Abstract:
The present invention relates to an operation mode switching apparatus comprising a structure for making it possible to minimize the waste of energy caused by troubles occurring in optical fiber lines and the like in order to efficiently utilize the energy consumed in an optical transmission system aimed for a long-term operation; a Raman amplifier including the operation mode switching apparatus; and an optical transmission system including the Raman amplifier. If an optical fiber line acting as an optical transmission medium breaks in the optical transmission system, the signal power detected from the optical fiber decreases, whereby it changes to a set value or less or the amount of decrease in signal power per unit time changes by a set value or more. The operation mode switching apparatus comprises a control section for monitoring such a change in signal power, thereby switching the pumping light supply operation from a communication mode to a wait mode for minimizing the waste of pumping energy in response to the occurrence of troubles.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a Raman amplifier for Raman-amplifying signal powers of a plurality of channels included in a signal wavelength band, an operation mode switching apparatus employed in the Raman amplifier, and an optical transmission system including the Raman amplifier.  
           [0003]    2. Related Background Art  
           [0004]    As optical amplifiers employed in optical transmission systems for amplifying signal powers, Raman amplifiers utilizing Raman effects have been known (see Japanese Patent Application Laid-Open No. 2000-98433, for example) in addition to rare-earth-doped optical fiber amplifiers whose optical waveguide region is doped with a rare earth element (e.g., Er element). A Raman amplifier is an optical device which supplies Raman amplification pumping light to an optical fiber acting as an optical waveguide, and Raman-amplifies the power of a signal propagating through the optical waveguide. When the signal wavelength falls within the band of 1.55 μm, for example, the Raman amplification pumping light has a wavelength in the vicinity of 1.45 μm.  
           [0005]    A rare-earth-doped optical fiber amplifier is formed into a module and installed in a repeater or the like in an optical transmission system. On the other hand, a Raman amplifier can not only be formed into a module and installed in a repeater or the like, but also supply high-power Raman amplification pumping light to an optical fiber line laid between repeating sections, so as to Raman-amplify the power of a signal propagating through the optical fiber line. Therefore, utilizing the Raman amplifier can lower the effective loss of signal power in the optical fiber line. Namely, the use of a Raman amplifier is favorable in terms of realizing long-distance transmission, since it can elongate the repeating section by effectively reducing the loss of signal power.  
         SUMMARY OF THE INVENTION  
         [0006]    The inventor studied the prior art mentioned above and, as a result, has found the following problems. Namely, in an optical transmission system for Raman-amplifying the signal power in an optical fiber line, if the optical fiber line to which Raman amplification light is supplied breaks, then high-power pumping light will leak out from thus broken part. If the pumping light continuously leaks out of the optical fiber line as such, the pumping energy will be wasted. Also, if an optical waveguide (optical fiber) for carrying out Raman amplification breaks or other optical components fail in a module Raman amplifier, high-power pumping light will leak out, thus making it impossible to maintain a communicating state, thereby wasting the pumping energy.  
           [0007]    For overcoming the problems mentioned above, it is an object of the present invention to provide an operation mode switching apparatus comprising a structure for making it possible to minimize the waste of energy caused by troubles occurring in optical fiber lines and the like in order to efficiently utilize the energy consumed in an optical transmission system aimed for a long-term operation; a Raman amplifier including the operation mode switching apparatus; and an optical transmission system including the Raman amplifier.  
           [0008]    For achieving the above-mentioned object, the operation mode switching apparatus according to the present invention comprises a first detector for monitoring a change in signal power in an optical transmission line, and a control section for switching between a communication mode and a wait mode as an operation for supplying pumping light so as to adjust a power of the pumping light supplied to an optical waveguide. The first detector detects the power P S  of a signal Raman-amplified in the optical waveguide to which the Raman amplification pumping light is supplied. The communication mode is an operation mode in which the power of pumping light supplied to the optical waveguide is maintained at a first level which enables Raman amplification or higher, whereas the wait mode is an operation mode in which the power of pumping light supplied to the optical waveguide is maintained at a second level, lower than the first level, including a stop of pumping light supply or less.  
