Abstract:
An exchange switch for an optical switching network utilizes the manipulation of fluid to selectively exchange outputs for two optical transmission paths. That is, when the switch is in a signal-exchange state, first and second transmission paths reciprocally exchange optical signals that are received at input waveguides of the transmission paths. In one embodiment, the exchange switch includes an optical switching arrangement having first and second switching members. The first switching member is positioned to selectively interrupt the continuation of signal propagation along the first transmission path, while the second switching member is positioned to selectively interrupt the continuation of signal propagation along the second transmission path. When fluid resides within the chambers of the two switching members, the exchange switch is in a signal-continuation state. However, by evacuating fluid from the chambers, a signal propagating along one of the transmission paths will be reflected into an exchange waveguide that transfers the optical signal to the other transmission path. In other embodiments, the exchange switch includes at least one fluid-manipulable chamber and a steady-state reflector. The different elements cooperate to enable either signal continuation or signal exchange, depending on the states of the fluid-manipulable chambers.

Description:
TECHNICAL FIELD 
     The invention relates generally to optical switching arrangements and more particularly to exchange switches that enable two or more transmission paths to exchange outputs. 
     BACKGROUND ART 
     It is conventional to transfer information between nodes of a telecommunications and data communications network by transmitting electrical signals via electrically conductive lines. However, an alternative medium of data exchange is the transmission of optical signals through optical fibers. Information is exchanged in the form of modulations of laser-produced light. The equipment for efficiently generating and transmitting the optical signals has been designed and implemented, but the design of optical switches for use in telecommunications and data communications networks is problematic. As a result, switching requirements within a network that transmits optical signals is often satisfied by converting the optical signals to electrical signals at the inputs of a switching network, then reconverting the electrical signals to optical signals at the outputs of the switching network. 
     Recently, reliable optical switching systems have been developed. U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention, describes a switching matrix that may be used for routing optical signals from parallel input optical fibers to parallel output optical fibers. Another such matrix of switching elements is described in U.S. Pat. No. 4,988,157 to Jackel et al. An isolated switching element  10  is shown in FIG. 1, while a 4×4 matrix  32  of switching elements is shown in FIG.  2 . The optical switch of FIG. 1 is formed on a substrate. The substrate may be a silicon substrate, but other materials may be used. The optical switch  10  includes planar waveguides defined by a lower cladding layer  14 , a core  16 , and an upper cladding layer, not shown. The core is primarily silicon dioxide, but with other materials that achieve a desired index of refraction for the core. The cladding layers should be formed of a material having a refractive index that is substantially different from the refractive index of the core material, so that optical signals are guided along the waveguides. 
     The material core  16  is patterned to form an input waveguide  20  and an output waveguide  26  of a first optical path and to define a second input waveguide  24  and a second output waveguide  22  of a second optical path. The upper cladding layer is then deposited over the patterned core material. A gap  28  is formed by etching a trench through the core material and the two cladding layers to the substrate. The waveguides intersect the trench at an angle of incidence greater than the critical angle of total internal reflection (TIR) when the location  30  aligned with the waveguides is filled with a vapor or gas. Thus, TIR diverts light from the input waveguide  20  to the output waveguide  22 , unless an index-matching fluid resides within the location  30  between the aligned waveguides  20  and  26 . The trench  28  is positioned with respect to the four waveguides such that one sidewall of the trench passes through or slightly offset from the intersection of the axes of the waveguides. 
     The above-identified patent to Fouquet et al. describes a number of alternative approaches to switching the switching element  10  between a transmissive state and a reflective state. The element includes at least one heater that can be used to manipulate fluid within the gap  28 . One approach is illustrated in FIG.  1 . The switching element  10  includes two microheaters  50  and  52  that control the position of a bubble within the fluid-containing gap. The fluid within the gap has a refractive index that is close to the refractive index of the core material  16  of the four waveguides  20 - 26 . Fluid fill-holes  54  and  56  may be used to provide a steady supply of fluid, but this is not critical. In the operation of the switching element, one of the heaters  50  and  52  is brought to a temperature sufficiently high to form a gas bubble. Once formed, the bubble can be maintained in position with a reduced current to the heater. In FIG. 1, the bubble is positioned at the location  30  of the intersection of the four waveguides. Consequently, an input signal along the waveguide  20  will encounter a refractive index mismatch upon reaching the gap  28 . This places the switching element in a reflective state, causing the optical signal along the waveguide  20  to be redirected to the output waveguide  22 . However, even in the reflective state, the second input waveguide  24  is not in communication with the output waveguide  26 . 
