Abstract:
An optical switch module includes: N first input ports to which a signal is input; M first output ports from which a signal is output; an M×N switch to include N second input ports and M second output ports, and to set a path between the second input ports and the second output ports, the second output ports coupling with the first output ports, respectively; a test-signal input port to which a test-signal is capable of being externally input; an expansion port from which one of the test-signal and the signal from any one of the first input ports is output; and an optical switch to selectively connect at least one of the test-signal and the signal from any one of the first input ports to at least one of the expansion port and any one of the second input ports, wherein both N and M are natural numbers.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-093148, filed on Apr. 30, 2015, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to an optical switch module and an optical relay apparatus and a path expansion method that use the optical switch module. 
       BACKGROUND 
       [0003]    A reconfigurable optical add/drop multiplexer (ROADM) used in a wavelength division multiplex (WDM) system is an optical relay apparatus that has optical switches to drop and add optical signals with different wavelengths. To increase the flexibility of an optical network, implementation of Colorless, Directionless, and Contentionless (CDC) functions with which wavelengths and paths are freely set in a ROADM and wavelength contention is avoided is being studied. A ROADM having CDC functions is called a CDC-ROADM. 
         [0004]      FIG. 1  illustrates the structure of a multicast switch (MCS) module  100  as an example of an optical switching structure that implements CDC functions. At an MCS  110 - 2  on the drop side, which receives WDM signals and drops optical signals, WDM signals received from M paths (deg 1 to deg M) are input to M 1×N optical couplers (represented as SPLs in  FIG. 1 )  13   1  to  13   m  (collectively referred to below as the optical couplers  13  at appropriate points) and are then dropped in N directions. The dropped WDM signals are connected to N M×1 optical switches  11   1  to  11   n  (collectively referred to below as the optical switches  11  at appropriate points) and are then output from M×1 optical switches  11   1  to  11   n  to N drop ports. 
         [0005]    An MCS  110 - 1  on the add side, which adds, to WDM signals, optical signals to be transmitted, has the same structure as the MCS  110 - 2 ; at the MCS  110 - 1 , optical signals are input from N add parts into N 1×M optical switches  11   1  to  11   n . Outputs from each 1×M optical switch  11  are output to M N×1 optical couplers  13   1  to  13   m  and are then output to M paths. The 1×M optical switches  11  on the add side and the M×1 optical switches  11  on the drop side have the same switch structure; they differ only in that the number of inputs and the number of outputs are reversed depending on the signal transmission direction. In  FIG. 1 , therefore, each of these optical switches is indicated as M×1 SW on both the add side and the drop side. Similarly, the N×1 optical couplers  13  on the add side and the 1×N optical couplers  13  on the drop side have the same optical coupler structure; they differ only in that the number of inputs and the number of outputs are reversed depending on the signal transmission direction. In  FIG. 1 , therefore, each of these optical couplers is indicated as 1×N SPL on both the add side and the drop side. In this description, an optical switching structure having n output ports or input ports for m input ports or output ports will be referred to as M×N optical switch (including optical coupler, optical selector, optical splitter, and the like), regardless of the input and output directions. 
         [0006]    In general, the MCS module  100  is used in such a way that the add side and drop side are paired as illustrated in  FIG. 1 . An MCS having N add ports or drop ports for M paths will be referred to as an M×N MCS. 
         [0007]      FIG. 2  illustrates the node structure of a two-path ROADM  1001  in which wavelength selective switches (WSSs)  105   a  and  105   b  and 2×2 MCSs  110 - 1  and  110 - 2  are combined together. In a previous ROADM, arrayed waveguide gratings (AWGs) have been used at portions equivalent to the MCSs  110 - 1  and  110 - 2 , so it has been possible to input only optical signal with a predetermined wavelength from each add port. However, the use of the MCSs  110 - 1  and  110 - 2  enables an optical signal with a desired wavelength to be input from each add port, so a Colorless function is achieved. This is also true on the drop side. 
         [0008]    Optical output signals from two transponders  102  are input to signal input ports. Paths for these optical output signals are selected by a 2×2 MCS  110 - 1 . Optical input signals from two paths are dropped to two transponders  102  by a 2×2 MCS  110 - 2 . Since a path can be selected independently for each signal, a Directionless function is achieved. With a wavelength assigned to an input port, it is also possible to input a signal with the same wavelength from another input port (however, the same path is unable to be selected). That is, a Contentionless function is achieved. 
