Abstract:
An optical amplifier may be quickly returned from a shutdown state to a regular state after getting recovery information of a fault. Gain setting by ASE is conducted to the repeaters on the up-stream side during the shutdown state, by outputting ASE light with the same intensity as the WDM signal. Accordingly, before realizing the recovery of shutdown, the gain setting is completed with the light whose intensity is within the safe criterion. After realizing the recovery of shutdown, the optical transmission system can be returned quickly to the regular operating state after recovery of shutdown state.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is related to and claims priority to Japanese Application No. 2005-51404 filed Jul. 15, 2005 in the Japanese Patent Office, the contents of which are incorporated by reference herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an optical transmission system and more particularly to an optical transmission system which is capable of quickly shifting to the regular state from a safe light state in which the shutdown state of the transmission path is detected.  
         [0004]     2. Description of the Related Art  
         [0005]     In an optical transmission system using wavelength division multiplexing (WDM) signal lights, it is very important to control an optical amplifier in accordance with the number of multiplexed WDM signal lights. In the optical amplifier, the control modes of the automatic gain control and automatic level control are widely utilized.  
         [0006]     Since the gain is kept at the constant level in the AGC (Automatic Gain Control) mode, a ratio of input level to output level of an optical amplifier is constant and an output level of an optical amplifier varies following an input level. For example, when the number of multiplexed WDM signal lightss input to an optical amplifier changes, a level of the input signal light changes, but since the gain of the signal light of each wavelength is constant, an output level of the signal light of each wavelength is not influenced.  
         [0007]     Since a gain of an optical amplifier is controlled to make constant an output level of an optical amplifier in the ALC mode, the information on the number of multiplexed signals is necessary to control an output level of the signal light of each wavelength of the wavelength-multiplexed signal to the target value. For example, if the number of multiplexed input signal lights of the WDM is varied due to the signal add/drop process, an output level of the signal light of each wavelength also changes, resulting in the possibility of generation of transmission error.  
         [0008]     In the optical amplifier used for the optical transmission system in which the number of multiplexed WDM signal lights to be transmitted varies, the control for switching the AGC mode and ALC (Automatic Level Control) mode is implemented.  
         [0009]     Since the transmission light is possibly released to the external side of an optical fiber in the optical transmission system due to breakdown of the transmission path or a fault such as opening or the like of an optical connector by erroneous operation of a worker, an output of the transmission light must be kept within the safety criterion when a fault is detected.  
         [0010]     There is known an automatic power shutdown (APSD) control in which the WDM light output is suspended only in the section wherein a fiber fault is generated by transmitting, if a fault in the transmission path is detected in the down-stream side of the fault point, the fault information to the up-stream side through the transmission path in the opposite direction in order to suppress an output of the optical amplifiers in the up-stream side. An example of this is shown in Japanese Patent Application JP-A No. 77056/2002.  
         [0011]     In this APSD control, an output light intensity is maintained within the safety criterion by suspending output of the WDM light from the up-stream side station and an optical supervisory channel (OSC) control light is output. Since recovery from the fault can be detected by receiving the OSC light in the down-stream side of the fault point, the transmitting state can be returned to the regular state by canceling the APSD control in the up-stream side terminal through the opposite transmission line.  
         [0012]      FIG. 4  illustrates flows of controls and signals when a fault occurs at the fault point  7  in the optical transmission path  5 A in the optical transmission system in which optical transmission is performed between a terminal  1 A and a terminal  1 B through the optical transmission paths  5 A and  5 B and the repeaters  2 A to  2 C are provided in the course of the transmission paths.  
         [0013]     In  FIG. 4 , the WDM signal light output from an optical switch  13 A is amplified with a post-amplifier  10 A and is then transmitted to the optical transmission path  5 A. The transmitted WDM signal light is amplified with the optical amplifiers  20 E,  20 C,  20 A of the repeaters  2 C,  2 B,  2 A and is then input to a pre-amplifier  11 A of the terminal  1 B.  
         [0014]     Moreover, the OSC light is output from an OSC transmitting unit  121 A, in addition to the WDM signal light. Each repeater is also provided the OSC receiving units  222 A,  222 C,  222 E and the OSC transmitting units  221 A,  221 C,  221 E. In the terminal and repeater, the OSC signal is branched to the OSC receiving unit before input to an optical amplifier. These terminals and repeaters receive the supervisory control signal from the up-stream side, combine the OSC signals from the OSC transmitting unit, and then output the combined OSC signals to the down-stream side of the transmission path.  
