Abstract:
An optical communication system and method thereof include outputting a wavelength division multiplexed (WDM) light to a transmission line, the WDM light being multiplexed from a plurality of signal lights, and one of the plurality of signal lights being converted from a pilot superimposed signal that has a data signal superimposed with a pilot signal and receiving the WDM light at a receiving station including a demultiplexer, a semiconductor optical amplifier, a photoelectric converter, a detection unit, and a controller. The plurality of signal lights are demultiplexed into a plurality of electric signals, respectively and the pilot signal is detected in the plurality of electric signals, with the exception of the pilot superimposed signal, and an amplification condition of the semiconductor optical amplifier is controlled based on the pilot signal detected.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-268483, filed on Oct. 17, 2008, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    The present invention relates to an optical communication apparatus and a method of controlling a semiconductor optical amplifier. 
         [0004]    2. Description of the Related Art 
         [0005]    In recent years, to increase the speed and capacity of networks, attention has been given to optical communication technology. In optical communication technology, information is transmitted not by electric signals but by signal lights, utilizing an optical fiber as a transmission line. For example, according to a 100-gigabit Ethernet (trademark) standard, communication of 100 Gbps is performed at 25.78125 Gbps×4 lights of different wavelengths. That is, the communication is performed by utilizing four lights of different wavelengths at a modulation speed of 25.78125 Gbps per channel. 
         [0006]    According to the 100-gigabit Ethernet standard with a transmission speed of 40 km, a semiconductor optical amplifier is used as a preamplifier, and signal lights of four wavelengths are collectively amplified. 
         [0007]    As a method of controlling the semiconductor optical amplifier, a technique is known that utilizes a pilot signal for the semiconductor optical amplifier in order to keep its gain constant. The technique is described, for example, in Electronics Letters No. 25, pp. 235-236, 1989. According to this technique, a transmission apparatus transmits a signal light to a reception apparatus, by superimposing a pilot signal to the signal light. The reception apparatus extracts the pilot signal from the signal light to which the pilot signal is superimposed. Then, the reception apparatus keeps a gain of the semiconductor optical amplifier within a certain range by utilizing the pilot signal, in order to avoid gain saturation in the semiconductor optical amplifier. 
         [0008]    Another typical technique superimposes a pilot signal to a plurality of signal lights with different wavelengths, multiplexes the plurality of signal lights, and transmits the multiplexed signal light to the reception apparatus. The technique is described, for example, in JP-A-11-41208. 
         [0009]    A semiconductor optical amplifier using the techniques above and others similar thereto cannot be used in the gain saturation region. Gain saturation occurs when high-power signal light is input to the semiconductor optical amplifier and the excitation carrier is reduced by a stimulated emission. As a result, the signal gain differs when the power of the input light differs and the gain saturation occurs. 
         [0010]    For example, when the gain saturation region of the semiconductor optical amplifier changes due to age deterioration or the like, the semiconductor optical amplification performs an amplification using the changed gain saturation region, and a crosstalk between channels occurs and decreases reception sensitivity. 
       SUMMARY 
       [0011]    An optical communication system includes a transmission station configured to output a wavelength division multiplexed (WDM) light to a transmission line, where the WDM light is multiplexed from a plurality of signal lights, and one of the plurality of signal lights is converted from a pilot superimposed signal having a data signal superimposed with a pilot signal, and a receiving station configured to input the WDM light from the transmission line, the receiving station including a demultiplexer, a semiconductor optical amplifier, a photoelectric converter, a detection unit, and a controller. 
         [0012]    The semiconductor optical amplifier has a gain saturation characteristic and amplifying the WDM light, where the demultiplexer is configured to demultiplex the WDM light amplified by the semiconductor optical amplifier into the plurality of signal lights, the photoelectric converter is configured to convert the plurality of signal lights demultiplexed by the demultiplexer into a plurality of electric signals, respectively, the detection unit is configured to detect the pilot signal in the plurality of electric signals, with the exception of the pilot superimposed signal, and the controller is configured to control an amplification condition of the semiconductor optical amplifier based on the pilot signal detected by the detection unit. 
