Patent Publication Number: US-10320483-B2

Title: Optical communication system, optical receiver, optical receiver control method, and non-transitory computer readable medium

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 14/904,023, filed on Jan. 8, 2016, which is a National Stage Entry of International Application No. PCT/JP2014/002737, filed May 26, 2014, which claims priority from Japanese Patent Application No. 2013-145488, filed Jul. 11, 2013. The entire contents of the above-referenced applications are expressly incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an optical communication system, an optical receiver, an optical receiver control method, and a non-transitory computer readable medium. 
     BACKGROUND ART 
     Along with an increase in demand of data communication service in recent years, the introduction of a longer-distance, larger-volume high-density wavelength multiplexing optical fiber communication system with higher reliability is being promoted. On such a background, higher performance of an optical communication system that uses optical fibers is being demanded. As part thereof, the adoption of a digital coherent reception system in which a digital processing technique is introduced to an optical receiver is being promoted. 
     In the digital coherent reception system, it is possible to correct, by digital processing, linear degradation of an optical waveform due to optical fiber transmission, and compensate transmission characteristic degradation due to a wavelength dispersion characteristic of an optical fiber as a transmission channel. However, due to the wavelength dispersion, an optical signal waveform collapses and expands in an amplitude direction, so in a digital conversion, a dynamic range for an analog/digital conversion becomes a problem. 
     As a communication system that takes the wavelength dispersion characteristic into consideration, Patent Literature 1 discloses a method of changing a dynamic range at a time when an analog electric signal is converted into a digital electric signal by monitoring wavelength dispersion. 
     CITATION LIST 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2012-129960 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the method of changing the dynamic range, when the wavelength dispersion is small, control is performed to reduce the dynamic range. Accordingly, a resolution in the analog/digital conversion is not changed, and sensitivity at a time when the digital electric signal is demodulated is not also changed. 
     The purpose of the present invention is to provide an optical communication system that controls reception sensitivity of an optical receiver in accordance with wavelength dispersion. 
     Solution to Problem 
     An example of a communication system comprising: 
     an optical transmitter to which an transmission signal is input, and which modulates the transmission signal to an optical signal and transmits the optical signal; and 
     an optical receiver that receives the optical signal and demodulates the optical signal to an transmission signal, wherein 
     the optical receiver includes a photoelectric conversion means for converting the optical signal into an analog electric signal, a conversion and demodulation means for converting the analog electric signal into a digital signal and demodulating the signal to the transmission signal, and an amplitude control means for controlling amplitude of the analog electric signal, and 
     the amplitude control means controls the amplitude of the analog electric signal in accordance with wavelength dispersion of the optical signal. 
     An example of an optical receiver comprising: 
     a photoelectric conversion means for converting an optical signal into an analog electric signal; 
     a conversion and demodulation means for converting the analog electric signal into a digital signal and demodulating the signal to an transmission signal; and 
     an amplitude control means for controlling amplitude of the analog electric signal, 
     wherein the amplitude control means controls the amplitude of the analog electric signal in accordance with wavelength dispersion of the optical signal. 
     An example of an optical receiver control method, 
     wherein an optical receiver is configured to convert an optical signal into an analog electric signal, convert the analog electric signal into a digital signal, and demodulate the signal to an transmission signal, 
     the control method comprising controlling amplitude of the analog electric signal in accordance with wavelength dispersion of the optical signal. 
     An example of a non-transitory computer readable medium for causing a computer to execute an optical receiver control method, wherein 
     an optical receiver is configured to convert an optical signal into an analog electric signal, convert the analog electric signal into a digital signal, and demodulate the digital signal to an transmission signal, and 
     the control method includes controlling amplitude of the analog electric signal in accordance with wavelength dispersion of the optical signal. 
     Advantageous Effects of Invention 
     According to the aspects described above, it is possible to provide an optical communication system that controls the reception sensitivity of the optical receiver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a structural example of an optical communication system  100  according to first embodiment of the present invention. 
         FIG. 2  is a diagram showing the structure of an optical transmitter  1  according to first embodiment of the present invention. 
         FIG. 3  is a diagram of an optical receiver  2  according to first embodiment of the present invention. 
         FIG. 4  is a diagram of an amplitude control unit  102  according to first embodiment of the present invention. 
         FIG. 5  is a flowchart of a method for communication according to first embodiment of the present invention. 
         FIG. 6  is a sequence diagram of a method for communication according to first embodiment of the present invention. 
         FIG. 7  is a diagram of a communication system  300  according to second embodiment of the present invention. 
         FIG. 8  is a diagram of an optical transmitter  3  according to second embodiment of the present invention. 
         FIG. 9  is a diagram of an optical receiver  4  according to second embodiment of the present invention. 
         FIG. 10  is a diagram of a amplitude control unit  302  according to second embodiment of the present invention. 
         FIG. 11  is a flowchart of a communication method according to second embodiment of the present invention. 
         FIG. 12  shows a communication system as a technique to which the present invention is relevant. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First, studied matters about relevant techniques until the present invention has been conceived will be described. 
       FIG. 12  shows a communication system  500  as a technique to which the present invention is relevant. A digital coherent receiver  560  shown in  FIG. 12  receives optical signal transmitted from an optical transmitter  550 , causes local light to be interfered in a 90-degree hybrid circuit  522 , converts the light into an analog electric signal by coherent detection in PDs (Photo Diodes)  523 , sets signal amplitude to be constant in analog amplifiers  524 , and inputs the signal to an ADC (Analog/Digital Converter) unit  501  of a DSP (Digital Signal Processor)  525 . 
