Abstract:
Apparatus for redirecting a plurality of optical beams emanating from a respective plurality of optical transmitters arranged in a pattern along a first dimension and a second dimension. The apparatus comprises a plurality of refractive regions. Each refractive region intercepts the optical beams emanating from an associated group of optical transmitters occupying a common position in the first dimension. In addition, each particular refractive region imparts to the intercepted optical beams an angular deflection in the first dimension, the angular deflection in the first dimension being a function of the common position in the first dimension occupied by the optical transmitters from which emanate the optical beams intercepted by the particular refractive region. Use of the refractive regions reduces the amount of available deflection area left unused when a parallel set of port cards is employed for switching optical signals.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   The present application is related in subject matter to two U.S. patent applications entitled “APPARATUS FOR SWITCHING OPTICAL SIGNALS” and “SYSTEM AND METHOD FOR CONTROLLING DEFLECTION OF OPTICAL BEAMS”, both to Alan Graves, both filed on the same date as the present application and both hereby incorporated by reference herein. 
   FIELD OF THE INVENTION  
   The present invention relates generally to optical communications and, more particularly, to an apparatus for redirecting optical signals in free space. 
   BACKGROUND OF THE INVENTION 
   As optical signals used in optical communications carry ever increasing data rates according to an ever widening variety of data standards, it becomes desirable to provide switching at the photonic level, i.e., without resorting to electronic circuitry for converting the optical signals into the electrical domain before switching is performed. These types of optical switches are referred to as photonic (or OOO—short for “Optical Input, Optically Switched, Optical Output”) switches. 
   The desirable characteristics of a photonic switch are scalability, robustness and the ability to provide non-blocking performance in a compact low-cost package. Generally speaking, first-generation photonic switches afford at most two of these benefits at the expense of the other(s) in packages compromised in size and cost due to the complex, usually fiber-guided, interconnect between the various modules of the switch. 
   For example, first-generation photonic switches that are scalable by virtue of a modular design (e.g., multiple planes on a per-wavelength, or per-wavelength-group, basis) typically require a wavelength conversion unit to provide a satisfactory level of residual blocking performance. This introduces inefficiencies in provisioning the switch. Also, since optical signals are converted into the electrical domain for the purposes of wavelength conversion, switches of this type lose the designation of being truly photonic in nature. Moreover, in lambda-plane switches, the optical interconnect requires up to thousands of individual optical fiber connections, which can be reduced in size somewhat by the provision of an orthogonal shuffle function, but this nevertheless results in a non-compact solution. 
   Other designs, such as multi-stage photonic switches (e.g., CLOS), can be made non-blocking through dilation or path rearranging, but do not scale well to accommodate an increase in the number of input signals. In particular, the complexity of the interconnect between stages becomes intractable as the number of input signals increases. Furthermore, in addition to introducing a delay, the multi-stage characteristic of these switches imparts a higher path loss due to multiple lossy switching operations in series that need to be compensated for in the design. 
   Still other first-generation photonic switch architectures, such as the Xros X-1000, utilize opposing arrays of independently controllable mirrors at the end of an optical chamber to achieve non-blocking performance. However, these switches tend to be large in size, have low tolerance to manufacturing error and also do not scale well due to a lack of modularity. In addition, such switches have a complex fiber-based interconnect. 
   Against this background, it is clear that there exists a need in the industry for improvement in the area of photonic switches. 
   SUMMARY OF THE INVENTION 
   In accordance with a first broad aspect, the present invention seeks to provide an apparatus for redirecting a plurality of optical beams emanating from a respective plurality of optical transmitters arranged in a pattern along a first dimension and a second dimension. The apparatus comprises a plurality of refractive regions. Each refractive region intercepts the optical beams emanating from an associated group of optical transmitters occupying a common position in the first dimension. In addition, each particular refractive region imparts to the intercepted optical beams an angular deflection in the first dimension, the angular deflection in the first dimension being a function of the common position in the first dimension occupied by the optical transmitters from which emanate the optical beams intercepted by the particular refractive region. 
   In accordance with a second broad aspect, the present invention seeks to provide an apparatus for switching optical signals. The apparatus comprises a transmit entity adapted to emit a plurality of optical beams along a first plurality of parallel planes of travel, the parallel planes of travel in the first plurality of parallel planes of travel occupying respective first positions along a normal to the first plurality of parallel planes of travel. The apparatus further comprises a deflection entity adapted to receive the optical beams from the transmit entity and to deflect the received optical beams into a plurality of deflected optical beams along a second plurality of parallel planes of travel, the parallel planes of travel in the second plurality of parallel planes of travel occupying respective second positions along said normal to the first plurality of parallel planes of travel, each of the second positions being distinct from each of the first positions. Finally, the apparatus comprises a receive entity adapted to receive the deflected optical beams from the deflection entity. 
   In accordance with a third broad aspect, the present invention seeks to provide an apparatus for switching optical signals. The apparatus comprises a transmit entity adapted to emit a plurality of optical beams having respective directions of travel. The apparatus also comprises a receive entity adapted to receive a plurality of deflected optical beams from respective directions of arrival. Finally, the apparatus comprises a reflective entity comprising a first reflective surface and a second reflective surface held in a fixed relative position to one another, the first reflective surface adapted to deflect the optical beams upon receipt from the transmit entity towards the second reflective surface, the second reflective surface being adapted to deflect the optical beams received from the first reflective surface towards the receive entity as the plurality of deflected optical beams. 
   These and other aspects and features of the present invention will now become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a block diagram showing the use of test cards for an out-of-service calibration procedure; 
       FIGS. 2A-2C  are, respectively, perspective, side elevational and plan views of an apparatus for switching optical signals in accordance with an embodiment of the present invention; 
       FIG. 2D  is a side elevational view of a port card for use in an apparatus for switching optical signals in accordance with an embodiment of the present invention; 
       FIGS. 3A-3C  are, respectively, perspective, side elevational and plan views of an apparatus for switching optical signals in accordance with an embodiment of the present invention, additionally comprising a prism plate; 
       FIG. 3D  is a perspective view of a dual faceted prism plate in accordance with an embodiment of the present invention; 
       FIG. 3E  is side elevational view of an apparatus for switching optical signals in accordance with an embodiment of the present invention, comprising a pair of reflective surfaces; 
       FIGS. 4A-4C  are, respectively, perspective, side elevational and plan views of an apparatus for switching optical signals in accordance with an embodiment of the present invention; 
       FIG. 4D  is a plan view of a switch for optical signals in accordance with an embodiment of the present invention, comprising a reflector with a pair of reflective surfaces; 
       FIG. 5A  is a plan view of an apparatus for switching optical signals in accordance with an embodiment of the present invention, comprising a reflector with a planar surface; 
       FIG. 5B  is a plan view of an apparatus for switching optical signals in accordance with an embodiment of the present invention, comprising a reflector with a curved surface; 
       FIG. 5C  is a plan view of an apparatus for switching optical signals in accordance with an embodiment of the present invention, comprising a reflector with multiple facets; 
       FIG. 5D  is a more detailed depiction of the reflector of  FIG. 5C ; 
       FIG. 6  is a perspective view of a beam steering element capable of causing controllable deflection of an optical beam; 
       FIG. 7A  is a block diagram of a transmit beam steering element array in accordance with an embodiment of the present invention; 
       FIG. 7B  is a side elevational view of an arrangement of beam steering elements in the transmit beam steering element array of  FIG. 7A ; 
       FIGS. 7C and 7D  are variants of  FIG. 7B ; 
       FIG. 7E  is a plan view of an arrangement of beam steering elements in the transmit beam steering element array of  FIG. 7A ; 
       FIGS. 7F and 7G  are variants of  FIG. 7E ; 
       FIG. 8A  is a flowchart illustrating operation of a control module responsible for controlling the beam steering element array of  FIG. 7A ; 
       FIGS. 8B and 8C  depict possible lookup table structures for use by the control module; 
       FIG. 9A  is a view of a transmit port card and a receive port card from the perspective of a reflector, illustrating misalignment of an optical beam sent in an unpredictable direction of departure; 
       FIG. 9B  illustrates misalignment of an optical beam arriving at a beam steering element in an unpredictable direction of arrival; 
       FIG. 9C  is a view of a transmit port card and a receive port card from the perspective of a reflector, illustrating precession of an optical beam under control of the transmit port card; 
       FIG. 10  shows, in block diagram form, a circuit for detecting characteristics of a received optical beam; 
       FIG. 11  is a flowchart illustrating operation of a control module responsible for executing a fine tuning process to steer an optical beam; 
       FIG. 12  illustrates the fine tuning process at various stages of execution. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   With reference to  FIGS. 2A-2C , an apparatus for photonically switching optical signals (i.e., the signals remain in a photonic form throughout the switch node paths) in accordance with an embodiment of the present invention, hereinafter referred to as a photonic switch  100 , includes a provisionable plurality of port cards  102 A,  102 B, also sometimes referred to as tributary cards, trib cards, input/output (I/O) cards, etc. From a mechanical standpoint, the port cards  102 A,  102 B of the switch  100  may stand on edge in a side-by-side manner, supported by slots of a card cage. The port cards  102 A,  102 B exchange optical signals with other elements of the overall photonic network of which the switch  100  is a part (not shown, but designated by the reference numeral  104 ) and which are external to the switch  100 . The port cards include transmit port cards  102 A (so called because they cause optical signals to be transmitted into an optical chamber  118 ) and receive port cards  102 B (so called because they receive optical signals from the optical chamber  118 ). In some embodiments of the present invention (including but not limited to  FIGS. 2A-2D  and  FIGS. 3A-3E ), the port cards are dual function port cards, i.e., the transmit port cards  102 A are the same as the receive port cards  102 B. In other embodiments of the present invention (including but not limited to  FIGS. 4A-4C ), the transmit port cards  102 A are distinct from the receive port cards  102 B. 
   The transmit port cards  102 A receive input optical signals, e.g., along fiber optic cables  106  and connectors  172 , from the external entities  104 . Various optical processing functions are performed in customized signal conditioning and processing functions of the transmit port cards  102 A. For example, in  FIGS. 2A and 2B , the input optical signal is an input multi-carrier optical signal such as a dense wavelength division multiplexed (DWDM) signal. Here, an input signal conditioning module  108  on each transmit port card  102 A provides demultiplexing and other processing of the input optical signals. Other functionalities are possible, dependent upon the signal conditioning and processing functions needing to be implemented on the transmit port card  102 A. In still other cases, for example, in  FIG. 2D , where individual optical carrier signals are received via a ribbon cable, there is no need for an input signal conditioning module. 
   The output of the input signal conditioning module  108  is a set of individual optical carrier signals sent to a set of respective optical coupling elements (such as rod lenses or “GRIN” lenses, hereinafter referred to as optical transmitter elements  110 ) to couple from a waveguide environment within the substrate of the transmit port card  102 A into a free-space parallel sided optical beam. Thus, the optical transmitter elements  110  on each transmit port card  102 A transform the individual optical carrier signals from their guided wave environment on the transmit port card  102 A into respective parallel (non-divergent) optical beams  112 . In an example implementation, the optical transmitter elements  110  comprise rod lenses or beam collimators aligned to the substrate waveguides of the transmit port card  102 A by the use of V-grooves etched into the edge of a silicon substrate, into which the rod lenses or beam collimators are placed. 
   Each of the optical beams  112  acquires an initial direction given by the corresponding optical transmitter element  110 , which is independently and individually modified by one or more beam steering elements in a transmit beam steering element array  114 . The resulting optical beams, hereinafter referred to as “oriented” optical beams  116 , are projected into a free-space optical chamber  118 , in the general direction of a reflector  120  although at distinct and precisely controlled individual angular directions of departure, each aimed at the virtual image (in the reflector  120 ) of a target receiver element on one of the receive port cards  102 B. The direction of each beam in three-dimensional space will have a horizontal component (denoted by a horizontal deflection angle α, see  FIGS. 2A ,  2 C) and a vertical component (denoted by a vertical deflection angle η, see  FIGS. 2A ,  2 B) for each oriented optical beam  116  emanating from a given transmit port card  102 A. Each of the oriented optical beams  116  then undergoes reflection by the reflector  120  and is received at one of the received port cards  102 B. It should be understood that the terms “horizontal” and “vertical” are chosen for convenience only, in order to describe two orthogonal dimensions, but these terms should not be considered as restrictive. 