     If the heater  50  at location  30  is deactivated and the second heater  52  is activated, the bubble will be attracted to the off-axis heater  52 . This allows index-matching fluid to fill the location  30  at the intersection of the waveguides  20 - 26 . The switching element  10  is then in a transmitting state, since the input waveguide  20  is optically coupled to the collinear waveguide  26 . 
     In the 4×4 matrix  32  of FIG. 2, any of the four input waveguides  34 ,  36 ,  38  and  40  may be optically coupled to any one of the four output waveguides  42 ,  44 ,  46  and  48 . The switching matrix is sometimes referred to as a “non-blocking” matrix, since any free input fiber can be connected to any free output fiber, regardless of which connections have already been made through the switching matrix. Each of the sixteen optical switches has a gap that causes TIR in the absence of a fluid at the location between collinear waveguides, but collinear waveguides of a particular waveguide path are optically coupled when the locations between the waveguides are filled with the fluid. Trenches that are in the transmissive state are represented by fine lines that extend at an angle through the intersections of the optical waveguides in the matrix. On the other hand, trenches of switching elements in a reflective state are represented by broad lines through points of intersection. 
     In FIGS. 1 and 2, the input waveguide  20  is in optical communication with the output waveguide  22 , as a result of TIR at the empty location  30  of the gap  28 . Since all other cross points for allowing the input waveguide  34  to communicate with the output waveguide  44  are in a transmissive state, a signal that is generated at input waveguide  34  will be received at output waveguide  44 . In like manner, the input waveguide  36  is optically coupled to the first output waveguide  42 , the third input waveguide  38  is optically coupled to the fourth output waveguide  48 , and the fourth input waveguide  40  is optically coupled to the third output waveguide  46 . 
     In FIG. 1, the second input waveguide  24  can be optically coupled to the second output waveguide  22 , but is isolated from the output waveguide  26 . This is because the axes of the waveguides  24  and  26  do not intersect at the trench wall where reflection will occur when the switching element is in the reflective state. That is, in the reflective state of FIG. 1, an optical signal along the second input waveguide  24  is unable to reach either of the two output waveguides  22  and  26 , since total internal reflection will occur at the interface of the second input waveguide  24  with the trench  28 . 
     It follows that the matrix  32  of FIG. 2 has limitations regarding transferring signals among waveguides. While any of the four input waveguides  34 - 40  can be connected to any one of the downwardly extending output waveguides  42 - 48 , each input waveguide can be optically coupled to only one of the rightwardly extending waveguides  58 ,  60 ,  62  and  64 . Specifically, each input waveguide can be connected to only the rightwardly extending waveguide that is optically aligned with that input waveguide. 
     What is needed is an exchange switch for an optical switching network, so that a greater versatility in directing optical signals among available transmission paths is achieved. 
     SUMMARY OF THE INVENTION 
     An exchange switch for an optical switching network manipulates fluid into and out of alignment with optical transmission paths in order to enable exchanges of optical signals between two separate transmission paths. That is, the fluid is manipulated to allow two inputs to exchange outputs. The exchange switch includes an optical switching arrangement that is defined by at least two fluid-manipulable chambers. The optical switching arrangement has a signal-continuation state in which optical signals remain on their respective transmission paths without interruption and has a signal-exchange state in which the optical coupling to outputs is reversed. 
     In a first embodiment, the optical switching arrangement includes first and second switching members. The transmission paths are formed of waveguides. The first switching member is positioned to selectively interrupt the continuation of the first transmission path, while the second switching member is positioned to selectively interrupt the continuation of the second transmission path. The switching members have transmissive conditions in which fluid resides along the transmission paths and have reflective conditions in which there is an absence of fluid along the transmission paths. The fluid has a refractive index that generally matches the refractive index of the core material of the waveguides. Thus, when the fluid of a switching member is aligned with the corresponding transmission path, optical signals propagate through the fluid from an input waveguide to an output waveguide. On the other hand, when the fluid of a switching member is misaligned with the corresponding transmission path, reflection occurs at the interface of the input waveguide and the fluid-manipulable chamber of the switching member. 
