Patent Publication Number: US-2023163856-A1

Title: Optical signal processing apparatus and optical signal processing method

Description:
TECHNICAL FIELD 
     The present invention relates to an optical signal processing apparatus and an optical signal processing method. 
     BACKGROUND ART 
     In the conventional optical communication, a user terminal superimposes a main signal and a control signal on one another in the form of an electrical signal, converts the superimposed electrical signal to an optical signal, and thereafter transmits the optical signal (see, for example, Non-Patent Literature 1). 
       FIG.  18    illustrates a configuration example of the conventional optical communication system. In the optical communication system illustrated in  FIG.  18   , a user terminal # 1  and a user terminal # 2  are connected by an optical fiber. An optical transceiver (TRx) #i installed in a user terminal #i (i=1, 2) communicates with an optical transceiver (TRx) #j installed in a counterpart user terminal #j (j=1, 2 and j≠i) by using an optical signal. During the communication, the user terminal #i superimposes a control signal Ci on a main signal Mi in which user data has been set, and transmits/receives the superimposed signal. For example, the control signal Ci is an alive monitoring signal, a signal including state information, a wavelength control signal, control signals other than the wavelength control signal, or the like. The state information indicates, for example, the wavelength, power, temperature, or the like of the TRx #i. The wavelength control signal provides instructions about the initial settings of the wavelength, wavelength offset calibration, wavelength switching, or the like. The control signals other than the wavelength control signal are, for example, an on/off control signal for a TRx, a bit-rate control signal, or the like. The wavelength to be used for communication between TRx # 1  and TRx # 2  is set in advance in the user terminal # 1  and the user terminal # 2 . There is also a case where one user terminal instructs the other user terminal about which wavelength to be used, such as a case where the user terminal # 1  instructs the user terminal # 2  about the wavelength. 
     The user terminal #i (i=1, 2) on the transmitting side superimposes the main signal Mi and the control signal Ci on one another in the form of an electrical signal. The TRx #i converts a superimposed electrical signal Ei to an optical signal Gi, and outputs the optical signal Gi to an optical fiber. The TRx #j in the user terminal #j (j=1, 2 and j≠i) on the receiving side receives an input of the optical signal Gi transmitted through the optical fiber, and converts the optical signal Gi to the electrical signal Ei by using a photo diode (PD) or other diodes. The user terminal #j separates the converted electrical signal Ei into the main signal Mi and the control signal Ci, and handles the main signal Mi and the control signal Ci as separate electrical signals. In the user terminal #i on the transmitting side, a main signal and a control signal are carried on different frequency carriers in an electricity field as with the auxiliary management and control channel (AMCC), so that the main signal and the control signal can be handled as physically separate signals. 
     CITATION LIST 
     Non-Patent Literature 
     Non-Patent Literature 1: K. Honda et al., “WDM Passive Optical Network Managed with Embedded Pilot Tone for Mobile Fronthaul,” 2015 European Conference on Optical Communication (ECOC), We.3.4.4 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     As described above, a user terminal on the transmitting side superimposes a main signal and a control signal in the form of an electrical signal on one another, and thereafter converts the superimposed signal to an optical signal and transmits the optical signal. Due to this operation, the user terminal cannot rewrite the control signal somewhere along a communication path between the user terminal and another user terminal. 
     The present invention is made in view of the above circumstances, and it is an object of the present invention to provide an optical signal processing apparatus and an optical signal processing method to allow a signal superimposed on an optical signal to be partially replaced with a different signal somewhere along a communication path. 
     Means for Solving the Problem 
     One aspect of the present invention is an optical signal processing apparatus including: a removal unit that receives, from a first optical transmission path, an optical signal converted from an electrical signal, in which a first signal and a second signal having different frequencies from each other have been superimposed, and removes the second signal from the optical signal which has been input; and a superimposition unit that superimposes a third signal having a frequency different from a frequency of the first signal on the optical signal in which the second signal has been removed by the removal unit, and outputs the optical signal in which the third signal has been superimposed to a second optical transmission path. 
     Another aspect of the present invention is an optical signal processing method including: a removing step of receiving, from a first optical transmission path, an optical signal converted from an electrical signal, in which a first signal and a second signal having different frequencies from each other have been superimposed, and removing the second signal from the optical signal which has been input; and a superimposing step of superimposing a third signal having a frequency different from a frequency of the first signal on the optical signal in which the second signal has been removed in the removing step, and outputting the optical signal in which the third signal has been superimposed to a second optical transmission path. 
     Effects of the Invention 
     The present invention makes it possible to replace a part of a signal superimposed on an optical signal with a different signal somewhere along a communication path. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating the configuration of an optical communication system according to a first embodiment of the present invention. 
         FIG.  2    is a diagram for explaining the processing in a control signal superimposition unit according to the first embodiment. 
         FIG.  3    is a diagram illustrating the configuration of the control signal superimposition unit according to the first embodiment. 
         FIG.  4    is a diagram illustrating the configuration of the control signal superimposition unit according to the first embodiment. 
         FIG.  5    is a graph illustrating response characteristics of an optical amplifier according to the first embodiment. 
         FIG.  6    is a diagram illustrating the configuration of an optical communication system according to a second embodiment. 
         FIG.  7    is a diagram illustrating the configuration of the optical communication system according to the second embodiment. 
         FIG.  8    is a diagram illustrating the configuration of an optical communication system according to a third embodiment. 
         FIG.  9    is a diagram illustrating the configuration of the optical communication system according to the third embodiment. 
         FIG.  10    is a diagram illustrating the configuration of an optical communication system according to a fourth embodiment. 
         FIG.  11    is a diagram illustrating the configuration of an optical communication system according to a fifth embodiment. 
         FIG.  12    is a diagram illustrating the configuration of an optical communication system according to a sixth embodiment. 
         FIG.  13    is a diagram illustrating the configuration of the optical communication system according to the sixth embodiment. 
         FIG.  14    is a diagram illustrating the configuration of an optical communication system according to a seventh embodiment. 
         FIG.  15    is a diagram illustrating the configuration of the optical communication system according to the seventh embodiment. 
         FIG.  16    is a diagram illustrating the configuration of an optical communication system according to an eighth embodiment. 
         FIG.  17    is a diagram illustrating the configuration of an optical communication system according to a ninth embodiment. 
         FIG.  18    is a diagram illustrating a configuration example of a conventional optical communication system. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that in a plurality of drawings, like component are denoted by like reference numerals, and therefore explanations thereof are omitted. 
     First Embodiment 
       FIG.  1    is a diagram illustrating the configuration of an optical communication system  1000  according to a first embodiment. The optical communication system  1000  includes a user terminal  1100 , a control signal superimposition unit (superimposer)  1200 , and a management and control functional unit  1300 . The user terminal  1100  is connected to the control signal superimposition unit  1200  by an optical fiber  1500 . 
     As the user terminal  1100 , an optical communication apparatus configured to transmit/receive an optical signal according to the conventional technique can be used. The user terminal  1100  includes a signal mixer  1110 , an optical transceiver (TRx)  1120 , and a signal divider  1130 . The signal mixer  1110  outputs, to the TRx  1120 , a transmission signal in which a main signal and a control signal that are both electrical signals have been superimposed on one another. The control signal has a lower frequency than the frequency of the main signal. For example, as the control signal, the AMCC is used. The TRx  1120  converts the transmission signal from an electrical signal to an optical signal, and outputs the optical signal to the optical fiber  1500 . The TRx  1120  receives an input of an optical signal transmitted through the optical fiber  1500 , and converts the input optical signal to an electrical signal to output the electrical signal. The signal divider  1130  separates the electrical signal output by the TRx  1120  into the main signal and the control signal based on their respective frequencies. 
     Hereinafter, two units of user terminals  1100 , each of which is a counterpart of the other, are described as “user terminal  1100 - 1 ” and “user terminal  1100 - 2 ,” and the signal mixer  1110 , the TRx  1120 , and the signal divider  1130  included in a user terminal  1100 - i  (i=1, 2) are described as “signal mixer  1110 - i ,” “TRx  1120 - i ,” and “signal divider  1130 - i ,” respectively. The optical fiber  1500  between the user terminal  1100 - i  and the control signal superimposition unit  1200  is described as “optical fiber  1500 - i .” The main signal and the control signal to be transmitted by the user terminal  1100 - i  are described as “main signal Mi” and “control signal Ci,” respectively. Similarly to the conventional technique, the control signal Ci is, for example, an alive monitoring signal, a signal including state information (the wavelength, power, temperature, or the like of the TRx  1120 - i ), a wavelength control signal, control signals other than the wavelength control signal, or the like. 
     The control signal superimposition unit  1200  includes transmission-reception separation devices  1210 - 1  and  1210 - 2 , optical amplifiers  1220 - 1  and  1220 - 2 , and optical modulators  1230 - 1  and  1230 - 2 . The transmission-reception separation devices  1210 - 1  and  1210 - 2  are connected by optical fibers  1240 - 1  and  1240 - 2 . On an optical fiber  1240 -i (i=1, 2), an optical amplifier  1220 - i  and an optical modulator  1230 - i  are provided. 
     The transmission-reception separation device  1210 - i  (i=1, 2) receives an input of an optical signal output by the user terminal  1100 - i  from the optical fiber  1500 - i,  and outputs the optical signal to the optical amplifier  1220 - i.  The transmission-reception separation device  1210 - i  receives an input of an optical signal output by the optical modulator  1230 - j  (j=1, 2 and j≠i), and outputs the optical signal to the optical fiber  1500 - i.    
     Each of the optical amplifiers  1220 - 1  and  1220 - 2  has the characteristics of high-pass filter, and removes a low-frequency signal. Each of the optical amplifiers  1220 - 1  and  1220 - 2  is, for example, a semiconductor optical amplifier (SOA). The optical amplifier  1220 - i  (i=1, 2) receives an input of an optical signal output by the transmission-reception separation device  1210 - i.  The optical amplifier  1220 - i  removes a control signal superimposed on the low-frequency side of the input optical signal, and outputs the optical signal with the control signal removed to the optical modulator  1230 - i.  Note that according to the transmittance characteristics of the optical amplifier  1220 - i  serving as a high-pass filter, a frequency that can be sufficiently attenuated in the optical amplifier  1220 - i  is selected as a carrier frequency of the control signal. The transmittance characteristics of the optical amplifier  1220 - i  serving as a high-pass filter will be described later with reference to  FIG.  5   . 
     Each of the optical modulators  1230 - 1  and  1230 - 2  is a lithium-niobate (LiNbO 3  (LN)) modulator, an electro-absorption (EA) modulator, or other types of modulators. The optical modulator  1230 - i  (i=1, 2) receives an input of an optical signal as the main signal from the optical amplifier  1220 - i,  and superimposes a control signal on the low-frequency side of the input optical signal. The optical modulator  1230 - i  outputs the optical signal with the control signal superimposed thereon to the transmission-reception separation device  1210 - j  (j=1, 2 and j≠i). 
     The management and control functional unit  1300  manages and controls the optical communication system  1000 . The management and control functional unit  1300  outputs a control signal to be superimposed on a main signal to the control signal superimposition unit  1200 . A control signal to be superimposed on the main signal Mi (i=1, 2) is described as “control signal C 3 - i.”   
       FIG.  2    is a diagram for explaining the processing in the control signal superimposition unit  1200 . A signal mixer  1110 - 1  in the user terminal  1100 - 1  illustrated in  FIG.  1    outputs the electrical signal E 1  in which the main signal M 1  and the control signal C 1  in the form of an electrical signal have been superimposed on one another. A TRx  1120 - 1  converts the electrical signal E 1  to an optical signal G 11 , and outputs the optical signal G 11  to an optical fiber  1500 - 1 . 
     The transmission-reception separation device  1210 - 1  receives an input of the optical signal G 11  transmitted through the optical fiber  1500 - 1 , and outputs the optical signal G 11  to the optical amplifier  1220 - 1 . The optical amplifier  1220 - 1  generates an optical signal G 12  with the control signal C 1  on the low-frequency side removed from the input optical signal G 11 , and outputs the optical signal G 12  to the optical modulator  1230 - 1 . The optical modulator  1230 - 1  superimposes the control signal C 3 - 1 , input from the management and control functional unit  1300 , on the low-frequency side of the optical signal G 12  to generate an optical signal G 13 , and outputs the optical signal G 13  to the transmission-reception separation device  1210 - 2 . The transmission-reception separation device  1210 - 2  outputs the optical signal G 13  to an optical fiber  1500 - 2 . A TRx  1120 - 2  in the user terminal  1100 - 2  illustrated in  FIG.  1    receives an input of the optical signal G 13  transmitted through the optical fiber  1500 - 2 , and converts the optical signal G 13  to an electrical signal E 1 ′. A signal divider  1130 - 2  separates the converted electrical signal E 1 ′ into the main signal M 1  and the control signal C 3 - 1 . 
