Abstract:
[Problem] To provide an optical transmission/reception device, an optical communication system, an optical communication method, and a program which are capable of securing the confidentiality of information included in an optical signal even when the optical signal is transferred to a device that is not an original transmission destination device. 
     [Solution] This optical transmission/reception device is provided with: a wave separation unit for receiving a wavelength-multiplexed optical signal and separating the same into a plurality of optical signals; a plurality of reception units for receiving each of the plurality of optical signals separated by the wave separation unit; a plurality of output units for outputting optical signals differing in wavelength from each other; a control unit for requesting, in response to the inclusion in the received wavelength-multiplexed optical signal of an optical signal to which a prescribed process has been applied, that a prescribed change be applied to the optical signal outputted by at least one of the plurality of output units; and a wave combining unit for combining the plurality of optical signals outputted from the plurality of output units and outputting the combined signal.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an optical transmission/reception device, an optical communication system, and an optical communication method. 
       BACKGROUND ART 
       [0002]    In recent years, with an increase in traffic, a submarine cable system is demanded to have a wide-band circuit (line) or to have a network having a high functionality. A technique such as an OADM (Optical Add-Drop Multiplexer) or an ROADM (Reconfigurable Optical Add-Drop Multiplexer) is therefore applied to a submarine cable system. 
         [0003]    A submarine ROADM system uses a wavelength division multiplexing (WDM: Wavelength Division Multiplexing) communication. In a submarine ROADM system, for example, a transmit device inputs a client signal as a wavelength multiplexed optical signal in a submarine cable to accommodate a plurality of paths in one optical fiber, thereby improving flexibility of a network. 
         [0004]    In a submarine cable system having an OADM function, the total power of a signal transmitted in a cable composed of optical fibers is set to be constant. When a part of wavelength components of a signal disappears by a breakage of the cable or the like, the signal amplifies other wavelength components, thereby keeping the total power of the signal constant. 
         [0005]    By increasing only the power of a specific wavelength component of a signal up to a predetermined value or larger, however, optical spectrum changes caused by deterioration of a waveform of the signal or the like by a nonlinear effect of an optical fiber, thereby deteriorating the transmission quality of the signal. 
         [0006]    An optical communication system described in PTL 1 discloses a technique in which, when a failure occurs in a cable, the total power of a signal is corrected by a dummy light to secure communication quality. In an optical communication system described in PTL 1, a terminal equipment (optical transmission device) comprises a dummy light generating unit which generates a dummy light corresponding to a location where an optical signal break has occurred when a cable break failure is generated, and keeps the intensity (power) of a signal to be transmitted constant. PTL 2 describes a configuration for enhancing a security function of a signal wavelength. 
       CITATION LIST 
     Patent Literature 
       [0007]    [PTL 1] Japanese Unexamined Patent Application Publication No. 2010-098547 
         [0008]    [PTL 2] Japanese Unexamined Patent Application Publication No. 2011-082751 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0009]    As mentioned above, a terminal equipment (optical transmission device) described in PTL 1 needs to generate a dummy light to compensate an optical signal, and has to comprise a dummy light generating unit. 
         [0010]    Accordingly, in order for a terminal equipment (optical transmission device) to compensate an optical signal without comprising a dummy light generating unit, it is considered to compensate a disappeared optical signal by an optical signal transmitted by another terminal equipment. When the optical signal transmitted by the other terminal equipment used for compensation is received by a device which is not an original communication destination of the optical signal, however, secrecy of information included in the optical signal cannot be secured, which is problematic. 
         [0011]    An object of the present invention is to solve the above-described problem, and to provide an optical transmission/reception device, an optical communication system, an optical communication method, and a program in which, even when an optical signal is forwarded to a device which is not an original transmission destination device, secrecy of information included in the optical signal can be secured. 
       Solution to Problem 
       [0012]    An optical transmission/reception device of the present invention comprises: a demultiplexing unit which receives a wavelength multiplexed optical signal, and demultiplexes the signal into a plurality of optical signals; a plurality of reception units which receive each of the plurality of optical signals demultiplexed by the demultiplexing unit; a plurality of output units which respectively output optical signals having different wavelengths; a control unit which requests to apply a predetermined change to an optical signal output from at least one of the plurality of output units, when the received wavelength multiplexed optical signal includes an optical signal to which a predetermined processing is applied; and a multiplexing unit which multiplexes the plurality of optical signals output from the plurality of output units and outputs the multiplexed signal. 
         [0013]    An optical communication system of the present invention is characterized by comprising: an optical communication device which outputs a wavelength multiplexed optical signal; and an optical transmission/reception device comprising a demultiplexing unit which receives the wavelength multiplexed optical signal, and demultiplexes the signal into a plurality of optical signals, a plurality of reception units which receive each of the plurality of optical signals demultiplexed by the demultiplexing unit, a plurality of output units which respectively output optical signals having different wavelengths, a control unit which requests to apply a predetermined change to an optical signal output from at least one of the plurality of output units when the received wavelength multiplexed optical signal includes an optical signal to which a predetermined processing is applied, and a multiplexing unit which multiplexes the plurality of optical signals output from the plurality of output units and outputs the multiplexed signal. 
         [0014]    An optical communication method of the present invention is characterized by comprising: a multiplexing unit of receiving a wavelength multiplexed optical signal; requesting to apply a predetermined change to an optical signal to be output, when the received wavelength multiplexed optical signal includes an optical signal to which a predetermined processing is applied; and multiplexing a plurality of optical signals having different wavelengths including an optical signal to which the predetermined change has been applied, and outputting the multiplexed signal. 
         [0015]    A program of the present invention is characterized by making a computer execute: a processing to receive a wavelength multiplexed optical signal; a processing which requests to apply a predetermined change to an optical signal to be output, when the received wavelength multiplexed optical signal includes an optical signal to which a predetermined processing is applied; and a processing to multiplex a plurality of optical signals having different wavelengths including an optical signal to which the predetermined change has been applied, and to output the multiplexed signal. 
       Advantageous Effects of Invention 
       [0016]    The present invention exhibits an effect capable of securing secrecy of information included in the optical signal even when an optical signal is forwarded to a device which is not an original transmission destination device. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]      FIG. 1  is a configuration example of an optical communication system in a first exemplary embodiment of the present invention. 
           [0018]      FIG. 2  is a diagram illustrating a configuration example of an optical branch device  3  in the first exemplary embodiment of the present invention. 
           [0019]      FIG. 3  is a flow chart illustrating an operational example of the optical branch device  3  in the first exemplary embodiment of the present invention. 
           [0020]      FIG. 4  is a diagram illustrating a configuration example of the optical transmission/reception device  1 - 3  in a second exemplary embodiment of the present invention. 
           [0021]      FIG. 5  is a flow chart illustrating an operational example of the optical transmission/reception device  1 - 3  in the second exemplary embodiment of the present invention. 
           [0022]      FIG. 6  is a diagram illustrating a configuration example of the optical branch device  3  in a third exemplary embodiment of the present invention. 
           [0023]      FIG. 7  is a diagram illustrating a state of an optical signal which transmits an optical communication system in the third exemplary embodiment of the present invention. 
           [0024]      FIG. 8  is a flow chart illustrating an operational example when the optical branch device  3  in the third exemplary embodiment of the present invention outputs a wavelength multiplexed optical signal to the optical transmission/reception device  1 - 3 . 
           [0025]      FIG. 9  is a flow chart illustrating an operational example when the optical branch device  3  in the third exemplary embodiment of the present invention outputs a wavelength multiplexed optical signal to an optical reception device  1 - 1 . 
           [0026]      FIG. 10  is a diagram illustrating a configuration example of the optical branch device  3  when WSS is used in the third exemplary embodiment of the present invention. 
           [0027]      FIG. 11  is a diagram illustrating a configuration example of the optical transmission/reception device  1 - 3  in a fourth exemplary embodiment of the present invention. 
           [0028]      FIG. 12  is a configuration example of a transponder  14  in the fourth exemplary embodiment of the present invention. 
           [0029]      FIG. 13  is a flow chart illustrating an operational example of the optical transmission/reception device  1 - 3  in the fourth exemplary embodiment of the present invention. 
           [0030]      FIG. 14  is another configuration example of the transponder  14  in the fourth exemplary embodiment of the present invention. 
           [0031]      FIG. 15  is a diagram illustrating another configuration example of the optical transmission/reception device  1 - 3  in the fourth exemplary embodiment of the present invention. 
           [0032]      FIG. 16  is a diagram illustrating a configuration example of an optical transmission device  1 - 2  in a fifth exemplary embodiment of the present invention. 
           [0033]      FIG. 17  is a flow chart illustrating an operational example of the optical transmission device  1 - 2  in the fifth exemplary embodiment of the present invention. 
           [0034]      FIG. 18  is a diagram illustrating a configuration example of the optical transmission device  1 - 2  in a sixth exemplary embodiment of the present invention. 
           [0035]      FIG. 19  is a configuration example of a transponder  124  in the sixth exemplary embodiment of the present invention. 
           [0036]      FIG. 20  is a flow chart illustrating an operational example of the optical transmission device  1 - 2  in the sixth exemplary embodiment of the present invention. 
           [0037]      FIG. 21  is another configuration example of the transponder  124  in the sixth exemplary embodiment of the present invention. 
           [0038]      FIG. 22  is a configuration example of the optical branch device  3  in a seventh exemplary embodiment of the present invention. 
           [0039]      FIG. 23  is a diagram illustrating a configuration example of the transponder  14  included in the optical transmission/reception device  1 - 3  in the seventh exemplary embodiment of the present invention. 
           [0040]      FIG. 24  is a diagram illustrating a configuration example of a variable DGD generator in an eighth exemplary embodiment of the present invention. 
           [0041]      FIG. 25  is a diagram illustrating another configuration example of a variable DGD generator in the eighth exemplary embodiment of the present invention. 
           [0042]      FIG. 26  is a configuration example of the optical branch device  3  in the eighth exemplary embodiment of the present invention. 
           [0043]      FIG. 27  is a configuration example of the optical branch device  3  in a ninth exemplary embodiment of the present invention. 
           [0044]      FIG. 28  is a diagram illustrating a configuration example of the optical branch device  3  in a tenth exemplary embodiment of the present invention. 
           [0045]      FIG. 29  is a flow chart illustrating an operational example of the optical branch device  3  in the tenth exemplary embodiment of the present invention. 
           [0046]      FIG. 30  is a diagram illustrating a configuration example of the optical branch device  3  in an eleventh exemplary embodiment of the present invention. 
           [0047]      FIG. 31  is a flow chart illustrating an operational example of the optical branch device  3  in the eleventh exemplary embodiment of the present invention. 
           [0048]      FIG. 32  is a diagram illustrating a configuration example of a communication system before a failure occurs in a transmission path  2  in a twelfth exemplary embodiment of the present invention. 
           [0049]      FIG. 33  is a table illustrating optical signals transmitted by an interval between an A base station  10 - 1  and the optical branch device  3 , and an interval between the optical branch device  3  and a B base station  10 - 2 , in the communication system of the twelfth exemplary embodiment of the present invention. 
           [0050]      FIG. 34  is a configuration example of a communication system when a failure occurs in a part of the transmission path  2  and an optical signal from a part of the base stations  10  disappears in the twelfth exemplary embodiment of the present invention. 
           [0051]      FIG. 35  is a table illustrating a connection relation of the transponder  14  which transmits/receives an optical signal in an interval between the A base station  10 - 1  and the optical branch device  3 , and an interval between the optical branch device  3  and the B base station  10 - 2  of a communication system in the twelfth exemplary embodiment of the present invention. 
           [0052]      FIG. 36  is a table illustrating a connection relation of the transponder  14  which transmits/receives an optical signal in an interval between the A base station  10 - 1  and the optical branch device  3 , and an interval between the optical branch device  3  and the B base station  10 - 2  of a communication system after the optical branch device  3  has switched a path in the twelfth exemplary embodiment of the present invention. 
           [0053]      FIG. 37  is a configuration example of an optical communication system in a thirteenth exemplary embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Exemplary Embodiment 
       [0054]    A summary of the first exemplary embodiment of the present invention will be described with reference to the drawings. It is noted that a reference sign for each of the drawings in the summary has been added for convenience to each element as one example for helping understanding, and the description of the summary should not be construed as limiting in any way. 
