Patent Publication Number: US-2022216934-A1

Title: Optical reception apparatus and monitor signal generating method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of U.S. patent application Ser. No. 17/060,364 filed on Oct. 1, 2020, which is a continuation application of U.S. patent application Ser. No. 16/741,496 filed on Jan. 13, 2020, which issued as U.S. Pat. No. 10,826,642, which is a continuation application of U.S. patent application Ser. No. 16/219,216 filed on Dec. 13, 2018, which issued as U.S. Pat. No. 10,554,323, which is a continuation application of U.S. patent application Ser. No. 15/601,103 filed on May 22, 2017, which issued as U.S. Pat. No. 10,187,174, which is a continuation application of U.S. patent application Ser. No. 14/904,031 filed on Jan. 8, 2016, which issued as U.S. Pat. No. 9,692,545, which is a National Stage Entry of international application PCT/JP2014/001793, filed on Mar. 27, 2014, which claims the benefit of priority from Japanese Patent Application 2013-145238 filed on Jul. 11, 2013, the disclosures of all of which are incorporated in their entirety by reference herein. 
    
    
     Technical Field 
     The present invention relates to an optical reception apparatus and a monitor signal generating method, and particularly to an optical reception apparatus and a monitor signal generating method using the coherent light transmission scheme. 
     BACKGROUND ART 
     The wavelength division multiplexing (WDM) communication belongs to the optical communication technology. In the wavelength division multiplexing communication, since a multiplexed optical signal in which optical signals of a plurality of wavelengths are multiplexed is used, large-volume information can be transmitted with a single optical fiber. Further, there is a technique of selectively extracting a particular optical signal from the multiplexed optical signal, which is referred to as the coherent light transmission scheme. In the coherent light transmission scheme, by allowing the multiplexed optical signal and local oscillation light to interfere with each other and performing a coherent detection, an optical signal corresponding to the wavelength of the local oscillation light is selectively extracted from the multiplexed optical signal. 
     Patent Literatures 1 and 2 each disclose a technique relating to the coherent light transmission scheme. Patent Literature 1 discloses a technique of stabilizing the absolute wavelength of a local oscillation light source and a transmission light source, thereby making it easier to set the wavelength. Patent Literature 2 discloses a technique for improving the S/N ratio in the reception characteristic while suppressing an increase in costs, even in the case where a multiplexed optical signal is selectively received by the wavelength of local oscillation light. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. H04-212530 
     Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2012-070234 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the coherent light transmission scheme disclosed in Patent Literatures 1 and 2, a multiplexed optical signal transmitted from an optical transmission apparatus is received using an optical reception apparatus. For example, the power of the multiplexed optical signal input to the optical reception apparatus can be monitored by branching the multiplexed optical signal input to the optical reception apparatus with an optical coupler or the like, and converting the branched multiplexed optical signal to an electric signal with a monitor-purpose photoelectric converter. 
     However, since a multiplexed optical signal is an optical signal in which optical signals of a plurality of wavelengths are multiplexed, when a multiplexed optical signal input to the optical reception apparatus is monitored, all the optical signals input to the optical reception apparatus are monitored. Hence, in this case, there is a problem that the power of an optical signal of a particular wavelength cannot be measured solely. 
     In view of the problem described above, an object of the present invention is to provide an optical reception apparatus and a monitor signal generating method, with which the power of an optical signal of a particular wavelength can be monitored. 
     Solution to Problem 
     An optical reception apparatus of the present invention includes: 
     a local oscillator outputting local oscillation light having a prescribed wavelength; 
     an optical mixer receiving a multiplexed optical signal in which optical signals being different in wavelength from each other are multiplexed and the local oscillation light, and selectively outputting an optical signal corresponding to the wavelength of the local oscillation light from the multiplexed optical signal; 
     a photoelectric converter converting the optical signal output from the optical mixer to an electric signal; 
     a variable gain amplifier amplifying the electric signal converted by the photoelectric converter, to generate an output signal whose output amplitude is amplified to a certain level; 
     a gain control signal generating circuit generating a gain control signal for controlling a gain of the variable gain amplifier; and 
     a monitor signal generating unit generating, using the gain control signal, a monitor signal corresponding to power of the optical signal output from the optical mixer. 
     A monitor signal generating method of the present invention is a monitor signal generating method for generating a monitor signal corresponding to power of an optical signal received by an optical reception apparatus, the method including: 
     causing a multiplexed optical signal in which optical signals being different in wavelength from each other are multiplexed and local oscillation light having a prescribed wavelength to interfere with each other, to extract an optical signal corresponding to the wavelength of the local oscillation light from the multiplexed optical signal; 
     converting the extracted optical signal into an electric signal; 
     amplifying the electric signal using a variable gain amplifier, to generate an output signal whose output amplitude is amplified to a certain level; 
     generating a gain control signal for controlling a gain of the variable gain amplifier; and 
     generating a monitor signal corresponding to the power of the optical signal using the gain control signal. 
