Abstract:
An optical transmission system including a transmitting unit to transmit an optical main signal, a receiving unit to receive the optical main signal, and a transmission line through which the optical main signal is transmitted, the optical transmission system includes: an optical transmitter unit configured to be activated or inactivated based on a control signal so as to transmit the control signal, the optical transmitter being included in the transmitting; and an optical receiver configured to receive light with the activate state or the inactivate state of the optical transmitter the optical receiver being included in the receiving unit, wherein the receiving unit regenerates the control signal, based on a power of the received light.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2012-186838 filed on Aug. 27, 2012, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiment discussed herein is related to an optical transmission system, a controls signal transmission method, and an optical module. 
       BACKGROUND 
       [0003]    In a transmission system to which an optical signal is applied, it is preferable that information can be transmitted and received in addition to a main signal between a transmitter and a receiver in a simple configuration. Japanese Laid-open Patent Publication No. 2002-271440 discloses a technique in which a transmitter transmits a low frequency ON/OFF signal, in addition to a main signal, to a receiver. Japanese Examined Utility Model Application Publication No. 8-4760 discloses a technique in which transmission/non-transmission to an optical fiber is performed by specifying drive/non-drive of a modulation unit according to a signal. 
       SUMMARY 
       [0004]    According to an aspect of the invention, an optical transmission system including a transmitting unit to transmit an optical main signal, a receiving unit to receive the optical main signal, and a transmission line through which the optical main signal is transmitted, the optical transmission system includes: an optical transmitter configured to be activated or inactivated based on a control signal so as to transmit the control signal, the optical transmitter being included in the transmitting unit; and an optical receiver configured to receive light with the activate state or the inactivate state of the optical transmitter, the optical receiver being included in the receiving unit, wherein the receiving unit regenerates the control signal, based on a power of the received light. 
         [0005]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0006]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0007]      FIG. 1  is a block diagram for explaining an entire configuration of an optical transmission system; 
           [0008]      FIGS. 2A and 2B  are diagrams for explaining an example of a LOS signal corresponding to a predetermined channel; 
           [0009]      FIG. 3  is a diagram for explaining details of transmission of control information; 
           [0010]      FIGS. 4A and 4B  are diagrams for explaining an example in which a control signal is transmitted by using channels; 
           [0011]      FIGS. 5A to 5C  are diagrams illustrating an example of change of the LOS signal; 
           [0012]      FIG. 6  is an example of a flowchart executed when the LOS signal is determined according to the examples of  FIGS. 5A to 5C ; 
           [0013]      FIGS. 7A and 7B  are diagrams illustrating an example of the LOS signal; 
           [0014]      FIG. 8  is an example of a flowchart executed when the LOS signal is determined according to the examples of  FIGS. 7A and 7B ; 
           [0015]      FIG. 9  is a diagram illustrating a time sequence for explaining details of transmission and reception of the control information; 
           [0016]      FIG. 10  is a diagram illustrating a time sequence when a certain transmission line fails; 
           [0017]      FIG. 11  is a diagram illustrating a time sequence when the number of operation channels is changed by an instruction from an external system; and 
           [0018]      FIG. 12  is a diagram illustrating a time sequence when a port state is changed to a stop state. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0019]    Embodiments are to provide an optical transmission system, a control signal transmission method, and an optical module which transmit and receive information different from the main signal between a transmitter and a receiver in a simple configuration. Hereinafter, embodiments will be described with reference to the drawings. 
         [0020]      FIG. 1  is a block diagram for explaining an entire configuration of an optical transmission system  200 . As illustrated in  FIG. 1 , the optical transmission system  200  has a configuration in which an optical module  100   a  and an optical module  100   b  are connected by a plurality of transmission lines such as optical fibers. Since the optical module  100   a  and the optical module  100   b  have the same configuration, the optical module  100   a  will be described. 
