Abstract:
Practical utilization of an optical signal processor configured by combining a plurality of optical components requires configurations and methods by which reflected light arising in the processor during assembly, installation, or operation thereof can be detected reliably and immediately to enable switching of optical signal paths and recovery actions (maintenance) such as replacement and repair of components, thereby improving the reliability, availability, and service ability of the processor. The present invention provides an optical switching system configured as a combination of optical components, including multistage-connected optical switching devices each having a plurality of optical reflection monitoring functions. These functions enable reliable and immediate detection and notification of reflected light arising in the optical switching system. The invention also provides a method of monitoring and reporting reflection.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the structure of an optical switching system including an optical reflection measuring system. More particularly, the invention relates to an optical switching system that enables immediate detection and notification of reflected light occurring on a plurality of optical signal transmission paths, and an optical reflection measuring system for measuring the reflected light.  
         BACKGROUND OF THE INVENTION  
         [0002]    In order to keep up with rapid increases in data traffic as typified by the Internet and in demands for multimedia communications combining image, voice, and data, the speed and capacity of transmission paths and communication nodes that form networks are being improved, and optical communication systems using optical fibers and optical signals are being brought into use. In addition, as an alternative to conventional communication equipment in which optical signals are processed through optical-to-electrical conversion, optical signal processors such as the optical cross-connect (referred to as an OXC below) and optical add drop multiplexer (referred to as an OADM below), in which switching operations such as transmission path switching and circuit switching are carried out without such conversion, are under consideration for practical use.  
           [0003]    The OXCs and OADMs mentioned above are configured by selectively using optical amplifiers, optical couplers, optical isolators and other optical components as required and combining (interconnecting) them with optical fibers and connectors. As can typically be seen in optical switches and other optical devices, it is difficult to increase their capacity as matters now stand, so high-capacity optical switching systems are generally implemented by combining a number of low-capacity optical components. A higher-capacity optical switch, for example, can be implemented by multistage-connecting low-capacity optical switches, such as 2×2 or 8×8 optical switches that are already in commercial use.  
           [0004]    As described above, an optical switching system is implemented by interconnecting a number of optical components and optical fibers with connectors and splices, so optical signals passing through the system suffer degradation due to optical loss in the components, and to various conditions at the connecting points, such as dirt, axial deviation, and open connection ends, which may give rise to the departure of part of an optical signal from the proper course. In particular, reflection in the direction opposite to the proper direction of propagation causes degradation of the optical signal.  
           [0005]    Some optical signal processors and optical components that detect optical reflection have already been introduced. optical switches such as the one disclosed in JP-A-358261/2000 have been suggested, which comprises a reflected light detector at the input terminal thereof and a reflector at the output terminal thereof, and checks internal paths by confirming that an optical signal input from the input terminal is reflected back to the input terminal.  
           [0006]    In optical signal processors configured by combining a plurality of optical components such as optical amplifiers, optical switches, optical couplers, and optical isolators as mentioned above, light reflected at a connection point of another optical component, resulting in multiply reflected light.  
           [0007]    This multiply reflected light becomes a delayed version of the intended optical signal, so it interferes with the intended optical signal (causing degradation of the optical signal). Recent studies by the present inventor(s) have resulted in the discovery that degradation of optical signals caused by such multiply reflected light has a major effect on the operation of optical signal processors configured by combining a plurality of optical components.  
           [0008]    More specifically, it was discovered that, in the optical switching system  300  in FIG. 2, when an optical (digital) signal  370  transmitted through optical fibers  310 - 1  to  310 -N proceeds from an input port  330 -N to an output port  340 -N, multiply reflected light  375  that has been delayed at a reflecting point  1  indicated by reference numeral  350  and a reflecting point  2  indicated by reference numeral  360  may superimpose itself on the optical signal  370 , causing coherent crosstalk, or interference between the optical signal and the multiply reflected light may form a resonator that is not actually present in the system. If a wavelength multiplexed signal is processed optically in an optical signal processor configured by combining a plurality of optical components, various types of optical degradation due to multiply reflected light may occur on a random basis: for example, (1) wavelength-dependent variations in optical-loss characteristics, (2) occurrence of signal amplitude noise due to wavelength fluctuations of an intended signal, and (3) wavelength dispersion. It has been found that these effects have a major effect on the operation of the system.  
