Abstract:
An optical communication system includes an optical transmission terminal including a first transmitter for transmitting an optical signal, and a second transmitter for transmitting information that indicates the first transmitter transmitting the optical signal; and an optical receiving terminal including an optical receiver for receiving the optical signal and the information transmitted from the first and second transmitter, respectively, a storage device for storing a power value of the optical signal monitored in response to receipt of the information transmitted from the second transmitter, and a fault detector for detecting fault of the optical signal by continuously monitoring a power value of the optical signal received by the optical receiver in comparison with the power value stored in the storage device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation of Application PCT/JP2007/065813, filed on Aug. 13, 2007, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The embodiments discussed herein are related to an optical communication system. 
       BACKGROUND 
       [0003]    Optical communication systems perform wavelength-division multiplexing (WDM) transmission in which a plurality of optical signals is transmitted with a plurality of optical channels having different wavelengths. There is an optical communication system for switching between a currently used line and a backup line using the Optical Unidirectional Path Switched Ring (OUPSR) method to increase the reliability thereof. 
         [0004]    Switching between a currently used line and a backup line is performed when an optical receiving terminal included in an optical communication system cannot receive an optical signal owing to the failure of an optical channel. Accordingly, in this case, the optical receiving terminal may receive light from an optical transmission path and detect the fault of the optical signal. In a standard such as SONET, it is required to perform switching between a currently used line and a backup line within 50 ms of an optical signal fault in an optical channel. 
         [0005]    On the other hand, it is difficult to actually detect an optical signal fault. In the case of optical communication using a transmission path formed of an optical fiber, in order to increase a transmission distance, each of an optical transmission terminal, an optical relay terminal, and an optical receiving terminal included in an optical communication system includes an optical amplifier for directly amplifying incident light. The optical amplifier generates amplified spontaneous emission (ASE) light when amplifying a received optical signal. Accordingly, since light received by the optical receiving terminal includes not only signal light but also ASE light, it cannot be determined whether received light is ASE light or signal light only by detecting light transmitted from an optical transmission path. 
         [0006]    Japanese Laid-open Patent Publication No. 2001-244900 discusses a technique for switching between a currently used line and a backup line. 
       SUMMARY 
       [0007]    According to an aspect of the embodiment, an optical communication system includes an optical transmission terminal including a first transmitter for transmitting an optical signal, and a second transmitter for transmitting information that indicates the first transmitter transmitting the optical signal; and an optical receiving terminal including an optical receiver for receiving the optical signal and the information transmitted from the first and second transmitter, respectively, a storage device for storing a power value of the optical signal monitored in response to receipt of the information transmitted from the second transmitter, and a fault detector for detecting fault of the optical signal by continuously monitoring a power value of the optical signal received by the optical receiver in comparison with the power value stored in the storage device. 
         [0008]    The object and advantages of the embodiment will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0009]    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 
         [0010]      FIG. 1  is a diagram illustrating a configuration of an optical communication system according to an embodiment; 
           [0011]      FIG. 2  is a diagram illustrating configurations of an optical transmission terminal according to a first embodiment and an optical receiving terminal according to the first embodiment; 
           [0012]      FIG. 3  is a diagram illustrating a line switching unit; 
           [0013]      FIG. 4  is a diagram illustrating a configuration of a fault detector; 
           [0014]      FIGS. 5A to 5E  are each time chart in a configuration according to an embodiment; 
           [0015]      FIG. 6  is a diagram illustrating the relationship between an optical channel and an optical signal; 
           [0016]      FIGS. 7A and 7B  are diagrams illustrating a first switching operation; 
           [0017]      FIG. 8  is a diagram illustrating a line switching control circuit for performing the first switching operation; 
           [0018]      FIGS. 9A ,  9 B, and  9 C are diagrams illustrating a second switching operation; 
           [0019]      FIG. 10  is a diagram illustrating a line switching control circuit for performing the second switching operation; 
           [0020]      FIG. 11  is a diagram illustrating a configuration of an optical receiving terminal; and 
           [0021]      FIGS. 12A and 12B  are diagrams describing the detection of an optical channel at an optical transmission terminal. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0022]    Embodiments of the present invention will be described below with reference to the accompanying drawings. A configuration according to an embodiment of the present invention is merely illustrative, and the present invention is not limited thereto. 
         [0023]      FIG. 1  is a diagram illustrating a configuration of an optical communication system according to an embodiment of the present invention. An optical ring network includes a first path  6   a  and a second path  6   b  which are capable of performing optical transmission and a plurality of optical terminals. Switching between a currently used line and a backup line can be performed by performing switching between the first path  6   a  and the second path  6   b  in the optical ring network. A description will be made using an Optical Unidirectional Path Switched Ring (OUPSR) system in this embodiment, but this embodiment is not limited to the OUPSR system. A plurality of optical terminals are disposed on an optical ring network. These optical terminals include an optical transmission terminal  2 , optical relay terminals  5   a  and  5   b , and an optical receiving terminal  7 . 