         [0015]     When a fault occurs at the fault point  7  in the optical transmission path  5 A, the OSC receiving unit  222 A of the repeater  2 A in the down-stream side detects shutdown of the OSC signal, while the optical amplifier  20 A detects shutdown of the main signal (WDM signal light) with an input light monitor (not illustrated). Accordingly, the control unit  22 A of the repeater  2 A detects generation of a fault.  
         [0016]     The APSD control unit  220 A transmits the APSD request signal to the repeater  2 B using the OSC signal of the opposed transmission line. The APSD request signal is transmitted to the APSD control unit  220 B from the OSC transmitting unit  221 B via the OSC receiving unit  222 D of the repeater  2 B.  
         [0017]     The APSD control unit  220 B shuts down the optical amplifier  20 C on the basis of the APSD request signal. Accordingly, the optical repeater  20 C is shutdown and an output of the WDM light is suspended only in the section between the repeater  2 B and the repeater  2 A where a fault is generated in the optical transmission path  5 A.  
         [0018]     Here, the OSC light is transmitted from the OSC transmitting unit  221  and recovery of the fault at the fault point  7  can be detected by receiving the OSC light with the OSC receiving unit  222 A.  
         [0019]     In the case of returning to the regular mode from the shutdown state by the APSD control or the like and switching to the backup line from the working line because of generation of a fault, or in the case of newly constituting a network system, gain setting is necessary for each optical amplifier.  
         [0020]     Here, there is also known the technology for setting the gain (gain setting by ASE) of an optical amplifier by utilizing the amplified spontaneous emission (ASE) light even if the signal light is not transmitted to the transmission path when the optical transmission system is constituted and/or when recovered from the defective state. An exmaple of this is shown in Japanese Patent Application JP-A No. 23437/2004).  
         [0021]      FIG. 5  and  FIG. 6  illustrate the procedures for setting the gain of each optical amplifier through gain setting by ASE when the fault at the fault point  7  is recovered from the shutdown state by the APSD control of  FIG. 4 .  
         [0022]     When the fault at the fault point  7  is recovered in  FIG. 5 , the OSC light transmitted from the OSC transmitting unit  221 C is received with the OSC receiving unit  222 A and the control unit  22 A of the repeater  2 A detects recovery of fault at the fault point  7 . The control unit  22 A transmits the APSD canceling signal to the repeater  2 B using the OSC signal of the opposed transmission line and also transmits the ASE setting request to the up-stream side terminal and the optical amplifier  20 A. The optical amplifier  20 A receives the ASE setting request and shifts to the ASE setting mode.  
         [0023]     The APSD canceling signal is transmitted to the APSD control unit  220 B from the OSC transmitting unit  221 B through the OSC receiving unit  222 D of the repeater  2 B. The APSD control unit  220 B cancels the shutdown state of the optical amplifier  20 C on the basis of the APSD canceling signal. The optical amplifier  20 C cancels the shutdown state with the APSD canceling signal and thereafter receives the ASE setting request to shift to the ASE setting mode.  
         [0024]     The repeater  2 C in the up-stream side also receives the ASE setting request via the OSC signal and the optical amplifier  20 E shifts to the ASE setting mode.  
         [0025]     When the terminal  1 A receive the ASE setting request, the optical switch  13 A shuts down the input WDM signal light to the post-amplifier  10 A. The terminal amplifier  10 A which has shifted to the ASE setting mode by receiving the ASE setting request controls the intensity of the excited light for the gain control in order, to output the ASE light in the intensity which is identical to that of the ASE light when a single light is amplified under the state that the input WDM signal light is shutdown.  
         [0026]     The down-stream optical amplifier which has shifted to the ASE setting mode with the ASE light output from the termination amplifier  10 A also controls the intensity of the excited light for the gain control in order to output the ASE light in the intensity which is identical to that of the ASE light when only a single light is amplified. When the gain control of the optical amplifier in the ASE setting mode is completed and the gain of the same amplifier is set, each optical amplifier transmits the gain setting end message through the OSC light of the opposed transmission line.  