   In the embodiment shown in  FIGS. 2A-2C , the optical beams received at a given receive port card  102 B, hereinafter referred to as “received optical beams” and denoted by the numeral  122 , impinge upon a receive beam steering element array  140  on the given receive port card  102 B. The receive beam steering element array  140  redirects the received optical beams  122  into respective deflected optical beams  142 . The receive beam steering element array  140  provides a controllable amount of deflection which causes each of the deflected optical beams  142  to impinge directly on a respective one of a plurality of optical receive elements  124 , allowing those elements to focus the beams  142  accurately on to the waveguide interface into the receive port card substrate at the far end of those elements. 
   In an example embodiment, the optical receive elements  124  can be constructed similarly to the optical transmitter elements  110 , e.g., as V-grooves etched into the edge of a silicon substrate which carries the optical waveguides connecting to the rest of the receive port card  102 B in combination with rod lenses mounted in those grooves. Each rod lens has the end of a respective waveguide at its focal point for the case where a parallel optical beam is input into the lens from free space in a direction along its axis. The optical receive elements  124  transform the deflected optical beams  142  into switched optical carrier signals, which are provided in a guided wave environment to an output signal conditioning module  126 . The output of the signal conditioning module  126  is a plurality of switched optical signals, which are provided to the external entities  104 , e.g., along fiber optic cables  128  via a connector  174 . In an example, the output signal conditioning module  126  may perform multiplexing of multiple single-carrier optical signals. Of course, the output signal conditioning module  126  may perform other optical processing functions as required. In still other cases, for example in  FIG. 2D , where a ribbon cable connects the optical receive elements  124  directly to the connector  174 , there is no need for an output optical signal conditioning module. 
   When the transmit port cards  102 A and the receive port cards  102 B are of the type shown in  FIGS. 2A-2C , the switch  100  operates as a linearly provisionable lambda plane switch. When the transmit port cards  102 A and the receive port cards  102 B are of the type shown in  FIG. 2D , the switch  100  operates as a linearly scalable non-blocking switch. A mix of the two types of port cards allows one to build a lambda plane switch with any amount of add-drop to a lambda converter, allowing a wide variety of switch configurations to be constructed on a common platform. It is noted that these advantages are enabled by placing the optical switch elements (particularly the transmit and receive beam steering element arrays  114 ,  140 ) on the port cards, which can be done in a small physical space using hybrid optical integrated circuits (HOICs). Specifically, with the advent of hybrid optical integration, complex optical functions can be monolithically integrated into a silica-on-silicon substrate (e.g., array waveguide multiplexers, optical attenuators, thermo-optic switches, and even, with Erbium doping of the silica, optical amplification). These functions can be further augmented by hybridized components such as lasers, detectors and electronic chips in order to achieve a relatively complex, electrically controlled optical functionality in a relatively small space, especially in “height” (the dimension orthogonal to the plane of the substrate of the port card), which translates into a reduction of the required inter-card spacing or pitch. This allows the realization of switching implementations, modularities and partitionings that may have been viewed as impractical in the past. 
   In operation, the photonic switch  100  achieves switching action by virtue of the deflection angles (α and η) acquired by each of the oriented optical beams  116  under the action of the transmit beam steering element array  114  (the oriented optical beams  116  being pointed at virtual images of respective receive elements  124 ), and also by virtue of the action of the reflector  120 . Alternatively, the optical receiver elements  124  themselves can be placed on the opposite side of the optical chamber  118  and the optical transmitter elements  110  then target the oriented optical beams  116  on the optical receiver elements  124  rather than on virtual images of the receivers in the reflector  120 , thereby allowing the reflector to be dispensed with. In any event, by precisely controlling the angle at which the oriented optical beams  116  are sent away from the transmit port cards  102 A, individual candidate beam steering elements within each receive beam steering element array  140  and the associated optical receive elements  124  can be reached as desired. 
   Control of the individual beam steering elements of the transmit and receive beam steering element arrays  114 ,  140  on each transmit and receive port card  102 A,  102 B is effected by a control module  130  responsible for the port card in question. It should be noted that the control module  130  responsible for a given transmit or receive port card  102 A,  102 B can be located on that port card itself, on another port card or on a separate “controller card”; alternatively, the various control modules  130  can be consolidated onto a smaller number of separate controller cards. 
   The control module  130  receives switching instructions from a switch controller  134 , which can be implemented as a central shared resource that receives and acts on connection requests by interacting with the control modules  130  responsible for the various transmit and receive port cards  102 A,  102 B. One non-limiting way of supplying the switching instructions to the control module  130  is by way of a shared data bus  138 . Other configurations are possible, including but not limited to a daisy chain among the port cards. The switching instructions identify individual combinations of optical transmitter elements  110  and optical receive elements  124  that are intended to be optically connected to one another, in order to satisfy some higher level switching function. For example, the switching instructions sent onto the data bus  138  may indicate “connect the A th  optical transmitter element  110  on the B th  transmit port card  102 A to the C th  optical receive element  124  on the D th  receive port card  102 B”. These switching instructions are sent to the control modules  130  on both the B th  transmit port card  102 A and the D th  port cards  102 B. On the B th  transmit port card  102 A, the switching instructions are used to control the transmit beam steering element array  114  via a link  136  on the B th  transmit port card  102 A, while on the D th  receive port card  102 B, the switching instructions are used to control the receive beam steering element array  140  via the link  136  on the D th  receive port card  102 B. 
   From the above, it will be appreciated that the switching action provided by the switch  100  is non-blocking, since there is nothing to prevent any optical transmitter element  110  from optically connecting to any optical receive element  124  via their associated beam steering element arrays  114 ,  140  and the reflector  120 . Also, it should be appreciated that as the number of transmit or receive port cards  102 A,  102 B is increased, the capacity of the switch  100  will grow in a linear fashion in proportion to the number of additional optical transmitter elements  110  and/or optical receive elements  124  located on the added port cards  102 A,  102 B. As an aside, it will be recognized that the number of port cards  102 A,  102 B, as well as the number of optical transmitter elements  110  per transmit port card  102 A and the number of optical receive elements  124  per receive port card  102 B, can have a wide range of values while remaining within the scope of the present invention. 
   From  FIG. 2C , it will be apparent that the horizontal deflection angle α for a given transmit port card  102 A ranges from a minimum horizontal deflection angle α MIN  to a maximum horizontal deflection angle α MAX , where the range depends on the position of the transmit port card  102 A within the card cage. For example, the optical transmitter elements  110  located on transmit port cards  102 A at the center (in the horizontal direction) have a range of potential horizontal deflection angles α that is symmetric about zero, while the optical transmitter elements  110  located on transmit port cards  102 A at the rightmost edge have a range of potential horizontal deflection angles α that is entirely to the left, and the optical transmitter elements  110  located on transmit port cards  102 A at the leftmost edge have a range of potential horizontal deflection angles α that is entirely to the right. 
   Now, in a design where all transmit port cards  102 A are intended to be identical, one will need to pre-design them to provide a range of potential deflection angles α that is greater than necessary, since one needs to account for the positive, symmetric and negative cases described above and illustrated in  FIG. 2C . The result is that, in use, each transmit port card  102 A is effectively left unable to exploit about half of the pre-designed range of potential horizontal deflection angles α. This may be problematic, depending on the technology chosen for fabricating the individual beam steering elements of the transmit and receive beam steering element arrays  114 ,  140 . For example, consider the case where the beam steering elements are 2-axis gimbaled MEMS mirrors having achievable deflection angles of +/−5-7 degrees of mechanical movement, resulting in +/−10-14 degrees of optical deflection. This can result in the requirement for a deep free-space optical chamber  118  and resultant long optical paths, with the commensurate difficulties in achieving the requisite pointing accuracy, as well as holding that pointing accuracy in the presence of mechanical vibration. 
   Fortunately, it is possible to reduce this ineffective use of potential range of horizontal deflection angle. Specifically, as shown in  FIGS. 3A-3C , an apparatus hereinafter referred to as a “prism plate”  300  can be introduced between the transmit port cards  102 A and the reflector  120 . The prism plate  300  has a number of refractive facetted vertical strips  302 , each associated with a different one of the transmit port cards  102 A. The vertical strips  302  may be coated with an anti-reflective material (or multiple layers of anti-reflective materials, each layer being of the order of (2n+1)/4 wavelengths thick at the center of the optical frequency band of interest, where n is an integer, usually 0,1 or 2 but not limited to those values) covering the wavelengths of interest, such as (but not limited to) 1500 nm to 1600 nm or a subset thereof. 
   Each of the vertical strips  302  presents a face having an angle relative to the general horizontal direction, which is a function of the position (along the horizontal direction) of the associated transmit port card  102 A, in addition to being a function of the refractive index of the material of the prism plate  300 , the physical geometry of the reflector  120  (planar mirror or otherwise), the total number of transmit port cards  102 A and the pitch, i.e., the spacing between the transmit port cards  102 A. This will translate into a right or left bias Δα for each given vertical strip  302  that depends on the horizontal position of the transmit port card  102 A associated with the given vertical strip  302 . More specifically, the transmitter elements  110  can be viewed as defining a two-dimensional array, i.e., in the horizontal and vertical directions. The transmitter elements  110  on a given transmit port card  102 A share the same horizontal position. Each vertical strip  302  will thus provide the same horizontal bias for the optical beams  112  emitted by the transmitter elements  110  sharing the same horizontal position, i.e., which are on the same transmit port card  102 A. 
   It will thus be appreciated that with the use of the prism plate  300 , it is not necessary to over-provision the beam steering elements of the transmit beam steering element array  114  on the various transmit port cards  102 A to provide a larger-than-necessary range of potential horizontal deflection angles α. Rather, the available range of potential horizontal deflection angles α will always be directed towards the optical chamber  118  by the prism plate  300 . This has the advantage of allowing a reduction in both the optical path length and the depth of the free-space optical chamber  118 , as well as allowing a reduction in the required pointing precision for the oriented optical beams  116  emanating from the transmit beam steering element array  114  to impinge on the desired beam steering element of the receive beam steering element array  140 . 
   With reference now to  FIG. 2B , it will also be apparent that the vertical deflection angle η may range from a minimum vertical deflection angle η MIN  (when a northernmost optical transmitter element  110  sends an oriented optical beam  116  to a southernmost receiver element  110  on any receive port card  102 B) to a maximum vertical deflection angle η MAX  close to zero (when a southernmost optical transmitter element  110  sends an oriented optical beam  116  to a northernmost receiver element  110  on any receive port card  102 B). In fact, each of the optical transmitter elements  110  on a given transmit port card  102 A has its own range of potential vertical deflection angles η, delending on the vertical position of each optical transmitter element  110  on the given transmit port card  102 A, although there is no dependence on the horizontal position of the given transmit port card  102 A within the card cage. 
   Because there is no dependency of the range of potential vertical deflection angles η on the horizontal position of a given transmit port card  102 A in the card cage, it may be of advantage to bias each optical transmitter element  110  “downwards” at all times, so as to point generally towards the image of a optical receive element  124  somewhere in the lower half of the switch  100 . This will translate into a downward vertical bias for each optical transmitter element  110  that depends on the relative vertical position of that optical transmitter element  110 . This downward vertical bias can be achieved in a variety of ways, some of which are now described. 
   In a first example, the downward vertical bias can be achieved by the control module  130  providing a bias drive voltage to the beam steering elements in the transmit beam steering element array  114 . The bias drive voltage can be such that the optical beam  112  emanating from each optical transmitter element  110  is steered via the reflector  120  towards an existing or fictitious beam steering element corresponding to an optical receive element  124  that is located midway between the uppermost and lowermost optical receive elements  124 . The bias drive voltage is then varied differentially (i.e., increased or reduced slightly) during actual operation so as to point to an actual beam steering element corresponding to the target optical receive element  124  specific in the switching instructions. However, this solution has the detrimental side-effect of eroding the useful deflection range of the beam steering elements (typically MEMS switch mirrors with +/−5-7 degrees of mechanical movement) in a manner similar to that described before. 