         [0021]    The optical module  100   a  includes a transmitting unit  10 , a receiving unit  20 , a control unit  30 , a determination unit  40 , and the like. The transmitting unit  10  includes an optical transmitter  11  in which light emitting elements are arranged, the number of which corresponds to the number of transmission lines (the number of channels) from the optical module  100   a  to the optical module  100   b . The transmitting unit  10  also includes a drive circuit  12  to drive the optical transmitter  11 . The optical transmitter  11  is, for example, an array laser in which a plurality of semiconductor lasers are arranged in parallel. The receiving unit  20  includes an optical receiver  21  in which light receiving elements are arranged, the number of which corresponds to the number of transmission lines (the number of channels) from the optical module  100   b  to the optical module  100   a . The receiving unit  20  also includes a processing circuit  22  that processes an electrical signal outputted from the optical receiver  21 . The optical receiver  21  is, for example, an array photodiode in which a plurality of photodiodes are arranged in parallel. The processing circuit  22  includes, for example, a Trans Impedance Amplifier (TIA), a limiting amplifier, and the like. The determination unit  40  includes a timer  41  and a decoder  42 . The control unit  30  and the determination unit  40  can be realized by a central processing unit (CPU) executing a program loaded in a random access memory (RAM) or the like. Or, the control unit  30  and the determination unit  40  may be configured by using a dedicated circuit or the like. 
         [0022]    Next, a transmission of a main signal between the optical module  100   a  and the optical module  100   b  will be described. In the optical module  100   a , digital electrical signals  51  corresponding to each channel are inputted into the drive circuit  12 . Wiring through which the digital electrical signals  51  are transmitted may be either serial or parallel. The drive circuit  12  drives the optical transmitter  11  according to the digital electrical signals  51 . For example, when the digital electrical signal  51  of a channel n is inputted into the drive circuit  12 , the drive circuit  12  drives the light emitting element of the channel n. Each light emitting element of the optical transmitter  11  outputs an optical main signal  52  to a transmission line corresponding to the channel of the light emitting element. Thereby, the optical main signal  52  is transmitted through each transmission line. 
         [0023]    In the optical module  100   b , each light receiving element of the optical receiver  21  outputs an electric current according to the optical main signal  52  transmitted through each transmission line. The processing circuit  22  converts the electric currents inputted from the optical receiver  21  into digital electrical signals  53  of each channel and outputs the digital electrical signals  53 . Wiring through which the digital electrical signals  53  are transmitted may be either serial or parallel. Through the process described above, the optical main signal  52  is transmitted from the optical module  100   a  to the optical module  100   b . Through a similar process, digital electrical signals  54  inputted into the optical module  100   b  are transmitted from the optical module  100   b  to the optical module  100   a  as an optical main signal  55  and outputted from the optical module  100   a  as digital electrical signals  56 . 
         [0024]    Next, output of Loss Of Signal (LOS) signals by the processing circuit  22  will be described. The processing circuit  22  outputs a LOS signal according to an optical power inputted into each light receiving element of the optical receiver  21  to the determination unit  40  for each channel.  FIGS. 2A and 2B  are diagrams for explaining an example of the LOS signal corresponding to a predetermined channel. 
         [0025]    As illustrated in  FIGS. 2A and 2B , the LOS signal becomes high level when the optical input power inputted into the light receiving element of the optical receiver  21  is smaller than a threshold value and becomes low level when the optical input power is greater than the threshold value. The threshold value is a threshold value for determining whether or not there is an input of the main signal for the light receiving element. The high level and the low level may be reversed. In the embodiment, the LOS signal becomes high level when the optical input power is smaller than the threshold value and becomes low level when the optical input power is greater than the threshold value. The processing circuit  22  outputs the LOS signal (LOS 1  to LOS n ) for each channel. For example, when the LOS signals of all the channels are high level, it can be determined that the main signal is not transmitted. When the LOS signal of a specific channel is high level, it can be determined that a failure occurs in the specific channel. 
         [0026]    In the embodiment, control information is transmitted between the optical module  100   a  and the optical module  100   b  by using the LOS signals. Hereinafter, the details will be described. When the control unit  30  receives the control information from an external system, the control unit  30  performs a process to realize transmission of the control information through the transmission lines in a period of time in which the main signal is not transmitted and received, for example a period of time before being linked up. 
