Abstract:
An optical switching device for switching arbitrarily polarized optical signal beams includes a plurality of polarization-independent switching cells arranged in matrix form. Each polarization-independent switching cell is independently controllable to selectively direct received optical beams along at least a selected one of two axes. The device can be three-dimensionally expanded to increase the number of output ports to which the optical beams can be selectively switched, in which case the matrices can share integrally constructed polarization-independent switching cells.

Description:
RELATED APPLICATIONS AND PATENTS 
     This application is related to the application entitled &#34;Optical Device with Spatial Light Modulators for Switching Polarized Light&#34;, Ser. No. 07/991,607, filed Dec. 16, 1992, assigned to the assignee of the present application, and which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     This invention relates to an optical switching device and, more particularly, to an electrically controlled switching device for individually directing a plurality of arbitrarily polarized optical beams to selected output ports. 
     Signal processing systems often employ multiple parallel processors which permit many operations to take place concurrently. These so called parallel processing architectures typically require switching devices capable of efficiently transferring the data signals between such parallel processors. It is particularly advantageous to use optical switching devices to selectively interconnect the multiple parallel processors. For example, optical signal beams provide greater operating bandwidths and superior immunity to electromagnetic interference as compared with electrical signals. 
     These optical switching devices must be capable of spatially switching large numbers of light beams while preserving the integrity of the signals communicated by such light beams. One approach generally used in the fabrication of these switching devices is to utilize a plurality of elementary switching cells which in combination provide the overall switching capacity of the device. In these switching devices, it is highly desirable to reduce the number of elementary switching cells required to switch a given number of light beams so that, for instance, the switching device becomes more compact while having lower fabrication costs. 
     An optical device for switching light beams is proposed by J. B. McManus, R. S. Putnam, and H. J. Caulfield in the paper entitled &#34;Switched holograms for reconfigurable optical interconnection: demonstration of a prototype device&#34;, Vol 27, Applied Optics, pp. 4244-4250, Oct. 15, 1988. The device proposed by McManus et al.,(hereinafter McManus), utilizes a plurality of polarization-dependent switching cells arranged in a two-dimensional matrix which, in general, can have M columns and N rows of switching cells. Each switching cell comprises a polarizing beamsplitter and a spatial light modulator (SLM) which cooperate to selectively direct light beams externally applied to the matrix to selected outputs. Several disadvantages of the device proposed by McManus are that use of polarization-dependent switching cells can reduce light beam intensity (as may occur if a polarizer is used to polarize light to be used in a polarization-dependent system) and connections between such polarization dependent switching devices may require the use of polarization-maintaining fibers. Moreover, the device proposed by McManus is confined to a two-dimensional arrangement of switching cells (e.g., a single two-dimensional switching matrix), as McManus does not suggest how his device can be expanded into a three-dimensional arrangement of switching cells (e.g., a succession of identical two-dimensional matrices positioned parallel to each other). A device having such three-dimensional expansion capability can significantly increase the universe of selectable output ports to which the light beams can be switched. For example, if Q is the number of successive two-dimensional matrices positioned parallel to each other, then the number of selectable output ports will increase by a Q factor. 
     Polarization-independent types of switching devices have been suggested that enable an arbitrarily polarized light beam (i.e. an unpolarized light beam) to be selectively directed along a predetermined path. One example of such a polarization-independent switch is described by Wagner and Cheng in &#34;Electrically Controlled Optical Switch For Multimode Fiber Applications,&#34; Applied Optics, Vol. 19, No. 17, September 1980, pp 2921-2925. However, Wagner et al. do not suggest any two-dimensional arrangement for their switches, and much less a three-dimensional switching arrangement. 
     Accordingly, one object of the invention is to provide a two-dimensional optical switching device which can be controlled to individually and simultaneously direct arbitrarily polarized light beams received by each of multiple polarization-independent switching cells thereof. 