   Alternatively, as shown in  FIG. 3D , the desired downward vertical bias can be achieved by providing refraction at the output of the optical transmitter elements  110 . Specifically, a modified prism plate  300 ′ (which already provides the requisite horizontal bias, described above) additionally introduces a variable vertical bias, being downward for the upper half of the shelf and, depending on operation requirements, upward for the lower half of the shelf. This can be achieved by providing vertical strips (providing horizontal bias) on one surface of the prism plate and horizontal prism facets (providing vertical bias) on the other surface. In an alternative embodiment to the one illustrated in  FIG. 3D , the same side of the prism plate provides both a horizontal bias and a vertical bias. In yet another embodiment, two prism plates could be placed in series, one providing the horizontal bias and one providing the vertical bias. Alternatively, instead of the prism plates being implemented as rows of horizontal and vertical prisms, more complex structures and facet angles with both varying horizontal and vertical components could be used on one or both surfaces, creating a two-dimensional array of angled prism facets on each surface of the prism plate. This would allow for an increased level of deflection from the prism plate. 
   In yet another embodiment, shown in  FIG. 3E , the desired vertical bias can be provided by splitting the reflector  120  into a pair of planar mirrors  320 ,  322  that act in a “periscope” fashion. In a specific embodiment, the reflective surfaces of the planar mirrors  320 ,  322  may be perpendicular to one another. This setup further helps to reduce the depth of the optical chamber  118 . Also, this embodiment is particularly advantageous where there is a pre-determined constant relationship between the vertical positions of all optical transmitter elements  110  and all optical receive elements  124 , i.e., when switching occurs only in the horizontal direction. For example, such a constraint may be in effect when different beams of received monochromatic light are being demultiplexed and re-multiplexed at the port cards. In such cases, the receive port cards  102 B may be designed such that an optical receive element  124  can only receive same-colored light which means light from optical transmitter elements  110  occupying a common position in the vertical direction on any given transmit port card  102 A. The use of planar mirrors  320 ,  322  effectively results in an inversion in the order in which colors are distributed in a vertical direction, between the optical transmitter elements  110  on one hand and the optical receive elements  124  on the other. Under these circumstances, the backplane mirror periscope structure of  FIG. 3E  provides the requisite vertical translation. 
   In the embodiments of  FIGS. 2A-2C  and  FIGS. 3A-3E  described above, each of the port cards  102 A,  102 B possesses both transmit and receive functionality. However, when the transmit port cards  102 A are distinct from the receive port cards  102 B, then the transmit port cards  102 A and the receive port cards  102 B can be interleaved while in other specific embodiments, for example with reference to  FIGS. 4A-4C , the transmit port cards  102 A are located generally towards one side (in this case the leftmost side of the card cage) and the receive port cards  102 B can be located generally towards the other side. It is also noted that when the transmit port cards  102 A are distinct from the receive port cards  102 B then, as best shown in  FIG. 4D , the transmit port cards  102 A and the receive port cards  102 B can be separated from one another by a one or more other cards  420  or empty slots, which can be used for control purposes or future expansion. 
   As before, a horizontal deflection angle α and a vertical deflection angle η for each oriented optical beam  116  emanating from a particular transmit port card  102 A is provided by the beam steering elements of the corresponding transmit beam steering element array  114  on that transmit port card  102 A. The oriented optical beam  116  then reflects off of the reflector  120  towards the appropriate receiver  124  on the appropriate receive port card  102 B via the appropriate beam steering element of the receive beam steering element array  140  on that receive port card  102 B. 
   From  FIG. 4C , it will be apparent that the horizontal deflection angle α may range from a minimum horizontal deflection angle α MIN  (when a optical transmitter element  110  on the leftmost of the transmit port cards  102 A sends an oriented optical beam  116  to the rightmost of the receive port cards  102 B) to a maximum horizontal deflection angle α MAX  close to zero (when an optical transmitter element  110  on the rightmost one of the transmit port cards  102 A sends an oriented optical beam  116  to the leftmost of the receive port cards  102 B). In fact, each of the port cards  102 A has its own range of potential horizonal deflection angles α, which will be different for transmit port cards  102 A occupying different slots in the card cage. 
   In a design where the transmit port cards  102 A are designed to be interchangeable, all of the transmit port cards  102 A would ideally to have the same capabilities of deflection. Therefore, in the design of  FIGS. 4A-4C , where all transmit port cards  102 A are identical and where the range of potential horizontal deflection angles is symmetric about zero and designed to account for the worst-case scenario, there will be erosion of a significant percentage of the available range of potential horizontal deflection angles α. 
   Now, recalling that with current deflection technologies such as MEMS, deflection angle is a scarce commodity, it is possible to pre-orient each optical transmitter element  110 , so as to point in a direction that corresponds to the image of a beam steering element on an imaginary receive port card located midway between the rightmost and leftmost ones of the receive port cards  102 B. This will translate into a rightward bias for the optical transmitter elements  110  on each of the transmit port cards  102 A that depends on the horizontal position of that transmit port card  102 A within the card cage. This rightward bias can be achieved by providing a prism plate (not shown) at the output of the transmit beam steering element arrays  114  of the various transmit port cards  102 A, in a manner similar to that described above with reference to the embodiment of  FIGS. 3A-3C . 
   As before, the use of such a prism plate allows one to forego over-provisioning the transmit beam steering element array  114  on each transmit port card  102 A to provide a larger-than-necessary range of potential horizontal deflection angles α. Moreover, due to the effect of the prism plate, the full range of potential horizontal deflection angles α of all the transmit port cards  102 A will remain inside the optical chamber  118 , allowing the optical path length and the chamber depth to be reduced. The path length and the chamber depth can be even further reduced by extending the prism plate to provide refraction of the received optical beams  122  (received via the reflector  120 ) towards the beam steering elements on the receive port cards  102 B. As an alternative, which allows the use of less powerful prism plates or even eliminates the need for such prism plates, one can use a horizontal periscope setup as shown in  FIG. 4D . Specifically, a pair of planar reflective surfaces  430 ,  440  are provided at the back of the optical chamber  118  are serve to provide a horizontal bias to the oriented optical beams  116  sent by the transmit port cards  102 A. 
   Returning now to  FIG. 4B , it will also be apparent that the vertical deflection angle η may range from a minimum vertical deflection angle η MIN  (when an uppermost optical transmitter element  110  sends an oriented optical beam  116  to a lowermost optical receive element  124 ) to a maximum vertical deflection angle η MAX  (when a lowermost optical transmitter element  110  sends an oriented optical beam  116  to an uppermost optical receive element  124 ). In fact, each of the optical transmitter elements  110  has its own range of potential vertical deflection angles η, which will be different for optical transmitter elements  110  at different vertical positions, but will not vary amongst the transmit port cards  102 A. For example, a optical transmitter element  110  located mid-way between the upper and lower extremes has a range of potential vertical deflection angles η that is symmetric about zero, while a lowermost optical transmitter element  110  has a range of potential vertical deflection angles η that is entirely upwards, and an uppermost optical transmitter element  110  has a range of potential vertical deflection angles η that is entirely downward. In this way, it is seen that the optical transmitter elements  110  will be left unable to exploit about half of their range of potential vertical deflection angles η. 
   However, it is possible to harness the unused portion of the range of potential vertical deflection angles η of the optical transmitter elements  110 . Specifically, a second prism plate (not shown, but similar to the prism plate  300 ′ of  FIG. 3D ) can be introduced between the transmit port cards  102 A and the reflector  120 . The second prism plate will have a number of refractive facetted horizontal strips (similar to the strips  312 ), each associated with an optical transmitter element  110  in a different position along the vertical direction. The horizontal strips of the second prism plate may comprise one or more coatings of anti-reflective material covering the wavelengths of interest, such as 1500 nm to 1600 nm or a subset thereof. Each of the horizontal strips has an angle relative to the general vertical direction, which is a function of the vertical position of the associated optical transmitter element  110 , in addition to being a function of the refractive index of the material of the second prism plate, the physical geometry of the reflector  120  (planar mirror or otherwise), the total number of optical receive elements  124  and the spacing therebetween. This will translate into a vertical bias for each of the optical transmitter elements  110  that depends on the vertical position of that optical transmitter element  110 . 
   It will thus be appreciated that with the use of the second prism plate, the full range of potential vertical deflection angles η of all the optical transmitter elements  110  will be utilized, allowing the optical path length and the chamber depth to be reduced. Also, it should be noted that the first and second prism plates can be placed one in front of the other, or they can be integrated to form a single composite prism plate, similar to the prism plate  300 ′ of  FIG. 3D , but adapted to account for the new geometry which separates the transmit port cards  102 A from the receive port cards  102 B. 
   The reflector  120  is now described in greater detail. The configuration of the reflector  120  has an influence on the depth of the optical chamber  118  as well as on the precise direction in which the transmit beam steering element array  114  must send the oriented optical beams  116  in order for them to reach their intended optical receive element  124 , as specified in the switching instructions. For example, the complete absence of a reflector is one possibility, where the transmit port cards  102 A and the receive port cards  102 B face one another at opposite ends of a optical chamber  118 . However, the depth of the optical chamber  118  is greater than in the presence of a reflector  120 . 
   When a reflector  120  is used, such may be planar or non-planar in nature. With reference to  FIG. 5A , there is shown a planar mirror  502  in plan view. The beam steering elements of the receive beam steering element arrays  140  are associated with virtual images that are “behind” the planar mirror  502 , and represent the points towards which the beam steering elements of each transmit beam steering element array  114  should aim when attempting to reach an actual target beam steering element. The target beam steering element and its image are equally far from the planar mirror  502 , and are of the same size. 
   Now with reference to  FIG. 5B , consider the case of a convex mirror  504 , smaller in size than the planar mirror  502 , placed at the back of the optical chamber  118  in place of the original planar mirror  502 . The use of the convex mirror  504  gives rise to a smaller virtual image, which is located closer to the convex mirror  504  than the originating object, the ratio of image magnification and front/back distances being equal. Hence, for a magnification of S (where S&lt;1), the ratio of front-back distances is S:1 and the ratio of the size of the image vis-à-vis the original is S:1. It can thus be shown that the ratio of the overall path length to the image is (1+S):2 when compared with the case of the planar mirror  502 , which means that the distance to the convex mirror  504  can be reduced by a factor of (2*S/(1+S)):1. Alternatively, if the distance to the convex mirror  504  is kept constant, then the arctangent of this ratio represents the available reduction in the total horizontal deflection angle range, although it is noted that stronger horizontal bias by a prism plate would be needed with this approach. 
   One side-effect from the convex mirror  504  approach of  FIG. 5B , is that the curvature of the surface of the convex mirror  504  will cause an astigmatic distortion, leading to an expansion (dispersion) of the optical beams  122  leaving the convex mirror  504 . This can be overcome by the solution in  FIG. 5C , which shows an alternative to both the planar mirror  502  and the convex mirror  504 , namely the use of a facetted backplane mirror  506 . Specifically, the facetted backplane mirror  506  comprises planar vertical facets  512  that have increasingly acute angles as the horizontal distance from the center of the facetted backplane mirror  506  increases. In a specific embodiment, the number of planar vertical facets  512  used to support a number (P) of port cards acting as both transmit and receive port cards is 2P−1. The facets can be designed so as to lie in a flat plane at a specific points determined by the magnification factor (S) required, but with the facet angles matching those of a curved mirror giving the same value of S. This gives the same benefit of the convex mirror  504 , namely a smaller deflection angle or a reduced chamber depth. However, because the facets  512  are planar, there will be no distortion of the received optical beams  122 , provided that each oriented optical beam  116  impinges upon only one of the facets  512  at any given time. 
   An example of how to design the facetted backplane  506  for a desired image/object ratio of S (which is equal to H/G or J/K) is now described. The 2P−1 facets  512  are denoted  512   1 ,  512   2 , . . . ,  512   2P-1 , while the P port cards are denoted  102   1 ,  102   2 , . . . ,  102   P . Facet  512   1  interconnects only one port card to itself, namely  102   1 . Facet  512   2  interconnects two port cards, namely port card  102   1  and port cards  102   2 . Facet  512   3  intercepts port card  102   2  to itself, as well as port  102   1  to  102   3 . This pattern continues, until one reaches the central (i.e., P th ) facet  512   P , which intercepts some connections from all port cards. Beyond this point, the number of port cards interconnected decreases until, at facet  512   2P-2 , where just port cards  102   P-1  and  102   P . In the case of a switch  100  with eight (P=8) port cards, as is shown in  FIG. 5C , this leads to: 
   