     In this first embodiment, an exchange path intersects the input waveguide of the first transmission path at the first switching member. Consequently, when the first switching member is in the reflective state, an optical signal along the input waveguide is reflected to the first exchange path. The opposite end of the first exchange path intersects the output waveguide of the second transmission path at the second switching member. As a result, when the first and second switching members are both in the reflective condition, an optical signal along the first transmission path is transferred to the output waveguide of the second transmission path. 
     A second exchange path is connected in a similar manner to that of the first exchange path, but is optically coupled between the input waveguide of the second transmission path and the output waveguide of the first transmission path. When both of the switching members are in the reflective condition, an optical signal along the second transmission path is transferred to the output waveguide of the first transmission path. 
     In a second embodiment, the switching arrangement includes steady-state reflectors in addition to the fluid-manipulable chambers. While not critical, the reflectors may be gas-filled chambers that are similar to the fluid-manipulable chambers. This minimizes the number of different types of components that must be fabricated in order to form a switching matrix. In the exchange of signals from input waveguides of first and second linear transmission paths to output waveguides of the transmission paths, at least one steady-state reflector and at least one fluid-manipulable chamber must be utilized. 
     A single fluid-manipulable chamber may be used in this second embodiment by forming the first and second transmission paths to define an X configuration in which the axis of the input waveguide of the first transmission path intersects the axis of the output waveguide of the second transmission path at one wall of the fluid-manipulable chamber. Thus, when the chamber is in the signal-exchange state, the optical signals from the input waveguide of the first transmission path will be reflected directly to the output waveguide of the second transmission path. However, since the input waveguide of the second transmission path will not intersect the output waveguide of the first transmission path, the steady-state reflector is utilized to optically couple the two waveguides when the signal-exchange state is established. A first exchange waveguide extends from the input waveguide of the second transmission path to the reflector, and a second exchange waveguide extends from the reflector to the intersection of the fluid-manipulable chamber with the output waveguide of the first transmission path. This causes the optical signals in the input waveguide to follow a W-shaped path to the output waveguide of the other transmission path. 
     As an alternative second embodiment, a number of separate fluid-manipulable chambers may be used with the reflector to form an optical switching arrangement for first and second transmission paths. For example, there may be three fluid-manipulable chambers, with one chamber intersecting both of the transmission paths. In the signal-continuation state of the switching arrangement, optical signals along each of the two transmission paths must pass through the common chamber and one of the other two chambers. In the signal-exchange state, optical signals from the first transmission path are reflected to the second transmission path by the common chamber, while optical signals from the second path must follow a W-shaped path similar to the one described above. 
     An advantage of switching arrangements in accordance with the invention is that optical signals are freely transferrable from one transmission path to another transmission path. This increases the versatility of switching matrixes in optical signal networks. Versatility can be further enhanced by utilizing a matrix of the optical switching arrangements with a matrix of switching elements described in the above-identified patent to Fouquet et al. (U.S. Pat. No. 5,699,462). The combination of the two types of matrixes allow test signals and signal “dumping” to be incorporated into the network. The Fouquet et al switching matrix may be incorporated at either or both of the input and output ends of the matrix of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of an optical switching element that utilizes total internal reflection in accordance with the prior art. 
     FIG. 2 is a 4×4 matrix of switching elements of FIG. 1 to allow connection of the input waveguides to the output waveguides of the matrix in accordance with the prior art. 
     FIG. 3 is a top view of a first embodiment of a signal exchange switch in accordance with the invention, with the switch being shown in a signal-continuation state. 
     FIG. 4 is a top view of the switch of FIG. 3 shown in a signal-exchange state. 
     FIG. 5 is a top view of a second embodiment of an optical switching arrangement shown in a signal-continuation state. 
     FIG. 6 is a top view of the switching arrangement of FIG. 5 shown in a signal-exchange state. 
     FIG. 7 is a top view of a third embodiment of a optical switching arrangement in accordance with the invention, shown in the signal-continuation state. 
     FIG. 8 is a top view of the switching arrangement of FIG. 7 shown in a signal-exchange state. 
     FIG. 9 is a schematic representation of the switching arrangement of FIGS. 3 and 4. 
     FIG. 10 is a schematic representation of a first topology of an optical switching network using a matrix of switching elements of FIGS. 3 and 4. 