         [0009]    For a CDC-ROADM node, there is a demand to increase the number (M) of selectable paths after an operation has been started. To meet this demand, at an MCS  210 - 2  on the drop side and an MCS  210 - 1  on the add side, each of 2×1 optical switches  12   1  to  12   n  (collectively referred to below as the optical switches  12  at appropriate points) are connected to one of the M×1 optical switches  11   1  to  11   n , as in an MCS module  200  illustrated in  FIG. 3 . Of the 2×1 optical ports  12 , N ports not connected to the M×1 optical switches  11  are collectively connected to an upgrade port  215  to increase the number of paths (see U.S. Patent Application Publication No. 2013/0108215, for example). 
       SUMMARY 
       [0010]    According to an aspect of the invention, an optical switch module includes: N first input ports to which an optical signal is input; M first output ports from which an optical signal is output; an M×N switch configured to include N second input ports and M second output ports, and to set an optical path between the N second input ports and the M second output ports, the M second output ports coupling with the M first output ports, respectively; a test signal input port to which a test signal is capable of being externally input; an expansion port from which one of the test signal and the optical signal from any one of the N first input ports is output; and an optical switch configured to selectively connect at least one of the test signal and the optical signal from any one of the N first input ports to at least one of the expansion port and any one of the N second input ports, wherein both N and M are natural numbers. 
         [0011]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0012]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  illustrates the structure of an ordinary MCS module; 
           [0014]      FIG. 2  illustrates an example of the structure of a ROADM in which MCS modules are used; 
           [0015]      FIG. 3  illustrates an example of the structure of an MCS module with path expansion functions; 
           [0016]      FIG. 4  illustrates a connection structure after path expansion; 
           [0017]      FIG. 5  illustrates a problem with a possible path expansion structure; 
           [0018]      FIG. 6  illustrates a problem with another possible path expansion structure; 
           [0019]      FIG. 7  illustrates a path expansion structure in which MCS modules in a first embodiment are used; 
           [0020]      FIGS. 8A and 8B  illustrate the states of 2×2 optical switches used in the structure in  FIG. 7 ; 
           [0021]      FIG. 9  illustrates an example of the structure of a drop circuit used for optical connection check; 
           [0022]      FIG. 10  illustrates connection check when the add side is operated; 
           [0023]      FIG. 11  illustrates connections to added paths; 
           [0024]      FIG. 12  illustrates a path expansion structure in which MCS modules in a second embodiment are used; 
           [0025]      FIG. 13  illustrates a modification of the second embodiment; 
           [0026]      FIG. 14  illustrates a path expansion structure in which MCS modules in a third embodiment are used; 
           [0027]      FIG. 15  illustrates a modification of the third embodiment; 
           [0028]      FIG. 16  illustrates a path expansion structure in which MCS modules in a fourth embodiment are used; 
           [0029]      FIG. 17  illustrates a modification of the fourth embodiment; 
           [0030]      FIG. 18  illustrates an example of the structure of a ROADM in which MCS modules in an embodiment are used; 
           [0031]      FIG. 19  illustrates a ROADM structure, in which path expansion has been carried out; and 
           [0032]      FIG. 20  illustrates a flowchart indicating a path expansion method in an embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0033]      FIG. 4  illustrates a structure in which an MCS module  200 B, which is a second MCS module in operation, is connected to an MCS module  200 A, which is a first MCS module. An upgrade port  215 A on the add side of the MCS  200 A is connected to add ports in the MCS  200 B through an optical cable  217 , and an upgrade port  215 A on the drop side of the MCS  200 A is similarly connected to drop ports in the MCS  200 B. When optical signals are to be connected to paths of the second MCS  200 B (or optical signals are to be connected from paths of the MCS  200 B), signal paths can be switched toward the upgrade port  215 A by switching 2×1 optical switches  12  to the MCS  200 B. During path expansion, only M paths have been selectable for optical signals input from add ports. After path expansion, however, 2×M paths are selectable. 
         [0034]    To increase paths by connecting a new MCS  200 B, it is desirable to check that the MCS  200 A in operation and the added MCS  200 B are correctly interconnected. On the drop side, test signals can be input from an upgrade port  215 B in the added MCS  200 B. On the add side, however, the first MCS  200 A is in operation, so it is difficult to send the test signals to the MCS  200 B. If the operation of the first MCS  200 A is stopped to check connections, optical signal transmission is impeded. 
         [0035]    Before explaining embodiments of the structures of optical switch modules with which it is possible to check connections between an optical switch module in operation and an optical switch module that is added to increase paths on an optical network without affecting the optical switch module in operation and explaining an embodiment of a path expansion method, problems with possible MCS module structures will described with reference to  FIGS. 5 and 6 . 