           [0009]    Therefore, practical utilization of an optical signal processor configured by combining a plurality of optical components requires configurations and methods by which reflected light arising in the processor during assembly, installation, or operation thereof can be detected reliably and immediately to enable alteration of optical signal paths and recovery actions (maintenance) such as replacement and repair of components, thereby improving the reliability, availability, and serviceability of the processor.  
           [0010]    The document mentioned above describes a configuration for detecting singly reflected light, but it does not provide configurations and methods for implementing systems that address the problems of multiply reflected light in an optical signal processor configured by combining a plurality of optical components.  
         SUMMARY OF THE INVENTIION  
         [0011]    An object of the present invention is to solve the above problems of optical switching systems configured by combining a plurality of optical components, by providing an optical switching system with functions enabling reliable and immediate detection and notification of reflected light, and providing methods enabling reliable and immediate detection and notification of reflected light arising in an optical switching system.  
           [0012]    Another object of the present invention is to provide a more highly reliable, available, and serviceable optical switching system by providing a simplified configuration enabling reliable and immediate detection and notification of reflected light arising therein, thus enabling the replacement of optical signal paths and recovery actions (maintenance) such as replacement and repair of the optical components.  
           [0013]    Another object of the present invention is to provide a method comprising simplified procedural steps for reliable and immediate detection and notification of reflected light arising in an optical switching system, thereby improving the reliability, availability, and serviceability thereof.  
           [0014]    Another object of the present invention is to provide an optical switching device with a plurality of optical input ports and a plurality of optical output ports, comprising optical reflection monitors with optical reflection monitoring functions provided between the plurality of optical input ports and the plurality of optical output ports.  
           [0015]    Another object of the present invention is to provide an optical switching system configured by multistage-connecting a plurality of optical switching devices, wherein each optical switching device comprises a plurality of optical reflection monitors having optical reflection monitoring functions, and the optical reflection monitors can detect reflected light on paths followed by optical signals input to the optical switching device and use the optical reflection monitoring function to locate the point of reflection on the path.  
           [0016]    Another object of the present invention is to provide a reflected light measuring system comprising a terminal with reflected light measuring software, an optical switching system including optical switching units that control switching of optical signals, reflected light meters that measure reflected light of optical signals, and port selectors that select the input path of an optical signal input to the optical switching unit, wherein the software can be executed to control the operation of the reflected light meters, port selectors, and optical switching system, and thereby measure the reflected light of the optical signal to locate reflection positions.  
           [0017]    Another object of the present invention is to provide an optical switching method capable of detecting reflected light, comprising steps of performing settings for switching of an optical switch and storing optical interconnection relationships; selecting a circuit board equipped with the optical switching device according to a command from an operation control unit and storing optical reflection alarm information; and locating a position at which reflection is occurring according to the stored optical interconnection relationships and optical reflection alarm information.  
           [0018]    Another object of the present information is to provide a method of setting optical switching information and optical reflection alarm information in an optical switching device, comprising steps in which a switching control unit in the optical switching device performs settings for switching of an optical switch and settings of a switching information register, and a CPU selects an optical reflection monitoring circuit, then transfers a signal from the optical reflection monitoring circuit, after analog-to-digital conversion, to a monitoring and control unit, and sets an optical reflection monitoring register therein.  
           [0019]    Another object of the present invention is to provide a reflection position measuring method using an optical reflection measuring system in an optical switching unit, comprising steps of transmitting a switching command to a port selector under control of a portable terminal; transmitting the switching command to an optical switching unit under control of the portable terminal; requesting a measured value from a reflected light meter; and searching in an optical reflection alarm control table and an interconnection control table to determine an abnormal alarm position.  