     
       
             
             
             
           
         
             
                 
                 
             
             
                 
               Facet # 
               Port Cards Interconnected 
             
             
                 
                 
             
           
           
             
                 
               512 1   
               102 1            102 1   
             
             
                 
               512 2   
               102 1            102 2   
             
             
                 
               512 3   
               102 1            102 3 , 102 2            102 2   
             
             
                 
               512 4   
               102 1            102 4 , 102 2            102 3   
             
             
                 
               512 5   
               102 1            102 5 , 102 2            102 4 , 102 3            102 3   
             
             
                 
               512 6   
               102 1            102 6 , 102 2            102 5 , 102 3            102 4   
             
             
                 
               512 7   
               102 1            102 7 , 102 2            102 6 , 102 3            102 5 , 102 4            102 4   
             
             
                 
               512 8   
               102 1            102 8 , 102 2            102 7 , 102 3            102 6 , 102 4            102 5   
             
             
                 
               512 9   
               102 2            102 8 , 102 7            102 3 , 102 6            102 4 , 102 5            102 5   
             
             
                 
               512 10   
               102 3            102 8 , 102 7            102 4 , 102 6            102 5   
             
             
                 
               512 11   
               102 4            102 8 , 102 7            102 5 , 102 6            102 6   
             
             
                 
               512 12   
               102 5            102 8 , 102 7            102 6   
             
             
                 
               512 13   
               102 6            102 8 , 102 7            102 7   
             
             
                 
               512 14   
               102 7            102 8   
             
             
                 
               512 15   
               102 8            102 8   
             
             
                 
                 
             
           
        
       
     