         [0027]     In  FIG. 6 , when the optical amplifier  20 A of the repeater  2 A located at the next stage of the fault point  7  shifts to the gain control from the ASE setting mode and sets the gain thereof, the OSC transmitting unit  221 B transmits the gain setting end message with the OSC light on the opposed transmission line. When the terminal  1 A receives the gain setting end message from the optical amplifier of the repeater  2 A located at the next stage of the fault point  7 , the optical switch  13 A of the terminal  1 A switches the input WDM signal light, which has been shutdown, to be transmitted to the post-amplifier  10 A. Thereby, the light including the WDM light is transmitted from the terminal  1 A.  
         [0028]     When the optical switch  13  of the terminal  1 A is switched for transmission of light, the optical amplifiers up to the optical amplifier  20 A from the post-amplifier have already completed the gain setting by the ASE setting mode. Accordingly, the light including the WDM light is transmitted from the terminal  1 A, resetting the regular operating state.  
         [0029]     In above description, the intensity of the ASE light in the ASE setting mode is set identically to the intensity of the light when a single light is amplified. However, the ASE light intensity is enough when it can be detected with an input monitor of the optical amplifier located in the next stage and it enables setting of the gain even if the signal light does not exist.  
         [0030]     The structure for detection of the shutdown state and recovery from the shutdown state has been described for the external transmission path of the terminal or repeater. Meanwhile, a unit structure is generally employed for each function in the WDM light transmitting apparatus.  
         [0031]     For example, the unit including a pre-amplifier for amplifying the WDM optical signal with a receiving unit of the terminal is formed in separation from the unit including a demultiplexing unit for demultiplexing the amplified WDM optical signal, consideration should be taken also for a fault in the fiber connecting the units.  
         [0032]      FIG. 7  and  FIG. 8  illustrate the procedures for detection and recovery of the shutdown state when a fault is generated in the optical fiber  50 A connecting the unit  3 A including the pre-amplifier  11 A and the unit including the DEMUX unit  31 A.  
         [0033]     In  FIG. 7 , a fault in the fiber  50 A is detected with a power detector (PD)  32 A for monitoring input of the unit  3 B. Based on the detection result of PD  32 A, the pre-amplifier  11 A performs the shutdown operation.  
         [0034]     Since the OSC light is not transmitted to the fiber  50 A, the fault in the fault point  7  is recovered by setting the pre-amplifier  11 A to the safe light state for transmitting light of a level satisfying the safety criterion.  
         [0035]      FIG. 8  illustrates the re-setting of the transmission system when a fault at the fault point  70  is recovered. When the fault is recovered, the light transmitted from the pre-amplifier  11 A in the safe light state is received with the PD  32 A. Accordingly, the control unit  12 B detects recover of fault in the fault point  70 .  
         [0036]     The control unit  12 B transmits the ASE setting request to the up-stream side terminals and pre-amplifier  11 A using the OSC signal on the opposed transmission line. The pre-amplifier  11 A shifts to the ASE setting mode by receiving the ASE setting request.  
         [0037]     When the terminal  1 A receives the ASE setting request, the optical switch  13 A shuts down the input WDM signal light to the post-amplifier  10 A. The post-amplifier  10 A, having shifted to the ASE setting mode by receiving the input WDM signal light, controls the intensity of the excited light for the gain control in order to output the ASE light in the intensity which is identical to that of the ASE light when a single signal light is amplified under the state that the input WDM signal light is shutdown.  
         [0038]     The optical amplifier in the down-stream side which has shifted to the ASE setting mode with the ASE light output from the post-amplifier  10 A also controls the intensity of the excited light for gain control to output the ASE light in the intensity which is identical to that of the ASE light when the input WDM signal light is amplified. When the gain control of the optical amplifier in the ASE setting mode is completed and the gain thereof is set, each amplifier transmits the gain setting end message via the OSC light on the opposed transmission line.  
         [0039]     The optical transmission systems illustrated in  FIG. 4  to  FIG. 6  have the problem that a longer time is required to set the gain for all optical amplifiers of the repeaters located in the next stage of the fault point from the post-amplifier of the terminal because the control by the APSD control is different from the gain control for recovery to the regular operating state.  