           [0020]    Another object of the present invention is to provide an optical switching device comprising a plurality of optical reflection monitors with optical reflection monitoring functions disposed between a plurality of optical input ports and a plurality of optical output ports, that receives an optical signal input through an optical input port and uses the optical reflection monitors to monitor reflected light arising at certain points along the transmission paths between the plurality of optical input ports and the plurality of optical output ports, thereby enabling immediate notification of abnormal conditions in connecting cables along the optical transmission paths. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a drawing showing an example of the structure of a communication network equipped with an optical switching system according to the present invention;  
         [0022]    [0022]FIG. 2 is a drawing explaining the effects of reflected light arising in optical switching systems;  
         [0023]    [0023]FIG. 3 is a drawing showing an example in which a reflection alarm is issued in an optical switching system with multistage-connected optical switching devices according to the present invention;  
         [0024]    [0024]FIG. 4 is a drawing showing another example in which a reflection alarm is issued in an optical switching system with multistage-connected optical switching devices according to the present invention;  
         [0025]    [0025]FIG. 5 is a drawing showing an exemplary block diagram of an optical switching device according to the present invention;  
         [0026]    [0026]FIG. 6 is a block diagram of an optical switching system with an external measuring instrument for measuring reflected light in the switching system and an external port selector;  
         [0027]    [0027]FIG. 7 is a drawing showing an example of the structure of an optical branching unit and optical detector in an optical switching device according to the present invention;  
         [0028]    [0028]FIG. 8 is a drawing showing an example of the structure of an optical branching unit, optical isolator, and optical detector in an optical switching device according to the present invention;  
         [0029]    [0029]FIG. 9 is a drawing showing an example of the structure of an optical circulator and optical detector in an optical switching device according to the present invention;  
         [0030]    [0030]FIG. 10 is a diagram showing the flow of operations based on the system configuration shown in FIGS. 3 and 4 according to the present invention;  
         [0031]    [0031]FIG. 11 is a flow diagram of operations based on the exemplary structure shown in FIG. 4 according to the present invention;  
         [0032]    [0032]FIG. 12 is a flow diagram of operations based on the exemplary structure shown in FIG. 5 according to the present invention;  
         [0033]    [0033]FIGS. 13A and 13B are drawings showing an optical reflection alarm information table according to the present invention; and  
         [0034]    [0034]FIG. 14 is a drawing showing an interconnection control table according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0035]    Embodiments illustrating the structure of optical signal switching apparatus according to the present invention and the use thereof will be described with reference to the attached drawings, in which like parts are indicated by like reference characters in FIGS. 3 and 4. FIG. 1 is a drawing of a network configuration that will be used as an example of a communication network adopting optical signal switching apparatus according to the present invention. Optical signal switching apparatus  100  ( 100 - 1  to  100 - 9 ) is interconnected with optical fibers  200  ( 200 - 1  to  200 - 12 , and others) to form a communication network. A more specific embodiment includes a pair of optical cross-connects (each referred to as an OXC below:  100 - 1  and  100 - 2 ) that switch and output multiplexed optical signals received from the optical fibers ( 200 - 1  to  200 - 5  and others) to appropriate destination optical fibers, and optical add drop multiplexers (each referred to as an OADM below:  100 - 3  to  100 - 9 ) that separate an optical signal from or insert an optical signal into the multiplexed optical signals received from the optical fibers ( 200 - 5  and  200 - 9 ) as required by the devices connected thereto and transmit and receive optical signals to and from the optical fibers ( 200 - 6  to  8 , and  200 - 10  to  12 ). A communication network is constructed by connecting the optical signal switching apparatus according to the present invention and these optical fibers having proper multiplexing degrees and transmission rates, as required.  
         [0036]    An optical signal switching system according to the present invention simplifies network construction by enabling proper selective use of components to construct flexible communication networks capable of supporting various optical signal transmission rates and multiplexing degrees. For example, the system can handle both optical signals with transmission rates exceeding the STM-0 (51.84 MHz) level established by an ITU-T Recommendation or unmodulated (dc) light, and places no limitations on the presence or absence of wavelength division multiplexing and the number of multiplexed wavelengths. An optical signal switching system providing 8 paths with 32 multiplexed wavelengths requires an OXC capable of 256×256 switching. In this case, it becomes impossible to implement a compact signal switching system by using electronic circuits, so the present invention provides a significant effect.  