   
   In accordance with the above, and as can be seen from  FIG. 5C , every second facet returns light from a port card back to that port card. Thus, facet  512   1  is in a plane perpendicular to a line from port card  102   1  to the center of facet  512   1 , facet  512   2  is in a plane perpendicular to the bisect of the angle formed at that facet between lines from port cards  102   1  and  102   2 , facet  512   3  is in a plane perpendicular to a line from port card  102   2  to the center of facet  512   3 , facet  512   4  is in a plane perpendicular to the bisect of the angle formed at that facet between lines from port cards  102   1  and  102   4 , etc. 
   As previously mentioned, the transmit beam steering element array  114  on a given transmit port card  102 A is responsible for deflecting the optical beams  112  into oriented optical beams  116 , causing the latter to acquire a desired direction towards the reflector  120  (if used). It is noted that the optical beams  112  deflected by the transmit beam steering element array  114  are closely spaced and arrive in parallel at the transmit beam steering element array  114  from the optical transmitter elements  110 .  FIG. 7A  shows generally how deflection is achieved while  FIGS. 7B through 7G  show various embodiments of the transmit beam steering element array  114 . As can be appreciated from  FIG. 7A  and the description to follow, a common feature of each of configurations in  FIGS. 7B through 7G  is that a plurality of points of deflection are provided for each of the optical beams  112 . It will be appreciated that a similar design can be used in the receive beam steering element array  140  on each receive port card  102 B. It is recalled that the receive beam steering element array  140  redirects the received optical beams  122  into deflected optical beams  142  that impinge on the optical receive elements  124 . 
   With specific reference to  FIG. 7B , there is shown a portion of a first example embodiment of the transmit beam steering element array  114  on a particular transmit port card  102 A, with the substrate of the transmit port card  102 A being in the plane of the page. Specifically, the optical transmitter elements  110  produce a plurality of optical beams  112  which impinge on a column of beam steering elements  702 ,  704 ,  706 . The beam steering elements  702 ,  704 ,  706  each comprise a respective first, movable reflective facet  702   1 ,  704   1 ,  706   1 . The beam steering elements  702 ,  704 ,  706  also each comprise a respective second, non-movable reflective facet  702   2 ,  704   2 ,  706   2 , which may be provided in a specific non-limiting example embodiment by a reflectively coated back wall of an enclosure. Contrary to the first reflective facet  702   1 ,  704   1 ,  706   1  of each of the beam steering elements  702 ,  704 ,  706 , which has a controllable deflection angle, the deflection angle of the second reflective facets  702   2 ,  704   2 ,  706   2  is fixed. Of course, it should be understood that the second reflective facets  702   2 ,  704   2 ,  706   2  may be provided as stand-alone mirrors not having any connection to the first reflective facets  702   1 ,  704   1 ,  706   1 . 
   As previously described, the transmit beam steering element array  114  provides at least two points of deflection for each of the optical beams  112 , as emitted by the optical transmitter elements  110  on the particular transmit port card  102 A of interest. In the specific embodiment of  FIG. 7B , the second reflective facet  702   2  of beam steering element  702  is fixed in a position where it intercepts the optical beam  112  emitted by a corresponding one of the optical transmitter elements  110  and deflects it towards the first reflective facet  704   1  of beam steering element  704 . Similarly, the second reflective facet  704   2  of beam steering element  704  is fixed in a position where it intercepts the optical beam  112  emitted by another one of the optical transmitter elements  110  and deflects it towards the first reflective facet  706   1  of beam steering element  706 . Beam steering in two axes (vertical deviations from the horizontal direction in the plane of the drawing and perpendicular to the drawing) is provided by the first reflective facets  702   1 ,  704   1 ,  706   1 , which in fact deliver the second of two points of deflection for each of the resultant optical beams  112 , resulting in the oriented optical beams  116 . 
   With reference now to  FIG. 7C , there is shown a portion of a second example embodiment of a transmit beam steering element array  114  on a particular transmit port card  102 A, again with the substrate of the transmit port card  102 A being in the plane of the page. This embodiment is similar to the one in  FIG. 7B , except that the beam steering elements  702 ,  704 ,  706  are inverted. Thus, the transmit beam steering element array  114  continues to provide at least two points of deflection for each of the optical beams  112  emitted by the optical transmitter elements  110  on the transmit port card  102 A. However, in the specific embodiment of  FIG. 7C , the first reflective facet  702   1  of beam steering element  702 , which has a controllable deflection angle, is positioned so as to intercept the optical beam  112  emitted by a corresponding one of the optical transmitter elements  110  and to controllably deflect it towards the second reflective facet  704   2  of beam steering element  704 . Similarly, the first reflective facet  704   1  of beam steering element  704  intercepts the optical beam  112  emitted by another one of the optical transmitter elements  110  and controllably deflects it towards the second reflective facet  706   2  of beam steering element  706 . Beam steering is again provided by the first reflective facets  702   1 ,  704   1 ,  706   1 , but which in this case deliver the first (rather than the second) of two points of deflection for each of the optical beams  112 , resulting in the oriented optical beams  116 . While it is clear that the total range of deflection angles is the same in the embodiment of  FIG. 7C , the second reflective facets  702   2 ,  704   2 ,  706   2  in the embodiment of  FIG. 7C  need to provide a reflective area that is slightly larger than the reflective area that needs to be provided in the embodiment of  FIG. 7B . 
   In another embodiment, each of the optical beams  112  is deflected by two separate beam steering elements having independently controllable deflection angles. Specifically, having regard to  FIG. 7D  and again with the substrate of the transmit port card  102 A being in the plane of the page, a back-to-back assembly is provided, whereby beam steering elements  702  and  703  each have a respective reflective first facet  702   1 ,  703   1  and are joined by a common second facet  702   2 , which need not be reflective. Similarly, beam steering elements  704  and  705  each have a respective reflective first facet  704   1 ,  705   1  and are joined by a common second facet  704   2 , while beam steering elements  706  and  707  each have a respective reflective first facet  706   1 ,  707   1  and are joined by a common second facet  706   2 . In another embodiment (not shown), there may be a separation between the second facet of each pair of back-to-back beam steering elements. In fact, the back facets of the various beam steering elements  702 - 707  are irrelevant to the optical path reflections in this particular embodiment. 
   Here again, the transmit beam steering element array  114  provides at least two points of deflection for each of the optical beams  112  emitted by the optical transmitter elements  110  on the transmit port card  102 A. Specifically, the first reflective facet  702   1 ,  703   1 ,  704   1 ,  705   1 ,  706   1 ,  707   1  of each of the beam steering elements  702 ,  703 ,  704 ,  705 ,  706 ,  707  has a controllable deflection angle. Thus, the first reflective facet  7021  of beam steering element  702  is located such as to intercept the optical beam  112  emitted by a corresponding one of the optical transmitter elements  110  and to deflect it towards the first reflective facet  705   1  of beam steering element  705 . Similarly, the first reflective facet  704   1  of beam steering element  704  is fixed in a position where it intercepts the optical beam  112  emitted by another one of the optical transmitter elements  110  and deflects it towards the first reflective facet  707   1  of beam steering element  706 . Beam steering is provided by each of the two reflective facets encountered by each of the optical beams  112 , which affords a substantially increased total range of possible deflection angles, approximately doubling the maximum beam deflection when compared with the embodiments of  FIGS. 7B and 7C . 
     FIGS. 7B-7D  were presented with the plane of the substrate of the transmit port card  102 A in the plane of the page. A further set of solutions is rendered possible by rotating the structures through 90 degrees relative to the substrate. These solutions are shown in  FIGS. 7E-7G , where the transmit port card  102 A is being viewed from above (i.e., plan view). 
   In  FIGS. 7E and 7F , the beam steering element array  114  utilizes a strip mirror for at least one of the two deflections. Specifically, with reference to  FIG. 7E , there is shown a plan view of an edge of a particular transmit port card  102 A, with the uppermost optical transmitter element  110  being visible in the drawing, and producing an optical beam  112  underneath which there is an entire column of optical beams  112 , effectively forming a parallel optical beam front. The optical beam front impinges upon a strip mirror  740 , which deflects the optical beams  112  into deflected optical beams  742 . The strip mirror  740  has a fixed deflection angle and may be formed of a single, monolithic piece of material. The deflected optical beams  742  each impinge upon an individual beam steering element  744 , which has a reflective facet  746  with a controllable deflection angle. Thus, the transmit beam steering element array  114  provides two points of deflection for each of the optical beams  112  emitted by the optical transmitter elements  110  on the transmit port card  102 A. 
     FIG. 7F  shows a similar setup to that of  FIG. 7E , except that the roles of the strip mirror  740  and the beam steering elements  744  have been reversed. Thus, the reflective facets  746  of the beam steering elements  744  provide the first deflection for each of the optical beams  112  in the optical beam front, while the second deflection is provided by the strip mirror  740 . In this case, it is the angle of the first deflection, rather than the angle of the second deflection, which is controllable. 
   Yet another non-limiting example embodiment of the transmit beam steering element array  114  is shown in  FIG. 7G , using two columns of individual beam steering elements  752 ,  754 . The beam steering elements  752  comprise respective reflective facets  752   1 ,  754   1 , which provide two independently controllable deflection angles for each of the optical beams  112 . The combination of the two independently controllable deflective surfaces approximately doubles the achievable deflection angle, with a commensurate shortening of required optical path length, relative to the examples with single controlled deflection surfaces. However it also requires that the area of the second deflective surface be enlarged slightly. 
   The beam steering elements in the above-described examples of the transmit beam steering element array  114  can be implemented in many ways, one of which is now described with reference to  FIG. 6 . For convenience, the various beam steering elements, which took on reference numerals  702 ,  703 ,  704 ,  705 ,  706 ,  707 ,  752 ,  744 ,  752  and  754 , will be hereinafter referred to under the numeral  600  in  FIG. 6 . The basic structure of the beam steering element  600  described below is similar to technologies such as part number ADN59102 or part number ADN59210 available from Analog Devices, Norwood, Mass., USA. However, other embodiments of the beam steering element  600  are possible, in which different mechanisms are used. 
   In this example implementation, not to be considered a limitation but rather an example of what can be achieved using readily available technologies, the beam steering element  600  comprises a 3-D MEMS mirror  602  linked to a housing via two sets of torsion members  604 ,  606  (for the X and Y directions, respectively). A set of four (4) quadrant electrodes  608  on a nearby substrate  610  underlies the back surface (not shown) of the mirror  602 . The electrodes  608 , which may be implemented as plates under the surface of the mirror  602 , are driven with electrostatic drive voltages to cause the mirror  602  to move to a desired position in three-dimensional space against the tension of the torsion bar springs  604  linking the mirror  602  to the annulus and of the torsion bar springs  606  linking the annulus to the mirror surround. Specifically, the mirror  602  is activated by placing analog control voltages on each of the four electrodes  608  and exploiting electrostatic attraction to point the mirror  602  in a desired direction. 