         [0013]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
         [0014]    Additional aspects and/or advantages will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    These and/or other aspects and advantages will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
           [0016]      FIG. 1  is a view illustrating a system configuration of an optical communication system; 
           [0017]      FIG. 2  is a view illustrating a transmission apparatus of an embodiment; 
           [0018]      FIG. 3  is a view illustrating a reception apparatus of an embodiment; 
           [0019]      FIG. 4  is a view illustrating a semiconductor optical amplifier; 
           [0020]      FIG. 5A  is a view illustrating a gain saturation characteristic of a semiconductor optical amplifier; 
           [0021]      FIG. 5B  is a view illustrating a gain saturation characteristic of a semiconductor optical amplifier when an amount of injected current is increased; 
           [0022]      FIG. 6  is a view illustrating another structure of a reception apparatus; 
           [0023]      FIG. 7  is a flowchart illustrating a procedure of processing including by a control unit; 
           [0024]      FIG. 8  is a view illustrating a transmission apparatus of an embodiment; 
           [0025]      FIG. 9  is a view illustrating a reception apparatus of an embodiment; 
           [0026]      FIG. 10  is a view illustrating a structure of a transmission apparatus of an embodiment, and illustrates a manner in which a light power of a light source is controlled by a transmission apparatus side control unit based on a notification from the control unit of a reception apparatus; 
           [0027]      FIG. 11  is a view illustrating another structure of a transmission apparatus; 
           [0028]      FIG. 12A  illustrates an example in which an excitation light source is provided in a preceding stage of a semiconductor optical amplifier; and 
           [0029]      FIG. 12B  illustrates an example in which an excitation light source is provided in a succeeding stage of a semiconductor optical amplifier. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0030]    Reference will now be made in detail to the embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures. 
         [0031]      FIG. 1  is a view illustrating a system configuration of an optical communication system of an embodiment. 
         [0032]    In  FIG. 1 , the optical communication system includes a transmission apparatus  10   a , a reception apparatus  20   a , and an optical transmission line  50 , such as an optical fiber, that connects the transmission apparatus  10   a  and the reception apparatus  20   a.    
         [0033]    Next,  FIG. 2  is a view illustrating a transmission apparatus of the transmission apparatus  10   a . The transmission apparatus  10   a  has a superimposing unit  11 , photoelectric conversion units  12   a  to  12   d , and a multiplexer  13 . The photoelectric conversion units  12   a  to  12   d  are provided, for example, for each communication channel. 
         [0034]    While  FIG. 1  illustrates a case where four channels are provided as communication channels, a number of communication channels is not limited to four. Hereinafter, a communication channel  1  is a communication channel that utilizes a signal light which is converted to an electric signal by the photoelectric conversion unit  12   a . Likewise, communication channels  2 ,  3 , and  4  are communication channels utilizing signal lights which are converted to electric signals by the photoelectric conversion units  12   b ,  12   c  and  12   d , respectively. The conversion units  12  correspond to each wavelength of the signals that are to be converted. 
         [0035]    A wavelength of λ 1  is a wavelength of a signal light used for the communication channel  1 , and a wavelength λ 2  is a wavelength of the signal light used for communication by the communication channel  2 . Likewise, a wavelength is a wavelength of the signal light used for the communication channel  3 , and a wavelength of λ 4  is a wavelength of the signal light used for the communication channel  4 . 
         [0036]    The superimposing unit  11  superimposes a pilot signal on a data signal, and outputs the superimposed signal to the photoelectric conversion unit  12   a . The modulation wavelength of the data signal may be several GHz to several tens of GHz. The wavelength of the signal used as the pilot signal may be several hundreds of Hz to several thousands of kHz, and a wavelength sufficiently lower than the wavelength of the data signal is used. The output of the superimposing unit  11  is output to the photoelectric conversion unit  12   a.    
         [0037]    The photoelectric conversion unit  12   a  converts the data signal, on which the pilot signal is superimposed, into a signal light. The photoelectric conversion unit  12   a  outputs the converted signal light to the multiplexer  13 . 
         [0038]    Likewise, the photoelectric conversion units  12   b  to  12   d  convert the data signals of the channels  2  to  4  into signal lights, respectively, and output the converted signal lights to the multiplexer  13 . 
