Patent Publication Number: US-2023155675-A1

Title: Failure detection device, failure detection method, and failure-detection-program recording medium

Description:
TECHNICAL FIELD 
     The present invention relates to a failure detection device, a failure detection method, and a failure-detection-program recording medium, and particularly relates to a failure detection device, a failure detection method, and a failure-detection-program recording medium in a wavelength multiplexing optical transmission system. 
     BACKGROUND ART 
     Among recent submarine cable systems, “spectrum sharing,” in which a plurality of wavelength bands are independently used by a plurality of users, is widespread. In a system in which the spectrum sharing is applied, a land-based MUX/DEMUX receives, from a terminal station of each of the plurality of users, a wavelength-multiplexed optical signal in a band that is allocated for each of the users. The MUX/DEMUX multiplexes these wavelength-multiplexed optical signals at a MUX and thereby generates a WDM signal, and sends out the WDM signal to a submarine cable. MUX/DEMUX is an abbreviation for a multiplexer/demultiplexer. MUX is an abbreviation for a multiplexer. WDM is an abbreviation for wavelength division multiplexing. 
     The MUX/DEMUX receives the WDM signal transmitted through the submarine cable, and demultiplexes, by using a demultiplexer (DEMUX), the WDM signal into WDM signals each having a wavelength band of each user. The demultiplexed WDM signal is sent out to a terminal station of each of the users. 
     In relation to the present invention, a technique for power measurement of a WDM signal is described in PTLs 1 and 2. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] International Publication No. WO 2018/051935 
     [PTL 2] Japanese Unexamined Patent Application Publication No. 2015-220553 
     SUMMARY OF INVENTION 
     Technical Problem 
     In an optical transmission system in which optical signals of a plurality of users are wavelength-multiplexed and then transmitted, for early recovery from a failure, it is desired that a failure in a line between a terminal station of a user and a MUX/DEMUX can be detected for each terminal station (specifically, for each user). However, since the optical signals of the users being transmitted from the terminal stations are multiplexed in a MUX, there is a problem that, in a general WDM optical transmission system, it is difficult to learn, from a multiplexed WDM signal, in which user&#39;s line a failure has occurred. 
     Object of Invention 
     An object of the present invention is to provide a technique of detecting, in a WDM optical transmission system in which optical signals transmitted by a plurality of terminal stations are wavelength-multiplexed and then transmitted, a terminal station of a line in which a failure has occurred. 
     Solution to Problem 
     A failure detection device according to the present invention includes: an input means for receiving, from a plurality of terminal stations, first optical signals each having a wavelength allocated, based on allocation information, to each of the plurality of terminal stations, and for coupling the received first optical signals; a monitoring means for outputting a monitoring signal being a signal according to intensity relevant to a wavelength of each of the coupled first optical signals; and an identification means for identifying a first terminal station from the plurality of terminal stations, based on the allocation information and the monitoring signal. 
     A failure detection method according to the present invention includes procedures of: receiving, from a plurality of terminal stations, first optical signals each having a wavelength allocated, based on allocation information, to each of the plurality of terminal stations; coupling the received first optical signals; outputting a monitoring signal being a signal according to intensity relevant to a wavelength of each of the coupled first optical signals; and identifying a first terminal station from the plurality of terminal stations, based on the allocation information and the monitoring signal. 
     A failure-detection-program recording medium according to the present invention records a program causing a computer of a failure detection device to execute: a procedure of receiving, from a plurality of terminal stations, first optical signals each having a wavelength allocated, based on allocation information, to each of the plurality of terminal stations; a procedure of coupling the received first optical signals; a procedure of outputting a monitoring signal being a signal according to intensity relevant to a wavelength of each of the coupled first optical signals; and a procedure of identifying a first terminal station from the plurality of terminal stations, based on the allocation information and the monitoring signal. 
     Advantageous Effects of Invention 
     The present invention enables, in a WDM optical transmission system, detection of a terminal station in which a failure has occurred. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating a configuration example of a failure detection device  10  according to a first example embodiment. 
         FIG.  2    is a flowchart illustrating an example of an operation process of the failure detection device  10 . 
         FIG.  3    is a flowchart illustrating an example of an operation process of a failure detection device  10  according to a second example embodiment. 
         FIG.  4    is a block diagram illustrating a configuration example of a failure detection device  10  according to a third example embodiment. 