   While it may be advantageous to have the substrate  610  close to the mirror  602  in order to achieve adequate deflection sensitivity without the use of inordinately high voltages, this proximity also limits the degree of deflection achievable with the mirror  602  before electrostatic attraction overcomes the torsion springs and the mirror “snaps-down” to make contact with the underlying electrode  608 . In current designs “snap-down” (whereby the electrostatic attraction overpowers the torsion of the torsion bar spring in a non-linear manner) can occur beyond 5-7 degrees of mechanical deflection, by which point the drive voltages may be approaching 150 volts, although it is envisaged that in future designs, the range of deflection may be greater due to the use of improved mechanisms for steering the mirror  602 . 
   As has been previously mentioned, the beam elements  600  in the transmit and receive beam steering element arrays  114 ,  140  on the transmit and receive port cards  102 A,  102 B are controlled by the control module  130  for the port card of interest, in response to switching instructions received from the switch controller  134 . Assume that the switching instructions require the A th  optical transmitter element  110  of the B th  transmit port card  102 A to emit an oriented optical beam  116  with the aim of eventually reaching the C th  optical receive element  124  of the D th  receive port card  102 B (via the reflector  120 , if any). The switching instructions are interpreted differently by the control module  130  on the B th  transmit port card  102 A and the control module  130  on the D th  receive port card  102 B. Specifically, the control module  130  on the B th  port card interpets the switching instructions as “connect the A th  optical transmitter element  110  to the C th  optical receive element  124  of the D th  receive port card  102 B”, whereas the control module  130  on the D th  receive port card  102 B interprets the switching instructions as “connect the C th  optical receive element  124  to the A th  optical transmitter element  110  of the B th  transmit port card  102 A”. The instructions to the B th  transmit port card  102 A ensure that the correct optical transmitter element  110  shines in the correct direction, while the instructions to the D th  receive port card  102 B ensure that the correct optical receive element  124  looks in the correct direction for incoming light. It is noted that the B th  transmit port card  102 A and the D th  receive port card  102 B may in fact be the same port card. 
   Reference is now made to  FIG. 8A , which shows the basic steps executed by the control module  130  responsible for the B th  transmit port card  102 A upon receipt of the switching instructions. At step  810 , the control module  130  responsible for the B th  transmit port card  102 A identifies the particular beam steering element in the transmit beam steering element array  114  responsible for providing a controllable deflection angle for the optical beam  112  emanating from the A th  optical transmitter element  110 . In addition, at step  820 , the control module  130  responsible for the B th  port card determines the X and Y drive voltages for that particular beam steering element, with the intent of establishing an optical path to the C th  optical receive element  124  on the D th  receive port card  102 B. 
   A similar process is carried out for the receive beam steering element array  140  on the D th  port card. Specifically, at step  810 , the control module  130  responsible for the D th  receive port card  102 B identifies the particular beam steering element in the receive beam steering element array  140  responsible for shining a beam into the rod lens of the C th  optical receive element  142 . In addition, at step  820 , the control module  130  responsible for the D th  port card determines the X and Y drive voltages for that particular beam steering element, with the intent of parallelizing a received optical beam  122  picked up in the direction from the A th  optical transmit element  110  on the B th  transmit port card  102 A. 
   Of course, it should be appreciated that if more than one beam steering element with a controllable deflection angle is used to deflect the optical beam  112  emanating from the A th  optical transmitter element  110  on the B th  transmit port card  102 A (or if more than one beam steering element with a controllable deflection angle is used to deflect the resultant received optical beam  122  at the receive beam steering element array  140 ), then step  810  would consist of identifying these plural beam steering elements and step  820  would consist of obtaining the X and Y drive voltages for each of these plural beam steering elements. However, for the sake of simplicity but without intending to limit the scope of the invention, it is hereinafter assumed that only one beam steering element in each of the transmit beam steering element array  114  and the receive beam steering element array  140  needs to be controlled for any given connection. 
   The control module  130  on either the B th  transmit port card  102 A or the D th  receive port card  102 B can perform step  820  in many ways. Consider the control module  130  on the B th  transmit port card  102 A for the sake of example. In one embodiment, step  820  will be performed by consulting a first lookup table (at step  822 ) followed by a second lookup table (at step  824 ). With reference to  FIG. 8B , the first lookup table  850  maps each combination of optical transmitter element  110  on the B th  transmit port card  102 A and possible optical receive element  124  (on any receive port card  102 B) to the required X and Y angular deflections for the beam steering element in the path of the optical beam  112  emanating from the optical transmitter element  110  in the combination. The second lookup table  860  (see  FIG. 8C ) maps the angular deflection per applied millivolt, for both the X and Y directions, for each beam steering element on the B th  transmit port card  102 A. Thus, consultation of the first lookup table  850  at step  822  results in obtaining the requisite angular deflection for the beam steering element in the transmit beam steering element array  114  located in the path of the A th  optical transmitter element  110 , whereas consultation of the second lookup table  860  at step  824  results in obtaining the drive voltages necessary for achieving the requisite angular deflection. A similar set of tables is used on the D th  port card to establish the drive voltages to the beam steering elements in the beam steering element array  140  so that they couple a beam from the appropriate direction into the optical receive elements  124  on the D th  port card. 
   The first lookup table  850  can be populated analytically from the physical geometry of the switch  100 , i.e., based on parameters such as the depth of the optical chamber  118 , the spacing between the port cards (i.e., pitch), as well as the presence or absence of a prism plate  300  (and its refractive characteristics, if present). Since the transmit port cards  102 A are interchangeable, it may be advantageous to store the first lookup table  850  in volatile memory to allow modification as the switch  100  is scaled, although this is not a requirement. 
   The second lookup table  860 , i.e., which maps angular deflection to applied voltage for the beam steering elements on a given transmit port card  102 A (e.g., the B th  transmit port card  102 A), can be populated during an initialization phase of the manufacturing process of the given transmit port card  102 A. By way of example, this initialization phase may entail pointing each beam steering element of the given transmit port card  102 A at a variety of test detectors in order to compute a “deflection sensitivity map” for the beam steering element. In one embodiment, this may require a large number of values for both the X, Y directions for each beam steering element. In another embodiment, a smaller number of vertical and horizontal locations is established, while the rest are computed by a polynomial “form-fit”. The voltages required to achieve specific deflections, whether obtained directly or through polynomial interpolation, form the second lookup table  860 . Since the second lookup table  860  is specific to the hardware on the given transmit port card  102 A, it may be advantageous to store the second lookup table  860  in non-volatile memory, although this is not a requirement. 
   As an alternative to maintaining the two lookup tables  850 ,  860 , a single composite lookup table could be created and stored in volatile memory, thus (in the case of the B th  transmit port card  102 A, for example) mapping each combination of optical transmitter element  110  (on the B th  transmit port card  102 A) and possible optical receive element  124  (on any receive port card  102 B) to the required X and Y voltages to be applied to the beam steering element in the path of the optical beam  112  emanating from the optical transmitter element  110  in the combination. Yet another alternative would be to fit a very high order polynomial to the values in such composite lookup table and to store the coefficients of the resultant polynomial. In this way, a polynomial computation is required on the part of the control module  130 , there will be a reduced need for memory, since only the coefficients of the polynomial need to be stored. 
   Regardless of the manner in which step  820  is performed, the result will be that (i) the oriented optical beam  116  resulting from action of the transmit beam steering element array  114  upon the optical beam  112  emanating from the A th  optical transmitter element  110  will be shone towards the reflector  120  in a direction that is intended to cause the received optical beam  122  to reach the C th  optical receive element  124  on the D th  receive port card  102 B; and (ii) the beam steering element array associated with the C th  optical receive element  124  on the D th  receive port card  102 B will capture an incoming optical beam  122  from the direction associated with the A th  optical transmitter element  110  on the B th  transmit port card  102 A and couple it into the C th  optical receive element  124 . With, say, a +/−7 degree full-scale deflection and a one-in-10,000 resolution, the use of the look up tables  850 ,  860  allows aiming of a particular beam steering element to a precision of about 0.7 milli-degrees, which, at the end of an optical path that may be of the order of a meter in length, results in an “aiming granularity” on the order of roughly 0.24 mm, i.e., the location of the end of the received optical beam  122  in three-dimensional space can be controlled with an initial pointing precision of 0.24 millimeters. 
   It should be noted that due to a variety of factors, one might not always be able to rely on the beam steering elements producing correctly aligned oriented optical beams  116  based on pre-computed lookup tables or polynomials. In other words, the above-defined pointing precision does not necessarily translate into a pointing accuracy. For instance, while it is possible to produce changes as small as 0.24 mm in the vertical or horizontal location of the received optical beams  122 , the initial oriented optical beam  116  may be misaligned to begin with, this despite the manufacturing calibration performed to produce the second lookup table  860 . Examples of possible error sources in obtaining consistent pointing accuracy include:
         repeatability of the setting of individual beam steering elements (e.g., MEMS mirrors  602 ); although it is expected to be excellent, there might be an unknown aging mechanism in the mirror deflection torsion members;   tolerances in the X, Y and Z positions occupied by individual transmit and receive port cards  102 A,  102 B as they are held in position by the slots of the card cage;   tolerances in the angular positions occupied by individual transmit and receive port cards  102 A,  102 B as they are held in position by the slots of the card cage;   errors in the angles of the strips  302  of the prism plate  300 ; with a refractive index of, say, 1.5, a one-degree facet angle will produce about 0.3-0.5 degree of pointing error, depending on the angle of incidence and other factors; based on what is commercially available for precision prisms it is reasonable to control facet angles (by precision grinding) to +/−0.01 degree or better giving rise to approximately 0.003-0.005 degrees of pointing error;   errors in the flatness of the reflector  120  and its angular positioning at the end of the optical chamber  118 ; assuming that the reflector  120  can be made optically flat, the main error will be the depth of the optical chamber  118 , which may have approximately 0.5 mm of depth error.       

