Abstract:
A light source error detection system and method including monitoring a plurality of characteristics of a light source and controlling the light source based on a plurality of monitored results. The system and method control the light source differently for a first case when only one of the monitored characteristics is abnormal than a second case when more than two of the monitored characteristics are abnormal. In controlling the light source, an output power of the light source is decreased with different time constant for the first case and the second case.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is related to and claims the benefit of foreign priority to Japanese application 2007-119363, filed on Apr. 27, 2007 in the Japan Patent Office, which is incorporated herein by reference in its entirety. 
   BACKGROUND 
   1. Field 
   The present invention is directed to light source anomaly state detection and, more particularly, to a light source anomaly state detection enabled to selectively control a light source based on monitored characteristics of the light source. 
   2. Description of the Related Art 
   A Wavelength Division Multiplexing (WDM) optical transmission is an example of one technique used in optical transmission systems.  FIG. 9  is a block diagram showing a light source configuration for use with the WDM technique. As shown in  FIG. 9 , a light source  1  monitors an output optical power from a laser diode (LD)  4  using a monitoring photodiode (PD)  2  and an LD output light monitor  3 , and monitors a wavelength of the output light from the LD  4  using a wavelength locker  5  and a wavelength monitor  6 . When an anomaly is detected in a monitored value for the output optical power or in a monitored value for the wavelength is detected by an optical output alarm detector  7  or a wavelength alarm detector  8 , the optical output is instantaneously blocked (shut down) by an optical power controller  9 . 
   Generally, in a WDM optical transmission system, optical amplifiers which make use of EDFs (Erbium Doped Fibers) are disposed in the transmission lines to extend transmission distances. The WDM light, which is a wavelength multiplexed light composed of a plurality of signal lights with a plurality of differing wavelengths, is input to the optical amplifiers and amplified therein. When a change in the number of input wavelengths, i.e. the number of multiplexed optical signals included in the input WDM light, or some other factor causes an increase in input optical power, the amount of excitation light input to the EDF increases in Automatic Gain Control (AGC). Similarly, when a change in the number of input wavelengths or the like causes a reduction in input optical power, the amount of excitation light is reduced. Hence an amount of output optical power is controlled so as to remain constant. 
     FIG. 10  is a set of schematic plots illustrating optical changes in the transmission path when output from a portion of the light sources is instantaneously shut down. As shown in  FIG. 10 , when an anomaly in the monitored value of the output optical power is detected in a light source outputting a light of a specific wavelength, an output optical power  11  of the LD module of the light source in which the anomaly has been detected is shut down. As a result, the input optical power  12  to the EDF optical amplifier drops steeply by precisely the amount of output optical power of the shut-down light source. 
   Since there is a transition time (EDF excitation light emission time constant), which is a time between a time when the excitation light input to the optical amplifier and a time when emission occurs at the EDF, even though control is performed to reduce the excitation light intensity corresponding to a decrease of the input light power to the optical amplifier, there is a time when the optical amplifier remains in an excitation state corresponding to the intensity before the input optical power is reduced. This excitation state is a state having a high gain with respect to the reduced input optical power, and so an output optical power  13  from the optical amplifier temporarily increases. Thereafter, for the period (AGC circuit tracking time constant) until the automatic gain control (AGC) begins to function normally, the output optical power  13  of the optical amplifier becomes unstable, and the output optical power  14 , which is the output optical power of the optical amplifier excluding the optical power of the shut-down light source, temporarily changes. 
   In WDM optical transmission systems, it is required that an anomaly occurring in a light source of a specific wavelength does not affect signal light of other wavelengths. Thus, even when optical output of the light source of the specific wavelength is shut down, the output optical power of the optical amplifier at other wavelengths must be maintained without any large changes. 
   A configuration in which an optical attenuator and an optical input monitor are provided at an optical input stage of the EDF optical amplifier is discussed in Japanese Patent Laid-Open No. H11-112435 (JP11-112435), for example. The optical attenuator allows variation based on an amount of change in the input optical power to the EDF optical amplifier. With this configuration, abrupt changes in the input optical power are absorbed by the optical attenuator, and temporary changes in the optical output power of the EDF optical amplifier are suppressed. 
