Abstract:
With respect to a coherent optical receiving apparatus, a polarization multiplexing light signal, whereupon a first signal is placed upon a first polarized wave light and a second signal is placed upon a second polarized wave light, is polarization divided upon the transmitting side thereof, and the first signal and the second signal cannot be received in correspondence with the transmitting side. Accordingly, disclosed is a coherent optical receiving apparatus, comprising a coherent light receiving unit that detects coherent light, and a signal processing unit that carries out signal processing that is set with control coefficients. The coherent light receiving unit receives a first polarized light that is modulated with a first transmitted signal and outputs a first detected signal, and simultaneously receives the first polarized light with a second polarized light that is modulated with a second transmitted signal, and outputs a second detected signal. The signal processing unit establishes a first control coefficient on the basis of the first detected signal, and establishes a second control coefficient on the basis of the first control coefficient and the second detected signal, and employs the second control coefficient to output a first received signal corresponding to the first transmitted signal, and a second received signal corresponding to the second transmitted signal.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to coherent optical receivers, coherent optical communications systems provided therewith, and coherent optical communications methods and, in particular, to a coherent optical receiver which receives polarization multiplexing optical signals by means of coherent detection and digital signal processing, and to a coherent optical communications system employing same and a coherent optical communications method. 
       BACKGROUND ART 
       [0002]    The data capacity in the network has been increasing year by year due to the wide spread of the Internet. In the trunk line connecting metropolitan areas, the optical transmission link whose transmission capacity per one channel is 10 Gb/s or 40 Gb/s has already been introduced. On-Off-Keying (OOK) is employed as a modulation scheme in 10 Gb/s transmission. On the other hand, the OOK scheme is unsuitable for long-haul transmission because the transmission characteristics are greatly influenced by the chromatic dispersion due to the narrow optical pulse width of 25 ps in 40 Gb/s transmission systems. Therefore, the multilevel modulation scheme using phase modulation has been adopted, and Quadrature-Phase-Shift-Keying (QPSK) scheme is mainly employed for 40 Gb/s transmission systems. 
         [0003]    In the 100 Gb/s class super high speed optical transmission, it is necessary to suppress the influence of chromatic dispersion by widening the optical pulse width, that is, by decreasing the baud rate by means of increasing the multiplicity. A polarization multiplexing scheme is a method for achieving the above. In the polarization multiplexing scheme, two systems of the optical signals are inputted into an optical fiber with the oscillation planes of electric field intensity E X  and E Y  orthogonal to each other. A signal light with electric field intensity of E X  and a signal light with electric field intensity of E Y  propagate with their oscillation planes rotating randomly keeping the orthogonal relation in an optical fiber. The orthogonal signal light E X +E Y  is obtained whose rotation angle θ is unknown at the output end of the optical fiber. In this specification, signal light E X  represents a signal light with electric field intensity of E X , and signal light E X +E Y  represents a signal light in which the oscillation directions of electric field intensity E X  and E Y  are orthogonal to each other. 
         [0004]    It is known that a polarization demultiplexing scheme includes an optical scheme and a signal processing scheme. In the optical scheme, the polarization demultiplexing is performed by using a polarization control element and a polarization splitter. When the orthogonal signal light E X +E Y  is projected on the polarization planes of E X ′ and E Y ′ which the polarization control element defines and is separated, the signal lights of E X ′=aE X +bE Y  and E Y ′=cE X +dE Y  are obtained. While monitoring the output signal after the separation, the rotation angle θ is estimated by providing feedback to the polarization control element so that the output signal will become maximum, that is, E X ′=aE X  (b=0) and E Y ′=dE Y  (c=0). However, the polarization control element is not able to follow fast polarization fluctuations because its control cycle is about 100 MHz. 
         [0005]    On the other hand, in the signal processing scheme, the polarization demultiplexing is performed after coherent detection of the orthogonal signal light and conversion into electric signal. When the orthogonal signal light E X +E Y  is projected on the polarization planes of X′ and Y′ which the local light defines and is detected, the electric field information of the signal light is obtained as electric signals. 
         [0006]    An example of the coherent optical receiver using such signal processing scheme is described in the patent literature 1. According to the coherent optical receiver in the patent literature 1, local oscillator light has orthogonal polarization components in which the optical frequencies are different to each other. The local oscillator light and the received signal light are combined by a 2×4 optical hybrid circuit. After that two differential optical detectors perform differential photoelectric conversion, and then analog-to-digital (AD) conversion circuits convert the analog received signals output from the differential optical detectors into digital signals. A digital processing circuit estimates received data by executing signal processing for the obtained digital signal. 
         [0000]    Patent Literature 1: Japanese Patent Application Laid-Open No. 2008-153863 (paragraph “0012” and  FIG. 1 )
 
Non Patent Literature 1: D. N. Godard, “Self-Recovering Equalization and Carrier Tracking in Two-Dimensional Data Communication Systems”, IEEE Transactions on Communications, The Institute of Electrical and Electronics Engineers, November, 1980, Vol. COM-28, No. 11, pp. 1867-1875.
 
       DISCLOSURE OF INVENTION 
     Problem to be Solved by the Invention 
       [0007]    First, the method for polarization demultiplexing by signal processing using a related coherent optical receiver will be described.  FIG. 10  is a block diagram showing the configuration of the related coherent optical receiver  500 . The polarization multiplexed signal light S XY (t)=E X +E Y  interferes with local light L X′Y′ (t) from a local oscillator (LO) light source  511  in a 90 degree hybrid circuit  512 , to be signal light E X ′ and E Y ′, which are detected by photo detectors (PD)  513 . The detection signal detected by the photo detector includes electric field information of the signal light. An analog-to-digital converter (ADC)  514  quantizes the detection signal and outputs quantized signals of e x ′ and e y ′ to a digital signal processor (DSP)  515 . In the digital signal processor  515 , the polarization rotation angle θ of e x ′ and e y ′ is canceled by means of a butterfly filter  516  to obtain polarization demultiplexed demodulation signals of e x  and e y . At that time, a CMA processing unit  517  determines the filter parameters by using the Constant Modulus Algorithm (CMA), for example (refer to non patent literature 1). 
         [0008]    In the related coherent optical receiver  500 , quantized demodulation signals of e x  and e y , which are obtained by the process in the digital signal processor (DSP)  515 , include electric field information of E X  and E Y  in the polarization multiplexed signal light S XY . However, it is not always true that the demodulated signal ex corresponds to the electric field information E X  and the demodulated signal e y  corresponds to the electric field information E Y . There are cases where the demodulated signal ex corresponds to the electric field information E Y  and the demodulated signal e y  corresponds to the electric field information E X . The reason is as follows. Since the CMA algorithm merely performs the control for keeping constant the electric field intensity of quantized signals e x ′ and e y ′, it is not able to control which of the electric field information E X  and E Y  the converged demodulated signal e x  or e y  corresponds to. That is to say, it is possible to separate the signals put on two multiplexed polarization lights into two signals by signal processing which just controls the amplitude including the electric field information. However, it is not always possible to receive the transmitted signals put on the X polarization light (or Y polarization light) recognizing on the receiving side that the signals have been put on the X polarization light (or Y polarization light). 