         [0036]      FIG. 5  is drawing to check connections between an MCS module  8 A, which is a first MCS module in operation, and an MCB module  8 B, which is a second MCS module connected to the MCS module  8 A. The MCS modules  8 A and  8 B have the same structure and the MCS module  8 B is appropriately added in response to a path expansion request. Therefore, the following description will focus on the MCS module  8 A. The MCS module  8 A has an MCS  10  on the drop side and an MCS  210  on the add side. The MCS  210  on the add side is the same as the MCS  210  in  FIGS. 3 and 4 . The MCS  10  on the drop side has a test access port (TAP) circuit  19 A between an upgrade port  15 A and unused 2×1 optical switches  12 . The TAP circuit  19 A has n monitor photodetectors (PDs). 
         [0037]    The upgrade port  15 A on the add side of the MCS module  8 A is connected to the add ports of the second MCS module  8 B through an optical cable  27 ; the upgrade port  15 A on the drop side is connected to the drop ports of the MCS module  8 B through an optical cable  17 . The optical cables  17  and  27  are each, for example, an optical fiber connector with optical connectors. If a connection is disconnected due to a broken optical fiber or a connection is made to an incorrect port due to, for example, an incorrect connection of an optical connector, no optical signal is transmitted between the MCS module  8 A and the MCS module  8 B. Alternatively, an optical signal is sent to an incorrect path. 
         [0038]    On the drop side, test signals for connection monitoring are received from the upgrade port  15 B of the second MCS module  8 B to monitor the test signals at the TAP circuit  19 A in the first MCS module  8 A. Even if the first MCS module  8 A is in operation, connections to the second MCS module  8 B can be checked. 
         [0039]    On the add side, however, the first MCS module  8 A is in operation, so it is difficult to input the test signals to the MCS  210  on the add side. To check connections on the add side, if the output ports of the 2×1 optical switches  12  are switched to the upgrade port  15 A to input the test signals to the MCS  210  on the add side, optical transmission is suspended while the switching is in progress. 
         [0040]    A similar problem arises in the structure in  FIG. 6  as well. To increase paths, MCS modules  9 A and  9 B in  FIG. 6  use n (M+1)×1 optical switches  14  instead of using n M×1 optical switches  11  and n 2×1 optical switches  12 . The MCS modules  9 A and  9 B have the same structure. Therefore, the following description will focus on the MCS module  9 A. The MCS module  9 A has an MCS  70  on the drop side and an MCS  310  on the add side. In the MCS  70 , an M+1st port of each optical switch  14  is connected to an upgrade port  75 A and a TAP circuit  19  is inserted between the upgrade port  75 A and the M+1st port of each optical switch  14 . When test signals for connection monitoring are input from an upgrade port  75 B of the second MCS module  9 B, connections between the MCS module  9 A and the MCS module  9 B can be checked on the drop side. Since the optical switches  14   1  to  14   n  on the add side are in operation, however, it is difficult to receive the test signals on the add side. 
         [0041]    In view of this situation, embodiments below will describe specific examples of an MCS module with path expansion functions that, even if a first MCS module is in operation, is capable of checking connections without affecting the operation. In the description and drawings, like elements will be denoted by like reference characters, and repeated descriptions will be omitted. 
       First Embodiment 
       [0042]      FIG. 7  schematically illustrates an MCS module  1  in a first embodiment. In the first embodiment, 2×2 optical switches  22  are used on the add side so that connections between an MCS module in operation and an additional MCS module can be checked during path expansion. 
         [0043]    The MCS module  1  has an MCS  20  on the add side and an MCS  10  on the drop side. The MCS  20  and MCS  10  may be structured by interconnecting optical switches and couplers through fibers. Alternatively, they may be of a planar light wave circuit (PLC) type in which they are structured by using waveguides made of quartz, silicon, or another semiconductor material. 
         [0044]    The MCS  20  on the add side has n 2×2 optical switches  22   1  to  22   n  (collectively referred to below as the 2×2 optical switches  22  at appropriate points), which are disposed in correspondence to n add ports, n M×1 optical switches  11 , m 1×N optical couplers  13 , a first upgrade port  21 , and a second upgrade port  25 . An M×N switch that interconnects m paths and n ports in a selectable manner is structured by using n M×1 optical switches  11  and m 1×N optical couplers  13 . 
         [0045]    One input port of each 2×2 optical switch  22  is connected to the corresponding add port, and the other input port is connected to the first upgrade port  21 . One output port of the 2×2 optical switch  22  is connected to the corresponding M×1 optical switch  11 , and the other output port is connected to the second upgrade port  25 . Both the first upgrade port  21  and the second upgrade port  25  are formed by combining a plurality of ports. Test signals for connection monitoring are input to the first upgrade port  21  as described later. In this sense, the first upgrade port  21  may be referred to as the test signal input port  21 . The second upgrade port  25  passes the test signals through new paths to check connections and increases paths. In this sense, the second upgrade port  25  may be referred to as the expansion port  25 . 
