmitted to a second switch element (not shown in FIG. 9) where it would it is subjected to another beam splitter (not shown). Similar to the first embodiment, the beam splitter in the second switch element would be adapted to reflect light in another range of wavelengths and transmit light not in that range to another switch element. 
     Light that is produced by other switch elements, outgoing light  435 , is transmitted back along the path of incoming light  428 . 
     Reflected light  432  is directed through an optional focusing lens  436 . In this embodiment, light  432  then falls on beam splitter  438 . Beam splitter  438  allows light  432  to pass 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 arc 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 optical signal. The signal passes through beam splitter  438  and lens  436  and is reflected by beam splitter  430  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. 11, 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 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 λ/2 and λ/4 wave plates, which are well known to those skilled in the art. A λ/4 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 λ/2 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. 
     Turing now to FIG. 13, 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 the 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 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 a λ/4 plate  360 . This converts the right-circularly polarized light from a state x+iy to x−y. However, 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 . 
     Similar to the embodiment disclosed in FIG. 10, switch element  350  may be utilized in an array of switch elements (not shown). A dichroic beam splitter may be provided between the switch element  350  and sources  351  to reflect light of a predetermined wavelength to the switch element and transmit not in the predetermined wavelength to other switch elements. 
     FIG. 14 and 15 illustrate embodiments that operate in substantially the same way as the embodiment illustrated in FIG.  13 . In the embodiment shown in FIG. 14, MEM array  370  is on the same side of switch element  349  as MEM array  366 . In switch element  348  in FIG. 15, MEM array  370  is positioned in line with polarizing beam splitter  362  and targets  384 . Thus, mirror  376  (seen in FIGS. 13 an  14 ) is not required. 
     FIG. 16 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 λ/4 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 arc larger than about 10 degrees from normal incidence at λ/4 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 λ/4 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 λ/4 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 λ/4 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.  4 . 
     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. 
     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.