         [0009]    As mentioned above, the related coherent optical receiver has the problem that it is not able to receive the first signal and the second signal included in the polarization multiplexed light signals by performing the polarization demultiplexing corresponding to the transmitter side on which the first signal has been put on the first polarization light, the second signal has been put on the second polarization light, and then these signals have been combined by the polarization multiplexing. 
         [0010]    The object of the present invention is to provide a coherent optical receiver, a coherent optical communications system employing same and a coherent optical communications method which solve the problem mentioned above that it is not able to receive the first signal and the second signal included in the polarization multiplexed light signals by performing the polarization demultiplexing corresponding to the transmitter side on which the first signal has been put on the first polarization light, the second signal has been put on the second polarization light, and then these signals have been combined by the polarization multiplexing. 
       Means for Solving a Problem 
       [0011]    A coherent optical receiving apparatus according to an exemplary aspect of the invention includes a coherent optical receiving unit performing coherent optical detection; and a signal processing unit performing signal processing defined by control parameters; wherein the coherent optical receiving unit outputs a first detection signal receiving a first polarization light modulated by a first transmission signal, and outputs a second detection signal receiving simultaneously the first polarization light and a second polarization light modulated by a second transmission signal; and the signal processing unit determines a first control parameter on the basis of the first detection signal, determines a second control parameter on the basis of the first control parameter and the second detection signal, and outputs a first received signal corresponding to the first transmission signal and a second received signal corresponding to the second transmission signal by using the second control parameter. 
         [0012]    A coherent optical communications system according to an exemplary aspect of the invention includes a transmitter; and a coherent optical receiving apparatus connected to the transmitter through an optical fiber; wherein the transmitter includes a light source; a first modulator modulating output light having first polarization from the light source with a first transmission signal and outputting first polarization light; a second modulator modulating output light having second polarization from the light source with a second transmission signal and outputting second polarization light; an orthogonal multiplexing unit orthogonally multiplexing the first polarization light and the second polarization light and transmitting to the optical fiber; and a transmission control unit controlling intensity of the second polarization light; wherein the coherent optical receiving apparatus includes a coherent optical receiving unit performing coherent optical detection; a signal processing unit performing signal processing defined by control parameters; and a receiving controller unit controlling an operation of the signal processing unit; wherein the coherent optical receiving unit receives the first polarization light and outputs a first detection signal, and receives simultaneously the first polarization light and the second polarization light and outputs a second detection signal; the receiving controller unit instructs the signal processing unit to start a processing to determine a first control parameter when confirming that the coherent optical receiving unit has received the first polarization light, and instructs the signal processing unit to start a processing to determine a second control parameter when confirming that the coherent optical receiving unit has received simultaneously the first polarization light and the second polarization light; and the signal processing unit determines the first control parameter on the basis of the first detection signal, determines the second control parameter on the basis of the first control parameter and the second detection signal, and outputs a first received signal corresponding to the first transmission signal and a second received signal corresponding to the second transmission signal by using the second control parameter. 
         [0013]    A coherent optical communications method according to an exemplary aspect of the invention includes the steps of: transmitting first polarization light obtained by modulating output light having first polarization with a first transmission signal; receiving the first polarization light and obtaining a first detection signal by performing coherent optical detection; transmitting second polarization light obtained by modulating output light having second polarization with a second transmission signal; receiving simultaneously the first polarization light and the second polarization light and obtaining a second detection signal by performing coherent optical detection; determining a first control parameter on the basis of the first detection signal; determining a second control parameter on the basis of the first control parameter and the second detection signal; and obtaining a first received signal corresponding to the first transmission signal and a second received signal corresponding to the second transmission signal by using the second control parameter. 
       Effect of the Invention 
       [0014]    According to the coherent optical receiving apparatus, the coherent optical communications system employing same and the coherent optical communications method by the present invention, it becomes possible to receive the first signal and the second signal included in the polarization multiplexed light signals by performing the polarization demultiplexing corresponding to the transmitter side on which the first signal has been put on the first polarization light, the second signal has been put on the second polarization light, and then these signals have been combined by the polarization multiplexing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a block diagram showing the configuration of a coherent optical receiving apparatus in accordance with the first exemplary embodiment of the present invention. 
           [0016]      FIG. 2  is a block diagram showing the configuration of a digital coherent optical communications system in accordance with the second exemplary embodiment of the present invention. 
           [0017]      FIG. 3  is a block diagram showing the configuration of a digital signal processor (DSP) in accordance with the second exemplary embodiment of the present invention. 
           [0018]      FIG. 4  is a sequence diagram illustrating the initial setting of filter coefficients in a digital signal processor (DSP) in accordance with the second exemplary embodiment of the present invention. 
           [0019]      FIG. 5  is a block diagram showing the configuration of a digital coherent optical communications system in accordance with the third exemplary embodiment of the present invention. 
           [0020]      FIG. 6  is a block diagram showing the configuration of a transmitter and a receiver in accordance with the third exemplary embodiment of the present invention. 
           [0021]      FIG. 7  is a sequence diagram illustrating the initial setting of filter parameters in a digital signal processor (DSP) in accordance with the third exemplary embodiment of the present invention. 
           [0022]      FIG. 8  is a block diagram showing the configuration of a digital coherent optical communications system in accordance with the fourth exemplary embodiment of the present invention. 
           [0023]      FIG. 9  is a block diagram showing the configuration of a digital signal processor (DSP) in accordance with to the fourth exemplary embodiment of the present invention. 
           [0024]      FIG. 10  is a block diagram showing the configuration of the related digital coherent receiver. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0025]    The exemplary embodiments of the present invention will be described with reference to drawings below. 
       The First Exemplary Embodiment 
       [0026]      FIG. 1  is a block diagram showing the configuration of a coherent optical receiving apparatus  100  in accordance with the first exemplary embodiment of the present invention. The coherent optical receiving apparatus  100  has a coherent optical receiving unit  110  performing coherent optical detection and a signal processing unit  120  performing signal processing defined by control parameters. 
         [0027]    The coherent optical receiving unit  110  outputs the first detection signal to the signal processing unit  120  receiving the first polarization light modulated by the first transmission signal, and outputs the second detection signal to the signal processing unit  120  receiving simultaneously the first polarization light and the second polarization light modulated by the second transmission signal. The signal processing unit  120  determines the first control parameters on the basis of the first detection signal and determines the second control parameters on the basis of the first control parameters and the second detection signal. And then the signal processing unit  120  outputs the first received signal corresponding to the first transmission signal and the second received signal corresponding to the second transmission signal by using the second control parameters. 