           [0009]    The Raman amplifier according to the present invention comprises an optical waveguide for Raman-amplifying a signal power when Raman amplification pumping light is supplied thereto, a pumping light source for supplying the Raman amplification pumping light to the optical waveguide, and an operation mode switching apparatus having the structure mentioned above (the operation mode switching apparatus according to the present invention). In this Raman amplifier, the power P S  of a Raman-amplified signal is monitored by the first detector of the operation mode switching apparatus and, according to a result of detection from the first detector, the control section of the operation mode switching apparatus determines whether or not to switch between the operation modes for supplying the pumping light and carries out an adjustment of pumping light power including a stop of pumping light supply.  
           [0010]    When the signal power P S  changes to a set value (signal detection level) or less, the control section in the operation mode switching apparatus adjusts the power level of pumping light to the optical waveguide such that a state of the first level or higher shifts to a state of the second level or less. In this case, the switching between operation modes for supplying the pumping light can be determined easily. Also, the control section adjusts the power level of pumping light to the optical waveguide when the amount of decrease in signal power P S  per unit time changes by a set value or more. In this case, a long-time fluctuation in signal power P S  is kept from being erroneously determined to be a break in the optical waveguide.  
           [0011]    The operation mode switching apparatus according to the present invention may further comprise a second detector for detecting the power P P  of pumping light supplied to the optical waveguide. In this case, the control section determines the switching of operation modes for supplying the pumping light according to both of the signal power P S  detected by the first detector and the pumping light power P P  detected by the second detector. For example, the control section may adjust the power of pumping light supplied to the optical waveguide (switch from the communication mode to the wait mode) when the ratio (P S /P P ) between the signal power P S  and the pumping light power P P  changes to a set value or less. Also, the control section may adjust the power of pumping light supplied to the optical waveguide (switch from the communication mode to the wait mode) when the amount of decrease in ratio (P S /P P ) between the signal power P S  and the pumping light power P P  per unit time changes by a set value or more. In each case, a fluctuation in signal power P S  caused by a long-time fluctuation in pumping light power P P  is kept from being erroneously determined to be a break in the optical waveguide.  
           [0012]    The switching between the communication mode and wait mode includes both of an operation for adjusting the pumping light power performed by the control section and an operation for connecting/blocking a pumping light propagation path itself. Namely, while an operation for adjusting the pumping light power in the control section such as the one mentioned above is carried out by directly controlling the driving current of the pumping light source, a shutter may be disposed between the pumping light source and the optical fiber line, and the control section may control the opening/closing of the shutter so as to block the propagation path of the pumping light emitted from the pumping light source if an operation for restoring the broken optical fiber line and the like can be completed in a short period of time.  
           [0013]    The optical transmission system according to the present invention comprises an optical fiber line for transmitting signals of a plurality of channels included in a signal wavelength band, and a Raman amplifier having the structure mentioned above (the Raman amplifier according to the present invention) for Raman-amplifying signal powers of these channels. This optical transmission system is excellent in transmission quality, since the signal powers are Raman-amplified during the signal propagation. Also, the optical transmission system may be configured such that the Raman amplifier supplies the Raman amplification pumping light to the optical fiber line. In this case, the signal powers are Raman-amplified in the laid optical fiber line, whereby its repeating sections can be elongated. The optical transmission system may further comprise a bypass optical fiber line other than the main optical fiber line, and a line switch for switching the signal transmission path from the main optical fiber line to the bypass optical fiber line when the pumping light supply to the main optical fiber line is stopped or reduced (in the wait mode). In this case, the signal transmission is kept from being interrupted by an operation for restoring the main optical fiber line. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a diagram showing the configuration of a first embodiment of the optical transmission system (including an operation mode switching apparatus and a Raman amplifier) according to the present invention;  
         [0015]    [0015]FIGS. 2A and 2B are graphs for showing respective changes in signal level and driving current of a pumping light source with time for explaining the switching between a communication mode and a wait mode;  
         [0016]    [0016]FIG. 3 is a diagram showing the configuration of a second embodiment of the optical transmission system (including an operation mode switching apparatus and a Raman amplifier) according to the present invention; and  
         [0017]    [0017]FIG. 4 is a diagram showing the configuration of a third embodiment of the optical transmission system (including an operation mode switching apparatus and a Raman amplifier) according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    In the following, embodiments of the operation mode switching apparatus, Raman amplifier, and optical transmission system according to the present invention will be explained in detail with reference to FIGS. 1, 2A,  2 B,  3 , and  4 . In the explanation of the drawings, constituents identical to each other will be referred to with numerals identical to each other without repeating their overlapping descriptions.  