     A signal mixer  1110 - 2  in the user terminal  1100 - 2  illustrated in  FIG.  1    outputs an electrical signal E 2  in which a main signal M 2  and a control signal C 2  in the form of an electrical signal have been superimposed on one another. The TRx  1120 - 2  converts the superimposed electrical signal E 2  to an optical signal G 21 , and outputs the optical signal G 21  to the optical fiber  1500 - 2 . 
     The transmission-reception separation device  1210 - 2  receives an input of the optical signal G 21  transmitted through the optical fiber  1500 - 2 , and outputs the optical signal G 21  to the optical amplifier  1220 - 2 . The optical amplifier  1220 - 2  generates an optical signal G 22  with the control signal C 2  on the low-frequency side removed from the input optical signal G 21 , and outputs the optical signal G 22  to the optical modulator  1230 - 2 . The optical modulator  1230 - 2  superimposes a control signal C 3 - 2 , input from the management and control functional unit  1300 , on the low-frequency side of the optical signal G 22  to generate an optical signal G 23 , and outputs the optical signal G 23  to the transmission-reception separation device  1210 - 1 . The transmission-reception separation device  1210 - 1  outputs the optical signal G 23  to the optical fiber  1500 - 1 . The TRx  1120 - 1  in the user terminal  1100 - 1  illustrated in  FIG.  1    receives an input of the optical signal G 23  transmitted through the optical fiber  1500 - 1 , and converts the optical signal G 23  to an electrical signal E 2 ′. A signal divider  1130 - 1  separates the converted electrical signal E 2 ′ into the main signal M 2  and the control signal C 3 - 2 . 
     Note that a signal to be superimposed in the control signal superimposition unit  1200  uses the same frequency as a control signal to be transmitted by the user terminal  1100 . At this time, a pilot tone with a frequency different from, and within a lower frequency range than, the frequency of the main signal may be used as a carrier, or a control signal remaining as a baseband signal may be superimposed. It is allowable that a control signal to be superimposed in the control signal superimposition unit  1200  uses a pilot tone within a higher frequency range than the frequency of the main signal as a carrier, provided that the control signal is separable from the main signal in the user terminal  1100 . 
     If the management and control functional unit  1300  does not superimpose a control signal on an optical signal transmitted from the user terminal  1100 - 1  and addressed to the user terminal  1100 - 2 , then the control signal superimposition unit  1200  may not be provided with the optical amplifier  1220 - 1  and the optical modulator  1230 - 1 . If the management and control functional unit  1300  does not superimpose a control signal on an optical signal transmitted from the user terminal  1100 - 2  and addressed to the user terminal  1100 - 1 , then the control signal superimposition unit  1200  may not be provided with the optical amplifier  1220 - 2  and the optical modulator  1230 - 2 . 
     The optical communication system  1000  may be provided with a control signal superimposition unit  1200   a  illustrated in  FIG.  3    or a control signal superimposition unit  1200   b  illustrated in  FIG.  4   , instead of the control signal superimposition unit  1200  illustrated in  FIG.  1   . 
       FIG.  3    is a diagram illustrating the configuration of the control signal superimposition unit  1200   a.  The control signal superimposition unit  1200   a  includes transmission-reception separation devices  1210   a - 1  and  1210   a - 2 , optical switches (SW)  1250 - 1  and  1250 - 2 , the optical amplifiers  1220 - 1  and  1220 - 2 , and the optical modulators  1230 - 1  and  1230 - 2 . 
     The transmission-reception separation device  1210   a - 1  and the optical SW  1250 - 1  are connected by an optical fiber  1260 - 1 . The transmission-reception separation device  1210   a - 1  and the optical SW  1250 - 2  are connected by optical fibers  1270 - 2  and  1280 - 2 . The transmission-reception separation device  1210   a - 2  and the optical SW  1250 - 2  are connected by an optical fiber  1260 - 2 . The transmission-reception separation device  1210   a - 2  and the optical SW  1250 - 1  are connected by optical fibers  1270 - 1  and  1280 - 1 . Each of the optical fibers  1270 - 1  and  1270 - 2  is a line on which a control signal is superimposed. Each of the optical fibers  1280 - 1  and  1280 - 2  is a line on which a control signal is not superimposed. On the optical fiber  1270 -i (i=1, 2), the optical amplifier  1220 - i  and the optical modulator  1230 - i  are provided. 
     The transmission-reception separation device  1210   a - i  (i=1, 2) receives an input of an optical signal transmitted through the optical fiber  1500 - i,  and outputs the input optical signal to an optical SW  1250 - i.  The transmission-reception separation device  1210   a - i  receives an input of an optical signal transmitted through an optical fiber  1270 - j  (j=1, 2 and j≠i) or through an optical fiber  1280 - j,  and outputs the input optical signal to the optical fiber  1500 - i.    
     The optical SW  1250 - i  (i=1, 2) includes one input port and two output ports. The input port of the optical SW  1250 - i  is connected to an optical fiber  1260 - i.  The two output ports are connected individually to the optical fibers  1270 - i  and  1280 - i.  The optical SW  1250 - i  outputs an optical signal input from the input port to either of the output ports. 
     Operation of the control signal superimposition unit  1200   a  is described below. The transmission-reception separation device  1210   a - 1  receives an input of the optical signal G 11  with the main signal M 1  and the control signal C 1  superimposed on one another from the optical fiber  1500 - 1 , and outputs the optical signal G 11  to the optical SW  1250 - 1 . The optical SW  1250 - 1  outputs the optical signal G 11 , which is a target from which a control signal is removed, to the optical amplifier  1220 - 1 , or outputs the optical signal G 11 , from which a control signal is not removed, to the transmission-reception separation device  1210   a - 2 . 
     The optical amplifier  1220 - 1  receives an input of the optical signal G 11  from the optical SW  1250 - 1 , and outputs the optical signal G 12  with the control signal C 1  on the low-frequency side removed from the optical signal G 11 . The optical modulator  1230 - 1  outputs the optical signal G 13  in which the control signal C 3 - 1  input from the management and control functional unit  1300  has been superimposed on the low-frequency side of the optical signal G 12 . The transmission-reception separation device  1210 - 2  outputs an optical signal G 14 , which is the optical signal G 13  input from the optical modulator  1230 - 1  or the optical signal G 11  input from the optical SW  1250 - 1 , to the optical fiber  1500 - 2 . The TRx  1120 - 2  in the user terminal  1100 - 2  illustrated in  FIG.  1    converts the optical signal G 14  to an electrical signal, and the signal divider  1130 - 2  separates the converted electrical signal into the main signal M 1  and the control signal C 1  or the control signal C 3 - 1 . 
     The transmission-reception separation device  1210   a - 2  receives an input of the optical signal G 21  with the main signal M 2  and the control signal C 2  superimposed on one another from the optical fiber  1500 - 2 , and outputs the optical signal G 21  to the optical SW  1250 - 2 . The optical SW  1250 - 2  outputs the optical signal G 21 , which is a target from which a control signal is removed, to the optical amplifier  1220 - 2 , or outputs the optical signal G 21 , from which a control signal is not removed, to the transmission-reception separation device  1210   a - 1 . 
     The optical amplifier  1220 - 2  receives an input of the optical signal G 21  from the optical SW  1250 - 2 , and outputs the optical signal G 22  with the control signal C 2  on the low-frequency side removed from the optical signal G 21 . The optical modulator  1230 - 2  outputs the optical signal G 23  in which the control signal C 3 - 2  input from the management and control functional unit  1300  has been superimposed on the low-frequency side of the optical signal G 22 . The transmission-reception separation device  1210   a - 1  outputs an optical signal G 24 , which is the optical signal G 23  input from the optical modulator  1230 - 2  or the optical signal G 21  input from the optical SW  1250 - 2 , to the optical fiber  1500 - 1 . The TRx  1120 - 1  in the user terminal  1100 - 1  illustrated in  FIG.  1    converts the optical signal G 24  to an electrical signal, and the signal divider  1130 - 2  separates the converted electrical signal into the main signal M 1  and the control signal C 2  or the control signal C 3 - 2 . 
       FIG.  4    is a diagram illustrating the configuration of the control signal superimposition unit  1200   b.  The control signal superimposition unit  1200   b  is different from the control signal superimposition unit  1200   a  illustrated in  FIG.  3    in that the transmission-reception separation devices  1210 - 1  and  1210 - 2  illustrated in  FIG.  1    are provided instead of the transmission-reception separation devices  1210   a - 1  and  1210   a - 2 , and further optical SWs  1290 - 1  and  1290 - 2  are provided. 
     An optical SW  1290 - i  (i=1, 2) includes two input ports and one output port. The optical SW  1290 -i is connected to the optical modulator  1230 - i  by the optical fiber  1270 - i,  connected to the optical SW  1250 - i  by the optical fiber  1280 - i,  and connected to the transmission-reception separation device  1210 - j  (j=1, 2 and j≠i) by an optical fiber  1295 - i.    
     The optical SW  1290 - 1  outputs the optical signal G 13  transmitted through the optical fiber  1270 - 1 , or the optical signal G 11  transmitted through the optical fiber  1280 - 1  to an optical fiber  1295 - 1  as the optical signal G 14 . The transmission-reception separation device  1210 - 2  receives an input of the optical signal G 14  transmitted through the optical fiber  1295 - 1 , and outputs the input optical signal G 14  to the optical fiber  1500 - 2 . The optical SW  1290 - 2  outputs the optical signal G 24 , which is the optical signal G 23  transmitted through the optical fiber  1270 - 2  or the optical signal G 21  transmitted through the optical fiber  1280 - 2  to an optical fiber  1295 - 2 . The transmission-reception separation device  1210 - 1  receives an input of the optical signal G 24  transmitted through the optical fiber  1295 - 2 , and outputs the input optical signal G 24  to the optical fiber  1500 - 1 . Note that in the control signal superimposition unit  1200   b,  each of the optical SWs  1290 - 1  and  1290 - 2  may be replaced with an optical coupler. 
       FIG.  5    is a graph illustrating response characteristics of the optical amplifiers  1220 - 1  and  1220 - 2 .  FIG.  5    illustrates the response characteristics of the optical amplifiers  1220 - 1  and  1220 - 2  when they are SOAs. The optical amplifiers  1220 - 1  and  1220 - 2  exhibit adequate response characteristics to a signal with a sufficiently high frequency equal to or higher than 10 GHz. In contrast, the response characteristics of the optical amplifiers  1220 - 1  and  1220 - 2  significantly deteriorate within the low frequency range equal or lower than several GHz. By taking advantage of these response characteristics, it is possible to remove a low-frequency signal. For example, a frequency Fa is set to the main signal, while a frequency Fb is set to the control signal, and this makes it possible to remove the control signal from an optical signal having passed through the optical amplifiers  1220 - 1  and  1220 - 2 . 
     Note that since the differences in response characteristics depend on the gain of the SOA, the length of the SOA is increased, or the SOAs are connected in multiple stages, so that this makes it possible to increase the differences in response characteristics. Also, the differences in response characteristics depend on input power to the SOA. It is possible to increase the differences in response characteristics by increasing the output power of the TRx  1120  installed in the user terminal  1100  such that the input power to the SOA is increased. Note that the details and other information on the SOA characteristics are described in, for example, Reference Literature 1 “K. Sato et al., “Reduction of Mode Partition Noise by Using Semiconductor Optical Amplifiers,” IEEE Journal of Selected Topics in Quantum Electronics, vol. 7, No. 2, pp. 328-333, 2001.” 
     According to the present embodiment, the optical communication system includes the control signal superimposition unit  1200 ,  1200   a,  or  1200   b,  unlike the conventional optical communication system, so that even after connection between the user terminals  1100  has completed, the management and control functional unit  1300  can still transmit a control signal such as a wavelength management and control signal to the user terminals  1100 . 
     Second Embodiment 
     An optical communication system according to the present embodiment superimposes a control signal that provides instructions about wavelength switching somewhere along a communication path between subscriber terminal, and switches the wavelength for the subscriber terminal to a different wavelength. 
       FIG.  6    is a diagram illustrating the configuration of an optical communication system  2000  according to a second embodiment. The optical communication system  2000  includes a subscriber terminal  2100 , an optical gateway (GW)  2200 , a management and control functional unit  2300 , an optical SW control functional unit  2400 , and an optical GW  2500 . The subscriber terminal  2100  is an optical communication apparatus according to the conventional technique. For example, the subscriber terminal  2100  is the user terminal  1100  of the first embodiment. 