         [0028]    Here, the signal processing unit  120  can include a filter unit  121  performing signal processing on the basis of control parameters and a control parameter processing unit  122  calculating the control parameters by a control parameter determination algorithm. At that time, the control parameter processing unit  122  determines the first control parameters so that the output signal of the filter unit  121  may converge at the first received signal for the input of the first detection signal. The first control parameter is changed so that the output signal of the filter unit  121  may converge at the second received signal for the input of the second detection signal, and the control parameters by which the output signal of the filter unit  121  converges at the second received signal are fixed as the second control parameters. The filter unit  121  outputs the first received signal and the second received signal on the basis of those second control parameters. 
         [0029]    Thus, according to the coherent optical receiving apparatus  100  of this exemplary embodiment, it becomes possible to receive the first transmission signal and the second transmission signal corresponding to the transmitter side by means of receiving the first polarization light modulated by the first transmission signal and the second polarization light modulated by the second transmission signal and performing the polarization demultiplexing. 
       The Second Exemplary Embodiment 
       [0030]    Next, the second exemplary embodiment of the present invention will be described.  FIG. 2  is a block diagram showing the configuration of a coherent optical communications system  200  in accordance with the second exemplary embodiment of the present invention. The coherent optical communications system  200  has a transmitter  210  and a receiver  220 . 
         [0031]    The transmitter  210  includes a signal light source (LD)  211 , a first phase modulator (PM X )  212  as a first modulator and a second phase. modulator (PM Y )  213  as a second modulator. In addition, it has a polarization beam splitter (PBS)  215  as an orthogonal multiplexing unit, and has a variable optical attenuator (VOA)  214  and a controller  216 , which compose a transmission control unit. 
         [0032]    The receiver  220  includes a local light source (LO)  221 , a 90 degree hybrid circuit  222 , and a photo detector (PD)  223 , which compose a coherent optical receiving unit. In addition, it has an analog-to-digital converter (ADC)  224  and a digital signal processor (DSP)  225 , which compose a signal processing unit, and has a receiving controller unit  226 . 
         [0033]    Here, the controller  216  controls the variable optical attenuator (VOA)  214  and the receiving controller unit  226  controls the digital signal processor (DSP)  225 , respectively. The transmitter  210  and the receiver  220  are connected through an optical fiber  230  and communication is performed thereby. Further, the coherent optical communications system  200  in accordance with this exemplary embodiment is provided with a line  240  which enables communication between the controller  216  and the receiving controller unit  226 . 
         [0034]    In the transmitter  210 , the output light from the signal light source (LD)  211  is separated into X polarization light composed of the first polarization light component X and Y polarization light composed of the second polarization light component Y, which are orthogonal to each other, and then they are input into the first phase modulator (PM X )  212  and the second phase modulator (PM Y )  213  respectively. The first phase modulator (PM X )  212  modulates the X polarization light with the first transmission signal and outputs the first signal light E X  with the electric field intensity E X . The second phase modulator (PM Y )  213  modulates the Y polarization light with the second transmission signal and outputs the second signal light E Y  with the electric field intensity E Y . The first signal light E X  and the second signal light E Y  are orthogonally multiplexed in the polarization beam splitter (PBS)  215 , and the orthogonal signal light S XY (=E X +E Y ) is output. Here, the variable optical attenuator (VOA)  214  performs on/off control of the output of the second signal light with its polarization in the Y direction, according to the instructions from the controller  216 . 
         [0035]    The orthogonal signal light S XY (=E X +E Y ) input into the receiver  220  interferes with the local light L X′Y′  from the local light source (LO)  221  in the 90 degree hybrid circuit  222  to be the signal light E X ′, E Y ′ which is projected on arbitrary polarization plane X′, Y′ of the local light L X′Y′ . The signal light E X ′, E Y ′ is detected in the photo detector (PD)  223 , and the electric field information on the signal light E X ′, E Y ′ is input into the analog-to-digital converter (ADC)  224  as a detection signal. The analog-to-digital converter (ADC)  224  quantizes the detection signals, and then outputs quantized signals of e x ′ and e y ′. The quantized signals of e x ′ and e y ′ are processed for polarization demultiplexing in the digital signal processor (DSP)  225 , and demodulated signals of ex and e y  are obtained. 
         [0036]    The configuration of the digital signal processor (DSP)  225  is shown in  FIG. 3 . The digital signal processor (DSP)  225  is provided with a butterfly filter  227 , a memory unit  228 , and a CMA processing unit (CMA)  229 . The butterfly filter  227  performs the matrix operation on the input quantized signals of e x ′ and e y ′ according to the following formula (1), and outputs demodulated signals of e x  and e y . 
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         [0037]    The matrix H is a rotation matrix for canceling the rotation angle of the polarization axis between the polarization plane XY of the transmission side signal light and the polarization plane X′Y′ of the receiver side signal light. Here, since the relation between the polarization axis on the polarization plane XY of the transmission side and that on the polarization plane X′Y′ of the receiver side is not determined uniquely, it is difficult to calculate the matrix presuming the rotation angle. One of the methods to calculate each element of this matrix H is a CMA algorithm (refer to a non patent literature 1, for example). As mentioned below, in this exemplary embodiment, the configuration is employed in which the CMA processing unit (CMA)  229  calculates each element of the matrix H (filter coefficients) by means of the CMA algorithm. The CMA processing unit (CMA)  229  outputs to the butterfly filter  227  the filter coefficients of h 11 , h 12 , h 21 , and h 22  calculated by using the CMA algorithm, when each element of the matrix H is obtained as h xx =h 11 , h xy =h 12 , h yx =h 21 , and h yy =h 22 . 
         [0038]    Next, the behavior of the CMA processing unit (CMA)  229  will be described in detail. The CMA processing unit (CMA)  229  calculates filter coefficients in the subsequent time period using the filter coefficients of h 11 , h 12 , h 21 , and h 22  stored in the memory unit  228 . That is to say, when the filter coefficients in the time period of k are set for h 11 (k), h 12 (k), h 21 (k), and h 22 (k), the CMA processing unit (CMA)  229  calculates the filter coefficients in the time period of k+1, that is, h 11 (k+1), h 12 (k+1), h 21 (k+1), and h 22 (k+1), according to the following formula (2). The CMA processing unit (CMA)  229  overwrites in the memory unit  228  with the calculation results of the filter coefficients in the time period of k+1. If an FIR filter is used for the calculation of the formula (2), the vector h in the formula (2) represents tap coefficients of the FIR filter. 