     The analog amplifier  524  is subjected to gain control in such a manner that average amplitude of the analog electric signal that is converted to the electric signal is constant, but the waveform of the optical signal received is expanded in a temporal axis direction by being affected by wavelength dispersion of an optical fiber as a transmission channel. The wavelength dispersion is such a phenomenon that, during transmission of optical pulses in an optical fiber, light beams with different wavelengths are transmitted at different speeds in the optical fiber, and thus a width of an optical signal is temporally expanded. Then, a number of waveforms that are expanded in the temporal axis direction are superposed, thereby increasing the amplitude of a reception signal and causing the waveform to change at random. 
     To perform digital processing for an analog signal in the DSP  525 , in the ADC unit  501 , it is necessary to perform a digital signal conversion for entire analog signal amplitude. Accordingly, it is necessary to control an output of the analog amplifier  524  in such a manner that a peak (maximum value) of the analog signal amplitude input to the ADC unit  501  is equal to or less than a dynamic range (width between the minimum value and the maximum value of an analog signal input range which can be identified) of the ADC unit  501 . 
     As a method of controlling the gain (input/output ratio) of the analog amplifier  524 , generally, used is such a method that the output amplitude of the analog amplifier  524  is monitored, a comparator  527  compares the output amplitude and a preset reference value output from a reference generation circuit  526  with each other, and a feedback is given to the gain control for the analog amplifier  524 . 
     In the relevant technique, the reference value output from the reference generation circuit  526  is fixed, and the reference value is set in accordance with the maximum value of wavelength dispersion of the transmission channel. Therefore, even in the case where a dispersion value is small, and distortion of a signal waveform is small, the signal amplitude is controlled to be small. In the case where the wave dispersion is small, and the distortion of the signal waveform is small, it is possible to increase the signal amplitude and enhance the resolution in the ADC relatively. However, because the signal amplitude is set to be constant, the transmission signal amplitude to the ADC unit  501  is not changed, which is a problem in that the reception sensitivity of the optical receiver is not changed as compared to the case where the wavelength dispersion is at maximum. 
     In a relevant digital coherent reception circuit, the input amplitude of the ADC is uniformly set irrespective of a wavelength dispersion value. In the case where the wavelength dispersion value of the transmission channel is large, the waveform of a reception signal is distorted due to the wavelength dispersion, resulting in an increase in the peak of the amplitude. Further, performing uniform amplitude control for the reception signal results in falling out of the dynamic range of the ADC. For this reason, when the reference value that is input to the comparator  527  is set to be small, and the analog signal amplitude that is input to the ADC is set to be small, it is confirmed that the receptions sensitivity is improved. 
     However, in the case where the reference value is set to be small, the signal amplitude after dispersion compensation also becomes small. In the case where the wavelength dispersion of the transmission channel is small, the distortion of the waveform is also small, and the peak of the amplitude is also small. But, there is a problem in that the analog signal amplitude input to the ADC remains small, and the reception sensitivity is not improved. The present invention has been made on the background as described above, and embodiments of the present invention will be described below. 
     Embodiment 1 
       FIG. 1  shows a structural example of an optical communication system  100  according to this embodiment.  FIG. 1  shows an optical transmitter with a DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) system and an optical receiver with a digital coherent system. The optical communication system  100  is provided with a DP-QPSK transmitter (optical transmitter)  1  and a digital coherent DP-QPSK receiver (optical receiver)  2 . 
       FIG. 2  is a diagram showing the structure of the optical transmitter  1 . The optical transmitter  1  is provided with a circuit for generating and transmitting a DP-QPSK signal. The optical transmitter  1  is provided with an LD (Laser Diode)  11  that generates CW (Continuous Wave) light as a light source of signal light and a DP-QPSK modulator  12  that performs DP-QPSK modulation for the CW light from the LD  11  with an input electric signal (input signal). 
     The DP-QPSK modulator  12  divides the CW light into two light beams, performs QPSK modulation for each light beam, then orthogonalizes polarization planes at 90 degrees, and performs multiplexing, thereby generating a polarization multiplexed signal. The optical transmitter  1  indicates the structure of a general DP-QPSK transmitter. 
       FIG. 3  is a diagram showing the structure of the optical receiver  2 . The optical receiver  2  is provided with a photoelectric conversion unit  101  to which an optical signal is input, and which causes the signal to be interfered with local light and converts the optical signal into an electric signal, a conversion and demodulation unit (DSP: Digital Signal Processor)  25  that converts the analog electric signal input from the photoelectric conversion unit  101  into a digital signal and performs wavelength dispersion compensation and demodulation of the DP-QPSK signal by digital processing, and an amplitude control unit  102 . 
     The photoelectric conversion unit  101  is provided with a local light generation unit (LO; Local Oscillator)  21  that generates local light, a 90-degree hybrid circuit  22  for inputting and causing the received DP-QPSK signal and the local light from the LO  21  to interfere with each other, PDs (Photo Diodes)  23  that perform coherent detection for the optical signal interfered in the 90-degree hybrid circuit  22  and convert the signal into an analog electric signal, and analog amplifiers (TIAs; Trans Impedance AMPs)  24  that input the analog electric signal from the PDs  23  and amplify the analog electric signal to a predetermined amplitude by a gain control signal from a comparator (COMP; Comparator)  27 . 