         [0039]    According to an embodiment, the multiplexer  13  has four ports inputting a plurality of lights with different wavelengths of λ 1 , λ 2 , λ 3  and λ 4 , and multiplexes the signal lights. The multiplexed signal light (wavelength-division multiplexed signal light) is output to the optical transmission line  50  ( FIG. 1 ), and input to the reception apparatus  20   a.    
         [0040]    Next,  FIG. 3  is a view illustrating the reception apparatus  20   a . The reception apparatus  20   a  has a semiconductor optical amplifier  21 , a demultiplexer  22 , photoelectric conversion units  23   a  to  23   d , filter units  24   b  to  24   d , a signal processing unit  25  and a control unit  26 . 
         [0041]    The signal light received through the optical transmission line  50  is input to the semiconductor optical amplifier  21 . The semiconductor optical amplifier  21  amplifies the input signal light and outputs to the demultiplexer  22 . 
         [0042]      FIG. 4  is a view illustrating the semiconductor optical amplifier  21 . The semiconductor optical amplifier  21  is an amplifier having a gain saturation characteristic. The semiconductor optical amplifier  21  has, for example, an active layer  213  made of a semiconductor, a p-type semiconductor layer  212 , an n-type semiconductor layer  214 , a substrate  215 , and electrodes  211  and  216  for current injection. The p-type semiconductor layer  212  and the n-type semiconductor layer  214  are disposed so as to sandwich the active layer  213   
         [0043]    In a case when no current is injected from a current source  27  (OFF), and when light is incident on the active layer  213 , electrons in the valence band absorbs the light, and the electrons themselves make a transition to the conduction band (absorption). 
         [0044]    When there are electrons in the conduction band and light having energy corresponding to the band gap passes near the electrons, the electrons make a transition from the conduction band to the valence band and, at the same time, emits light whose wavelength, phase, and direction are the same as those of the input light (stimulated emission). 
         [0045]    In the semiconductor optical amplifier  21 , by forming a p-n junction and creating an inverted distribution, where the carrier density is high, by current injection (ON), amplification (stimulated-emission) of the incident light, such as WDM light, is realized. 
         [0046]    The demultiplexer  22  demultiplexes the signal light (WDM light) amplified by the semiconductor optical amplifier  21 , to signal lights of the different wavelengths (λ 1 , λ 2 , λ 3  and λ 4 ). The signal lights demultiplexed by the demultiplexer  22  are output to the photoelectric conversion units  23   a  to  23   d , respectively. 
         [0047]    The photoelectric conversion units  23   a  to  23   d  are provided for the communication channels, respectively, and convert the signal lights of the corresponding communication channels into electric signals. The data signal converted into an electric signal by the photoelectric conversion unit  23   a  is output to the signal processing unit  25 . The data signals converted into electric signals by the photoelectric conversion units  23   b  to  23   d  are output to the signal processing unit  25  and output to the filter units  24   b  to  24   d , respectively. 
         [0048]    The signal processing unit  25  receives the data signals output from the photoelectric conversion units  23   a  to  23   d , and demodulates the signals to thereby obtain binary format data. Then, the signal processing unit  25  performs processing set for each communication channel. 
         [0049]    The filter units  24   b  to  24   d  perform filtering processing on the data signals, which are converted into electric signals by the photoelectric conversion units  23   b  to  23   d , thereby extracting the pilot signals contained in the data signals. The extracted pilot signals are output to the control unit  26 . 
         [0050]    The control unit  26  calculates an average power of each of the pilot signals detected on the communication channels  2  to  4 , and controls the semiconductor optical amplifier  21  based on the calculated average power. For example, when pilot signals are detected by the filter units  24   b  to  24   d , the control unit  26  adds the signal powers of the pilot signals for each of the communication channels  2  to  4 , and divides the sum by the number of pilot signals whose signal powers are added, thereby obtaining the average power of the pilot signals for each communication channel. 
         [0051]    The transmission apparatus  10   a  superimposes a pilot signal on the data signal of the communication channel  1 , and transmits the superimposed signal to the reception apparatus  20   a . The reception apparatus  20   a  receives the signal light containing the pilot signal through the optical transmission path  50 , and inputs it to the semiconductor optical amplifier  21 . 
