Abstract:
An apparatus and method of failure detection in optical amplifier is disclosed. The optical amplifier includes an optical amplifying fiber configured to receive input light so that the input light travels through the optical amplifying fiber, an excitation light generator configured to supply excitation light to the optical amplifying fiber to optically amplify the input light as the input light travels through the optical amplifying fiber, and a processor configured to determine whether a failure has occurred in the optical amplifier, based on a change in power of the excitation light and a change in power of the input light.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2012-267145, filed on Dec. 6, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an optical amplifier and a method for detecting a failure in an optical amplifier. 
     BACKGROUND 
     Optical communication systems have recently become widespread, and optical amplifiers are being used in various communication devices. For example, optical amplifiers are used in optical transmission devices, such as an optical transmitter, an optical receiver, an optical repeater and an optical add-drop multiplexer. 
     When used in optical transmission devices as mentioned above, optical amplifiers sometimes operate in an automatic gain control (AGC) mode in which an optical signal is amplified while the gain is maintained to be fixed. In the AGC mode, for example, the power of light input to an optical amplification medium (hereinafter referred to as “input light power”) and the power of light output from the optical amplification medium (hereinafter referred to as “output light power”) are monitored, and excitation light is controlled so that the ratio of the input light power to the output light power (that is, gain) is maintained to be fixed. Accordingly, when the input light power decreases, the control system of the optical amplifier reduces the power of excitation light so that the gain is maintained to be fixed. Similarly, when the input light power increases, the control system of the optical amplifier increases the power of excitation light so that the gain is maintained to be fixed. 
     An optical amplifier performs control as mentioned above and therefore is designed to include many optical components. When any one optical component breaks down, it becomes impossible for the optical amplifier to amplify an optical signal so as to obtain desired gain. For this reason, methods for detecting a failure in an optical amplifier have been proposed. 
     For example, when a laser light source for generating excitation light of an optical amplifier deteriorates, a control signal for increasing the power of excitation light is provided to the laser light source in order to maintain fixed gain. Accordingly, in this configuration, breakdown of a laser light source (or deterioration of the laser light source) is detected if the control signal mentioned above is monitored. In addition to this, for example, a method in which a failure in an optical amplifier is detected based on a change in output light power or a change in gain is known. 
     Japanese National Publication of International Patent Application No. 11-507189 and Japanese Laid-open Patent Publication No. 2003-174420 disclose examples of the related art. 
     If the input light power of an optical amplifier varies, the control system controls excitation light power in accordance with the variation of the input light power in order to maintain the gain to be fixed. In some cases, however, it is not possible to determine whether such a change in excitation light is caused by a variation in input light power or caused by a failure in an optical amplifier, only by monitoring excitation light or a control signal for generating excitation light. That is, with the related art technologies, whether a failure has occurred in an optical amplifier is not able to be correctly determined in some cases. 
     SUMMARY 
     According to an aspect of the invention, an optical amplifier includes an optical amplifying fiber configured to receive input light so that the input light travels through the optical amplifying fiber; an excitation light generator configured to supply excitation light to the optical amplifying fiber to optically amplify the input light as the input light travels through the optical amplifying fiber; and a processor configured to determine whether a failure has occurred in the optical amplifier, based on a change in power of the excitation light and a change in power of the input light. 
     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. 
     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 
         FIG. 1  illustrates an example of an optical transmission system according to embodiments of the present disclosure; 
         FIG. 2  illustrates an example of the configuration of a WDM transmission device; 
         FIG. 3  illustrates an example of an optical amplifier of the embodiments of the present disclosure; 
         FIG. 4  illustrates a configuration of an optical amplifier of another embodiment; 
         FIG. 5  illustrates operations of the optical amplifier in a first embodiment (part 1); 
         FIG. 6  illustrates operations of the optical amplifier in the first embodiment (part 2); 
         FIG. 7  is a flowchart illustrating a failure detection method in the first embodiment; 
         FIG. 8  is a flowchart illustrating a failure detection method in a second embodiment; and 
         FIG. 9  is a flowchart illustrating a failure detection method in a third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  illustrates an example of an optical transmission system according to embodiments of the present disclosure. In this example, an optical transmission system  1  is capable of transmitting wavelength division multiplexing (WDM) signals. That is, the optical transmission system  1  includes a plurality of WDM transmission devices  2 . Each WDM transmission device  2  is a reconfigurable optical add-drop multiplexer (ROADM), for example. The WDM transmission device  2  is capable of causing a desired signal to branch off from a WDM signal, and guiding the desired signal to a device subordinate to the WDM transmission device  2 . The WDM transmission device  2  is capable of inserting a signal received from a device subordinate thereto into a WDM signal. The ROADM also has a function of amplifying WDM signals in a collective manner. Each WDM transmission device  2  may be an optical in-line amplifier (ILA) provided on an optical transmission path. In this case, the WDM transmission device  2  amplifies WDM signals in a collective manner. 