       FIG. 3  shows a block diagram of the internal structure of the conversion and demodulation unit  25 . The conversion and demodulation unit  25  is provided with an ADC unit  201  that converts the analog electric signal into a digital signal, a dispersion estimation and compensation unit  202  that estimates and compensates wavelength dispersion and polarization dispersion, a carrier estimation unit  203  that extracts a carrier from the signal that has been digital conversion and reproduces a clock, and a QPSK demodulation unit  204  that demodulates a QPSK signal and generates an electric signal (output signal) having the same waveform as an original signal. 
     Here, the polarization dispersion is such a phenomenon that, due to distortion of an optical fiber, a reflection direction of light in the fiber is divided into slow components and rapid components, with the result that a difference is generated in the time of arrival, and a signal component width is increased. 
     The ADC unit  201  monitors average amplitude of the input analog electric signal and outputs a monitored value to the comparator  27 . 
     The dispersion estimation and compensation unit  202  outputs a dispersion estimation result to a reference generation circuit  26 . The carrier estimation unit  203  detects phase states of the signal light and local light of the signal that has been subjected to the dispersion compensation and performs phase correction. The QPSK demodulation unit  204  demodulates the phase-corrected signal and restores the signal to a data signal that is identical to the transmission signal in the optical transmitter. 
       FIG. 4  is a block diagram showing the internal structure of the amplitude control unit  102 . The amplitude control unit  102  is provided with the reference generation circuit  26  to which a dispersion estimation result is input from the conversion and demodulation unit  25  and which generates a reference value and the comparator (COMP)  27  to which an amplitude monitored value of the analog electric signal is input from the conversion and demodulation unit  25 , and which compares the value with the reference value input from the reference generation circuit  26  and outputs a gain control signal to the analog amplifier  24 . 
     As shown in  FIG. 4 , the reference generation circuit  26  is provided with an input unit  104  serving as an interface to which the dispersion estimation value output from the dispersion estimation and compensation unit  202  is input, a computation unit  105  that calculates an optimal reference value on the basis of the dispersion estimation value, and an output unit  106  serving as an interface for outputting the reference value to the comparator  27 . The structure described above is merely an example, and another device structure may be used. 
     As shown in  FIG. 4 , the comparator  27  is provided with an input unit  107  serving as an interface to which the reference value output from the reference generation circuit  26  and the amplitude monitored value output from the ADC unit are input, a comparison unit  108  that compares the reference value and the amplitude monitored value with each other, and an output unit  109  serving as an interface for outputting a gain control signal based on a result of the comparison between the reference value and amplitude monitored value to the analog amplifier  24 . The structure described above is merely an example, and another device structure may be used. 
     Explanation of Processing 
     Subsequently, with reference to  FIG. 2  and  FIG. 3 , processing of a communication system  100  according to this embodiment will be described. The optical transmitter  1  uses an input electric signal to perform DP-QPSK modulation for CW light from the LD  11  and transmits the light to the transmission channel. The optical receiver  2  inputs a DP-QPSK optical signal received from the transmission channel and local light generated in the local light generation unit  21  to the 90-degree hybrid circuit  22 . The 90-degree hybrid circuit  22  respectively divides the input optical signal and local signal into four and adjusts the polarization and the phase in such a manner that the DP-QPSK signal can be demodulated to multiplex the optical signal and the local light. 
     The 90-degree hybrid circuit  22  inputs the multiplexed light to the four PDs  23  and causes the signal light and the local light to interfere with each other, thereby converting phase modulation into amplitude modulation and generating an analog electric signal. The analog electric signal is faint, so the signal is amplified to a predetermined amplitude by the analog amplifiers  24 . At this time, the comparator  27  uses a control signal to adjust a gain of the analog amplifier  24  in such a manner that the signal amplitude in the input of the ADC unit  201  in the conversion and demodulation unit  25  becomes constant. 
       FIG. 5  is a flowchart showing the processing of a method for controlling the communication system  100 . The optical signal transmitted from the optical transmitter  1  is received by the optical receiver  2 . The optical receiver performs computation of a wavelength dispersion estimation value of the received optical signal (S 100 ). The optical receiver  2  compares the wavelength dispersion estimation value with the predetermined reference value to determine whether the wavelength dispersion estimation value is larger than the reference value or not (S 101 ). 
     In the case where the optical receiver  2  determines that the wavelength dispersion value is larger than the reference value (S 101 : yes), the optical receiver  2  sets the reference value to be small (S 102 ). On the basis of the reference value, the optical receiver  2  controls the amplitude of the analog electric signal to be small (S 103 ). In the case where the optical receiver  2  determines that the dispersion value is smaller than the reference value (S 101 : no), the optical receiver  2  sets the reference value to be large (S 104 ). On the basis of the reference value, the optical receiver  2  controls the amplitude of the analog electric signal to be large (S 105 ). 
       FIG. 6  is a sequence diagram showing processing of each unit of the optical receiver  2 . 
     As an initial state, the reference generation circuit  26  generates a reference value in the case where the wavelength dispersion estimation value is at the maximum, that is, a minimum reference value, and the comparator  27  uses the reference value to set the gain of the analog amplifier  24 . Here, in the case of the wavelength dispersion of the input transmission signal is at the maximum, the maximum peak value of the signal amplitude is obtained. The comparator  27  performs control to set the gain of the TIA  24  to be small with the minimum reference value, so the input amplitude to the ADC unit  201  does not exceed the dynamic range. 
     The analog amplifier  24  amplifies the analog electric signal and inputs the signal to the conversion and demodulation unit  25 . 