         [0055]      FIG. 1  is a configuration example of an optical communication system in the first exemplary embodiment of the present invention. As illustrated in  FIG. 1 , the optical communication system includes an optical reception device  1 - 1  which receives a wavelength multiplexed optical signal, an optical transmission device  1 - 2  which transmits a wavelength multiplexed optical signal, and an optical transmission/reception device  1 - 3  which transmits/receives a wavelength multiplexed optical signal. The optical communication system illustrated in  FIG. 1  includes a transmission path  2  which transmits a wavelength multiplexed optical signal, and an optical branch device (Branch Unit: BU)  3  which multiplexes and branches a wavelength multiplexed optical signal. 
         [0056]      FIG. 2  is a diagram illustrating a configuration example of the optical branch device  3 . The optical branch device  3  comprises a reception unit  30 , a demultiplexing unit  32 , a processing unit  33 , and a multiplexing unit  34 . 
         [0057]    The reception unit  30  receives a wavelength multiplexed optical signal input from the transmission path  2 . 
         [0058]    The optical reception unit  30  may be a branch unit  31 . In this case, the branch unit  31  branches a wavelength multiplexed optical signal input from the transmission path  2 , inputs one wavelength multiplexed optical signal to the demultiplexing unit  32 , and outputs the other wavelength demultiplexing signal to another external device (for example, the optical reception device  1 - 1 ). The other wavelength multiplexed optical signal is output to another external device (for example, the optical reception device  1 - 1 ) after a predetermined processing is applied in another device (not illustrated) included in the optical branch device  3 . The other wavelength multiplexed optical signal is output as it is to another external device when a predetermined processing does not need to be applied. 
         [0059]    The demultiplexing unit  32  demultiplexes a wavelength multiplexed optical signal into a first demultiplexed light including a first wavelength and a second demultiplexed light including a second wavelength. The demultiplexing unit  32  inputs the first demultiplexed light to the processing unit  33 , and inputs the second demultiplexed light to the multiplexing unit  34 . 
         [0060]    The processing unit  33  applies a predetermined processing to the first demultiplexed light input from the demultiplexing unit  32 , and outputs the light to the multiplexing unit  34 . As the predetermined processing, the processing unit  33  adds a predetermined pattern to an optical signal to be output. The predetermined pattern is, for example, a dummy pattern in which 0 and 1 are randomly arranged, or a fixed pattern in which 0 and 1 are arranged in a specific pattern. As the predetermined processing, the processing unit  33  may scramble an optical signal to be output. As the predetermined processing, the processing unit  33  may deteriorate transmission characteristics of an optical signal to be output. 
         [0061]    The multiplexing unit  34  multiplexes the first demultiplexed light on which a predetermined processing has been performed input from the processing unit  33  and the second demultiplexed light input from the demultiplexing unit  32 , and outputs the multiplexed light to the transmission path  2 . 
         [0062]      FIG. 3  is a flow chart illustrating an operational example of the optical branch device  3  in the first exemplary embodiment of the present invention. 
         [0063]    The reception unit  30  receives a wavelength multiplexed optical signal input from the transmission path  2 , and inputs the wavelength multiplexed optical signal to the demultiplexing unit  32  (S 101 ). 
         [0064]    The demultiplexing unit  32  demultiplexes a wavelength multiplexed optical signal into the first demultiplexed light including the first wavelength and the second demultiplexed light including the second wavelength, inputs the first demultiplexed light to the processing unit  33 , and inputs the second demultiplexed light to the multiplexing unit  34  (S 102 ). 
         [0065]    The processing unit  33  performs a predetermined processing on the first demultiplexed light input from the demultiplexing unit  32 , applies a predetermined change thereto, and outputs the light to the multiplexing unit  34  (S 103 ). 
         [0066]    The multiplexing unit  34  multiplexes the first demultiplexed light on which a predetermined processing has been performed input from the processing unit  33  and the second demultiplexed light input from the demultiplexing unit  32 , and outputs the multiplexed light to the transmission path  2  (S 104 ). 
         [0067]    As mentioned above, for example, when an optical signal including data is forwarded to a device which is not an original transmission destination, the optical branch device  3  of the first exemplary embodiment of the present invention performs a predetermined processing on an optical signal including the data. An optical signal on which a predetermined processing has been performed is, for example, an optical signal including a predetermined pattern, or a scrambled optical signal, and it is difficult to reproduce data (read data) included in an original optical signal. As a result, even when an optical signal including data is forwarded to a device which is not an original transmission destination, secrecy of data included in the optical signal can be secured. 
       Second Exemplary Embodiment 
       [0068]    A second exemplary embodiment of the present invention will be described with reference to the drawings. In the second exemplary embodiment of the present invention, a description of a configuration similar to the first exemplary embodiment of the present invention will be omitted. 
         [0069]    A configuration example of an optical communication system in the second exemplary embodiment of the present invention is similar to the optical communication system illustrated in  FIG. 1 . 
         [0070]    In the second exemplary embodiment of the present invention, a wavelength multiplexed optical signal received by the optical transmission/reception device  1 - 3  may include an optical signal to which a predetermined processing has been applied. The predetermined processing is applied for the purpose that data superimposed on an optical signal cannot be extracted, and is applied by an optical transmission device  1 - 1  or the optical branch device  3 . In other words, the optical transmission device  1 - 1  or the optical branch device  3  applies the predetermined processing to an optical signal which is not destined for the optical transmission/reception device  1 - 3 , thereby securing secrecy of data superimposed on the optical signal. 
         [0071]    For this reason, it is highly probable that, to a reception unit which has received an optical signal to which a predetermined processing is applied in the optical transmission/reception device  1 - 3 , a channel which the optical transmission/reception device  1 - 3  does not use for transmitting/receiving data is assigned. It is also highly probable that, to an output unit (an output unit corresponding to the reception unit) which outputs an optical signal using the same channel as that of the reception unit in the optical transmission/reception device  1 - 3 , a channel which the optical transmission/reception device  1 - 3  does not use for transmitting/receiving data is assigned. 
         [0072]    Here, when an output unit to which a channel which the optical transmission/reception device  1 - 3  does not use for transmitting/receiving data is assigned outputs an optical signal on which data is superimposed, the optical signal on which data is superimposed is received by a device which is not an original destination. In this case, secrecy of data superimposed on the optical signal may not be secured. 
         [0073]    Accordingly, the optical transmission/reception device  1 - 3  of the second exemplary embodiment of the present invention applies a predetermined change to an optical signal output from an output unit corresponding to (whose channel is the same as that of) a reception unit which has received an optical signal to which a predetermined processing has been applied. The optical transmission/reception device  1 - 3  applies a predetermined change to an optical signal from an output unit to which a channel which own device does not use for transmitting/receiving data is assigned such that data cannot be extracted, thereby securing secrecy of data superimposed on the optical signal. 
         [0074]      FIG. 4  is a diagram illustrating a configuration example of the optical transmission/reception device  1 - 3 . The optical transmission/reception device  1 - 3  comprises a plurality of output units  11 - 1  to  11 -N (when there is no particular distinction, referred to as “output unit  11 ”), a multiplexing unit  12 , a control unit  13 , a plurality of reception units  17 - 1  to  17 -N (when there is no particular distinction, referred to as “reception unit  17 ”), and a demultiplexing unit  18 . 
         [0075]    The demultiplexing unit  18  demultiplexes a wavelength multiplexed optical signal received from the transmission path  2 , and outputs the demultiplexed signals to each of the plurality of reception units  17 . To each of the plurality of reception units  17 , a wavelength of an optical signal to be received is assigned. The demultiplexing unit  18  outputs an optical signal having the assigned wavelength to each of the plurality of reception units  17 . 
         [0076]    In the second exemplary embodiment of the present invention, a wavelength multiplexed optical signal received by the demultiplexing unit  18  includes an optical signal to which a predetermined processing is applied. The optical signal to which a predetermined processing is applied is, for example, an optical signal including a predetermined pattern. The predetermined pattern is a dummy pattern in which 0 and 1 are randomly arranged, or a fixed pattern in which 0 and 1 are arranged in a specific pattern, and data cannot be extracted (read) from an optical signal including the predetermined pattern. The optical signal to which a predetermined processing is applied may be, for example, a scrambled optical signal or an optical signal whose transmission characteristics are deteriorated, and data cannot be extracted (read) also from such an optical signal. 
         [0077]    The output unit  11  outputs an optical signal having a predetermined wavelength. The output unit  11  includes, for example, a laser or the like, and can output an optical signal obtained by superimposing data on a light output from the laser. The type of the structure or the like of the laser is not restricted, and may be, for example, a variable wavelength laser or the like in which an output wavelength can be changed. 
         [0078]    Among the plurality of output units  11 , the control unit  13  specifies the reception unit  17  which has received an optical signal to which a predetermined processing is applied, and requests the output unit  11  corresponding to the specified reception unit  17  to output an optical signal to which a predetermined change has been applied. The output unit  11  corresponding to the specified reception unit  17  is the output unit  11  which outputs an optical signal by using the same channel as that of the reception unit  17 . 
         [0079]    The control unit  13  watches (monitors) an optical signal received by each of the plurality of reception units  17 , and specifies the reception unit  17  which has received an optical signal to which a predetermined processing is applied. The control unit  13  may receive a control signal to notify a wavelength of an optical signal to which a predetermined change has been applied, and specify the reception unit  17  which has received an optical signal having the wavelength notified by the control signal as the reception unit  17  which has received an optical signal to which a predetermined processing is applied. The control unit  13  may receive a notification that an optical signal to which a predetermined processing has been applied is received from at least one of the reception units  17 , and specify the notified reception unit  17  as an input/output unit which has received the optical signal to which a predetermined processing has been applied. 
         [0080]    The output unit  11  applies a predetermined change to an optical signal to be output in response to a request from the control unit  13 . As the predetermined change, the output unit  11  adds a predetermined pattern to an optical signal to be output. The predetermined pattern is, for example, a dummy pattern in which 0 and 1 are randomly arranged, or a fixed pattern in which 0 and 1 are arranged in a specific pattern. As the predetermined change, the output unit  11  may scramble an optical signal to be output. As the predetermined change, the output unit  11  may deteriorate transmission characteristics of an optical signal to be output. 
         [0081]    The output unit  11 , when not instructed by the control unit  13 , converts an electric signal input from a device (not illustrated in  FIG. 2 ) in a previous stage into an optical signal, and outputs the optical signal to the multiplexing unit  12 . In other words, the output unit  11  outputs an optical signal including data when not requested from the control unit  13 . 
         [0082]    The multiplexing unit  12  multiplexes a plurality of optical signals input from each of the plurality of output units  11 , and outputs the multiplexed signal. 
         [0083]      FIG. 5  is a flow chart illustrating an operational example of the optical transmission/reception device  1 - 3  in the second exemplary embodiment of the present invention. 
         [0084]    The demultiplexing unit  18  demultiplexes a wavelength multiplexed optical signal received from the transmission path  2 , and outputs the demultiplexed signals to each of the plurality of reception units  17  (S 201 ). 
         [0085]    Each of the plurality of reception units  17  receives an optical signal having a predetermined wavelength (S 202 ). 
         [0086]    Among the plurality of reception units  17 , the control unit  13  specifies the reception unit  17  which has received an optical signal to which a predetermined processing is applied, and requests the output unit  11  corresponding to the specified reception unit  17  to output an optical signal to which a predetermined change has been applied (S 203 ). 
         [0087]    The output unit  11 , when requested from the control unit  13 , outputs an optical signal to be output to which a predetermined change has been applied, and, when not requested from the control unit  13 , outputs an optical signal including data to the multiplexing unit  12  (S 204 ). 
         [0088]    The multiplexing unit  12  multiplexes a plurality of optical signals input from each of the plurality of output units  11 , and outputs the multiplexed signal to the transmission path  2  (S 205 ). 
         [0089]    As mentioned above, the optical transmission/reception device  1 - 3  of the second exemplary embodiment outputs an optical signal to which a predetermined change has been applied from the output unit  11  corresponding to the reception unit  17  which has received an optical signal to which a predetermined processing is applied among the plurality of reception units  17 . 
         [0090]    As mentioned above, in the second exemplary embodiment of the present invention, the optical transmission/reception device  1 - 3  applies a predetermined change to an optical signal output from the output unit  11  corresponding to (whose channel is the same as that of) a reception unit  17  which has received an optical signal to which a predetermined processing has been applied. For this reason, the optical transmission/reception device  1 - 3  can secure secrecy of data superimposed on an optical signal output from the output unit  11  to which a channel which is not used for transmitting/receiving data is assigned. 