     Advantageous Effects of Invention 
     The present invention can provide an optical reception apparatus and a monitor signal generating method, with which the power of an optical signal of a particular wavelength can be monitored. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing an optical reception apparatus according to a first embodiment; 
         FIG. 2  is a circuit diagram showing one example of an amplifier circuit included in the optical reception apparatus according to the first embodiment; 
         FIG. 3  is a block diagram showing an optical reception apparatus according to a second embodiment; 
         FIG. 4  is a diagram showing a 90-degree optical hybrid circuit included in the optical reception apparatus according to the second embodiment; 
         FIG. 5  is a block diagram for describing details of an amplifier circuit included in the optical reception apparatus according to the second embodiment; 
         FIG. 6  is a block diagram showing an optical reception apparatus according to a third embodiment; 
         FIG. 7  is a block diagram showing an optical reception apparatus according to a fourth embodiment; 
         FIG. 8  is a block diagram showing an optical reception apparatus according to a fifth embodiment; and 
         FIG. 9  is a block diagram showing an optical reception apparatus according to Comparative Example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     In the following, with reference to the drawings, a description will be given of embodiments of the present invention.  FIG. 1  is a block diagram showing an optical reception apparatus  1  according to a first embodiment. As shown in  FIG. 1 , the optical reception apparatus  1  according to the present embodiment includes a local oscillator (LO)  11 , an optical mixer  12 , a photoelectric converter  13 , a variable gain amplifier  15 , a gain control signal generating circuit  16 , and a monitor signal generating unit  17 . Here, the variable gain amplifier  15  and the gain control signal generating circuit  16  configure an amplifier circuit  14 . 
     The optical reception apparatus  1  receives a multiplexed optical signal  21  generated on the transmission apparatus side (not shown). The multiplexed optical signal  21  is an optical signal in which optical signals being different in wavelength from each other are multiplexed. That is, the multiplexed optical signal  21  is an optical signal in which a plurality of optical signals respectively having different wavelengths λ 1 , λ 2 , . . . , λ n  (n is an integer equal to or greater than 2) are multiplexed. In the WDM communication, since such a multiplexed optical signal is used, large-volume information can be transmitted with a single optical fiber. 
     The local oscillator  11  outputs local oscillation light  22  having a prescribed wavelength λ m  (m=1 to n) to the optical mixer  12 . That is, the local oscillator  11  outputs, to the optical mixer  12 , the local oscillation light  22  of a wavelength λm corresponding to the wavelength of an optical signal to be extracted from the multiplexed optical signal  21 . For example, the local oscillator  11  includes a wavelength variable laser, and is capable of changing the wavelength λ m  of the local oscillation light  22  output from the local oscillator  11  so as to correspond to the wavelength of an optical signal to be extracted from the multiplexed optical signal  21 . 
     The optical mixer  12  receives the multiplexed optical signal  21  and the local oscillation light  22 , and selects an optical signal  23  corresponding to the wavelength of the local oscillation light  22  from the multiplexed optical signal  21 . Then, the optical mixer  12  outputs the selected optical signal  23  to the photoelectric converter  13 . In the coherent light transmission scheme, by causing the multiplexed optical signal  21  and the local oscillation light  22  to interfere with each other and performing a coherent detection, an optical signal corresponding to the wavelength λ m  of the local oscillation light  22  can be selectively extracted from the multiplexed optical signal  21  in which a plurality of optical signals having the wavelengths λ 1 , λ 2 , . . . , λ n  are multiplexed. Hence, by changing the wavelength λ m  of the local oscillation light  22  output from the local oscillator  11 , an optical signal to be extracted from the multiplexed optical signal  21  can be arbitrarily selected. 
     The photoelectric converter  13  converts the optical signal  23  output from the optical mixer  12  to an electric signal  24 , and outputs the electric signal  24  to the amplifier circuit  14 . The photoelectric converter  13  may be, for example, a photodiode. 
     The amplifier circuit  14  includes the variable gain amplifier  15  and the gain control signal generating circuit  16 . The amplifier circuit  14  configures an AGC (Automatic Gain Control) circuit. 
     The variable gain amplifier  15  amplifies the electric signal  24  output from the photoelectric converter  13 , and generates an output signal whose output amplitude is amplified to a certain level. At this time, the variable gain amplifier  15  adjusts the gain of the variable gain amplifier  15  in accordance with a gain control signal  26  generated by the gain control signal generating circuit  16 . 