         [0037]    [0037]FIGS. 3 and 4 are block diagrams of an optical switching system  300  with multistage-connected optical switches.  
         [0038]    A system control and monitoring unit  405  comprises an interconnecting structure with a bus  436 , an I/O unit  410  that communicates with an operation control unit  400 , a CPU  415  that controls the overall system control and monitoring unit  405 , a switching information memory  420 , an optical reflection alarm information memory  425 , and an I/O unit  430  that interconnects multistage-connected circuit boards (CBs)  700 - 1  to  700 - 6  with conducting wire  435 . Each of the CBs includes a circuit board control and monitoring unit  440 -N that controls and monitors the CB, an optical reflection monitor  460 -X-N that monitors reflected light in the CB, and an optical switching unit  465 -N that switches optical paths in the CB. The switching information memory unit  420  stores an interconnection control table shown in FIG. 14; the optical reflection alarm information memory  425  stores an optical reflection alarm control table shown in FIGS.  12 ( a ) and ( b ).  
         [0039]    The CPU  415  accesses these control tables to control switching and other processing of the circuit boards  700 - 1  to  700 - 6 . The operation control unit  400  collects circuit board status information from the multistage-connected circuit boards  700 - 1  to  700 - 6 , switching status information from the optical switching unit, alarm information provided from the optical reflection monitors when reflection occurs, and other information through the I/O unit  430  and relays switching settings and other commands to each of the circuit boards. The multistage-connected circuit boards  700 - 1  to  700 - 6  are used for optical transmission and switching. The multistage connection structure includes several redundant paths, providing alternative detour paths in the case of a failure in a circuit board.  
         [0040]    In the example of the system structure shown in FIGS. 3 and 4, an optical signal  690 - 1 , for example, is transmitted on a transmission path sequentially from an input port  455 - 1 - 1 , through the optical switching unit  465 - 1 , an output port  475 - 1 - 1  of the circuit board  700 - 1 , an input port  455 - 3 - 1 , the optical switching unit  465 - 3 , an output port  475 - 3 - 1  of the circuit board  700 - 3 , an input port  455 - 5 - 1 , and the optical switching unit  465 - 5 , to an output port  475 - 5 - 1  of the circuit board  700 - 5 .  
         [0041]    In the example of FIG. 3, when there is a reflecting point in the input port  455 - 3 - 1  of CB 700 - 3  and reflected light is monitored in optical reflection monitors  470 - 1 - 1  and  460 - 1 - 1 , a value indicating an abnormal condition is written into both ingress and egress columns under CB 1  in the optical reflection alarm control table shown in FIG. 13A. Based on the monitoring results, maintenance or other personnel can replace optical cables and circuit boards with new ones at the position where the reflection is occurring.  
         [0042]    [0042]FIG. 5 is a block diagram showing the details of one of the circuit boards  700 - 1  to  700 - 6  included in the optical switching system described with reference to FIGS. 3 and 4. In FIG. 5, an I/O unit  710 , a CPU  715 , a switching information register unit  720 , an optical reflection monitoring register  725 , a switch control unit  730  that controls switching of switches in an optical switching unit  465 -X, a monitoring control unit  765 , analog-to-digital (A/D) converters  735  and  770 , and a driver  755  that drives the optical switching unit are interconnected via a bus  713 , forming a circuit board control and monitoring unit  440 -X that is controlled by the CPU  715 . The monitoring control unit  765  monitors digital signals obtained through detection of reflected light in the optical detectors  750 - 1  to  750 -N and  775 - 1  to  775 -N and A/D conversion of the detected reflected light in A/D converters  735  and  770 .  
         [0043]    The optical detectors  750 - 1  to  750 -N and  775 - 1  to  775 -N monitor reflected light of an optical signal that has been branched out from the optical signal in optical branching circuits  745 - 1  to  745 -N and  780 - 1  to  780 -N, each of which comprises an optical coupler and other optical components, and transmits it to the A/D converters  735  and  770  as a monitored signal. The behaviors of the switching information register unit  720  and the optical reflection monitoring register unit  725  will be described later with reference to the flow diagram shown in FIG. 11.  