       FIG. 2  illustrates an example of the configuration of the WDM transmission device  2 . The WDM transmission device  2  includes, as illustrated in  FIG. 2 , a plurality of transmitters (Txp)  11 , a multiplexer (MUX)  12 , optical amplifiers (AMPs)  13  and  14 , a demultiplexer (DEMUX)  15 , and a plurality of receivers (Rxp)  16 . 
     The plurality of transmitters  11  generate optical signals at different wavelengths. The wavelength range of the plurality of transmitters  11  is from 1531 to 1563 nm, for example. The multiplexer  12  multiplexes a plurality of optical signals generated by the plurality of transmitters  11  to generate a WDM signal. The optical amplifier  13  amplifies the WDM signal generated by the multiplexer  12 . The optical amplifier  14  amplifies received WDM signals. The demultiplexer  15  separates, by wavelength, the WDM signals amplified by the optical amplifier  14 . The plurality of receivers  16  receive optical signals at the respective wavelengths. 
     The WDM transmission device  2 , when used as an ILA, does not have to include the transmitters  11 , the multiplexer  12 , the demultiplexer  15 , and the receivers  16 . In this case, the WDM transmission device  2  includes an optical amplifier  17  for amplifying WDM signals. The intervals at which ILAs are provided on an optical transmission path are about 40 to 100 km, for example. 
       FIG. 3  illustrates an example of an optical amplifier of the embodiments of the present disclosure. An optical amplifier  20 , in the example illustrated in  FIG. 2 , is used as the optical amplifier  13 ,  14 , or  17 . 
     The optical amplifier  20  includes an input connector  21 , an erbium doped fiber (EDF)  22 , and an output connector  23 . WDM signals input through the input connector  21  are guided to the EDF  22 . The EDF  22  acts as an optical amplification medium when excitation light is supplied thereto. Here, the EDF  22  is designed to amplify WDM signals transmitted in the optical transmission system  1  in a collective manner. Accordingly, the EDF  22  amplifies the input WDM signals in a collective manner. Then, the WDM signals amplified in the EDF  22  are output through the output connector  23 . 
     The optical amplifier  20  further includes a central processing unit (CPU)  24 . The CPU  24  operates as a controller that controls operations of the optical amplifier  20 . The CPU  24  may control operations of the optical amplifier  20  utilizing programs stored in a memory (read-only memory (ROM))  25 , for example. The memory  25  may store other programs, data, and information. The CPU  24  stores measured values of input light power and values representing excitation light power in a memory (random access memory (RAM))  26 . Additionally, the CPU  24  is capable of reading the measured values of input light power and values representing excitation light power stored in the memory  26 . The memory  26  may be used for storing other data or information. 
     The optical amplifier  20  includes an input power measuring circuit  27  that measures the input light power of the optical amplifier  20  (or the input light power of the EDF  22 ). The input power measuring circuit  27  includes an optical beam splitter  28 , a photodetector (PD)  29 , an amplifier  30 , and an analog-to-digital (A/D) converter  31 . An optical beam splitter  28  causes an input WDM signal to branch off and guides the WDM signal to the photodetector  29 . The photodetector  29  generates a current signal representing the light power of the WDM signal guided from the optical beam splitter  28 . The amplifier  30  converts the current signal generated by the photodetector  29  into a voltage signal. The A/D converter  31  converts the voltage signal generated by the amplifier  30  into digital data. This digital data represents the light power of the input WDM signal. Then, the CPU  24  acquires digital data representing light power of the input WDM signal at predetermined time intervals from the input power measuring circuit  27 . 