         [0027]      FIG. 3  is a diagram for explaining the details of the transmission of the control information from the optical module  100   a  to the optical module  100   b . As illustrated in  FIG. 3 , in the optical module  100   a , the control unit  30  converts the control information into a digital control signal including High (1) and Low (0) and transmits the digital signal to the drive circuit  12 . The drive circuit  12  converts the inputted control signal into an electrical signal including Enable and Disable to turn on and off the light emitting elements and outputs the electrical signal to the optical transmitter  11 . Thereby, the optical transmitter  11  outputs an optical signal for each channel according to the electrical signal. The optical power outputted from the light emitting element in the optical transmitter  11  may be set to a level so that the LOS signal of the facing optical module  100   b  is low level when the light emitting element is ON. When the light emitting element is turned off, no optical signal is outputted, so that the LOS of the facing optical module  100   b  signal becomes high level. The light emitting element does not have to be turned off. The output of the optical signal may be controlled to a level so that the LOS signal becomes high level. 
         [0028]    In the optical module  100   b , the processing circuit  22  outputs the LOS signal according to the optical input power of the optical signal inputted into the optical receiver  21 . In this way, the control information is transmitted between the optical module  100   a  and the optical module  100   b  by using the LOS signals. The main signal and the control signal might not have the same bit rate. For example, the control signal may have a bit rate lower than that of the main signal. 
         [0029]    The control signal may be transmitted by using a transmission line of a specific channel. However, the control signal may be transmitted by using transmission lines of a plurality of channels.  FIGS. 4A and 4B  are diagrams for explaining an example in which the control signal is transmitted by using channels 1 to 4. By referring to  FIG. 4A , the same control signal may be transmitted by using the transmission lines of each channel and the logical OR may be retrieved as the control signal. By referring to  FIG. 4B , even if a failure occurs in a transmission line of a certain channel, the control signal can be retrieved by using the logical OR. 
         [0030]    In the embodiment, the control signal is converted into the high level and the low level of the LOS signal, so that it is preferable to be able to determine whether the high level of the LOS signal indicates a part of the control signal or the LOS signal indicates that no control signal is transmitted. Therefore, the determination unit  40  determines the LOS signal. For example, when the control signal is transmitted, the LOS signal changes from the high level to the low level or the low level to the high level. Therefore, if the LOS signal does not change over a period of time in which a predetermined specified period of time Tm (for example, one bit of the control signal) repeats a predetermined number of times (for example, four times), it is possible to determine that no control signal is transmitted. Information related to time can be obtained from the timer  41 . The determination unit  40  outputs alarm to an external system if the LOS signal does not change over a period of time in which the specified period of time Tm repeats a predetermined number of times. 
         [0031]      FIGS. 5A to 5C  are diagrams illustrating an example of change of the LOS signal. As illustrated in  FIGS. 5A and 5B , as an example, the high level and the low level of the LOS signal are included in a period of time in which four bits are transmitted. In this case, the determination unit  40  determines that the control signal is transmitted and instructs the decoder  42  to decode the control signal. Thereby, the decoder  42  retrieves the control information by decoding the control signal and outputs the control information to the control unit  30 . On the other hand, in  FIG. 5C , the LOS signal does not change in a period of time in which four bits are transmitted. In this case, the determination unit  40  determines that the control signal is not transmitted and outputs alarm to the external system. 
         [0032]      FIG. 6  is an example of a flowchart executed when the LOS signal is determined according to the examples of  FIGS. 5A to 5C . As illustrated in  FIG. 6 , when the determination unit  40  receives the LOS signal (High), the determination unit  40  substitutes “1” for “n” (operation  51 ). Next, the determination unit  40  waits for the period of time Tm (1 bit) (operation S 2 ). Next, the determination unit  40  adds “1” to “n” (operation S 3 ). Next, the determination unit  40  determines whether or not the LOS signal switches to Low (operation S 4 ). If it is determined to be “Yes” in operation S 4 , the determination unit  40  instructs the decoder  42  to decode the control signal. Thereby, the decoder  42  decodes the LOS signal (operation S 5 ). 