         [0044]    The driver  755  and the switching control unit  730  set optical transmission paths from the input ports  455 -X- 1  to  455 -X-N to the output ports  475 -X- 1  to  475 -X-N of the optical switching units  465 -X.  
         [0045]    Circuits  460 -X- 1  to  460 -X-N and  470 -X- 1  to  470 -X-N (shown in the boxes enclosed with a broken line in FIG. 5), which are combinations of optical branching circuits  745 - 1  to  745 -N and  780 - 1  to  780 -N and optical detectors  750 - 1  to  750 -N and  775 - 1  to  755 -N respectively correspond to the optical reflection monitor circuits (or optical reflection monitors)  460 - 1 - 1  to  460 - 6 -N and  470 - 1 - 1  to  470 - 6 -N in each circuit board shown in FIGS. 3 and 4. Each optical reflection monitor can detect reflected light on an optical transmission path followed by an optical signal input to the optical switching device mounted on the circuit board, and locate the reflecting positions along the path.  
         [0046]    FIGS.  7  to  9  show specific examples of the structure of the optical detector  750  and an optical branching circuit that form the optical reflection monitor connected to the optical switching devices mounted on CB  700  in FIG. 5 described above.  
         [0047]    The optical reflection monitor shown in FIG. 7 comprises an optical detector  1010  for monitoring optical power of an optical signal  1015 - 1  input to an optical branching circuit  1000  (provided to separate the optical signal and reflected light thereof), and an optical detector  1005  that monitors reflected light  1020 - 1  or  1020 - 2  of the optical signal  1015 - 1  or  1015 - 2  at the optical connector. This structure makes it possible to determine the amount of reflection loss accurately as the ratio of input power to reflected light power.  
         [0048]    [0048]FIG. 8 shows an optical reflection monitor with a structure comprising a combination of an optical isolator  1100 , an optical branching circuit  1105  that separates reflected light of optical signals, and an optical detector  1110 . The optical isolator  1100  allows an optical signal  1115 - 1  to pass but blocks the reflected light  1120 - 1  that arises at the optical connector. Providing the optical isolator  1100  can prevent reflected light from proceeding beyond the optical detector  1110  (toward the left in the drawing). The optical detector  1110  monitors reflected light  1120 - 3  as described with reference to FIG. 7.  
         [0049]    [0049]FIG. 9 shows an optical reflection monitor with a structure comprising a combination of an optical circulator  1200  that allows the passage of an optical signal and circulates or blocks reflected light thereof and an optical detector  1205 . An optical signal  1210 - 1  is passed through the optical circulator  1200  to the optical connector and other components, while reflected light  1215 - 2  that arises in the optical connector is circulated clockwise in the optical circulator  1200  and transmitted to the optical detector  1205  to be monitored. The optical circulator  1200  has an advantage in that it produces less reflection loss than occurs in the optical coupler used in the optical branching circuit described above, and consequently never weakens the reflected light power.  
         [0050]    [0050]FIG. 10 is a flow diagram showing the procedures for switching operations, collecting optical reflection alarms, and locating abnormal conditions. OPERATION 1 shows a procedure of switching operation, OPERATION 2 shows a procedure for collecting optical reflection alarms, and OPERATION 3 shows a procedure of finding and calculating optical reflection alarm positions and other operations.  
         [0051]    In OPERATION 1, the CPU  415  performs settings for switching optical switches as commanded by the operation control unit  400  in FIGS. 3 and 4 (Step S 10 ); transfers the switching command to an optical switching device mounted on one of the multistage-connected circuit boards concerned (Step S 11 ); and completes required setting for the switching of the optical switch (Step S 12 ). Then the CPU  415  updates the contents of the interconnection control table shown in FIG. 14, which is stored in the switching information memory  420 , in accordance with switching information transferred from the optical switching device (Step S 13 ), and if all settings for switching of optical switches required are completed, terminates OPERATION 1, or otherwise, returns to Step S 10  and repeats the switching setting operation in accordance with the switching command from the operation control unit  400 .  