         [0000]        h   11 ( k+ 1)= h   11 ( k )+με x   e   x ( k )    e   x ′ ( k )
 
         [0000]        h   12 ( k+ 1)= h   12 ( k )+με x   e   x ( k )    e   y ′ ( k )
 
         [0000]        h   21 ( k+ 1)= h   21 ( k )+με y   e   y ( k )    e   x ′ ( k )
 
         [0000]        h   22 ( k+ 1)= h   22 ( k )+με y   e   y ( k )    e   y ′ ( k )  (2)
 
         [0039]    Here, ε x  and ε y  represent error functions, which are expressed by the following formula. 
         [0000]      ε x =1−| e   x  ( k )| 2 , ε y =1−| e   y  ( k )| 2   (3)
 
         [0040]    Here, μ is a constant and a bar represents conjugate complex number. 
         [0041]    The CMA algorithm performs control of keeping the intensity of the quantized signals of e x ′ and e y ′ constant using the error functions of ε x  and ε y . However, based solely on the information on the electric field intensity, it is indistinguishable whether the data in the quantized signals correspond to the information put on the X polarization light or the information put on the Y polarization light. Therefore, as mentioned above, when using the filter coefficients of h 11 , h 12 , h 21 , and h 22  calculated by using the CMA algorithm, there can be cases where the signal component E X  of the first signal light with X polarization is converged on the demodulated signal e y , and the signal component E Y  of the second signal light with Y polarization is converged on the demodulated signal e x . 
         [0042]    Therefore, in this exemplary embodiment, by setting up an order for calculation of the filter coefficients, the signal components which converge on the demodulated signals of e x  and e y  are controlled. Here, the phenomenon that the demodulated signals are switched with the transmission side does not occur at every updating of the filter coefficients. Therefore, by inputting the correct filter coefficients into the butterfly filter  227  at first and then updating them according to the formula (2) successively, it is possible to determine the filter coefficients with which to enable input signals to converge on the demodulated signals corresponding to the transmission side. A training method will be described below, which is the method for determining the filter coefficients of h 11 , h 12 , h 21 , and h 22  of the butterfly filter  227  with which the signal component E X  with X polarization is converged on the demodulated signal e x  and the signal component E Y  with Y polarization is converged on the demodulated signal e y . 
         [0043]      FIG. 4  is a sequence diagram illustrating the initial setting of the filter coefficients. First, the receiving controller unit  226  in the receiver  220  sets arbitrary filter coefficients, that is h 11 =h 11  (0) and h 12 =h 12  (0), for the memory unit  228  (step S 101 ). In this exemplary embodiment, those are set as h 11  (0)=1 and  12  (0)=0. On the other hand, the controller  216  in the transmitter  210  puts the output signal light with Y polarization into a non-output (OFF) state and puts only output signal light with X polarization into an output (ON) state by controlling the variable optical attenuator (VOA)  214  (step S 102 ). At that time, the only X polarization light is transmitted to the receiver  220  through the optical fiber  230  (step S 103 ). 
         [0044]    The CMA processing unit (CMA)  229  starts calculating CMA algorithm using the initial setting values h 11  (0), h 12  (0) of the filter coefficients (step S 104 ). The CMA processing unit (CMA)  229  sequentially updates the filter coefficients by using the first and second formulas of the formula (2). At that time, the signal light S XY  (=E X ) input into the receiver  120  is separated into the signal lights of E X ′ and E Y ′ which are projected on the polarization planes of X′ and Y′ of the local light L X′Y′ . Here, if E X ′&gt;E Y ′, the relation between the filter coefficients becomes h 11 &gt;h 12  and they are converged because the output e x  is mainly composed of the quantized signal e x ′. On the other hand, if E X ′&lt;E Y ′, the relation between the filter coefficients becomes h 11 &lt;h 12  and they are converged because the output ex is mainly composed of the quantized signal e y ′. Here, h 11  (1) and h 12  (1) are obtained as the converged values of the filter coefficients. At this time, the receiving controller unit  226  stops the calculation once (step S 105 ) and notifies the controller  216  of that effect through the line  240  (step S 106 ). The output e x  of the butterfly filter  227  at this time is expressed in the following formula using the converged values of the filter coefficients of h 11  (1) and h 12  (1). 
         [0000]        e   x   =h   11 (1)· e   x   ′+h   12 (1)· e   y ′  (4)
 
         [0045]    The receiving controller unit  226  sets filter coefficients as h 11 =h 11  (1), h 12 =h 12  (1), h 21 =−h 12  (1), and h 22 =h 11  (1) for the memory unit  228  (step S 107 ). 
         [0046]    Next, the controller  216  in the transmitter  210  gets the transmitter  210  outputting the light signal with Y polarization along with the light signal with X polarization by controlling the variable optical attenuator (VOA)  214  (step S108), and notifies the receiving controller unit  226  of that effect through the line  240  (step S 109 ). 
         [0047]    After the receiving controller unit  226  has received this notification (step S 109 ), the CMA processing unit (CMA)  229  resumes calculating CMA algorithm and then updates the filter coefficients according to the formula (4) (step S 110 ). Here, the quantized signal e x ′ includes the components of both the signal light E X  and E Y . For example, if the quantized signal e x ′ includes more components of the signal light E X , the quantized signal e y ′ will include more components of the signal light E Y . When the calculations according to the formula (4) is performed in this condition, since the relation between the filter coefficients set in step  107  is h 11 &gt;h 12 , the quantized signal e x ′ becomes dominant in the output e x . As a result, more components of the signal light E X  will be included in the output e x . This tendency gets stronger by updating the filter coefficients repeatedly, and finally the output e x  converges on the signal corresponding to the signal light E X , the h 11  (k) and h 12  (k) are obtained as the filter coefficients. With regard to the output e y , similarly, since the relation between the filter coefficients is h 21 &lt;h 22 , the quantized signal e y ′ becomes dominant in the output e y . As a result, the output e y  converges on the signal corresponding to the signal light E Y , then h 21  (k) and h 22  (k) are obtained as the filter coefficients. 
         [0048]    If the quantized signal e x ′ includes more components of the signal light E Y  and the quantized signal e y ′ includes more components of the signal light E X , since the relation between the filter coefficients set in step  107  is h 11 &lt;h 12 , the quantized signal e y ′, which includes more components of the signal light E X , becomes dominant in the output e x . As a result, the output ex converges on the signal corresponding to the signal light E X . With regard to the output e y , similarly, since the relation between the filter coefficients is h 21  &lt;h 22 , the quantized signal e x ′, which includes more components of the signal light E Y , becomes dominant in the output e y . As a result, the output e y  converges on the signal corresponding to the signal light E Y , then h 21  (k) and h 22  (k) are obtained as the filter coefficients (step S 111 ). 