     Another object of the present invention is to provide a polarization-independent optical switching device which can be three-dimensionally expanded to provide additional selectable output ports for the light beams switched by the device. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, an optical switching device capable of simultaneously and individually directing a plurality of externally derived arbitrarily polarized optical signal beams to a selected plurality of output ports is provided. The device comprises at least a fast switching matrix which includes a plurality of polarization-independent switching cells arranged in columns and rows. Each of the polarization-independent switching cells is independently controllable such that individual optical beams received by any given polarization-independent switching cell can respectively emerge therefrom along at least a selected one of first and second axes, in accordance with the polarization orientation of the optical beams passing therethrough. Each polarization-independent switching cell in the first column of the matrix is positioned to receive a portion of the externally derived optical beams. In a reverse mode of operation, each polarization-independent switching cell in the last row of the matrix is also capable of receiving a portion of the externally derived optical beams. Each polarization-independent switching cell in a column subsequent to the first column is positioned to receive or to pass optical beams propagating along the selected second axis with respect to a switching cell located in a preceding column of the same row. Each polarization-independent switching cell in a row preceding the last row is positioned to receive or to pass optical beams propagating along the selected first axis with respect to a cell located in the subsequent row of the same column. 
     Each polarization-independent switching cell comprises a first switch unit optically coupled to a spatial light modulator which in turn is optically coupled to a second switch unit. The spatial light modulator preferably includes an array of individually controllable liquid crystal pixels. The spatial light modulator and the respective switch units of each polarization-independent switching cell cooperate to propagate through the matrix the externally derived optical beams applied simultaneously to the first column or to the last row of the matrix so as to provide bidirectional communication between selected ports without mutual interference. 
     In one embodiment of the invention the switching device comprises a succession of substantially identical switching matrices identified by alternate odd and even designations. Each of the matrices is positioned in parallel alignment with each other across a common axis perpendicular to their rows and columns. In this embodiment, row reflecting means is capable of optically coupling each pair of consecutive matrices in which an odd designated matrix (hereinafter &#34;odd matrix &#34;) is followed by an even designated matrix, (hereinafter &#34;even matrix&#34;) and column reflecting means is capable of optically coupling each pair of consecutive matrices in which an even matrix is followed by an odd matrix. In accordance with this embodiment, in each polarization-independent switching cell of an odd matrix the spatial light modulator and the respective switch units cooperate to propagate optical beams through the odd matrix in ascending column order within a row and in ascending row order within a column, while in each polarization-independent switching cell of an even matrix the spatial light modulator and the respective switch units therein cooperate to propagate signal beams through the even matrix in descending row order within a column and in descending column order within a row. 
     In another aspect of the invention, each polarization-independent switching cell is capable of simultaneously directing each received optical beam along each of the two selected axes. In accordance with this aspect of the invention, each of the externally derived optical signal beams can be simultaneously directed to at least two selected output ports. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description in conjunction with the accompanying drawings in which like characters represent like pans throughout the drawings, and in which: 
     FIG. 1 is a schematic diagram showing a front elevation view of an optical switching device in accordance with an embodiment of the present invention; 
     FIGS. 2A through 2D are schematic diagrams showing respective front elevation views of a polarization-independent switch in accordance with the prior art; 
     FIG. 3A is a schematic perspective view of another embodiment in accordance with the present invention; 
     FIG. 3B is a side elevation view of the row reflecting means shown in FIG. 3A; and 
     FIG. 3C is a top plan view of the column reflecting means shown in FIG. 3A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an optical switching device 10 capable of simultaneously and individually directing a plurality of externally derived arbitrarily polarized optical signal beams, e.g., K 1  &amp; K 2 , to a selected plurality of output ports, e.g., L 1  &amp; L 2 . As illustrated in FIG. 1, switching device 10 comprises four polarization-independent switching cells (PISCs) S 1 ,1, S 1 ,2, . . . S 2 ,2 arranged to form a two-dimensional switching matrix having two columns C 1  -C 2  and two rows R 1  -R 2 . In general S i ,j identifies a PISC located in the i th  row and the j th  column of a switching matrix comprising a plurality MxN of PISCs arranged in M columns and N rows. 
     Each PISC S i ,j, as illustrated in FIGS. 2A-2D, typically comprises a first switch unit 130 optically coupled to a spatial light modulator (SLM) 140 which is in turn optically coupled to a second switch unit 150. As used herein, &#34;optically coupled&#34; refers to an arrangement in which one or more optical beams are directed from one optical component to another in a manner which maintains the integrity of the signal carried by the optical beams. First switch unit 130 comprises respective polarizing beamsplitter (PBS) means, such as a cube PBS 132 coupled to an associated reflector 134; second switch unit 150 similarly comprises respective polarizing beamsplitter means, such as a cube PBS 152 coupled to an associated reflector 154. As used herein polarizing beamsplitter means refers to the entire PBS assembly, not just the interface of the prisms at which light separation or combination occurs. 