         [0019]    First Embodiment  
         [0020]    [0020]FIG. 1 is a diagram showing the configuration of a first embodiment in the optical communication system according to the present invention. This optical communication system  1  comprises an optical fiber line  120 , a Raman amplifier employing the optical fiber line  120  as a Raman amplification medium, and an operation mode switching apparatus A included in the Raman amplifier. In the optical communication system  1 , the optical fiber line  120 , an optical multiplexer  111 , and an optical branching device  112  are arranged successively from a transmitter  130  to a receiver  140 . Also, the optical transmission system  1  includes a pumping light source  113  (LD) for outputting Raman amplification pumping light to the optical multiplexer  111 , a driving section  114  for driving the pumping light source  113 , a photodetector  115  (PD) for receiving signals having arrived from the optical branching device  112 , and a control section  116  for driving and controlling the driving section  114  according to a signal outputted from the photodetector  115 .  
         [0021]    In the optical transmission system  1  according to the first embodiment, the operation mode switching apparatus A is constituted, at least, by the optical branching device  112 , photodetector  115 , and control section  116 . The Raman amplifier is constituted, at least, by the optical multiplexer  111 , pumping light source  113 , driving section  114 , and optical fiber line  120  together with the operation mode switching apparatus A.  
         [0022]    The pumping light source  113  is a semiconductor laser (LD), for example, and is driven by the driving current supplied from the driving section  114 , so as to output Raman amplification pumping light. The optical multiplexer  111  feeds the optical fiber line  120  with the pumping light outputted from the pumping light source  113 , and the optical branching device  112  with the light (signal) having arrived from the optical fiberline  120 . The optical branching device  112  causes a part of the light having arrived from the optical multiplexer  111  to branch out into the photodetector  115 , while outputting the remainder to the receiver  140 . The photodetector  115  is a photodiode (PD), for example, which receives the light having arrived from the optical branching device  112  and outputs an electric signal at a value corresponding to the power of thus received light. The control section  116  inputs the electric signal outputted from the photodetector  115 , and drives and controls the driving section  114  according to the value of the electric signal so as to make it shift to one of the operation modes of the communication mode and wait mode.  
         [0023]    For example, the following inspection is carried out in the optical transmission system  1  in order to verify the status of installation of the optical fiber line  120  acting as an optical transmission line. Namely, inspection light (inspection signal), outputted from the transmitter  130 , for inspecting the optical fiber line (optical fiber to be inspected)  120  is lead to the optical fiber line  120 . The inspection light propagates through the optical fiber line  120 , which is an object to be inspected, and is Raman-amplified upon the propagation. Substantially all the inspection light outputted from the optical fiber line  120  passes through the optical multiplexer  111  and the optical branching device  112  in succession, thereby reaching the receiver  140 . Thus, the receiver  140  can detect the optical power Raman-amplified in the optical fiber line  120  even in the case where the optical fiber line  120  to be inspected is long or where the optical fiber line  120  yields a large loss, whereby the optical transmission system  1  can inspect the optical fiber line  120  with a sufficient accuracy.  
         [0024]    In the optical transmission system  1 , the optical branching device  112  and photodetector  115  function as a detector for detecting the inspection light power Raman-amplified in the optical fiber line  120 , i.e., the signal power P S . According to the signal power P S  detected by the photodetector  115 , the control section  116  determines the state of pumping light supply to the optical fiber line  120  (operation mode) . Namely, the control section  116  determines whether or not to switch from the communication mode in which the pumping light power is maintained at such a level that it can be Raman-amplified or higher to the wait mode in which the pumping light power is maintained at a low-output state including the pumping light supply stop state. In conformity to the determination made by the control section  116 , the driving section  114  functions as a regulator for the pumping light power.  