         [0049]    After the above steps have ended, the receiving controller unit  226  notifies through the line  240  the controller  216  in the transmitter  210  of the effect that CMA processing has finished (step S 112 ). 
         [0050]    As mentioned above, first of all, the only signal light with X polarization is transmitted, and then the coefficients of the butterfly filter  227  are temporarily determined. Next, the signal light with Y polarization is transmitted multiplexed with the signal light with X polarization, then the coefficients of the butterfly filter  227  are determined. As a result, the polarization demultiplexing becomes possible where the output e x , which is obtained by signal processing in the digital signal processor (DSP)  225 , surely corresponds to the signal light E X  with X polarization and the output e y  surely corresponds to the signal light E Y . That is to say, according to the coherent optical communications system  200  in this exemplary embodiment, it becomes possible to perform the polarization demultiplexing for the polarization multiplexed optical signals in which the first signal has been put on the first polarization light and the second signal has been put on the second polarization light on the transmitter side, and to receive the first signal and the second signal corresponding to the transmitter side. 
       The Third Exemplary Embodiment 
       [0051]    Next, the third exemplary embodiment of the present invention will be described.  FIG. 5  is a block diagram showing the configuration of a coherent optical communications system  300  according to the third exemplary embodiment of the present invention. As shown in  FIG. 5 , the coherent optical communications system  300  includes a terminal station  300 A and a terminal station  300 B. The terminal station  300 A is provided with a transmitter  310 A and a receiver  320 A, and the terminal station  300 B is provided with a receiver  320 B and a transmitter  310 B. The transmitter  310 A and the receiver  320 B, the transmitter  310 B and the receiver  320 A are connected through an optical fiber  330  respectively, and mutually communicate. The coherent optical communications system  300  according to this exemplary embodiment is composed of a first coherent optical communications system  301  including the transmitter  310 A, the receiver  320 B and the optical fiber  330 , and a second coherent optical communications system  302  including the transmitter  310 B, the receiver  320 A and the optical fiber  330 . 
         [0052]    The configuration of the first coherent optical communications system  301  according to this exemplary embodiment is shown in  FIG. 6 . The configuration of the transmitter  310 A is the same as that of the transmitter  210  in the second exemplary embodiment with the exception that a controller  316  also controls a signal light source (LD)  311 . The configuration of the receiver  320 B is the same as that of the receiver  220  of the second exemplary embodiment with the exception that a photo detector (PD)  323  has a power monitoring function and notifies a receiving controller unit  326  of the monitoring results. The transmitter  310 B and the receiver  320 A, which compose the second coherent optical communications system  302 , are similarly configured. In this exemplary embodiment, the line  240  in the coherent optical communications system  200  according to the second exemplary embodiment is unnecessary. 
         [0053]    The configuration of the digital signal processor (DSP) provided for the receiver  320 B is the same as that of the digital signal processor (DSP)  225  in the second exemplary embodiment shown in  FIG. 3 . Here, the coefficients of the butterfly filter in the digital signal processor (DSP)  225  are set at bh 11  (k), bh 12  (k), bh21 (k), and bh 22  (k). As mentioned above, when using these filter coefficients, there can be cases where the signal component E X  of the first signal light with X polarization transmitted from the transmitter  310 A is converged on the demodulated signal e y , and the signal component E Y  of the second signal light with Y polarization is converged on the demodulated signal e x . 
         [0054]    Therefore, in this exemplary embodiment, by setting up an order for calculation of the filter coefficients, the signal components which converge on the demodulated signals of e x  and e y  are controlled. Here, the phenomenon that the demodulated signals are switched with the transmission side does not occur at every updating of the filter coefficients. Therefore, by inputting the correct filter coefficients into the butterfly filter at first and then updating them according to the formula (2) successively, it is possible to determine the filter coefficients with which to enable input signals to converge on the demodulated signals corresponding to the transmission side. A training method will be described below, which is the method for determining the filter coefficients of h 11 , h 12 , h 21 , and h 22  of the butterfly filter with which the signal component E X  with X polarization is converged on the demodulated signal e x  and the signal component E Y  with Y polarization is converged on the demodulated signal e y . 
         [0055]      FIG. 7  is a sequence diagram illustrating the initial setting of the filter coefficients. First, the receiving controller unit  326 B provided for the receiver  320 B in the terminal station  300 B sets arbitrary filter coefficients, that is h 11 =bh 11  (0) and h 12 =bh 12  (0), for the memory unit  228 B (step S 201 ). In this exemplary embodiment, those are set as bh 11  (0)=1 and bh 12  (0)=0. 
         [0056]    On the other hand, the controller  316 A provided for the transmitter  310 A in the terminal station  300 A puts the signal light source (LD)  311 A into an OFF state. After controlling the variable optical attenuator (VOA)  214 A and setting the signal light with Y polarization not outputting, the controller  316 A puts the signal light source (LD)  311 A into an ON state to put the X polarization light into an output (ON) state and put the Y polarization light into a non-output (OFF) state (step S 202 ). At this time, the only X polarization light is transmitted to the receiver  320 B through the optical fiber  330  (step S 203 ). 
         [0057]    When the receiving controller unit  326 B provided for the receiver  320 B in the terminal station  300 B confirms that the photo detector (PD)  323 B has received the light signal and outputs the receiving light signal, it instructs the CMA processing unit  229 B to start calculating CMA algorithm (step S 204 ). The CMA processing unit (CMA)  229 B sequentially updates the filter coefficients by using the first and second formulas of the formula (2). At this time, the converged values of the filter coefficients are set as bh 11  (1) and bh 12  (1), and the CMA processing unit (CMA)  229 B stops the calculation (step S 205 ). 
         [0058]    On the other hand, the receiving controller unit  326 A provided for the receiver  320 A in the terminal station  300 A sets arbitrary filter coefficients, that is h 11 =ah 11  (0) and h 12 =ah 12  (0), for the memory unit  228 A (step S 206 ). In this exemplary embodiment, those are set as ah 11  (0)=1 and ah 12  (0)=0. 
         [0059]    Next, the controller  316 B provided for the transmitter  310 B in the terminal station  300 B sets the signal light with Y polarization not outputting by controlling the variable optical attenuator (VOA)  214 B. After that, the controller  316 B puts the signal light source (LD)  311 B into an ON state to put the X polarization light into an output (ON) state and put the Y polarization light into a non-output (OFF) state (step S 207 ). At this time, the only X polarization light is transmitted to the receiver  320 A through the optical fiber  330  (step S 208 ). 