   In the above-described and other similar configurations, such as the light source shown in  FIG. 9 , when an anomaly is detected in the monitored value, even if the cause is an anomaly in the various monitors and the optical output of the LD is in a controllable state, the optical output is uniformly shut down and a large level change temporarily occurs in the EDF optical amplifier in spite of the LD optical output being in a controllable state. 
   Further, in the above-described related art, such as the optical amplifier with the configuration disclosed in JP11-112435, it is necessary to provide the optical attenuator and the optical input monitor at an optical input stage of the optical amplifier. That causes an increase of a number of parts, requires a more complex configuration, and deterioration of noise properties. 
   SUMMARY 
   A light source monitored by a monitoring unit for a plurality of characteristics of the light source and controlled by a control unit based on a plurality of monitored results by the monitoring unit. The control unit controls the light source differently for a first case when only one of characteristics monitored is abnormal than for a second case when more than two of the characteristics monitored is abnormal. 
   Additional aspects and/or advantages will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the invention. 
   The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and/or other aspects and advantages will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
       FIG. 1  is a block diagram illustrating a configuration of a light source according to an embodiment; 
       FIG. 2  is a flowchart illustrating an optical output suspending processing procedure used upon detection of an anomaly in a light source according to an embodiment; 
       FIG. 3  is a set of schematic plots illustrating optical change(s) in a transmission path of a system using a light source according to an embodiment; 
       FIG. 4  is a block diagram illustrating a configuration of a light source according to an embodiment; 
       FIG. 5  is a flowchart illustrating an optical output suspending processing procedure used upon detection of an anomaly in a light source according to an embodiment; 
       FIG. 6  is a set of schematic plots illustrating optical change(s) in a transmission path of a system using a light source according to an embodiment; 
       FIG. 7  is a block diagram illustrating a configuration of a light source according to an embodiment; 
       FIG. 8  is a flowchart illustrating an optical output suspending processing procedure used upon detection of an anomaly in a light source according to an embodiment; 
       FIG. 9  is a block diagram illustrating a configuration of a light source in a related art; and 
       FIG. 10  is a set of schematic plots illustrating optical changes in the transmission path of the system of a related art. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Reference will now be made in detail to the embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures. 
     FIG. 1  is a block diagram showing a configuration of a light source according to an embodiment of the present invention. As shown in  FIG. 1 , a light source  21  includes an LD module  22  outputting laser light, an LD control block  23  controlling operation(s) of the LD module  22 , an LN modulator  24  which is an external modulator for modulating output light from the LD module  22 , an LN control block  25  controlling operation(s) of the LN modulator  24 , and an alarm detection control block  26  detecting an occurrence of an anomaly. A light source as referred to herein may include any device, system or apparatus serving as a source of and/or controlling light including but not limited to an optical apparatus or a similar device. 
   The LD module  22  includes an LD  31  and a monitoring PD  32 . The LD  31  outputs main output light in a forward direction and outputs monitor-use output light in a backward direction. Here, the forward direction of the LD  31  refers to the side of the LD module  22  from which output light is outputted. PD  32  converts the monitor-use output light to a current signal. The current signals vary depending on the power of the monitor-use output light. 
   The LD control block  23  includes an LD output light monitor  33 , an LD driver  34 , a current monitor  35 , and an optical power controller  36 . The LD output light monitor  33  monitors a power of the monitor-use output light based on an output signal from the PD  32 . Hence, the PD  32  and the LD output light monitor  33  may be considered as functioning as a first monitoring unit. The LD driver  34  outputs a driving signal for causing the LD  31  to operate. The output power of the LD  31  is controlled by the driving signal. 
   The current monitor  35  monitors an output current of the LD driver  34 . The optical power controller  36  outputs a control signal based on an output signal of the LD output light monitor  33  and an output signal of a current monitor  35 . The LD driver  34  outputs a driving signal for the LD  31  based on the control signal. In a normal steady-state operating state, automatic power control (APC) is performed by the PD  32 , the LD output light monitor  33 , the optical power controller  36  and the LD driver  34 . 
   The LN modulator  24  includes a modulator  37  having, for example, a Mach-Zehnder configuration and a monitoring PD  38 . In an embodiment, the modulator  37  is a phase modulator that modulates main output light from the LD  31 . The PD  38  converts output light of the phase modulator  37  to a current signal. The current signal varies depending on the output optical power of the phase modulator  37 . 