         [0025]    When it is detected at a timing t 1 +Δt that the signal power P S  changed to a set value (signal detection level) or less at a timing t 1  as shown in FIG. 2A in particular, the control section  116  controls the driving section  114  (in operation mode switching control) such that the driving current supplied to the pumping light source  113  shifts from a first level at which the driving current can be Raman-amplified (the level of driving current to be supplied to the pumping light source at the time of communication mode) or higher to a second level (a driving current level, including the state where the pumping light supply is stopped, at which an environment enabling an operation for restoring the optical fiber line can be provided) or less. In this case, operation modes for supplying pumping light can easily be determined by comparing the detected signal power and the set value with each other.  
         [0026]    The control section  116  may also carry out operation mode switching control for pumping light supply (see FIG. 2B) when it is detected that the amount of decrease (ΔP) in signal power P S  per unit time (Δt) changed by a set value or more (see FIG. 2A). In this case, a long-time fluctuation in signal power P S  is kept from being erroneously determined to be a break in the optical fiber line  120 .  
         [0027]    When the optical fiber line  120  breaks in such an optical transmission system  1 , the signal power P S  detected by the photodetector  115  decreases, whereby the signal power P S  changes to a set value or less, or the amount of decrease in the signal power P S  per unit time drastically changes by a set value or more. In response to the decrease in signal power P S , the control section  116  determines the operation mode switching of pumping light supply to the optical fiber line  120 . The driving section  114  having received the instruction from the control section  116  reduces the power of pumping light supplied to the optical fiber line  120  to a predetermined level or less, or stops the pumping light supply itself. Since the pumping light supply operation mode is switched from the communication mode to the wait mode as such at the time when the occurrence of a failure is detected in the optical fiber line  120 , the pumping energy can effectively be kept from being wasted.  
         [0028]    Second Embodiment  
         [0029]    [0029]FIG. 3 is a diagram showing the configuration of a second embodiment in the optical transmission system according to the present invention. This optical transmission system  2  also includes a Raman amplifier and an operation mode switching apparatus B. The optical transmission system  2  comprises a transmitter  230 , a repeater  210 , and optical fiber lines  221 ,  222  laid between the transmitter  230  and repeater  210 . The transmitter  230  includes a signal source  231  (LD) and an optical switch section (line switch)  232 . The signal source  231  sends out a signal having a predetermined wavelength. The optical switch section  232  guides the signal sent out from the signal source  231  to one of the optical fiber lines  221  and  222 .  
         [0030]    In the repeater  210 , an optical switch section (line switch)  219 , an optical multiplexer  211 , and an optical branching device  212  are arranged successively from its entrance end to exit end. Also, the repeater  210  includes a pumping light source (LD)  213  for outputting Raman amplification pumping light to the optical multiplexer  211 , a driving section  214  for driving the pumping light source  213 , a photodetector (PD)  215  for receiving light having arrived from the optical branching device  212 , an optical branching device  217  disposed between the pumping light source  213  and the optical multiplexer  211 , a photodetector (PD)  218  for receiving light having arrived from the optical branching device  217 , and a control section  216  for controlling the driving section  214  (in operation mode switching control of pumping light supply) according to respective electric signals outputted from the photodetectors  215  and  218 .  
         [0031]    In the optical transmission system according to the second embodiment, the operation mode switching apparatus B is constituted, at least, by the optical branching device  212 , photodetector  215 , optical branching device  217 , photodetector  218 , and control section  216 . The Raman amplifier is constituted, at least, by the optical multiplexer  211 , pumping light source  213 , optical switch section  219 , and optical fiber line  221  (or optical fiber line  222 ) together with the operation mode switching apparatus.  
         [0032]    The optical switch section  232  in the transmitter  230  and the optical switch section  219  in the repeater  210  operate in synchronization with each other. When the transmitter  230  is set such that the signal sent out from the signal source  231  is made incident on the optical fiber line  221  by the optical switch section  232 , for example, the repeater  210  is set such that the Raman amplification pumping light having arrived from the optical multiplexer  211  is made incident on the optical fiber line  221  by the optical switch  219 .  