         [0046]      FIGS. 8A and 8B  illustrate the states of the 2×2 optical switch  22  used in the MCS  20 . The 2×2 optical switch  22  is an optical switch of crossbar type. In a straight state in  FIG. 8A , an input port 1 is connected to an output port 1 and an input port 2 is connected to an output port 2. In a cross state in  FIG. 8B , the input port 1 is connected to the output port 2 and the input port 2 is connected to the output port 1. During path expansion, the 2×2 optical switch  22  is in the straight state, so an optical signal from the corresponding add port is input to the input port 1 and is output from the output port 1. The input port 2 and output port 2 are not used. 
         [0047]    In a case as well in which connections are checked during path expansion, the 2×2 optical switch  22  is in the straight state. An optical signal is input from the corresponding add port to the input port 1 and is output from the output port 1 to the corresponding M×1 optical switch  11 . A test signal for connection monitoring is input from the first upgrade port  21  and is then input to the input port 2 of the 2×2 optical switch  22 . The test signal is sent from the output port 2 through the second upgrade port  25  to the additionally connected MCS module. When connections have been checked and optical signals are sent to the added paths, the 2×2 optical switches  22  are switched to the cross state. The states of the 2×2 optical switches  22  during connection check and after path expansion will be described later in detail. 
         [0048]    The MCS  10  on the drop side in the MCS module  1  is the same as the MCS  10  in the MCS modules  8 A and  8 B in  FIG. 5 . Therefore, it is possible to detect test signals input from the upgrade port  15  in the additionally connected MCS module at the TAP circuit  19  in the MCS module  1  in operation and conform connections. 
         [0049]      FIG. 9  illustrates an example of the structure of the TAP circuit  19  used in the MCS  10  on the drop side. The TAP circuit  19  has n monitor PDs  18   1  to  18   n  (collectively referred to below as the monitor PDs  18  at appropriate points), which are disposed in a one-to-one correspondence to n optical fibers  16   1  to  16   n  (collectively referred to below as the optical fibers  16  at appropriate points), which extend from the upgrade port  15 . Each monitor PD  18  monitors an optical component dropped from the corresponding optical fiber  16  and outputs a current according to the intensity of the test signal. If the intensity of the test signal detected by the monitor PD  18  is equal to or greater than a certain level, it can be decided that a connection to the MCS module used for path expansion has been established. 
         [0050]      FIG. 10  illustrates connection check when the second MCS module  1 B has been connected to the first MCS module  1 A. At the time of connection check, each 2×2 optical switch  22  in the MCS  20  is in the straight state (see  FIG. 8A ). An optical signal is input from the corresponding add port to the first input port of the 2×2 optical switch  22 , is output from the first output port to the corresponding M×1 optical switch  11 , and is transmitted to any one of M paths. 
         [0051]    A test signal for connection monitoring is input from a first upgrade port  21 A on the add side to the second input port of the 2×2 optical switch  22 . The test signal is connected from the second output port to a second upgrade port  25 A on the add side, after which the test signal is led to a first upgrade port  21 B on the add side in the second MCS module  1 B through the optical cable  27 . The 2×2 optical switch  22  in the MCS module  1 B is also in the straight state, so the test signal is monitored at a second upgrade port  25 B. By observing whether the test signal input from the first upgrade port  21 A on the add side in the first MCS module  1 A has been output to the second upgrade port  25 B on the add side in the second MCS module  1 B, a connection between the MCS module  1 A and the MCS module  1 B can be checked. Since n optical fibers in the first upgrade port  21 A and first upgrade port  21 B on the add side are connected to the corresponding 2×2 optical switches  22 , all n test signals can be checked at the second upgrade port  25 B on the add side in the second MCS module  1 B. 
         [0052]    Although, in  FIG. 10 , the upgrade ports  15 A,  15 B,  25 A, and  25 B are schematically drawn with a plurality of lines, each of these ports may be a plurality of ports connected to the optical cable  17  or  27  through optical connectors (not illustrated). 
         [0053]      FIG. 11  illustrates signal transmission to paths that have been added after their connections had been confirmed. When an optical signal is transmitted to an added path, the corresponding 2×2 optical switches  22  in both the MCS module  1 A and MCS module  1 B are switched to the cross state. An optical signal from a transponder  102  (see  FIG. 2 ) is input from one add port in the MCS module  1 A to the first input port of the corresponding 2×2 optical switch  22 , after which the optical signal is led from the second output port, which is diagonally opposite to the first input port, to the upgrade port  25 A. The optical signal is input to the first upgrade port  21 B in the MCS  20  in the second MCS module  1 B through the optical cable  27 , after which the optical signal is input to the corresponding M×1 optical switch  11  by the corresponding 2×2 optical switch  22  in the cross state and is sent to any one of an M+1st path to a 2×Mth path. 