         [0040]     Moreover, such optical transmission systems also have a problem in that the control is complicated because the control by the APSD control must be conducted in separation from the gain control such as the ASE setting or the like.  
         [0041]     In the optical transmission system illustrated in  FIG. 7  and  FIG. 8 , it is required to transmit the light in a level satisfying the safety criterion and to detect recovery of the shutdown state with a pre-amplifier because the OSC light is not transmitted to the fiber connecting the units.  
         [0042]     Moreover, these transmission systems also have a problem that a longer time is required for total gain setting because the gain control such as the ASE setting or the like must be conducted from the post-amplifier located at the other end of the transmission path for the recovery from the shutdown state.  
       SUMMARY OF THE INVENTION  
       [0043]     The present invention reduces the time needed to recover from a fault in an optical transmission system. An optical amplifier may be quickly returned from a shutdown state to a regular state after getting recovery information of a fault. Gain setting by ASE is conducted to the repeaters on the up-stream side during the shutdown state, by outputting ASE light with the same intensity as the WDM signal. Accordingly, before realizing the recovery of shutdown, the gain setting is completed with the light whose intensity is within the safe criterion. After realizing the recovery of shutdown, the optical transmission system can be returned quickly to the regular operating state after recovery of shutdown state.  
         [0044]     In one embodiment of the present inventionm an optical transmission system comprises a first terminal transmitting a wavelength division multiplex signal light amplified by a post-amplifier to a first transmission path, a second transmission path opposite to the first transmission path transmitting the signal light from the second terminal and a plurality of repeaters provided in the first and the second transmission paths, having optical amplifiers amplifying the signal light from the first transmission path, wherein, when a fault in the first transmission path occurs between a first repeater located on an up-stream side and a second repeater located on a down-stream side and the second repeater detects this fault, the second repeater transmits fault information to the second transmission path, the repeaters located on the up-stream side from the first repeater for the first transmission path stops an amplifying operation of the signal light based on the fault information and transmits a stop request to the second transmission path, the first terminal stops transmission of the signal light on the basis of the stop request, controls the post-amplifier so that an intensity of an amplified spontaneous emission (ASE) light output by the post-amplifier can be detected by an input monitor of an optical amplifier located at a next stage and controls an intensity of an output light within a range satisfying a safety criterion, and transmits the ASE light to the first transmission path, and a repeater located on the up-stream side of the first transmission path from the first repeater controls, on the basis of the ASE light, an optical amplifier so that an intensity of the ASE light output by the optical amplifier can be detected with an input monitor of an optical amplifier located in a next stage and controls an intensity of the output light within a range for satisfying a safety criterion.  
         [0045]     In one aspect of the present invention, the second repeater detects a recovery of the fault, the second repeater controls the optical amplifier so that the intensity of the ASE light output by the optical amplifier can be detected with the input monitor of the optical amplifier located in the next stage and the intensity of the output light within the range satisfying the safety criterion, and when the second repeater completes the control of the optical amplifier, the second repeater transmits completion information to the second transmission path and the first repeater starts transmission of the signal light based on the completion information.  
         [0046]     In one embodiment of the present invention, a repeater provided in the course of a first transmission path and a second transmission path provided opposite to the first transmission path having a first optical amplifier amplifying a signal light from the first transmission path, wherein when fault information on a down-stream side of the first transmission path from the second transmission path is received, an amplifying operation of the signal light is stopped and the stop request is transmitted to the second transmission path, and when the ASE light transmitting signal is received from a terminal of the first transmission path, an optical amplifier is controlled so that an intensity of the ASE light output by the optical amplifier can be detected with an input monitor of the optical amplifier located at a next stage and controls the intensity within a range satisfying a safety criterion.  