     FIG. 11 is a schematic representation of a second optical switching network that utilizes a matrix of the switching arrangement of FIGS. 3 and 4. 
     FIG. 12 is a schematic representation of a switching arrangement in accordance with the second or third embodiments shown in FIGS. 5-8. 
     FIG. 13 is a schematic representation of a steady-state reflector for use in the network topologies of FIGS. 14 and 15. 
     FIGS. 14 and 15 are schematic representations of two examples of network topologies using the optical arrangements of FIGS.  5 - 8 . 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 3, a preferred embodiment of an exchange switch  66  is shown as including two separate fluid-manipulable chambers  68  and  70 . The chambers are shown in the transmissive state, i.e., the condition in which fluid resides within the chamber. In contrast, the chambers are shown in the reflective state in FIG. 4, since the fluid has been removed from a substantial portion of the chamber. 
     The approach for manipulating the fluid within the chambers  68  and  70  is not critical to the invention. One acceptable approach is to utilize a number of microheaters that control the position of a bubble within a fluid supply. This approach was described above, with reference to FIG.  1 . At least one heater may be brought to a temperature sufficiently high to form a gas bubble. Once formed, the bubble can be maintained in position with a reduced current to the heater. If the heater is deactivated and a second heater is activated, the bubble will be attracted to the second heater. By properly positioning the different heaters, the bubble can be manipulated to toggle each chamber of the exchange switch  66  between transmissive and reflective states. Another approach to manipulating the fluid is to use inkjet techniques for firing fluid into and/or out of the chambers. 
     The exchange switch  66  includes a first input waveguide  72  and a first output waveguide  74  to define a transmission path that intersects the fluid-manipulable chamber  68 . The waveguides of the exchange switches to be described below may be formed of a core material of silicon dioxide, but with one or more dopants that are selected to achieve a desired index of refraction for the core. While not shown, cladding layers are formed to encapsulate the core material. The cladding material is selected to provide a substantially different refractive index, so that optical signals are guided along the core material. 
     In FIGS. 3 and 4, a second transmission path is defined by coaxial second input and output waveguides  76  and  78 . Two exchange waveguides  80  and  82  intersect the input waveguides  72  and  76  at first ends and intersect the output waveguides  74  and  78  at the opposite ends. The axes of the exchange waveguides intersect the axes of the input and output waveguides at the interfaces with the surfaces of the fluid-manipulable chambers  68  and  70 . The intersecting axes are at angles greater than 90 degrees and less than 150 degrees. A more preferred range is 96 degrees to 135 degrees. 
     In operation, when the fluid within the chambers  68  and  70  fills the region along the two transmission paths, optical signals from the first input waveguide  72  will be output via the first output waveguide  74  and optical signals along the second input waveguide  76  will be output via the second output waveguide  78 . The fluid is selected such that there is a close match between the indices of refraction of the fluid and the core material of the waveguides. Thus, in the signal-continuation state of the exchange switch  66 , the exchange waveguides  80  and  82  will be inactive. In FIG. 3, signal propagation that occurs while the exchange switch is in this state is represented by arrows  84  and  86 . 
     In FIG. 4, the exchange switch  66  is shown in the signal-exchange state. Fluid within the chambers  68  and  70  is no longer aligned with the transmission paths through the chambers. As a result, total internal reflection (TIR) occurs when an optical signal impinges upon a surface of one of the chambers. This is represented by arrows  88 ,  89 ,  90  and  91 . Arrows  88  and  90  represent an exchange of an optical signal from the input waveguide  72  to the output waveguide  78 , while arrows  89  and  91  represent an exchange of an optical signal from the input waveguide  76  to the output waveguide  74 . 
     FIGS. 3 and 4 show the output waveguides  74  and  78  as being axially aligned with the corresponding input waveguides  72  and  76 . However, there may be advantages to providing some offset between the corresponding input and output waveguides. If the refractive index of the fluid within the chambers  68  and  70  is imprecisely matched with the refractive index of the core material of the waveguides, some refraction will occur as the optical signals enter and exit the chambers. An offset may be used to compensate for the refraction. 
     While the waveguides have been described as being layers formed on a substrate, other structures for guiding optical signals may be utilized in the exchange switch  66 , without departing from the invention. Moreover, while the waveguides have been described as being unidirectional with respect to conducting optical signals, the device of FIGS. 3 and 4 may be used in bidirectional communication systems. 