   The LN control block  25  includes a modulator output monitor (LN output light monitor)  39 , a bias controller  40 , a biasing circuit  41 , and a modulation signal driver  42 . The modulator output monitor  39  monitors an output optical power of the phase modulator  37  based on the output signal of the PD  38 . Hence, the PD  38  and the modulator output monitor  39  may be considered as functioning as a second monitoring unit. The bias controller  40  outputs a control signal based on the output signal from the modulator output monitor  39 . 
   The biasing circuit  41  applies a bias voltage to the phase modulator  37  based on the output signal from the bias controller  40 . In a steady-state operating state, automatic biasing control (ABC) is performed by the PD  38 , the modulator output monitor  39 , the bias controller  40 , and the biasing circuit  41 . Further, the biasing circuit  41  controls a phase of the main output light from the LD  31  in the phase modulator  37 , based on a modulation signal supplied from the modulation signal driver  42 . The main output light is modulated by controlling the phase thereof. 
   The alarm detection control block  26  includes an optical output alarm detector  43 . The optical output alarm detector  43  detects anomaly in the light source  21  based on an output signal of the LD output light monitor  33  and an output signal of the modulator output monitor  39 . The optical output alarm detector  43  then judges or determines, based on content of the detected anomaly, whether to instantaneously attenuate the optical output of the LD  31  or gradually attenuate the optical output of the LD  31 . Hence, the optical output alarm detector  43  functions as a detection unit. The optical output alarm detector  43  outputs a control signal to the optical power controller  36  based on a result of the judgment or determination. 
   The optical power controller  36  instantaneously attenuates the optical output of the LD  31  or gradually attenuates the optical output of the LD  31  based on the output signal from the optical output alarm detector  43 . When the optical output of the LD  31  is to be gradually attenuated, the optical power controller  36  causes gradual attenuation of the optical output of the LD  31  using a time constant which is at least the sum of the EDF excitation emission time constant and the automatic gain control circuit tracking time constant. 
     FIG. 2  is a flow chart showing an optical output suspending processing procedure used upon detection of an anomaly in a light source according to an embodiment of the present invention. As shown in  FIG. 2 , in the normal steady-state operating state, the above-described automatic power control (APC) is performed, and the output optical power, for example, from the LD  31  is stable (operation S 1 ). In this state, the optical output alarm detector  43  acquires a value from the LD output light monitor  33  (operation S 2 ), and judges (determines) whether an anomaly in the monitored value for the output optical power has been detected (operation S 3 ). When no anomaly is detected in the monitored value for the output optical power (No in operation S 3 ), the processing returns to operation S 1 . 
   When an anomaly in the monitored value for the output optical power is detected (Yes in operation S 3 ), the optical output alarm detector  43  fixes a voltage applied to the LD driver  34  by the optical power controller  36  at a value immediately before detection of the anomaly (operation S 4 ). As a result, the feedback control (APC) of the output optical power is suspended. Next, the optical output alarm detector  43  acquires and checks a value (LN output light monitor value) from the modulator output monitor  39  (operation S 5 ). Next, the optical output alarm detector  43  judges whether the value from the modulator output monitor  39  is in a normal range (operation S 6 ). 
   When the value is in the normal range (Yes in operation S 6 ), the optical output alarm detector  43  judges that there is an anomaly in the function of the LD output light monitor. In this case, it is inferred that the LD  31  is in a state in which control by the optical power controller  36  and the LD driver  34  is possible. Hence, the optical output alarm detector  43  controls the voltage applied by the optical power controller  36  to the LD driver  34 , so that the output light of LD  31  is gradually attenuated using the time constant which is at least the sum of the EDF excitation emission time constant and the automatic gain control circuit tracking time constant (operation  7 ). As a result, it is possible to suspend the output of light from LD  31  through gradual attenuation (operation S 9 ). 
   When, on the other hand, the value of the modulator output monitor  39  is outside the normal range (No in operation S 6 ), the optical output alarm detector  43  judges that the anomaly is in the LD output control function. In this case, it is inferred that the LD  31  is in a state in which control by the optical power controller  36  and the LD driver  34  is not possible. Hence, the optical output alarm detector  43  controls the voltage applied by the optical power controller  36  to the LD driver  34 , so that the output light from the LD  31  is instantaneously attenuated. As a result, it is possible to suspend the output of light from the LD  31  instantaneously (operation S 9 ). 