         [0024]    The optical transmission terminal  2  is connected to an optical transmitter  1 . An optical coupler  3  included in the optical transmission terminal  2  splits an optical signal transmitted from the optical transmitter  1 . Although not illustrated, the same number of the optical transmitters  1  and the same number of the optical couplers  3  as that of optical channels of optical wavelengths used in Wavelength Division Multiplexing (WDM) are present. The optical signals obtained by the optical coupler  3  are individually input into WDM couplers  4   a  and  4   b . The optical signal input into the WDM coupler  4   a  is wavelength-division multiplexed with an optical signal of another optical channel and is then input into the first path  6   a . The optical signal input into the WDM coupler  4   b  is wavelength-division multiplexed with an optical signal of another optical channel and is then input into the second path  6   b.    
         [0025]    The optical relay terminal (add/drop multiplexer)  5   a  is on the first path  6   a , and the optical relay terminal (add/drop multiplexer)  5   b  is on the second path  6   b . Each of the optical relay terminals  5   a  and  5   b  wavelength-division demultiplexes a wavelength-division multiplexed optical signal into optical signals corresponding to optical channels so as to drop a predetermined optical channel. Furthermore, each of the optical relay terminals  5   a  and  5   b  adds a new optical signal to the dropped optical channel and transmits it to a downstream. 
         [0026]    An optical receiving terminal  7  includes WDM couplers  10   a  and  10   b  and a line switching unit  8 . The optical receiving terminal  7  individually receives wavelength-division multiplexed optical signals from the first path  6   a  and the second path  6   b . Each of the WDM couplers  10   a  and  10   b  included in the optical receiving terminal  7  demultiplexes the received wavelength-division multiplexed optical signal into optical signals corresponding to optical channels. Each of the optical signals corresponding to optical channels demultiplexed by the WDM coupler  10   a  is input into the line switching unit  8  along with a corresponding backup optical signal (a corresponding optical signal demultiplexed by the WDM coupler  10   b ). The output of the line switching unit  8  is input into an optical receiver  9 . The line switching unit  8  detects whether there is an optical signal and selects one of the optical signal transmitted from the first path  6   a  and the optical signal transmitted from the second path  6   b.    
         [0027]      FIG. 2  is a diagram illustrating configurations of an optical transmission terminal according to a first embodiment and an optical receiving terminal according to the first embodiment. The same numerals are used for like components so as to avoid repeated explanation. 
       [Optical Transmission Terminal] 
       [0028]    The optical transmission terminal  2  includes the WDM couplers  4   a  and  4   b , the optical coupler  3 , a plurality of light-receiving elements  80   a , a plurality of light-receiving elements  80   b , optical amplifiers  15   a  and  15   b , and optical supervisory channel (OSC) transmission units  11   a  and  11   b.    
         [0029]    The optical coupler  3  splits an optical signal transmitted from the optical transmitter  1  illustrated in  FIG. 1  into optical signals and transmits one of the optical signals to the WDM coupler  4   a  and the light-receiving elements  80   a  and the other one of the optical signals to the WDM coupler  4   b  and the light-receiving elements  80   b.    
         [0030]    The light-receiving elements  80   a  and the light-receiving elements  80   b  detect whether the optical signals supplied to the WDM couplers  4   a  and  4   b  are present in an optical channel, respectively. Light detected by the light-receiving elements  80   a  and  80   b  does not include an ASE light component, since it has yet to be processed by the optical amplifiers  15   a  and  15   b . The optical amplifiers  15   a  and  15   b  are each optical transmitter for transmitting the optical signal. The detection results of the optical signals performed by the light-receiving elements  80   a  and  80   b  are used as wavelength channel failure (WCF) information representing that an optical signal of an optical channel for which optical signal fault detection is performed is output or is not output. In this embodiment, for example, the WCF information indicates “0” in a case where an optical signal is present, and “1” in a case where no optical signal is present. The WCF information is transmitted to the OSC transmission unit  11   a.    
         [0031]    The OSC transmission unit  11   a  transmits an OSC signal used for supervisory control of the first path  6   a . The OSC transmission unit  11   a  transmits the OSC signal to the optical receiving terminal  7  through the first path  6   a  or another path. The OSC signal includes wavelength channel service (WCS) information B that is information about a used optical channel for which a carrier for managing the optical communication system determines to provide a service and WCF information A. In this embodiment, for example, WCS information indicates “1” for an in-service optical channel, and “0” for an out-of-service optical channel. In this embodiment, the WCF information A and the WCS information B are used on the side of the first path  6   a.    
         [0032]    The OSC transmission unit  11   b  transmits an OSC signal used for supervisory control of the second path  6   b . The OSC transmission unit  11   b  transmits the OSC signal to the optical receiving terminal  7  through the second path  6   b  or another path. The OSC signal includes wavelength channel service (WCS) information D that is information about a used optical channel for which a carrier for managing the optical communication system determines to provide a service and WCF information C. In this embodiment, for example, WCS information indicates “1” for an in-service optical channel, and “0” for an out-of-service optical channel. In this embodiment, the WCF information C and the WCS information D are used on the side of the second path  6   b.    
         [0033]    The WDM couplers  4   a  and  4   b  individually wavelength-division multiplex the optical signals transmitted from the optical coupler  3 . 