     The optical amplifier  20  includes an output power measuring circuit  32  for measuring output light power of the optical amplifier  20 . The output power measuring circuit  32  includes an optical beam splitter  33 , a photodetector (PD)  34 , an amplifier  35 , and an A/D converter  36 . The optical beam splitter  33  causes a WDM signal amplified by the EDF  22  to branch off and guides the WDM signal to the photodetector  34 . The photodetector  34  generates a current signal representing the light power of the WDM signal guided from the optical beam splitter  33 . The amplifier  35  converts the current signal generated by the photodetector  34  into a voltage signal. The A/D converter  36  converts the voltage signal generated by the amplifier  35  into digital data. The digital data represents the light power of the WDM signal amplified by the EDF  22 . The CPU  24  is capable of acquiring digital data representing the light power of the amplified WDM signal at predetermined time intervals from the output power measuring circuit  32 . 
     The CPU  24  controls excitation light based on the light power of the input WDM signal measured by the input power measuring circuit  27 , and the light power of the output WDM signal measured by the output power measuring circuit  32 . At this point, the CPU  24  generates a current control value representing a driving current for controlling excitation light. 
     The optical amplifier  20  includes an excitation light generator  37  for supplying excitation light to the EDF  22 . The excitation light generator  37  includes a digital-to-analog (D/A) converter  38 , an amplifier  39 , a laser light source  40 , and a WDM coupler  41 . The D/A converter  38  converts the current control value generated by the CPU  24  into an analog signal. The amplifier  39  converts the analog signal output from the D/A converter  38  into a current signal. The laser light source  40  is driven by the current signal supplied from the amplifier  39  to generate continuous light, that is, excitation light at a predetermined wavelength. The WDM coupler  41  guides the excitation light generated by the laser light source  40  to the EDF  22 . 
     The optical amplifier  20  may include an optical isolator  42  between the optical beam splitter  28  and the WDM coupler  41 . The optical amplifier  20  may include an optical isolator  43  between the EDF  22  and the optical beam splitter  33 . Additionally, the optical amplifier  20  may include a gain equalizer (GEQ)  44  between the EDF  22  and the optical beam splitter  33 . 
     The optical amplifier  20  amplifies a WDM signal in the automatic gain control (AGC) mode, for example. In this case, the CPU  24  controls power of excitation light so that the gain in the EDF  22  is maintained to be fixed. The gain in the EDF  22  is calculated based on the ratio of the light power of the input WDM signal to the light power of the output WDM signal. 
     In the optical amplifier  20  having the configuration described above, the CPU  24  stores measured values of input light power obtained by the input power measuring circuit  27  in the memory  26 . Thus, the CPU  24  may refer to the measured values of past input light power by accessing the memory  26 . The input power measuring circuit  27  continuously measures input light power. Then, the CPU  24  acquires the measured values of input light power from the input power measuring circuit  27  at regular intervals, and stores the acquired values in the memory  26 . Therefore, the CPU  24  may learn changes in the past input light power by accessing the memory  26 . 
     The CPU  24  stores a current control value given from the CPU  24  to the excitation light generator  37  in the memory  26 . Here, the excitation light generator  37  generates excitation light with a power corresponding to the current control value and supplies the generated excitation light to the EDF  22 . That is, the current control value substantially represents excitation light power. The CPU  24  generates a current control value at regular intervals and provides it to the excitation light generator  37 , and stores the generated current control value in the memory  26 . Therefore, the CPU  24  may learn changes in the past excitation light power by accessing the memory  26 . 
     However, data (the measured value of input light power and the current control value) for which a predetermined time period has elapsed since the data was written in the memory  26  may be deleted from the memory  26 . That is, the memory  26  may have a configuration in which only data written therein during a predetermined time period immediately preceding the current time point is held. 