     FIG. 3  is a set of schematic plots illustrating optical changes in a transmission path of a system that makes use of a light source of an embodiment of the present invention. As shown in  FIG. 3 , in a light source of a specific wavelength, when the monitored value of the output optical power rises to a threshold value and an anomaly is detected, an output optical power  51  of the LD module of the light source for which the anomaly was detected is, after an anomaly state diagnosis period, gradually reduced using the time constant which is greater than or equal to the sum of the EDF excitation emission time constant and the automatic gain control circuit tracking time constant, and the output of light is suspended. 
   Hence, after the anomaly state diagnosis period, an input optical power  52  to the EDF optical amplifier gradually drops by an amount exactly corresponding to the output optical power of the light source in which the anomaly has occurred. An output optical power  53  from the EDF optical amplifier also drops gradually after the anomaly state diagnosis by an amount exactly corresponding to the output optical power of the light source in which the anomaly has occurred. During this period, an output optical power  54 , which is the optical output power of the EDF optical amplifier excluding the output power of the light source for which the anomaly occurred, remains constant. 
   According to the above-described embodiment, when an anomaly is detected in the monitored value of the monitor-use output optical power, it is possible, by checking the monitored value of the main output optical power, to judge whether the operating state of the LD  31  is actually abnormal or the LD  31  is operating normally and the anomaly has occurred in the monitoring system. When, it is judged that the LD  31  is normal but the monitoring system is abnormal, the optical output power of the LD  31  can be gradually attenuated. Consequently, it is possible to prevent excitation light from being left over in the EDF as a result of the extinguishing or immediate suspension of the output light from the LD  31 . 
   Hence, it is possible to prevent a temporary change in the output optical power from other light sources which contribute to the optical output power from the optical amplifier. Further, when the monitored value for the main output optical power is abnormal and the monitored value for the power of the monitor-use output light is normal, it is possible to judge that the LD  31  is normal and the monitoring system is abnormal, and the optical output of the LD  31  can therefore be gradually attenuated. Note that a monitoring function for monitoring the main output optical power may be provided separately from the LN modulator  24  and the LN control block  25 . 
     FIG. 4  is a block diagram showing a configuration of a light source according to an embodiment of the present invention. As shown in  FIG. 4 , a light source  61  includes, in addition to the light source configuration of  FIG. 1 , a wavelength locker  62  having a wavelength-dependent permittivity constituted by a filter or the like, a thermoelectric cooler (TEC)  63  which may be a Peltier device or the like, and a temperature sensor  64  which is a thermistor (Rth) or the like in the LD module  22 . Further, the LD control block  23  includes a thermoelectric cooler driver (TEC driver)  65 , a temperature monitor  66 , a wavelength monitor  67  and a wavelength controller  68 . The alarm detection control block  26  includes a wavelength alarm detector  69  in place of the optical output alarm detector  43  shown in  FIG. 1   
   The wavelength monitor  67  monitors a wavelength of the output light from the LD  31  based on an output signal from the wavelength locker  62 . Hence, the wavelength locker  62  and the wavelength monitor  67  may be considered as functioning as a first monitoring unit. The temperature monitor  66  monitors a temperature of the thermoelectric cooler  63  based on an output signal from the temperature sensor  64 . Since the LD  31  is provided in relation to the thermoelectric cooler  63 , the temperature of the thermoelectric cooler  63  is equal to the temperature of the LD  31 . Hence, the temperature sensor  64  and the temperature monitor  66  may be considered as functioning as the second monitoring unit. 
   The wavelength controller  68  outputs the control signal based on the output signal from the temperature monitor  66  and the output signal from the wavelength monitor  67 . The thermoelectric cooler driver  65  outputs a driving signal for thermoelectric cooler  63  based on the control signal. In the normal steady-state operating state, automatic frequency control (AFC) is performed by the wavelength locker  62 , the wavelength monitor  67 , the temperature sensor  64 , the temperature monitor  66 , the wavelength controller  68 , the thermoelectric cooler driver  65  and the thermoelectric cooler  63 . 