         [0033]    If it is determined to be “No” in operation S 4 , the determination unit  40  determines whether or not “n” is smaller than “k (for example, 4)” (operation S 6 ). If it is determined to be “Yes” in operation S 6 , the flowchart is re-executed from operation S 2 . If it is determined to be “No” in operation S 6 , the determination unit  40  determines that the control signal is not transmitted and outputs alarm to the external system (operation S 7 ). After operation S 5  or operation S 7  is executed, the flowchart ends. 
         [0034]    Or, it is possible to determine the LOS signal by determining start data of the control signal in advance. For example, it is possible to determine that the control signal is transmitted by adding a header “10” to the control signal. As an example, when the data of the control signal is “100110”, the data is changed to “10100110”, so that it is possible to determine that the control signal is transmitted. 
         [0035]      FIGS. 7A and 7B  are diagrams illustrating an example of the LOS signal. In  FIG. 7A , “1” and “0” appear for each bit, so that it is possible to determine that the control signal is transmitted. In this case, the determination unit  40  instructs the decoder  42  to decode the control signal. Thereby, the decoder  42  retrieves the control information by decoding the control signal by removing the first “10” and outputs the control information to the control unit  30 . On the other hand, in  FIG. 7B , “1” and “0” do not appear, so that the determination unit  40  determines that the control signal is not transmitted and outputs alarm to the external system. 
         [0036]      FIG. 8  is an example of a flowchart executed when the LOS signal is determined according to the examples of  FIGS. 7A and 7B . As illustrated in  FIG. 8 , when the determination unit  40  receives the LOS signal (High) (operation S 11 ), the determination unit  40  waits for the period of time Tm (1 bit) (operation S 12 ). Next, the determination unit  40  determines whether or not the LOS signal switches to low level (operation S 13 ). If it is determined to be “Yes” in operation S 13 , the determination unit  40  instructs the decoder  42  to decode the control signal. Thereby, the decoder  42  decodes the LOS signal (operation S 14 ). If it is determined to be “No” in operation S 13 , the determination unit  40  determines that the control signal is not transmitted and outputs alarm to the external system (operation S 15 ). After operation S 14  or operation S 15  is executed, the flowchart ends. 
         [0037]    Next, the details of the control information will be described. The control information is information related to operation conditions of the optical module  100   a  and the optical module  100   b . The control information may include information that specifies operation channels. For example, the control information may include information that specifies the signal transmission channels from the optical module  100   a  to the optical module  100   b  to be the channels 1 to 4 and specifies the signal transmission channels from the optical module  100   b  to the optical module  100   a  to be the channels 1 to 8. When the control unit  30  of the optical module  100   a  receives the control information from the external system, the control unit  30  converts the control information into the control signal and instructs the drive circuit  12  to transmit the control signal. Thereby, in the optical module  100   b , the decoder  42  decodes the control signal into the control information and the control unit  30  receives the control information. After the process described above, the signal transmission channels from the optical module  100   a  to the optical module  100   b  are set to the channels 1 to 4 and the signal transmission channels from the optical module  100   b  to the optical module  100   a  are set to the channels 1 to 8. Thereafter, the channels 1 to 4 are used to transmit the main signal from the optical module  100   a  to the optical module  100   b  and the channels 1 to 8 are used to transmit the main signal from the optical module  100   b  to the optical module  100   a.    