     The optical GW  2200  includes an optical SW  2210  and one or more control signal superimposition units  2220 . The optical SW  2210  includes N (N is an integer equal to or larger than 1) first ports  2211 , and M (M is an integer equal to or larger than 2) second ports  2212 . The N first ports  2211  are described as “first ports  2211 - 1  to  2211 -N,” respectively. The M second ports  2212  are described as “second ports  2212 - 1  to  2212 -M,” respectively. 
     The first port  2211  is connected to the subscriber terminal  2100  through an optical transmission path  2600 . Hereinafter, the optical transmission path  2600  connected to a first port  2211 - n  (n is an integer equal to or larger than 1, and equal to or smaller than N) is described as “optical transmission path  2600 - n .” The subscriber terminal  2100  connected to the optical transmission path  2600 - n  is described as “subscriber terminal  2100   a - n .” Subscriber terminals  2100   a - 1  to  2100   a -N are collectively referred to as “subscriber terminal  2100   a ,” or are described as “subscriber terminal  2100   a ” when any of the subscriber terminals  2100   a - 1  to  2100   a -N is not specified. 
     The second port  2212  is connected to an optical transmission path  2700 . Hereinafter, the optical transmission path  2700  connected to a second port  2212 - k  (k is an integer equal to or larger than 1, and equal to or smaller than M) is described as “optical transmission path  2700 - k .” Some of the second ports  2212  are connected to the management and control functional unit  2300 , while the other second ports  2212  are connected to an additional optical communication apparatus. In the present embodiment, the second port  2212 - 1  is connected to the management and control functional unit  2300  through an optical transmission path  2700 - 1 , while the second port  2212 - m  (m is an integer equal to or larger than 2, and equal to or smaller than M) is connected to an additional optical communication apparatus such as an optical SW through an optical transmission path  2700 - m.    
     The optical SW  2210  outputs an optical signal, input from any of the first ports  2211 , through the second port  2212  corresponding to the destination of this optical signal. The optical SW  2210  outputs an optical signal, input from any of the second ports  2212 , through the first port  2211  corresponding to the subscriber terminal  2100   a  as the destination of this optical signal. 
     The control signal superimposition unit  2220  is provided on the optical transmission path  2700 - m  (m is an integer equal to or larger than 2, and equal to or smaller than M). On some of the optical transmission paths  2700 - 2  to  2700 -M, the control signal superimposition unit  2220  may not be provided. The control signal superimposition unit  2220  superimposes a control signal, output by the management and control functional unit  2300 , on one or both of an optical signal whose transmission source is the subscriber terminal  2100   a  and an optical signal addressed to the subscriber terminal  2100   a.    
     As the control signal superimposition unit  2220 , the control signal superimposition unit  1200  of the first embodiment illustrated in  FIGS.  1  and  2    is used. In the control signal superimposition unit  2220 , the transmission-reception separation device  1210 - 1  ( FIGS.  1  and  2   ) is connected to the optical transmission path  2700  between the transmission-reception separation device  1210 - 1  and the optical SW  2210 , instead of being connected to the optical fiber  1500 - 1 , while the transmission-reception separation device  1210 - 2  ( FIGS.  1  and  2   ) is connected to the optical transmission path  2700  between the transmission-reception separation device  1210 - 2  and an optical SW different from the optical SW  2210 , instead of being connected to the optical fiber  1500 - 2 . If the management and control functional unit  2300  does not superimpose a control signal on an optical signal transmitted from the subscriber terminal  2100   a,  then the control signal superimposition unit  1200  that does not include the optical amplifier  1220 - 1  and the optical modulator  1230 - 1  may be used as the control signal superimposition unit  2220 . If the management and control functional unit  2300  does not superimpose a control signal on an optical signal addressed to the subscriber terminal  2100   a,  then the control signal superimposition unit  1200  that does not include the optical amplifier  1220 - 2  and the optical modulator  1230 - 2  may be used as the control signal superimposition unit  2220 . Alternatively, as the control signal superimposition unit  2220 , the control signal superimposition unit  1200   a  illustrated in  FIG.  3    or the control signal superimposition unit  1200   b  illustrated in  FIG.  4    may be used. 
     The management and control functional unit  2300  sets the wavelength to the subscriber terminal  2100  connected to the management and control functional unit  2300  through the optical SW  2210 . The management and control functional unit  2300  switches the path for the subscriber terminal  2100   a  to a different path in the optical SW  2210 , and thereafter outputs a control signal to be superimposed on an optical signal to the control signal superimposition unit  2220 . Examples of the control signal include a wavelength control signal, an on/off control signal for a TRx, and a bit-rate control signal. The wavelength control signal is a signal that provides instructions about the initial settings of the wavelength, wavelength offset calibration, wavelength switching, or the like. Note that the control signal is not limited to those signals described above. The optical SW control functional unit  2400  sets a path between the first port  2211  and the second port  2212  of the optical SW  2210 . 
     The optical GW  2500  includes an optical SW  2510 . The optical SW  2510  includes one or more first ports  2511 , and one or more second ports  2512 . The optical SW  2510  outputs an optical signal, input from any of the first ports  2511 , through the second port  2512  corresponding to the destination of this optical signal. The optical SW  2510  outputs an optical signal, input from any of the second ports  2512 , through the first port  2511  corresponding to the destination of this optical signal. The second port  2512  is connected to the subscriber terminal  2100  or an additional optical SW. The subscriber terminal  2100  connected to the optical GW  2500  is described as “subscriber terminal  2100   b .” The subscriber terminal  2100   b  may be located inside the optical GW  2500 , or may be located outside the optical GW  2500 . In addition, the optical GW  2500  may be of an identical configuration to the optical GW  2200 . 
     Operation of the optical communication system  2000  at the time of newly connecting the subscriber terminal  2100   a - 1  to a network is described below. First, the subscriber terminal  2100   a - 1  is connected to the management and control functional unit  2300  by a path P 0  through the optical SW  2210 . For example, the subscriber terminal  2100   a - 1  uses a wavelength λ 11  for transmission, while using a wavelength λ 12  for reception. The management and control functional unit  2300  transmits a control signal to the subscriber terminal  2100   a - 1  to instruct the subscriber terminal  2100   a - 1  to set the wavelength corresponding to a communication destination. For example, the communication destination is represented as a service to be utilized, or as a user at the communication destination. In this example, the communication destination for the subscriber terminal  2100   a - 1  is the subscriber terminal  2100   b,  and the subscriber terminal  2100   a - 1  sets a wavelength λ 21  for transmission, while setting a wavelength λ 22  for reception. In accordance with the timing of wavelength switching operation performed by the subscriber terminal  2100   a - 1 , the optical SW control functional unit  2400  switches the path P 0  between the first port  2211 - 1  and the second port  2212 - 1  in the optical SW  2210  to a path P 1  between the first port  2211 - 1  and the second port  2212 - 2 . Upon this switching, the subscriber terminal  2100   a - 1  starts communicating with the subscriber terminal  2100   b  that is a desired connection destination for the subscriber terminal  2100   a - 1 . 
     When the subscriber terminal  2100   a - 1  connects to an additional connection destination during communication with the subscriber terminal  2100   b,  it is necessary to set a wavelength to be used for the subscriber terminal  2100   a - 1 . Thus, the management and control functional unit  2300  outputs a control signal that instructs the subscriber terminal  2100   a - 1  to change the wavelength to a different wavelength to the control signal superimposition unit  2220  on the optical transmission path  2700 - 2 . The control signal superimposition unit  2220  removes a control signal from an optical signal transmitted from the subscriber terminal  2100   b  and addressed to the subscriber terminal  2100   a - 1 , and superimposes the control signal, output by the management and control functional unit  2300 , on the optical signal. According to the control signal superimposed on the received optical signal, the subscriber terminal  2100   a - 1  sets a wavelength λ 31  for transmission, while setting a wavelength λ 32  for reception. 
     The management and control functional unit  2300  outputs a control signal that instructs the subscriber terminal  2100   b  to change the wavelength or stop transmission at this wavelength, to the control signal superimposition unit  2220  on the optical transmission path  2700 - 2 . The control signal superimposition unit  2220  removes a control signal from an optical signal transmitted from the subscriber terminal  2100   a - 1  and addressed to the subscriber terminal  2100   b,  and superimposes the control signal, output by the management and control functional unit  2300 , on the optical signal. According to the control signal superimposed on the received optical signal, the subscriber terminal  2100   b  changes the wavelength or stops transmission at this wavelength. 
     In accordance with the timing of wavelength switching operation performed by the subscriber terminal  2100   a - 1 , the optical SW control functional unit  2400  switches the path P 1  to a path P 2  between the first port  2211 - 1  and the second port  2212 - 3  in the optical SW  2210 . Upon this switching, the subscriber terminal  2100   a - 1  starts communicating with a new communication destination. The control signal superimposition unit  2220  that superimposes a control signal on an optical signal to be transmitted/received by the subscriber terminal  2100   a - 1  switches from the control signal superimposition unit  2220  on the optical transmission path  2700 - 2  to the control signal superimposition unit  2220  on the optical transmission path  2700 - 3 . 
     Note that a splitter may be provided in the control signal superimposition unit  1200 , which is used as the control signal superimposition unit  2220 , between the transmission-reception separation device  1210 - 1  and the optical amplifier  1220 - 1  and between the transmission-reception separation device  1210 - 2  and the optical amplifier  1220 - 2 , or a splitter may be provided in the control signal superimposition unit  1200   a,  which is used as the control signal superimposition unit  2220 , between the transmission-reception separation device  1210 a- 1  and the optical SW  1250 - 1  and between the transmission-reception separation device  1210   a - 2  and the optical SW  1250 - 2 , or a splitter may be provided in the control signal superimposition unit  1200   b,  which is used as the control signal superimposition unit  2220 , between the transmission-reception separation device  1210 - 1  and the optical SW  1250 - 1  and between the transmission-reception separation device  1210 - 2  and the optical SW  1250 - 2 , such that the splitter may be used to split a part of an optical signal, and output it to the management and control functional unit  2300 . The management and control functional unit  2300  monitors a control signal set in the part of the split optical signal. 
     In a case where the subscriber terminal  2100  is connected by using a passive optical network (PON), even when the communication destination is changed, the same path and the same wavelength may still be used continuously, or different wavelengths may be used on the same path. In the above descriptions, the control signal provides instructions about wavelength switching, however, the control signal is not limited thereto. 
     In the present embodiment, the control signal superimposition unit  2220  is provided, so that even after the subscriber terminal  2100  has completed connection with its desired connection destination, the management and control functional unit  2300  can still transmit a wavelength management and control signal. 
     Note that an optical SW may use different ports for transmission to and for reception from a single unit of subscriber terminal  2100   a.    FIG.  7    is a diagram illustrating the configuration of an optical communication system  2000   a  when an optical SW uses different ports for transmission to and for reception from a single unit of subscriber terminal  2100   a.  The optical communication system  2000   a  illustrated in  FIG.  7    is different from the optical communication system  2000  illustrated in  FIG.  6    in that an optical GW  2200   a  is provided instead of the optical GW  2200 , and the subscriber terminal  2100   a  is connected to an optical transmission path  2650  instead of the optical transmission path  2600 . While  FIG.  7    only illustrates a single unit of subscriber terminal  2100   a,  a plurality of units of subscriber terminals  2100   a  can possibly be connected to the optical GW  2200   a.    
     The optical GW  2200   a  includes a transmission-reception separation device  2230 , the optical SW  2210 , and a control signal superimposition unit  2220   a.  While  FIG.  7    illustrates only a single control signal superimposition unit  2220   a,  the optical GW  2200   a  can possibly include a plurality of control signal superimposition units  2220   a.  The transmission-reception separation device  2230  is connected to the optical transmission path  2650 , and also connected to an optical transmission path  2600 - n   1  (n 1  is an integer equal to or larger than 1, and equal to or smaller than N), and an optical transmission path  2600 - n   2  (n 2 ≠n 1 , and n 2  is an integer equal to or larger than 2, and equal to or smaller than N). The transmission-reception separation device  2230  receives an input of an optical signal transmitted by the subscriber terminal  2100   a  from the optical transmission path  2650 , and outputs the input optical signal to the first port  2211 - n   1  of the optical SW  2210 . The transmission-reception separation device  2230  outputs the optical signal, output from the first port  2211 - n   2  of the optical SW  2210 , to the optical transmission path  2650 . 