         FIG.  5    is a block diagram illustrating a configuration example of a failure detection device  11  according to a fourth example embodiment. 
         FIG.  6    is a flowchart illustrating an example of an operation process of the failure detection device  11 . 
         FIG.  7    is a flowchart illustrating another example of an operation process of the failure detection device  11 . 
         FIG.  8    is a block diagram illustrating an example in which an output unit  140  includes a demultiplexing unit  141 . 
         FIG.  9    is a diagram for describing an example of control of the demultiplexing unit  141  by an output control unit  150 . 
         FIG.  10    is a diagram illustrating a sixth example embodiment. 
         FIG.  11    is a block diagram illustrating a configuration example of an optical transmission system  1  according to a seventh example embodiment. 
     
    
    
     EXAMPLE EMBODIMENT 
     Example embodiments of the present invention are described blow with reference to the drawings. An allow illustrated in the drawings exemplifies a direction of a signal or an order of processing, and no limitation on the direction of a signal or the order of processing is intended. In the example embodiments and the drawings, same reference signs are given to already described elements, and overlapping description is omitted. 
     First Example Embodiment 
       FIG.  1    is a diagram illustrating a configuration example of a failure detection device  10  according to a first example embodiment of the present invention. The failure detection device  10  includes an input unit  110 , a monitoring unit  120 , and an identifying unit  130 . Each of n terminal stations  201  to  20   n  installed for each user is connected to the input unit  110  via each of optical transmission paths  211  to  21   n . N is a natural number equal to or greater than two. The terminal stations  201  to  20   n  send out optical signals (first optical signals) having a wavelength that is allocated, based on allocation information, for each of the optical signals to the terminal stations to the optical transmission paths  211  to  21   n . The wavelengths of the optical signals transmitted by the terminal stations  201  to  20   n  do not overlap. Specifically, the terminal stations  201  to  20   n  transmit the first optical signals each having a wavelength allocated for each user in such a way as not to overlap with one another, to the failure detection device  10  via the optical transmission paths  211  to  21   n.    
     The input unit  110  receives the first optical signal from each of the optical transmission paths  211  to  21   n , and couples the received first optical signals and outputs the coupled first optical signals. The input unit  110  is one example of an input means. The monitoring unit  120  generates a monitoring signal, and outputs the generated monitoring signal. The monitoring signal is a signal according to intensity at each wavelength of an optical signal included in the first optical signals coupled at the input unit  110 . The monitoring unit  120  branches, for example, a part of an input optical signal, converts the branched part of the input signal into a signal having an amplitude indicating intensity of the branched optical signal at each wavelength, and outputs the signal. An optical channel monitor (OCM) commonly used in a WDM optical transmission system may be used as the monitoring unit  120 . The monitoring unit  120  is one example of a monitoring means. 
     The identifying unit  130  has a function of identifying the first terminal station from the plurality of terminal stations, based on the allocation information and the monitoring signal. An identification result (information of the identified first terminal station) may be output to outside of the failure detection device  10 , or may be used for internal control of the failure detection device  10 . The allocation information may be stored in the identifying unit  130 . The identifying unit  130  is one example of an identification means. 
       FIG.  2    is a flowchart illustrating an example of operation process of the failure detection device  10 . The input unit  110  receives first optical signals from the terminal stations  201  to  20   n , and couples the received first optical signals (step S 01  in  FIG.  2   ). The monitoring unit  120  outputs a monitoring signal according to intensity relevant to a wavelength of each of the first optical signals (step S 02 ). Then, the identifying unit  130  identifies a first terminal station from the plurality of terminal stations, based on the monitoring signal output from the monitoring unit  120 . 
     The failure detection device  10  provided with such a configuration can identify the first terminal station from the plurality of terminal stations, based on the allocation information of a wavelength for a terminal station and the monitoring signal according to intensity relevant to a wavelength of the received optical signal. For example, the failure detection device  10  can identify, according to intensity of the first optical signal indicated by the monitoring signal relevant to a wavelength allocated for a certain terminal station, the terminal station as a terminal station in which a failure has occurred. 