       FIG. 4  illustrates a configuration of an optical amplifier of another embodiment. In the configuration illustrated in  FIG. 3 , the current control value is stored as data representing excitation light power in the memory  26 . In contrast to this, in the configuration illustrated in  FIG. 4 , a value representing a back power monitor current of the laser light source  40  is stored as data representing excitation light power in the memory  26 . 
     In  FIG. 4 , the laser light source  40  outputs a back power monitor current. The back power monitor current substantially represents the power of excitation light generated by the laser light source  40 . The amplifier  51  converts the back power monitor current into a voltage signal. An A/D converter  52  converts the voltage signal generated by the amplifier  51  into digital data. This digital data represents the back power monitor current. That is, this digital data indicates the excitation light power. Then, the CPU  24  stores the back power monitor current value as data indicating the excitation light power in the memory  26 . 
     The CPU  24  acquires a back power monitor current value from the excitation light generator  37  at regular intervals, and stores the acquired value in the memory  26 . Therefore, the CPU  24  may learn changes in the past excitation light power by accessing the memory  26 . The configurations in  FIG. 3  and  FIG. 4  are substantially the same except for the circuit for monitoring excitation light power. 
     As such, in the configuration illustrated in  FIG. 3 , excitation light power is represented by the current control value that indicates the driving current of the laser light source  40 . In the configuration illustrated in  FIG. 4 , excitation light power is represented by the back power monitor current value of the laser light source  40 . That is, the current control value is an example of a value representing excitation light power, and the back power monitor current value is another example of a value representing excitation light power. 
     In the optical amplifier  20  illustrated in  FIG. 3  or  FIG. 4 , if input light power changes, the CPU  24  controls excitation light so that gain specified in advance is maintained. For example, if input light power falls, the CPU  24  decreases excitation light power so that the gain specified in advance is maintained. Conversely, if input light power rises, the CPU  24  increases excitation light power so that the gain specified in advance is maintained. In this way, the optical amplifier  20  may operate in the AGC mode in which the gain is maintained to be fixed. 
     The optical amplifier  20  in the above configuration has the function of detecting a failure in the optical amplifier  20  based on excitation light power and input light power. For example, it is assumed that the gain of the optical amplifier  20  has fallen because of a failure occurring inside the optical amplifier  20 . In this case, the CPU  24  increases excitation light power in order to recover the gain. Accordingly, the change in excitation light power may be used as one parameter for detecting a failure in the optical amplifier  20 . 
     In the AGC mode, however, excitation light power changes even when the input light power of the optical amplifier  20  changes. For example, the input light power of the optical amplifier  20  increases when the number of wavelengths of a WDM signal increases, and therefore excitation light power for securing the fixed gain also increases. Accordingly, in the failure detection method of the embodiments, it is determined whether a change in excitation light power is caused by a change in input light power or caused by a failure in the optical amplifier  20 . Then, if the change in excitation light power is caused by a change in input light power, it is determined that there is no failure in the optical amplifier  20 . Otherwise, if the change in excitation light power is not caused by a change in input light power, it is determined that there is a failure in the optical amplifier  20 . 
     However, when the change in excitation light power is caused by a change in input light power, the input light power has changed before the change of the excitation light power. That is, in order to determine whether the change in excitation light power is caused by a change in input light power, the measured values of past input light power are used. Accordingly, the optical amplifier  20  of the embodiments stores the measured values of the past input light power in the memory  26 . Then, when excitation light power changes such that the change amount is larger than a predetermined threshold, it is determined whether a failure has occurred in the optical amplifier  20 , using measured values of the past input light power stored in the memory  26 . 
     First Embodiment 
       FIG. 5  and  FIG. 6  illustrate operations of the optical amplifier  20  in the first embodiment. The optical amplifier  20  is assumed to operate in the AGC mode in which the gain is maintained to be fixed. 
     As described above, the optical amplifier  20  acquires the measured value of input light power and the value representing excitation light power at regular intervals, and stores the acquired values in the memory  26 . The value representing excitation light power is a current control value with the configuration illustrated in  FIG. 3 , and is a back power monitor current value with the configuration illustrated in  FIG. 4 . The measured value of input light power and the value representing excitation light power are stored in the memory  26  at intervals of 1 ms, for example. In the below description, the value representing excitation light power (a current control value or a back power monitor current value) is sometimes referred to simply as “excitation light power”. 