         [0034]    The optical amplifier  15   a  amplifies the optical signal wavelength-division multiplexed by the WDM coupler  4   a  and outputs the amplified light to the first path  6   a . The optical amplifier  15   b  similarly amplifies the optical signal wavelength-division multiplexed by the WDM coupler  4   b  and outputs the amplified light to the second path  6   b.    
       [Optical Receiving Terminal] 
       [0035]    The optical receiving terminal  7  includes optical amplifiers  16   a  and  16   b , the WDM couplers  10   a  and  10   b , OSC receiving units  12   a  and  12   b , and the line switching unit  8 . 
         [0036]    The OSC receiving units  12   a  and  12   b  receive the OSC signals from the OSC transmission units  11   a  and  11   b , respectively. The OSC receiving unit  12   a  extracts the WCF information A and the WCS information B from the OSC signal and transmits them to the line switching unit  8 . The OSC receiving unit  12   b  extracts the WCF information C and the WCS information D from the OSC signal and transmits them to the line switching unit  8 . 
         [0037]    The optical amplifier  16   a  couples to the first path  6   a . The optical amplifier  16   b  couples to the second path  6   b . The optical amplifier  16   a  amplifies the light transmitted through the first path  6   a  and supplies the amplified light to the WDM coupler  10   a . The optical amplifier  16   b  amplifies the light transmitted through the second path  6   b  and supplies the amplified light to the WDM coupler  10   b.    
         [0038]    The WDM couplers  10   a  and  10   b  demultiplex the wavelength-division multiplexed optical signals transmitted from the optical amplifiers  16   a  and  16   b  in units of wavelengths of optical channels, respectively. Each of the WDM couplers  10   a  and  10   b  supplies the demultiplexed light to the line switching unit  8 . As a result, a pair of light transmitted through the first path  6   a  corresponding to a currently used optical channel and light transmitted through the second path  6   b  corresponding to a backup channel is supplied to the line switching unit  8 . 
         [0039]    The line switching unit  8  includes a switch  14 , optical signal fault detection units  13   a  and  13   b , and a line switching control circuit  21 . 
         [0040]    The switch  14  receives light supplied from the WDM coupler  10   a  and light supplied from WDM coupler  10   b  as a pair of lights, and outputs one of the received lights in response to a switching signal transmitted from the line switching control circuit  21 . 
         [0041]    The optical signal fault detection unit  13   a  receives the light of an optical channel demultiplexed by the WDM coupler  10   a  and the WCS information B and the WCF information A which have been extracted by the OSC receiving unit  12   a , and performs optical signal fault detection on the basis of them. The optical signal fault detection unit  13   b  receives the light of the optical channel demultiplexed by the WDM coupler  10   b  and the WCS information D and the WCF information C which have been extracted by the OSC receiving unit  12   b , and performs optical signal fault detection on the basis of them. The results of the optical signal fault detection performed by the optical signal fault detection units  13   a  and  13   b  are input into the line switching control circuit  21 . 
         [0042]    The line switching control circuit  21  determines, on the basis of the output from the optical signal fault detection units  13   a  and  13   b , which of the light transmitted from the WDM coupler  10   a  and the light transmitted from the WDM coupler  10   b  is selected, and outputs a signal used to control the switch  14 . 
       [Line Switching Unit] 
       [0043]      FIG. 3  illustrates the line switching unit  8 . An optical coupler  17   a  splits the light transmitted from the WDM coupler  10   a  into two lights. One of the lights is input into the switch  14 , and the other one of the lights is input into the optical signal fault detection unit  13   a.    
         [0044]    The optical signal fault detection unit  13   a  includes a light-receiving element  18   a , an analog-to-digital converter (ADC)  19   a , and a fault detector  20   a . The light-receiving element  18   a  converts the light transmitted from the optical coupler  17   a  into a current. The ADC  19   a  converts a value of the current transmitted from the light-receiving element  18   a  into a digital current value. The fault detector  20   a  performs optical signal fault detection on the basis of the digital value obtained by the ADC  19   a  and the WCF information A and the WCS information B which have been transmitted through the first path  6   a . The fault detector  20   a  will be described in detail later with reference to an accompanying drawing. 
         [0045]    An optical coupler  17   b  splits the light transmitted from the WDM coupler  10   b  into two lights. One of the lights is input into the switch  14 , and the other one of the lights is input into the optical signal fault detection unit  13   b . The optical signal fault detection unit  13   b  includes a light-receiving element  18   b , an analog-to-digital converter (ADC)  19   b , and a fault detector  20   b . The light-receiving element  18   b  converts the light transmitted from the optical coupler  17   b  into a current. The ADC  19   b  converts a value of the current transmitted from the light-receiving element  18   b  into a digital current value. The fault detector  20   b  performs optical signal fault detection on the basis of the digital value obtained by the ADC  19   b  and the WCF information C and the WCS information D which have been transmitted through the second path  6   b.    