     Second Example Embodiment 
       FIG.  3    is a flowchart illustrating an example of an operation process of a failure detection device  10  according to a second example embodiment of the present invention. A configuration of the failure detection device  10  is similar to that in  FIG.  1   . In the process according to the present example embodiment, an identifying unit  130  compares, for each wavelength allocated for terminal stations  201  to  20   n , intensity of a first optical signal indicated by a monitoring signal with a predetermined threshold value. Specifically, an input unit  110  receives the first optical signals from the terminal stations  201  to  20   n , and couples the received first optical signals (step S 11  in  FIG.  3   ). A monitoring unit  120  outputs a monitoring signal according to intensity relevant to a wavelength of each of the coupled first optical signals (step S 12 ). The identifying unit  130  acquires, from the monitoring signal, the intensity of the first optical signal, and compares the intensity for each wavelength indicated by the monitoring signal with a predetermined threshold value (step S 13 ). When the intensity of the first optical signal at a certain wavelength is less than the predetermined threshold value (step S 13 : YES), the identifying unit  130  refers to allocation information, searches a terminal station for which the wavelength is allocated, and identifies the searched terminal station as a first terminal station (step S 14 ). When intensity of the first optical signal at the wavelength is equal to or more than the predetermined threshold value (step S 13 : NO), the identifying unit  130  does not identify the terminal station, and continues the comparison in step S 13 . 
     When a failure occurs in any one of the terminal stations  201  to  20   n  or in any one of optical transmission paths  211  to  21   n , it is likely that intensity of the first optical signal transmitted through a path including a point where the failure has occurred decreases. Therefore, when intensity of a wavelength allocated for a certain terminal station becomes less than a predetermined threshold value, the failure detection device  10  can identify, by using the process illustrated in  FIG.  3   , the terminal station as a terminal station that may affect a user due to a failure. 
     Note that, each of the first optical signals transmitted by the terminal stations  201  to  20   n  may be a WDM signal including a plurality of optical carriers. In this case, the monitoring unit  120  outputs a monitoring signal according to intensity of each of the optical carriers included in first optical signals coupled at the input unit  110 . For example, the monitoring unit  120  outputs a monitoring signal having an amplitude according to the intensity of each of the optical carriers included in the coupled optical signals. When a plurality of optical carriers having different wavelengths are allocated for a single terminal station, the identifying unit  130  may identify the first terminal station, based on all or some of intensities of the allocated plurality of optical carriers. For example, the identifying unit  130  identifies the first terminal station, based on a maximum value, a minimum value, a median value, or an average value of the intensities of the plurality of optical carriers included in each of the first optical signals received from the terminal stations  201  to  20   n.    
     Third Example Embodiment 
       FIG.  4    is a block diagram illustrating a configuration example of a failure detection device  10  according to a third example embodiment of the present invention. In the present example embodiment, an input unit  110  includes a multiplexing unit  111 . The multiplexing unit  111  includes at least n input ports A 1  to An, and receives, at each of the input ports A 1  to An, each of first optical signals received from terminal stations  201  to  20   n . The multiplexing unit  111  couples the received first optical signals and outputs the coupled optical signals to a monitoring unit  120 . 
     The multiplexing unit  111  is a multiplexer (MUX) that is capable of wavelength-multiplexing optical signals having n or more wavelength bands in accordance with allocation information, and is, for example, an arrayed waveguide grating (AWG) or a wavelength selective switch (WSS). When wavelength bands of the first optical signals that are input to the input unit  110  do not overlap, an n×1 optical coupler (star coupler) may be used as the multiplexing unit  111 . 
     Fourth Example Embodiment 
       FIG.  5    is a block diagram illustrating a configuration example of a failure detection device  11  according to a fourth example embodiment of the present invention. In the present example embodiment, a form in which optical transmission paths  211  to  21   n  between a plurality of terminal stations  201 - 20   n  and the failure detection device  11  are fiber pairs is described. One fiber pair includes two optical fibers. The two optical fibers are each used for transmission of an optical signal (specifically, a first optical signal) transmitted by a user device and transmission of an optical signal (a second optical signal, which is described later) received by the user device. 
     When intensity of a first optical signal input to the failure detection device  11  from at least one of the terminal stations  201  to  20   n  decreases, there may be a failure such as disconnection in an optical transmission path (specifically, a fiber pair) between the terminal station and the failure detection device  11 . In order to ensure safety of maintenance work on the fiber pair having a potential failure, it is preferable that output of an optical signal transmitted from the failure detection device  11  to the terminal station via the fiber pair can be stopped. Further in this case, in order to avoid effect on an optical signal transmitted to another terminal station of which fiber pair is normal, it is preferable that transmission of an optical signal to a terminal station can be stopped only for the fiber pair having a potential failure. 