         [0054]    When, on the drop side, an optical signal is to be supplied from an added path to a transponder  102 , the corresponding 2×1 optical switch  12  in the first MCS  10  is switched to the upgrade port  15 A and the corresponding 2×1 optical switch  12  in the second MCS  10  is connected to the corresponding M×1 optical switch  11 . Thus, an optical signal that has been sent from any one of the M+1st path to the 2×Mth path is received at the transponder  102 . 
         [0055]    When the MCS module  1 B is added to the MCS module  1 A for path expansion as described above, even if the first MCS module  1 A is in operation, connections of optical paths between the MCS modules  1 A and  1 B can be checked without affection the operation. After the connections have been checked, an optical signal can be sent to a desired path in a state in which there is no problem such as an incorrect connection or a broken fiber. 
       Second Embodiment 
       [0056]      FIG. 12  illustrates a path expansion structure in which MCS modules in a second embodiment are used. An MCS module  2 A is a module in operation and an MCS module  2 B is an additionally connected module. In the second embodiment, 2×2 optical switches  22  of crossbar type are used on the drop side as well. In this structure, the drop side can lack a TAP circuit. 
         [0057]    The MCS modules  2 A and  2 B have the same structure. Therefore, the following description will focus on the MCS module  2 A. The MCS module  2 A has an MCS  20 - 1  on the add side and an MCS  20 - 2  on the drop side. The MCSs  20 - 1  and  20 - 2  have the same structure. 
         [0058]    When connections are checked during path expansion, test signals are input from the second upgrade port  25 B in the MCS- 20 - 2  in the second MCS module  2 B. During connection check, the 2×2 optical switches  22  in both the MCS modules  2 A and  2 B are in the straight state. The input test signals are further input from the first upgrade port  21 B in the MCS  20 - 2  in the MCS module  2 B to the second upgrade port  25 A in the MCS  20 - 2  in the MCS module  2 A through the optical cable  17 . The input test signals are output to the first upgrade port  21 A in the MCS  20 - 2  by the 2×2 optical switches  22  and are monitored. This connection check can be performed without affecting the operation of the first MCS module  2 A. The structure and connection check on the add side are the same as in the first embodiment. 
         [0059]    When optical signals are sent to an M+1st path to a 2×Mth path after connection check, the 2×2 optical switches  22  in both the MCS  20 - 1  in the MCS module  2 A and the MCS  20 - 1  in the MCS module  2 B are switched to the cross state. When optical signals are received from the M+1st path to the 2×Mth path on the drop side, the 2×2 optical switches  22  in both the MCS  20 - 2  in the MCS module  2 A and the MCS  20 - 2  in the MCS module  2 B are similarly switched to the cross state. 
         [0060]    In this structure, it is possible to check connections of the additionally connected MCS module  2 B while the first MCS module  2 A is in operation. After the connections have been confirmed, optical signals can be transmitted and received to and from added paths. In the structure in  FIG. 12 , the add side and drop side can have the same structure in each MCS module  2 , so manufacturing is simplified. If MCSs  20 - 1  and  20 - 2  of PLC type are used, cutouts of PLCs manufactured on the same wafer can be used. 
         [0061]      FIG. 13  illustrates a modification of the second embodiment. In  FIG. 13 , a TAP circuit is placed on at least one of the add side and drop side for connection check, besides the structure in  FIG. 12 . MCS modules  3 A and  3 B have the same structure. Therefore, the following description will focus on the MCS module  3 A. 
         [0062]    The MCS module  3 A has an MCS  30  on the add side and an MCS  40  on the drop side. In the MCS  30 , a TAP circuit  39 A is placed between the first upgrade port  21 A and n 2×2 optical switches  22 . In the MCS  40 , the TAP circuit  19 A is placed between a second upgrade port  45 A and n 2×2 optical switches  22 . 
         [0063]    On the drop side, test signals are input from an upgrade port  45 B in the second MCS module  3 B and are then monitored at the TAP circuit  19 A on the drop side in the first MCS module  3 A. On the add side, test signals are input from the upgrade port  21 A in the first MCS module  3 A and are then monitored at a TAP circuit  39 B on the add side in the second MCS module  3 B. 
         [0064]    This structure enables the MCS modules themselves on an external add side and drop side to have optical signal monitoring functions for connection check. Theoretically, even in a structure in which the TAP circuit  39 B is placed only in the MCS  30  in the second MCS module  3 B on the add side and the TAP circuit  19 A is placed only in the MCS  40  in the first MCS module  3 A on the drop side, connections can be checked. From the viewpoint of achieving path expansion and connection check only by connecting the MCS modules  3 A and  3 B having the same structure, however, a convenient way for path expansion is to use MCS modules of the same type in which a TAP circuit is placed on both the add site and the drop side. 