         [0046]    The line switching control circuit  21  determines which of the light transmitted from the WDM coupler  10   a  and the light transmitted from the WDM coupler  10   b  is output on the basis of the results of the optical signal fault detection performed by the optical signal fault detection units  13   a  and  13   b . At that time, it is required to perform switching in consideration of the status of an optical signal on a path to be switched. Accordingly, control processing is performed using not only the WCF information A and the WCS information B for the first path  6   a  that is a currently used path but also the WCF information C and the WCS information D for the second path  6   b  that is a back up path. The line switching control circuit  21  will be described in detail later with reference to an accompanying drawing. 
         [0047]    A driving circuit  22  controls the switch  14  on the basis of a control signal transmitted from the line switching control circuit  21 . The switch  14  outputs one of the lights transmitted from the WDM couplers  10   a  and  10   b  on the basis of a control signal transmitted from the driving circuit  22 . 
       [Optical Signal Fault Detection Circuit] 
       [0048]      FIG. 4  is a diagram illustrating the configuration of the fault detector  20   a.    
         [0049]    The fault detector  20   a  includes registers  22   b  and  22   a , a processor (ALU)  23 , a comparator  24 , an OR circuit  25 , pulse creation circuits  26   a  and  26   b , and counter circuits  27   a  and  27   b.    
         [0050]    The registers  22   a  and  22   b  are each storage device. The registers  22   a  and  22   b  receive a value from the ADC  19   a  illustrated in  FIG. 3 . The register  22   b  stores the current intensity (level) of light obtained in the sampling period of the ADC  19   a . The register  22   a  also stores the power value of light obtained in the sampling period of the ADC  19   a  until the WCF information and the WCS information are input. When an enable signal is input into the register  22   a , the register  22   a  stores a power value at that time. The registers  22   a  and  22   b  may be arranged out of the fault detector  20   a.    
         [0051]    For example, the ALU  23  subtracts a value output from the register  22   a  from a value output from the register  22   b . In an initial state and a normal state (in which received light includes an optical signal), the values output from the registers  22   b  and  22   a  are the same. Accordingly, the ALU  23  outputs zero as a computation result. When a failure occurs on a used path, a value output from the register  22   b  becomes small since received light does not include an optical signal. Accordingly, the ALU  23  outputs a negative value as a computation result. 
         [0052]    The comparator  24  compares the value of the computation result with a predetermined value. When the value of the comparison result is smaller than a predetermined value, the comparator  24  determines that the fault of an optical signal has occurred and outputs a trigger signal to the line switching control circuit  21  illustrated in  FIG. 3 . In this embodiment, at the time of occurrence of an optical signal fault, the trigger signal is changed from “0” to “1”. The predetermined value used by the comparator  24  is a fault detection threshold value. Received light usually includes an ASE light component that is several dB to 10 dB lower than an optical signal component. Accordingly, as the fault detection threshold value, a value with which only remaining ASE light can be detected may be set. In this embodiment, 8 dB is set as the fault detection threshold value. 
         [0053]    The counter circuit  27   a  receives the WCF information A for a corresponding optical channel from the OSC receiving unit  12   a  and detects whether a change occurs within a predetermined time period after the WCF information A has been changed. The counter circuit  27   a  includes an inverter  40   a , flip-flops  41   a ,  42   a , and  43   a , and an AND gate  44   a.    
         [0054]    The inverter  40   a  inverts the WCF information A and transmits the inverted WCF information A to the flip-flop  41   a . The inverter  40   a  performs inversion so as to match the logic of WCF information to that of WCS information. That is, the WCF information indicates “0” when an optical signal is present and “1” when no optical signal is present, while the WCS information indicates “1” when an optical signal is present and “0” when no optical signal is present. 
         [0055]    The flip-flop  41   a  receives a signal output from the inverter  40   a  and a clock and transmits an output signal to the flip-flop  42   a  and the AND gate  44   a . The flip-flop  42   a  receives the signal output from the inverter  41   a  and a clock and transmits an output signal to the flip-flop  43   a  and the AND gate  44   a . The flip-flop  43   a  receives the signal output from the flip-flop  42   a  and a clock and transmits an output signal to the AND gate  44   a . The AND gate  44   a  outputs the logical AND among the signals output from the flip-flops  41   a ,  42   a , and  43   a . The output of the AND gate  44   a  is input into the pulse creation circuits  26   a  and  26   b  as the output of the counter circuit  27   a.    
         [0056]    The pulse creation circuit  26   a  is a circuit for creating a pulse functioning as an enable signal. The pulse creation circuit  26   a  includes flip-flops  28   a  and  29   a , an inverter  30   a , and an AND gate  31   a . The flip-flop  28   a  receives the output of the counter circuit  27   a  and a clock, and transmits an output signal to the flip-flop  29   a  and the AND gate  31   a . The flip-flop  29   a  receives the output of the flip-flop  28   a  and a clock, and transmits an output signal to the inverter  30   a . The AND gate  31   a  outputs the logical AND among the signals output from the inverter  30   a , the flip-flop  28   a , and the counter circuit  27   b . That is, the pulse creation circuit  26   a  outputs a single pulse when the outputs of the counter circuits  27   a  and  27   b  are “1” and the counter circuit  27   a  is changed from “0” to “1”. 