     The failure detection device  11  includes an output unit  140  and an output control unit  150 , in addition to the failure detection device  10  illustrated in  FIG.  1   . In the present example embodiment, the optical transmission paths  211  to  21   n  are fiber pairs. One optical fiber included in each of the fiber pairs is connected in such a way as that the first optical signal transmitted by the terminal stations  201  to  20   n  is received at an input unit  110 . The other optical fiber included in each of the fiber pairs is connected in such a way as that an optical signal (second optical signal) output by the output unit  140  is received at the terminal stations  201  to  20   n.    
     The output unit  140  outputs a second optical signal of which destination is each of the terminal stations  201  to  20   n . The second optical signal may be an optical signal generated by the failure detection device  11 , according to an optical signal received from outside. The second optical signal is transmitted to each of the terminal stations, via the same fiber pair as that for the first optical signal. The second optical signal may be a signal different for each of the terminal stations being a destination. The output unit  140  is one example of an output means. 
     The output control unit  150  receives information of a first terminal station from an identifying unit  130 . The information of the first terminal station includes identification information of the terminal stations  201  to  20   n  and an instruction to output the second optical signal to the first terminal station or an instruction to stop outputting the second optical signal to the first terminal station. Further, the output control unit  150  instructs the output unit  140  to output the second optical signal to the first terminal station or to stop outputting the second optical signal to the first terminal station. The output control unit  150  is one example of an output control means. 
       FIG.  6    is a flowchart illustrating an example of an operation process of the failure detection device  11 . In step S 21  to step S 23  in  FIG.  6   , a first terminal station is identified from a plurality of terminal stations, based on a monitoring signal. Step S 21  to step S 23  is a process similar to that in step S 01  to step S 03  in  FIG.  2    of the first example embodiment. Further, the failure detection device  11  may execute a process in step S 11  to step S 14  in  FIG.  3    instead of the process in step S 21  to step S 23 . The output control unit  150  instructs the output unit  140  to stop outputting the second optical signal to a terminal station identified as the first terminal station (step S 24  in  FIG.  6   ). 
     With such a configuration, the output unit  140  stops output of a second optical signal to the first terminal station. Consequently, the failure detection device  11  can stop transmitting an optical signal to a terminal station only for a fiber pair having a potential failure, while maintaining communication with a terminal station of a user having no failure. 
       FIG.  7    is a flowchart illustrating an example of another operation process of the failure detection device  11 . The process in  FIG.  7    includes a process of restarting output of a second optical signal to a first terminal station. The identifying unit  130  identifies a first terminal station of which intensity of a first optical signal is less than a predetermined threshold value (step S 31  in  FIG.  8   ). The process in steps S 11  to S 14  in  FIG.  3    is applicable to the process of identifying the first terminal station in step S 31 . The output control unit  150  instructs the output unit to stop outputting the second optical signal to the first terminal station, according to a result of the identification by the identifying unit  130  (step S 32 ). 
     After step S 32 , the identifying unit  130  monitors intensity of the first optical signal by using a monitoring signal, and when there is a first terminal station of which intensity of first optical signal has increased to equal to or more than a predetermined threshold value (step S 33 : YES), notifies the output control unit  150  of information of the first terminal station. This information includes an instruction to output the second optical signal to the first terminal station. The output control unit  150  instructs, in accordance with the notified information, the output unit  140  to output the second optical signal of which destination is the first terminal station (step S 34 ). The output unit  140  outputs, according to the instruction from the output control unit  150 , the second optical signal of which destination is the first terminal station (step S 35 ). The identifying unit  130  may exclude, from the first terminal station, the terminal station being the destination of the second optical signal instructed to be output. When there is no first terminal station of which intensity of first optical signal has increased to equal to or more than the predetermined threshold value (step S 33 : NO), the identifying unit  130  continues the monitoring of intensity of the first optical signal using the monitoring signal. The output control unit  150  is notified of information of the first terminal station. 
     Fifth Example Embodiment 
       FIG.  8    is a block diagram illustrating an example in which the output unit  140  of the failure detection device  11  described in the fourth example embodiment includes a demultiplexing unit  141 . The demultiplexing unit  141  is a demultiplexer (DEMUX) that separates a WDM signal input from outside the failure detection device  11  by each wavelength, based on allocation information. Further, the demultiplexing unit  141  outputs each of the demultiplexed optical signals as a second optical signal. Generally, the second optical signal is an optical signal that is different for each terminal station being a destination. However, all or some of the second optical signals to the terminal stations may be identical optical signals. The failure detection device  11  transmits the second optical signal output from the demultiplexing unit  141  to terminal stations  201  to  20   n.    