         [0033]    The pumping light source  213  is a semiconductor laser (LD), for example, and is driven by the current supplied from the driving section  214 , so as to output Raman amplification pumping light. The optical branching device  217  causes a part of the light outputted from the pumping light source  213  to branch out into the photodetector  218 , while outputting the remainder to the optical multiplexer  211 . The photo detector  218  is a photodiode (PD), for example, which receives the light having arrived from the optical branching device  217  and outputs an electric signal at a value corresponding to the power of thus received light. The optical multiplexer  211  feeds the optical switch section  219  with the pumping light having arrived from the optical branching device  217 , and the optical branching device  212  with the light (signal) having arrived from the optical switch section  219 . The optical branching device  212  causes a part of the light outputted from the optical multiplexer  211  to branch out into the photodetector  215 , while outputting the remainder to a downstream stage. The photodetector  215  is a photodiode (PD), for example, which receives the light having arrived from the optical branching device  212  and outputs an electric signal at a value corresponding to the power of thus received light. The control section  216  inputs the respective electric signals outputted from the photodetectors  215  and  218 , and controls the driving section  214  (in operation mode switching control of pumping light supply).  
         [0034]    In the transmitter  230 , the light (signal) sent out from the signal source  231  is transmitted to the optical fiber line  221  by way of the optical switch section  232 . The light having entered the optical fiber line  221  propagates through the optical fiber line  221  and reaches the repeater  210  after being Raman-amplified upon the propagation. The light having reached the repeater  210  passes through the optical switch section  219  and, with substantially all its power passing through the optical multiplexer  211  and optical branching device  212  in succession, is outputted to a downstream stage. Thus, the optical transmission system  2  is excellent in transmission quality, since the signal Raman-amplified in the optical fiber line  221  reaches the repeater  210  even in the case where the optical fiber line  221  is long or where the optical fiber line  221  yields a large loss.  
         [0035]    In the optical transmission system  2  according to the second embodiment, the optical branching device  212  and photodetector  215  function as a detector (first detector) for detecting the signal power P S  Raman-amplified in the optical fiber line  221 . The optical branching device  217  and photodetector  218  function as a detector (second detector) for detecting the power P P  of pumping light supplied to the optical fiber line  221 . According to both of the signal power P S  detected by the photodetector  215  and pumping light power P P  detected by the photodetector  218 , the control section  216  determines whether or not the operation mode switching is necessary between the communication mode and wait mode as an operation of supplying pumping light to the optical fiber line  221 , and carries out operation mode switching control for the driving section  214  according to the result of determination.  
         [0036]    While monitoring the signal power P S , the control section  216  may carry out the operation mode switching control of pumping light supply as in the first embodiment when the signal power P S  changes to a set value or less or when the amount of decrease in signal power P S  per unit time changes by a set value or more (see FIG. 2A). Alternatively, it may carry out the operation mode switching control for the driving section  214  according to both of the signal power P S  and the pumping light power P P . Namely, in the optical transmission system according to the second embodiment, the control section  216  carries out the operation mode switching control for the driving section  214  from the communication mode to the wait mode so as to suppress the power of pumping light outputted from the pumping light source  213  when the ratio (P S /P P ) between the signal power P S  and pumping light power P P  changes to a set value or less. The control section  216  may also carry out the operation switching control similarly when the amount of decrease in ratio (P S /P P ) per unit time changes by a set value or more. In each case, a fluctuation in signal power P S  caused by a long-time fluctuation in pumping light power P P  is kept from being erroneously determined to be a break in the optical fiber line  220  to be inspected. In the optical transmission system  2  according to the second embodiment, the control section  216  may determine whether or not the operation mode switching between the communication mode and wait mode as an operation for supplying pumping light to the optical fiber line  221  is necessary according to a value of a function in which the signal power P S  and pumping light power P P  are variables.  