     The optical SW  2210  receives an input of an optical signal, whose transmission source is the subscriber terminal  2100   a,  from the first port  2211 - n   1 , and outputs the input optical signal from the second port  2212 - m   1  (m 1  is an integer equal to or larger than 3, and equal to or smaller than M). The optical SW  2210  receives an input of an optical signal addressed to the subscriber terminal  2100   a  from the second port  2212 - m   2  (m 2 ≠m 1 , and m 2  is an integer equal to or larger than 3, and equal to or smaller than M), and outputs the input optical signal from the first port  2211 - n   2 . Note that although not illustrated in  FIG.  7   , the second ports  2212 - 1  and  2212 - 2  of the optical SW  2210  are connected to the management and control functional unit  2300 . The second port  2212 - 1  outputs a signal addressed to the management and control functional unit  2300  from the subscriber terminal  2100   a.  The second port  2212 - 2  receives an input of a signal transmitted from the management and control functional unit  2300  and addressed to the subscriber terminal  2100   a.    
     The control signal superimposition unit  2220   a  is connected to the second port  2212 - m   1  of the optical SW  2210  by the optical transmission path  2700 - m   1 , while being connected to the second port  2212 - m   2  of the optical SW  2210  by the optical transmission path  2700 - m   2 . The control signal superimposition unit  2220   a  includes splitters  2221 - 1  and  2221 - 2 , the optical amplifiers  1220 - 1  and  1220 - 2 , the optical modulators  1230 - 1  and  1230 - 2 , and a transmission-reception separation device  2222 . The splitter  2221 - 1 , the optical amplifier  1220 - 1 , and the optical modulator  1230 - 1  are provided on the optical transmission path  2700 - m   1 . The splitter  2221 - 2 , the optical amplifier  1220 - 2 , and the optical modulator  1230 - 2  are provided on the optical transmission path  2700 - m   2 . The transmission-reception separation device  2222  is connected to the optical transmission paths  2700 - m   1  and  2700 - m   2 , and also connected to an optical fiber between the transmission-reception separation device  2222  and an additional optical SW. 
     The splitter  2221 - 1  receives an input of an optical signal output by the optical SW  2210 , and splits the input optical signal into parts. The splitter  2221 - 1  outputs a part of the split optical signal to the management and control functional unit  2300 , while outputting the remaining part of the split optical signal to the optical amplifier  1220 - 1 . The splitter  2221 - 2  splits an optical signal input from the transmission-reception separation device  2222  into parts. The splitter  2221 - 2  outputs a part of the split optical signal to the management and control functional unit  2300 , while outputting the remaining part of the split optical signal to the optical amplifier  1220 - 2 . The transmission-reception separation device  2222  outputs a signal input from the optical transmission path  2700 - m   1  to an optical fiber connected to an additional optical SW, and outputs a signal input from the optical fiber connected to the additional optical SW to the optical transmission path  2700 - m   2 . 
     Operation of the optical communication system  2000   a  is described below. The transmission-reception separation device  2230  receives an input of an optical signal transmitted by the subscriber terminal  2100   a,  and outputs the input optical signal to the first port  2211 - n   1  of the optical SW  2210 . The optical SW  2210  outputs an optical signal, input from the first port  2211 - n   1 , through the second port  2212 - m   1  corresponding to the subscriber terminal  2100   b  as a destination of this optical signal. The splitter  2221 - 1  in the control signal superimposition unit  2220   a  splits an optical signal output from the second port  2212 - m   1  into parts. The splitter  2221 - 1  outputs a part of the split optical signal to the management and control functional unit  2300 , while outputting the remaining part of the split optical signal to the optical amplifier  1220 - 1 . The management and control functional unit  2300  monitors a control signal set in the part of the optical signal split by the splitter  2221 - 1 . 
     The optical amplifier  1220 - 1  removes the control signal on the low-frequency side from the input optical signal, and outputs the optical signal to the optical modulator  1230 - 1 . The optical modulator  1230 - 1  superimposes a control signal, input from the management and control functional unit  2300 , on the optical signal input from the optical amplifier  1220 - 1 , and outputs the superimposed signal to the transmission-reception separation device  2222 . The transmission-reception separation device  2222  outputs a signal input from the optical modulator  1230 - 1  to an additional optical SW connected to the subscriber terminal  2100   b.    
     The transmission-reception separation device  2222  receives an input of an optical signal transmitted by the subscriber terminal  2100   b  from the additional optical SW, and outputs the input optical signal to the optical transmission path  2700 - m   2 . The splitter  2221 - 2  receives an input of the optical signal output by the transmission-reception separation device  2222 , and splits the input optical signal into parts. The splitter  2221 - 2  outputs a part of the split optical signal to the management and control functional unit  2300 , while outputting the remaining part of the split optical signal to the optical amplifier  1220 - 2 . The management and control functional unit  2300  monitors a control signal set in the part of the optical signal split by the splitter  2221 - 2 . 
     The optical amplifier  1220 - 2  removes the control signal on the low-frequency side from the input optical signal, and outputs the optical signal to the optical modulator  1230 - 2 . The optical modulator  1230 - 2  superimposes a control signal, input from the management and control functional unit  2300 , on the optical signal input from the optical amplifier  1220 - 2 , and inputs the superimposed signal to the second port  2212 - m   2  of the optical SW  2210 . The optical SW  2210  outputs the optical signal, input from the second port  2212 - m   2 , through the first port  2211 - n   2  corresponding to the subscriber terminal  2100   a  as a destination of this optical signal. The transmission-reception separation device  2230  receives an input of the optical signal output from the first port  2211 - n   2 , and outputs the input optical signal to the optical transmission path  2650  between the transmission-reception separation device  2230  and the subscriber terminal  2100   a.    
     Note that if the management and control functional unit  2300  does not superimpose a control signal on an optical signal transmitted from the subscriber terminal  2100   a  and addressed to the subscriber terminal  2100   b,  then the control signal superimposition unit  2220   a  may not be provided with the optical amplifier  1220 - 1  and the optical modulator  1230 - 1 . If the management and control functional unit  2300  does not superimpose a control signal on an optical signal transmitted from the subscriber terminal  2100   b  and addressed to the subscriber terminal  2100   a,  then the control signal superimposition unit  2220   a  may not be provided with the optical amplifier  1220 - 2  and the optical modulator  1230 - 2 . 
     There is a case where the subscriber terminal  2100   b  is connected to an optical GW  2200   a  (described as “optical GW  2200   a - 2 ”) different from the optical GW  2200   a  (described as “optical GW  2200   a - 1 ”) connected to the subscriber terminal  2100   a.  In that case, the control signal superimposition unit  2220   a  in the optical GW  2200   a - 1  may not be provided with the optical amplifier  1220 - 2  and the optical modulator  1230 - 2 , and simultaneously, the control signal superimposition unit  2220   a  in the optical GW  2200   a - 2  may not be provided with the optical amplifier  1220 - 2  and the optical modulator  1230 - 2 . In this case, the control signal superimposition unit  2220   a  in the optical GW  2200   a - 1  superimposes a control signal for the subscriber terminal  2100   b  on an optical signal to be transmitted from the subscriber terminal  2100   a  to the subscriber terminal  2100   b.  In addition, the control signal superimposition unit  2220   a  in the optical GW  2200   a - 2  superimposes a control signal for the subscriber terminal  2100   a  on an optical signal to be transmitted from the subscriber terminal  2100   b  to the subscriber terminal  2100   a.    
     Alternatively, the control signal superimposition unit  2220   a  in the optical GW  2200   a - 1  may not be provided with the optical amplifier  1220 - 1  and the optical modulator  1230 - 1 , and simultaneously, the control signal superimposition unit  2220   a  in the optical GW  2200   a - 2  may not be provided with the optical amplifier  1220 - 1  and the optical modulator  1230 - 1 . In this case, the control signal superimposition unit  2220   a  in the optical GW  2200   a - 2  superimposes a control signal for the subscriber terminal  2100   b  on an optical signal to be transmitted from the subscriber terminal  2100   a  to the subscriber terminal  2100   b.  In addition, the control signal superimposition unit  2220   a  in the optical GW  2200   a - 1  superimposes a control signal for the subscriber terminal  2100   a  on an optical signal to be transmitted from the subscriber terminal  2100   b  to the subscriber terminal  2100   a.    
     According to the present embodiment, the optical communication system includes the control signal superimposition unit  2220  or  2220   a,  so that even after connection between the subscriber terminals  2100  has completed, the management and control functional unit  2300  can still transmit, to the subscriber terminals  2100 , a control signal for controlling an item included in the state information, such as a wavelength management and control signal. 
     Third Embodiment 
     In the second embodiment, a control signal is superimposed at the subsequent stage to an optical SW. In the present embodiment, a control signal is superimposed at the previous stage to an optical SW. 
       FIG.  8    illustrates the configuration of an optical communication system  3000  of the present embodiment. The optical communication system  3000  is different from the optical communication system  2000  of the second embodiment illustrated in  FIG.  6    in that an optical GW  3200  is provided instead of the optical GW  2200 . 
     The optical GW  3200  includes a control signal superimposition unit  3210  and the optical SW  2210 . The control signal superimposition unit  3210  is provided on the optical transmission path  2600 - n  between the subscriber terminal  2100   a - n  and the optical SW  2210 . The control signal superimposition unit  3210  superimposes a control signal, output by the management and control functional unit  2300 , on one or both of an optical signal transmitted by the subscriber terminal  2100   a - n  and an optical signal transmitted to the subscriber terminal  2100   a - n.    
     As the control signal superimposition unit  3210 , the control signal superimposition unit  1200  of the first embodiment illustrated in  FIGS.  1  and  2    is used. In the control signal superimposition unit  3210 , the transmission-reception separation device  1210 - 1  ( FIGS.  1  and  2   ) is connected to the optical transmission path  2600  between the transmission-reception separation device  1210 - 1  and the subscriber terminal  2100   a,  instead of being connected to the optical fiber  1500 - 1 , while the transmission-reception separation device  1210 - 2  ( FIGS.  1  and  2   ) is connected to the optical transmission path  2600  between the transmission-reception separation device  1210 - 2  and the optical SW  2210 . If the management and control functional unit  2300  does not superimpose a control signal on an optical signal transmitted from the subscriber terminal  2100   a,  then the control signal superimposition unit  1200  that does not include the optical amplifier  1220 - 1  and the optical modulator  1230 - 1  may be used as the control signal superimposition unit  3210 . If the management and control functional unit  2300  does not superimpose a control signal on an optical signal addressed to the subscriber terminal  2100   a,  then the control signal superimposition unit  1200  that does not include the optical amplifier  1220 - 2  and the optical modulator  1230 - 2  may be used as the control signal superimposition unit  3210 . Alternatively, as the control signal superimposition unit  3210 , the control signal superimposition unit  1200   a  of the first embodiment illustrated in  FIG.  3    or the control signal superimposition unit  1200   b  of the first embodiment illustrated in  FIG.  4    may be used. 
     The optical communication system  3000  operates in the same manner as in the second embodiment at the time of performing the initial settings and wavelength switching on the subscriber terminal  2100   a - 1  to be newly connected to the network. That is, the subscriber terminal  2100   a - 1  is connected to the management and control functional unit  2300  by the path P 0  between the first port  2211 - 1  and the second port  2212 - 1  through the optical SW  2210 . The management and control functional unit  2300  outputs a control signal for the initial settings to the optical SW  2210  to instruct the optical SW  2210  to set the wavelength and other information corresponding to a communication destination. The optical SW  2210  transmits the control signal output by the management and control functional unit  2300  to the subscriber terminal  2100   a - 1  through the path P 0 . In accordance with the timing of wavelength switching operation performed by the subscriber terminal  2100   a - 1 , the optical SW control functional unit  2400  switches the path P 0  to the path P 1  between the first port  2211 - 1  and the second port  2212 - 2 . The subscriber terminal  2100   a - 1  performs the initial settings such as wavelength switching according to the received control signal, and thereafter starts communicating with its desired connection destination through the path P 1 . 
     When the subscriber terminal  2100   a - 1  connects to an additional connection destination during the communication using the path P 1  in the optical SW  2210 , the management and control functional unit  2300  outputs a control signal that instructs the control signal superimposition unit  3210  on the optical transmission path  2600 - 1  to change the wavelength to a different wavelength. The control signal superimposition unit  3210  removes a control signal from an optical signal output by the optical SW  2210  and addressed to the subscriber terminal  2100   a - 1 , and superimposes the control signal, output by the management and control functional unit  2300 , on the optical signal. According to the control signal superimposed on the received optical signal, the subscriber terminal  2100   a - 1  sets the wavelength to be used for transmission, and the wavelength to be used for reception. 