     The demultiplexing unit  141  demultiplexes a WDM signal of n or more wavelength bands by each of the bands. For example, the demultiplexing unit  141  includes an AWG or a WSS that is conform to the allocation information. Further, in a case in which the WDM signal input to the demultiplexing unit  141  is distributed to the terminal stations  201  to  20   n  without being demultiplexed, a 1×n optical coupler may be used instead of the demultiplexing unit  141 . The 1×n optical coupler is a star coupler that distributes the input WDM signal as n signals. 
       FIG.  9    is a diagram for describing an example of control of the demultiplexing unit  141  by an output control unit  150 . The demultiplexing unit  141  illustrated in  FIG.  9    includes a demultiplexing element  143  and optical switches  142 . The demultiplexing element  143  demultiplexes the input WDM signal into n signals. The optical switches  142  are each provided between n outputs of the demultiplexing element  143  and n output ports B 1  to Bn of the demultiplexing unit  141 . The output control unit  150  controls the n optical switches  142  independently. According to an instruction from the output control unit  150  to output or to stop outputting the second optical signal, the n optical switches  142  transmit or block each of the demultiplexed second optical signals individually.  FIG.  9    illustrates a case in which a failure occurs in an optical transmission path  211  between the terminal station  201  and an input unit  110 , and only the output port B 1  does not output the second optical signal. The identifying unit  130  detects, based the allocation information and a monitoring signal input from a monitoring unit  120 , that intensity of an optical signal having a wavelength that is allocated for the terminal station  201  is less than a predetermined threshold value. Consequently, the identifying unit  130  identifies the terminal station  201  as a first terminal station, and outputs information of the first terminal station to the output control unit  150 . In the case illustrated in  FIG.  9   , the information of the first terminal station includes identification information of the first terminal station (for example, “terminal station  201 ”) and an instruction to stop output of the second optical signal from the output unit  140  to the first terminal station. The output control unit  150  controls, in accordance with the information of the first terminal station, the output port B 1  of the demultiplexing unit  141  connected to the optical transmission path  211  to which the terminal station  201  is connected, in such a way as to block the second optical signal. 
     In this way, transmission of the second optical signal from the output unit  140  to the fiber pair to which the terminal station  201 , which transmits the first optical signal having decreased intensity, is connected is stopped. Consequently, safety in repairing the fiber pair is improved. Note that, the terminal station  201  may stop transmitting the first optical signal, by being triggered by loss of the second optical signal. The terminal station  201  stops transmission of the first optical signal, and thereby no optical signal is input from any device to the fiber pair connecting the terminal station  201  to the failure detection device  11 . Consequently, safety in repairing the fiber pair can be further improved. 
     The optical switch  142  may be installed outside the demultiplexing unit  141 . Even when the optical switch  142  is external to the demultiplexing unit  141 , the optical switch  142  is controlled by the output control unit  150 . Further, some WSSs have a function of setting each of n output ports to be valid or to be invalid. When the demultiplexing unit  141  is a WSS having such a function, the output control unit  150  may achieve the function of the optical switch  142  by controlling the function of the WSS. For example, the second optical signal is transmitted to the terminal station  201  connected to the output port B 1  by setting the output port B 1 , which outputs the second optical signal, to be “valid”. Further, output of the second optical signal to the terminal station  201  connected to the output port B 1  is stopped by setting the output port B 1 , which outputs the second optical signal, to be “invalid”. 
     As described in the fourth example embodiment, when intensity of the first optical signal from the first terminal station becomes equal to or more than the predetermined threshold value, the identifying unit  130  may instruct the output control unit  150  to cause the demultiplexing unit  141  to restart output of the second optical signal to the first terminal station. Then, the output unit  140  controls, according to an instruction from the output control unit  150 , the optical switch  142  in such a way as that the second optical signal is transmitted to the first terminal station. 