     The conversion and demodulation unit  25  samples the input analog electric signal in the ADC unit  201  and converts amplitude information into a digital signal. 
     The ADC unit  201  monitors the amplitude of the analog electric signal and outputs a monitored value to the comparator  27 . The ADC unit  201  performs sampling on the basis of the waveform of the analog signal, converts the analog signal into a digital signal, and inputs the signal to the dispersion estimation and compensation unit  202 . The dispersion estimation and compensation unit  202  estimates a wavelength dispersion value and a polarization dispersion value by a numerical value calculation and compensates the wavelength dispersion on the basis of the estimation result. 
     The dispersion estimation and compensation unit  202  also outputs the wavelength dispersion estimation value to the reference generation circuit  26 . The carrier estimation unit  203  detects phase states of the signal light and the local light of the signal that has been subjected to the dispersion compensation and performs phase correction. The QPSK demodulation unit  204  demodulates the signal that has been subjected to the phase correction and restores the signal to the original signal. 
     The reference generation circuit  26  inputs the wavelength dispersion estimation value from the dispersion estimation and compensation unit  202  of the conversion and demodulation unit  25  through the input unit  104 . The input unit  104  outputs the wavelength dispersion estimation value to the computation unit  105 . The computation unit calculates an optimal reference value corresponding to the wavelength dispersion value in such a manner that the input amplitude to the ADC unit  201  is optimized in accordance with the wavelength dispersion estimation value from the dispersion estimation and compensation unit  202 . As the reference value computation means, a table may be referred to, or calculation may be used. The computation unit outputs the reference value to the comparator  27  through the output unit  106 . 
     Here, in the case where the wavelength dispersion of the transmission signal is smaller than a predetermined reference value, a difference between signal amplitude after the dispersion compensation and a peak value of signal amplitude of an electric signal before the dispersion compensation becomes small. As a result, in order to enhance reception sensitivity of a signal, it is possible to increase the input amplitude of the analog signal to the ADC unit  201 . In view of this, in signal processing in the conversion and demodulation unit  25 , the gain of the TIA is increased so as not to be outside of the dynamic range, and the input amplitude to the ADC unit is optimized. 
     The comparator  27  inputs the amplitude monitored value of the analog electric signal from the ADC unit  201  of the conversion and demodulation unit  25  and the reference value from the reference generation circuit  26  through the input unit  107 . The input unit  107  outputs the amplitude monitored value and the reference value to the comparison unit  108 . The comparison unit  108  compares the amplitude monitored value and the reference value with each other. 
     Through the output unit  109 , the comparison unit  108  performs control to set the gain of the analog amplifier  24  to be small in the case where the amplitude monitored value is larger than the reference value and performs control to set the gain of the analog amplifier  24  to be large in the case where the amplitude monitored value is smaller than the reference value. 
     That is, in the case where the amplitude monitored value is larger than the reference value, the comparison unit  108  performs control to set the amplitude of the analog electric signal to be small. Further, in the case where the amplitude monitored value is smaller than the reference value, the comparison unit  108  performs control to set the amplitude of the analog electric signal to be large. Note that in the case where the reference value is equal to the amplitude monitored value, the comparison unit  108  does not control the amplitude of the analog electric signal. By this control, the analog amplifier  24  makes an adjustment in such a manner that the amplitude of the analog signal to be output is equal to the reference value output from the reference generation circuit  26 . 
     In the analog electric signal output from the analog amplifier  24 , a waveform thereof is distorted due to the dispersion of the transmission channel, a temporal distribution of the signal is expanded along with an increase in the wavelength dispersion value, and a variation of the amplitude is increased due to superposition of the signals. As a result, in the case where the wavelength dispersion of the transmission channel is large, the peak value of the electric signal before the dispersion compensation is increased relative to the signal amplitude after the dispersion compensation. On the other hand, in the case where the wavelength dispersion value of the transmission channel is small, the temporal distribution of the signal is small, and the variation of the amplitude becomes small, so it is possible to increase the input amplitude of the analog signal to the ADC unit  201  within an acceptable range of the ADC unit  201 . 
     Further, as an example, the control method for the amplitude of the analog electric signal may be a method as follows: in the case where a first amplitude corresponding to a first wavelength dispersion value as a reference is provided, if a second wavelength dispersion value is smaller than the first wavelength dispersion value, control is performed to set a second amplitude to be larger relative to the first amplitude, and if the second wavelength dispersion value is larger than the first wavelength dispersion value, control is performed to set the second amplitude to be smaller relative to the first amplitude. 
     Explanation of Effects 
     According to this embodiment, the wavelength dispersion value of the reception signal estimated by the dispersion estimation and compensation unit  202  of the conversion and demodulation unit  25  is input to the reference generation circuit, and in accordance with the estimated wavelength dispersion value, the reference value can be changed. As a result, the gain control for the analog amplifier  24  is performed in such a manner that the input amplitude of the analog electric signal to the ADC unit  201  in accordance with the wavelength dispersion value of the reception signal, making it possible to optimally adjust the reception sensitivity. 
     Through the processing described above, the DP-QPSK optical receiver  2  adjusts the amplitude of the analog electric signal in the input of the ADC unit  201  of the optical receiver  2  to be the optimal value at all times in accordance with the wavelength dispersion value of the transmission channel of the optical signal, with the result that a reduction in the reception sensitivity of the signal can be avoided. 