         [0037]    When the optical fiber line  221  breaks in such an optical transmission system  2 , the signal power P S  detected by the photodetector  215  decreases, whereby the ratio (P S /P P ) changes to a set value or less, or the amount of decrease in ratio (P S /P P ) per unit time changes by a set value or more. When the control section  216  determines it necessary to adjust the power of pumping light supplied to the optical fiber line  221  (switch from the communication mode to the wait mode) in response to the decrease in ratio (P S /P P ), the driving section  214  having received the instruction from the control section  216  reduces the driving current supplied to the pumping light source  213  to a level attaining the wait mode or less (see FIG. 2B). The time when the driving current from the driving section  214  to the pumping light source  213  becomes 0 mA corresponds to the state where the supply of pumping light is stopped. As in the foregoing, a failure such as a break of the optical fiber line  221  by which the communication state cannot be maintained is detected, and the pumping light supply operation is rapidly switched from the communication mode in which such driving current that the pumping light power can be Raman-amplified is supplied to the pumping light source  213  to the wait mode in which the driving current is lowered (the pumping light power outputted from the pumping light source  213  is lowered or the supply of pumping light is stopped), whereby an optical transmission system which is operable for a long term is realized more efficiently.  
         [0038]    When the optical fiber line  221  breaks in the optical transmission system  2  according to the second embodiment, each of the optical switch sections  219  and  232  maybe changed over, whereby the optical fiber line  222  can be utilized for optical transmission in place of the broken optical fiber line  221 . This can keep the signal transmission from being interrupted by an operation for restoring the optical fiber line  221 .  
         [0039]    Third Embodiment  
         [0040]    [0040]FIG. 4 is a diagram showing the configuration of a third embodiment in the optical transmission system according to the present invention. The optical transmission system  3  also includes an operation mode switching apparatus C and a Raman amplifier. This optical transmission system  3  includes a repeater  310 , an optical fiber line  321  laid upstream the repeater  310 , and an optical fiber line  322  laid downstream the repeater  310 . Signals propagated through the optical fiber line  321  further propagate through the optical fiber line  322  with its power being Raman-amplified in the repeater  310 .  
         [0041]    In the repeater  310 , a Raman amplification optical fiber  318 , an optical multiplexer  311 , and an optical branching device  312  are arranged in succession from its entrance end to exit end. Also, the repeater  310  includes a pumping light source (LD)  313  for outputting the Raman amplification pumping light to the optical multiplexer  311 , a driving section  314  for driving the pumping light source  313 , a photodetector (PD)  315  for receiving the light (signal) having arrived from the optical branching device  312 , a shutter  317  disposed between the pumping light source  313  and the optical multiplexer  311 , and a control section  316  for controlling the opening/closing of the shutter  317  according to a signal outputted from the photodetector  315 .  
         [0042]    In the optical transmission system  3  according to the third embodiment, the operation mode switching apparatus C is constituted, at least, by the optical branching device  312 , photodetector  315 , control section  316 , and shutter  317 . The Raman amplifier is constituted, at least, by the optical multiplexer  311 , pumping light source  313 , driving section  314 , and Raman amplification optical fiber  318  in addition to the operation mode switching apparatus C.  
         [0043]    The pumping light source  313  is a semiconductor laser LD, for example, and is driven by the driving current supplied from the driving section  314 , so as to output the Raman amplification pumping light. The optical multiplexer  311  feeds the Raman amplification optical fiber  318  with the pumping light passing through the shutter  317  after being outputted from the pumping light source  313 , and the optical branching device  312  with the light (signal) having arrived from the Raman amplification optical fiber  318 . The optical branching device  312  causes a part of the light having arrived from the optical multiplexer  311  to branch out into the photodetector  315 , and outputs the remainder to a downstream stage. The photo detector  315  is a photodiode PD, for example, which receives the light having arrived from the optical branching device  312  and outputs an electric signal at a value corresponding to the power of thus received light. The control section  316  inputs the electric signal outputted from the photodetector  315 , and controls the opening/closing of the shutter  317  (corresponding to the switching control between the communication mode for supplying the pumping light and the wait mode for stopping the pumping light supply) according to the value of electric signal.  