         [0057]    The counter circuit  27   b  receives the WCS information B for a corresponding optical channel from the OSC receiving unit  12   b  and detects whether a change occurs within a predetermined time period after the WCS information B has been changed. The counter circuit  27   b  includes flip-flops  41   b ,  42   b , and  43   b  and an AND gate  44   b.    
         [0058]    The flip-flop  41   b  receives a signal output from an inverter  40   b  and a clock and transmits an output signal to the flip-flop  42   b  and the AND gate  44   b.    
         [0059]    The flip-flop  42   b  receives the signal output from the flip-flop  41   b  and a clock and transmits an output signal to the flip-flop  43   b  and the AND gate  44   b . The flip-flop  43   b  receives the signal output from the inverter  42   b  and a clock and transmits an output signal to the AND gate  44   b . The AND gate  44   b  outputs the logical AND among the signals output from the flip-flops  41   b ,  42   b , and  43   b . The output of the AND gate  44   b  is input into the pulse creation circuits  26   a  and  26   b  as the output of the counter circuit  27   b.    
         [0060]    The pulse creation circuit  26   b  is a circuit for creating a pulse functioning as an enable signal. The pulse creation circuit  26   b  includes flip-flops  28   b  and  29   b , an inverter  30   b , and an AND gate  31   b . The flip-flop  28   b  receives the output of the counter circuit  27   b  and a clock, and transmits an output signal to the flip-flop  29   b  and the AND gate  31   b . The flip-flop  29   b  receives the output of the flip-flop  28   b  and a clock, and transmits an output signal to the inverter  30   b . The AND gate  31   b  outputs the logical AND among the signals output from the inverter  30   b , the flip-flop  28   b , and the counter circuit  27   a . That is, the pulse creation circuit  26   b  outputs a single pulse when the outputs of the counter circuits  27   a  and  27   b  are “1” and the counter circuit  27   b  is changed from “0” to “1”. 
         [0061]    The OR circuit  25  sets the OR condition of pulses output from the pulse creation circuits  26   a  and  26   b  as an enable signal used to fix data of the register  22   a.    
         [0062]    It is desirable that a time set for the counter circuits  27   a  and  27   b  be sufficiently longer than a transmission delay of an OSC signal. 
         [0063]    An OSC signal is used for supervisory control, and a speed at which an OSC signal is transmitted from a transmission terminal is lower than a speed at which an optical signal is transmitted from the transmission terminal. When information indicating that an optical signal is coming continues for a time longer than the transmission time of an OSC signal, it can be determined that the state is power of a state in which an optical signal is certainly coming. 
         [0064]    Accordingly, when the power of the state in which an optical signal is certainly coming is stored in the register  22   a  using an enable signal, the power of a state in which an optical signal is certainly present is automatically stored. 
         [0065]    In this embodiment, the fault detector  20   a  uses both the WCF information A and the WCS information B, but may use only the WCF information A. In this case, the counter circuit  27   b , the pulse creation circuit  26   b , and the OR circuit  25  are removed, the AND gate  31   a  obtains the logical AND between signals output from the flip-flop  29   a  and the inverter  30   a , and the output of the AND gate  31   a  is transmitted to the register  22   a  as an enable signal. 
         [0066]    The configuration of the fault detector  20   a  has been described. The fault detector  20   b  has the same configuration as that of the fault detector  20   a  except that the value of the ADC  19   b  is supplied to the registers  22   b  and  22   a  and the WCF information C and the WCS information D for the second path  6   b  are supplied to the counter circuits  27   a  and  27   b , respectively. 
         [0000]    [Time Chart in this Configuration] 
         [0067]      FIG. 5  is a time chart in this configuration. The time chart illustrates the signal state of each unit on a first path. Since this time chart can be applied to the second path  6   b , the description thereof is therefore omitted. 
         [0068]      FIG. 5A  illustrates the state of the WCS information B in the OSC transmission unit  11   a .  FIG. 5B  illustrates the state of the WCS information B in the OSC receiving unit  12   a .  FIG. 5C  illustrates the state of the light-receiving elements  80   a .  FIG. 5D  illustrates the state of the WCF information A in the OSC transmission unit  11   a .  FIG. 5E  illustrates the state of the WCF information A in the OSC receiving unit  12   a.    
         [0069]    At a point A in  FIG. 5A , a carrier sets starting of a service for the optical transmitter  1  illustrated in  FIG. 1  and the WCS information B is changed from “0” to “1”. At a point B in  FIG. 5B  after a transmission delay T has elapsed from the point A at which the WCS information B was changed from “0” to “1”, the WCS information B reaches the OSC receiving unit  12   a . When the operation of the optical transmitter  1  is started, the light-receiving elements  80   a  start to detect optical power at a point C in  FIG. 5C . When the light-receiving elements  80   a  detect optical power, the WCF information A is changed from “1” to “0” at a point E in  FIG. 5D . The change of the WCF information A at the point E is received by the OSC receiving unit  12   a  at a point H in  FIG. 5E  after the transmission delay T has elapsed from the point E. The counter circuit  27   a  starts a count operation at the point H in  FIG. 5E . The optical signal fault detection unit  13   a  generates an enable signal at a point I in  FIG. 5E  at which the transmission delay T has elapsed from the point H and the WCS information B illustrated in  FIG. 5B  is “1”. 