       Third Exemplary Embodiment 
       [0091]    A summary of a third exemplary embodiment of the present invention will be described with reference to the drawings. In the third exemplary embodiment of the present invention, a description of a configuration similar to the above-described exemplary embodiments of the present invention will be omitted. 
         [0092]    A configuration example of an optical communication system in the third exemplary embodiment of the present invention is similar to  FIG. 1 . 
         [0093]      FIG. 6  is a diagram illustrating a configuration example of the optical branch device  3 . The optical branch device  3  comprises a branch unit  31 , a demultiplexing unit  32 , a processing unit  33 , a first multiplexing unit  39 , a control unit  35 , a first filter  36 , a second filter  37 , and a second multiplexing unit  38 . The first multiplexing unit  39  corresponds to the multiplexing unit  34  in the first exemplary embodiment of the present invention. 
         [0094]    The branch unit  31  branches a wavelength multiplexed optical signal input from the transmission path  2 , inputs one wavelength multiplexed optical signal to the demultiplexing unit  32 , and inputs the other wavelength multiplexed optical signal to the first filter  36 . 
         [0095]    The control unit  35  notifies the demultiplexing unit  32  of information about an optical signal to be input to the processing unit  33 . The control unit  35  may receive a control signal which notifies of a wavelength of an optical signal to perform a predetermined processing, and notify the demultiplexing unit  32  of a wavelength of a first demultiplexed light which the demultiplexing unit  32  inputs to the processing unit  33 . 
         [0096]    The demultiplexing unit  32  demultiplexes a wavelength multiplexed optical signal into the first demultiplexed light including the first wavelength and the second demultiplexed light including the second wavelength based on the notification from the control unit  35 . The demultiplexing unit  32  inputs a first demultiplexed light to the processing unit  33 , and a second demultiplexed light to the first multiplexing unit  39 . 
         [0097]    The processing unit  33  performs a predetermined processing on the first demultiplexed light input from the demultiplexing unit  32 , and inputs the light to the first multiplexing unit  39 . 
         [0098]    The first multiplexing unit  39  multiplexes the first demultiplexed light on which a predetermined processing has been performed input from the processing unit  33  and the second demultiplexed light input from the demultiplexing unit  32 , and outputs the multiplexed light to the transmission path  2 . 
         [0099]    Among a plurality of optical signals included in a wavelength multiplexed optical signal, the first filter  36  blocks an optical signal which is destined for the optical transmission/reception device  1 - 3 , and passes an optical signal which is destined for the optical reception device  1 - 1 . 
         [0100]    Among wavelength multiplexed optical signals from the optical transmission/reception device  1 - 3 , the second filter  37  blocks an optical signal to which a predetermined change has been applied, and passes the other optical signal. 
         [0101]    The second multiplexing unit  38  outputs a wavelength multiplexed optical signal obtained by multiplexing an optical signal input from the first filter  36  and an optical signal input from the second filter  37  to the transmission path  2 . 
         [0102]      FIG. 7  is a diagram illustrating a state of an optical signal which transmits an optical communication system in the third exemplary embodiment of the present invention.  FIG. 7  illustrates an example in which the processing unit  33  of the optical branch device  3  adds a predetermined pattern to a first demultiplexed light. 
         [0103]    An optical transmission device  1 - 2 , as illustrated in  FIG. 7 , outputs a wavelength multiplexed optical signal including an optical signal Sub-band  1  (block arrow “1” in  FIG. 7 ) and an optical signal Sub-band  2  (block arrow “2” in  FIG. 7 ). In an example of  FIG. 7 , the optical signal Sub-band  1  is an optical signal destined for the optical reception device  1 - 1 , and the optical signal Sub-band  2  is an optical signal destined for the optical transmission/reception device  1 - 3 . 
         [0104]    The branch unit  31  of the optical branch device  3  inputs one wavelength multiplexed optical signal output from the optical transmission device  1 - 2 , and inputs the other to the first filter  36 . 
         [0105]    The demultiplexing unit  32  inputs the optical signal Sub-band  1  included in a wavelength multiplexed optical signal to the processing unit  33 , and inputs the optical signal Sub-band  2  to the first multiplexing unit  39 . 
         [0106]    The processing unit  33  outputs an optical signal Sub-band  1 ′ (block arrow “predetermined pattern” in  FIG. 7 ) obtained by applying a predetermined processing to the input optical signal Sub-band  1 . The processing unit  33  adds, for example, a predetermined pattern as a predetermined processing to the optical signal Sub-band  1 . Since the optical signal Sub-band  1  is an optical signal destined for the optical reception device  1 - 1 , the optical branch device  3  outputs an optical signal (i.e., optical signal Sub-band  1 ′) obtained by applying a predetermined processing to the optical signal Sub-band  1  so as not to be received by the optical transmission/reception device  1 - 3 . 
         [0107]    The first multiplexing unit  39  outputs a wavelength multiplexed optical signal obtained by multiplexing the optical signal Sub-band  2  input from the demultiplexing unit  32  and the optical signal Sub-band  1 ′ input from the processing unit  33  to the optical transmission/reception device  1 - 3 . 
         [0108]    The optical transmission/reception device  1 - 3  outputs a wavelength multiplexed optical signal obtained by multiplexing an optical signal Sub-band  3  (block arrow “3” in  FIG. 7 ) destined for the optical reception device  1 - 1  and an optical signal (block arrow “predetermined pattern” in  FIG. 7 ) obtained by applying a predetermined change to an optical signal whose wavelength corresponds to the optical signal Sub-band  1 ′. The optical transmission/reception device  1 - 3  adds, for example, a predetermined pattern as a predetermined change to the optical signal Sub-band  1 . 
         [0109]    Among wavelength multiplexed optical signals input from the optical transmission/reception device  1 - 3 , the second filter  37  passes only the optical signal Sub-band  3  destined for the optical reception device  1 - 1 , and blocks an optical signal to which a predetermined change has been applied. 
         [0110]    On the other hand, among the input wavelength multiplexed optical signals, the first filter  36  passes only the optical signal Sub-band  1  destined for the optical reception device  1 - 1 , and blocks the optical signal Sub-band  2 . 
         [0111]    The second multiplexing unit  38  outputs a wavelength multiplexed optical signal obtained by multiplexing the optical signal Sub-band  1  input from the first filter and the optical signal Sub-band  3  input from the second filter to the optical reception device  1 - 1 . 
         [0112]      FIG. 8  and  FIG. 9  are flow charts illustrating an operational example of the optical branch device  3  in the third exemplary embodiment of the present invention.  FIG. 8  is a flow chart illustrating an operational example when the optical branch device  3  outputs a wavelength multiplexed optical signal to the optical transmission/reception device  1 - 3 . 
         [0113]    The branch unit  31  branches a wavelength multiplexed optical signal input from the transmission path  2 , inputs one signal to demultiplexing unit  32 , and inputs the other signal to the first filter  36  (S 301 ). 
         [0114]    The control unit  35  notifies a wavelength of an optical signal to be input to the processing unit  33  to the demultiplexing unit  32  (S 302 ). 
         [0115]    The demultiplexing unit  32  demultiplexes a wavelength multiplexed optical signal into the first demultiplexed light including the first wavelength and the second demultiplexed light including the second wavelength based on the notification from the control unit  35 , inputs a first demultiplexed light to the processing unit  33 , and a second demultiplexed light to the first multiplexing unit  39  (S 303 ). 
         [0116]    The processing unit  33  performs a predetermined processing on the first demultiplexed light input from the demultiplexing unit  32 , and outputs the light to the first multiplexing unit  39  (S 304 ). 
         [0117]    The first multiplexing unit  39  multiplexes the first demultiplexed light on which a predetermined processing has been performed input from the processing unit  33  and the second demultiplexed light input from the demultiplexing unit  32 , and outputs the multiplexed light to the transmission path  2  (S 305 ). 
         [0118]      FIG. 9  is a flow chart illustrating an operational example when the optical branch device  3  outputs a wavelength multiplexed optical signal to the optical reception device  1 - 1 . 
         [0119]    The branch unit  31  branches a wavelength multiplexed optical signal input from the transmission path  2 , inputs one wavelength multiplexed optical signal to the demultiplexing unit  32 , and inputs the other wavelength multiplexed optical signal to the first filter  36  (S 401 ). 
         [0120]    Among a plurality of optical signals included in a wavelength multiplexed optical signal, the first filter  36  blocks an optical signal which is destined for the optical transmission/reception device  1 - 3 , and passes an optical signal which is destined for the optical reception device  1 - 1  (S 402 ). 
         [0121]    The second filter  37  passes an optical signal other than an optical signal to which a predetermined change has been applied included in a wavelength multiplexed optical signal from the optical transmission/reception device  1 - 3  (S 403 ). 
         [0122]    The second multiplexing unit  38  outputs a wavelength multiplexed optical signal obtained by multiplexing an optical signal input from the first filter  36  and an optical signal input from the second filter  37  to the transmission path  2  (S 404 ). 
         [0123]    The demultiplexing unit  32  may be a 3-port wavelength filter module. Among a wavelength multiplexed optical signal input from the first port, the 3-port wavelength filter module, including three ports, for example, reflects an optical signal Sub-band  1  and passes an optical signal Sub-band  2  from the second port. The 3-port wavelength filter module outputs the reflected optical signal Sub-band  1  from the third port. Accordingly, by connecting the second port of the 3-port wavelength filter module to the first multiplexing unit  39  and connecting the third port to the processing unit  33 , only the optical signal Sub-band  1  can be input to the processing unit  33 . The 3-port wavelength filter module is a variable filter which can change an optical signal which reflects or passes based on an instruction from the control unit  35 . 
         [0124]    In the optical branch device  3 , the demultiplexing unit  32  may be a WSS (Wavelength Selective Switch). In the optical branch device  3 , in place of the first filter  36 , the second filter  37 , and the second multiplexing unit  38 , a WSS may be employed. 
         [0125]    A WSS is a switch which can independently set any path for each of a plurality of wavelengths included in a wavelength multiplexed optical signal. Here, as the WSS, a WSS described in Japanese Patent No. 5128254 can be used. The WSS includes a multiplexing/demultiplexing function, and can independently demultiplex or multiplex each of a plurality of optical signals included in a wavelength multiplexed optical signal. 
         [0126]      FIG. 10  is a diagram illustrating a configuration example of the optical branch device  3  when a WSS is used. As illustrated in  FIG. 10 , the optical branch device  3  comprises a first WSS  40  in place of the demultiplexing unit  32 . The first WSS  40  inputs at least one optical signal included in a wavelength multiplexed optical signal based on a notification from the control unit  35 , and inputs the other optical signal (an optical signal which is not input to the processing unit  33 ) to the first multiplexing unit  39 . 
         [0127]    As illustrated in  FIG. 10 , the optical branch device  3  comprises a second WSS  41  in place of the first filter  36 , the second filter  37 , and the second multiplexing unit  38 . The second WSS  41  multiplexes an optical signal destined for the optical reception device  1 - 1  among a plurality of optical signals included in a wavelength multiplexed optical signal from the optical transmission device  1 - 2  and an optical signal destined for the optical reception device  1 - 1  among a plurality of optical signals included in a wavelength multiplexed optical signal from the optical transmission/reception device  1 - 3 . The second WSS  41  blocks an optical signal destined for the optical transmission/reception device  1 - 3  among a plurality of optical signals included in a wavelength multiplexed optical signal from the optical transmission device  1 - 2 . The second WSS blocks an optical signal to which a predetermined change has been applied among a plurality of optical signals included in a wavelength multiplexed optical signal from the optical transmission/reception device  1 - 3 . 
         [0128]    As mentioned above, the optical branch device  3  in the third exemplary embodiment of the present invention performs a predetermined processing on the optical signal in such a way that, for example, an optical signal destined for the optical reception device  1 - 1  is not forwarded to the optical transmission/reception device  1 - 3 . As a result, the optical branch device  3  can secure secrecy of data included in the optical signal even when an optical signal destined for the optical reception device  1 - 1  is forwarded to the optical transmission/reception device  1 - 3 . 
       Fourth Exemplary Embodiment 
       [0129]    A summary of a fourth exemplary embodiment of the present invention will be described with reference to the drawings. In the fourth exemplary embodiment of the present invention, a description of a configuration similar to the first exemplary embodiment of the present invention and the second exemplary embodiment of the present invention will be omitted. 
         [0130]    A configuration example of an optical communication system in the fourth exemplary embodiment of the present invention is similar to  FIG. 1 . 