     Each SLM typically includes a two-dimensional array of individually controllable pixels illuminated by optical beams passing through a respective PISC. Each pixel comprises a material capable of altering the polarization orientation of linearly polarized light passing therethrough. For explanatory purposes, and not by way of limitation, it will be assumed that each pixel comprises a twisted nematic liquid crystal (LC) material. It will be understood by those skilled in the an that other materials will be equally effective as polarization rotators (e.g., materials exhibiting the property known as the Faraday effect or ferroelectric smectic LCs). Alternatively, a linear nematic LC (i.e., untwisted) acting as a half-wave retarder can be used to rotate the polarization of light. The LC in each pixel may operate so that when a control signal (not shown) applied thereto has a zero voltage the molecular orientation of the LC has a helical twist angle of 90°. In this twisted mode of operation, the molecular orientation of the LC causes a 90° rotation of the polarization orientation of the optical beam passing through the pixel. When the control signal applied to the pixel reaches a predetermined maximum voltage, the molecular orientation of the LC aligns approximately parallel to the optical beam passing therethrough and thus the optical beam passing through the pixel does not experience a polarization rotation. Thus, each pixel in the SLMs can be set to selectively rotate or not the polarization orientation of the optical beam passing therethrough. 
     As seen in FIG. 1, each PISC in the first column C 1  is positioned to receive a portion of the externally derived optical signal beams, e.g., K 1  &amp; K 2 . As indicated by the single headed arrows along the optical beams shown in FIG. 1, and as will be explained in greater detail, optical beams applied to the first column of PISCs can be selectively directed through the switching matrix in ascending column order within a row and in ascending row order within a column so that externally applied optical beams reach selected output ports L 1  &amp; L 2  associated with the last row of PISCs. In a reverse mode of operation, externally derived optical signal beams can be applied to each PISC in the last row R 2  through ports L 1  and L 2 , here functioning as input ports. The input or output function of such ports being solely defined in terms of the direction of the light beams passing therethrough. In the reverse mode of operation, the applied beams can be selectively directed through the switching matrix in descending row order within a column and in descending column order within a row (i.e., in directions opposite to the single headed arrows along the optical beams shown in FIG. 1) so as to reach selected ports M 1  &amp; M 2  associated with the first column of PISCs. 
     The conventional PISC illustrated in FIGS. 2A-2D enables arbitrarily polarized optical beams received by a fast or a second input face 135 and 137, respectively, of the PISC to be selectively switched along either a first or a second axis (here the X and Y axes, respectively). By way of example and not of limitation, a representative arbitrarily polarized optical beam &#34;B&#34; is illustrated in FIG. 2A incident on the first input face 135 of PISC S i ,j. This arbitrarily polarized optical beam may comprise both &#34;p&#34; polarized light (e.g., having a polarization orientation parallel to the plane of the figure and represented by the double headed arrows across the optical beam) and &#34;s&#34; polarized light (e.g., having a polarization orientation perpendicular to the plane of the figure and represented by the solid dots on the light beam). 
     Beam B is split into a constituent pair of beams of opposite linear polarization in polarizing beamsplitter 132, represented as B&#39; and B&#34;. Beam B&#39; passes through first switch unit 130 along substantially the same path as incident beam B and into a respective pixel in SLM 140; beam B&#34; is deflected in PBS 132 such that it is incident on reflector 134 and then optically coupled to a different respective pixel in SLM 140. In FIGS. 2A-2D, cube PBSs are shown and thus the angle of deflection is typically about 90°; alternatively, other types of PBSs may be used, such as Thompson PBSs, in which the deflection angle is other than 90°. As illustrated in FIG. 2A, the two respective pixels in SLM 140 through which beams B&#39; and B&#34; pass are set not to rotate their polarization orientation and thus such beams emerge from SLM 140 with their respective original polarization orientation which is carried to second switch unit 150. Beam B&#39; is incident on reflector 154 and is deflected into PBS 152; beam B&#34; passes from SLM 140 directly into PBS 152. The polarization orientation of beams B&#39; and B&#34; is such that they are combined in PBS 152 to form one unpolarized beam B that emerges from PISC S i ,j through a first output face 155 along the selected first axis (here the X axis). 