         [0038]    The control information may include information that specifies an operation rate. When the control information is transmitted and received between the optical module  100   a  and the optical module  100   b , the bit rate of the main signal transmitted through each transmission line is changed. The control information may include a transition command of a port state. When a transition command to a stop state is transmitted and received between the optical module  100   a  and the optical module  100   b , each unit in the optical module  100   a  and the optical module  100   b  goes to a power off state. When a transition command to a dormant state is transmitted and received between the optical module  100   a  and the optical module  100   b , partial units in the optical module  100   a  and the optical module  100   b  (for example, the transmitting units) go to the power off state and the other units go to a stand-by state. The transition command to the dormant state may include information that specifies operation channels and increases or decreases the number of channels which go to the stand-by state in the receiving unit  20 . The control information includes information that indicates setting of the operation conditions of the optical modules, such as the channel specification and the operation rate, will be completed (accepted) or information that indicates the setting is rejected. 
         [0039]      FIG. 9  is a diagram illustrating a time sequence for explaining the details of the transmission and reception of the control information between the optical module  100   a  and the optical module  100   b . Before the transmission and reception of the control information, the receiving unit  20  is in a state of waiting for input of an optical signal in both the optical module  100   a  and the optical module  100   b . Therefore, the LOS signal outputted from the processing circuit  22  maintains the high level. Thereby, the determination unit  40  outputs alarm to the external system. 
         [0040]    When the control information is inputted into the control unit  30  of the optical module  100   a , the control unit  30  converts the control information into a digital control signal and outputs the digital control signal to the transmitting unit  10 . The transmitting unit  10  transmits a control signal according to the control signal to the optical module  100   b  through each transmission line. 
         [0041]    In the optical module  100   b , the receiving unit  20  outputs a digital LOS signal to the determination unit  40  according to an inputted optical input power. The determination unit  40  determines the LOS signal and the decoder  42  decodes the control signal. The determination unit  40  outputs the control information obtained by the decoding to the control unit  30 . The control unit  30  sets the operations of the transmitting unit  10  and the receiving unit  20  by setting parameters of the transmitting unit  10  and the receiving unit  20  according to the received control information. Also, the control unit  30  converts the control information inputted to the optical module  100   b  into a digital control signal and outputs the digital control signal to the transmitting unit  10 . The transmitting unit  10  transmits a control signal according to the control signal to the optical module  100   a  through each transmission line. 
         [0042]    In the optical module  100   a , the receiving unit  20  outputs a digital LOS signal to the determination unit  40  according to an inputted optical input power. The determination unit  40  determines the LOS signal and the decoder  42  decodes the control signal. The determination unit  40  outputs the control information obtained by the decoding to the control unit  30 . The control unit  30  sets the operations of the transmitting unit  10  and the receiving unit  20  by setting parameters of the transmitting unit  10  and the receiving unit  20  according to the received control information. Through the process described above, the operations of the optical module  100   a  and the optical module  100   b  are set. Although, in the example of  FIG. 9 , the control signal is transmitted from the optical module  100   b  to the optical module  100   a , the control unit  30  of the optical module  100   a  may set parameters of each unit on the basis of control information received from the external system. Although, in the example of  FIG. 9 , the transfer of the control information between the optical modules  100   a  and  100   b  is completed by one transfer, for example, when the operation channels notified from the optical module  100   a  may not be used by the optical module  100   b , the control information may be transferred a plurality of times, such as, the optical module  100   b  notifies the optical module  100   a  that the setting is rejected and the optical module  100   a  newly specifies the operation channels. 
         [0043]      FIG. 10  is a diagram illustrating a time sequence when a certain transmission line from the optical module  100   a  to the optical module  100   b  fails. When a transmission line fails, through the transmission line, no light is inputted into the receiving unit  20  of the optical module  100   b . Thereby, in the optical module  100   b , the determination unit  40  determines the LOS signal and outputs alarm to the external system and the control unit  30 . Thereby, the control unit  30  stops the transmission and reception performed by the transmitting unit  10  and the receiving unit  20 . In this case, the determination unit  40  specifies the failed channel on the basis of the LOS signal. 
         [0044]    In this case, no light is inputted into the receiving unit  20  of the optical module  100   a . Thereby, in the optical module  100   a , the determination unit  40  determines the LOS signal and outputs alarm to the external system and the control unit  30 . Thereby, the control unit  30  stops the transmission and reception performed by the transmitting unit  10  and the receiving unit  20 . Through the process described above, the transmission and reception of the main signal are stopped between the optical module  100   a  and the optical module  100   b.    