       Third Embodiment 
       [0065]      FIG. 14  illustrates a path expansion structure in which MCS modules in a third embodiment are used. In the third embodiment, 2×1 optical switches  42   1  to  42   n  (collectively referred to below as the 2×1 optical switches  42  at appropriate points) and a verify port  51  are used on the add side to check connections. An MCS module  4 A is a module in operation and an MCS module  4 B is an additionally connected module. In the example in  FIG. 14 , the MCS modules  4 A and  4 B have the same structure. Therefore, the following description will focus on the MCS module  4 A. 
         [0066]    The MCS module  4 A has an MCS  50  on the add side and the MCS  10  on the drop side. The MCS  10  is the same as the MCS  10  in  FIG. 7  (first embodiment). That is, paths are added by using 2×1 optical switches  12  and the upgrade port  15 A, and connections are checked by using the TAP circuit  19 A. 
         [0067]    The MCS  50  has n 2×1 optical switches  12 , n M×1 optical switches  11 , m 1×N optical couplers  13 , a TAP circuit  55 A, and the verify port  51  used to input test signals. The verify port  51  may be referred to as the test signal input port  51 . One output port of each 2×1 optical switch  12  is connected to the corresponding M×1 optical switch  11 , and a normal add operation is performed. 
         [0068]    The TAP circuit  55 A has n 2×1 optical switches  42 . Each 2×1 optical switch  42  has two input ports, one of which is used for a connection to the corresponding 2×1 optical switch  12  and the other of which is used for a connection to the verify port  51 . 
         [0069]    When paths are to be added while the MCS module  4 A, which is a first MCS module, is in operation, the MCS module  4 B, which is a second MCS module, is connected with the optical cable  17  and optical cable  27 . On the add side, an upgrade port  59 A in the MCS module  4 A is connected to add ports in the MCS module  4 B. To check connections, the input ports of the 2×1 optical switches  42  of the TAP circuit  55 A in the MCS module  4 A are connected to the verify port  51 , and test signals (optical signals) for connection monitoring are input from the verify port  51 . These test signals are led to the second MCS module  4 B through the optical cable  27 . 
         [0070]    In the second MCS module  4 B, the 2×1 optical switches  12  used for path selection are set so that input test signals are connected to the upgrade port  59 B. The input port setting of each 2×1 optical switch  42  in a TAP circuit  55 B is switched to the corresponding 2×1 optical switch  12 . When it is confirmed that test signals are output from the upgrade port  59 B, it is confirmed that optical paths on the add side have been connected between the first MCS module  4 A and the second MCS module  4 B. Upon the completion of the connection confirmation, the input port setting of each 2×1 optical switch  42  in the TAP circuit  55 A in the first MCS module  4 A is switched back from the verify port  51  to the corresponding 2×1 optical switch  12 . 
         [0071]    On the drop side, the upgrade port  15  in the MCS module  4 A is connected to the drop ports of the MCS module  4 B. When connections are to be checked, test signals are input from the upgrade port  15 B in the second MCS module  4 B and the test signals are monitored at the TAP circuit  19 A in the first MCS module  4 A, as in the first embodiment. 
         [0072]    Theoretically, the MCS  50  in the second MCS module  4 B can lack the TAP circuit  55 B and verify port  51 ; instead, the MCS  210  in  FIG. 5  may be used. From the viewpoint of achieving path expansion and connection check only by connecting MCS modules having the same structure, however, it is desirable to manufacture the MCS modules  4 A and  4 B having the same structure and use them. 
         [0073]      FIG. 15  illustrates a modification of the third embodiment. In this modification, the TAP circuit  55 A in which 2×1 optical switches  42  are used is placed in the drop side as well. An additional MCS module  5 B is connected to an MCS module  5 A in operation for path expansion. In this example, the MCS modules  5 A and  5 B have the same structure. Therefore, the following description will focus on the MCS module  5 A. 
         [0074]    The MCS module  5 A has an MCS  50 - 1  on the add side and an MCS  50 - 2  on the drop side. When connections are to be checked on the drop side, the setting of each 2×1 optical switch  42  is switched to the verify port  51  at the TAP circuit  55 A in the MCS  50 - 2  in the MCS module  5 A, which is a first MCS module. At the TAP circuit  55 B in the MCS  50 - 2  in the MCS module  5 B, which is a second MCS module, the setting of each 2×1 optical switch  42  is switched to the corresponding 2×1 optical switch  12 . 