         [0044]    The signal having reached the repeater  310  after propagating through the optical fiber line  321  propagates through the Raman amplification optical fiber  318  with its power being Raman-amplified upon the propagation. Substantially all the power-amplified signal successively passes through the optical multiplexer  311  and optical branching device  312 , so as to be outputted to a further downstream stage. The optical transmission system  3  according to the third embodiment is excellent in transmission quality, since the signal having its power Raman-amplified in the Raman amplification optical fiber  318  in the repeater  310  is thus outputted to a further downstream stage even in the case where the optical fiber line  321  is long or where the optical fiber line  321  yields a large loss.  
         [0045]    In the optical transmission system  3 , the optical branching device  312  and the photodetector  315  function as a detector for detecting the power P S  of the signal Raman-amplified in the Raman amplification optical fiber  318 . According to the signal power P S  detected by the photodetector  315 , the control section  316  determines whether or not to stop supplying the pumping light to the Raman amplification optical fiber  318 , and controls the opening/closing of the shutter  317  according to the result of determination. Namely, the control section  316  carries out the switching control from the communication mode to the wait mode according to the result of detection from the photodetector  315 . In the case where an operation for restoring the optical fiber line  321  or the like is accomplished in a short period of time, a configuration which blocks the supply of pumping light itself without controlling the pumping light power as such is also effective.  
         [0046]    When the signal power P S  changes to a set value or less or the amount of decrease in signal power P S  per unit time changes by a set value or more in particular, the control section  316  determines that the supply of pumping light to the Raman amplification optical fiber  318  should be stopped.  
         [0047]    When the optical fiber line  321  or Raman amplification optical fiber  318  breaks in such an optical transmission system  3 , the signal power P S  detected by the photodetector  315  decreases, whereby the signal power P S  changes to a set value or less or the amount of decrease in signal power P S  per unit time changes by a predetermined value or more. When the control section  316  determines that the supply of pumping light to the Raman amplification optical fiber  318  should be stopped in response to the decrease in signal power P S , the shutter  317  having received the instruction from the control section  316  is closed (switching from the communication mode to the wait mode), whereby the supply of pumping light to the Raman amplification optical fiber  318  is stopped.  
         [0048]    In accordance with the present invention, as in the foregoing, the operation mode switching apparatus monitors changes in signal power P S , so that the pumping light supply operation (operation mode) can be switched from the communication mode to the wait mode including the state where the supply is stopped, whereby the waste of pumping energy can effectively be suppressed.  
         [0049]    If the pumping power supply operation is switched from the communication mode to the wait mode when the signal power P S  changes to a set value or less, the operation mode switching can easily be determined. If the pumping power supply operation is switched from the communication mode to the wait mode when the amount of decrease in signal power P S  per unit time changes by a set value or more, a long-time fluctuation in signal power P S  will be kept from being erroneously determined to be a break in the optical waveguide.  
         [0050]    Further, the switching of pumping light supply operations to the optical waveguide may be determined according to both of the power P P  of pumping light supplied to the optical waveguide and the signal power P S . It is preferred that the operation mode switching control be carried out, for example, when the ratio (P S /P P ) between the signal power P S  and pumping light power P P  changes to a set value or less or when the amount of decrease in the ratio (P S /P P ) between the signal power P S  and pumping light power P P  per unit time changes by a set value or more. In each case, a fluctuation in signal power P S  caused by a long-time fluctuation in pumping light power P P  is kept from being erroneously determined to be a break in the optical waveguide.  
         [0051]    The optical transmission system according to the present invention comprises an optical fiber line for transmitting signals and a Raman amplifier for Raman-amplifying powers of these signals, whereas the signals propagate with their powers being Raman-amplified, thereby being excellent in transmission quality. When the Raman amplifier is configured so as to supply Raman amplification pumping light to the optical fiber line (cause the optical fiber line itself, which is an optical transmission medium, to function as a Raman amplification optical fiber), the signal powers are Raman-amplified in the laid optical fiber line, whereby repeating sections can be elongated. When a line switch for switching between the optical fiber line and a bypass optical fiber line different therefrom is provided, the signal transmission is kept from being interrupted by an operation for restoring the broken optical fiber line.