     A second embodiment of an exchange switch  92  is shown in FIGS. 5 and 6. In this embodiment, there is only one chamber  94  in which fluid is manipulated in order to toggle the switch  92  from a signal-continuation state of FIG. 5 to a signal-exchange state of FIG.  6 . 
     In FIG. 5, a first transmission path is formed by an input waveguide  96  and an output waveguide  98 . An arrow  100  is used to illustrate the signal propagation along the first transmission path when the switch  92  is in the signal-continuation state, i.e., the chamber  94  is filled with fluid. A second transmission path is formed by coaxial input and output waveguides  102  and  104 . Arrow  106  illustrates signal propagation from the second input waveguide to the second output waveguide when the switch  92  is in the signal-continuation state. 
     Referring now to FIG. 6, the region of the chamber  94  that is along the first and second transmission paths is shown as being occupied by a bubble  108 . As a result, TIR will occur at the interfaces of the waveguides and the walls of the chamber  94 . Arrow  110  represents the reflection of optical signals from the input waveguide  96  to the output waveguide  104 , as optical signals from the first transmission path are transferred to the second transmission path. A steady-state reflector  112  is used to transfer the signals from the input waveguide  102  of the second transmission path to the output waveguide  98  of the first transmission path. The optical signals are reflected at the chamber  94  to a first exchange waveguide  114 . The signals are redirected by the reflector  112  to a second exchange waveguide  116 . Consequently, the signals again impinge the wall of the trench  94  and are redirected to the output waveguide  98  of the first transmission path. 
     The steady-state reflector  112  may be a chamber that is similar to the fluid-manipulable chamber  94 . Alternatively, the reflector may be another type of structure that provides total internal reflection. However, the preferred embodiment is one in which the reflector  112  and the chamber  94  are formed simultaneously using the same fabrication techniques, since this facilitates the fabrication process. 
     A third embodiment of an exchange switch  118  is shown in FIGS. 7 and 8. This embodiment utilizes three fluid-manipulable chambers  120 ,  122  and  124 , as well as a steady-state reflector  126 . A first linear transmission path is defined by an input waveguide  128 , an intermediate waveguide  130  and an output waveguide  132 . A second linear transmission path is formed by an input waveguide  134 , a second intermediate waveguide  136  and an output waveguide  138 . The exchange switch  118  also includes a pair of exchange waveguides  140  and  142  that intersect at the steady-state reflector  126 . 
     When the exchange switch  118  is in the signal-continuation state of FIG. 7, there is fluid within each of the fluid-manipulable chambers  120 ,  122  and  124 . Consequently, optical signals will propagate through the three chambers. Each of the transmission paths intersects two of the chambers, since the chamber  122  is common to both paths. An optical signal that is received along the input waveguide  128  of the first linear transmission path propagates through the chamber  122  to the intermediate waveguide  130  and through the chamber  124  to the output waveguide  132 . Correspondingly, the optical signals received along the input waveguide  134  of the second transmission path propagate through the chamber  120  to the intermediate waveguide  136  and through the chamber  122  to the output waveguide  138 . 
     When the exchange switch  118  is toggled to the signal-exchange state of FIG. 8, bubbles  144 ,  146  and  148  are formed in the three chambers  120 ,  122  and  124 . Optical signals that are input at the input waveguide  128  of the first transmission path are reflected at the wall of the lower chamber  122  and to the output waveguide  138  of the second transmission path. On the other hand, optical signals that are received at the input waveguide  134  of the second transmission path are reflected at the chamber  120  to the exchange waveguide  140 , are reflected at the reflector  126  to the second exchange waveguide  142 , and are reflected at the chamber  124  to the output waveguide  132  of the first transmission path. Thus, when the switch  118  is in the signal-exchange state of FIG. 8, optical signals are reciprocally exchanged by the two transmission paths. 
     The embodiments of FIGS. 5-8 have been described with reference to unidirectional communication. However, both of the embodiments may be utilized with waveguides that provide bidirectional communications. 
     The exchange switch  66  of FIGS. 3 and 4 is represented by the symbol shown in FIG.  9 . The horizontal lines in the symbol represent the two parallel transmission paths of the switch, while the crossing diagonal lines represent the exchange waveguides  80  and  82 . 