     FIG. 2B illustrates a PISC S i ,j with the respective pixels in SLM 140 set to cause a 90° polarization rotation to light passing therethrough. The operation is as described above with the exception that beams B&#39; and B&#34; undergo a 90° polarization rotation as they pass through SLM 140. In accordance with such 90° polarization orientation beams B&#39; and B&#34; are combined in PBS 152 to form one unpolarized beam B that emerges from PISC S i ,j through a second output face 157 along the selected second axis (here the Y axis). 
     FIGS. 2C and 2D illustrate the operation of PISC S i ,j with its SLM 140 set for no polarization rotation, and 90° polarization rotation, respectively, when optical beam B is received along the second input face 137 of PISC S i ,j. The operation is identical to that described above with the exception that the no polarization rotation setting of SLM 140 results in the unpolarized output beam emerging along the second axis (here the Y axis) and the 90° polarization rotation setting of SLM 140 results in an unpolarized output beam emerging along the first axis (here the X axis). 
     FIG. 1 illustrates the operation of the switching matrix in the context of optical signal beams K 1  and K 2  received, respectively, by PISCs S 1 ,1 and S 2 ,1, each in the first column of the switching matrix. For purposes of explanation of operation, it will be assumed that the respective pixels in SLM 140 of PISCs S 1 ,1 and S 2 ,2 are set to cause a 90° polarization rotation to optical beams passing therethrough (the 90° polarization rotation being indicated by the .sup.[/2 label in the SLMs shown in FIG. 1), while the SLM of all remaining PISCs are set not to rotate the polarization of light passing therethrough (the no polarization setting being indicated by the 0 label in the SLMs shown in FIG. 1). It should be understood that while FIG. 1 illustrates only one light beam received by each PISC for simplicity of illustration, each PISC can simultaneously handle many light beams, for example, more than about 10 4  light beams per cm 2 . 
     In accordance with the aforementioned selected settings, recombined arbitrarily polarized beam K 1  emerges from second switch unit 150 of PISC S 1 ,1 along the Y axis. PISC S 1 ,2 located in a column subsequent to the first column C 1  is optically coupled to receive the optical beam from PISC S 1 ,1 passed along the Y axis. Since the respective pixels in the SLM of PISC S 1 ,2 are set not to rotate the polarization orientation of optical beams passing therethrough, the recombined beam K 1  emerges from PISC S 1 ,2 along the X axis. PISC S 2 ,2 located in a row subsequent to the first row is optically coupled to receive the optical beam from cell S 1 ,2 passed along the X axis and since the respective pixels of SLM 140 in cell S 2 ,2 are set to rotate by 90° optical beams passing therethrough, recombined beam K 1  emerges from PISC S 2 ,2 along the X axis so as to reach a selected output port (e.g., L 2 ). Again referring to FIG. 1, analysis analogous to the one described in reference to beam K 1  indicates that beam K 2  propagates through the switching matrix to a selected output port (e.g., L 1 ). In FIG. 1, a phantom line between PISCs indicates other possible optical paths for light beams propagating through the matrix but not selected here, given the assumed selected settings. Focusing of the multiple light beams passing between each pair of adjacent PISCs is desirable to minimize crosstalk. An optical lens 170, or alternatively a lens array, can be advantageously optically coupled between adjacent PISCs to focus the light beams. 
     FIG. 3A illustrates a switching device 10 which comprises a succession of stacked switching matrices identified by alternate odd and even numbers (e.g., P 1 , P 2 , P 3  . . . P Q ). In general device 10 comprises a plurality of Q matrices substantially similar to the one illustrated and described in the context of FIG. 1. Each of the Q matrices is positioned in parallel alignment with each other across a common axis perpendicular to their rows and columns (e.g. the Z axis extending perpendicular the X and Y axes). As illustrated in FIG. 3A, respective optical components of successive PISCs S i ,j in each of the Q switching matrices may be constructed integral to each other. Thus, each SLM located in the same ith row and jth column of successive matrices (P 1 , P 2  . . . P Q ) may be formed by a single elongated SLM extending parallel to the Z axis (e.g., SLM 140). A similar integral construction may also be provided for each of the first and second switch units of successive PISCs S i ,j (e.g., first switch unit 130 and second switch unit 150). This integral construction reduces manufacturing costs as well as optical misalignment between each successive switching cell. FIG. 3A further illustrates row reflecting means 200 1 , . . . 200 Q/2  each capable of optically coupling an associated pair of consecutive switching matrices in which an odd matrix is followed by an even matrix. For example, row reflecting means 200 1  is capable of optically coupling switching matrix P 1  to matrix P 2 . 