         [0075]    Test signals are input from the upgrade port  15 B in the second MCS module  5 B, pass through drop ports in the MCS module  5 B, are led to the upgrade port  15 A in the first MCS module  5 A through the optical cable  17 , and are input to the TAP circuit  55 A. Since the setting of each 2×1 optical switch  42  in the TAP circuit  55 A has been switched to the verify port  51 , when an output optical signal is monitored at the verify port  51 , connections between the MCS modules  5 A and  5 B on the drop side are checked. The structure and connection check on the add side are the same as in  FIG. 14 . 
         [0076]    In the structure in  FIG. 15 , the MCS  50 - 1  and MCS  50 - 2 , which have the same structure, can be used on the add side and drop side in each MCS module  5 , the manufacturing process is simplified. 
       Fourth Embodiment 
       [0077]      FIG. 16  illustrates a path expansion structure in which MCS modules in a fourth embodiment are used. In the fourth embodiment, (M+1)×1 optical switches  14  are used for path expansion, instead of using a combination of M×1 optical switches and 2×2 optical switches or 2×1 optical switches. 
         [0078]    In this example, an MCS module  6 A is a module in operation and an MCS module  6 B is an additionally connected module. The MCS modules  6 A and  6 B have the same structure. Therefore, the following description will focus on the MCS module  6 A. 
         [0079]    The MCS module  6 A has an MCS  60  on the add side and an MCS  70  on the drop side. The MCS  60  has n (M+1)×1 optical switches  14  and m 1×N optical couplers  13 , a TAP circuit  55 , and the verify port  51 . M output ports of each (M+1)×1 optical switch  14  are connected to the 1×N optical couplers  13 , and an M+1st output port is connected to the upgrade port  59 A. The TAP circuit  55  is inserted between the upgrade port  59 A and the M+1st output port of each (M+1)×1 optical switch  14 . The TAP circuit  55  has n 2×1 optical switches  42 . The first input port of each 2×1 optical switch  42  is connected to the M+1st output port of the corresponding (M+1)×1 optical switch  14 , and the second input port is connected to the verify port  51 . The output port of the 2×1 optical switch  42  is connected to the upgrade port  59 A. 
         [0080]    The MCS  70  on the drop side has n (M+1)×1 optical switches  14 , m 1×N optical couplers  13 , and the TAP circuit  19 . M input ports of each (M+1)×1 optical switch  14  are connected to the  1 ×N optical couplers  13 , and an M+1st input port is connected the upgrade port  45 A. The TAP circuit  19  is inserted between the upgrade port  45 A and the M+1st input port of each (M+1)×1 optical switch  14 . The TAP circuit  19  has n monitor photodetectors (PDs). 
         [0081]    When paths are to be added, the MCS module  6 B, which is a second MCS module, is connected to the MCS module  6 A, which is a first MCS module, through the optical cables  17  and  27 . When connections between them are to be checked, the settings of the 2×1 optical switches  42  in the TAP circuit  55  in the first MCS module  6 A are switched to the verify port  51  on the add side and the settings of the 2×1 optical switch  42  in the second MCS module  6 B are switched to the (M+1)×1 optical switches  14 . When test signals are input from the verify port  51  in the first MCS module  6 A and are monitored at the upgrade port  59 B in the second MCS module  6 B, connections between the first MCS module  6 A and the second MCS module  6 B can be checked. 
         [0082]    On the drop side, test signals are input from the upgrade port  45 B in the second MCS module  6 B and are monitored at the TAP circuit  19  in the first MCS module  6 A. 
         [0083]    In this structure, connections can be checked on both the add side and the drop side before path expansion, without affecting the operation of the first MCS module  6 A. 
         [0084]      FIG. 17  illustrates a modification of the fourth embodiment. In this modification, the TAP circuit  55  in which 2×1 optical switches  42  are used is employed instead of the TAP circuit  19  in which PDs are used. In this example, an MCS module  7 A is a module in operation and an MCS module  7 B is an additionally connected module. The MCS module  7 A and MCS module  7 B have the same structure. Therefore, the following description will focus on the MCS module  7 A. 
         [0085]    The MCS module  7 A has an MCS  60 - 1  on the add side and an MCS  60 - 2  on the drop side. The MCS  60 - 1  has the same structure as the MCS  60  in  FIG. 16 , and the method of checking connections during path expansion is also the same. 
         [0086]    The MCS  60 - 2  has the same structure as the MCS  60 - 1 . The MCS module  7 B, which is a second MCS module, is connected to the MCS module  7 A, which is a first MCS module, through the optical cables  17  and  27 , after which connections between them are checked. On the drop side, the output port setting of each 2×1 optical switch  42  in the second MCS module  7 B is switched to the corresponding (M + 1)×1 optical switch  14  and the output port setting of each 2×1 optical switch  42  in the first MCS module  7 A is switched to the verify port  51 . Test signals are input from the upgrade port  45 B in the second MCS module  7 B and are monitored at the verify port  51  on the drop side in the first MCS module  7 A. 