     The management and control functional unit  2300  outputs, to the control signal superimposition unit  3210  on the optical transmission path  2600 - 1 , a control signal that instructs the subscriber terminal  2100  (not illustrated) as a communication destination for the subscriber terminal  2100   a - 1  to change the wavelength or stop transmission at this wavelength. The control signal superimposition unit  3210  removes a control signal from an optical signal transmitted from the subscriber terminal  2100   a - 1  and addressed to the subscriber terminal  2100  as a communication destination, and superimposes the control signal, output by the management and control functional unit  2300 , on the optical signal. In accordance with the timing of wavelength switching operation performed by the subscriber terminal  2100   a - 1 , the optical SW control functional unit  2400  switches the path P 1  to the path P 2  in the optical SW  2210 . Upon this switching, the subscriber terminal  2100   a - 1  starts communicating with a new communication destination. 
     Note that in the optical communication system  3000 , the control signal superimposition unit  3210  may superimpose a control signal for performing the initial settings on the subscriber terminal  2100   a - 1  to be newly connected to the network, and transmit the superimposed control signal. The subscriber terminal  2100   a - 1  is connected to the management and control functional unit  2300  by the path P 0  through the optical SW  2210 . The management and control functional unit  2300  outputs unmodulated light (or modulated light) to the optical SW  2210 . The optical SW  2210  outputs the light output by the management and control functional unit  2300  through the path P 0 . Further, the management and control functional unit  2300  outputs, to the control signal superimposition unit  3210  on the optical transmission path  2600 - 1 , a control signal for the initial settings to be transmitted to the subscriber terminal  2100   a - 1 . The control signal superimposition unit  3210  on the optical transmission path  2600 - 1  superimposes the control signal input from the management and control functional unit  2300  on the light input from the optical SW  2210 . The control signal superimposition unit  3210  outputs the optical signal with the control signal superimposed on the light. The subscriber terminal  2100   a - 1  receives the optical signal transmitted through the optical transmission path  2600 - 1 , and performs the initial settings according to the control signal set in the received optical signal. 
     Note that a splitter may be provided in the control signal superimposition unit  1200 , which is used as the control signal superimposition unit  3210 , between the transmission-reception separation device  1210 - 1  and the optical amplifier  1220 - 1  and between the transmission-reception separation device  1210 - 2  and the optical amplifier  1220 - 2 , or a splitter may be provided in the control signal superimposition unit  1200   a,  which is used as the control signal superimposition unit  3210 , between the transmission-reception separation device  1210   a - 1  and the optical SW  1250 - 1  and between the transmission-reception separation device  1210   a - 2  and the optical SW  1250 - 2 , or a splitter may be provided in the control signal superimposition unit  1200   b,  which is used as the control signal superimposition unit  3210 , between the transmission-reception separation device  1210 - 1  and the optical SW  1250 - 1  and between the transmission-reception separation device  1210 - 2  and the optical SW  1250 - 2 , such that the splitter may be used to split a part of an optical signal, and output it to the management and control functional unit  2300 . The management and control functional unit  2300  monitors a control signal set in the part of the split optical signal. 
     An optical SW may use different ports for transmission to and for reception from a single unit of subscriber terminal  2100   a.    FIG.  9    is a diagram illustrating the configuration of an optical communication system  3000   a  when an optical SW uses different ports for transmission to and for reception from a single unit of subscriber terminal  2100   a.  The optical communication system  3000   a  illustrated in  FIG.  9    is different from the optical communication system  3000  illustrated in  FIG.  8    in that an optical GW  3200   a  is provided instead of the optical GW  3200 , and the subscriber terminal  2100   a  is connected to the optical transmission path  2650  instead of the optical transmission path  2600 . While  FIG.  9    only illustrates a single unit of subscriber terminal  2100   a,  a plurality of units of subscriber terminals  2100   a  can possibly be connected to the optical GW  3200   a.    
     The optical GW  3200   a  includes a control signal superimposition unit  3210   a  and the optical SW  2210 . While  FIG.  9    illustrates only a single control signal superimposition unit  3210   a,  the optical GW  3200   a  can possibly include a plurality of control signal superimposition units  3210   a.  The control signal superimposition unit  3210   a  includes a transmission-reception separation device  3211 , the splitters  2221 - 1  and  2221 - 2 , the optical amplifiers  1220 - 1  and  1220 - 2 , and the optical modulators  1230 - 1  and  1230 - 2 . The splitter  2221 - 1 , the optical amplifier  1220 - 1 , and the optical modulator  1230 - 1  are provided on the optical transmission path  2600 - n   1  between the transmission-reception separation device  3211  and the first port  2211 - n   1  of the optical SW  2210 . The splitter  2221 - 2 , the optical amplifier  1220 - 2 , and the optical modulator  1230 - 2  are provided on the optical transmission path  2600 - n   2  between the transmission-reception separation device  3211  and the first port  2211 - n   2  of the optical SW  2210 . The transmission-reception separation device  3211  is connected to the optical transmission path  2650 , and to the optical transmission paths  2600 - n   1  and  2600 - n   2 . 
     The transmission-reception separation device  3211  receives an input of an optical signal transmitted by the subscriber terminal  2100   a  from the optical transmission path  2650 , and outputs the input optical signal to the optical transmission path  2600 - n   1 . The splitter  2221 - 1  receives an input of the optical signal output by the transmission-reception separation device  3211 , and splits the input optical signal into parts. The splitter  2221 - 1  outputs a part of the split optical signal to the management and control functional unit  2300 , while outputting the remaining part of the split optical signal to the optical amplifier  1220 - 1 . The optical amplifier  1220 - 1  removes the control signal on the low-frequency side from the input optical signal, and outputs the optical signal to the optical modulator  1230 - 1 . The optical modulator  1230 - 1  superimposes a control signal, input from the management and control functional unit  2300 , on the optical signal input from the optical amplifier  1220 - 1 , and outputs the superimposed signal to the optical transmission path  2600 - n   1 . 
     The splitter  2221 - 2  receives an input of an optical signal addressed to the subscriber terminal  2100   a  from the optical SW  2210 , and splits the input optical signal into parts. The splitter  2221 - 2  outputs a part of the split optical signal to the management and control functional unit  2300 , while outputting the remaining part of the split optical signal to the optical amplifier  1220 - 2 . The optical amplifier  1220 - 2  removes the control signal on the low-frequency side from the input optical signal, and outputs the optical signal to the optical modulator  1230 - 2 . The optical modulator  1230 - 2  superimposes a control signal, input from the management and control functional unit  2300 , on the optical signal input from the optical amplifier  1220 - 2 , and outputs the superimposed signal to the transmission-reception separation device  3211 . The transmission-reception separation device  3211  outputs the optical signal input from the optical modulator  1230 - 2  to the optical transmission path  2650  between the transmission-reception separation device  3211  and the subscriber terminal  2100   a.    
     If the management and control functional unit  2300  does not superimpose a control signal on an optical signal transmitted from the subscriber terminal  2100   a,  then the control signal superimposition unit  3210   a  may not be provided with the optical amplifier  1220 - 1  and the optical modulator  1230 - 1 . If the management and control functional unit  2300  does not superimpose a control signal on an optical signal addressed to the subscriber terminal  2100   a,  then the control signal superimposition unit  3210   a  may not be provided with the optical amplifier  1220 - 2  and the optical modulator  1230 - 2 . 
     There is a case where the subscriber terminal  2100  as a communication destination for the subscriber terminal  2100   a  is connected to an optical GW  3200   a  (described as “optical GW  3200   a - 2 ”) different from the optical GW  3200   a  (described as “optical GW  3200   a - 1 ”) connected to the subscriber terminal  2100   a.  In that case, the control signal superimposition unit  3210   a  in the optical GW  3200   a - 1  may not be provided with the optical amplifier  1220 - 2  and the optical modulator  1230 - 2 , and simultaneously, the control signal superimposition unit  3210   a  in the optical GW  3200   a - 2  may not be provided with the optical amplifier  1220 - 2  and the optical modulator  1230 - 2 . In this case, the control signal superimposition unit  3210   a  in the optical GW  3200   a - 1  superimposes a control signal for the subscriber terminal  2100  as a communication destination on an optical signal to be transmitted from the subscriber terminal  2100   a  to the subscriber terminal  2100  as the communication destination. In addition, the control signal superimposition unit  3210   a  in the optical GW  3200   a - 2  superimposes a control signal for the subscriber terminal  2100   a  on an optical signal to be transmitted from the subscriber terminal  2100  as the communication destination to the subscriber terminal  2100   a.    
     Alternatively, the control signal superimposition unit  3210   a  in the optical GW  3200   a - 1  may not be provided with the optical amplifier  1220 - 1  and the optical modulator  1230 - 1 , and simultaneously, the control signal superimposition unit  3210   a  in the optical GW  3200   a - 2  may not be provided with the optical amplifier  1220 - 1  and the optical modulator  1230 - 1 . In this case, the control signal superimposition unit  3210   a  in the optical GW  3200   a - 2  superimposes a control signal for the subscriber terminal  2100  as the communication destination on an optical signal to be transmitted from the subscriber terminal  2100   a  to the subscriber terminal  2100  as the communication destination. The control signal superimposition unit  3210   a  in the optical GW  3200   a - 1  superimposes a control signal for the subscriber terminal  2100   a  on an optical signal to be transmitted from the subscriber terminal  2100  as the communication destination to the subscriber terminal  2100   a.    
     According to the present embodiment, the optical communication system includes the control signal superimposition unit  3210  or  3210   a,  so that even after the subscriber terminal  2100  has completed connection with its desired connection destination, the management and control functional unit  2300  can still transmit a control signal, such as a wavelength management and control signal, to the subscriber terminal  2100 . In addition, since each control signal superimposition unit  3210  or  3210   a  is dedicated for each individual subscriber terminal  2100 , the management and control is more simplified compared to the second embodiment. 
     Fourth Embodiment 
     In the above embodiments, a control signal superimposed on an optical signal transmitted by a user terminal is eliminated by a filter somewhere along a communication path, and then an additional control signal is superimposed on the optical signal. In the present embodiment, a control signal superimposed on an optical signal transmitted by a user terminal is removed somewhere along a communication path by an inverted control signal, and then an additional control signal is superimposed on the optical signal. 
       FIG.  10    is a diagram illustrating the configuration of an optical communication system  4000  according to a fourth embodiment. The optical communication system  4000  includes a user terminal  4100 , a control signal superimposition unit  4200 , the management and control functional unit  1300 , and a user terminal  4300 . The user terminal  4100  and the user terminal  4300  are connected by an optical fiber  4400 . The control signal superimposition unit  4200  is provided on the optical fiber  4400 . 
     As the user terminal  4100 , an optical communication apparatus configured to transmit an optical signal according to the conventional technique can be used. The user terminal  4100  includes a signal mixer  4110  and an optical transmitter (Tx)  4120 . The signal mixer  4110  outputs the electrical signal E 1  to the Tx  4120 . In the electrical signal E 1 , the main signal M 1  and the control signal C 1  that are electrical signals with different frequencies have been superimposed on one another. The frequency of the control signal C 1  is lower than the frequency of the main signal M 1 . For example, as the control signal C 1 , the AMCC is used. The Tx  4120  converts the electrical signal E 1  to the optical signal G 11 , and outputs the optical signal G 11  to the optical fiber  4400 . 
     The control signal superimposition unit  4200  includes a splitter  4210 , a photo diode (PD)  4220 , a signal processing unit  4230 , an optical modulator  4240 , and an optical modulator  4250 . The splitter  4210 , the optical modulator  4240 , and the optical modulator  4250  are provided on the optical fiber  4400 . 
     The splitter  4210  receives an input of the optical signal G 11  output by the user terminal  4100  from the optical fiber  4400 . The splitter  4210  splits the input optical signal G 11  into parts, and outputs a part of the split optical signal G 11  to the PD  4220 , while outputting the remaining part of the split optical signal G 11  to the optical modulator  4240 . 
     The PD  4220  converts the part of the optical signal G 11  split by the splitter  4210  to an electrical signal E 11 , and outputs the converted electrical signal E 11  to the signal processing unit  4230 . 
     The signal processing unit  4230  reads the control signal C 1  from the electrical signal E 11  output by the PD  4220 , and generates an inverted signal E 12  that is the inverted control signal C 1 . The inverted signal E 12  is a signal with its phase inverted from the original control signal C 1 . Since the control signal C 1  falls within the low frequency range, it is possible for the signal processing unit  4230  to generate the inverted signal E 12  without performing costly signal processing. The signal processing unit  4230  outputs the generated inverted signal E 12  to the optical modulator  4240 . 