         [0047]     In one embodiment of the present invention, an optical transmission system comprises a first terminal operable to transmit a wavelength division multiplex signal light amplified with a post-amplifier to a first transmission path and receiving a signal light from a second transmission path opposite to the first transmission path, a second terminal operable to amplify the wavelength division multiplex signal light received from the first transmission path with a preamplifier, input the amplified wavelength division multiplex signal light to a branching filter for demultiplexing via a unit-to-unit fiber, and transmitting the signal light to the second transmission path, and a plurality of repeaters including optical amplifiers operable to amplify the signal lights from the first transmission path and provided in the course of the first and the second transmission paths, and wherein the second terminal includes a monitor operable to monitor the output light of the unit-to-unit fiber in the preceding stage of the branching filter, the second terminal stops an amplifying operation of the signal light of the preamplifier and transmits fault information to the second transmission path, the repeater stops an amplifying operation of the signal lights on the basis of the fault information and transmits a stop request to the second transmission path, the first terminal stops transmission of the signal light based on the stop request, controls the post-amplifier so that an intensity of an ASE light output by the post-amplifier can be detected with an input monitor of an optical amplifier located at the next stage and controls the intensity in a range satisfying a safety criterion, and transmits the ASE light transmitting signal to the first transmission path, and the repeater controls the optical amplifier based on a naturally emitted light transmitting signal so that an intensity of the naturally emitted light by the optical amplifier can be detected with the input monitor of the optical amplifier and controls the intensity within the range satisfying the safety criterion.  
         [0048]     In one aspect of the present invention, the monitoring means has detected recovery of the fault, the second repeater transmits completion information to the second transmission path and the first repeater starts transmission of signal light based on the completion information.  
         [0049]     According to the present invention, since the gain control for returning the regular operating state can be realized in parallel under the safe light state after detection of a fault in the transmission path, the regular operating state can be recovered quickly after recovery from the fault.  
         [0050]     Moreover, since the control for the safe light state and the gain control may be conducted simultaneously, the control can be simplified.  
         [0051]     Spread of shutdown state can be detected and the regular state can be returned quickly after recovery from a fault, without separately providing a transponder unit, for the fault of fiber connecting the units wherein the supervisory control light is not transmitted. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0052]      FIG. 1  is a diagram illustrating operations when shutdown of the transmission path is detected in the optical transmission system of the present invention.  
         [0053]      FIG. 2  is a diagram illustrating operations when the fault in the transmission path is recovered in the optical transmission system of the present invention.  
         [0054]      FIG. 3  is a diagram illustrating operations when shutdown of fiber connecting units of terminal is detected in the optical transmission system of the present invention.  
         [0055]      FIG. 4  is a diagram illustrating operations by APSD when shutdown of transmission path is detected in the optical transmission system based on the related art.  
         [0056]      FIG. 5  is a diagram illustrating the ASE setting operation when shutdown of transmission path is recovered in the optical transmission system based on the related art.  
         [0057]      FIG. 6  is a diagram illustrating the ASE setting operation when shutdown of transmission path is recovered in the optical transmission system based on the related art.  
         [0058]      FIG. 7  is a diagram illustrating operations when shutdown of fiber connecting units of the terminal is detected in the optical transmission system based on the related art.  
         [0059]      FIG. 8  is a diagram illustrating the ASE setting operation when shutdown of fiber connecting units of the terminal is recovered in the optical transmission system based on the related art.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0060]     The embodiments for carrying out the present invention will be described with reference to the accompanying drawings.  
       [1] Description of the First Embodiment  
       [0061]      FIG. 1  illustrates an optical transmission system of a first embodiment of the present invention. In the optical transmission system illustrated in  FIG. 1 , optical transmission is conducted between a terminal  1 A and a terminal  1 B using an optical transmission path  5 A and an optical transmission path  5 B forming the oppos ite transmission line and repeaters  2 A to  2 C are provided in the course of the transmission paths.  
         [0062]     The terminal  1 A comprises an optical switch  13 A to which the WDM signal light to be transmitted is input, a post-amplifier  10 A to amplify the light input from the optical switch, and a pre-amplifier  11 B to amplify the signal light input from the optical transmission path  5 B. Moreover, a control unit  12 A comprises an OSC transmitting unit  121 A to output the OSC light to the optical transmission path  5 A and an OSC receiving unit  122 B to receive the OSC light from the optical transmission path  5 B. Similarly, the terminal  1 B comprises an optical switch  13 B, a post-amplifier  10 B, a pre-amplifier  11 A, and a control unit  12 B.  
         [0063]     Moreover, the repeater  2 A comprises optical amplifiers  20 A,  20 B for amplifying the lights to be transmitted through the transmission paths  5 A,  5 B and a control unit  22 A. The control unit  22 A comprises an OSC transmitting unit  221 A to output the OSC light to the optical transmission path  5 A, an OSC receiving unit  222 A to receive the OSC light from the optical transmission path  5 A, an OSC transmitting unit  221 B to output the OSC light to the optical transmission path  5 B, and an OSC receiving unit  222 B to receive the OSC light from the optical transmission path  5 B.  