         [0052]      FIG. 5A  is a view illustrating a gain saturation characteristic of a semiconductor optical amplifier. The semiconductor optical amplifier  21  has a gain saturation characteristic. As illustrated in  FIG. 5A , the semiconductor optical amplifier  21  has a region where the amplification characteristic exhibits linearity (hereinafter, referred to as linear region) and a region where the amplification characteristic does not exhibit linearity (hereinafter, referred to as nonlinear region, or gain saturation region). When the input light power of the signal light is in the linear region, the signal light is amplified and output with a gain corresponding to the input light power. When the input light power of the signal light is in the nonlinear region, the gain is reduced when the input light power becomes equal to or more than a given extent, and the signal gain do not correspond to the power of the input light. 
         [0053]    When the power of the input light input to the semiconductor optical amplifier  21  is in the linear region, the semiconductor optical amplifier  21  amplifies the signal light with a gain corresponding to the input light power, and outputs the amplified output light to the demultiplexer  22 . However, when the power of the input light input to the semiconductor optical amplifier  21  is in the nonlinear region (gain saturation region), the gain of the semiconductor optical amplifier  21  is reduced. When the range of the linear region of the semiconductor optical amplifier  21  is decreased, due to age deterioration or the like, for example, there are cases where the power of the input light input to the semiconductor optical amplifier  21  is in the nonlinear region. 
         [0054]    When the input light in the nonlinear area of the semiconductor optical amplifier  21  is amplified, the component of the pilot signal is superimposed in polarity-inverted state on the signal light of another communication channel by cross-gain modulation. 
         [0055]    Accordingly, in an embodiment, the pilot signal(s) superimposed on the data signals of the communication channels  2  to  4  are detected by the filter units  24   b  to  24   d  provided on the communication channels  2  to  4 . The control unit  26  calculates the average power of the detected pilot signals for each of the channels  2  to  4 . Then, the control unit  26  compares the average power of the pilot signals calculated for each of the channels  2  to  4  with a threshold value. When the average power of a communication channel is detected to be higher than the threshold value, the control unit  26  controls the saturation output power of the semiconductor optical amplifier  21  or controls the power of the input light of the semiconductor optical amplifier  21 . By the control, the amplification of the signal light is controlled so as to be performed in the region of the input power where the amplification characteristic of the semiconductor optical amplifier  21  exhibits linearity. 
         [0056]      FIG. 5B  is a view illustrating the gain saturation characteristic of the semiconductor optical amplifier when an amount of injected current is increased. To enhance the saturation output power of the semiconductor optical amplifier  21  (that is, to expand the linear region), the current source  27  is controlled so that the amount of injected current supplied to the semiconductor optical amplifier  21  is increased. The control unit  26  controls the amount of injected current according to a difference between the average power of the pilot signals and the threshold value. As illustrated in  FIG. 5B , the gain saturation characteristic of the semiconductor optical amplifier  21  changes when the injected current is increased. As is apparent when  FIGS. 5A and 5B  are compared with each other, by increasing the amount of injected current supplied to the semiconductor optical amplifier  21 , the region of the input power where the amplification characteristic of the semiconductor optical amplifier  21  exhibits linearity can be expanded. 
         [0057]      FIG. 6  is a view illustrating another structure of the reception apparatus. The control unit  26  may perform control so that the input light power of the semiconductor optical amplifier  21  is restricted. 
         [0058]    As illustrated in  FIG. 6 , a variable optical attenuator  28  is provided in a preceding stage of the semiconductor optical amplifier  21 . The control unit  26  outputs the signal light to the semiconductor optical amplifier  21  after attenuating the light power of the signal light by the variable optical attenuator  28  based on a difference between the average power of the pilot signals and the threshold value. Consequently, the input light power of the semiconductor optical amplifier  21  is weakened, and only the region of the input light power where the amplification characteristic of the semiconductor optical amplifier  21  exhibits linearity can be used for amplification. 