   The wavelength alarm detector  69  detects an anomaly in the light source  61  based on an output signal from the wavelength monitor  67  and an output signal from the temperature monitor  66 . The wavelength alarm detector  69  judges, based on content of the detected anomaly, whether to instantaneously attenuate the optical output of the LD  31  or gradually attenuate the optical output of the LD  31 . Hence, the wavelength alarm detector  69  functions as the detection unit. The wavelength alarm detector  69  outputs a control signal to the optical power controller  36  based on a result of the judgment. 
   Based on the output signal from the wavelength alarm detector  69 , the optical power controller  36  either instantaneously attenuates the optical output of the LD  31  or gradually attenuates the optical output of the LD  31  using a time constant which is at least the sum of the EDF excitation emission time constant and the automatic gain control circuit tracking time constant. Note that since an optical output alarm detector is not provided in this embodiment, anomaly detection in the light source  61  based on the output signal of the LD output light monitor  33  and the output signal of the modulator output monitor  39  described in  FIG. 1  is not required to be performed. 
   In an embodiment shown in  FIG. 4 , automatic power control (APC) is performed by the PD  32 , the LD output light monitor  33 , the optical power controller  36 , and the LD driver  34 , and automatic biasing control (ABC) is performed by the PD  38 , the modulator output monitor  39 , the bias controller  40 , and the biasing circuit  41 . Other portions of the configuration are the same as the embodiment shown in  FIG. 1 . 
     FIG. 5  is a flowchart showing an optical output suspending processing procedure used upon detection of an anomaly in the light source according to the embodiment shown in  FIG. 4 . As shown in  FIG. 5 , in a normal steady-state operating state, the above-described automatic frequency control (AFC) is performed, and the wavelength of the output light from the LD  31  is stable (operation S 11 ). In this state, the wavelength alarm detector  69  acquires a value (LD output wavelength monitor value) from the wavelength monitor  67  (operation S 12 ), and judges whether an anomaly in the monitored value for the optical output wavelength has been detected (operation S 13 ). When no anomaly is detected in the monitored value of the optical output wavelength (No in operation S 13 ), the processing returns to operation S 11 . 
   When an anomaly in the monitored value for the optical output wavelength has been detected (Yes in operation  513 ), the wavelength alarm detector  69  fixes a voltage applied to the thermoelectric cooler (TEC) driver  65  by the wavelength controller  68  at a value immediately before detection of the anomaly (operation S 14 ). As a result, the feedback control (AFC) of the output optical wavelength is suspended. Next, the wavelength alarm detector  69  acquires and checks a value (temperature monitor value) from the temperature monitor  66  (operation S 15 ). Next, the wavelength alarm detector  69  judges whether the value from temperature monitor  66  is in a normal range (operation S 16 ). 
   When the value is in the normal range (Yes in operation S 16 ), the wavelength alarm detector  69  judges that the anomaly is in the optical output wavelength monitoring function. In this case, it is inferred that the LD  31  is in a state in which control by the optical power controller  36  and the LD driver  34  is possible. Hence, the wavelength alarm detector  69  controls the voltage applied to the LD driver  34  by optical power controller  36 , so that the output light of LD  31  is gradually attenuated using a time constant which is at least the sum of the EDF excitation emission time constant and the automatic gain control circuit tracking time constant (operation S 17 ). As a result, it is possible to suspend the output of light from the LD  31  through gradual attenuation (operation S 19 ). 
   On the other hand, when the value from the temperature monitor  66  is outside the normal range (No in operation S 16 ), the wavelength alarm detector  69  judges that the optical output wavelength monitoring function is normal and the anomaly is in the optical output wavelength. In other words, the wavelength alarm detector  69  judges that the thermoelectric cooler (TEC) output control function is abnormal. In this case, the wavelength alarm detector  69  controls the voltage applied by the optical power controller  36  to the LD driver  34  so that the output light from the LD  31  is attenuated instantaneously (operation S 18 ). As a result, it is possible to suspend the output of light from the LD  31  instantaneously (operation S 19 ). 