         [0060]    When the receiving controller unit  326 A provided for the receiver  320 A in the terminal station  300 A confirms that the photo detector (PD)  323 A has received the light signal and outputs the receiving light signal, it instructs the CMA processing unit  229 A to start calculating CMA algorithm (step S 209 ). The CMA processing unit (CMA)  229 A sequentially updates the filter coefficients by using the first and second formulas of the formula (2). At this time, the converged values of the filter coefficients are set as ah 11  (1) and ah 12  (1), and the CMA processing unit (CMA)  229 A stops the calculation (step S 210 ). 
         [0061]    On the other hand, the receiving controller unit  326 B provided for the receiver  320 B in the terminal station  300 B sets filter coefficients as h 11 =bh 11  (1), h 12 =bh 12  (1), h 21 =bh 12  (1), and h 22 =bh 11  (1) for the memory unit  228 B (step S 211 ). 
         [0062]    Next, the controller  316 A provided for the transmitter  310 A in the terminal station  300 A puts the light signal with Y polarization along with the light signal with X polarization into an output (ON) state by controlling the variable optical attenuator (VOA)  214 A (step S 212 ). At this time, both the X polarization light and the Y polarization light are transmitted to the receiver  320 B through the optical fiber  330  (step S 213 ). 
         [0063]    When the photo detector (PD)  323 B outputs the receiving light signal about twice as large as that in the step  204 , the receiving controller unit  326 B provided for the receiver  320 B in the terminal station  300 B instructs the CMA processing unit  229 B to resume calculating CMA algorithm (step S 214 ). The CMA processing unit  229 B updates the filter coefficients according to the formula (4). As a result, the filter coefficients converge, and bh 11  (k), bh 12  (k), bh 21  (k) and bh 22  (k) are obtained as the filter coefficients at that time (step S 215 ). 
         [0064]    In the same way, the receiving controller unit  326 A provided for the receiver  320 A in the terminal station  300 A sets filter coefficients as h 11 =ah 11  (1), h 12 =ah 12  (1), h 21 =−ah 12  (1), and h 22 =ah 11  (1) for the memory unit  228 A (step S 216 ). 
         [0065]    Next, the controller  316 B provided for the transmitter  310 B in the terminal station  300 B puts the light signal with Y polarization along with the light signal with X polarization into an output (ON) state by controlling the variable optical attenuator (VOA)  214 B (step S 217 ). At this time, both the X polarization light and the Y polarization light are transmitted to the receiver  320 A through the optical fiber  330  (step S 218 ). 
         [0066]    When the photo detector (PD)  323 A outputs the receiving light signal about twice as large as that in the step  209 , the receiving controller unit  326 A provided for the receiver  320 A in the terminal station  300 A instructs the CMA processing unit  229 A to resume calculating CMA algorithm (step S 219 ). The CMA processing unit  229 A updates the filter coefficients according to the formula (4). As a result, the filter coefficients converge, and ah 11  (k), ah 12  (k), ah 21  (k) and ah 22  (k) are obtained as the filter coefficients at that time (step S 220 ). 
         [0067]    As mentioned above, first of all, the only light signal with X polarization is transmitted from the transmitter  310 A, and then the coefficients of the butterfly filter, which is provided for the digital signal processor (DSP)  225 B in the receiver  320 B, are temporarily determined. Next, the light signal with Y polarization is transmitted from the transmitter  310 A multiplexed with the light signal with X polarization, then the coefficients of the butterfly filter of the digital signal processor (DSP)  225 B are determined. As a result, the polarization demultiplexing becomes possible where the output signal e x , which is obtained by signal processing in the digital signal processor (DSP)  225 B, surely corresponds to the signal component E X  with X polarization and the output signal e y  surely corresponds to the signal component E Y  with Y polarization. Similarly, in the receiver  320 A, it is possible to perform the polarization demultiplexing for the polarization multiplexed signal light transmitted from the transmitter  310 B and to receive them. 
         [0068]    According to this exemplary embodiment, since the line  140  in the digital coherent optical communications system  100  of the first exemplary embodiment is unnecessary, it is possible to simplify the configuration of the coherent optical communications system which is able to perform the polarization demultiplexing corresponding to the transmitter side. 
       The Fourth Exemplary Embodiment 
       [0069]    Next, the fourth exemplary embodiment of the present invention will be described.  FIG. 8  is a block diagram showing the configuration of a digital coherent optical communications system  400  in accordance with the fourth exemplary embodiment of the present invention. The digital coherent optical communications system  400  includes a transmitter  410  and a receiver  420 . 
         [0070]    The transmitter  410  is provided with a signal light source (LD)  411 , a first phase modulator (PM X )  412  as a first modulator, and a second phase modulator (PM Y )  413  as a second modulator. In addition, it has a polarization beam splitter (PBS)  415  as an orthogonal multiplexing unit and has a variable optical attenuator (VOA)  414  and a controller  416 , which compose a transmission control unit. 
         [0071]    The receiver  420  includes a local light source (LO)  421 , a 90 degree hybrid circuit  422 , and a photo detector (PD)  423 , which compose a coherent optical receiving unit. In addition, it has an analog-to-digital converter (ADC)  424  and a digital signal processor (DSP)  425 , which compose a signal processing unit, and has a receiving controller unit  426 . 
         [0072]    Here, the controller  416  controls the variable optical attenuator (VOA)  414  and the receiving controller unit  426  controls the digital signal processor (DSP)  425 , respectively. The transmitter  410  and the receiver  420  are connected through an optical fiber  430  and communication is performed thereby. In addition, the digital coherent optical communications system  400  is provided with a line  440  which enables communication between the controller  416  and the receiving controller unit  426 . 
         [0073]    This exemplary embodiment differs from the second exemplary embodiment that the first phase modulator (PM X )  412  provided for the transmitter  410  modulates the X polarization light and the second phase modulator (PM Y )  413  modulates the Y polarization light respectively, using the QPSK (Quadrature Phase Shift Keying) method. The orthogonal multiplexed signal light S XY  (=E X +E Y ) input into the receiver  420  interferes with the local light L X′Y′  from the local light source (LO)  421  in the 90 degree hybrid circuit  422  to be projected on arbitrary polarization plane X′, Y′ of the local light L X′Y′ . At the same time, the 90 degree hybrid circuit  422  detects the phase difference between the orthogonal multiplexed signal light S XY  and the local light L X′Y′ , and outputs to the photo detector  423  an in-phase output I X ′ and a quadrature-phase output Q X ′ which are X′ polarization light, and an in-phase output I Y ′ and a quadrature-phase output Q Y ′ which are Y′ polarization light. Each output light is detected by the photo detector  423 , and the detection signal is input into the analog-to-digital converter (ADC)  424 . The analog-to-digital converter (ADC)  424  quantizes these detection signals and then outputs quantized signals of i x ′, q x ′, i y ′, and q y ′. The quantized signals of i x ′, q x ′, i y ′, and q y ′ are processed for polarization demultiplexing in the digital signal processor (DSP)  425 , and demodulated signals of i x , q x , i y , and q y  are obtained. 