         [0059]      FIG. 7  is a flowchart illustrating a procedure of processing, for example, by a control unit  26 . As illustrated in  FIG. 7 , when optical communication between the transmission apparatus  10   a  and the reception apparatus  20   a  is started, the filter units  24   b  to  24   d  of the reception apparatus  20   a  detect the pilot signals from the electric signals of the communication channels  2  to  4 , respectively. The detected pilot signals are output from the filter units  24   b  to  24   d  to the control unit  26 . The control unit  26  calculates the average power of the pilot signals obtained from the filter units  24   b  to  24   d  for each of the channels  2  to  4 . For example, every time a pilot signal is input from the filter units  24   b  to  24   d , the signal power of the input pilot signal is added, and the average value of the signal power is obtained for each communication channel (operation S 1 ). Then, the control unit  26  compares the calculated average power of the pilot signals with the threshold value (operation S 2 ). The control unit  26  compares the average power of the pilot signals calculated for each communication channel with the threshold value, and determines whether or not there is a communication channel where the average power is higher than the threshold value (operation S 2 ). When the average power of the pilot signals in a communication channel is detected to be higher than the threshold value (operation S 2 /YES), the control unit  26  obtains the difference between the average power of the pilot signals and the threshold value. Then, the control unit  26  controls the current source  27  based on the obtained difference, thereby increasing the amount of current supplied to the semiconductor optical amplifier  21  (operation S 3 ). 
         [0060]    When the components of the pilot signals are superimposed on the data signals of the communication channels  2  to  4 , it can be determined whether the semiconductor optical amplifier  21  amplifies the input light by using the saturation region. Therefore, the control unit  26  increases the amount of current supplied to the semiconductor optical amplifier  21  to thereby expand the linear region where the amplification characteristic of the semiconductor optical amplifier  21  exhibits linearity. 
         [0061]    As described above, according to an embodiment, the pilot signals, which are contained in the data signals of the communication channels other than the data signal on which the pilot signal is superimposed by the transmission apparatus  10   a , are detected by the reception apparatus. When the average power of the detected pilot signals is equal to or higher than the threshold value, it is determined that amplification using the gain saturation region of the semiconductor optical amplifier  21  is performed, and an operation range of the input light power of the semiconductor optical amplifier  21  is increased. Consequently, even if the gain saturation region of the semiconductor optical amplifier  21  is changed, due to age deterioration or the like, for example, the semiconductor optical amplifier  21  can be controlled so that no gain saturation occurs. 
         [0062]    Moreover, since the injected current supplied to the semiconductor optical amplifier  21  is controlled to thereby increase the operation range of the input light power of the semiconductor optical amplifier  21 , the control of the semiconductor optical amplifier  21  is facilitated. Moreover, by providing the variable optical attenuator  28  ( FIG. 6 ) in the preceding stage of the semiconductor optical amplifier  21 , the signal light can be input to the semiconductor optical amplifier  21  after the power of the signal light is attenuated by the variable optical attenuator  28 . Consequently, the signal light can be amplified by using only the region where the amplification characteristic of the semiconductor optical amplifier  21  exhibits linearity. 
         [0063]      FIG. 8  is a view illustrating a transmission apparatus of an embodiment. With respect to this embodiment, descriptions of parts similar to those of the above-described embodiment are omitted. 
         [0064]    As illustrated in  FIG. 8 , the transmission apparatus  10   b  of an embodiment superimposes the pilot signal on the data signals of all of the communication channels  1  to  4 . The superimposing unit  11 , which is provided only for the communication channel  1  in the above-described embodiment, is also provided for each of the communication channels  2  to  4 . The frequencies of the pilot signals superimposed on the data signals of the communication channels are different from one another. For example, as the frequency of the pilot signal superimposed on the data signal of the communication channel  1 , 700 Hz is used (the pilot signal of the frequency is referred as f 1 ). As the frequency of the pilot signal superimposed on the data signal of the communication channel  2 , 1300 Hz is used (the pilot signal of the frequency is referred as f 2 ). As the frequency of the pilot signal superimposed on the data signal of the communication channel  3 , 1900 Hz is used (the pilot signal of the frequency is referred as f 3 ). As the frequency of the pilot signal superimposed on the data signal of the communication channel  4 , 2500 Hz is used (the pilot signal of the frequency is referred as f 4 ). 