         [0131]      FIG. 11  is a diagram illustrating a configuration example of the optical transmission/reception device  1 - 3 . The optical transmission/reception device  1 - 3  comprises a multiplexing unit  12 , a control unit  13 , a plurality of transponders  14 - 1  to  14 -N (when there is no particular need for distinction, referred to as “transponder  14 ”), a plurality of reception units  17 - 1  to  17 -N, and a demultiplexing unit  18 . In the fourth exemplary embodiment of the present invention, the transponder  14  corresponds to the output unit  11  in the second exemplary embodiment of the present invention. 
         [0132]    Among the plurality of transponders  14 , the control unit  13  specifies the reception unit  17  which has received an optical signal to which a predetermined processing is applied, and requests the transponder  14  corresponding to the specified reception unit  17  to output an optical signal to which a predetermined change has been applied. 
         [0133]    Each of the transponders  14  outputs an optical signal of a predetermined wavelength. 
         [0134]      FIG. 12  is a configuration example of the transponder  14  in the fourth exemplary embodiment of the present invention. The transponder  14  comprises a client module  141 , a Framer LSI  142 , a processing unit  143 , and a line module  144 . LSI is an abbreviation of large scale integration (large scale integrated circuit). 
         [0135]    The client module  141  converts an optical signal received from a client device (not illustrated) into an electric signal, and outputs the electric signal as a signal received from a client to the Framer LSI  142 . 
         [0136]    The Framer LSI  142  accommodates the client signal input from the client module  141  in a frame for a line signal, and outputs the signal to the processing unit  143 . 
         [0137]    The processing unit  143  outputs an electric signal including a predetermined pattern (such as a dummy pattern or a fixed pattern) in place of the frame for a line signal to the line module  144  in response to a request from the control unit  13 . On the other hand, when a control signal which requests to output an optical signal of a predetermined pattern is not received, the frame for a line signal received from the Framer LSI  142  is output to the line module  144 . 
         [0138]    The processing unit  143 , in response to a request from the control unit  13 , may as a scrambler randomly reshuffle a bit string of an electric signal input from the Framer LSI  142 . 
         [0139]    The line module  144  converts the input electric signal (a frame for a line signal or an electric signal of a predetermined pattern) into an optical signal of a predetermined wavelength, and outputs the signal to the multiplexing unit  12 . 
         [0140]      FIG. 13  is a flow chart illustrating an operational example of the optical transmission/reception device  1 - 3  in the fourth exemplary embodiment of the present invention.  FIG. 13  is an example when the optical transmission/reception device  1 - 3  transmits an optical signal. 
         [0141]    The client module  141  converts an optical signal received from a client device (not illustrated) into an electric signal, and outputs the electric signal as a signal received from a client to the Framer LSI  142  (S 501 ). 
         [0142]    The Framer LSI  142  accommodates the client signal input from the client module  141  in a frame for a line signal, and outputs the signal to the processing unit  143  (S 502 ). 
         [0143]    The processing unit  143  determines whether the control unit  13  has requested to output an optical signal to which a predetermined change has been applied (S 503 ). 
         [0144]    The processing unit  143 , in response to a request from the control unit  13  (YES in S 503 ), applies a predetermined change to the received line signal frame, and outputs the frame to the line module  144  ( 5504 ). 
         [0145]    On the other hand, the processing unit  143 , when not requested from the control unit  13  (NO in S 503 ), outputs the received line signal frame as it is to the line module  144  (S 505 ). 
         [0146]    The line module  144  converts the frame for a line signal input from the processing unit  143  into an optical signal, and outputs the optical signal to the multiplexing unit  12  (S 506 ). 
         [0147]    The multiplexing unit  12  outputs a wavelength multiplexed optical signal obtained by multiplexing a plurality of optical signals input from the plurality of transponders  14  to the transmission path  2  (S 507 ). 
         [0148]    The transponder  14  may be a configuration example illustrated in  FIG. 14 . In this case, the processing unit  143 , in response to a request from the control unit  13 , outputs an electric signal including a predetermined pattern (such as a dummy pattern or a fixed pattern) to the line module  144 . The line module  144 , when an electric signal including a predetermined pattern is input from the processing unit  143 , converts, in place of the frame for a line signal, an electric signal including a predetermined pattern into an optical signal of a predetermined wavelength, and outputs the signal to the multiplexing unit  12 . 
         [0149]    The optical transmission/reception device  1 - 3  may be a configuration example illustrated in  FIG. 15 . As illustrated in  FIG. 15 , the optical transmission/reception device  1 - 3  comprises a transponder  14 , a control unit  13 , and an optical multiplexing/demultiplexing unit  19 . 
         [0150]    The optical multiplexing/demultiplexing unit  19  demultiplexes a wavelength multiplexed optical signal received from the transmission path  2 , and outputs the demultiplexed signals to each of the plurality of transponders  14 . To each of the plurality of transponders  14 , a wavelength of an optical signal to be input/output is assigned. The optical multiplexing/demultiplexing unit  19  outputs an optical signal having the assigned wavelength to each of the plurality of transponders  14 . The optical multiplexing/demultiplexing unit  19  multiplexes a plurality of optical signals input from each of the plurality of transponders  14 , and outputs the multiplexed signal. In the fourth exemplary embodiment of the present invention, a wavelength multiplexed optical signal received by the optical multiplexing/demultiplexing unit  19  includes an optical signal to which a predetermined processing has been applied. 
         [0151]    In an example of  FIG. 15 , a case in which the transponder  14  of the optical transmission/reception device  1 - 3  has received an optical signal from the optical multiplexing/demultiplexing unit  19  will be described. 
         [0152]    The line module  144  converts an optical signal received from the multiplexing unit  12  into an electric signal, and outputs the signal as a line-received signal (line signal frame) to the processing unit  143 . The processing unit  143  outputs the line-received signal (frame for a line signal) input from the line module  144  to the Framer LSI  142 . 
         [0153]    The Framer LSI  142  extracts a client signal from the frame for a line signal input from the processing unit  143 , and outputs the extracted signal as an electric signal to the client module  141 . 
         [0154]    The client module  141  converts the electric signal input from the Framer LSI  42  into an optical signal, and transmits the converted signal as a client signal to a client device (not illustrated). 
         [0155]    In the optical transmission/reception device  1 - 3  illustrated in  FIG. 15 , a configuration and an operation when the transponder  14  of the optical transmission/reception device  1 - 3  transmits an optical signal to the optical multiplexing/demultiplexing unit  19  are similar to the case of  FIG. 11  described above. 
         [0156]    As mentioned above, in the fourth exemplary embodiment of the present invention, the optical transmission/reception device  1 - 3  applies a predetermined change to an optical signal output from the transponder  14  corresponding to (whose channel is the same as that of) a reception unit  17  which has received an optical signal to which a predetermined processing has been applied. For this reason, the optical transmission/reception device  1 - 3  can secure secrecy of data superimposed on an optical signal output from the transponder  14  to which a channel which is not used for transmitting/receiving data is assigned. 
       Fifth Exemplary Embodiment 
       [0157]    A summary of a fifth exemplary embodiment of the present invention will be described with reference to the drawings. 
         [0158]    In the fifth exemplary embodiment of the present invention, an optical transmission device  1 - 2  outputs an optical signal to which a predetermined change has been applied based on a predetermined condition (for example, an optical signal output from the transmission unit  121  has been forwarded to a device which is not an original transmission destination). 
         [0159]    In the fifth exemplary embodiment of the present invention, a configuration example of an optical communication system is similar to a configuration example of an optical communication system illustrated in  FIG. 1 . 
         [0160]      FIG. 16  is a diagram illustrating a configuration example of the optical transmission device  1 - 2 . The optical transmission device  1 - 2  comprises a transmission unit  121 , a multiplexing unit  122 , and a control unit  123 . The optical transmission device  1 - 2  comprises a plurality of transmission units  121 . 
         [0161]    The multiplexing unit  122  outputs a wavelength multiplexed signal light obtained by multiplexing optical signals input from each of the plurality of transmission units  121  to the transmission path  2 . 
         [0162]    The control unit  123 , when a predetermined condition is satisfied, requests the transmission unit  121  to apply a predetermined change to an optical signal to be output. 
         [0163]    The predetermined condition is, for example, that a control signal which requests to output an optical signal of a predetermined pattern has been received. The control signal is transmitted to the control unit  123  when a failure occurs in the transmission path  2  and when an optical signal output from the optical transmission device  1 - 2  is forwarded to a device which is not an original transmission destination. 
         [0164]    The transmission unit  121 , when instructed from the control unit  123 , applies a predetermined change to an optical signal to be output, and outputs the signal to the multiplexing unit  122 . The optical signal to which a predetermined change has been applied is, for example, an optical signal including a predetermined pattern. The predetermined pattern is, for example, a dummy pattern in which 0 and 1 are randomly arranged, or a fixed pattern in which 0 and 1 are arranged in a specific pattern. 
         [0165]    The transmission unit  121 , when not instructed by the control unit  123 , converts an electric signal input from a device (not illustrated in  FIG. 16 ) in a previous stage into an optical signal, and outputs the optical signal to the multiplexing unit  122 . In other words, the transmission unit  121  outputs an optical signal including data when not instructed by the control unit  123 . 
         [0166]      FIG. 17  is a flow chart illustrating an operational example of the optical transmission device  1 - 2  in the fifth exemplary embodiment of the present invention. 
         [0167]    The control unit  123 , when a predetermined condition is satisfied, requests the transmission unit  121  to apply a predetermined change to an optical signal to be output (S 601 ). 
         [0168]    The transmission unit  121  determines whether the control unit  123  has instructed (S 602 ). 
         [0169]    The transmission unit  121 , when instructed from the control unit  123  (YES in S 602 ), outputs an optical signal to which a predetermined change has been applied to the multiplexing unit  122  (S 603 ). 
         [0170]    The transmission unit  121 , when not instructed by the control unit  123  (NO in S 602 ), converts the input electric signal into an optical signal, and outputs the signal to the multiplexing unit  122  (S 604 ). 
         [0171]    The multiplexing unit  122  outputs a wavelength multiplexed signal light obtained by multiplexing a plurality of optical signals input from a plurality of transmission units  121  to the transmission path  2  (S 605 ). 
         [0172]    As mentioned above, the optical transmission device  1 - 2  of the fifth exemplary embodiment of the present invention outputs an optical signal to which a predetermined change has been applied based on a predetermined condition (for example, an optical signal output from the transmission unit  121  has been forwarded to a device which is not an original transmission destination). An optical signal to which a predetermined change has been applied is, for example, an optical signal including a dummy pattern in which 0 and 1 are randomly arranged, or a fixed pattern in which 0 and 1 are arranged in a specific pattern, and it is difficult to reproduce an original optical signal. As a result, the optical transmission device  1 - 2  can secure secrecy of data included in the optical signal even when an optical signal including data is forwarded to a device which is not an original transmission destination. 
       Sixth Exemplary Embodiment of the Present Invention 
       [0173]    A summary of a sixth exemplary embodiment of the present invention will be described with reference to the drawings. In the sixth exemplary embodiment of the present invention, a description of a configuration similar to the above-described exemplary embodiments of the present invention will be omitted. 
         [0174]    A configuration example of an optical communication system in the sixth exemplary embodiment of the present invention is similar to  FIG. 1 . 
         [0175]      FIG. 18  is a diagram illustrating a configuration example of the optical transmission device  1 - 2 . The optical transmission device  1 - 2  comprises a multiplexing unit  122 , a control unit  123 , and a plurality of transponders  124 - 1  to  124 -N (when there is no particular need for distinction, referred to as “transponder  124 ”). The transponder  124  in the sixth exemplary embodiment of the present invention corresponds to the transmission unit  121  in the fifth exemplary embodiment of the present invention. 
         [0176]    The control unit  123 , when a predetermined condition is satisfied, instructs the transponder  124  to output an optical signal including a predetermined pattern. 
         [0177]    The predetermined condition is, for example, that a control signal which requests to output an optical signal of a predetermined pattern has been received. The control signal is transmitted to the control unit  123  when a failure occurs in the transmission path  2  and when an optical signal output from the optical transmission device  1 - 2  is forwarded to a device which is not an original transmission destination. 
         [0178]    The transponder  124 , when instructed from the control unit  123 , outputs an optical signal of a predetermined pattern to the multiplexing unit  122 . 
         [0179]    The transponder  124 , when not instructed by control unit  123 , converts an electric signal input from a device in a previous stage (for example, a client device) into an optical signal, and outputs the signal to the multiplexing unit  122 . 