         [0087]    In this structure as well, connections can be checked on both the add side and the drop side, without affecting the operation of the first MCS module  7 A during path expansion. 
         [0088]    Optical Relay Apparatus 
         [0089]      FIG. 18  illustrates an example of the structure of a ROADM  80 A in which MCS modules in an embodiment and 1×9 WSSs  105   a  and  105   b  are combined. The MCS modules may be any one of the MCS modules  1  to  7  described in the first to fourth embodiments and their modifications. 
         [0090]    As an example, the MCS module  1  (or any one of MCS modules  2  to  7 ) is a module that uses 4×4 MCSs having an upgrade function. For each installed transponder (TRPN), adding and dropping are possible by using colorless, directionless, and contentionless (CDC) functions adaptable to up to four paths. 
         [0091]    The ROADM  80 A is used on, for example, a ring network having six paths 1 to 6. For optical signals transmitted from a path 1 on the drop side, the 1×9 WSSs  105   b  on the drop side selects the four drop ports of the MCS module  1  (or any one of MCS modules 2 to 7) and five paths 2 to 6 (or a network). Another WSS, which is not illustrated in  FIG. 18  to simplify it, selects optical signals sent from the add ports of the MCS module 1 (or any one of MCS modules  2  to  7 ) toward directions other than the path 1. 
         [0092]    On the add side, optical signals destined for the path 1 are selected by the 1×9 WSSs  105   a.  For example, four inputs from the add ports of the MCS module  1  (or any one of MCS modules 2 to 7) and optical signals from the five paths 2 to 6 are selected. Another WSS, which is not illustrated in  FIG. 18  to simplify it, selects signals to be dropped from the paths 2 to 6 to the MCS module  1  (or any one of MCS modules 2 to 7). 
         [0093]      FIG. 19  illustrates a ROADM  80 B, in which path expansion has been carried out, indicating an example of expansion in a case in which adding and dropping adaptable to up to eight paths are desirable to increase network flexibility in the MCS module  1  (or any one of MCS modules  2  to  7 ). To increase the number of paths from 4 to 8, an upgrade port is used to connect a new MCS module  1 B to the MCS module  1 A in operation. In a case as well in which any one of MCS modules  2 A to  7 A is used, the corresponding one of MCS modules  2 B to  7 B having the same structure as the MCS modules  2 A to  7 A is additionally connected. 
         [0094]    Although, in the examples of the structures in  FIGS. 18 and 19 , 4×4 MCSs and 1×9 WSSs have been used, M×N MCSs and 1×K WSSs (M, N, and K are an arbitrary integer) may be used instead. 
         [0095]      FIG. 20  illustrates a flowchart indicating a path expansion method in an embodiment. First, a new MCS module (second MCS module, for example) to be added to an MCS module in operation (first MCS module, for example) is prepared (S 11 ). The first and second MCS modules may have any one of the structures described in the first to fourth embodiments. 
         [0096]    The first MCS module and second MCS module are interconnected with optical fibers such as in the form of an optical cable (S 12 ). Settings for connection check are made at each MCS module on a demand basis (S 13 ). If, for example, the TAP circuit  55  in which 2×1 optical switches  42  are used is placed for connection check, it is checked whether the settings of the 2×1 optical switches  42  in the first MCS module are switched to the verify port  51  and the settings of the 2×1 optical switches  42  in the second MCS modules are switched to ports other than the verify port  51 . 
         [0097]    After the settings have been checked, optical signals (test signals) for connection monitoring are input (S 14 ), after which whether the test signals have been monitored is checked (S 15 ). If, for example, test signals at a prescribed level or higher are detected (the result in S 15  is Yes), the processing is terminated, assuming that the test signals have been confirmed. If the test signals fail to be confirmed (the result in S 15  is No), the connection states of the optical fibers and optical connectors, for example, are checked (S 16 ), and test signals are input and checked again (S 14  and S 15 ). When S 14  and S 15  are repeated until the test signals are confirmed, reliable connection of the additional MCS module is assured and it is suppressed that an optical signal is lost or is sent in an incorrect direction. Connection checks on the add site and drop side may be performed one at a time or simultaneously. 
         [0098]    Upon completion of connection confirmation, the second MCS module is operated. Signals that have been sent from the transponders to M paths can now be sent to 2×M paths. It also becomes possible for the transponders to receive any optical signals from 2×M paths. 
         [0099]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.