     As illustrated in FIG. 3B, each row reflecting means (e.g., 200 1 ) comprises a first row end polarizing beamsplitter 250 optically coupled to receive arbitrarily polarized optical beams from the last row of a predetermined odd matrix (e.g., matrix P 1 ). First row end polarizing beamsplitter 250 splits each received optical beam (e.g., beam B) into a constituent pair of beams (e.g. beams B&#39; and B&#34;) having mutually orthogonal polarization orientations. First row end polarizing beamsplitter 250 cooperates with an associated first reflector 252 to direct beams B&#39; and B&#34; along the common axis (here the Z axis). A row end SLM 260 is optically coupled to receive beams B&#39; and B&#34; directed along the common axis. 
     The row end SLM 260 typically includes an army of individually controllable pixels capable of selectively rotating the polarization orientation of each beam pair passing therethrough. The row reflecting means further comprises a second row end polarizing beamsplitter 270 and an associated second reflector 272 optically coupled to combine each constituent beam pair received from the row end SLM 260. In particular a beam pair having its polarization orientation simultaneously rotated by 90° in SLM 260 is recombined in the second row end PBS 270 to form an arbitrarily polarized optical beam (represented by the dashed line) directed to a selected output port (e.g. L 1 ) associated with the row reflecting means, while a beam pair not experiencing polarization rotation (as specifically exemplified in FIG. 3B) is recombined to form an arbitrarily polarized optical beam directed along the Z axis. A third reflector 274 is optically coupled to receive the arbitrarily polarized beam along the Z axis and to deflect each received optical beam to the last row of PISCs of the next subsequent even matrix (e.g., matrix P 2 ) for further propagation within that subsequent even matrix. 
     FIG. 3A also illustrates column reflecting means 300 1  . . . 300 Q/2  each capable of optically coupling an associated pair of consecutive switching matrices in which an even matrix is followed by an odd matrix. For example, column reflecting means 300 1  is capable of optically coupling switching matrix P 2  to matrix P 3 . 
     In FIG. 3C, each column reflecting means is shown to comprise a first column end polarizing beamsplitter 350 optically coupled to receive arbitrarily polarized optical beams from the first column of a predetermined even matrix (e.g., matrix P 2 ). First column end polarizing beamsplitter 350 splits each received optical beam (e.g., beam B) into a constituent pair of beams (e.g. beams B&#39; and B&#34;) having mutually orthogonal polarization orientations. First column end polarizing beamsplitter 350 cooperates with an associated first reflector 352 to direct beams B&#39; and B&#34; along a common axis (here the Z axis). A column end SLM 360 is optically coupled to receive beams B&#39; and B&#34; directed along the common axis. 
     The column end SLM 360 typically includes an array of individually controllable pixels capable of selectively rotating the polarization orientation of each beam pair passing therethrough. The column reflecting means further comprises a second column end polarizing beamsplitter 370 and an associated second reflector 372 optically coupled to combine each constituent beam pair received from the row end SLM 360. In particular a beam pair having its polarization orientation simultaneously rotated by 90° recombines to form an arbitrarily polarized optical beam (represented by the dashed line) directed to a selected output port (e.g., N 1 ) associated with the column reflecting means, while a beam pair not experiencing polarization rotation (as specifically exemplified in FIG. 3C) is recombined to form an arbitrarily polarized optical beam directed along the common axis (here the Z axis). A third reflector 374 is optically coupled to receive the arbitrarily polarized optical beam along the common axis and to deflect each received optical beam to the first column of PISCs of the next subsequent odd matrix (e.g., matrix P 3 ) for further propagation within that subsequent odd matrix. 
     In accordance with the embodiment of FIG. 3A, in each PISC of an odd matrix the SLM and first and second switch units cooperate to propagate optical beams through the odd matrix in ascending column order within a row and in ascending row order within a column, while in each PISC of an even matrix the SLM and first and second switch units cooperate to propagate signal beams through the even matrix in descending row order within a column and in descending column order within a row so that redirected optical beams may be switched to selected output ports. 
     In another aspect of the invention, the respective pixels of the SLM of a PISC may be set to rotate the polarization orientation of the optical beams passing them through by an angle of about 45°. Optical beams with this 45° polarization orientation recombine to form two optical beams simultaneously emerging along both the first and second axes for further propagation within the switching matrix. In accordance with this aspect of the invention each of the externally derived optical beams can be simultaneously directed to at least two selected output ports of the switching matrix. 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.