     FIG. 6  is a set of schematic plots illustrating optical changes in the transmission path of the system using the light source according to the embodiment shown in  FIG. 4 . As shown in  FIG. 6 , in a light source of a specific wavelength, when the monitored value of the output optical wavelengths rises to a threshold and an anomaly is detected, an output optical power  72  of the light source in which the anomaly has been detected is, after an anomaly state diagnosis period, gradually reduced using a time constant which is greater than or equal to the sum of the EDF excitation emission time constant and the automatic gain control circuit tracking time constant, and the output of light is suspended. 
   Hence, after the anomaly state diagnosis period, an input optical power  73  to the EDF optical amplifier is gradually reduced by an amount of optical output power exactly corresponding to the output optical power of the light source in which the anomaly has occurred. After the anomaly state diagnosis period, an output optical power  74  from the EDF optical amplifier also drops gradually by an amount exactly corresponding to the output optical power of the light source in which the anomaly has occurred. During this period, an optical power  75 , which is the output optical power in the EDF optical amplifier from light sources other than the light source in which the anomaly has occurred, remains constant. 
   Note that the uppermost characteristic plot in  FIG. 6  is a plot showing an optical output wavelength  71  of the LD module of the light source in which the anomaly has been detected, and λc is the target wavelength. As shown in the characteristic plot of the optical output wavelength  71 , after the anomaly state diagnosis period, the wavelength remains high until the output light from the light source in which the anomaly has been detected is suspended. After the suspension of the output of light from the light source in which the anomaly has been detected, detection of the wavelength ceases. This state in which the wavelength is not detected is shown as a low level near to the horizontal axis in the characteristic plot of the optical output wavelength  71  in  FIG. 6 . 
   According to the embodiment shown in  FIG. 4 , when an anomaly is detected in the monitored value for optical output wavelength, it is possible to judge, by checking the monitored value for the temperature of the thermoelectric cooler  63 , whether the operating state of the LD  31  is actually abnormal or the LD  31  is operating normally and the anomaly has occurred in the monitoring system. When the monitored value for the temperature of the thermoelectric cooler  63  is anomalous and the monitored value of the optical output wavelength is normal, it is judged that the LD  31  is normal and the monitoring system is abnormal. Hence, the similar effects to the embodiment shown in  FIG. 1  are obtained. 
     FIG. 7  is a block diagram showing a configuration of a light source according to an embodiment of the present invention. A light source  81  shown in  FIG. 7  includes, in addition to the light source configuration of the embodiment shown in  FIG. 4 , a wavelength tuning device  82  enabled to control the output wavelength of the LD  31  based on an applied voltage in the LD module  22 . Further, a wavelength driver  83  and a voltage monitor  84  are provided in the LD control block  23 , the thermoelectric cooler, the temperature sensor, and the thermoelectric cooler driver are omitted. 
   The wavelength locker  62  and the wavelength monitor  67  in  FIG. 7  may be considered as functioning as the first monitoring unit. The wavelength driver  83  outputs a voltage which is applied to the wavelength tuning device  82 . The voltage monitor  84  monitors the output voltage from the wavelength driver  83 , which is the voltage applied to the wavelength tuning device  82 . Since the output wavelength of the LD  31  is controlled based on the applied voltage, the voltage monitor  84  functions as the second monitoring unit. 
   The wavelength controller  68  outputs the control signal based on the output signal of the voltage monitor  84  and the output signal of the wavelength monitor  67 . Based on the control signal, the wavelength driver  83  outputs the voltage which is applied to the wavelength tuning device  82 . In the normal steady-state operating state, automatic frequency control (AFC) is performed by the wavelength locker  62 , the wavelength monitor  67 , the voltage monitor  84 , the wavelength controller  68 , the wavelength driver  83  and the wavelength tuning device  82 . The wavelength alarm detector  69  detects an anomaly in the light source  81  based on the output signal of the wavelength monitor  67  and the output signal of the voltage monitor  84 . Other elements of the configuration are the same as in the above-discussed embodiment. 