         [0065]      FIG. 9  is a view illustrating a reception apparatus  20   b  of an embodiment. As illustrated in  FIG. 9 , a filter unit  24   a  is also provided for the communication channel  1 . 
         [0066]    The filter unit  24   a  detects the frequency components of the pilot signals (f 2 , f 3  and f 4 ) superimposed on the data signals of the communication channels  2  to  4 , from the data signal of the communication channel  1 . The detected pilot signals of the frequencies (f 2 , f 3  and f 4 ) are output from the filter unit  24   a  to the control unit  26 . The filter unit  24   b  detects the frequency components of the pilot signals (that is, f 1 , f 3  and f 4 ) superimposed on the data signals of the communication channels  1 ,  3  and  4 , from the data signal of the communication channel  2 . The detected pilot signals of the frequencies (f 1 , f 3  and f 4 ) are output from the filter unit  24   b  to the control unit  26 . The filter unit  24   c  detects the frequency components of the pilot signals (f 1 , f 2  and f 4 ) superimposed on the data signals of the communication channels  1 ,  2  and  4 , from the data signal of the communication channel  3 . The detected pilot signals of the frequencies (f 1 , f 2  and f 4 ) are output from the filter unit  24   c  to the control unit  26 . The filter unit  24   d  detects the frequency components of the pilot signals (f 1 , f 2  and f 3 ) superimposed on the data signals of the communication channels  1  to  3 , from the data signal of the communication channel  4 . The detected pilot signals of the frequencies (f 1 , f 2  and f 3 ) are output from the filter unit  24   a  to the control unit  26 . 
         [0067]    Based on the pilot signals obtained from the filter units  24   a  to  24   d , the control unit  26  identifies the communication channel that causes the power of the light input to the semiconductor optical amplifier  21  to be in the nonlinear region (gain saturation region) of the semiconductor optical amplifier  21 . 
         [0068]    First, the control unit  26  obtains the average power of the pilot signals detected by each of the filter units  24   a  to  24   d.    
         [0069]    From the pilot signals detected by the filter unit  24   a , the average power of the pilot signals of the frequencies f 2 , f 3  and f 4  is obtained. Likewise, from the pilot signals detected by the filter unit  24   b , the average power of the pilot signals of the frequencies f 1 , f 3  and f 4  is obtained, and from the pilot signals detected by the filter unit  24   c , the average power of the pilot signals of the frequencies f 1 , f 2  and f 4  is obtained. Moreover, from the pilot signals detected by the filter unit  24   d , the average power of the pilot signals of the frequencies f 1 , f 2  and f 3  is obtained. 
         [0070]    The control unit  26  compares the calculated average powers of the pilot signals of the frequencies f 1 , f 2 , f 3  and f 4 , and determines the communication channel the signal light of which causes the input light power to be in the nonlinear region (gain saturation region) of the semiconductor optical amplifier  21 . 
         [0071]    That is, when the average power of the pilot signal of the frequency f 1  is higher than the threshold value, it can be determined that the light power of the communication channel  1  is high. Likewise, when the average power of the pilot signal of the frequency f 2  is higher than the threshold value, it can be determined that the light power of the communication channel  2  is high. 
         [0072]    When identifying the communication channel with high light power, the control unit  26  of the reception apparatus  20   b  calculates the difference between the average power of the pilot signal superimposed on the data signal of the identified communication channel and the threshold value. The control unit  26  notifies a transmission apparatus side control unit  15  shown in  FIG. 10  of the calculated difference between the average power of the pilot signal and the threshold value. 
         [0073]      FIG. 10  is a view illustrating a structure of a transmission apparatus of an embodiment, and illustrates a manner in which the light power of the light source is controlled by a transmission apparatus side control unit based on a notification from the control unit of the reception apparatus. As illustrated in  FIG. 10 , the photoelectric conversion unit  12   a  of the transmission apparatus  10   b  has a light source  121   a  and a modulation unit  122   a . Likewise, the photoelectric conversion unit  12   b  has a light source  121   b  and a modulation unit  122   b , the photoelectric conversion unit  12   c  has a light source  121   c  and a modulation unit  122   c , and the photoelectric conversion unit  12   d  has a light source  121   d  and a modulation unit  122   d.    