         [0180]      FIG. 19  is a configuration example of the transponder  124  in the sixth exemplary embodiment of the present invention. The transponder  124  comprises a client module  241 , a Framer LSI  242 , a processing unit  243 , and a line module  244 . 
         [0181]    The client module  241  converts an optical signal received from a client device (not illustrated) into an electric signal, and outputs the signal as a signal received from a client to the Framer LSI  242 . 
         [0182]    The Framer LSI  242  accommodates the client signal input from the client module  241  in a frame for a line signal, and outputs the signal to the processing unit  243 . 
         [0183]    The processing unit  243 , when a control signal which requests to output a signal of a predetermined pattern is received, outputs an electric signal including a predetermined pattern (such as a dummy pattern or a fixed pattern) in place of the frame for a line signal to the line module  244 . On the other hand, when a control signal which requests to output an optical signal of a predetermined pattern is not received, the frame for a line signal received from the Framer LSI  242  is output to the line module  244 . 
         [0184]    The processing unit  243 , in place of generation of a dummy pattern and a fixed pattern, may as a scrambler randomly reshuffle a bit string of an electric signal input from the Framer LSI  242 . 
         [0185]    The line module  244  converts the input electric signal (a frame for a line signal or an electric signal of a predetermined pattern) into an optical signal of a predetermined wavelength, and outputs the signal to the multiplexing unit  122 . 
         [0186]      FIG. 20  is a flow chart illustrating an operational example of the optical transmission device  1 - 2  in the sixth exemplary embodiment of the present invention. 
         [0187]    The client module  241  converts an optical signal received from a client device (not illustrated) into an electric signal, and outputs the electric signal as a signal received from a client to the Framer LSI  242  (S 701 ). 
         [0188]    The Framer LSI  242  accommodates the client signal input from the client module  241  in a frame for a line signal, and outputs the signal to the processing unit  243  (S 702 ). 
         [0189]    The control unit  123 , when a predetermined condition is satisfied, instructs the processing unit  243  in the transponder  124  to output an optical signal including a predetermined pattern (S 703 ). 
         [0190]    The processing unit  243  determines whether the control unit  123  has instructed (S 704 ). 
         [0191]    The processing unit  243 , when instructed from the control unit  123  (YES in S 704 ), outputs an optical signal of a predetermined pattern (S 705 ). 
         [0192]    On the other hand, the processing unit  243 , when not instructed by the control unit  123  (NO in S 704 ), converts a frame for a line signal input from the Framer LSI  242  into an optical signal, and outputs the signal to the multiplexing unit  122  (S 706 ). 
         [0193]    The multiplexing unit  122  outputs a wavelength multiplexed signal light obtained by multiplexing a plurality of optical signals input from the plurality of transponders  124  to the transmission path  2  (S 707 ). 
         [0194]    The transponder  124  may be a configuration example illustrated in  FIG. 21 . In this case, the processing unit  243 , when a control signal which requests to output a signal of a predetermined pattern is received, outputs an electric signal including a predetermined pattern (such as a dummy pattern or a fixed pattern) to the line module  244 . The line module  244 , when an electric signal including a predetermined pattern is input from the processing unit  243 , converts, in place of the frame for a line signal, an electric signal including a predetermined pattern into an optical signal of a predetermined wavelength, and outputs the signal to the multiplexing unit  122 . 
         [0195]    As mentioned above, the optical transmission device  1 - 2  of the sixth exemplary embodiment of the present invention, when a control signal which requests to output an optical signal of a predetermined pattern is received, outputs an optical signal of a predetermined pattern (such as a dummy pattern or a fixed pattern). For this reason, the optical transmission device  1 - 2  can transmit a signal of a predetermined pattern in place of an optical signal including data, for example, when an optical signal including data has been forwarded to a device which is not an original transmission destination. As a result, the optical transmission device  1 - 2  can prevent an optical signal including data from being forwarded to a device which is not an original transmission destination, thereby securing secrecy of data included in the optical signal. 
       Seventh Exemplary Embodiment 
       [0196]    A summary of a seventh exemplary embodiment of the present invention will be described with reference to the drawings. In the seventh exemplary embodiment of the present invention, a description of a configuration similar to the above-described exemplary embodiments of the present invention will be omitted. 
         [0197]    The seventh exemplary embodiment of the present invention is an exemplary embodiment when a processing unit is a polarization scrambler. In other words, the seventh exemplary embodiment of the present invention is an exemplary embodiment when the processing unit  33  of the optical branch device  3  is a polarization scrambler  42  of  FIG. 22  described below, and the processing unit  143  of the optical transmission/reception device  1 - 3  is a polarization scrambler  145  of  FIG. 23  described below. The processing unit  243  of the optical transmission device  1 - 1  may also be a polarization scrambler. 
         [0198]    In the seventh exemplary embodiment of the present invention, data is superimposed on an optical signal by polarization modulation. 
         [0199]    Here, the polarization scramblers  42  and  145  polarization-modulate the input optical signal, and output the polarization-modulated optical signal. 
         [0200]    For the polarization scramblers  42  and  145 , a polarization scrambler described, for example, in Japanese Patent No. 3777045 can be used. The polarization scrambler is obtained by forming an optical waveguide on a substrate having an electrooptic effect such as lithium niobate, providing an electrode on the substrate, and optically coupling an optical fiber to an input/output portion of the optical waveguide. In a polarization scrambler having such a configuration, a signal light of a linearly polarized wave enters the optical waveguide with its polarization direction inclined at 45 degrees with respect to the vertical direction of the optical waveguide, whereby the linearly polarized wave is decomposed into a vertical component and a horizontal component. At this time, by applying a modulating signal such as a sine wave to an electrode provided on the optical waveguide, the refractive indices of the vertical component and the horizontal component of the optical waveguide change by an electrooptic effect, thereby changing the velocity of each direction component of propagation in the optical waveguide. By this, a phase difference is generated between the vertical component and the horizontal component of a signal light, which makes the polarization state of an optical signal to be emitted random. 
         [0201]    As mentioned above, in the seventh exemplary embodiment of the present invention, data is superimposed on an optical signal by a polarization modulation. For this reason, when the polarization state of an optical signal is made random by the polarization scramblers  42  or  145 , data cannot be decoded from the optical signal. As a result, since data cannot be restored even when the optical signal is forwarded to a device which is not an original transmission destination, secrecy of the data is secured. 
         [0202]    A configuration example of an optical communication system in the seventh exemplary embodiment of the present invention is similar to  FIG. 1 . 
         [0203]      FIG. 22  is a configuration example of the optical branch device  3  in the seventh exemplary embodiment of the present invention. As illustrated in  FIG. 22 , the optical branch device  3  includes the polarization scrambler  42  in place of the processing unit  33 . The polarization scrambler  42  polarization-modulates an optical signal input from the demultiplexing unit  32 , and inputs the polarization-modulated optical signal to a first multiplexing unit  39 . 
         [0204]    A configuration example of the optical transmission/reception device  1 - 3  in the seventh exemplary embodiment of the present invention is similar to  FIG. 11 . 
         [0205]      FIG. 23  is a diagram illustrating a configuration example of the transponder  14  included in the optical transmission/reception device  1 - 3  in the seventh exemplary embodiment of the present invention. The transponder  14  includes a polarization scrambler  145 . The polarization scrambler  145  polarization-modulates an optical signal input from the line module  144  based on a control signal from the control unit  13 . In this case, the control unit  13 , when a control signal which requests to output an optical signal of a predetermined pattern is received, requests the polarization scrambler  145  to polarization-modulates an optical signal. 
         [0206]    As mentioned above, the optical branch device  3  in the seventh exemplary embodiment of the present invention polarization-modulates a predetermined optical signal included in a wavelength multiplexed optical signal to be transmitted to the optical transmission/reception device  1 - 3 . For example, the optical branch device  3  polarization-modulates an optical signal destined for the optical reception device  1 - 1  by the polarization scrambler  42 , thereby making the optical signal irreproducible even when the optical signal is received by the optical transmission/reception device  1 - 3 . As described above, the optical branch device  3  in the seventh exemplary embodiment of the present invention can make the optical signal irreproducible even when an optical signal including data is forwarded to a device which is not an original transmission destination, thereby securing secrecy of data included in the optical signal. 
         [0207]    The optical transmission/reception device  1 - 3  of the seventh exemplary embodiment outputs an optical signal to which a polarization modulation has been applied from the transponder  14  corresponding to the reception unit  17  which has received an optical signal to which a predetermined processing is applied among a plurality of transponders  14 . As a result, the optical transmission/reception device  1 - 3  can use an optical signal output from at least one of the plurality of transponders as a dummy light, and can secure secrecy of data which has been superimposed on the optical signal. 
         [0208]    Further, when the processing unit  243  is a polarization scrambler, the optical transmission device  1 - 2  can apply a polarization modulation to an optical signal including data and transmit the data, for example, when an optical signal including data is forwarded to a device which is not an original transmission destination. As a result, the optical transmission device  1 - 2  can make the optical signal irreproducible even when an optical signal including data is forwarded to a device which is not an original transmission destination, thereby securing secrecy of data included in the optical signal. 
       Eighth Exemplary Embodiment 
       [0209]    A summary of an eighth exemplary embodiment of the present invention will be described with reference to the drawings. In the eighth exemplary embodiment of the present invention, a description of a configuration similar to the above-described exemplary embodiments will be omitted. 
         [0210]    The eighth exemplary embodiment of the present invention is an exemplary embodiment when the processing unit  33  of the optical branch device  3  is a PMD (Polarization Mode Dispersion) addition device  43  of  FIG. 26  described below. In the eighth exemplary embodiment of the present invention, the PMD addition device  43  generates a primary PMD and a secondary PMD with respect to an optical signal to be input, thereby making an optical signal output from the PMD addition device  43  irreproducible from the input optical signal. 
         [0211]    An optical signal to which a PMD is added splits into polarized waves having different oscillating directions by 90 degrees to propagate. The split two polarized waves propagate at different velocities to cause a difference in time at which the waves reach the optical reception device  1 - 1 . Such a difference is referred to as a DGD (Differential Group Delay), which is a scale for indicating the degree of a PMD. Since, as a DGD of an optical signal increases, adjacent data in time overlap, a digital data of “0” and “1” superimposed on the optical signal cannot be correctly distinguished. In other words, the PMD addition device  43  purposely generates a PMD causing a large DGD with respect to an optical signal, thereby making data superimposed on the optical signal irreproducible. 
         [0212]    Here, for the PMD addition device  43 , a variable DGD generator described in Japanese Patent No. 4142300 can be used.  FIG. 24  is a diagram illustrating a configuration example of a variable DGD generator described in Japanese Patent No. 4142300. The variable DGD generator comprises a collimator  431 , a birefringent medium  432 , a variable faraday rotator  433 , and a reflection mirror  434 . 
         [0213]    An optical signal is converted into a light beam in the collimator  431 , and enters the birefringent medium  432 . The propagation velocities of polarized wave components in the directions of the fast axis and the slow axis of the birefringent medium  432  of the light beam which has entered the birefringent medium  432  differ. In other words, a DGD is added to a light beam which has entered the birefringent medium  432 . 
         [0214]    A light beam to which a DGD has been added enters the variable faraday rotator  433 , rotated at a predetermined rotation angle (θ/2), and is emitted from the variable faraday rotator  433 . The optical signal emitted from the variable faraday rotator  433  is reflected by the reflection mirror  434 , and reenters the variable faraday rotator  433 . A plane of polarization of the light beam which has reentered the variable faraday rotator  433  is further rotated by a rotation angle of θ/2, and the light beam is emitted from the variable faraday rotator  433 . A DGD is added to the light beam whose plane of polarization has been rotated by θ, and the light beam is converted into an optical signal in the collimator  431 . As described above, since an optical path is turned back by the reflection mirror  434 , a DGD is added twice. In other words, the variable DGD generator generates a primary PMD and a secondary PMD with respect to an optical signal. 
         [0215]    In the PMD addition device  43 , the properties of the birefringent medium  432  or the rotation angle θ of the variable faraday rotator  433  is set in such a way that a PMD can be generated to a degree to which the input optical signal cannot be reproduced from an optical signal to be output. 
         [0216]    For the birefringent medium  432 , for example, an electrooptic crystal such as lithium niobate or lanthanum lead titanate zirconate can be used. As illustrated in  FIG. 25 , the variable DGD generator may be configured by arranging the collimator  431 , the birefringent medium  432 , and the variable faraday rotator  433  in series without using the reflection mirror  434 . 