     In the example illustrated in  FIG. 5 , the input light power changes from P IN1  to P IN2  at a time point T2. When the input light power changes, the CPU  24  controls excitation light power so that specified gain is maintained. That is, feedback control in the AGC mode is carried out. Thereby, at a time point T3, the excitation light power changes from P P1  to P P2 . The time period between the time point T2 and the time point T3 is equivalent to a delay in feedback control. 
     The CPU  24  monitors changes in excitation light power at all times. For example, the CPU  24  monitors changes in excitation light power by detecting excitation light power at intervals of 1 ms. At this point, every time the CPU  24  detects excitation light power, the CPU  24  calculates a difference between excitation light power newly detected and the immediately preceding excitation power, for example, as the amount of change in excitation light power. Alternatively, the CPU  24  may calculate a difference between the past excitation light power and the current excitation light power, as an amount of change in excitation light power. The term “past excitation light power” refers to excitation light power at a time point preceding the current time point by a predetermined time period. Then, the CPU  24  determines whether the amount of change in excitation light power exceeds an excitation light change threshold TH1 specified in advance. The excitation light change threshold TH1 is not limited, and is determined based on the current excitation light power, for example. By way of an example, the excitation light change threshold TH1 is set to a value of about 5% of the current excitation light power. Alternatively, the excitation light change threshold TH1 may be a fixed value specified in advance. The excitation light change threshold TH1, however, is preferably set to, for example, a value larger than the maximum variation range of excitation light power due to temperature change or the like. 
     In the embodiment illustrated in  FIG. 5 , the CPU  24  detects an amount of change ΔP P  larger than the excitation light change threshold TH1 at the time point T3. Here, ΔP P  indicates a difference between the excitation light power P P1  and the excitation light power P P2 , and P P2  indicates excitation light power newly detected at the time point T3. The CPU  24  is able to acquire the excitation light power P P1  by accessing the memory  26 . 
     When detecting a change in excitation light power larger than the excitation light change threshold TH1, the CPU  24  accesses the memory  26  and examines measured values of past input light power. At this point, the CPU  24  determines whether the amount of change in input light power exceeds the input power change threshold TH2 specified in advance in a time period from a time point at which the amount of change in excitation light power exceeds the excitation light change threshold TH1 to a time point preceding that time point by the predetermined time period ΔT. The input light change threshold TH2 is not limited, and is determined based on the current input light power, for example. By way of an example, the input light change threshold TH2 is set to a value of about 5% of the current input light power. Alternatively, the input light change threshold TH2 may be a fixed value specified in advance. The input light change threshold TH2, however, is preferably set to a value with which the change in the number of wavelengths of a WDM signal is able to be detected. 
     In the example illustrated in  FIG. 5 , at the time point T3, the amount of change in excitation light power exceeds the excitation light change threshold TH1. Here, the time point preceding the time point T3 by the time period ΔT is T1. Accordingly, the CPU  24  calculates the amount of change in input light power in a time period from T1 to T3. As a result, ΔP IN  is obtained as illustrated in  FIG. 5 . Here, ΔP IN  indicates a difference between the input light power P IN1  measured at the time point T1 and the input light power P IN2  measured at the time point T3. The CPU  24  is able to obtain the input light power P IN1  measured at the time point T1 by accessing the memory  26 . 
     The CPU  24  determines whether a failure has occurred inside the optical amplifier  20 , based on a change in input light power in the time period from T1 to T3. In the example illustrated in  FIG. 5 , the amount of change ΔP IN  in input light power in the time period from T1 to T3 is larger than the input power change threshold TH2. In this case, it is considered that the change in excitation light power at the time point T3 is caused by a change in input light power. Accordingly, in this case, the CPU  24  determines that a failure has not occurred inside the optical amplifier  20 . 
     Also in the embodiment illustrated in  FIG. 6 , the amount of change in excitation light power larger than the excitation light change threshold TH1 is detected at the time point T3. For this reason, the CPU  24  calculates the amount of change in input light power in the time period from T1 to T3. 