     The optical modulator  4240  is an LN modulator, an EA modulator, an SOA, or the like. The optical modulator  4240  modulates the optical signal G 11  input from the splitter  4210  again based on the inverted signal E 12  input from the signal processing unit  4230 , and removes the control signal C 1  from the optical signal G 11 . The main signal M 1  is set in the optical signal G 12  with the control signal C 1  removed. The optical modulator  4240  outputs the optical signal G 12  to the optical modulator  4250 . 
     The optical modulator  4250  superimposes the control signal C 3 , input from the management and control functional unit  1300 , on the low-frequency side of the optical signal G 12  to generate an optical signal G 13 , and outputs the optical signal G 13  to the user terminal  4300 . As the optical modulator  4250 , the optical modulator  1230 - 1  in the first embodiment illustrated in  FIGS.  1  and  2    can be used. 
     As the user terminal  4300 , an optical communication apparatus configured to receive an optical signal according to the conventional technique can be used. The user terminal  4300  includes an optical receiver (Rx)  4310  and a signal divider  4320 . The Rx  4310  receives the optical signal G 13 , output by the control signal superimposition unit  4200 , from the optical fiber  4400 , converts the received optical signal G 13  to an electrical signal E 13 , and outputs the electrical signal E 13 . The signal divider  4320  separates the electrical signal E 13 , output by the Rx  4310 , into the main signal M 1  and the control signal C 3  based on their respective frequencies. 
     Note that some or all of the functional units in the control signal superimposition unit  4200  may be included in the management and control functional unit  1300 . According to the present embodiment, the optical communication system includes the control signal superimposition unit  4200 , so that even after connection between the user terminal  4100  and the user terminal  4300  has completed, the management and control functional unit  1300  can still transmit a control signal to the user terminal  4300 . 
     Although not illustrated in  FIG.  10   , the PD  4220  or the signal processing unit  4230  may be connected to the management and control functional unit  1300 . The PD  4220  or the signal processing unit  4230  transmits information in a control signal superimposed in the user terminal  4100  to the management and control functional unit  1300 . This allows the management and control functional unit  1300  to monitor information in a control signal transmitted between user terminals. 
     Note that if the control signal superimposition unit  4200  does not superimpose a control signal, then the control signal superimposition unit  4200  allows a signal transmitted from the user terminal  4100  to directly pass through the transmission path without applying a signal to the optical modulator  4240  and the optical modulator  4250 . In this case, an SOA cannot be used in principle. 
     Fifth Embodiment 
     In the fourth embodiment, a control signal superimposed on an optical signal transmitted by a user terminal is removed somewhere along a communication path by an inverted control signal, and then an additional control signal is superimposed on the optical signal. In the present embodiment, an optical signal is modulated somewhere along a communication path by using a signal in which an inverted control signal and an additional control signal have been superimposed on one another. The present embodiment is described below, mainly focusing on the differences from the fourth embodiment. 
       FIG.  11    is a diagram illustrating the configuration of an optical communication system  4000   a  of the present embodiment. The optical communication system  4000   a  illustrated in  FIG.  11    is different from the optical communication system  4000  of the fourth embodiment illustrated in  FIG.  10    in that a control signal superimposition unit  4200   a  is provided instead of the control signal superimposition unit  4200 . 
     The control signal superimposition unit  4200   a  includes the splitter  4210 , the PD  4220 , the signal processing unit  4230 , a superimposition unit  4260 , and an optical modulator  4270 . The superimposition unit  4260  outputs a superimposed signal E 14  to the optical modulator  4270 . The superimposed signal E 14  is an electrical signal obtained by superimposing the inverted signal E 12  input from the signal processing unit  4230 , and the control signal C 3  input from the management and control functional unit  1300  on one another. The optical modulator  4270  modulates the optical signal G 11  input from the splitter  4210  again based on the superimposed signal E 14  input from the superimposition unit  4260 , and generates the optical signal G 13  with the control signal C 1  removed and with the control signal C 3  superimposed on the main signal M 1 . The optical modulator  4270  outputs the generated optical signal G 13  to the user terminal  4300 . 
     Note that some or all of the functional units in the control signal superimposition unit  4200   a  may be included in the management and control functional unit  1300 . According to the present embodiment, the number of optical modulators to be used in a control signal superimposition unit can be reduced, compared to the fourth embodiment. 
     In the same manner as in the fourth embodiment, the PD  4220  or the signal processing unit  4230  may be connected to the management and control functional unit  1300 . The PD  4220  or the signal processing unit  4230  transmits information in a control signal superimposed in the user terminal  4100  to the management and control functional unit  1300 . This allows the management and control functional unit  1300  to monitor information in a control signal transmitted between user terminals. Note that if the control signal superimposition unit  4200   a  does not superimpose a control signal, then the control signal superimposition unit  4200   a  allows a signal transmitted from the user terminal  4100  to directly pass through the transmission path without applying a signal to the optical modulator  4270 . 
     Sixth Embodiment 
     In the present embodiment, an optical communication system that switches the wavelength to a different wavelength during communication between user terminals uses the control signal superimposition unit of the fifth embodiment or a sixth embodiment to superimpose a control signal somewhere along a communication path. The present embodiment is described below, mainly focusing on the differences from the above embodiments. 
       FIG.  12    is a diagram illustrating the configuration of an optical communication system  5000  according to the sixth embodiment. The optical communication system  5000  is different from the optical communication system  2000  of the second embodiment illustrated in  FIG.  6    in that an optical GW  5200  is provided instead of the optical GW  2200 . The optical GW  5200  is different from the optical GW  2200  illustrated in  FIG.  6    in that a control signal superimposition unit  5220  is provided instead of the control signal superimposition unit  2220 . In the optical GW  5200 , the control signal superimposition unit  5220  is provided on the optical transmission path  2700 - m  (m is an integer equal to or larger than 2, and equal to or smaller than M), while  FIG.  12    illustrates only a single control signal superimposition unit  5220 . On some of the optical transmission paths  2700 - 2  to  2700 -M, the control signal superimposition unit  5220  may not be provided. 
     The control signal superimposition unit  5220  includes transmission-reception separation devices  5221 - 1  and  5221 - 2 , and superimposition units  5222 - 1  and  5222 - 2 . The transmission-reception separation device  5221 - 1  outputs an optical signal, output by the optical SW  2210 , to the superimposition unit  5222 - 1 , and outputs an optical signal, output by the superimposition unit  5222 - 2 , to the optical SW  2210 . The transmission-reception separation device  5221 - 2  outputs an optical signal, output by the superimposition unit  5222 - 1 , to an additional optical SW such as the optical SW  2510 , and outputs an optical signal, input from an additional optical SW such as the optical SW  2510 , to the superimposition unit  5222 - 2 . 
     The superimposition units  5222 - 1  and  5222 - 2  are either the control signal superimposition unit  4200  of the fourth embodiment illustrated in  FIG.  10    or the control signal superimposition unit  4200   a  of the fifth embodiment illustrated in  FIG.  11   . The superimposition unit  5222 - 1  removes a control signal set by the subscriber terminal  2100   a  from an optical signal transmitted by the subscriber terminal  2100   a,  and superimposes a control signal, output by the management and control functional unit  2300 , on the optical signal. In the superimposition unit  5222 - 1 , the splitter  4210  ( FIGS.  10  and  11   ) splits the optical signal input from the transmission-reception separation device  5221 - 1  into parts, while the optical modulator  4250  ( FIG.  10   ) or the optical modulator  4270  ( FIG.  11   ) outputs the optical signal to the transmission-reception separation device  5221 - 2 . The superimposition unit  5222 - 2  removes a control signal set by a transmission source of an optical signal addressed to the subscriber terminal  2100   a  from the optical signal, and superimposes a control signal, output by the management and control functional unit  2300 , on the optical signal. In the superimposition unit  5222 - 2 , the splitter  4210  ( FIGS.  10  and  11   ) splits the optical signal input from the transmission-reception separation device  5221 - 2  into parts, while the optical modulator  4250  ( FIG.  10   ) or the optical modulator  4270  ( FIG.  11   ) outputs the optical signal to the transmission-reception separation device  5221 - 1 . 
     Note that an optical SW may use different ports for transmission to and for reception from a single unit of subscriber terminal  2100   a.    FIG.  13    is a diagram illustrating the configuration of an optical communication system  5000   a  when an optical SW uses different ports for transmission to and for reception from a single unit of subscriber terminal  2100   a.  The optical communication system  5000   a  illustrated in  FIG.  13    is different from the optical communication system  2000   a  illustrated in  FIG.  7    in that an optical GW  5200   a  is provided instead of the optical GW  2200   a.  While  FIG.  13    only illustrates a single unit of subscriber terminal  2100   a,  a plurality of units of subscriber terminals  2100   a  can possibly be connected to the optical GW  5200   a.    
     The optical GW  5200   a  includes the transmission-reception separation device  2230 , the optical SW  2210 , and a control signal superimposition unit  5220   a.  While  FIG.  13    illustrates only a single control signal superimposition unit  5220   a,  the optical GW  5200   a  can possibly include a plurality of control signal superimposition units  5220   a.  The control signal superimposition unit  5220   a  is different from the control signal superimposition unit  2220   a  illustrated in  FIG.  7    in that the superimposition unit  5222 - 1  is provided instead of the optical amplifier  1220 - 1  and the optical modulator  1230 - 1 , and the superimposition unit  5222 - 2  is provided instead of the optical amplifier  1220 - 2  and the optical modulator  1230 - 2 . 
     Operation of the optical communication system  5000   a  is described below. An optical signal transmitted by the subscriber terminal  2100   a  is output from the second port  2212 - m   1  of the optical SW  2210  in the same manner as the optical communication system  2000   a  illustrated in  FIG.  7   . The splitter  2221 - 1  in the control signal superimposition unit  5220   a  splits the optical signal, output from the second port  2212 - m   1  by the optical SW  2210 , into parts. The splitter  2221 - 1  outputs a part of the split optical signal to the management and control functional unit  2300 , while outputting the remaining part of the split optical signal to the superimposition unit  5222 - 1 . The management and control functional unit  2300  monitors a control signal set in the part of the optical signal split by the splitter  2221 - 2 . 
     The superimposition unit  5222 - 1  removes a control signal set by the subscriber terminal  2100   a  from the optical signal transmitted by the subscriber terminal  2100   a,  and superimposes an additional control signal, output by the management and control functional unit  2300 , on the optical signal. The superimposition unit  5222 - 1  outputs the optical signal with the control signal removed and with the additional control signal superimposed to the transmission-reception separation device  2222 . The transmission-reception separation device  2222  outputs the signal, input from the superimposition unit  5222 - 1 , to an additional optical SW connected to the subscriber terminal  2100   b.    
     The transmission-reception separation device  2222  receives an input of an optical signal, transmitted by the subscriber terminal  2100   b  and addressed to the subscriber terminal  2100   a,  from an additional optical SW, and outputs the input optical signal to the optical transmission path  2700 - m   2 . The splitter  2221 - 2  splits the optical signal input from the transmission-reception separation device  2222  into parts. The splitter  2221 - 2  outputs a part of the split optical signal to the management and control functional unit  2300 , while outputting the remaining part of the split optical signal to the superimposition unit  5222 - 2 . The management and control functional unit  2300  monitors a control signal set in the part of the optical signal split by the splitter  2221 - 2 . 
     The superimposition unit  5222 - 2  removes a control signal set by a counterpart subscriber terminal  2100   b  from an optical signal addressed to the subscriber terminal  2100   a,  and superimposes an additional control signal, output by the management and control functional unit  2300 , on the optical signal. The superimposition unit  5222 - 2  inputs the optical signal with the control signal removed and with the additional control signal superimposed to the second port  2212 - m   2  of the optical SW  2210 . The optical SW  2210  outputs the optical signal, input from the second port  2212 - m   2 , through the first port  2211 - n   2  corresponding to the subscriber terminal  2100   a  as a destination of this optical signal. The transmission-reception separation device  2230  outputs the optical signal output from the first port  2211 - n   2  to the optical transmission path  2650 . 
     Note that if the management and control functional unit  2300  does not superimpose a control signal on an optical signal transmitted from the subscriber terminal  2100   a,  then each of the control signal superimposition units  5220  and  5220   a  may not be provided with the superimposition unit  5222 - 1 . If the management and control functional unit  2300  does not superimpose a control signal on an optical signal addressed to the subscriber terminal  2100   a,  then each of the control signal superimposition units  5220  and  5220   a  may not be provided with the superimposition unit  5222 - 2 . 