         [0064]     Structures of the repeaters  2 B,  2 C are also similar to that of the repeater  2 A and comprise optical amplifiers for the lights transmitted through the optical transmission paths  5 A,  5 B and control units.  
         [0065]     Operation of the optical transmission system of  FIG. 1 , control and flow of signal when a fault is generated at the fault point  7  in the optical transmission path  5 A will be described below.  
         [0066]     In  FIG. 1 , the WDM signal light output from the optical switch  13 A is amplified with the post-amplifier  10 A and is then transmitted to the optical transmission path  5 A. The WDM signal light transmitted is amplified with the optical amplifiers  20 E,  20 C,  20 A of the repeaters  2 C,  2 B,  2 A and is then input to the pre-amplifier  11 A of the terminal  1 B.  
         [0067]     Moreover, the OSC light is output, in addition to the WDM signal light, from the OSC transmitting unit  121 A of the terminal  1 A and the supervisory control signal is transmitted by the OSC receiving unit and OSC transmitting unit of each repeater.  
         [0068]     When a fault is generated at the fault point  7  in the optical transmission path  5 A (S 11 ), the OSC receiving unit  222 A detects shutdown of the OSC signal (S 12 A) in the repeater  2 A as the repeater located at the down-stream side of the fault point and the optical amplifier  20 A detects the shutdown state of the main signal (WDM signal light) with an input optical monitor (not illustrated). Accordingly, the control unit  22 A of the repeater  2 A detects generation of a fault.  
         [0069]     In this embodiment, the control unit  22 A of the repeater  2 A having detected generation of a fault sends the ASE setting request, namely conducts gain setting to the optical amplifier of the repeater or terminal located at the up-stream side of the optical transmission path  5 A using the ASE light output with the optical amplifier through the optical transmission path  5 B opposed to the optical transmission path  5 A having generated a fault (S 13 ). Namely, the ASE setting request is used as the fault information.  
         [0070]     The control unit  22 A transmits the ASE setting request to the repeater  2 B using the OSC signal of the opposed transmission line. The ASE setting request is transmitted to the control unit  22 B from the OSC transmitting unit  221 B via the OSC receiving unit  222 D of the repeater  2 B.  
         [0071]     The optical amplifier  20 C shifts to the ASE setting mode on the basis of the ASE setting request (S 14 ) from the control unit  22 B.  
         [0072]     The optical amplifier  20 C having shifted to the ASE setting mode conducts the shutdown process. Accordingly, output of optical signal is suspended in the section between the repeater  2 B and repeater  2 A in which a fault of optical transmission path  5 A is generated.  
         [0073]     When the shutdown process of the optical amplifier  20 C is completed, the signal light transmission end request is transmitted to the post-amplifier  10 A of the terminal  1 A via the OSC transmitting unit  221 D.  
         [0074]     Similarly, the optical repeater in the up-stream side of the repeater  2 B shifts to the ASE setting mode by receiving the ASE setting request, shuts down the optical amplifier to stop output of the signal light and also transmits the stop request to the post-amplifier  10 A of the terminal  1 A.  
         [0075]     When the terminal  1 A receives the stop request, the optical switch  13 A shuts down the input WDM signal light to the post-amplifier  10 A (S 15 ). The post-amplifier  10 A having shifted to the ASE setting mode by receiving the ASE setting request (S 16 ) controls the intensity of the excited light for gain control to output the ASE light in the intensity which is identical to that of the signal light when a single signal light is amplified under the state that the input WDM signal light is shutdown. Accordingly, the ASE light transmitting signal is transmitted to the repeater in the down-stream side using the OSC light of the optical transmission path  5 A.  
         [0076]     The optical amplifier in the down-stream side in the shutdown state having shifted to the ASE setting mode controls, upon reception of the ASE optical light transmitting signal transmitted from the terminal  1 A, the intensity of the excited light for gain control using the ASE light output from the post-amplifier  10 A in order to output the ASE light of an intensity which is identical to that of the light when a signal light is amplified. When the gain of the optical amplifier in the ASE setting mode is set with completion of the gain control thereof, the optical amplifiers  20 E,  20 C up to the repeater  2 B just in the up-stream side of the fault point  7  transmit the gain setting end message using the OSC light of the opposed transmission line.  