     The symbol of FIG. 9 is used in the representation of a network topology in FIG.  10 . By using five exchange switches  150 ,  152 ,  154 ,  156  and  158 , any one of four input waveguides  160 ,  162 ,  164  and  166  can be optically coupled to any one of four output waveguides  168 ,  170 ,  172  and  174 . For example, an optical signal that is received along waveguide  164  may be transferred to the top output waveguide  168  by placing the exchange switch  154  in the signal-exchange state while the switch  152  is in the signal-continuation state. On the other hand, the same optical signal can be transferred to the lowermost output waveguide  174  by placing the exchange switch  152  in the signal-exchange state and the exchange switch  156  in the signal-continuation state. With the switches  152  and  154  in the signal-continuation state and the switch  158  in the signal-exchange state, the same optical signals will be output via the output waveguide  170 . Dependent upon the state of the five exchange switches  150 - 158 , all possible arrangements of signal exchanges are realizable. 
     A more complex exchange switching arrangement is shown in FIG.  11 . Nine transmission paths  176 ,  178 ,  180 ,  182 , 184 ,  186 ,  188 ,  190  and  192  are shown as being linked by a matrix of the exchange switches of FIGS. 3 and 4. By selectively setting the states of the various switches, all possible arrangements of signal exchanges are realizable. 
     The arrangements of FIGS. 10 and 11 are not considered to be inventive. The topologies are only illustrated for purposes of example. The exchange switch  66  of FIGS. 3 and 4 may be used in other topologies as well. 
     FIG. 12 is used to represent either the exchange switch  92  of FIGS. 5 and 6 or the exchange switch  118  of FIGS. 7 and 8. The symbol will be referred to generically as an optical switching arrangement  200 . The switching arrangement may be the combination of the trench  94  and the reflector  112  in FIGS. 5 and 6, may be the combination of the three trenches  120 ,  122  and  124  and the reflector  126  in FIGS. 7 and 8, or may be a different combination of reflection-enabling elements that achieve the reciprocal exchange operation. 
     FIG. 13 is a symbol of a steady-state reflector  202  that is used in addition to any single-state reflectors that are included in the optical switching arrangements. That is, the reflector that is symbolized by FIG. 13 is a reflection-enabling optical element that is used in addition to any reflectors of a switching arrangement such as the ones illustrated in FIGS. 5-8. 
     FIG. 14 is an example of a network topology that utilizes optical switching arrangements  204 ,  206 ,  208 ,  210  and  212  and reflectors  214  and  216  to achieve signal exchanges. The topology includes four input waveguides  218 ,  220 ,  222  and  224  and four output waveguides  226 ,  228 ,  230  and  232 . When all five of the switching arrangements  204 - 212  are in the signal-continuation state, signals introduced to the input waveguides  218 ,  220 ,  222  and  224  will be output via waveguides  232 ,  228 ,  230  and  226 , respectively. By placing the optical switching arrangement  204  in the signal-exchange state, input waveguides  218  and  222  exchange optical coupling to the output waveguides  232  and  230 . Alternatively, the input waveguide  218  can exchange output waveguides with input waveguide  220  by placing the optical switching arrangement  210  in the signal-exchange state. As a third alternative, optical switching arrangements  204 ,  206 ,  208  and  210  can all be placed in the signal-exchange state, causing input waveguides  218  and  224  to exchange output waveguides  232  and  226 . As in FIGS. 10 and 11, all possible exchanges of signals are realizable. 
     An alternative topology is shown in FIG.  15 . Twelve optical switching arrangements are utilized in this topology. With all of the switching arrangements in a signal-continuation state, a first array of input waveguides  234 ,  236  and  238  is coupled to a first array of output waveguides  244 ,  242  and  240 , respectively. Similarly, a second array of input waveguides  246 ,  248  and  250  is coupled to a second array of output waveguides  256 ,  254  and  252 , respectively. However, by selectively changing the state of one or more of the optical switching arrangements, any two input waveguides will exchange output waveguides. 
     While not shown in the figures, the versatility of the switching networks that utilize the exchange switches described above can be further enhanced by incorporating a matrix such as the one shown in FIG. 2 at either or both of the input and output ends of the exchange signal matrix. This enables test signals to be channeled into and out of the matrix and allows “dumping” of unused signals.