     Embodiment 2 
     In Embodiment 1, the example by the DP-QPSK communication system is described. In this embodiment, an example using a DP-BPSK (Dual Polarization-Binary Phase Shift Keying) system is described.  FIG. 7  shows a structural example of an optical communication system  300  according to this embodiment.  FIG. 7  shows a structure in the case where the embodiment of the present invention is applied to a DP-BPSK modulation system. In  FIG. 7 , an optical transmitter and a digital coherent receiver with the DP-BPSK system are shown as an example. The optical communication system  300  is provided with a DP-BPSK transmitter (optical transmitter)  3  and a digital coherent DP-BPSK receiver (optical receiver)  4 . 
       FIG. 8  is a diagram showing the structure of the optical transmitter  3 . The optical transmitter  3  is provided with a circuit for generating and transmitting a DP-BPSK signal. The optical transmitter  3  is provided with a LD (Laser Diode)  31  that generates CW (Continuous Wave) light as a light source of signal light and a DP-BPSK modulator  32  that performs DP-BPSK modulation for CW light of the LD  31  with an input electric signal (input signal). 
     In the DP-BPSK modulator  32 , the CW light is divided into two light beams, the two light beams are respectively subjected to BPSK modulation, then polarization planes are orthogonalized at 90 degrees, and performs multiplexing, thereby generating a polarization multiplexed signal. The optical transmitter  3  indicates the structure of a general DP-BPSK transmitter. 
       FIG. 9  is a diagram showing the structure of the optical receiver  4 . The optical receiver  4  is provided with a photoelectric conversion unit  301  to which an optical signal is input and which causes the light to be interfered with local light and converts the optical signal into an electric signal, a conversion and demodulation unit  45  that converts the analog electric signal that is input from the photoelectric conversion unit  301  into a digital signal and performs wavelength dispersion compensation and demodulation for the DP-BPSK signal by digital processing, and an amplitude control unit  302 . 
     The photoelectric conversion unit  301  is provided with a local light generation unit (LO; Local Oscillator)  41  that generates local light, a 90-degree hybrid circuit  42  to which the local light from the LO  41  and the received DP-BPSK signal are input and which causes the light and the signal to be interfered with each other, PDs (Photo Diodes)  43  that perform coherent detection for the optical signal interfered in the 90-degree hybrid circuit  42  and convert the signal into an analog electric signal, and analog amplifiers (TIAs; Trans Impedance AMPs)  44  to which the analog electric signal from the PDs  43  is input and which amplify the analog electric signal to a predetermined amplitude by a gain control signal from a comparator (COMP; Comparator)  47 . 
       FIG. 9  shows a block diagram of the internal structure of the conversion and demodulation unit  45 . The conversion and demodulation unit  45  is provided with an ADC unit  401  that converts the analog electric signal into a digital signal, a dispersion estimation and compensation unit  402  that performs estimation and compensation for wavelength dispersion and polarization dispersion, a carrier estimation unit  403  that extracts a carrier from the signal that has been subjected to the digital conversion and reproduces a clock, and a BPSK demodulation unit  404  that demodulates a BPSK signal to generate an electric signal (output signal) having the same waveform as the original signal. 
     The ADC unit  401  monitors average amplitude of the input analog electric signals and outputs a monitored value to the comparator  47 . 
     The dispersion estimation and compensation unit  402  outputs a dispersion estimation result to a reference generation circuit  46 . The carrier estimation unit  403  detects phase states of the local light and the signal light that have been subjected to the dispersion compensation and performs phase correction. The BPSK demodulation unit  404  demodulates the phase-corrected signal and restores the signal to an transmission signal having the same waveform as the original signal. 
       FIG. 10  is a diagram showing the internal structure of the amplitude control unit  302 . The amplitude control unit  302  is provided with the reference generation circuit  46  to which a wavelength dispersion estimation result is input from the conversion and demodulation unit  45  and which generates a reference value and the comparator (COMP)  47  to which the amplitude monitored value of the analog electric signal is input from the conversion and demodulation unit  45  and which compares the reference value that is input from the reference generation circuit  46  therewith and outputs a gain control signal to the analog amplifier  44 . 
     As shown in  FIG. 10 , the reference generation circuit  46  is provided with an input unit  304  serving as an interface to which the wavelength dispersion estimation value output from the dispersion estimation and compensation unit  402  is input, a computation unit  305  that calculates an optimal reference value on the basis of the wavelength dispersion estimation value, and an output unit  306  serving as an interface for outputting the reference value to the comparator  47 . The structure described above is merely an example, and another device structure may be used. 
     As shown in  FIG. 10 , the comparator  47  is provided with an input unit  307  serving as an interface to which the reference value output from the reference generation circuit  46  and the amplitude monitored value output from the ADC unit are input, a comparison unit  308  that compares the reference value and the amplitude monitored value with each other, and an output unit  309  serving as an interface for outputting a gain control signal based on a result of the comparison between the reference value and the amplitude monitored value to the analog amplifier  44 . The structure described above is merely an example, and another device structure may be used. 
     Subsequently, with reference to  FIG. 11  and  FIG. 6 , processing of the communication system  300  according to this embodiment will be described. The optical transmitter  3  uses the input electric signal to perform the DP-BPSK modulation for the CW light from the LD  31  and transmits the light to the transmission channel. The optical receiver  4  inputs the DP-BPSK optical signal input from the transmission channel and the local light generated in the local light generation unit  41  to the 90-degree hybrid circuit  42 . The 90-degree hybrid circuit  42  divides the input optical signal and the local light into two, respectively, adjusts the polarization and the phase so as to be able to demodulate the DP-BPSK signal, and multiplexes the optical signal and the local light. 