         [0077]     In this operating process, the optical amplifier  20 C shifts, from the shutdown state, to the state to control the intensity of the excited light and the gain in order to output the ASE light of the intensity which is identical to that of the signal light when a single light is amplified. In any state, the light output from the fault point  7  is within the range of the safety criterion.  
         [0078]     Moreover, in the shutdown state, recovery of fault at the fault point  7  can be detected by receiving the OSC light with the OSC receiving unit  222 A and by receiving the ASE light due to the input optical monitor or the like (not illustrated) of the optical amplifier  20 C under the state that the gain setting has been completed.  
         [0079]     Therefore, the optical amplifiers up to the optical amplifier  20 C of the repeater  2 B located in the up-stream side of the fault point  7  have completed the gain setting under the safe light state after detection of the fault at the fault point  7 .  
         [0080]     Next, control and flows of signals when the fault in the fault point  7  is recovered will be described below.  
         [0081]     In  FIG. 2 , when the fault in the fault point  7  is recovered (S 21 ), the ASE light output from the optical amplifier  20 C having completed the gain setting is received with the optical amplifier  20 A and the control unit  22 A of the repeater  2 A detects recovery of the fault at the fault point  7  (S 22 ). The control unit  22 A performs the ASE setting to the optical amplifier  20 A and transmits the gain setting end message using the OSC light on the opposite transmission line when the gain setting is completed (S 23 ).  
         [0082]     When the gain setting end message of the optical amplifier  20 A of the repeater  2 A is received by the terminal  1 A, the optical switch  13 A of the terminal  1 A switches the input WDM light in the shutdown state to pass to the post-amplifier  10 A. Thereby, the light including the WDM signal light is transmitted from the terminal  1 A.  
         [0083]     When the optical switch  13 A of the terminal  1 A is switched to transmit the light, the optical amplifiers up to the optical amplifier  20 A from the post-amplifier  10 A are reset to regular operating state by transmitting the light including the WDM light from the terminal  1 A because these amplifiers have completed the gain setting based on the ASE setting mode.  
         [0084]     As described above, the optical transmission system of the first embodiment uses the ASE setting request as the fault information and thereby completes the gain setting of the optical amplifiers in the up-stream side of the fault point in the safe light state. Therefore, the time required until start of the regular operating state from recovery of a fault can be shortened, when the fault is recovered, because the regular operating state can be restarted by conducting the gain setting for ASE setting to the optical amplifier of the repeater just in the down-stream side of the fault point and then transmitting the gain setting end message to the terminal.  
         [0085]     Moreover, each optical repeater is not required to provide the control mode for APSD when it is provided with only the ASE setting mode. Accordingly, the structure for controlling the optical amplifier in the repeater may be simplified.  
       [b] Description of Second Embodiment  
       [0086]      FIG. 3  illustrates a structure of a second embodiment of the present invention and procedures for detecting the shutdown state and recovery when a fault is generated in the fiber connecting the units in a receiving terminal formed in the unit structure for each function.  
         [0087]     In  FIG. 3 , a terminal  1 B comprises a unit  3 A including a pre-amplifier  11 A and a unit  3 B including a demultiplexing unit  31 A. The units  3 A and  3 B are connected with a fiber  50 A.  
         [0088]     When a fault is generated at a fault point  70  of the fiber  50 A, the fault in the fiber  50 A is detected with a PD  32 A for monitoring input of the unit  3 B (S 31 ) The fault information is transmitted to the unit  3 A on the basis of the detection result of the PD  32 A (S 32 ).  
         [0089]     When a control unit  12 B of the unit  3 A detects the fault information for the fiber  50 A, the pre-amplifier  11 A shifts to the ASE setting mode (S 34 ) and conducts the shutdown operation.  
         [0090]     Moreover, the control unit  12 B transfers the ASE setting request to the repeaters in the up-stream side using the OSC signal of the opposite transmission line.  
         [0091]     When the shutdown process of an optical amplifier  20 C is completed, the stop request is transmitted to a post-amplifier  10 A of a terminal  1 A via an OSC transmitting unit  121 B.  