         [0070]    At a point D in  FIG. 5C , the fault of an optical signal output from the optical transmitter  1  occurs. Since the optical signal is not transmitted, the value of the register  22   b  is also changed at the point D in  FIG. 5C . After the point D in  FIG. 5C , the WCF information A is changed from “0” to “1” at a point F in  FIG. 5D . This change of the WCF information A at the point F reaches the OSC receiving unit  12   a  at a point  3  in  FIG. 5E  after the transmission delay T has elapsed from the point F. 
         [0071]    When the fault of the optical signal output from the optical transmitter  1  is recovered, the WCF information A is changed from “1” to “0” at a point G in  FIG. 5D . This change of the WCF information A at the point G reaches the OSC receiving unit  12   a  at a point K in  FIG. 5E  after the transmission delay T has elapsed from the point G. 
         [0072]    At the point K in  FIG. 5E , the counter circuit  27   a  starts a count operation. The optical signal fault detection unit  13   a  generates an enable signal at a point L in  FIG. 5E  at which the transmission delay T has elapsed from the point K and the WCS information B illustrated in  FIG. 5B  is “1”. 
       [Relationship Between Optical Channel and Optical Signal] 
       [0073]      FIG. 6  is a diagram illustrating the relationship between an optical channel and an optical signal according to an embodiment of the present invention. In  FIG. 6 , there are optical channels λ 1  to λ 10 . That is, in this case, ten optical transmitters  1  corresponding to the number of wavelengths of these optical channels are required. Furthermore, the WDM couplers  4   a  and  4   b  are required to have the capability to multiplex ten wavelengths of the ten optical channels. 
         [0074]    In  FIG. 6 , an optical channel having WCS information of “1” represents an optical channel of a wavelength for which a carrier provides a service, and an optical channel having WCF information of “1” represents an optical channel of a wavelength that is not detected by the light-receiving elements  80   a . The optical channels λ 5  and λ 9  indicate a state in which the optical transmitter  1  is operated, the light-receiving elements  80   a  do not detect light, and a line is disconnected. 
       [First Switching Operation] 
       [0075]    A switching operation performed in the sate of the optical channels λ 5  and λ 9  or when the optical signal fault detection unit  13   a  detects the fault of an optical signal will be described below.  FIGS. 7A and 7B  are diagrams illustrating the first switching operation. In  FIGS. 7A and 7B , optical channels corresponding to the first path  6   a  and the second path  6   b  are in-service optical channels supported by a carrier, and have the WCS information of “1”. 
         [0076]    In  FIG. 7A , first, a failure occurs in an optical channel corresponding to the first path  6   a . Subsequently, the line switching unit  8  switches a line (optical channel) from a line (optical channel) on the first path  6   a  to a line (optical channel) on the second path  6   b . Subsequently, a failure occurs in the optical channel corresponding to the second path  6   b . At that time, if the optical channel corresponding to the first path  6   a  has yet to be recovered, line switching is not performed. 
         [0077]    In  FIG. 7B , first, a failure occurs in an optical channel corresponding to the first path  6   a . Subsequently, the line switching unit  8  switches an optical channel from the optical channel corresponding to the first path  6   a  to an optical channel corresponding to the second path  6   b . If the optical channel corresponding to the first path  6   a  is recovered when the optical channel corresponding to the second path  6   b  is used, the line switching unit  8  reselects the optical channel corresponding to the first path  6   a . At that time, the line switching unit  8  does not take the presence of a failure in the optical channel corresponding to the second path  6   b  into account. 
         [0078]    A control method of performing the switching operation illustrated in  FIGS. 7A and 7B  is referred to as a revertive control method.  FIG. 8  is a diagram illustrating an example of the line switching control circuit  21  for performing the revertive control method. 
         [0079]    The line switching control circuit  21  includes an OR gate  50 , inverters  51 ,  53 ,  55 ,  59 ,  60 ,  61 , and  62 , latching circuits  52  and  54 , and AND gates  56 ,  57 , and  58 . 
         [0080]    The inverter  59  receives the WCF information A and outputs an inversion signal. The inverter  60  receives a trigger signal from the fault detector  20   a  and outputs an inversion signal. The AND gate  57  receives the output of the inverter  59 , the output of the inverter  60 , and the WCS information B and outputs the logical AND among the three inputs. When the output of the AND gate  57  is 1, the first path  6   a  is in a normal state. Accordingly, in this case, the output of the AND gate  57  functions as a signal for selecting an optical channel corresponding to the first path  6   a.    
         [0081]    The inverter  61  receives the WCF information C and outputs an inversion signal. The inverter  62  receives a trigger signal from the fault detector  20   b  and outputs an inversion signal. The AND gate  58  receives the output of the inverter  61 , the output of the inverter  62 , and the WCS information D and outputs the logical AND among the three inputs. When the output of the AND gate  58  is 1, the second path  6   b  is in a normal state. 