         [0217]      FIG. 26  is a configuration example of the optical branch device  3  in the eighth exemplary embodiment of the present invention. As illustrated in  FIG. 26 , the optical branch device  3  includes the PMD addition device  43  in place of the processing unit  33 . The PMD addition device  43  generates a PMD in an optical signal input from the demultiplexing unit  32 , and inputs the optical signal to the first multiplexing unit  39 . 
         [0218]    The optical branch device  3  in the eighth exemplary embodiment of the present invention passes, for example, an optical signal destined for the optical reception device  1 - 1 , among optical signals included in a wavelength multiplexed optical signal to be transmitted to the optical transmission/reception device  1 - 3 , to the PMD addition device  43 , and generates a PMD with respect to the optical signal. As a result, the optical transmission/reception device  1 - 3  which has received an optical signal to which the PMD has been added cannot extract data superimposed on the optical signal. As a result, the optical branch device  3  in the eighth exemplary embodiment can make the optical signal irreproducible even when an optical signal including data is forwarded to a device which is not an original transmission destination, thereby securing secrecy of data included in the optical signal. 
       Ninth Exemplary Embodiment 
       [0219]    A summary of a ninth exemplary embodiment of the present invention will be described with reference to the drawings. In the ninth exemplary embodiment of the present invention, a description of a configuration similar to the above-described exemplary embodiments of the present invention will be omitted. 
         [0220]    The ninth exemplary embodiment of the present invention is an exemplary embodiment when the processing unit  33  of the optical branch device  3  is a highly nonlinear fibers (HNLF: Highly Nonlinear Fibers)  44  of  FIG. 27  described below. 
         [0221]    Here, for a highly nonlinear fibers  44 , for example, a non-linear dispersion-shifting optical fiber of Japanese Patent No. 4070083 can be used. The non-linear dispersion-shifting optical fiber adds a nonlinear optics phenomenon to an input optical signal, thereby generating a phase modulation, a wavelength conversion, or the like. Examples of a nonlinear optics phenomenon in the non-linear dispersion-shifting optical fiber include four-wave mixing, self-phase modulation, cross phase modulation, and stimulated Brillouin scattering. When such a phenomenon is caused in the non-linear dispersion-shifting optical fiber, a wavelength conversion, phase modulation, scattering or the like occurs in the input optical signal, and a noise component in the optical signal increases, or the input optical signal cannot be sufficiently transmitted. For this reason, as the degree of wavelength conversion, phase modulation, scattering or the like generated in an optical signal increases, a receiving side device cannot reproduce information (data) superimposed on the input optical signal. 
         [0222]    As mentioned above, the ninth exemplary embodiment of the present invention inputs an optical signal to the highly nonlinear fibers  44  to generate wavelength conversion, phase modulation, scattering or the like in the optical signal, thereby making data superimposed on the optical signal irreproducible. 
         [0223]    A configuration example of an optical communication system in the ninth exemplary embodiment of the present invention is similar to  FIG. 1 . 
         [0224]      FIG. 27  is a configuration example of the optical branch device  3  in the ninth exemplary embodiment of the present invention. As illustrated in  FIG. 27 , the optical branch device  3  includes the highly nonlinear fibers  44  in place of the processing unit  33 . The waveform of an optical signal input from the demultiplexing unit  32  deteriorates while the optical signal transmits through the highly nonlinear fibers  44 . The highly nonlinear fibers  44  input an optical signal whose waveform is deteriorated to the first multiplexing unit  39 . In the ninth exemplary embodiment of the present invention, the length and properties of the highly nonlinear fibers  44  are set (adjusted) in such a way that the waveform of the input optical signal can be deteriorated to a degree to be irreproducible from an optical signal to be output. 
         [0225]    The optical branch device  3  in the ninth exemplary embodiment of the present invention passes, for example, an optical signal destined for the optical reception device  1 - 1  through the highly nonlinear fibers  44 , thereby making the optical signal irreproducible in the optical transmission/reception device  1 - 3 . As a result, the optical branch device  3  in the ninth exemplary embodiment of the present invention can make the optical signal irreproducible even when an optical signal including data is forwarded to a device which is not an original transmission destination, thereby securing secrecy of data included in the optical signal. 
       Tenth Exemplary Embodiment 
       [0226]    A summary of a tenth exemplary embodiment of the present invention will be described with reference to the drawings. In the tenth exemplary embodiment of the present invention, a description of a configuration similar to the above-described exemplary embodiments will be omitted. 
         [0227]    In the tenth exemplary embodiment of the present invention, a variable filter is arranged in the optical branch device  3  illustrated in  FIG. 6  in place of the demultiplexing unit  32 , and the control unit  35  specifies a wavelength of an optical signal which passes through the variable filter. The control unit  35  instructs the processing unit  33  to output an optical signal having a wavelength corresponding to the wavelength of an optical signal to be blocked by the variable filter and having a predetermined pattern. The wavelength corresponding to the wavelength of an optical signal is, for example, the same wavelength as that of the optical signal. 
         [0228]      FIG. 28  is a diagram illustrating a configuration example of the optical branch device  3  in the tenth exemplary embodiment of the present invention. The optical branch device  3  comprises a variable filter  45  in place of the demultiplexing unit  32 . 
         [0229]    The variable filter  45  passes only an optical signal having a predetermined wavelength among wavelength multiplexed optical signals input from the branch unit  31  based on an instruction from the control unit  35 . The variable filter  45  inputs an optical signal which has passed therethrough to the first multiplexing unit  39 . 
         [0230]    The control unit  35 , for example, notifies a wavelength of an optical signal to pass among wavelength multiplexed optical signals based on a control signal. On the other hand, the control unit  35  specifies to the processing unit  33  a wavelength of an optical signal to be output. The wavelength of an optical signal to be output from the processing unit  33  is a wavelength corresponding to the wavelength of an optical signal blocked by the variable filter  45 . 
         [0231]    The processing unit  33  outputs an optical signal to which a predetermined processing has been applied to the first multiplexing unit  39 . 
         [0232]    The first multiplexing unit  39  outputs a wavelength multiplexed optical signal obtained by multiplexing an optical signal input from the variable filter  45  and an optical signal to which a predetermined processing has been applied input from the processing unit  33  to the optical transmission/reception device  1 - 3 . 
         [0233]      FIG. 29  is a flow chart illustrating an operational example of the optical branch device  3  in the tenth exemplary embodiment of the present invention.  FIG. 29  is an operational example when the optical branch device  3  outputs a wavelength multiplexed optical signal including an optical signal to which a predetermined processing has been applied. 
         [0234]    The branch unit  31  branches a wavelength multiplexed optical signal input from the transmission path  2 , inputs one wavelength multiplexed optical signal to the variable filter  45 , and inputs the other to the first filter  36  (S 801 ). 
         [0235]    To the variable filter  45 , the control unit  35  specifies a wavelength of an optical signal to pass, and specifies a wavelength of an optical signal to output to the processing unit  33  (S 802 ). 
         [0236]    The variable filter  45  passes only an optical signal having a predetermined wavelength among wavelength multiplexed optical signals input from the branch unit  31  based on an instruction from the control unit  35 , and inputs the signal to the first multiplexing unit  39  (S 803 ). 
         [0237]    The processing unit  33  outputs an optical signal which has a wavelength corresponding to the wavelength specified by the control unit  35  and to which a predetermined processing has been applied (S 804 ). 
         [0238]    The first multiplexing unit  39  outputs a wavelength multiplexed optical signal obtained by multiplexing an optical signal input from the variable filter  45  and an optical signal to which a predetermined processing has been applied input from the processing unit  33  to the transmission path  2  (S 805 ). 
         [0239]    As mentioned above, the optical branch device  3  in the tenth exemplary embodiment of the present invention outputs the optical signal to which a predetermined processing has been applied in such a way that, for example, an original optical signal cannot be restored even when an optical signal destined for the optical reception device  1 - 1  is forwarded to the optical transmission/reception device  1 - 3 . As a result, the optical branch device  3  can make a device which is not an original transmission destination not reproduce an optical signal including data, thereby securing secrecy of data included in the optical signal. 
       Eleventh Exemplary Embodiment 
       [0240]    A summary of an eleventh exemplary embodiment of the present invention will be described with reference to the drawings. In the eleventh exemplary embodiment of the present invention, a description of a configuration similar to the above-described exemplary embodiments of the present invention will be omitted. 
         [0241]      FIG. 30  is a diagram illustrating a configuration example of the optical branch device  3  in the eleventh exemplary embodiment of the present invention. The optical branch device  3  comprises a first filter  36 , a first branch unit  46 , and a third filter  47 . The first branch unit  46  branches a wavelength multiplexed optical signal from the optical transmission device  1 - 2 , inputs one to the third filter  47 , and inputs the other to the first filter  36 . The third filter  47  passes only an optical signal of an optical signal Sub-band  2  destined for the optical transmission/reception device  1 - 3  among wavelength multiplexed optical signals. On the other hand, first filter  36  passes only an optical signal of an optical signal destined for the optical reception device  1 - 1  Sub-band  1  among wavelength multiplexed optical signals. 
         [0242]    The optical branch device  3  comprises a processing unit  33 , a control unit  35 , a second filter  37 , a second multiplexing unit  38 , a first multiplexing unit  39 , and a second branch unit  48 . The second branch unit  48  inputs an optical signal (Sub-band  1 ) input from the first filter  36  to the processing unit  33  and the second multiplexing unit  38 . 
         [0243]    The control unit  35  notifies a wavelength of an optical signal to pass to the first filter  36  and the third filter  47 . The control unit  35  may receive a control signal for requesting replacement to an optical signal of a predetermined pattern, and specify a wavelength of an optical signal which passes through each of the first filter  36  and the third filter  47  based on the received control signal. 
         [0244]    The processing unit  33  outputs an optical signal which has a wavelength corresponding to a wavelength of an optical signal to be output from the first filter  36  and to which a predetermined processing has been applied to the first multiplexing unit  39 . The first multiplexing unit  39  outputs a wavelength multiplexed optical signal obtained by multiplexing an optical signal input from the third filter  47  and an optical signal to which a predetermined processing has been applied input from the processing unit  33  to the transmission path  2 . 
         [0245]    The second filter  37  passes an optical signal other than an optical signal to which a predetermined change has been applied included in a wavelength multiplexed optical signal from the optical transmission/reception device  1 - 3 . The second multiplexing unit  38  outputs a wavelength multiplexed optical signal obtained by multiplexing an optical signal input from the second branch unit  48  and an optical signal input from the second filter  37  to the transmission path  2 . 
         [0246]      FIG. 31  is a flow chart illustrating an operational example of the optical branch device  3  in the eleventh exemplary embodiment of the present invention.  FIG. 31  is an operational example when the optical branch device  3  outputs a wavelength multiplexed optical signal including an optical signal to which a predetermined processing is applied. 
         [0247]    The first branch unit  46  branches a wavelength multiplexed optical signal input from the transmission path  2 , inputs one wavelength multiplexed optical signal to the third filter  47 , and inputs the other to the first filter  36  (S 901 ). 
         [0248]    The control unit  35  specifies a wavelength of an optical signal to pass to the first filter  36  and the third filter  47  (S 902 ). 
         [0249]    The third filter  47  passes only an optical signal having a predetermined wavelength among wavelength multiplexed optical signals input from the first branch unit  46  based on an instruction from the control unit  35 , and inputs the signal to the first multiplexing unit  39  (S 903 ). 
         [0250]    The first filter  36  passes only an optical signal having a predetermined wavelength among wavelength multiplexed optical signals input from the first branch unit  46  based on an instruction from the control unit  35 , and inputs the signal to the second branch unit  48  (S 904 ). 
         [0251]    The second branch unit  48  inputs an optical signal input from the first filter  36  to the processing unit  33  and the second multiplexing unit  38  (S 905 ). 
         [0252]    The processing unit  33  outputs an optical signal obtained by applying a predetermined processing to a light input from the second branch unit  48  (S 906 ). 
         [0253]    The first multiplexing unit  39  outputs a wavelength multiplexed optical signal obtained by multiplexing an optical signal input from the third filter  47  and an optical signal of a predetermined pattern input from the processing unit  33  to the transmission path  2  (S 907 ). 
         [0254]    As mentioned above, the optical branch device  3  in the eleventh exemplary embodiment of the present invention outputs the optical signal to which a predetermined processing has been applied in such a way that, for example, an original optical signal cannot be restored even when an optical signal destined for the optical reception device  1 - 1  is forwarded to the optical transmission/reception device  1 - 3 . As a result, the optical branch device  3  can make a device which is not an original transmission destination of an optical signal including data not reproduce an optical signal including data, thereby securing secrecy of data included in the optical signal. 