         [0092]     An optical repeater in the transmission path shifts to the ASE setting mode by receiving the ASE setting request to shutdown in view of stopping an output of the signal light. The optical repeater also transmits the stop request to the post-amplifier  10 A of the terminal  1 A.  
         [0093]     When the terminal  1 A receives the signal light transmission stop request, an optical switch  13 A shuts down the input WDM signal light to the post-amplifier  10 A (S 35 ). The post-amplifier  10 A having shifted to the ASE setting mode by receiving (S 36 ) the ASE setting request controls the intensity of the excited light for the gain control in order to output the ASE light of an intensity which is identical to that of the signal light when a single signal light is amplified under the state that the input WDM signal light is shutdown. Thereby, the ASE light transmitting signal can be transmitted to the repeater in the down-stream side through the OSC light of the optical transmission path  5 A.  
         [0094]     When the optical amplifier in the down-stream side in the shutdown state which has shifted to the ASE setting mode controls, upon reception of the ASE light transmitting signal transmitted from the terminal  1 A, the intensity of the excited light for the gain control with the ASE light output from the post-amplifier  10 A in order to output the ASE light in an intensity which is identical to that of the signal light when a single signal light is amplified. When the gain control of the optical amplifier in the ASE setting mode is completed and the gain is set, the optical amplifier of the repeater transmits the gain setting end message through the OSC light of the opposed transmission line.  
         [0095]     Here, the pre-amplifier  11 A in the unit  3 A controls with the gain control the intensity of the excited light with the ASE light transmitting signal from the terminal  1 A in order to output the ASE light having an intensity which is identical to that of the light when a single signal light is amplified. However, the gain setting end message is not transmitted until the fault in the fault point  70  is recovered.  
         [0096]     In above operation process, the pre-amplifier  11 A shifts, from the shutdown state, to the state to control for the gain control the intensity of the excited light in order to output the ASE light in an intensity which is identical to that of the signal light when a single signal light is amplified. In any state, the light output from the fault point  70  is within the range of the safe light criterion.  
         [0097]     Moreover, under the state having completed the gain setting, PD  32 A detects recovery of the fault at the fault point  70 . Namely, in the safe light state in which the gain setting of the pre-amplifier  11 A has completed after detection of the fault at the fault point  70 , the gain setting of the optical amplifier and pre-amplifier  11 A of the repeater has been completed.  
         [0098]     When the fault at the fault point  70  is recovered, the ASE light output from the pre-amplifier  11 A which has completed the gain setting is received with the PD 32 A to detect recovery of the fault at the fault point  70 .  
         [0099]     When the gain setting of the pre-amplifier  11 A is completed, the control unit  12 B conducts ASE setting of the optical amplifier  20 A and when the gain setting is completed, the control unit  12 B transmits the gain setting end message using the OSC light in the opposite transmission line.  
         [0100]     When the terminal  1 A receives the gain setting end message of the pre-amplifier  11 A of the terminal  2 B, the optical switch  13 A of the terminal  1 A switches the input WDM signal light, in the shutdown state, to the post-amplifier  10 A to the transmitting state and thereby transmits the light including the WDM light from the terminal  1 A.  
         [0101]     When the optical switch  13 A of the terminal  1 A is switched for transmission of light, since the optical amplifiers up to the pre-amplifier  11 A from the post-amplifier  10 A have completed the gain setting based on the ASE setting, the light including the WDM signal light is transmitted from the terminal  1 A and thereby the regular operating state is performed again.  
         [0102]     As described above, in the optical transmission system of the second embodiment, the regular operating state may be started again by transmitting the gain setting end message to the terminal when the fault is recovered. Accordingly, the time required until the regular operating state from recovery of fault can be shortened.  
         [0103]     Moreover, when only the ASE mode is included in the pre-amplifier, it is no longer required to provide the control mode for transmission of the safe light. Accordingly, the structure for controlling the optical amplifier provided in the pre-amplifier can be simplified.  
         [0104]     The intensity of the ASE light in the ASE setting is set identically to that of the signal light when a single signal light is amplified in the above description. However, the intensity of the ASE light can be detected with an input monitor of the optical amplifier located at the next stage. This is sufficient intensity of the ASE light that it can set the gain even when the light does not exist.