         [0082]    The AND gate  56  receives the output of the AND gate  58  and a trigger signal transmitted from the fault detector  20   a  and outputs the logical AND between the two inputs. When the trigger signal input into the AND gate  56  is 1, there is no optical signal on the first path  6   a . When the output of the AND gate  58  input into the AND gate  56  is 1, the second path  6   b  is in a normal state. Accordingly, when the output of the AND gate  56  is 1, the output of the AND gate  56  functions as a signal for changing the optical channel corresponding to the first path  6   a  to an optical channel corresponding to the second path  6   b.    
         [0083]    The AND gate  57  receives the inversion signal of the trigger signal output from the fault detector  20   a . The AND gate  56  receives the trigger signal output from the fault detector  20   a . Accordingly, the AND gates  56  and  57  do not output “1” at the same time. 
         [0084]    The inverter  53  inverts the logical AND output from the AND gate  56  and supplies a result of the inversion to the latching circuit  52 . 
         [0085]    The latching circuit  52  receives the logical AND output from the AND gate  57  and performs latching in response to a signal transmitted from the inverter  53 . The inverter  55  inverts the logical AND output from the AND gate  57  and supplies a result of the inversion to the latching circuit  54 . The latching circuit  54  receives the logical AND output from the AND gate  56  and performs latching in response to a signal output from the inverter  55 . The inverter  51  inverts the output of the latching circuit  52  and outputs a result of the inversion to the OR gate  50 . The OR gate  50  outputs the logical OR between the output of the inverter  51  and the output of the latching circuit  54 . 
         [0086]    In the line switching control circuit  21  having the above-described configuration, it is possible to change an optical channel corresponding to the second path  6   b  back to an optical channel corresponding to the first path  6   a  at the time of failure recovery of the optical channel corresponding to the first path  6   a . The line switching control circuit  21  can perform switching to a path having WCS information (in-service line). 
       [Second Switching Operation] 
       [0087]      FIGS. 9A ,  9 B, and  9 C are diagrams illustrating the second switching operation. Optical channels corresponding to the first path  6   a  and the second path  6   b  are in-service optical channels supported by a carrier and have the WCS information of “1”. 
         [0088]    In  FIG. 9A , first, a failure occurs in an optical channel corresponding to the first path  6   a . Subsequently, the line switching unit  8  changes the optical channel corresponding to the first path  6   a  to an optical channel corresponding to the second path  6   b . When a failure occurs in the optical channel corresponding to the second path  6   b , the optical channel corresponding to the second path  6   b  is changed back to the optical channel corresponding to the first path  6   a  under the condition that the optical channel corresponding to the first path  6   a  is recovered. 
         [0089]    In  FIG. 9B , first, communication is performed with the optical channel corresponding to the first path  6   a . Subsequently, a failure occurs in the optical channel corresponding to the second path  6   b . At that time, the failure in the optical channel corresponding to the second path  6   b  does no have an effect on the communication performed with the optical channel corresponding to the first path  6   a . Subsequently, the optical channel corresponding to the second path  6   b  is recovered. After that, a failure occurs in the optical channel corresponding to the first path  6   a . At that time, since the optical channel corresponding to the second path  6   b  is recovered, the line switching unit  8  switches a line (optical channel) from the optical channel corresponding to the first path  6   a  to the optical channel corresponding to the second path  6   b.    
         [0090]    In  FIG. 9C , first, a failure occurs in the optical channel corresponding to the first path  6   a . Subsequently, the line switching unit  8  switches a line (optical channel) from the optical channel corresponding to the first path  6   a  to the optical channel corresponding to the second path  6   b . After that, a failure also occurs in the optical channel corresponding to the second path  6   b . In this state, since both the optical channel corresponding to the first path  6   a  and the optical channel corresponding to the second path  6   b  are unusable, the line switching unit  8  does not perform the optical channel switching operation. When the optical channel corresponding to the second path  6   b  is recovered, communication is performed with the optical channel corresponding to the second path  6   b.    
         [0091]    A control method of performing the switching operation illustrated in  FIGS. 9A ,  9 B, and  9 C is referred to as a non-revertive control method.  FIG. 10  is a diagram illustrating an example of the line switching control circuit  21  for performing the non-revertive control method. 
         [0092]    The line switching control circuit  21  includes the OR gate  50 , the inverters  51 ,  53 , and  55 , inverters  65   a ,  65   b ,  66   a , and  66   b , the latching circuits  52  and  54 , and AND gates  63   a ,  63   b ,  64   a , and  64   b.    
         [0093]    The inverter  65   a  receives the WCF information A and outputs an inversion signal. The inverter  66   a  receives a trigger signal from the fault detector  20   a  and outputs an inversion signal. The AND gate  64   a  receives the output of the inverter  65   a , the output of the inverter  66   a , and the WCS information B and outputs the logical AND among the three inputs. When the output of the AND gate  64   a  is 1, the first path  6   a  is in a normal state. 
         [0094]    The AND gate  63   a  receives the output of the AND gate  64   a  and a trigger signal transmitted from the fault detector  20   b  and outputs the logical AND between the two inputs. When the logical AND output from the AND gate  63   a  is 1, the output of the AND gate  63   a  functions as a signal for changing the optical channel corresponding to the second path  6   b  to the optical channel corresponding to the first path  6   a.    