     In the example illustrated in  FIG. 6 , however, the input light power is almost fixed in the time period from T1 to T3. That is, the amount of change in input light power in the time period from T1 to T3 is smaller than the input power change threshold TH2 mentioned above. In this case, it is considered that the change in excitation light power at the time point T3 is not caused by a change in input light power. In other words, it is considered that the cause of the change in excitation light power at the time point T3 lies in the optical amplifier  20 . Accordingly, in the case illustrated in  FIG. 6 , the CPU  24  determines that a failure has occurred inside the optical amplifier  20 . 
     The predetermined time period ΔT mentioned above is set to be longer than the operating time of a feedback system for controlling gain in the optical amplifier  20 , for example. By way of an example, when input light power and excitation light power are sampled at intervals of 1 ms, ΔT is about 10 ms. 
       FIG. 7  is a flowchart illustrating a failure detection method in the first embodiment. The process of this flowchart is performed by the CPU  24 . The CPU  24  performs a process of acquiring a measured value of input light power and a value representing excitation light power at regular intervals and storing the values in the memory  26 , in parallel with the failure detection process illustrated in  FIG. 7 . 
     At step (hereinafter abbreviated as “S”)  1 , the CPU  24  monitors excitation light power. As described above, in the configuration illustrated in  FIG. 3 , the CPU  24  acquires a current control value, which indicates the driving current of the laser light source  40 , as a value representing excitation light power. In the configuration illustrated in  FIG. 4 , the CPU  24  detects a back power monitor current as a value representing excitation light power. 
     At S 2 , the CPU  24  compares the amount of change in excitation light power with the excitation light change threshold TH1. Then, if the amount of change in excitation light power is larger than the excitation light change threshold TH1, the process of the CPU  24  proceeds to S 3 . Otherwise, if the amount of change in excitation light power is equal to or less than the excitation light change threshold TH1, the process of the CPU  24  returns to S 1 . That is, the operations of S 1  to S 2  are repeatedly performed during the period in which the amount of change in excitation light power is equal to or less than the excitation light change threshold TH1. 
     At S 3 , the CPU  24  refers to changes in the past input light power. The data indicating changes in the past input light power is stored in the memory  26 . At this point, the CPU  24  refers to a change in input light power in a time period from a time point at which the amount of change in excitation light power exceeds the excitation light change threshold TH1 to a time point preceding that time point by the predetermined time period ΔT. 
     At S 4 , the CPU  24  compares the amount of change in input light power with the input power change threshold TH2. Then, if the amount of change in input light power is smaller than the input power change threshold TH2, the process of the CPU  24  proceeds to S 5 . In this case, at S 5 , the CPU  24  outputs a signal or message indicating that a failure has occurred inside the optical amplifier  20 . 
     If the amount of change in input light power is equal to or larger than the input power change threshold TH2, it is considered that the change in excitation light power detected at S 2  is caused by a change in input light power. In this case, the CPU  24  determines that the change in excitation light power is not caused by a failure in the optical amplifier  20 . Accordingly, the process of the CPU  24  returns to S 1 . 
     In this way, with the configuration of the embodiment of the present disclosure, the influence resulting from a change in input light power is removed using a method of detecting a failure in an optical amplifier by making use of a change in excitation light power. That is, it is possible to make a determination as to whether a change in excitation light power is caused by a change in input light power or by a failure in an optical amplifier. Accordingly, a failure in an optical amplifier may be appropriately detected. 
     In a WDM transmission system, a change in the number of wavelengths of a WDM signal is notified to the corresponding node devices. For this reason, when the excitation light power of an optical amplifier changes in response to a change in the number of wavelengths of a WDM signal, the CPU  24  is able to recognize that the change in excitation light power is caused by the change in the number of wavelengths of the WDM signal, based on the above notification. However, when a failure occurs in a channel used for notification of the number of wavelengths of a WDM signal, for example, the CPU  24  is not able to recognize a change in the number of wavelengths of a WDM signal in some cases. Accordingly, the detection method of the embodiment of the present disclosure also contributes to improving the accuracy of detecting a failure in an optical amplifier in a transmission system in which node devices are notified of the number of wavelengths of a WDM signal. 