         [0052]    In OPERATION 2, the CPU  415  selects a circuit board (CB) (Step S 30 ) and requests optical reflection alarm acquisition (Step S 31 ); then the optical reflection alarm information is transferred from an optical switching device mounted on the selected CB to the operation control unit  400  through the CPU  415  (Step S 32 ). At the same time, the contents of the optical reflection alarm information table shown in FIGS. 13A and 13B, which is stored in the optical reflection alarm information memory  425 , are updated (Step S 33 ). For example, if there is an optical reflection alarm, “1” is written into the optical reflection information table in the optical reflection alarm information memory  425  to indicate the presence of an optical reflection alarm. If the monitoring of all circuit boards in OPERATION 2 is completed, the CPU  415  terminates OPERATION 2; otherwise, it returns to Step S 30  to repeat the procedure.  
         [0053]    Finally, in OPERATION 3, if no optical reflection alarm has been generated through OPERATIONs 1 and 2, the CPU  415  terminates the operation. If there is an optical reflection alarm, the optical reflection alarm control table shown in FIGS. 13A and 13B is searched (Step S 20 ), all alarm positions are detected (Step S 21 ), the interconnection control table shown in FIG. 14 is searched (Step S 22 ), a suspected abnormal optical interconnection path is selected (Step S 23 ), the rearmost interconnection of the connecting path on which reflected light is arising is determined and the reflection position is reported to the operation control unit  400  (Step S 24 ). If the rearmost interconnections for all optical reflection alarms have been found and reported, then the CPU  415  terminates operations in OPERATION 3; otherwise, it repeats the procedure of OPERATION 3.  
         [0054]    [0054]FIG. 11 is a flow diagram of the operation of the CPU  715 , the switching control unit  730 , the monitoring and control unit  765 , the switching information register unit  720 , and the optical reflection monitoring register  725  in FIG. 5 showing the details of each circuit board in the system structure shown in FIGS. 3 and 4.  
         [0055]    In OPERATION 1, the switching control unit  730  performs settings for required switching of an optical switch in an optical switching unit (Step S 40 ), the CPU  715  sets the switching information register unit  720  in accordance with the switching information (Step S 41 ), and if switching for all the settings is completed (Step S 42 ), then terminates the operation, or otherwise, returns to Step S 40  and repeats the procedure. These operations and settings can be executed directly by the system control and monitoring unit  405 .  
         [0056]    In OPERATION 2, the CPU  715  selects an optical reflection monitoring circuit (Step S 50 ); compares an A/D-converted output value from a designated optical reflection monitoring circuit to a threshold stored in the CPU  715 , the monitoring and control unit  765 , or the optical reflection monitoring register  725  (Step S 51 ); writes “1” for an abnormal condition and “0” for a normal condition into a memory in the monitoring control unit  765  and sets the optical reflection monitoring register  725  (Step S 52 ); and if the settings for all the optical reflection monitoring circuits are completed (Step S 53 ), then terminates OPERATION 2, or otherwise, returns to Step S 50  and repeats the procedure.  
         [0057]    [0057]FIGS. 13A and 13B show optical reflection alarm control tables. The tables are stored in the optical reflection alarm information memory  425  shown in FIGS. 3 and 4, and indicate the presence or absence of reflected light arising at the ingress and egress ports  1  to N of each of the circuit boards (CB 1  to CB 6 ) on which the multistage-connected optical switching units are mounted in the system structures (FIGS. 3 and 4) of the optical switching system  300 , as abnormal or normal condition information. In these tables, CB 1 , CB 3 , and CB 5  correspond to circuit boards  700 - 1 ,  700 - 3 , and  700 - 5 ; CB 2 , CB 4 , and CB 6  correspond to circuit boards  700 - 2 ,  700 - 4 , and  700 - 6 .  
         [0058]    The condition information (normal or abnormal) at the input and output ports  1  to N of each circuit board is monitored in the CPU  415  shown in FIGS. 3 and 4, and switching of the optical switching unit in the circuit board is carried out in accordance with the condition information. The control table shown in FIG. 13A indicates that there is a reflecting point at the input port  455 - 3 - 1  of circuit board  700 - 3  in the system structure in FIG. 3 and the reflected light has been monitored. The control table shown in FIG. 13B indicates that there is a reflecting point in the optical switching unit  465 - 3  of circuit board  700 - 3  in the system structure in FIG. 4 and the reflected light has been monitored.  