     Sixth Example Embodiment 
       FIG.  10    is a diagram for describing a sixth example embodiment of the present invention. In the present example embodiment, a management device  200  is externally connected to a failure detection device  11 . The management device  200  is, for example, a computer, and notifies an output control unit  150  of contents of control on an output unit  140 . The management device  200  may be operated by an administrator of a failure detection device  11 . The output control unit  150  instructs, according to control from the management device  200 , the output unit  140  to start outputting a second optical signal. In the present example embodiment, in a case in which transmission of the second optical signal to a first terminal station is stopped, the transmission of the second optical signal to the first terminal station can be restarted with an instruction from the management device  200  without an instruction for the output control unit  150  from the identifying unit  130 . 
     Seventh Example Embodiment 
       FIG.  11    is a block diagram illustrating a configuration example of an optical transmission system  1  according to a seventh example embodiment of the present invention. The optical transmission system  1  includes terminal stations  201  to  20   n  and  231  to  23   n , failure detection devices  11  and  21 , and a repeater  50 . The terminal stations  201  to  20   n  and the failure detection device  11  have been described in the already-described example embodiments. The terminal stations  201  to  20   n  are each connected to the failure detection device  11  via each of fiber pairs  221  to  22   n,  in such a way as to be able to perform bidirectional communication. The terminal stations  231  to  23   n  and the failure detection device  21  are provided with configurations similar to those of the terminal stations  201  to  20   n  and the failure detection device  11 . In the optical transmission system  1 , the terminal stations  201  to  20   n  each perform bidirectional communication with each of the terminal stations  231  to  23   n.  Specifically, the failure detection device  11  couples first optical signals transmitted from the terminal stations  201  to  20   n  , and sends out the coupled first optical signals from a monitoring unit  120  to the repeater  50 , as a WDM signal. The repeater  50  is a relay device for an optical signal and, for example, amplifies the WDM signal by using an optical amplifier and sends out the amplified WDM signal to the failure detection device  21 . 
     The failure detection device  21  is provided with a configuration similar to that of the failure detection device  11  in  FIG.  8   . The failure detection device  21  receives the WDM signal sent out by the repeater  50 . An output unit  140  included in the failure detection device  21  outputs an optical signal acquired by demultiplexing the WDM signal to the terminal stations  231  to  23   n , as a second optical signal. Likewise, an optical signal is also transmitted from the terminal stations  231  to  23   n  to the terminal stations  201  to  20   n  via the repeater  50 . 
     The optical transmission system  1  provided with such a configuration includes the failure detection device  11  and the failure detection device  21  having a function similar to that of the failure detection device  11 . Therefore, when intensity of an optical signal received by the failure detection device  11  or the failure detection device  21  decreases due to a failure in any one of the fiber pairs  221  to  22   n  and fiber pairs  241  to  24   n , the optical transmission system  1  can identify a terminal station connected to the optical transmission path in which the failure has occurred. Further, since the optical transmission system  1  can stop transmission of an optical signal from the failure detection device that has identified the terminal station to the identified terminal station, safety when dealing with the failure in the fiber pair can be improved. Further, when intensity of the optical signal from the identified terminal station recovers, the optical transmission system  1  can restart the transmission of the optical signal from the failure detection device to the terminal station. 
     Note that, the example embodiments of the present invention may be described as the following supplementary notes, but are not limited thereto. 
     (Supplementary Note 1) 
     A failure detection device including: 
     an input means for receiving, from a plurality of terminal stations, a first optical signal having a wavelength allocated, based on allocation information, to each of the plurality of terminal stations, and for coupling the received first optical signals; 
     a monitoring means for outputting a monitoring signal being a signal according to intensity relevant to a wavelength of each of the coupled first optical signals; and 
     an identification means for identifying a first terminal station from the plurality of terminal stations, based on the allocation information and the monitoring signal. 
     (Supplementary Note 2) 
     The failure detection device according to supplementary note 1, wherein the identification means identifies the first terminal station, according to the monitoring signal indicating that intensity of the first optical signal is less than a predetermined threshold value. 
     (Supplementary Note 3) The failure detection device according to supplementary note 1 or 2, wherein 
     the first optical signal to be received by the input means is a wavelength-multiplexed optical signal including a plurality of optical carriers, and 
     the monitoring signal is a signal according to intensity of each of the plurality of optical carriers included in the first optical signal coupled in the input means. 
     (Supplementary Note 4) 
     The failure detection device according to any one of supplementary notes 1 to 3, further including: 
     an output means for being capable of outputting a second optical signal to each of the plurality of terminal stations; and 
     an output control means for instructing the output means to stop outputting the second optical signal to the first terminal station identified by the identification means. 