     The 90-degree hybrid circuit  42  inputs the multiplexed light to the two PDs  43  and causes the signal light and the local light to be interfered with each other, thereby converting the phase modulation into an amplitude modulation and generating an analog electric signal. The analog electric signal is faint, so the signal is amplified to a predetermined amplitude by the analog amplifiers  44 . At this time, the comparator  47  uses a control signal to adjust a gain of the analog amplifier  44  in such a manner that the signal amplitude in the input of the ADC unit  401  in the conversion and demodulation unit  45  becomes constant. 
       FIG. 11  is a flowchart showing the processing of a communication method for the communication system  300 . The optical signal transmitted from the optical transmitter  3  is received by the optical receiver  4 . The optical receiver performs computation of a wavelength dispersion estimation value of the received optical signal (S 200 ). The optical receiver  4  compares the wavelength dispersion estimation value with the predetermined reference value to determine whether the wavelength dispersion estimation value is larger than the reference value or not (S 201 ). 
     In the case where the optical receiver  4  determines that the dispersion value is larger than the reference value (S 201 : yes), the optical receiver  4  sets the reference value to be small (S 202 ). On the basis of the reference value, the optical receiver  4  controls the amplitude of the analog electric signal to be small (S 203 ). In the case where the optical receiver  4  determines that the dispersion value is smaller than the reference value (S 201 : no), the optical receiver  4  sets the reference value to be large (S 204 ). On the basis of the reference value, the optical receiver  4  controls the amplitude of the analog electric signal to be large (S 205 ). 
     The processing of the units of the optical receiver  4  is the same as the sequence diagram shown in  FIG. 6 . Hereinafter, as shown in  FIG. 6 , the processing of each component of the communication system  300  will be described in detail. 
     As an initial state, the reference generation circuit  46  generates a reference value in the case where the wavelength dispersion estimation value is at the maximum, that is, a minimum reference value, and the comparator  47  uses the reference value to set the gain of the analog amplifier  44 . In the case of the maximum wavelength dispersion of the transmission signal, the maximum peak value of the signal amplitude is obtained. The comparator  47  performs control to set the gain of the TIA  44  to be small with the minimum reference value, so the input amplitude to the ADC unit  401  does not exceed the dynamic range. 
     The analog amplifier  44  amplifies the analog electric signal and inputs the signal to the conversion and demodulation unit  45 . The conversion and demodulation unit  45  samples the input analog electric signal in the ADC unit  401  and converts amplitude information to a digital signal. 
     The ADC unit  401  monitors the amplitude of the electric signal and outputs a monitored value to the comparator  47 . The ADC unit  401  performs sampling on the basis of the waveform of the analog signal, converts the analog signal into a digital signal, and inputs the signal to the dispersion estimation and compensation unit  402 . The dispersion estimation and compensation unit  402  estimates a wavelength dispersion value and a polarization dispersion value by a numerical value calculation and compensates the dispersions on the basis of the estimation result. 
     The dispersion estimation and compensation unit  402  also outputs the wavelength dispersion estimation value to the reference generation circuit  46 . The carrier estimation unit  403  detects phase states of the signal light and the local light of the signal that has been subjected to the dispersion compensation and performs phase correction. The BPSK demodulation unit  404  demodulates the signal that has been subjected to the phase correction and restores the signal to the original signal. 
     The reference generation circuit  46  inputs the wavelength dispersion estimation value from the dispersion estimation and compensation unit  402  of the conversion and demodulation unit  45  through the input unit  304 . The input unit  304  outputs the wavelength dispersion estimation value to the computation unit  305 . The computation unit calculates an optimal reference value corresponding to the wavelength dispersion value in such a manner that the input amplitude to the ADC unit  301  becomes optimal, in accordance with the wavelength dispersion estimation value from the dispersion estimation and compensation unit  402 . As the reference value computation means, a table may be referred to, or calculation may be used. The computation unit outputs the reference value to the comparator  47  through the output unit  306 . 
     Here, in the case where the wavelength dispersion of the transmission signal is smaller than the predetermined reference value, a difference between signal amplitude after the dispersion compensation and a peak value of signal amplitude of an electric signal before the dispersion becomes small. As a result, in order to enhance reception sensitivity of a signal, it is possible to increase the input amplitude of the analog signal to the ADC unit  301 . In view of this, in signal processing in the conversion and demodulation unit  45 , the gain of the TIA is increased so as not to be outside of the dynamic range, and the input amplitude to the ADC unit is optimized. 
     The comparator  47  inputs the amplitude monitored value of the analog electric signal from the ADC unit  301  of the conversion and demodulation unit  45  and the reference value from the reference generation circuit  46  through the input unit  307 . The input unit  307  outputs the amplitude monitored value and the reference value to the comparison unit  308 . The comparison unit  308  compares the amplitude monitored value and the reference value with each other. 
     Through the output unit  309 , the comparison unit  308  performs control to set the gain of the analog amplifier  44  to be small in the case where the amplitude monitored value is larger than the reference value and performs control to set the gain of the analog amplifier  44  to be large in the case where the amplitude monitored value is smaller than the reference value. 