         [0059]    The interconnection control table in FIG. 14 is stored in the switching information memory  420  shown in FIGS. 3 and 4, which indicates the input-to-output port interconnection information within each of the circuit boards CB 1  to CB 6  described with reference to FIGS. 13A and 13B and CB-to-CB port interconnection information. For example, the table shows interconnection between input port  1  and output port  1  and between input port N and output port  1  within CB 1 . The CB-to-CB interconnection column indicates CB 1 -to-CB 3  or CB 4 , CB 2 -to-CB 3  or CB 4 , CB 3 -to-CB 5  or CB 6 , and CB 4 -to-CB 5  or CB 6  interconnection information.  
         [0060]    Interconnecting conditions indicated by the interconnection control table conform to the optical cabling of CB 1  to CB 6  shown in FIGS. 3 and 4. The content of this interconnection control table is also monitored by the CPU  415  shown in FIGS. 3 and 4 as is the case with the optical reflection alarm information table shown in FIGS. 13A and 13B.  
         [0061]    In the structure shown in FIG. 3, it is possible to use the optical reflection alarm control table (FIG. 13A) and the interconnection control table (FIG. 14) in accordance with the flow diagram (FIG. 10) to locate reflecting points. Specifically, FIG. 13A indicates an alarm from the optical reflection monitors  460 - 1 - 1  and  470 - 1 - 1  of CB 1  and no alarm from the optical reflection monitor  460 - 3 - 1 . Reference to the interconnection control table in FIG. 14 shows that there may be abnormal conditions in the optical fiber from CB 1  output port  1  to CB 3  input port  1  or in the connectors of this optical fiber. In the case of FIG. 4, the table in FIG. 13B also indicates an alarm from optical reflection monitor  460 - 3 - 1  of CB 3 . Reference to the interconnection control table in FIG. 14 shows there may be an abnormal condition in the connection path from input port  1  to output port  1  within CB 3 . In this way, the optical reflection alarm control table and interconnection control table can be used in accordance with the procedure of the flow diagram shown in FIG. 10 to identify failures. As a result of identifying the failures, alarms can be issued and signals can be switched over to paths that are still normal.  
         [0062]    [0062]FIG. 6 is a block diagram of a structure for providing a reflected light measuring function by combining a personal computer (PC)  801  that operates as a portable terminal with stored software such as a reflected light measuring program  870 , an optical switching system  300 , outboard devices including a reflected light meter  800  and a port selector  830 . The personal computer  801  executes the reflected light measuring program  870  using an I/O cable  877 - 1  to send a reflected light measuring command to the optical switching system  300 , the port selector  830 , and the reflected light meter  800  through a bus  877  to measure reflected light.  
         [0063]    The reflected light meter  800  includes a laser diode or other electronic device as a light source  820  for generating test light. The optical signal  880 - 2  is transferred to the optical switching unit through a port  925 - 1 , for example, which is selected in the port selector  830 . The port selector  830  receives reflected light  885 - 1  from a port  930 - 1  of the optical switching unit, and transmits it back to a reflected light separating unit  825  in the reflected light meter  800  via a port  915 .  
         [0064]    A control unit  840  in the port selector  830  controls the port selecting unit that selects a port in accordance with command information sent via a bus  877 ; an I/O unit  835  is connected to the system control and monitoring unit  850  and reflected light meter  800  via the bus  877 .  
         [0065]    A control and monitoring unit  810  in the reflected light meter  800  monitors reflected light  885 - 4  that has been separated in the reflected light separating unit  825  in the optical detector  815 , and supervises the monitored signal. The control and monitoring unit  810  controls inside the reflected light meter  800  in accordance with the command information sent via the bus  877 , and it can also store the reflected light measuring program  870 .  