     (Supplementary Note 5) 
     The failure detection device according to supplementary note 4, wherein the output means includes a demultiplexing means for demultiplexing an input second wavelength-multiplexed optical signal and for outputting, based on the allocation information, each demultiplexed optical signal to the plurality of terminal stations, as the second optical signal. 
     (Supplementary Note 6) 
     The failure detection device according to supplementary note 4 or 5, wherein the output control means instructs, when intensity of the first optical signal received from the first terminal station changes from less than the predetermined threshold value to equal to or more than the predetermined threshold value, the output means to start outputting the second optical signal. 
     (Supplementary Note 7) 
     The failure detection device according to any one of supplementary notes 4 to 6, wherein the output control means instructs, in response to control from an outside, the output means to start outputting the second optical signal. 
     (Supplementary Note 8) 
     An optical transmission system including: 
     the failure detection device according to any one of supplementary notes 1 to 7; and 
     the plurality of terminal stations. 
     (Supplementary Note 9) 
     A failure detection method including: 
     receiving, from a plurality of terminal stations, a first optical signal having a wavelength allocated, based on allocation information, to each of the plurality of terminal stations; 
     coupling the received first optical signals, 
     outputting a monitoring signal being a signal according to intensity relevant to a wavelength of each of the coupled first optical signals; and 
     identifying a first terminal station from the plurality of terminal stations, based on the allocation information and the monitoring signal. 
     (Supplementary Note 10) 
     The failure detection method according to supplementary note 9, further including identifying the first terminal station, according to the monitoring signal indicating that intensity of the first optical signal is less than a predetermined threshold value. 
     (Supplementary Note 11) 
     The failure detection method according to supplementary note 9 or 10, wherein 
     the first optical signal to be received from the plurality of terminal stations is a wavelength-multiplexed optical signal including a plurality of optical carriers, and 
     the monitoring signal is a signal according to intensity of each of the plurality of optical carriers included in the coupled first optical signals. 
     (Supplementary Note 12) 
     The failure detection method according to any one of supplementary notes 9 to 11, further including instructing stop of outputting a second optical signal to the first terminal station. 
     (Supplementary Note 13) 
     The failure detection method according to supplementary note 12, further including demultiplexing an input second wavelength-multiplexed optical signal, and outputting, based on the allocation information, each demultiplexed optical signal to the plurality of terminal stations, as the second optical signal. 
     (Supplementary Note 14) 
     The failure detection method according to supplementary note 12 or 13, further including instructing, when intensity of the first optical signal received from the first terminal station changes from less than the predetermined threshold value to equal to or more than the predetermined threshold value, start of outputting the second optical signal. 
     (Supplementary Note 15) 
     A failure detection program for causing a computer of a failure detection device to execute: 
     a procedure of receiving, from a plurality of terminal stations, a first optical signal having a wavelength allocated, based on allocation information, to each of the plurality of terminal stations; 
     a procedure of coupling the received first optical signals; 
     a procedure of outputting a monitoring signal being a signal according to intensity relevant to a wavelength of each of the coupled first optical signals; and 
     a procedure of identifying a first terminal station from the plurality of terminal stations, based on the allocation information and the monitoring signal. 
     While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims. 
     The configuration described in each example embodiment is not necessarily is not exclusive of one another. Action and the advantageous effect of the present invention may be achieved by a configuration in which all or some of the above-described example embodiments are combined. 
     The function and the process described in each example embodiment described above may be achieved by a central processing unit (CPU) included in the failure detection device  10  and  11  executing a program. The program is recorded on a tangible, non-transitory storage medium. A semi-conductor memory or a fixed magnetic disk is used as the recording medium, but the recording medium is not limited thereto. 
     REFERENCE SIGNS LIST 
     
         
           1  Optical transmission system 
           10 ,  11 ,  21  Failure detection device 
           50  Repeater 
           110  Input unit 
           111  Multiplexing unit 
           120  Monitoring unit 
           130  Identifying unit 
           140  Output unit 
           141  Demultiplexing unit 
           142  Optical switch 
           150  Output control unit 
           200  Management device 
           201  to  20   n ,  231  to  23   n  Terminal station 
           211  to  21   n  Optical transmission path 
           221  to  22   n ,  241  to  24   n  Fiber pair