         [0045]    Subsequently, in the optical module  100   b , the determination unit  40  notifies the control unit  30  of the failed channel. Thereby, the control unit  30  sets the operation channels of the transmitting unit  10  and the receiving unit  20 . Further, the control unit  30  forms the control information from the information of the set operation channels and the operation rate, converts the control information into a digital control signal, and outputs the digital control signal to the transmitting unit  10 . The transmitting unit  10  transmits a control signal according to the control signal to the optical module  100   a  through each transmission line. 
         [0046]    In the optical module  100   a , the receiving unit  20  outputs a digital LOS signal to the determination unit  40  according to an inputted optical input power. The determination unit  40  determines the LOS signal and the decoder  42  decodes the control signal. The determination unit  40  outputs the control information obtained by the decoding to the control unit  30 . The control unit  30  sets the operation channels of the transmitting unit  10  and the receiving unit  20  according to the received control information. Also, the control unit  30  converts information for notifying that the setting is completed into a digital control signal and outputs the digital control signal to the transmitting unit  10 . The transmitting unit  10  transmits a control signal according to the control signal to the optical module  100   b  through each transmission line. Also, the control unit  30  causes the transmitting unit  10  and the receiving unit  20  to operate. 
         [0047]    In the optical module  100   b , the receiving unit  20  outputs a digital LOS signal to the determination unit  40  according to an inputted optical input power. The determination unit  40  determines the LOS signal and the decoder  42  decodes the control signal. The determination unit  40  outputs the control information obtained by the decoding to the control unit  30 . The control unit  30  recognizes that the setting is completed and causes the transmitting unit  10  and the receiving unit  20  to operate. 
         [0048]      FIG. 11  is a diagram illustrating a time sequence when the control unit  30  of the optical module  100   a  changes the number of operation channels by an instruction from the external system. In the optical module  100   a , when the control unit  30  receives an instruction to change the number of operation channels from the external system, the control unit  30  causes the transmitting unit  10  to stop the transmission of the main signal. 
         [0049]    In this case, in the optical module  100   b , no light is inputted into the receiving unit  20 . Thereby, the determination unit  40  determines the LOS signal and outputs alarm to the external system and the control unit  30 . Thereby, the control unit  30  stops the transmission and reception performed by the transmitting unit  10  and the receiving unit  20 . 
         [0050]    In this case, no light is inputted into the receiving unit  20  of the optical module  100   a . Thereby, in the optical module  100   a , the determination unit  40  determines the LOS signal and outputs alarm to the external system and the control unit  30 . Thereby, the control unit  30  recognizes that the transmission and reception of the main signal is also stopped in the optical module  100   b  and sets the operation channels of the transmitting unit  10  and the receiving unit  20 . Further, the control unit  30  forms the control information from the information of the set operation channels, converts the control information into a digital control signal, and outputs the digital control signal to the transmitting unit  10 . The transmitting unit  10  transmits a control signal according to the control signal to the optical module  100   b  through each transmission line. 
         [0051]    In the optical module  100   b , the receiving unit  20  outputs a digital LOS signal to the determination unit  40  according to an inputted optical input power. The determination unit  40  determines the LOS signal and the decoder  42  decodes the control signal. The determination unit  40  outputs the control information obtained by the decoding to the control unit  30 . The control unit  30  sets the operation channels of the transmitting unit  10  and the receiving unit  20  according to the received control information. Also, the control unit  30  converts information for notifying that the setting is completed into a digital control signal and outputs the digital control signal to the transmitting unit  10 . The transmitting unit  10  transmits a control signal according to the control signal to the optical module  100   a  through each transmission line. Also, the control unit  30  causes the transmitting unit  10  and the receiving unit  20  to operate. 