         [0066]    The system control and monitoring unit  850  in the optical switching system  300  comprises an I/O unit  855  that sends measurement commands to the reflected light meter  800  and the port selector  830  that are outboard equipment, an I/O unit  899  that sends switching commands to the optical switching unit  851  in the optical switching system  300 , a CPU  860  that globally controls the optical switching system  300 , a switching information memory  865  that stores the optical interconnection relationships in the optical switching unit, and the optical reflection alarm information memory  875  that stores the optical reflection alarm control table shown in FIGS. 13A and 13B and stores alarm information on reflected light that is transmitted from the reflected light meter; these elements are interconnected via a bus  856  etc. If the optical switching unit  851  is equivalent to the unit comprising a plurality of circuit boards shown in FIGS. 3 and 4, the switching information memory  865  stores the interconnection control table shown in FIG. 14. The reflected light measuring program  870 , including a testing program for measuring reflected light, can also be provided within the optical switching system  300 . In that case, the program issues measuring commands to be executed by the port selector  830  and the reflected light meter  800  to be executed and controls the optical switching unit with reference to the test results obtained from the reflected light meter  800  via bus  877 . Output ports  935 - 1  to  935 - 4  of the optical switching unit are terminated in the optical isolator and other components during measurement.  
         [0067]    [0067]FIG. 12 shows a flow diagram of operations in each block that operates under control of the CPU  860  in the reflected light measuring system shown in FIG. 6. In an environment in which the reflected light measuring program is executed on the personal computer  801 , the CPU  860  in the optical switching system  300  transfers a switching command to the port selector  830  (Step S 60 ); transfers the switching command to the optical switching unit (Step S 61 ); sends the reflected light meter  800  a request to acquire a reflected light measurement value (Step S 62 ); receives the reflected light measurement value transferred from the reflected light meter (Step S 63 ); then compares the reflected light measurement value with the threshold stored in the optical reflection alarm information memory  875  or the CPU  860  (Step S 64 ); if the measurement value is smaller than the threshold, writes “1” indicating an abnormal condition, or otherwise, writes “0” indicating a normal condition into the optical reflection alarm information memory  875 , thereby updating the memory (Step S 65 ); if measurements have been completed for all paths in the optical switching unit (Step S 66 ), references the optical reflection alarm information memory  875 ; and if there is a reflection alarm, sets a reflection alarm indication (Step S 67 ), or otherwise, returns to the starting point. After that, the CPU  860  searches a table similar to the optical reflection alarm control table in FIGS. 13A and 13B that indicate the conditions (indicated as normal or abnormal) of reflected light on each input and output port of the optical switching unit  851  (FIG. 6) (Step S 68 ), detects all optical reflection alarm positions in the abnormal conditions (Step S 69 ), searches a table similar to the interconnection control table in FIG. 14 that indicates interconnection status on each input and output port of the optical switching unit  851  (FIG. 6) (Step S 70 ),selects suspected abnormal cable connections (Step S 71 ), and determines the rearmost interconnection having reflection in its connection cable (Step S 72 ). If searching of all optical reflection alarms is completed, the CPU  860  terminates the operation; otherwise, it returns to Step S 67 .  
         [0068]    As described above, the present invention simplifies the detection of optical reflection causing degradation of signals, and consequently simplifies the installation and maintenance of the system. In addition, it becomes possible to provide functions enabling reliable and immediate detection and notification of reflected light in optical switching system configured by combining a plurality of optical components. Furthermore, the invention provides a method of reliable and immediate detection and notification of reflection in an optical switching system.  
         [0069]    It also becomes possible to provide optical switching systems with higher reliability, availability, and serviceability in a simplified configuration in which reliable and immediate detection and notification of reflected light makes possible the switching of optical signal paths and recovery actions (maintenance) including replacement and repair of components.  
         [0070]    It also becomes possible to provide a method enabling reliable and immediate detection and notification of reflected light arising in an optical switching system with simpler procedures, and improve the reliability, availability, and serviceability of the system.  
         [0071]    Furthermore, combination with an optical reflection prevention circuit (isolator) makes it possible to confine reflecting positions within a certain range, and the use of circulators can improve utilization efficiency and facilitate design of optical power monitors.  
         [0072]    In addition, if a circulator is used, it can also function as a reflection prevention circuit.