     That is, in the case where the amplitude monitored value is larger than the reference value, the comparison unit  308  performs control to set the amplitude of the analog electric signal to be small. Further, in the case where the amplitude monitored value is smaller than the reference value, the comparison unit  308  performs control to set the amplitude of the analog electric signal to be large. Note that in the case where the reference value is equal to the amplitude monitored value, the comparison unit  308  does not control the amplitude of the analog electric signal. By this control, the analog amplifier  44  makes an adjustment in such a manner that the amplitude of the analog signal to be output is equal to the reference value output from the reference generation circuit  46 . 
     In the analog electric signal output from the analog amplifier  44 , a waveform thereof is distorted due to the dispersion of the transmission channel, a temporal distribution of the signal is expanded along with an increase in the wavelength dispersion value, and a variation of the amplitude is increased due to superposition of the signals. As a result, in the case where the wavelength dispersion of the transmission channel is large, the peak value of the electric signal before the dispersion compensation is increased relative to the signal amplitude after the dispersion compensation. On the other hand, in the case where the wavelength dispersion value of the transmission channel is small, the temporal distribution of the signal is small, and the variation of the amplitude becomes small, so it is possible to increase the input amplitude of the analog signal to the ADC unit  401  within an acceptable range of the ADC unit  401 . 
     Further, as an example, the control method for the amplitude of the analog electric signal may be a method as follows: in the case where a first amplitude corresponding to a first wavelength dispersion value as a reference is provided, if a second wavelength dispersion value is smaller than the first wavelength dispersion value, control is performed to set a second amplitude to be larger relative to the first amplitude, and if the second wavelength dispersion value is larger than the first wavelength dispersion value, control is performed to set the second amplitude to be smaller relative to the first amplitude. 
     Explanation of Effects 
     According to this embodiment, the wavelength dispersion value of the reception signal which is estimated by the dispersion estimation and compensation unit  402  of the conversion and demodulation unit  45  is input to the reference generation circuit, and in accordance with the estimated wavelength dispersion value, the reference value can be changed. As a result, it is possible to perform the gain control for the analog amplifier  44  in such a manner that the input amplitude to the ADC unit  401  becomes optimal in accordance with the wavelength dispersion value of the reception signal, and optimally adjust the reception sensitivity. 
     Through the above processing, in accordance with the wavelength dispersion value of the transmission channel of the optical signal, the DP-BPSK optical receiver  2  can adjust the amplitude of the analog electric signal in the input of the ADC unit  401  of the optical receiver  4  to an optimal value at all times, with the result that a reduction in the reception sensitivity of the signal can be avoided. 
     Other Embodiments 
     The optical communication method described in the above embodiment is realized using semiconductor processing units each including an application specific integrated circuit (ASIC). These types of processing may be performed by causing a computer system including at least one processor (e.g. microprocessor, MPU, or digital signal processor (DSP)) to execute a program. Specifically, these types of processing may be performed by generating one or more programs including instructions for causing a computer system to execute an algorithm about the types of processing and then providing these programs to a computer. 
     These programs may be stored in various types of non-transitory computer-readable media and then provided to a computer. Such non-transitory computer-readable medium include various types of tangible storage media. 
     Examples of the non-transitory computer-readable media include magnetic storage media (e.g. flexible disks, magnetic tapes, hard disk drives), magneto-optical storage media (e.g. magneto-optical disks), compact disc read-only memory (CD-ROM), CD-R, CD-R/W, semiconductor memory (e.g. mask ROM, programmable ROM (PROM), erasable PROM (EPROM), flash ROM, and random access memory (RAM). 
     The programs may be provided to a computer by various types of transitory computer-readable media. Examples of such transitory computer-readable media include electric signals, optical signals, and electromagnetic waves. Such transitory computer-readable media can provide the programs for a computer through a wire communication path such as an electric line or an optical fiber, or a wireless communication path. 
     Note that the present invention is not limited to the above embodiments and can be appropriately changed without departing from the gist of the present invention. For example, the present invention may be applied to a quadrature amplitude modulation (QAM), an orthogonal frequency division multiplexing (OFDM), or other digital communication systems. 
     Further, the above embodiment is only illustrative of the application of the technical idea obtained by the present inventors. That is, the technical idea, is not limited to only the above embodiment, and various changes can, of course, be made to the embodiment. 
     For example, part or all of the above embodiment can be described as the Supplementary Notes below, but the embodiment is not limited thereto. 
     (Supplementary Note 1) 
     A communication system which comprises an optical transmitter to which an transmission signal is input, and which modulates the transmission signal to an optical signal and transmits the optical signal: and 
     an optical receiver that receives the optical signal and demodulates the optical signal to an transmission signal, wherein 
     the optical receiver includes a photoelectric conversion means for converting the optical signal into an analog electric signal, a conversion and demodulation means for converting the analog electric signal into a digital signal and demodulating the signal to the transmission signal, and an amplitude control means for controlling amplitude of the analog electric signal, and 
     the amplitude control means controls the amplitude of the analog electric signal in accordance with wavelength dispersion of the optical signal. 
     (Supplementary Note 2) 
     The communication system according to Supplementary Note 1, wherein the amplitude control means controls the amplitude of the analog electric signal to become smaller, as the wavelength dispersion of the optical signal is increased. 
     (Supplementary Note 3) 
     The communication system according to Supplementary Note 1 or 2, wherein 
     the photoelectric conversion means includes an amplifier that amplifies the analog electric signal input to the conversion and demodulation means, and 
     the amplitude control means estimates a wavelength dispersion value of the optical signal and determines, on the basis of the estimated wavelength dispersion value, a reference value for controlling a gain of the amplifier. 