     There is a case where the subscriber terminal  2100  as a communication destination for the subscriber terminal  2100   a  is connected to an optical GW  5200  or  5200   a  different from the optical GW  5200  or  5200   a  connected to the subscriber terminal  2100   a.  In that case, the superimposition unit  5222 - 2  may not be provided in the control signal superimposition unit  5220  or  5220   a  in the optical GW  5200  or  5200   a  to which the subscriber terminal  2100   a  is connected, and simultaneously, the superimposition unit  5222 - 2  may not be provided in the control signal superimposition unit  5220  or  5220   a  in the optical GW  5200  or  5200   a  to which the subscriber terminal  2100  as a communication destination is connected. In this case, the control signal superimposition unit  5220  or  5220   a  in the optical GW  5200  or  5200   a,  to which the subscriber terminal  2100   a  is connected, superimposes a control signal for the subscriber terminal  2100  as a communication destination on an optical signal to be transmitted from the subscriber terminal  2100   a  to the subscriber terminal  2100  as a communication destination. In addition, the control signal superimposition unit  5220  or  5220   a  in the optical GW  5200  or  5200   a,  to which the subscriber terminal  2100  as a communication destination is connected, superimposes a control signal for the subscriber terminal  2100   a  on an optical signal to be transmitted from the subscriber terminal  2100  as a communication destination to the subscriber terminal  2100   a.    
     Likewise, there is a case where the subscriber terminal  2100  as a communication destination for the subscriber terminal  2100   a  is connected to an optical GW  5200  or  5200   a  different from the optical GW  5200  or  5200   a  connected to the subscriber terminal  2100   a.  In that case, the superimposition unit  5222 - 1  may not be provided in the control signal superimposition unit  5220  or  5220   a  in the optical GW  5200  or  5200   a  to which the subscriber terminal  2100   a  is connected, and simultaneously, the superimposition unit  5222 - 1  may not be provided in the control signal superimposition unit  5220  or  5220   a  in the optical GW  5200  or  5200   a  to which the subscriber terminal  2100  as a communication destination is connected. In this case, the control signal superimposition unit  5220  or  5220   a  in the optical GW  5200  or  5200   a,  to which the subscriber terminal  2100  as a communication destination is connected, superimposes a control signal for the subscriber terminal  2100  as a communication destination on an optical signal to be transmitted from the subscriber terminal  2100   a  to the subscriber terminal  2100   b.  In addition, the control signal superimposition unit  5220  or  5220   a  in the optical GW  5200  or  5200   a,  to which the subscriber terminal  2100   a  is connected, superimposes a control signal for the subscriber terminal  2100   a  on an optical signal to be transmitted from the subscriber terminal  2100  as a communication destination to the subscriber terminal  2100   a.    
     According to the present embodiment, the optical communication system includes the control signal superimposition unit  5220  or  5220   a,  so that even after connection between the subscriber terminals  2100  has completed, the management and control functional unit  2300  can still transmit a control signal, such as a wavelength management and control signal, to the subscriber terminals  2100 . 
     Seventh Embodiment 
     In the sixth embodiment, a control signal is superimposed at the subsequent stage to an optical SW. In the present embodiment, a control signal is superimposed at the previous stage to an optical SW. The present embodiment is described below, mainly focusing on the differences from the above embodiments. 
       FIG.  14    is a diagram illustrating the configuration of an optical communication system  6000  according to a seventh embodiment. The optical communication system  6000  is different from the optical communication system  3000  of the third embodiment illustrated in  FIG.  8    in that an optical GW  6200  is provided instead of the optical GW  3200 . The optical GW  6200  is different from the optical GW  3200  illustrated in  FIG.  8    in that the control signal superimposition unit  5220  according to the sixth embodiment illustrated in 
       FIG.  12    is provided instead of the control signal superimposition unit  3210 . 
     Note that the management and control functional unit  2300  may transmit a control signal for performing the initial settings on the subscriber terminal  2100   a  through the optical SW  2210  in the same manner as in the third embodiment, or may transmit the control signal superimposed by the control signal superimposition unit  5220 . In a case where a control signal is superimposed and transmitted by the control signal superimposition unit  5220 , the management and control functional unit  2300  outputs unmodulated light (or modulated light) to the optical SW  2210 , and further outputs the control signal to the control signal superimposition unit  5220 . The optical SW  2210  outputs the light input from the management and control functional unit  2300  to the optical transmission path  2600 . The control signal superimposition unit  5220  superimposes the control signal, input from the management and control functional unit  2300 , on the light input from the optical SW  2210 , and outputs the optical signal with the control signal superimposed to the subscriber terminal  2100   a.    
     Note that an optical SW may use different ports for transmission to and for reception from a single unit of subscriber terminal  2100   a.    FIG.  15    is a diagram illustrating the configuration of an optical communication system  6000   a  when an optical SW uses different ports for transmission to and for reception from a single unit of subscriber terminal  2100   a.  The optical communication system  6000   a  illustrated in  FIG.  15    is different from the optical communication system  3000   a  illustrated in  FIG.  9    in that an optical GW  6200   a  is provided instead of the optical GW  3200   a.  While  FIG.  15    only illustrates a single unit of subscriber terminal  2100   a,  a plurality of units of subscriber terminals  2100   a  can possibly be connected to the optical GW  6200   a.    
     The optical GW  6200   a  includes a control signal superimposition unit  6210  and the optical SW  2210 . While  FIG.  15    illustrates only a single control signal superimposition unit  6210 , the optical GW  6200   a  can possibly include a plurality of control signal superimposition units  6210 . The control signal superimposition unit  6210  is different from the control signal superimposition unit  3210   a  illustrated in  FIG.  9    in that the superimposition unit  5222 - 1  is provided instead of the optical amplifier  1220 - 1  and the optical modulator  1230 - 1 , and the superimposition unit  5222 - 2  is provided instead of the optical amplifier  1220 - 2  and the optical modulator  1230 - 2 . 
     The superimposition unit  5222 - 1  removes a control signal set by the subscriber terminal  2100   a  from an optical signal transmitted by the subscriber terminal  2100   a,  and superimposes a control signal, output by the management and control functional unit  2300 , on the optical signal. In the superimposition unit  5222 - 1 , the splitter  4210  ( FIGS.  10  and  11   ) splits an optical signal input from the optical transmission path  2650  into parts, and the optical modulator  4250  ( FIG.  10   ) or the optical modulator  4270  ( FIG.  11   ) outputs the optical signal to the optical SW  2210 . The superimposition unit  5222 - 2  removes a control signal from an optical signal addressed to the subscriber terminal  2100   a,  and superimposes a control signal, output by the management and control functional unit  2300 , on the optical signal. In the superimposition unit  5222 - 2 , the splitter  4210  ( FIGS.  10  and  11   ) splits an optical signal input from the optical SW  2210  into parts, and the optical modulator  4250  ( FIG.  10   ) or the optical modulator  4270  ( FIG.  11   ) outputs the optical signal to the optical transmission path  2650 . 
     According to the present embodiment, even after the subscriber terminal  2100  has completed connection with its desired connection destination, the management and control functional unit  2300  can still transmit a control signal, such as a wavelength management and control signal, to the subscriber terminal  2100 . In addition, since each control signal superimposition unit  5220  or  6210  is dedicated for each individual subscriber terminal  2100 , the management and control is more simplified compared to the sixth embodiment. 
     Eighth Embodiment 
     In the present embodiment, a user utilizes multiple services.  FIG.  16    is a diagram illustrating the configuration of an optical communication system  7000  according to an eighth embodiment. The optical communication system  7000  includes a user terminal  7100 , a control signal superimposition unit  7200 , the management and control functional unit  1300 , a wavelength division multiplexer  7300 , a user terminal  7400 , and a user terminal  7500 . The user terminal  7100  and the control signal superimposition unit  7200  are connected by an optical fiber  7610 . The wavelength division multiplexer  7300  is connected to the control signal superimposition unit  7200  by an optical fiber  7620 , connected to the user terminal  7400  by an optical fiber  7630 , and connected to the user terminal  7500  by an optical fiber  7640 . 
     The user terminal  7100  utilizes two or more services. An example case is now described in which the user terminal  7100  utilizes a service # 1  provided by the user terminal  7400 , and a service # 2  provided by the user terminal  7500 . For example, the user terminal  7400  provides a data communication service, while the user terminal  7500  provides a video-related service of analog signals and other signals. Note that the multiple services to be utilized by the user terminal  7100  are not limited to those described above, but the user terminal  7100  can utilize any service. These services use optical signals with different wavelengths. Any of the multiple services uses a low-frequency control signal. 
     The user terminal  7100  includes the signal mixer  1110 , the TRx  1120 , the signal divider  1130 , a TRx  7110 , and a wavelength division multiplexer  7120 . The signal mixer  1110  outputs, to the TRx  1120 , a transmission signal for the service # 1  in which the main signal M 1  and the control signal  01  that are both electrical signals have been superimposed on one another. The TRx  1120  converts the transmission signal from an electrical signal to an optical signal, and outputs the optical signal to the wavelength division multiplexer  7120 . The TRx  1120  converts the optical signal for the service # 1 , input from the wavelength division multiplexer  7120 , to an electrical signal, and outputs the electrical signal. The signal divider  1130  separates the electrical signal converted by the TRx  1120  into the main signal and the control signal based on their respective wavelengths. The TRx  7110  converts a transmission signal for the service # 2  from an electrical signal to an optical signal, and outputs the optical signal to the wavelength division multiplexer  7120 . The TRx  7110  converts the optical signal for the service # 2 , input from the wavelength division multiplexer  7120 , to an electrical signal, and outputs the electrical signal. 
     The wavelength division multiplexer  7120  multiplexes the optical signal output by the TRx  1120  and the optical signal output by the TRx  7110  together, and outputs the multiplexed optical signal to the optical fiber  7610 . The wavelength division multiplexer  7120  receives an input of the optical signal transmitted through the optical fiber  7610 , and divides the input optical signal into an optical signal for the service # 1  and an optical signal for the service # 2  based on their respective wavelengths. The wavelength division multiplexer  7120  outputs the optical signal for the service # 1  to the TRx  1120 , while outputting the optical signal for the service # 2  to the TRx  7110 . 
     The control signal superimposition unit  7200  is either the control signal superimposition unit  1200  of the first embodiment illustrated in  FIGS.  1  and  2   , the control signal superimposition unit  1200   a  of the first embodiment illustrated in  FIG.  3   , the control signal superimposition unit  1200   b  of the first embodiment illustrated in  FIG.  4   , the control signal superimposition unit  4200  of the fourth embodiment illustrated in  FIG.  10   , the control signal superimposition unit  4200   a  of the fifth embodiment illustrated in  FIG.  11   , or the control signal superimposition unit  5220  of the sixth embodiment illustrated in  FIG.  12   . 
     The wavelength division multiplexer  7300  receives an input of an optical signal transmitted through the optical fiber  7620 , and divides the input optical signal into an optical signal for the service # 1  and an optical signal for the service # 2  based on their respective wavelengths. The wavelength division multiplexer  7300  outputs the optical signal for the service # 1  to the optical fiber  7630 , while outputting the optical signal for the service # 2  to the optical fiber  7640 . The wavelength division multiplexer  7120  multiplexes the optical signal for the service # 1  transmitted through the optical fiber  7630 , and the optical signal for the service # 2  transmitted through the optical fiber  7640  together, and outputs the multiplexed optical signal to the optical fiber  7620 . 
     The user terminal  7400  includes a TRx  7410 . The TRx  7410  converts the optical signal for the service # 1  transmitted through the optical fiber  7630  to an electrical signal, and converts the electrical signal for the service # 1  to an optical signal to output the optical signal to the optical fiber  7630 . As the user terminal  7400 , for example, the user terminal  1100  may be used. 
     The user terminal  7500  includes a TRx  7510 . The TRx  7510  converts the optical signal for the service # 2  transmitted through the optical fiber  7640  to an electrical signal, and converts the electrical signal for the service # 2  to an optical signal to output the optical signal to the optical fiber  7640 . 
     Operation of the optical communication system  7000  is described below. The user terminal  7100  outputs an optical signal obtained by multiplexing an optical signal for the service # 1  and an optical signal for the service # 2  together. In the optical signal for the service # 1 , the main signal M 1  and the control signal C 1  that are both electrical signals have been superimposed on one another. The control signal superimposition unit  7200  removes the control signal C 1  for the service # 1  from the optical signal transmitted by the user terminal  7100 , and superimposes the control signal C 3 - 1 , output by the management and control functional unit  1300 , on the optical signal to output the superimposed optical signal to the optical fiber  7620 . 
     The wavelength division multiplexer  7300  divides the optical signal output by the control signal superimposition unit  7200  into an optical signal for the service # 1  and an optical signal for the service # 2  based on their respective wavelengths. The wavelength division multiplexer  7300  outputs the optical signal for the service # 1  to the optical fiber  7630 , while outputting the optical signal for the service # 2  to the optical fiber  7640 . The TRx  7410  in the user terminal  7400  receives the optical signal for the service # 1  to convert the optical signal to an electrical signal, and separates the electrical signal into the main signal M 1  and the control signal C 3 - 1 . The TRx  7510  in the user terminal  7500  converts an electrical signal for the service # 2  to an optical signal. 