         [0052]    In the optical module  100   a , the receiving unit  20  outputs a digital LOS signal to the determination unit  40  according to an inputted optical input power. The determination unit  40  determines the LOS signal and the decoder  42  decodes the control signal. The determination unit  40  outputs the control information obtained by the decoding to the control unit  30 . The control unit  30  recognizes that the setting is completed and causes the transmitting unit  10  and the receiving unit  20  to operate. 
         [0053]      FIG. 12  is a diagram illustrating a time sequence when a port state is changed to a stop state. In the optical module  100   a , when the control unit  30  receives an instruction to change the port state to the stop state from the external system, the control unit  30  causes the transmitting unit  10  to stop the transmission of the main signal. 
         [0054]    In this case, in the optical module  100   b , no light is inputted into the receiving unit  20 . Thereby, the determination unit  40  determines the LOS signal and outputs alarm to the external system and the control unit  30 . Thereby, the control unit  30  stops the transmission and reception performed by the transmitting unit  10  and the receiving unit  20 . 
         [0055]    In this case, no light is inputted into the receiving unit  20  of the optical module  100   a . Thereby, in the optical module  100   a , the determination unit  40  determines the LOS signal and outputs alarm to the external system and the control unit  30 . Thereby, the control unit  30  recognizes that the transmission and reception of the main signal is also stopped in the optical module  100   b , forms the control information indicating that the port state is changed to the stop state, converts the control information into a digital control signal, and outputs the digital control signal to the transmitting unit  10 . The transmitting unit  10  transmits a control signal according to the control signal to the optical module  100   b  through each transmission line. 
         [0056]    In the optical module  100   b , the receiving unit  20  outputs a digital LOS signal to the determination unit  40  according to an inputted optical input power. The determination unit  40  determines the LOS signal and the decoder  42  decodes the control signal. The determination unit  40  outputs the control information obtained by the decoding to the control unit  30 . The control unit  30  turns off the power of the transmitting unit  10  and the receiving unit  20  according to the received control information. Also, the control unit  30  converts information for notifying that the stop is accepted into a digital control signal and outputs the digital control signal to the transmitting unit  10 . The transmitting unit  10  transmits a control signal according to the control signal to the optical module  100   a  through each transmission line. 
         [0057]    In the optical module  100   a , the receiving unit  20  outputs a digital LOS signal to the determination unit  40  according to an inputted optical input power. The determination unit  40  determines the LOS signal and the decoder  42  decodes the control signal. The determination unit  40  outputs the control information obtained by the decoding to the control unit  30 . The control unit  30  recognizes that the stop is accepted and turns off the power of the transmitting unit  10  and the receiving unit  20 . Through the process described above, the optical transmission system  200  goes into the stop state. 
         [0058]    According to the embodiment, a plurality of transmission lines are provided, so that it is possible to transmit and receive the control signal even if a failure occurs in a certain transmission line. The control signal includes the high level greater than the threshold value of the optical power for determining whether or not there is an input of the main signal and the low level smaller than the threshold value, so that it is possible to distinguish between the main signal and the control signal. It is detected that the high level and the low level is included in the control signal in a predetermined period of time, so that whether or not the control signal is inputted can be determined. The transmission lines for the main signal are used, so that another line for the control signal is not typically used. The control signal different from the main signal is transmitted and received, so that it is possible to avoid band limitation and data delay of the main signal. In this way, it is possible to transmit and receive information different from the main signal between a transmitter and a receiver in a simple configuration. 
         [0059]    If the control signal can be transmitted and received even when a failure occurs in a certain transmission line, the number of channels and the bit rate can be changed. In this case, it is possible to continue transmission and reception of the main signal by performing a reduction operation, such as reducing the number of channels. As a result, it is possible to avoid system halt. 
         [0060]    The control signal is decoded from the LOS signal, so that it is possible to use an existing device including a LOS signal detection mechanism. In other words, a new configuration is not typically added. A signal other than the LOS signal may be used. For example, the control signal may be decoded by using monitor currents of each light receiving element. 
         [0061]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment of the present invention has been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.