     (Supplementary Note 4) 
     An optical receiver comprising: 
     a photoelectric conversion means for converting an optical signal into an analog electric signal; 
     a conversion and demodulation means for converting the analog electric signal into a digital signal and demodulating the signal to an transmission signal; and 
     an amplitude control means for controlling amplitude of the analog electric signal, 
     wherein the amplitude control means controls the amplitude of the analog electric signal in accordance with wavelength dispersion of the optical signal. 
     (Supplementary Note 5) 
     The optical receiver according to Supplementary Note 4, wherein the amplitude control means controls the amplitude of the analog electric signal to become smaller, as the wavelength dispersion of the optical signal is increased. 
     (Supplementary Note 6) 
     The optical receiver according to Supplementary Note 4 or 5, wherein 
     the photoelectric conversion means includes an amplifier that amplifies the analog electric signal input to the conversion and demodulation means, and 
     the amplitude control means estimates a wavelength dispersion value of the optical signal and determines, on the basis of the estimated wavelength dispersion value, a reference value for controlling a gain of the amplifier. 
     (Supplementary Note 7) 
     An optical receiver control method, 
     wherein an optical receiver is configured to convert an optical signal into an analog electric signal, convert the analog electric signal into a digital signal, and demodulate the signal to an transmission signal, 
     the control method comprising controlling amplitude of the analog electric signal in accordance with wavelength dispersion of the optical signal. 
     (Supplementary Note 8) 
     The optical receiver control method according to Supplementary Not 7, wherein the controlling includes controlling the amplitude of the analog electric signal to become smaller, as the wavelength dispersion of the optical signal is increased. 
     (Supplementary Note 9) 
     The optical receiver control method according to Supplementary Note 7 or 8, wherein 
     the optical receiver includes an amplifier that amplifies the analog electric signal input to a conversion and demodulation means, and 
     the controlling includes estimating a wavelength dispersion value of the optical signal and determining, on the basis of the estimated wavelength dispersion value, a reference value for controlling a gain of the amplifier. 
     (Supplementary Note 10) 
     A non-transitory computer readable medium for causing a computer to execute an optical receiver control method, wherein 
     an optical receiver is configured to convert an optical signal into an analog electric signal, convert the analog electric signal into a digital signal, and demodulate the digital signal to an transmission signal, and 
     the control method includes controlling amplitude of the analog electric signal in accordance with wavelength dispersion of the optical signal. 
     (Supplementary Note 11) 
     The non-transitory computer readable medium according to Supplementary Not 10, wherein the controlling includes controlling the amplitude of the analog electric signal to become smaller, as the wavelength dispersion of the optical signal is increased. 
     (Supplementary Note 12) 
     The non-transitory computer readable medium according to Supplementary Note 10 or 11, wherein 
     the optical receiver includes an amplifier that amplifies the analog electric signal input to a conversion and demodulation means, and 
     the controlling includes estimating a wavelength dispersion value of the optical signal and determining, on the basis of the estimated wavelength dispersion value, a reference value for controlling a gain of the amplifier. 
     While the invention of the present application has been described with reference to the embodiment, the invention is not limited thereto. Various changes understandable by those skilled in the art can be made to the configuration or details of the invention of the present application without departing from the scope of the invention. 
     The present application claims priority based on Japanese Patent Application No. 2013-145488, filed on Jul. 11, 2013, the disclosure of which is incorporated herein in its entirety. 
     REFERENCE SIGNS LIST 
     
         
           1  optical transmitter 
           2  optical receiver 
           3  optical transmitter 
           4  optical receiver 
           11  LD (Laser Diode) 
           12  DP-QPSK modulator 
           21  local light generation unit (LO; Local Oscillator) 
           22  90-degree hybrid circuit 
           23  PD (Photo Diode) 
           24  analog amplifier (TIA; Trans Impedance AMP) 
           25  conversion modulation unit (DSP) 
           26  reference generation circuit 
           27  comparator (COMP) 
           31  LD (Laser Diode) 
           32  DP-BPSK modulator 
           41  local light generation unit (LO; Local Oscillator) 
           42  90-degree hybrid circuit 
           43  PD (Photo Diode) 
           44  analog amplifier (TIA; Trans Impedance AMP) 
           45  conversion and demodulation unit (DSP) 
           46  reference generation circuit 
           47  comparator (COMP) 
           100  optical communication system 
           101  photoelectric conversion unit 
           102  amplitude control unit 
           104  input unit 
           105  computation unit 
           106  output unit 
           107  input unit 
           108  comparison unit 
           109  output unit 
           201  ADC unit 
           202  dispersion estimation and compensation unit 
           203  carrier estimation unit 
           204  QPSK demodulation unit 
           300  optical communication system 
           301  photoelectric conversion unit 
           302  amplitude control unit 
           304  input unit 
           305  computation unit 
           306  output unit 
           307  input unit 
           308  comparison unit 
           309  output unit 
           401  ADC unit 
           402  dispersion estimation and compensation unit 
           403  carrier estimation unit 
           404  BPSK demodulation unit 
           500  optical communication system 
           501  ADC unit 
           521  local light generation unit (LO; Local Oscillator) 
           522  90-degree hybrid circuit 
           523  PD (Photo Diode) 
           524  analog amplifier (TIA; Trans Impedance AMP) 
           525  conversion and demodulation unit (DSP) 
           526  reference generation circuit 
           527  comparator (COMP) 
           550  optical transmitter 
           560  optical receiver