       Twelfth Exemplary Embodiment 
       [0255]    A twelfth exemplary embodiment of the present invention will be described with reference to the drawings. In the twelfth exemplary embodiment of the present invention, a description of a configuration similar to the above-described exemplary embodiments will be omitted. 
         [0256]    In the twelfth exemplary embodiment of the present invention, when a failure is generated in a part of the transmission path  2  and an optical signal from a part of base stations disappears, a path is switched in the BU (optical branch device)  3  to compensate the disappeared optical signal by an optical signal from another base station. In this case, the optical transmission device  1 - 2  outputs an optical signal which is to be transmitted to a device which is not an original communication counterpart by converting the signal into an optical signal to which a predetermined change has been applied in advance to prevent the device which is not an original communication counterpart from receiving the optical signal used for the compensation. 
         [0257]    The total power of an optical signal propagating in the transmission path  2  is set to be constant. When a part of wavelength components of the optical signal disappears by a breakage of the transmission path  2  or the like, the total power of the optical signal is kept constant by amplifying another wavelength component of the optical signal. 
         [0258]    By increasing only the power of a specific wavelength component of an optical signal up to a predetermined value or larger, however, optical spectrum changes caused by deterioration of a waveform of the optical signal or the like by a nonlinear effect of an optical fiber, thereby deteriorating the transmission quality of the optical signal. 
         [0259]    Accordingly, in the twelfth exemplary embodiment of the present invention, when an optical signal from a part of base stations is blocked and a part of wavelength components of an optical signal which is carried in the transmission path  2  disappears, the disappeared wavelength component is compensated by an optical signal from another base station. This prevents only the power of a specific wavelength component of an optical signal from increasing, thereby inhibiting deterioration of the transmission quality of the optical signal. 
         [0260]    The optical signal used for compensation is, however, forwarded to a device which is not an original communication counterpart. Accordingly, in the twelfth exemplary embodiment of the present invention, the transponder  14  of the optical transmission device  1 - 2  converts an optical signal to be forwarded to a device which is not an original communication counterpart into an optical signal to which a change is applied in advance, and transmits the converted signal. As a result, an optical signal used for the compensation is not received by a device which is not an original transmission destination. In other words, in the twelfth exemplary embodiment of the present invention, a disappeared part of wavelength components of an optical signal propagating in the transmission path  2  can be compensated, and at the same time, an original optical signal is made irreproducible even when an optical signal used for compensation is received by a device which is not an original communication counterpart. 
         [0261]      FIG. 32  is a diagram illustrating a configuration example of a communication system before a failure occurs in the transmission path  2  in the twelfth exemplary embodiment of the present invention. As illustrated in  FIG. 32 , the communication system includes the A base station  10 - 1 , the B base station  10 - 2 , a C base station  10 - 3 , a D base station  10 - 4 , and the optical branch device  3  to which these base stations are connected. Each base station  10  includes the transponder  14 , a client device  15 , and wavelength multiplexing units  16 - 1  to  16 - 4 . In  FIG. 32 , the client devices  15  are described as devices  15 - 1 -A to  15 - 5 -A, devices  15 - 1 -B to  15 - 5 -B, devices  15 - 1 -C to  15 - 3 -C, devices  15 - 1 -D to  15 - 3 -D. Similarly, the transponders  14  are also described in  FIG. 32  distinguished by reference signs. 
         [0262]      FIG. 33  is a table illustrating optical signals transmitted by an interval between an A base station  10 - 1  and the optical branch device  3  and an interval between the optical branch device  3  and a B base station  10 - 2  in the communication system illustrated in  FIG. 32 . Since a failure has not occurred in the transmission path  2 , in both of an interval between the A base station  10 - 1  and the optical branch device  3  and an interval between the optical branch device  3  and the B base station  10 - 2 , five pairs of the transponders  14  perform communication. 
         [0263]    On the other hand,  FIG. 34  is a configuration example of a communication system when a failure occurs in a part of the transmission path  2  and an optical signal from a part of the base stations  10  disappears in the twelfth exemplary embodiment of the present invention. 
         [0264]    An example of a communication system of  FIG. 34  is, for example, an example in which a failure has occurred in the transmission paths  2 - 2  and  2 - 3  between the C base station  10 - 3  and the D base station  10 - 4  and the optical branch device  3 , and an optical signal from the C base station and the D base station to the optical branch device  3  has disappeared. Specifically, among optical signals transmitted in an interval between the A base station  10 - 1  and the optical branch device  3 , and an interval between the optical branch device  3  and the B base station  10 - 2 , a wavelength component of an optical signal which has been transmitted/received between the A base station  10 - 1  and the C base station  10 - 3 , and between the B base station  10 - 2  and the D base station  10 - 4  disappears. 
         [0265]      FIG. 35  is a table illustrating a connection relation of the transponder  14  which transmits/receives an optical signal in an interval between the A base station  10 - 1  and the optical branch device  3  and an interval between the optical branch device  3  and the B base station  10 - 2  of the communication system illustrated in  FIG. 34 . As described above, when a failure occurs in the transmission path  2  between the C base station and the D base station, and the optical branch device  3 , a connection between the A base station  10 - 1  and the C base station  10 - 3 , and a connection between the B base station  10 - 2  and the D base station  10 - 4  break. As a result, as illustrated in  FIG. 35 , in an interval between the A base station  10 - 1  and the optical branch device  3  and an interval between the optical branch device  3  and the B base station  10 - 2 , only a communication between the A base station  10 - 1  and the B base station  10 - 2  survives. 
         [0266]    When an optical signal is continued to be transmitted in the states of  FIG. 34  and  FIG. 35  (in a state in which a part of wavelength components disappears in a wavelength multiplexed optical signal), however, the transmission quality of an optical signal deteriorates as described above. Accordingly, in the twelfth exemplary embodiment of the present invention, the optical branch device  3  switches a path to compensate the disappeared optical signal by an optical signal from another base station. 
         [0267]    In the twelfth exemplary embodiment of the present invention, the optical branch device  3  switches a path in such a way that an optical signal transmitted from the transponders  14 - 3 -A,  14 - 4 -A, and  14 - 5 -A of the A base station  10 - 1  is received by the transponders  14 - 3 -B,  14 - 4 -B, and  14 - 5 -B of the B base station  10 - 2 . 
         [0268]      FIG. 36  is a table illustrating a connection relation of the transponder  14  which transmits/receives an optical signal in an interval between the A base station  10 - 1  and the optical branch device  3  and an interval between the optical branch device  3  and the B base station  10 - 2  of a communication system after the optical branch device  3  has switched a path. When the optical branch device  3  switches a path, an optical signal transmitted/received between the transponder  14  of the A base station  10 - 1  and the transponder  14  of the C base station  10 - 3  is forwarded to the transponder  14  of the B base station  10 - 2 , and a disappeared optical signal is compensated. An optical signal transmitted/received between the transponder  14  of the B base station  10 - 1  and the transponder  14  of the D base station  10 - 4  is forwarded to the transponder  14  of the A base station  10 - 1 , and a disappeared optical signal is compensated. 
         [0269]    An optical signal used for compensation is, however, forwarded to a device (i.e., the transponder  14  of the B base station  10 - 2  or the transponder  14  of the A base station  10 - 1 ) which is not an original communication counterpart. 
         [0270]    Accordingly, in the twelfth exemplary embodiment of the present invention, an optical signal used for compensation may be an optical signal to which a predetermined processing has been applied by the optical branch device  3 . Examples of the predetermined change include generating a primary PMD and a secondary PMD to an optical signal to be input or polarization modulation of an optical signal to be output. In other words, the demultiplexing unit  32  of the optical branch device  3  inputs an optical signal (for example, an optical signal output from the transponder  14 - 3 -A of the A base station  10 - 1 ) used for compensation to the processing unit  33 , and outputs the signal in place of an optical signal to which a predetermined change has been applied. By outputting an optical signal to which a predetermined change has been applied, the transponder  14 - 3 -B of the B base station  10 - 2  cannot reproduce an original optical signal even when an optical signal including data (i.e., an optical signal output from the transponder  14 - 3 -A of the A base station  10 - 1 ) is received. 
         [0271]    In the twelfth exemplary embodiment of the present invention, the transponder  14  in the optical transmission device  1 - 2  may output an optical signal to which a predetermined change has been applied in place of an optical signal including data. For example, an optical signal transmitted from the transponder  14 - 3 -A of the A base station  10 - 1  is used for compensation, and is received by the transponder  14 - 3 -B of the B base station  10 - 2 . A device of an original communication counterpart of the transponder  14 - 3 -B of the B base station  10 - 2  is a transponder  14 - 1 -D of the D base station  10 - 4 . In this case, the transponder  14 - 3 -A of the A base station  10 - 1  outputs an optical signal to which a predetermined change has been applied in place of an optical signal (optical signal to be forwarded to a device which is not a device of an original communication counterpart) used for compensation. By outputting an optical signal to which a predetermined change has been applied, the transponder  14 - 3 -B of the B base station  10 - 2  cannot reproduce an original optical signal even when the optical signal is received. 
         [0272]    In the twelfth exemplary embodiment of the present invention, as mentioned above, when a failure is generated in the transmission path  2  and an optical signal from a part of base stations disappears, a path in the optical branch device  3  is switched to compensate the disappeared optical signal by an optical signal from anther base station. This prevents only the power of a specific wavelength component of an optical signal from increasing, thereby inhibiting deterioration of the transmission quality of the optical signal. In this case, an optical signal used by the optical branch device  3  for compensation is an optical signal to which a predetermined processing has been applied. As a result, a device which is not a device of an original communication counterpart cannot reproduce an original optical signal even when an optical signal including data is received. As a result, the optical branch device  3  can prevent a device which is not an original transmission destination from receiving an optical signal including data, thereby securing secrecy of data included in the optical signal. 
         [0273]    In the twelfth exemplary embodiment of the present invention, the transponder  14  on the transmitting side converts an optical signal used for compensation, which is an optical signal including data, into an optical signal to which a predetermined change has been applied. As a result, a device which is not a device of an original communication counterpart cannot reproduce an original optical signal even when the optical signal is received. As a result, the optical transmission device  1 - 2  can prevent a device which is not an original transmission destination from receiving an optical signal including data, thereby securing secrecy of data included in the optical signal. 
       Thirteenth Exemplary Embodiment 
       [0274]    A thirteenth exemplary embodiment of the present invention will be described with reference to the drawings. In the thirteenth exemplary embodiment of the present invention, a description of a configuration similar to the above-described exemplary embodiment will be omitted. 
         [0275]      FIG. 37  is a configuration example of an optical communication system in the thirteenth exemplary embodiment of the present invention. As illustrated in  FIG. 37 , the optical communication system includes the optical reception device  1 - 1 , the optical transmission device  1 - 2 , the optical transmission/reception device  1 - 3 , the transmission path  2 , the optical branch device  3 , and an EMS (Element Management System)  4 . The configurations of the optical reception device  1 - 1 , the optical transmission device  1 - 2 , the optical transmission/reception device  1 - 3 , the transmission path  2  and the optical branch device  3  are similar to those of the optical reception device  1 - 1 , the optical transmission device  1 - 2 , the optical transmission/reception device  1 - 3 , the transmission path  2 , and the optical branch device  3  of the above-described exemplary embodiments. 
         [0276]    The EMS  4  is a device which performs network management of an optical communication system, and collects information about a communication path of an optical signal from a device included in the optical communication system. The EMS  4  detects a failure which has occurred in the transmission path  2  based on the information about a communication path, and requests the optical branch device  3  to switch a path. The EMS  4  requests to switch a path in such a way that an optical signal transmitted from the transponders  14 - 3 -A,  14 - 4 -A and  14 - 5 -A of the A base station  10 - 1  is received by the transponders  14 - 3 -B,  14 - 4 -B, and  14 - 5 -B of the B base station  10 - 2  when a failure occurs in the transmission path  2  between the C base station and the D base station and the optical branch device  3 , as illustrated in  FIG. 34 . 