     FIG. 8  is a flowchart showing an optical output suspending processing procedure used upon detection of an anomaly in the light source according to the embodiment shown in  FIG. 7 . As shown in  FIG. 8 , in the normal steady-state operating state, the above-described automatic frequency control (AFC) is performed, and the wavelength of output light from the LD  31  is stable (operation S 21 ). In this state, the wavelength alarm detector  69  acquires a value (LD output wavelength monitor value) from the wavelength monitor  67  (operation S 22 ), and judges whether an anomaly in the monitored value of the optical output wavelength has been detected (operation S 23 ). When no anomaly is detected in the monitored value of the optical output wavelength (No in operation S 23 ), the processing returns to operation S 21 . 
   When an anomaly in the monitored value of the optical output wavelength has been detected (Yes in operation S 23 ), the wavelength alarm detector  69  fixes a voltage applied by the wavelength controller  68  to the wavelength driver  83  at a value immediately before detection of the anomaly (operation S 24 ). As a result, the feedback control (AFC) of the optical output wavelength is suspended. Next, the wavelength alarm detector  69  acquires and checks a value from the voltage monitor  84 , which is the value of the voltage applied to the wavelength tuning device  82  (operation S 25 ). Next, the wavelength alarm detector  69  judges whether the value from voltage monitor  84  is in a normal range (operation S 26 ). 
   When the value is in the normal range (Yes in operation S 26 ), the wavelength alarm detector  69  judges that the anomaly is in the optical output wavelength monitoring function. In this case, it is inferred that the LD  31  is in a state in which control by the optical power controller  36  and the LD driver  34  is possible. Hence, the wavelength alarm detector  69  controls the voltage applied to the LD driver  34  by the optical power controller  36 , so that the output light of LD  31  is gradually attenuated using the time constant which is at least the sum of the EDF excitation emission time constant and the automatic gain control circuit tracking time constant (operation S 27 ). As a result, it is possible to suspend the output of light from the LD  31  through gradual attenuation (operation S 29 ). 
   On the other hand, when the value from the voltage monitor  84  is outside the normal range (No in operation S 26 ), the wavelength alarm detector  69  judges that the optical output wavelength monitoring function is normal and the anomaly is in the optical output wavelength. In other words, the wavelength alarm detector  69  judges that the wavelength output control function by the wavelength tuning device  82  is abnormal. In this case, the wavelength alarm detector  69  controls the voltage applied by the optical power controller  36  to the LD driver  34  so that the output light from the LD driver  34  is instantaneously attenuated (operation S 28 ). As a result, it is possible to suspend the output of light from the LD  31  instantaneously (operation S 29 ). The optical changes in the transmission path of the system which makes use of the light source of the embodiment shown in  FIG. 7  are the same as the optical changes shown in  FIG. 6  for the embodiment shown in  FIG. 4 . 
   According to the embodiment shown in  FIG. 7 , when an anomaly is detected in the monitored value for the optical output wavelength, it is possible to judge, by checking the voltage applied to the wavelength tuning device  82  using the voltage monitor  84 , whether the operating state of the LD  31  is actually abnormal or the LD  31  is operating normally and the anomaly has occurred in the monitoring system. Further, when the monitored value from the voltage monitor  84  is abnormal and the monitored value for the optical output wavelength is normal, it is possible to judge that the LD  31  is normal and the monitoring system is abnormal. Hence, the similar effects to the embodiment shown in  FIG. 1  are obtained. 
   As described above, when an anomaly is detected in a monitored value, the light source of the embodiments of the present invention controls the optical output from the LD in accordance with a state of the anomaly, and allows suppression of a level change(s) in the output optical power of the optical amplifier caused by shutting down of the optical output as a result of the detection of the anomaly in the monitored value. 
   Since the light source according to the embodiments of the present invention monitors an operating state of the LD using two different systems, when an anomaly is detected in the monitored value, it is possible to judge whether the operating state of the LD  31  is actually abnormal or the LD  31  is operating normally and the anomaly has occurred in the monitoring system. Thus, depending on the judgment, it is possible to instantaneously attenuate or gradually attenuate the optical output of the LD. In the case that the attenuation of the optical output from the LD is gradual, the attenuation is performed using an optical output attenuation time constant that is greater than or equal to the tracking time constant of the optical amplifier, and it is therefore possible to prevent excitation light from being left over in the EDF as a result of the extinguishing of output light from the LD. Hence, it is possible to prevent a temporary change in the output optical power from other light sources which contribute to the optical output power from the optical amplifier. 
   Although a few embodiments have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.