         [0074]    The configuration of the digital signal processor (DSP)  425  is shown in  FIG. 9 . The digital signal processor (DSP)  425  is provided with a CPE (Carrier Phase Estimation) unit  450  in addition to a butterfly filter  427 , a memory unit  428 , and a CMA processing unit (CMA)  429 . 
         [0075]    The quantized signals of i x ′, q x ′, i y ′, and q y ′ input into the digital signal processor (DSP)  425  are added with respect to each of X′ polarization and Y′ polarization and then are input into the butterfly filter  427  as e x ′=i x ′+q x ′, e y ′=i y ′+q y ′. The butterfly filter  427  performs the matrix operation on the input signals of e x ′ and e y ′ according to the formula (1), and outputs demodulated signals of e x  and e y . 
         [0076]    One of the methods to calculate each element of this matrix H is a 
         [0077]    CMA algorithm (refer to a non patent literature 1, for example). As mentioned below, in this exemplary embodiment, the configuration is employed in which the CMA processing unit (CMA)  429  calculates each element of the matrix H (filter coefficients) by means of the CMA algorithm. The CMA algorithm performs control of keeping the intensity of the quantized signals of e x ′ and e y ′ constant using the error functions of ε x  and ε y  as shown in the formula (3). However, based solely on the information on the electric field intensity, it is indistinguishable whether the data in the quantized signals correspond to the information put on the X polarization light or the information put on the Y polarization light. Therefore, as mentioned above, when using the filter coefficients of h 11 , h 12 , h 21 , and h 22  calculated by using the CMA algorithm, there can be cases where the signal component E X  of the first signal light with X polarization is converged on the demodulated signal e y , and the signal component E Y  of the second signal light with Y polarization is converged on the demodulated signal e x . 
         [0078]    Therefore, in this exemplary embodiment, by setting up an order for calculation of the filter coefficients, the signal components which converge on the demodulated signals of e x  and e y  are controlled. Here, the phenomenon that the demodulated signals are switched with the transmission side does not occur at every updating of the filter coefficients. Therefore, by inputting the correct filter coefficients into the butterfly filter  427  at first and then updating them according to the formula (2) successively, it is possible to determine the filter coefficients with which to enable input signals to converge on the demodulated signals corresponding to the transmission side. In this exemplary embodiment, by means of the training method used in the second exemplary embodiment, the filter coefficients of h 11 , h 12 , h 21 , and h 22  of the butterfly filter  427  are determined, with which the signal component E X  with X polarization is converged on the demodulated signal e x  and the signal component E Y  with Y polarization is converged on the demodulated signal e y . 
         [0079]    The CPE unit  450  extracts phase information from the demodulated signals e x  and e y  obtained by the CMA processing, separates I-channel and Q-channel demodulated signals i x , q x  from the demodulated signal e x  with X polarization, separates demodulated signals i y , q y  from the demodulated signal e y  with Y polarization respectively, and then outputs them. 
         [0080]    In this exemplary embodiment, QPSK modulation scheme is employed as a modulation scheme for two-stream signals polarization multiplexed. However, the modulation scheme is not limited to this, other multilevel modulation schemes can be applied such as 8PSK (8-Phase Shift Keying) modulation scheme and 16QAM (Quadrature Amplitude Modulation) modulation scheme. 
         [0081]    As mentioned above, according to this exemplary embodiment, it becomes possible to perform the polarization demultiplexing for the polarization multiplexed optical signals and to receive the first signal and the second signal corresponding to the transmission side, even if the first polarization light is multilevel modulated with the first signal and the second polarization light is multilevel modulated with the second signal at the transmission side respectively. 
         [0082]    In the first to third exemplary embodiments, CMA algorithm is used for determining the filter coefficients. However, the algorithm is not limited to that, other algorithms can be used as long as they are filter coefficient determination algorithms for the butterfly filter such as an LMS (Least Mean Square) algorithm. 
         [0083]    In addition, although the variable optical attenuator (VOA) is used for controlling the output of one polarization light in the above-mentioned exemplary embodiments, but not limited to this, the output of the modulator can be controlled by adjusting its bias. 
         [0084]    The present invention is not limited to the above-mentioned exemplary embodiments and can be variously modified within the scope of the invention described in the claims. It goes without saying that these modifications are also included in the scope of the invention. 
         [0085]    This application is based upon and claims the benefit of priority from Japanese patent application No. 2010-002501, filed on Jan. 8, 2010, the disclosure of which is incorporated herein in its entirety by reference. 
         [0086]    The whole or part of the exemplary embodiments disclosed above can be described as, but not limited to, the following supplementary notes. 
         [0087]    (Supplementary note 1) A coherent optical receiving apparatus, comprising: a coherent optical receiving unit performing coherent optical detection; and a signal processing unit performing signal processing defined by control parameters; wherein the coherent optical receiving unit outputs a first detection signal receiving a first polarization light modulated by a first transmission signal, and outputs a second detection signal receiving simultaneously the first polarization light and a second polarization light modulated by a second transmission signal; and the signal processing unit determines a first control parameter on the basis of the first detection signal, determines a second control parameter on the basis of the first control parameter and the second detection signal, and outputs a first received signal corresponding to the first transmission signal and a second received signal corresponding to the second transmission signal by using the second control parameter. 
         [0088]    (Supplementary note 2) The coherent optical receiving apparatus according to Supplementary note 1, wherein the signal processing unit comprises a filter unit performing signal processing on the basis of control parameters and a control parameter processing unit calculating the control parameters by a control parameter determination algorithm, wherein the control parameter processing unit determines the first control parameter so that an output signal may converge at the first received signal for an input of the first detection signal, changes the first control parameter so that the output signal may converge at the second received signal for an input of the second detection signal, and fixes a control parameter by which the output signal converges at the second received signal as the second control parameter, and the filter unit outputs the first received signal and the second received signal on the basis of the second control parameter. 
         [0089]    (Supplementary note 3) The coherent optical receiving apparatus according to Supplementary note 1 or 2, further comprising a receiving controller unit controlling an operation of the signal processing unit; wherein the receiving controller unit instructs the signal processing unit to start a processing to determine the first control parameter when confirming that the coherent optical receiving unit has received the first polarization light, and instructs the signal processing unit to start a processing to determine the second control parameter when confirming that the coherent optical receiving unit has received simultaneously the first polarization light and the second polarization light. 