         [0074]    When notified of the communication channel with high light power and the difference between the average power of the pilot signal and the threshold value by the control unit  26  on the side of the reception apparatus  20   b , the transmission apparatus side control unit  15  controls the light power of the light source of the communication channel concerned. That is, the light source  121   a ,  121   b ,  121   c  or  121   d  of the communication channel with high optical power is reduced according to the difference between the average power of the pilot signal and the threshold value. 
         [0075]    As described above, according to the present embodiments, when the input light power is in the region of the input light power where the amplification characteristic of the semiconductor optical amplifier  21  does not exhibit linearity, the communication channel using the signal light that increases the input light power is identified. Consequently, by reducing the light power of the signal light of the identified communication channel, the signal light can be amplified by using only the region where the amplification characteristic of the semiconductor optical amplifier  21  exhibits linearity. 
         [0076]    In the above-described embodiment, to increase the saturation output power of the semiconductor optical amplifier  21  (that is, to expand the linear region), the amount of injected current supplied to the semiconductor optical amplifier  21  is controlled. 
         [0077]      FIG. 11  is a view illustrating another structure of a transmission apparatus. As illustrated in  FIG. 11 , when detecting that the average power of the pilot signal detected on any of the communication channels  2  to  4  exceeds the threshold value, the control unit  26  notifies a transmission apparatus  10   c  of the difference between the average power of the pilot signal and the threshold value. The transmission apparatus  10   c  side has the transmission apparatus side control unit  15  and an excitation light source  16 . The transmission apparatus side control unit  15  controls the excitation light source  16  according to the difference notified by the control unit  26  of the reception apparatus  20   a  side, and controls the input light power of the excitation light source input to the semiconductor optical amplifier  21  through the optical transmission line  50 . That is, by increasing the light power of the excitation light output to the semiconductor optical amplifier  21  together with the signal light, the saturation output power of the semiconductor optical amplifier  21  can be increased. 
         [0078]    The excitation light source may be provided in the preceding or succeeding stage of the semiconductor optical amplifier  21 .  FIG. 12A  illustrates an example in which an excitation light source  31  and a variable optical attenuator  32  are provided on the preceding side of the semiconductor optical amplifier  21 .  FIG. 12B  illustrates an example in which an excitation light source  33  and a variable optical attenuator  34  are provided on the succeeding side of the semiconductor optical amplifier  21 . 
         [0079]    From the excitation light source  31  (or  33 ), the excitation light input to the active layer  213  of the semiconductor optical amplifier  21  is always output. The control unit  26  controls the variable optical attenuator  32  to thereby control the power of the excitation light input to the active layer  213  of the semiconductor optical amplifier  21 . That is, to increase the saturation output power of the semiconductor optical amplifier  21 , the control unit  26  reduces the excitation light of the excitation light source  31  (or  33 ) attenuated by the variable optical attenuator  32  (or  34 ). 
         [0080]    As described above, according to the optical communication apparatus of the embodiments, the semiconductor optical amplifier can be controlled so that no gain saturation occurs in the semiconductor optical amplifier. 
         [0081]    The embodiments can be implemented in computing hardware (computing apparatus) and/or software, such as (in a non-limiting example) any computer that can store, retrieve, process and/or output data and/or communicate with other computers. The results produced can be displayed on a display of the computing hardware. A program/software implementing the embodiments may be recorded on computer-readable media comprising computer-readable recording media. The program/software implementing the embodiments may also be transmitted over transmission communication media. Examples of the computer-readable recording media include a magnetic recording apparatus, an optical disk, a magneto-optical disk, and/or a semiconductor memory (for example, RAM, ROM, etc.). Examples of the magnetic recording apparatus include a hard disk device (HDD), a flexible disk (FD), and a magnetic tape (MT). Examples of the optical disk include a DVD (Digital Versatile Disc), a DVD-RAM, a CD-ROM (Compact Disc-Read Only Memory), and a CD-R (Recordable)/RW. An example of communication media includes a carrier-wave signal. 
         [0082]    Further, according to an aspect of the embodiments, any combinations of the described features, functions and/or operations can be provided. 
         [0083]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention, the scope of which is defined in the claims and their equivalents.