   Assuming a 28 degree optical deflection cone (i.e., +/−7 degrees mechanical movement in each of the X and Y directions) and a path length of 1 meter, a tally of the worst-case error from the above sources may resemble the following: 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               digitization resolution/presets: 
               +/−0.12 mm 
             
             
                 
               card slot tolerance in X, Y, Z dimensions: 
                +/−0.2 mm 
             
             
                 
               card slot tolerance (angular): 
               +/−0.17 mm 
             
             
                 
               prism facet angle: 
               +/−0.09 mm 
             
             
                 
               reflector placement: 
               +/−0.53 mm 
             
             
                 
               TOTAL 
               +/−1.11 mm 
             
             
                 
                 
             
           
        
       
     
   
   With a mirror having dimensions of roughly 1 mm in diameter, the above worst-case cumulative error is sufficient for the received optical beam  122  to miss the target beam steering element in the receive beam steering element array  140 . 
   Now, using some example dimensions not indicative of any limitation or restriction of the present invention, if the pitch of the port cards is 7.5 mm and the spacing between adjacent optical transmitter elements  110  is greater than about 1.5 mm, then the use of the lookup tables  850  and  860  will orient the beam steering element in the transmit beam steering element array  114  so that the ensuing received optical beam  122  points somewhere in an imaginary circle of diameter 2.2 mm, centered on the target beam steering element in the receive beam steering element array  140 . 
   With reference to  FIG. 9A , this imaginary circle, hereinafter referred to as a “circle of uncertainty”  900 , surrounds an “area of detectability”  910  representative of the available detection area of the target beam steering element in the receive beam steering element array  140 . Assuming that the width of the received optical beam  122  at the end of its optical path to be 650 microns, it becomes apparent that although the received optical beam  122  might not be pointing directly towards the area of detectability  910 , it is nonetheless “close by”, i.e., somewhere in the surrounding circle of uncertainty  900 . Thus, the first challenge is to control the appropriate beam steering element in the transmit beam steering element array  114  so as to cause the received optical beam  122  to point directly at the area of detectability  910 , i.e., towards the center of the circle of uncertainty  900 . 
   Still, even with the received optical beam  122  pointing directly at the area of detectability  910 , it is possible that the target beam steering element in the receive beam steering element array  140  will cause an error in deflecting the received optical beam  122  towards the corresponding optical receive element  124  having its own “area of detectability”. With reference to  FIG. 9B , the use of the lookup tables  850  and  860  will orient the beam steering element in the receive beam steering element array  140  so that the ensuing deflected optical beam  142  will point somewhere in a circle of uncertainty  950  but not necessarily directly at the area of detectability  952  associated with the corresponding optical receive element  124  and whereby the optical receive element  124  can correctly focus the deflected optical beam  142  on to the exiting waveguide into the rest of the receive port card  102 B. Thus, the second challenge is to control the appropriate beam steering element in the receive beam steering element array  140  so as to cause the deflected optical beam  142  to impinge directly on the appropriate optical receive element  124 . It is noted that the circle of uncertainty  950  is somewhat smaller than the circle of uncertainty  900  due to the shorter distance between the beam steering element and the optical receive element  124 . 
   In order to shine the oriented optical beam  116  of interest onto the target beam steering element in the receive beam steering element array  140 , or in order to shine the received optical beam  122  onto the corresponding optical receive element  124 , the control module  130  on the appropriate transmit or receive port card  102 A,  102 B performs a “fine tuning process”, which is optional. In other words, it should be understood that the discussion to follow is merely illustrative of an example way to improve the pointing accuracy when such improvement is desired, and in no way implies the necessity to improve the pointing accuracy. Depending on the quality and tolerances of the components of the switch  100 , it may or may not be sought to improve the pointing accuracy afforded by straightforward execution of step  820  in the control module  130  of both the transmit port card  102 A and the receive port card  102 B. 
   Expressed in general terms, the fine tuning process solves the problem of locating an area of detectability (e.g.,  910 ,  952 ) from somewhere in a surrounding circle of uncertainty (e.g.,  900 ,  950 ). To this end, the controller  130  on the transmit port card  102 A causes a controlled and variable level of sinusoidal modulation voltages (tones) to be added in phase quadrature to the X and Y drive voltages applied to the beam steering element in the transmit beam steering element array  114  which emits the oriented optical beam  116  of interest. Similarly, the controller  130  on the receive port card  102 B causes a controlled and variable level of sinusoidal modulation voltages (tones) to be added in phase quadrature to the X and Y drive voltages applied to the beam steering element in the receive beam steering element array  140  which deflects the received optical beam  122  of interest towards the corresponding optical receive element  124 . 
   In the case of each or either beam steering element being implemented as a mirror  602  (see  FIG. 6 ), the modulation voltages are applied by sinusoidally varying the voltages applied to the electrodes  608  responsible for movement in the +X and +Y directions, but at opposite phases between the +X, −X electrodes and between the +Y and −Y electrodes, and in phase-quadrature between the +X, +Y electrodes. As shown in  FIG. 9C  for example, the addition of modulation voltages in phase quadrature in the above described way causes the oriented optical beam  116  deflected by the mirror  602  to be driven into an angular displacement (“wobble” or “precession”) which sweeps an orbital trajectory  902  with a period corresponding to the frequency of the modulation voltages. 
   The amplitude of the modulation voltages are designed (or can be controlled) to make the orbital trajectory  902  sufficiently wide so as to intersect the area of detectability  910 . For ease of understanding, it will be assumed in what follows that the modulation voltages applied to the X and Y drive voltages cause the oriented optical beam  116  to precess at a frequency (or “precession tone”) f T . However, applying different modulation voltages to the X and Y drive voltages changes the trajectory  902  and controls the precession orbit diameter, and it should be understood that such modifications to the trajectory  902  are well within the scope of the present invention. 
   A similar technique process is applied to when deflecting the received optical beam  122  towards the corresponding optical receive element  124 . In this case, the amplitude of the modulation voltages are designed (or can be controlled) to make the orbital trajectory of the deflected optical beam  142  sufficiently wide so as to intersect the area of detectability  952  of the optical receive element  124 . For ease of understanding but without limiting the scope of the present invention, it will be assumed in what follows that the modulation voltages applied to the X and Y drive voltages cause the deflected optical beam  952  to precess at a frequency ƒ R . 
   By tapping a small amount of the received optical signal into the receive port card  102 B and detecting that signal in an opto-electronic receiver, after the optical signal has completed its transition into the waveguide environment of the receive port card and by analyzing the frequency, amplitude and phase of the precession tones ƒ T , ƒ R  present in the optical signal detected as being received at the optical receive element  124 , and comparing these parameters to those of the precession tone expected to be received by the optical receive element  124 , one can compute the “pointing error”, both in directing the oriented optical beam  116  at the transmit beam steering element array  114 , and in deflecting the received optical beam  122  at the receive beam steering element array  142 . 
   Specifically, the presence of a precession tone at frequency ƒ T  in the received optical signal indicates that the received optical beam  122  is in the correct circle of uncertainty  900  to begin with, while the amplitude of the received optical signal is indicative of the radial distance of the center of the trajectory  902  from the area of detectability  910 , and the relative phase of the received optical signal is indicative of the angle at which the center of the trajectory  902  is located relative to the area of detectability  910 . This allows computation of a horizontal displacement correction dH T  and a vertical displacement correction dV T  required to properly align the oriented optical beam  116 . 
   Similarly, the presence of a precession tone at frequency ƒ R  in the received optical signal indicates that the received optical beam  122  is in the correct circle of uncertainty  950  to begin with, while the amplitude of the received optical signal is indicative of the radial distance of the center of the deflected optical beam  142  from the area of detectability  952 , and the relative phase of the received optical signal is indicative of the angle at which the center of the deflected optical beam  142  is located relative to the area of detectability  952 . This allows computation of a horizontal displacement correction dH R  and a vertical displacement correction dV R  required to properly align the deflected optical beam  142 . 
   It may be convenient to assign different sets (or ranges) of potential values to ƒ T  and ƒ R , in order to assist in discriminating between transmit and receive precession tone frequencies. Furthermore, to simplify the separation of the composite signal from the detector into the components ƒ T  and ƒ R  of the resultant detected signal (which will contain both of the transmit and receive precession tone frequencies), it may be convenient to use techniques including but not limited to separating the ranges of f T  and ƒ R  by a substantial factor (e.g., 10:1 or more) or to use a form of orthogonal modulation of the ƒ T , ƒ R  components to simplify detectability of each in the presence of the other. 
   Detection of the precession tones at frequencies ƒ T  and ƒ R  in the optical signal received at the optical receive element  124  can be achieved using a circuit as shown in  FIG. 10 , which comprises an optical detector  920  connected to the output of the optical receive element  124  via an optical coupler  922 . The optical detector  920  may be implemented as a photodiode, while the optical coupler  922  may be implemented as a fractional tap coupler. A processing unit  924  is connected to the optical detector  922  and possibly other optical detectors associated with other optical receive elements  124 . The processing unit  924  is shown as residing on the receive port card  102 B, although it should be appreciated that the processing unit  924  associated with a given receive port card  102 B can be located on that receive port card  102 B itself, on another port card, on a separate “controller card”, or multiple processing units  924  can be consolidated onto a smaller number of separate controller cards, which may be the same controller cards that support the control units  130  if these are consolidated as well. 
   The processing unit  924  has the role of determining the frequency, amplitude and phase of the precession tones present in the optical signal detected as being received at the optical receive element  124 . It is assumed that the processing unit  924  knows ƒ R  and ƒ T  based on the connection map. In the manner described above, the processing unit  924  computes dH T , dV T , dH R  and dV R . The values dH T  and dV T  are supplied to control module  130  responsible for the transmit beam steering element array  114  that emits the oriented optical beam  116 . This can be achieved by using the same data bus  138  used to carry the switching instructions to the various transmit port cards  102 A, for example. The values dH R  and dV R  are supplied to control module  130  responsible for the receive beam steering element array  140  that emits the deflected optical beam  142 . 
   In the context of the fine tuning process, the behaviour of the control module  130  responsible for the transmit beam steering element array  114  that emits the oriented optical beam  116  for a particular combination of optical transmitter element  110  and optical receive element  124  is now described with reference to the flowchart in  FIG. 11 . A virtually identical flowchart applies to the control module  130  responsible for the receive beam steering element array  140  that provides the deflected optical beam  142  to the optical receive element  124  of this combination, based on the values dH R  and dV R . For simplicity, only the process for controlling the transmit beam steering element array  114  will be described in detail, it being assumed that a person skilled in the art will be able to modify this process and apply it to the receive beam steering element array  140 . 
   It will be seen that step  822  is the same as in  FIG. 8 , and consists of consulting of the first lookup table  850  to obtain the requisite angular deflection for the beam steering element which outputs the oriented optical beam  116 . At this point, the control module  130  executes step  1110 , which consists of receiving the values dH T  and dV T  from the processing unit  924  on the receive port card  102 B which houses the optical receive element  124  of the particular combination in question. At step  1112 , the control module  130  checks to see whether the fine tuning process has previously been started for the particular combination of optical transmitter element  110  and optical receive element  124 . If not, then step  1114  is executed, where the values dH T  and dV T  are used to compute a pointing error that is compared to a “trigger threshold”. The trigger threshold  1114  is selected to represent a pointing error that is sufficiently large to require the fine tuning process to be initiated or re-initiated. Clearly, the trigger threshold is an arbitrary design parameter based upon the specific tolerances, dimensions and sensitivities of specific design implementations and its selection would be a matter of routine for a person of ordinary skill in the art. 