         [0277]    When a path of the optical branch device  3  is switched, in order to make an optical signal to be used for compensation an optical signal to which a predetermined change has been applied, the EMS  4  instructs the demultiplexing unit  32  in the optical branch device  3  to input the optical signal to be used for compensation to the processing unit  33 . When a failure occurs in the transmission path  2  between the C base station and the D base station, and the optical branch device, as illustrated in  FIG. 34 , the EMS  4  requests demultiplexing unit  32  to convert an optical signal output from the transponders  14 - 3 -A,  14 - 4 -A, and  14 - 5 -A of the A base station  10 - 1  into an optical signal to which a predetermined change has been applied. This prevents the B base station from receiving an optical signal output from the transponders  14 - 3 -A,  14 - 4 -A and  14 - 5 -A of the A base station  10 - 1 . 
         [0278]    The demultiplexing unit  32  requested from the EMS  4  inputs an optical signal to be used for compensation to the processing unit  33  based on the request. 
         [0279]    When a predetermined processing is applied to an optical signal having a predetermined wavelength in the optical branch device  3 , the EMS may notify the predetermined wavelength to the optical transmission/reception device  1 - 3 . When the notification is received, the optical transmission/reception device  1 - 3  can apply a predetermined change to an optical signal output from the output unit  11  (transponder  14 ) which receives an optical signal having the notified predetermined wavelength. 
         [0280]    The EMS  4 , when a failure occurred in the transmission path  2  is detected, requests the processing unit  33  of the transponder  14  included in the optical transmission device  1 - 2  to add a predetermined processing to an optical signal. The EMS  4 , when a failure occurs in the transmission path  2  between the C base station and the D base station and the optical branch device as illustrated in  FIG. 34 , requests the processing unit  33  of the transponders  14 - 3 -A,  14 - 4 -A and  14 - 5 -A of the A base station  10 - 1  to add a predetermined processing to an optical signal. 
         [0281]    In response to a request from the EMS  4 , the processing unit  33  requested from the EMS  4  outputs, in place of the input electric signal, an electric signal including a dummy pattern in which 0 and 1 are randomly arranged, or a fixed pattern in which 0 and 1 are arranged in a specific pattern, or an electric signal in which a bit string is randomly reshuffled. 
         [0282]    As mentioned above, in the thirteenth exemplary embodiment of the present invention, the EMS  4  instructs the demultiplexing unit  32  of the optical branch device  3  to input an optical signal used for compensation to the processing unit  33 . As a result, the optical branch device  3  can prevent an optical signal including data from being forwarded to a device which is not an original transmission destination, thereby securing secrecy of data included in the optical signal. 
         [0283]    The EMS  4  performs network management, and requests the optical branch device  3  to switch a path or requests the optical transmission device  1 - 2  to add a predetermined processing to an optical signal and transmit the signal. As a result, the optical transmission device  1 - 2  can prevent an optical signal including data from being forwarded to a device which is not an original transmission destination while keeping the power of an optical signal transmitted in the transmission path  2  constant, thereby securing secrecy of data included in the optical signal. 
       Fourteenth Exemplary Embodiment 
       [0284]    A fourteenth exemplary embodiment of the present invention will be described. In the fourteenth exemplary embodiment, a computer, a CPU (Central Processing Unit), an MPU (Micro-Processing Unit), or the like of the optical transmission device  1 - 2 , the optical branch device  3 , or the optical transmission/reception device  1 - 3  executes a software (program) which realizes a function of each of the above-described exemplary embodiments. In the fourteenth exemplary embodiment of the present invention, a device which executes the software (program) is not limited to the optical transmission device  1 - 2 , the optical branch device  3 , or the optical transmission/reception device  1 - 3 , and any device can be employed. 
         [0285]    In the fourteenth exemplary embodiment of the present invention, the optical transmission device  1 - 2 , the optical branch device  3 , or the optical transmission/reception device  1 - 3  acquires a software (program) which realizes a function of each of the above-described exemplary embodiments via various storage media such as a CD-R (Compact Disc Recordable) or via a network. A program which the optical transmission device  1 - 2 , the optical branch device  3 , or the optical transmission/reception device  1 - 3  acquires, or a storage medium storing the program constitutes the present invention. The software (program) may be stored, for example, in a predetermined storage unit included in the optical transmission device  1 - 2 , the optical branch device  3  or the optical transmission/reception device  1 - 3 . 
         [0286]    A computer, a CPU, an MPU, or the like of the optical transmission device  1 - 2 , the optical branch device  3 , or the optical transmission/reception device  1 - 3  reads a program code of the acquired software (program), and executes the program code. As a result, the same processing as the processing of the optical transmission device  1 - 2 , the optical branch device  3  or the optical transmission/reception device  1 - 3  in each of the above-described exemplary embodiments is executed. 
         [0287]    The fourteenth exemplary embodiment of the present invention can be applied to uses such as a program realized in a computer, a CPU, an MPU, or the like of the optical transmission device  1 - 2 , the optical branch device  3 , or the optical transmission/reception device  1 - 3 . 
         [0288]    Although exemplary embodiments of the present invention have been described, the present invention is not limited to the above-described exemplary embodiments. The present invention can be carried out based on variations, substitutions, or adjustment of each exemplary embodiment. The present invention can also be carried out in any combination of the exemplary embodiments. In other words, the present invention involves various variations or modifications which can be realized in accordance with all the content of the disclosure and technical ideas herein. 
         [0289]    This application claims priority based on Japanese Patent Application No. 2014-066137 filed on Mar. 27, 2014, the entire disclosure of which is herein incorporated. 
         [0290]    A part or the whole of the above-described exemplary embodiments may also be described as the following supplementary notes, but the present invention is not limited thereto. 
         [0291]    [Supplementary Note 1] 
         [0292]    An optical transmission/reception device comprising: 
         [0293]    a demultiplexing unit which receives a wavelength multiplexed optical signal, and demultiplexes the signal into a plurality of optical signals; 
         [0294]    a plurality of reception units which receive each of the plurality of optical signals demultiplexed by the demultiplexing unit; 
         [0295]    a plurality of output units which respectively output optical signals having different wavelengths; 
         [0296]    a control unit which requests to apply a predetermined change to an optical signal output from at least one of the plurality of output units when the received wavelength multiplexed optical signal includes an optical signal to which a predetermined processing is applied; and 
         [0297]    a multiplexing unit which multiplexes the plurality of optical signals output from the plurality of output units and outputs the multiplexed signal. 
         [0298]    [Supplementary Note 2] 
         [0299]    The optical transmission/reception device according to supplementary note 1, wherein the control unit requests to apply the predetermined change to an optical signal output from an output unit corresponding to a reception unit which has received the optical signal to which a predetermined processing is applied. 
         [0300]    [Supplementary Note 3] 
         [0301]    The optical transmission/reception device according to supplementary note 1 or 2, wherein each of the plurality of output units, when requested from the control unit, adds a predetermined pattern to the optical signal to be output as the predetermined change. 
         [0302]    [Supplementary Note 4] 
         [0303]    The optical transmission/reception device according to any one of supplementary notes 1 to 3, wherein each of the plurality of output units, when requested from the control unit, scrambles the optical signal to be output as the predetermined change. 
         [0304]    [Supplementary Note 5] 
         [0305]    The optical transmission/reception device according to any one of supplementary notes 1 to 4, wherein each of the plurality of output units, when requested from the control unit, deteriorates transmission characteristics of the optical signal to be output as the predetermined change. 
         [0306]    [Supplementary Note 6] 
         [0307]    The optical transmission/reception device according to any one of supplementary notes 1 to 5, wherein each of the plurality of output units comprises a processing unit which applies a predetermined change to the optical signal to be output when requested from the control unit. 
         [0308]    [Supplementary Note 7] 
         [0309]    An optical communication system comprising: 
         [0310]    an optical communication device which outputs a wavelength multiplexed optical signal; and 
         [0311]    an optical transmission/reception device comprising:
       a demultiplexing unit which receives the wavelength multiplexed optical signal, and demultiplexes the signal into a plurality of optical signals;   a plurality of reception units which receive each of the plurality of optical signals demultiplexed by the demultiplexing unit;   a plurality of output units which respectively output optical signals having different wavelengths;   a control unit which requests to apply a predetermined change to an optical signal output from at least one of the plurality of output units when the received wavelength multiplexed optical signal includes an optical signal to which a predetermined processing is applied; and   a multiplexing unit which multiplexes the plurality of optical signals output from the plurality of output units and outputs the multiplexed signal.       
 
         [0317]    [Supplementary Note 8] 
         [0318]    The optical communication system according to supplementary note 7, wherein the optical communication device adds the predetermined processing to an optical signal other than an optical signal destined for the optical transmission/reception device among optical signals included in the wavelength multiplexed optical signal. 
         [0319]    [Supplementary Note 9] 
         [0320]    The optical communication system according to supplementary note 7 or 8, further comprising 
         [0321]    a control device which notifies a wavelength of an optical signal to which the optical communication device has applied the predetermined processing to the optical transmission/reception device, 
         [0322]    wherein the control unit requests an output unit which outputs an optical signal having a wavelength notified by the control device to apply a predetermined change to the optical signal to be output. 
         [0323]    [Supplementary Note 10] 
         [0324]    An optical communication method comprising: 
         [0325]    receiving a wavelength multiplexed optical signal, 
         [0326]    requesting to apply a predetermined change to an optical signal to be output when the received wavelength multiplexed optical signal includes an optical signal to which a predetermined processing is applied, 
         [0327]    multiplexing a plurality of optical signals having different wavelengths including an optical signal to which the predetermined change has been applied, and outputting the multiplexed signal. 
         [0328]    [Supplementary Note 11] 
         [0329]    The optical communication method according to supplementary note 10, the method comprising adding a predetermined pattern to the optical signal to be output in response to the request. 
         [0330]    [Supplementary Note 12] 
         [0331]    The optical communication method according to supplementary note 10 or 11, the method comprising scrambling the optical signal to be output in response to the request. 
         [0332]    [Supplementary Note 13] 
         [0333]    The optical communication method according to any one of supplementary notes 10 to 12, the method comprising deteriorating transmission characteristics of the optical signal to be output in response to the request. 
         [0334]    [Supplementary Note 14] 
         [0335]    A program which allows a computer to execute: 
         [0336]    a processing to receive a wavelength multiplexed optical signal; 
         [0337]    a processing to request to apply a predetermined change to an optical signal to be output when the received wavelength multiplexed optical signal includes an optical signal to which a predetermined processing is applied; and 
         [0338]    a processing to multiplex a plurality of optical signals having different wavelengths including the optical signal to which a predetermined change has been applied, and to output. 
         [0339]    [Supplementary Note 15] 
         [0340]    The program according to supplementary note 14, comprising a processing to add a predetermined pattern to the optical signal to be output in response to the request. 
         [0341]    [Supplementary Note 16] 
         [0342]    The program according to supplementary note 14 or 15, comprising a processing to scramble the optical signal to be output in response to the request. 
         [0343]    [Supplementary Note 17] 
         [0344]    The program according to any one of supplementary notes 14 to 16, comprising a processing to deteriorate transmission characteristics of the optical signal to be output in response to the request. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1 - 1  optical reception device 
               1 - 2  optical transmission device 
               1 - 3  optical transmission/reception device 
               2 ,  2 - 1 ,  2 - 2 ,  2 - 3  transmission path 
               3  optical branch device 
               4  EMS 
               11 ,  11 - 1 ,  11 -N output unit 
               12  multiplexing unit 
               13  control unit 
               14 ,  14 - 1 ,  14 -N transponder 
               15 ,  15 - 1 ,  15 -N client device 
               16 ,  16 - 1 ,  16 -N wavelength multiplexing unit 
               17  reception unit 
               18  demultiplexing unit 
               19  optical multiplexing/demultiplexing unit 
               30  reception unit 
               31  branch unit 
               32  demultiplexing unit 
               33  processing unit 
               34  multiplexing unit 
               35  control unit 
               36  first filter 
               37  second filter 
               38  second multiplexing unit 
               39  first multiplexing unit 
               40  first WSS 
               41  second WSS 
               42  polarization scrambler 
               43  PMD addition device 
               44  highly nonlinear fibers 
               45  variable filter 
               46  first branch unit 
               47  third filter 
               48  second branch unit 
               121  transmission unit 
               122  multiplexing unit 
               123  control unit 
               124 ,  124 - 1 ,  124 -N transponder 
               141  client module 
               142  Framer LSI 
               143  processing unit 
               144  line module 
               145  polarization scrambler 
               241  client module 
               242  Framer LSI 
               243  processing unit 
               244  line module 
               431 ,  431 - 1 ,  431 - 2  collimator 
               432 ,  432 - 1 ,  432 - 2 ,  432 - 3  birefringent medium 
               433 ,  433 - 1 ,  433 - 2  variable faraday rotator 
               434  reflection mirror