         [0095]    The inverter  65   b  receives the WCF information C and outputs an inversion signal. The inverter  66   b  receives a trigger signal from the fault detector  20   b  and outputs an inversion signal. The AND gate  64   b  receives the output of the inverter  65   b , the output of the inverter  66   b , and the WCS information D and outputs the logical AND among the three inputs. When the output of the AND gate  64   b  is 1, the second path  6   b  is in a normal state. 
         [0096]    The AND gate  63   b  receives the output of the AND gate  64   b  and a trigger signal transmitted from the fault detector  20   a  and outputs the logical AND between the two inputs. When the logical AND output from the AND gate  63   b  is 1, the output of the AND gate  63   b  functions as a signal for changing the optical channel corresponding to the first path  6   a  to the optical channel corresponding to the second path  6   b.    
         [0097]    The inverter  53  inverts the logical AND output from the AND gate  63   b  and supplies a result of the inversion to the latching circuit  52 . 
         [0098]    The latching circuit  52  receives the logical AND output from the AND gate  63   a  and performs latching in response to a signal transmitted from the inverter  53 . The inverter  55  inverts the logical AND output from the AND gate  63   a  and supplies a result of the inversion to the latching circuit  54 . The latching circuit  54  receives the logical AND output from the AND gate  63   b  and performs latching in response to a signal transmitted from the inverter  55 . The inverter  51  inverts the output of the latching circuit  52  and outputs a result of the inversion to the OR gate  50 . The OR gate  50  outputs the logical OR between the output of the inverter  51  and the output of the latching circuit  54 . 
         [0099]    In the line switching control circuit  21  having the above-described configuration, it is possible to perform the switching operation illustrated in  FIGS. 9A ,  9 B, and  9 C. The line switching control circuit  21  can perform switching to a path having WCS information (in-service line). 
         [0100]      FIG. 11  is a diagram illustrating the configuration of an optical receiving terminal. This optical receiving terminal differs from the optical receiving terminal illustrated in  FIG. 2  in that a 2×2 switch  14   b  is used instead of the switch  14 , optical receivers  9   a  and  9   b  to which two optical outputs of the 2×2 switch  14   b  are individually transmitted are disposed, and an error determination circuit  70  for determining whether an optical signal has been received using bit error values that are results of demodulation performed by the optical receivers  9   a  and  9   b.    
         [0101]    At the time of occurrence of a failure, the 2×2 switch  14   b  switches between the optical channel corresponding to the first path  6   a  and the optical channel corresponding to the second path  6   b  and outputs signals. 
         [0102]    The error determination circuit  70  outputs an error determination signal (Bit Err) indicating whether the optical receivers  9   a  and  9   b  have received an optical signal transmitted from an optical transmitter. The error determination signal Bit Err is supplied to the optical signal fault detection units  13   a  and  13   b  and the line switching control circuit  21  instead of WCF information. In this embodiment, instead of the WCF information illustrated in  FIG. 6 , the error determination signal Bit Err illustrated in  FIG. 6  can be used. 
         [0103]    The optical signal fault detection units  13   a  and  13   b  store optical power obtained at the time of input of an optical signal using the output of the error determination circuit  70  instead of the WCF information. On the basis of a value that is a predetermined value below the stored power, the determination of a signal fault is performed. 
         [0104]      FIGS. 12A and 12B  are diagrams describing the detection of an optical channel at an optical transmission terminal. In  FIG. 2 , the light-receiving elements  80   a  receive light input into the WDM coupler  4   a  so as to detect whether an optical signal is present in an optical channel, and the light-receiving elements  80   b  receive light input into the WDM coupler  4   b  so as to detect whether an optical signal is present in an optical channel.  FIGS. 12A and 12B  describe the detection of an optical channel in which a plurality of light-receiving elements are not used. 
         [0105]    In  FIG. 12A , an optical spectrum analyzer  71  analyzes the output of the WDM coupler  4   a , thereby creating the WCF information of an optical channel of each light wavelength. The WCF information created by the optical spectrum analyzer  71  is transmitted by the OSC transmission unit  11   a  illustrated in  FIG. 2 . Although the detection of an optical channel has been described using components on the side of the first path  6   a , the same processing is performed on the side of the second path  6   b.    
         [0106]    In  FIG. 12B , a light wavelength tunable filter  72  divides the output of the WDM coupler  4   a  in units of wavelengths of optical channels. More specifically, a driver  73  performs wavelength shifting in accordance with the wavelength of an optical channel. 
         [0107]    At a predetermined time, a light-receiving element  74  receives light extracted by the light wavelength tunable filter  72  and converts the light into an electrical signal. An ADC  75  converts the electrical signal transmitted from the light-receiving element  74  into a digital signal. A processor  76  creates WCF information for each optical channel on the basis of a value output from the ADC  75  and transmits the created WCF information to downstream components via the OSC transmission units  11   a  and  11   b.    
         [0108]    The above-described embodiments can be combined as necessary. 
         [0109]    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 inventions 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.