         [0090]    (Supplementary note 4) The coherent optical receiving apparatus according to Supplementary note 3, wherein the coherent optical receiving unit comprises a photoelectric conversion unit connected to the receiving controller unit, and wherein the receiving controller unit confirms that the coherent optical receiving unit has received the first polarization light when the photoelectric conversion unit outputs a first receiving light signal, and confirms that the coherent optical receiving unit has received simultaneously the first polarization light and the second polarization light when the photoelectric conversion unit outputs a receiving light signal about twice as large as the first receiving light signal. 
         [0091]    (Supplementary note 5) A coherent optical communications system, comprising: a transmitter; and a coherent optical receiving apparatus connected to the transmitter through an optical fiber; wherein the transmitter comprises a light source; a first modulator modulating output light having first polarization from the light source with a first transmission signal and outputting first polarization light; a second modulator modulating output light having second polarization from the light source with a second transmission signal and outputting second polarization light; an orthogonal multiplexing unit orthogonally multiplexing the first polarization light and the second polarization light and transmitting to the optical fiber; and a transmission control unit controlling intensity of the second polarization light; wherein the coherent optical receiving apparatus comprises a coherent optical receiving unit performing coherent optical detection; a signal processing unit performing signal processing defined by control parameters; and a receiving controller unit controlling an operation of the signal processing unit; wherein the coherent optical receiving unit receives the first polarization light and outputs a first detection signal, and receives simultaneously the first polarization light and the second polarization light and outputs a second detection signal; the receiving controller unit instructs the signal processing unit to start a processing to determine a first control parameter when confirming that the coherent optical receiving unit has received the first polarization light, and instructs the signal processing unit to start a processing to determine a second control parameter when confirming that the coherent optical receiving unit has received simultaneously the first polarization light and the second polarization light; and the signal processing unit determines the first control parameter on the basis of the first detection signal, determines the second control parameter on the basis of the first control parameter and the second detection signal, and outputs a first received signal corresponding to the first transmission signal and a second received signal corresponding to the second transmission signal by using the second control parameter. 
         [0092]    (Supplementary note 6) The coherent optical communications system according to Supplementary note 5, wherein the signal processing unit determines the first control parameter so that an output signal may converge at the first received signal for an input of the first detection signal, changes the first control parameter so that the output signal may converge at the second received signal for an input of the second detection signal, and fixes a control parameter by which the output signal converges at the second received signal as the second control parameter. 
         [0093]    (Supplementary note 7) The coherent optical communications system according to Supplementary note 5 or 6, further comprising a line connecting the transmission control unit to the receiving controller unit; wherein the receiving controller unit transmits a first notification to the transmission control unit through the line when the first control parameter is determined; the transmission control unit gets the transmitter outputting simultaneously the first polarization light and the second polarization light by increasing the intensity of the second polarization light when receiving the first notification, and transmits a second notification to the receiving controller unit through the line; and the receiving controller unit confirms that the coherent optical receiving unit has received simultaneously the first polarization light and the second polarization light when receiving the second notification. 
         [0094]    (Supplementary note 8) The coherent optical communications system according to Supplementary note 5 or 6, wherein the coherent optical receiving unit comprises a photoelectric conversion unit connected to the receiving controller unit, wherein the receiving controller unit confirms that the coherent optical receiving unit has received the first polarization light when the photoelectric conversion unit outputs a first receiving light signal, and confirms that the coherent optical receiving unit has received simultaneously the first polarization light and the second polarization light when the photoelectric conversion unit outputs a receiving light signal about twice as large as the first receiving light signal. 
         [0095]    (Supplementary note 9) A coherent optical communications method, comprising the steps of: transmitting first polarization light obtained by modulating output light having first polarization with a first transmission signal; receiving the first polarization light and obtaining a first detection signal by performing coherent optical detection; transmitting second polarization light obtained by modulating output light having second polarization with a second transmission signal; receiving simultaneously the first polarization light and the second polarization light and obtaining a second detection signal by performing coherent optical detection; determining a first control parameter on the basis of the first detection signal; determining a second control parameter on the basis of the first control parameter and the second detection signal; and obtaining a first received signal corresponding to the first transmission signal and a second received signal corresponding to the second transmission signal by using the second control parameter. 
         [0096]    (Supplementary note 10) The coherent optical communications method according to Supplementary note 9, wherein, in the step of determining the first control parameter, setting control parameter so that an output signal may converge at the first received signal for an input of the first detection signal; and in the step of determining the second control parameter, changing the first control parameter so that the output signal may converge at the second received signal for an input of the second detection signal, and fixing a control parameter by which the output signal converges at the second received signal as the second control parameter. 
         [0097]    (Supplementary note 11) The coherent optical communications method according to Supplementary note 9 or 10, wherein, in the step of transmitting the second polarization light, using the determination of the first control parameter as a trigger to start transmitting the second polarization light. 
         [0098]    (Supplementary note 12) The coherent optical communications method according to any one of Supplementary notes 9, 10, and 11, wherein, in the step of determining the second control parameter, using the transmission of the second polarization light as a trigger to start determining the second control parameter. 
       DESCRIPTION OF THE CODES 
       [0099]      100  coherent optical receiving apparatus 
         [0100]      110  coherent optical receiving unit 
         [0101]      120  signal processing unit 
         [0102]      121  filter unit 
         [0103]      122  control parameter processing unit 
         [0104]      200 ,  300 ,  400  coherent optical communications system 
         [0105]      210 ,  310 A,  310 B,  410  transmitter 
         [0106]      211 ,  311 ,  411  signal light source (LD) 
         [0107]      212 ,  412  first phase modulator (PM X ) 
         [0108]      213 ,  413  second phase modulator (PM Y ) 
         [0109]      214 ,  414  variable optical attenuator (VOA) 
         [0110]      215 ,  415  polarization beam splitter (PBS) 
         [0111]      216 ,  316 ,  416  controller 
         [0112]      220 ,  320 A,  320 B,  420  receiver 
         [0113]      221 ,  421 ,  511  local light source (LO) 
         [0114]      222 ,  422 ,  512  90 degree hybrid circuit 
         [0115]      223 ,  323 ,  423 ,  513  photo detector (PD) 
         [0116]      224 ,  424 ,  514  analog-to-digital converter (ADC) 
         [0117]      225 ,  425 ,  515  digital signal processor (DSP) 
         [0118]      226 ,  326 ,  426  receiving controller unit 
         [0119]      227 ,  427 ,  516  butterfly filter 
         [0120]      228 ,  428  memory unit 
         [0121]      229 ,  429 ,  517  CMA processing unit (CMA) 
         [0122]      230 ,  330 ,  430  optical fiber 
         [0123]      240 ,  440  line 
         [0124]      300 A,  300 B terminal station 
         [0125]      301  first coherent optical communications system 
         [0126]      302  second coherent optical communications system 
         [0127]      450  CPE unit 
         [0128]      500  related coherent optical receiving apparatus