     Second Embodiment 
     In the first embodiment, failure in such a manner that the excitation light power (or gain) of the optical amplifier  20  abruptly changes is detected. In an optical amplifier, however, failure sometimes occurs in such a manner that the performance deteriorates slowly over a long time. For example, failure might occur in such a manner that the performance of a laser light source gradually deteriorates for about one week. 
     Accordingly, in a second embodiment, the optical amplifier  20  holds the measured valued of input light power and values representing excitation light power over a long time period. By way of an example, the optical amplifier  20  holds data indicating input light power and excitation light power for the past 200 hours. 
     However, the capacity of a memory would be large if the memory is intended to store the 200-hour data. For this reason, the optical amplifier  20  reduces memory capacity using a method described below. In the below description, it is assumed that the measured value of input light power and the value representing excitation light power are sampled at intervals of 1 ms and are stored in the memory  26 . 
     The sampling data stored in the memory  26  is held in its original state in the memory  26  for 1 min. That is, sampling data that is collected for 1 min immediately preceding the current time point is stored in the memory  26  at intervals of 1 ms. Here, provided that the amount of information of one sampling data is 2 bytes, the memory capacity for storing the data for the immediately preceding 1 min is as follows. 60×1000×2=120 kbyte 
     In contrast, sampling data that is collected in the past preceding the current time point by more than 1 min is left in the memory  26  at intervals of 1 min. In this case, the memory capacity for storing the 200-hour data is as follows. 200×60×2=24 kbyte 
       FIG. 8  is a flowchart illustrating a failure detection method in the second embodiment. The process of this flowchart is performed by the CPU  24 . The CPU  24  performs a process of acquiring a measured value of input light power and a value representing excitation light power at regular intervals and storing the values in the memory  26 , in parallel with the failure detection process illustrated in  FIG. 8 . As a result, as described above, measured values of input light power and values representing excitation light power for the past 200 hours with respect to the current time point are stored in the memory  26 . 
     At S 11 , the CPU  24  calculates reference excitation light power p 0 . The reference excitation light power p 0  is obtained by calculating the average of excitation light power detected at a plurality of different time points, for example. 
     At S 12 , the CPU  24  calculates reference input light power i 0 . The reference input light power i 0  is obtained by calculating the average of input light power detected at a plurality of different time points, for example. The CPU  24  may perform the operation of S 11  and the operation of S 12  in parallel. 
     At S 13 , the CPU  24  determines whether the number of wavelengths of a WDM signal has changed. The number of wavelengths of a WDM signal is notified from a network management system or the adjacent node, for example. Alternatively, the CPU  24  may detect the number of wavelengths of a WDM signal by monitoring the power of each wavelength channel of an input WDM signal. Then, if the number of wavelengths of a WDM signal has not changed, the process of the CPU  24  proceeds to S 14 . 
     At S 14 , the CPU  24  determines whether 200 hours have elapsed since the operations of S 11  and S 12  were performed. Then, if 200 hours have not yet elapsed since the operations of S 11  and S 12  were performed, the process of the CPU  24  proceeds to S 15 . 
     When the number of wavelengths of a WDM signal has changed, or when 200 hours have elapsed since the operations of S 11  and S 12  were performed, the process of the CPU  24  returns to S 11 . That is, when the number of wavelengths of a WDM signal has changed, or when 200 hours have elapsed since the operations of S 11  and S 12  were performed, new reference excitation light power p 0  and new reference input light power i 0  are calculated. 
     At S 15 , the CPU  24  detects excitation light power p c . The excitation light power p c  indicates excitation light power newly detected. The excitation light power p c  may be excitation light power that has been finally detected. In this case, the CPU  24  is able to read the excitation light power p c  from the memory  26 . 
     At S 16 , the CPU  24  determines whether the difference between the reference excitation light power p 0  and the excitation light power p c  is larger than a predetermined threshold. The threshold may be the same as the excitation light change threshold TH1 of the first embodiment. Then, if the difference between the reference excitation light power p 0  and the excitation light power p c  is larger than the threshold, the process of the CPU  24  proceeds to S 17 . Otherwise, if the difference is equal to or less than the threshold, the process of the CPU  24  returns to S 13 . 