   If the pointing error is indeed greater than the trigger threshold, the control module  130  executes step  1116 , where the fine tuning process is formally started, followed by step  1118 , by virtue of which the control module  130  begins the act of monitoring the “net angular compensation” as applied (to be seen in later steps) to the X and Y angular deflection for the current combination of optical transmitter element  110  and optical receive element  124 . While not used right away, the value of this “net angular compensation” at the end of the fine tuning process will indicate by how much the angular deflection shown in the first lookup table  850  should have been varied in order to cause the received optical beam  122  to have been shone directly onto the area of detectability  910 . 
   After execution of step  1118 —or if execution of step  1114  indicates that the pointing error was not greater than the trigger threshold, the control module  130  proceeds to step  1120 , where an angular compensation for the pointing error is computed. The computed angular compensation can be as great in absolute value as the pointing error computed from the values dH T  and dV T  received from the processing unit  924 ; however, it can be less in absolute value, so as to encourage stability of the feedback control loop having been created. At step  824 , the X and Y drive voltages are obtained from the second lookup table  860  by looking up the angular deflection obtained at step  822  but compensated by the value found at step  1120 . Finally, as the final step in the fine tuning process, step  1122  is executed, where each of the X and Y drive voltages is modulated by a precession tone having a particular frequency ƒ T  and a particular amplitude as discussed herein below. The fine tuning process subsequently returns to step  1110 , where new values dH T  and dV T  are received from the processing unit  924 . If the fine tuning process is running successfully, then it is expected that the pointing error that is computed from the values dH T  and dV T  received during the next iteration of step  1110  will be no greater (in absolute value terms) than the one during the previous iteration of step  1110 . 
   As indicated above, each of the X and Y drive voltages is modulated at step  1122  by a precession tone, which has a particular “precession amplitude” that should not be excessively large or exceedingly small. Specifically, it will be appreciated that when the center of the trajectory  902  of the received optical beam  122  is far off from the center of the area of detectability  910 , then too small a precession amplitude will cause the trajectory  902  of the received optical beam  122  to make small circles that never intersect the area of detectability  910 . On the other hand, too large a precession amplitude once the center of the trajectory  902  of the received optical beam  122  has become aligned with the center of the area of detectability  910  (i.e., after “convergence” has been achieved) will cause the trajectory  902  of the received optical beam  122  to make big circles that also never intersect the area of detectability  910 . For this reason, it may be advantageous during the fine tuning process to begin with a larger amplitude before convergence and to gradually decrease the amplitude of the precession tone as convergence is achieved, and to continue doing so until it is noticed that the pointing error has dropped to below a convergence threshold. 
   The above described approach translates into additional steps in the flowchart of  FIG. 11 . Specifically, returning to step  1112  and assuming that execution of this step indicates that the fine tuning process has already been started (i.e., due to previous execution of step  1116 ), then the pointing error computed at step  1110  is compared to a “convergence threshold” at step  1124 . The convergence threshold represents the amount of pointing error considered to be sufficiently small to indicate that the received optical beam  122  is satisfactorily centered within the area of detectability  910 . Clearly, the convergence threshold is an arbitrary design parameter and its selection would be a matter of routine for a person of ordinary skill in the art. 
   If the pointing error is greater than the convergence threshold, then there continues to be a need to center the received optical beam  122  within the area of detectability  910 . Thus, the control module  130  proceeds to step  1126 , where the value of the pointing error computed during the current iteration of step  1110  is compared to the value of the pointing error computed during the previous iteration of step  1110 . If it is greater, then this effectively means that the received optical beam  122  has moved further from the center of the area of detectability  910 , in which case it may be desirable to increase the amplitude of the precession tone (step  1130 ), so as to ensure that it will intersect the area of detectability  910 . If it is less, then this effectively means that the received optical beam  122  has moved closer to the center of the area of detectability  910 , in which case it may be desirable to decrease the amplitude of the precession tone (step  1128 ), so as to ensure that the optical beam will not remain entirely outside the area of detectability  910  as it precesses. 
   Upon having decided on how to modify the amplitude of the precession tone, steps  1120 ,  824  and  1122  are executed as previously described. It should be understood that control of the amplitude of the precession tone (steps  1126 - 1130 ) can be effected using a more sophisticated algorithm, and in some cases the amplitude of the precession tone need not be varied at all, or it may be varied differently in the X and Y directions, or it may be varied in a manner that is independent of the magnitude of the pointing error received at step  1110 . 
   As centering of the received optical beam  122  within the area of detectability  910  is achieved over time, the pointing error will eventually fall below the convergence threshold, and the “YES” branch emanating from step  1124  is taken, followed by stoppage of the fine tuning process at step  1132 . Next, step  1134  provides for the tallying of the net angular compensation (computed at each execution of step  1120 ) over the duration of the fine tuning process since it was started at step  1116 . The net angular deflection so tallied represents a correction to the angular deflection that is currently maintained in “row” of the first lookup table  850  corresponding to the current combination of optical transmitter element  110  and optical receive element  124 . Accordingly, step  1136  provides the option of modifying this “row” of the first lookup table  850  by the amount of the net angular compensation. By making this modification to the first lookup table  850 , the fine tuning will be accelerated in the event that the current combination of optical transmitter element  110  and optical receive element  124  is disconnected but then needs to be re-connected at a future time. Finally, since convergence has been achieved, there is no need to compensate the angular deflection that was initially obtained at step  822  (i.e., step  1120  can be skipped). Also, the X and Y drive voltages remain the same as before (i.e., step  824  can be skipped) and the precession amplitude need not be changed (i.e., step  1122  can be skipped). The algorithm thus returns to step  1110 , where new values dH T  and dV T  are received from the processing unit  924 . 
   To illustrate the effects of fine tuning process, reference is had to  FIG. 12 , which shows corrections being applied over time (at instants T 1 , T 2 , T 3 , T 4 ) to fine tune the angular deflections of the beam steering element producing the oriented optical beam  116 . It is apparent that the precession amplitude is reduced until, eventually, the detected precession tone is small enough in amplitude that the received optical beam  122 , when precessing but locked on target, remains fully within the area of detectability  910 . In other words, at instant T 4 , the processing unit  924  detects a full-strength signal but no precession tone. 
   By continuing the precessing motion during normal operation of the switch  100  (i.e., even after convergence), it is possible to detect if and when the deflected optical beam  120  ceases to be fully within the area of detectability  910 . If the received optical beam  122  partly exits the area of detectability  910 , the precessing motion of the oriented optical beam  116  will cause the received optical beam  122  to oscillate in and out of the area of detectability  910 , and this will be detected by the processing unit  924 . However, in other embodiments, the control module  130  may, in response to convergence, end the fine tuning process by simply stopping the precessing motion of the received optical beam  122 . 
   It will be appreciated that when the switching instructions change, the control module  130  responsible for each transmit port card  102 A responds accordingly by re-executing the algorithm in  FIG. 8  and obtaining new X and Y drive voltages for the various beam steering elements associated with the optical transmitter elements  110  on the transmit port card  102 A in question. If appropriate, the fine tuning process described above and with reference to  FIG. 11  is re-initiated for each new combination of optical transmitter element  110  and optical receive element  124 . It is noted that if step  1136  has previously been executed for the new combination of optical transmitter element  110  and optical receive element  124 , then the fine tuning process will be dramatically shortened, since the first lookup table  850  will already be pre-compensated. Any further fine tuning will be due to equipment aging having occurred since the previous time the particular combination of optical transmitter element  110  and optical receive element  124  needed to be connected, and re-execution of the fine tuning process allows the effects of such equipment aging to be detected, tracked and compensated. 
   As an alternative or an enhancement of the fine tuning process, an out-of-service calibration procedure can be used, as now described with reference to  FIG. 1 . Under such circumstances, one or more test cards  180 ,  182  are provided. For example, in the case where there are two test cards  180 ,  182 , one could be positioned in each of the leftmost and rightmost slots of the card cage. In an alternative embodiment, individual ones of the transmit or receive port cards  102 A,  102 B could be temporarily removed and replaced with a single test card. Each of the test cards  180 ,  182 , rather than containing beam steering elements for providing the requisite parallelization of the received optical beams  122 , contains a fixed array  184  of small photodiodes (not unlike a CCD in a digital camera) which will allow for the measuring of the actual location and distribution of a received optical beam  122 . The test card  180  also comprises a control module  186  which processes the output of the photodiode array  184 . 
   As part of the out-of-service calibration procedure, the A th  optical transmitter element  110  on the B th  transmit port card  102 A is selected (and this selection is known to the control module  130  on the B th  port card as well as the control module  186  on the test card  180 ), and the corresponding optical beam  112  is deflected by the transmit beam steering element array  114  on the B th  port card with the intention of reaching a chosen “C th  optical receive element” on one of the test cards, say test card  180 . However, the “C th  optical receive element on test card  180 ” is imaginary because the test card  180  does not contain optical receive elements but instead contains a photodiode array  184  which might conveniently be implemented as an array similar to a small CCD array as is used in digital camera technology. The photodiode array  184  detects the exact location of the received optical beam  122 , which may (but likely will not) correspond to the position that would have been occupied by the beam steering element corresponding to the “C th  optical receive element” had it been present. This difference in positions represents a pointing error, which is then converted into a compensation signal, and the process is repeated until the pointing error is sufficiently low to be considered satisfactory. 
   The above-described approach allows the major sources of tolerances (e.g., due to prism wedge angle and backplane spacing) to be compensated for before the first “in-traffic” connection, thereby either simplifying the precession routine or permitting less stringent tolerances on the backplane mirror, prism sheet, card positioning, etc. 
   Those skilled in the art will appreciate that in some embodiments, the functionality of the control module  130  and the processing module  924  may be implemented as pre-programmed hardware or firmware elements (e.g., application specific integrated circuits (ASICs), electrically erasable programmable read-only memories (EEPROMs), etc.), or other related components. In other embodiments, the control module  130  and the processing module  924  may be implemented as an arithmetic and logic unit (ALU) having access to a code memory (not shown) which stores program instructions for the operation of the ALU. The program instructions could be stored on a medium which is fixed, tangible and readable directly by the control module  130  and the processing module  924 , (e.g., removable diskette, CD-ROM, ROM, or fixed disk), or the program instructions could be stored remotely but transmittable to the control module  130  and the processing module  924  via a modem or other interface device (e.g., a communications adapter) connected to a network over a transmission medium. The transmission medium may be either a tangible medium (e.g., optical or analog communications lines) or a medium implemented using wireless techniques (e.g., microwave, infrared or other transmission schemes). 
   While specific embodiments of the present invention have been described and illustrated, it will be apparent to those skilled in the art that numerous modifications and variations can be made without departing from the scope of the invention as defined in the appended claims.