     The TRx  7410  in the user terminal  7400  converts the electrical signal for the service # 1  to an optical signal, and outputs the converted optical signal. The TRx  7410  may convert an electrical signal, in which the main signal M 2  and the control signal C 2  have been superimposed on one another, to an optical signal. The TRx  7510  in the user terminal  7500  converts the electrical signal for the service # 2  to an optical signal, and outputs the converted optical signal. The wavelength division multiplexer  7300  multiplexes the optical signal for the service # 1  output by the user terminal  7400 , and the optical signal for the service # 2  output by the user terminal  7500  together, and outputs the multiplexed optical signal to the optical fiber  7620 . 
     The control signal superimposition unit  7200  removes the control signal C 2  for the service # 1  from the optical signal output by the wavelength division multiplexer  7300 , and superimposes the control signal C 3 - 2 , output by the management and control functional unit  1300 , on the optical signal to output the superimposed optical signal to the optical fiber  7610 . The wavelength division multiplexer  7120  in the user terminal  7100  receives an input of the optical signal output by the control signal superimposition unit  7200 , and divides the input optical signal into an optical signal for the service # 1  and an optical signal for the service # 2  based on their respective wavelengths. The TRx  1120  receives the optical signal for the service # 1  to convert the optical signal to an electrical signal. The signal divider  1130  separates the electrical signal converted by the TRx  1120  into the main signal M 2  and the control signal C 3 - 2 . The TRx  7110  converts an electrical signal for the service # 2  to an optical signal. 
     Ninth Embodiment 
     In the eighth embodiment, optical signals for multiple services are multiplexed into an optical signal to be input to the control signal superimposition unit. In the present embodiment, an optical signal for a control-signal rewriting service, which is one of the multiple services to be utilized by a user, is input to the control signal superimposition unit. In the present embodiment, the differences from the eighth embodiment are mainly described below. 
       FIG.  17    is a diagram illustrating the configuration of an optical communication system  7000   a  according to a ninth embodiment. The optical communication system  7000   a  includes the user terminal  7100 , a wavelength division multiplexer  7600 , the control signal superimposition unit  7200 , the user terminal  7400 , and a service providing apparatus  7700 . The wavelength division multiplexer  7600  is connected to the user terminal  7100  by an optical fiber  7810 , connected to the control signal superimposition unit  7200  by an optical fiber  7820 , and connected to the service providing apparatus  7700  by an optical fiber  7830 . The user terminal  7400  is connected to the control signal superimposition unit  7200  by an optical fiber  7840 . 
     The wavelength division multiplexer  7600  receives an input of an optical signal transmitted through the optical fiber  7810 , and divides the input optical signal into an optical signal for the service # 1  and an optical signal for the service # 2  based on their respective wavelengths. The wavelength division multiplexer  7600  outputs the optical signal for the service # 1  to the optical fiber  7820 , while outputting the optical signal for the service # 2  to the optical fiber  7830 . The wavelength division multiplexer  7600  multiplexes the optical signal for the service # 1  transmitted through the optical fiber  7820 , and the optical signal for the service # 2  transmitted through the optical fiber  7830  together, and outputs the multiplexed optical signal to the optical fiber  7810 . 
     The service providing apparatus  7700  provides the service # 2 . The service providing apparatus  7700  may be installed on the network such as in a communications station in the same manner as the control signal superimposition unit  7200 . The service providing apparatus  7700  includes a TRx  7710 . The TRx  7710  converts the optical signal for the service # 2  transmitted through the optical fiber  7830  to an electrical signal, and converts the electrical signal for the service # 2  to an optical signal to output the optical signal to the optical fiber  7830 . 
     Operation of the optical communication system  7000   a  is described below. In the same manner as in the eighth embodiment, the user terminal  7100  outputs an optical signal obtained by multiplexing an optical signal for the service # 1  and an optical signal for the service # 2  together. In the optical signal for the service # 1 , the main signal M 1  and the control signal C 1  that are both electrical signals have been superimposed on one another. The wavelength division multiplexer  7600  divides the optical signal output by the user terminal  7100  into an optical signal for the service # 1  and an optical signal for the service # 2  based on their respective wavelengths. The wavelength division multiplexer  7600  outputs the optical signal for the service # 1  to the optical fiber  7820 , while outputting the optical signal for the service # 2  to the optical fiber  7830 . The TRx  7710  in the service providing apparatus  7700  converts an electrical signal for the service # 2  to an optical signal. The control signal superimposition unit  7200  removes the control signal C 1  from the optical signal for the service # 1  divided by the wavelength division multiplexer  7600 , and superimposes the control signal C 3 - 1 , output by the management and control functional unit  1300 , on the optical signal to output the superimposed optical signal to the optical fiber  7840 . The TRx  7410  in the user terminal  7400  receives the optical signal for the service # 1  to convert the optical signal to an electrical signal, and separates the electrical signal into the main signal M 1  and the control signal C 3 - 1 . 
     The TRx  7410  in the user terminal  7400  converts the electrical signal for the service # 1  to an optical signal, and outputs the converted optical signal. The TRx  7410  may convert an electrical signal, in which the main signal M 2  and the control signal C 2  have been superimposed on one another, to an optical signal. The control signal superimposition unit  7200  removes the control signal C 2  for the service # 1  from the optical signal output by the user terminal  7400 , and superimposes the control signal C 3 - 2 , output by the management and control functional unit  1300 , on the optical signal to output the superimposed optical signal to the optical fiber  7820 . 
     In contrast, the TRx  7710  in the service providing apparatus  7700  converts the electrical signal for the service # 2  to an optical signal, and outputs the optical signal. The wavelength division multiplexer  7600  multiplexes the optical signal for the service # 1  output by the control signal superimposition unit  7200 , and the optical signal for the service # 2  output by the service providing apparatus  7700  together, and outputs the multiplexed optical signal to the optical fiber  7810 . The user terminal  7100  receives an input of the optical signal output by the wavelength division multiplexer  7600  to perform the same processing as in the eighth embodiment. 
     Note that each of the control signal superimposition units  1200 ,  1200   a,    1200   b,    2220 ,  2220   a,    3210 ,  3210   a,    4200 ,  4200   a,    5220 ,  5220   a,    6210 , and  7200  may be provided physically in a single unit of signal control apparatus, or may be virtually a single unit of signal control apparatus made up of physically distributed functional units. 
     A user terminal configured to perform optical communication according to the conventional technique superimposes a main signal and a control signal on one another in the form of an electrical signal, converts the superimposed electrical signal to an optical signal, and then transmits the optical signal. Due to this operation, the user terminal cannot extract only a control signal from the optical signal, cannot add another control signal, or cannot rewrite the control signal somewhere along a communication path. In order to change the optical wavelength to be used by a user terminal, it is necessary to make a change to the settings of the optical wavelength directly on the user terminal. This makes it difficult for telecommunications carriers and other providers who provide communication services to flexibly change the wavelength through the network. According to the embodiments described above, a control signal provided to a user terminal is removed from an optical signal somewhere along a communication path of the optical signal, and then another control signal is superimposed on the optical signal. In the manner as described above, it is possible to replace a portion of the signal superimposed on the optical signal with a different signal somewhere along the communication path. Thus, this makes it possible for telecommunications carriers and other providers to flexibly transmit a control signal that instructs a user terminal to make a change to the settings, and to allow the user terminal to change the wavelength and other settings. 
     According to the embodiments described above, an optical signal processing apparatus includes a removal unit and a superimposition unit. The removal unit receives, from a first optical transmission path, an optical signal converted from an electrical signal, in which a first signal and a second signal having different frequencies from each other have been superimposed, and removes the second signal from the optical signal which has been input. The superimposition unit superimposes a third signal having a frequency different from a frequency of the first signal in an electricity field on the optical signal in which the second signal removed by the removal unit, and outputs the optical signal in which the third signal has been superimposed to a second optical transmission path. For example, the superimposition unit is either of the optical modulators  1230 - 1 ,  1230 - 2 ,  4250 , and  4270  described in the above embodiments. 
     As the removal unit, an amplifier having lower response characteristics to a frequency of the second signal than response characteristics to a frequency of the first signal can be used. For example, the removal unit is either of the optical amplifiers  1220 - 1  and  1220 - 2  described in the above embodiments. 
     The removal unit may read the second signal from the optical signal which has been input from the first optical transmission path, modulate the optical signal by using an inverted signal obtained by inverting the second signal which has been read, and remove the second signal from the optical signal. For example, the removal unit is the PD  4220 , the signal processing unit  4230 , and the optical modulator  4240  described in the above embodiments. 
     The removal unit may read the second signal from the optical signal which has been input from the first optical transmission path, and generate an inverted signal by inverting the second signal which has been read. The superimposition unit modulates the optical signal which has been input from the first optical transmission path by using a signal, in which the inverted signal and the third signal have been superimposed, so as to remove the second signal from the optical signal and to superimpose the third signal on the optical signal. For example, the removal unit is the PD  4220  and the signal processing unit  4230  described in the above embodiments, while the superimposition unit is the superimposition unit  4260  and the optical modulator  4270 . 
     Each of the second signal and the third signal may have a frequency lower than or higher than a frequency of the first signal. The first signal may be a main signal, while the second signal and the third signal may be control signals. 
     The embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to these embodiments, but may also cover the design and the like without departing from the scope of the present invention. 
     REFERENCE SIGNS LIST 
       1000 ,  2000 ,  2000   a,    3000 ,  3000   a,    4000 ,  4000   a,    5000 ,  5000   a,    6000 ,  6000   a,    7000 ,  7000   a  Optical communication system 
       1100 - 1 ,  1100 - 2 ,  4100 ,  4300 ,  7100 ,  7400 ,  7500  User terminal 
       1110 ,  1110 - 1 ,  1110 - 2 ,  4110  Signal mixer 
       1120 ,  1120 - 1 ,  1120 - 2 ,  7110 ,  7410 ,  7510 ,  7710  Optical transceiver 
       1130 ,  1130 - 1 ,  1130 - 2 ,  4320  Signal divider 
       1200 ,  1200   a,    1200   b,    2220 ,  2220   a,    3210 ,  3210   a,    4200 ,  4200   a,    5220 ,  5220   a,    6210 ,  7200  Control signal superimposition unit 
       1210 - 1 ,  1210 - 2 ,  1210   a - 1 ,  1210   a - 2 ,  2222 ,  2230 ,  3211 ,  5221 - 1 ,  5221 - 2 ,  6222 - 2  Transmission-reception separation device 
       1220 - 1 ,  1220 - 2  Optical amplifier 
       1230 - 1 ,  1230 - 2 ,  4240 ,  4250 ,  4270  Optical modulator 
       1240 - 1 ,  1240 - 2 ,  1260 - 1 ,  1260 - 2 ,  1270 - 1 ,  1270 - 2 ,  1280 - 1 ,  1280 - 2 ,  1295 - 1 ,  1295 - 2 ,  1500 - 1 ,  1500 - 2 ,  4400 ,  7610 ,  7620 ,  7630 ,  7640 ,  7810 ,  7820 ,  7830 ,  7840  Optical fiber 
       1250 - 1 ,  1250 - 2 ,  1290 - 1 ,  1290 - 2 ,  2210 ,  2510  Optical switch 
       1300 ,  2300  Management and control functional unit 
       2100   a,    2100   a - 1  to  2100   a - 3 ,  2100   b  Subscriber terminal 
       2200 ,  2200   a,    2500 ,  3200 ,  3200   a,    5200 ,  5200   a,    6200 ,  6200   a  Optical gateway 
       2211 - 1  to  2211 -N,  2211 - n   1 ,  2211 - n   2 ,  2511  First port 
       2212 - 1  to  2212 -M,  2212 - m   1 ,  2212 - m   2 ,  2512  Second port 
       2221 - 1 ,  2221 - 2  Splitter 
       2400  Optical SW control functional unit 
       2600 - n   1 ,  2600 - n   2 ,  2650 ,  2700 - 1  to  2700 - 3 ,  2700 - m   1 ,  2700 - m   2  Optical transmission path 
       4120  Optical transmitter 
       4210  Splitter 
       4230  Signal processing unit 
       4260 ,  5222 - 1 ,  5222 - 2  Superimposition unit 
       4310  Optical receiver 
       7120 ,  7300 ,  7600  Wavelength division multiplexer 
       7700  Service providing apparatus