     At S 17 , the CPU  24  detects input light power i c . The input light power i c  indicates input light power newly detected. The input light power i c  may be input light power that has been detected finally. In this case, the CPU  24  is able to read the input light power i c  from the memory  26 . 
     At S 18 , the CPU  24  determines whether the difference between the reference input light power i 0  and the input light power i c  is smaller than a predetermined threshold. The threshold may be the same as the input power change threshold TH2 of the first embodiment. 
     If the difference between the reference input light power i 0  and the input light power i c  is smaller than the threshold, it is considered that the change in excitation light power detected at S 16  is not caused by a change in input light power. In this case, at S 19 , the CPU  24  outputs a signal or message indicating that a failure has occurred inside the optical amplifier  20 . 
     If the difference between the reference input light power i 0  and the input light power i c  is equal to or larger than the threshold, it is considered that the change in excitation light power detected at S 16  is caused by a change in input light power. In this case, the CPU  24  determines that the change in excitation light power is not caused by a failure in the optical amplifier  20 . Accordingly, in this case, the process of the CPU  24  returns to S 13 . 
     In this way, with the configuration of the embodiment of the present disclosure, a failure in such a manner that the performance deteriorates slowly over a long time may also be detected in an optical amplifier. At this point, the influence resulting from a change in input light power is removed, and therefore a failure in the optical amplifier is detected appropriately. 
     The optical amplifier  20  may perform both of the detection method of the first embodiment and the detection method of the second embodiment. In this case, the optical amplifier  20  is able to detect both of a failure in which the performance deteriorates abruptly and a failure in which the performance deteriorates slowly. 
     Third Embodiment 
       FIG. 9  is a flowchart illustrating a failure detection method in a third embodiment. The failure detection method of the third embodiment is a variation of the second embodiment. Accordingly, description of operations common to the second and third embodiments is omitted. In the failure detection method of the third embodiment, the operation of S 12  illustrated in  FIG. 8  does not have to be performed. 
     In the third embodiment, if a difference between the reference excitation light power p 0  and the excitation light power p c  is larger than the threshold (Yes at S 16 ), the process of the CPU  24  proceeds to S 21 . At S 21 , the CPU  24  identifies the change pattern of excitation light power. The change pattern of excitation light power is obtained by reading values representing the past excitation light power stored in the memory  26  and arranging the read values in time sequence. At S 22 , the CPU  24  identifies the change pattern of input light power. The change pattern of input light power is obtained by reading measured values of the past input light power stored in the memory  26  and arranging the read values in time sequence. As mentioned above, the measured values of input light power and the values representing excitation light power for the past 200 hours with respect to the current time point are stored in the memory  26 . 
     At S 23 , the CPU  24  calculates a correlation between the change pattern of excitation light power and the change pattern of input light power. For example, if a timing at which excitation light power changes and a timing at which input light power changes are coincident or almost coincident, it is determined that the correlation between the two change patterns is high. 
     If the correlation between the two change patterns is low, it is considered that the change in excitation light power is not caused by a change in input light power. In this case, at S 19 , the CPU  24  outputs a signal or message indicating that a failure has occurred inside the optical amplifier  20 . 
     If the correlation between the two change patterns is high, it is considered that the change in excitation light power is caused by a change in input light power. In this case, the CPU  24  determines that the change in excitation light power is not caused by a failure in the optical amplifier  20 . Accordingly, in this case, the process of the CPU  24  returns to S 13 . 
     In this way, with the failure detection method of the third embodiment, based on the correlation between the change pattern of excitation light power and the change pattern of input light power, it is determined whether the change in excitation light power is caused by a change in input light power. Accordingly, with the method of the third embodiment, the accuracy of detecting a failure in an optical amplifier is higher as compared with the method of the second embodiment. 
     The optical amplifier  20  may perform both of the detection method of the first embodiment and the detection method of the third embodiment. In this case, the optical amplifier  20  is able to detect both of a failure in which the performance deteriorates abruptly and a failure in which the performance deteriorates slowly. 
     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.