     The gain control signal generating circuit  16  generates the gain control signal  26  for controlling the gain of the variable gain amplifier  15 . For example, the gain control signal generating circuit  16  generates, based on the amplitude voltage of the output signal  25  output from the variable gain amplifier  15  and a preset target voltage Vt, the gain control signal  26  for feedback-controlling the variable gain amplifier  15 . 
     For example, the gain control signal generating circuit  16  generates the gain control signal  26  with which the amplitude voltage of the output signal  25  output from the variable gain amplifier  15  (in other words, the absolute value of the amplitude voltage of the output signal  25 ) and the target voltage Vt become equal to each other. Specifically, when the amplitude voltage of the output signal  25  is greater than the target voltage Vt, the gain control signal generating circuit  16  generates the gain control signal  26  with which the gain of the variable gain amplifier  15  reduces. Conversely, when the amplitude voltage of the output signal  25  is smaller than the target voltage Vt, the gain control signal generating circuit  16  generates the gain control signal  26  with which the gain of the variable gain amplifier  15  increases. 
     Note that, in the present embodiment, as an amplifier circuit  14 ′ shown in  FIG. 2 , a transimpedance amplifier  18  may be provided between the photoelectric converter  13  and the variable gain amplifier  15 . For example, when the electric signal  24  output from the photoelectric converter  13  is a current signal, by providing the transimpedance amplifier  18 , the current signal can be converted to a voltage signal. 
     The monitor signal generating unit  17  generates a monitor signal  27  using the gain control signal  26 . The monitor signal  27  is a signal corresponding to the power of the optical signal  23  output from the optical mixer  12  (that is, the optical signal  23  selected from the multiplexed optical signal  21 ). 
     For example, when the variable gain amplifier  15  is configured such that the amplification factor of the variable gain amplifier  15  increases as the signal voltage of the gain control signal  26  becomes higher, the relationship between the power of the optical signal  23  and the gain control signal  26  is as follows. When the power of the optical signal  23  is excessively small, the amplitude voltage of the electric signal  24  also becomes small. In this case, since the difference between the amplitude voltage of the output signal  25  and the target voltage Vt becomes great, the amplification factor of the variable gain amplifier  15  must be increased. Hence, the signal voltage of the gain control signal  26  generated by the gain control signal generating circuit  16  becomes high. On the other hand, when the power of the optical signal  23  is close to the target value, the difference between the amplitude voltage of the output signal  25  and the target voltage Vt becomes small. In this case, since the amplification factor of the variable gain amplifier  15  becomes small, the signal voltage of the gain control signal  26  generated by the gain control signal generating circuit  16  becomes low. 
     Further, for example, when the variable gain amplifier  15  is configured such that the amplification factor of the variable gain amplifier  15  increases as the signal voltage of the gain control signal  26  becomes lower, the relationship between the power of the optical signal  23  and the gain control signal  26  is as follows. When the power of the optical signal  23  is excessively small, the amplitude voltage of the electric signal  24  also becomes small. In this case, since the difference between the amplitude voltage of the output signal  25  and the target voltage Vt becomes great, the amplification factor in the variable gain amplifier  15  must be increased. Hence, the signal voltage of the gain control signal  26  generated by the gain control signal generating circuit  16  becomes low. On the other hand, when the power of the optical signal  23  is close to the target value, the difference between the amplitude voltage of the output signal  25  and the target voltage Vt becomes small. In this case, since the amplification factor in the variable gain amplifier becomes small, the signal voltage of the gain control signal  26  generated by the gain control signal generating circuit  16  becomes high. 
     In this manner, the gain control signal  26  varies in accordance with the power of the optical signal  23 . The monitor signal generating unit  17  can generate the monitor signal  27  corresponding to the power of the optical signal  23  using the gain control signal  26  which varies in this manner. For example, the monitor signal generating unit  17  may include an analog-digital converter circuit. In this case, the gain control signal  26  being an analog signal can be converted into a digital signal. 
     Note that, the optical reception apparatus  1  according to the present embodiment may further include an analog-digital converter circuit (not shown) that converts the output signal  25  from an analog signal to a digital signal, and a digital signal processing circuit (not shown) that processes the output signal converted into a digital signal. 
     In the coherent light transmission scheme disclosed in Patent Literatures 1 and 2, a multiplexed optical signal transmitted from an optical transmission apparatus is received using an optical reception apparatus. For example, the power of a multiplexed optical signal input to the optical reception apparatus can be monitored by branching the multiplexed optical signal input to the optical reception apparatus with an optical coupler or the like, and converting the branched multiplexed optical signal to an electric signal with a monitor-purpose photoelectric converter. 
       FIG. 9  is a block diagram showing an optical reception apparatus  100  according to Comparative Example. The optical reception apparatus  100  shown in  FIG. 9  includes an optical coupler  101 , a photoelectric converter  104 , a local oscillator (LO)  111 , an optical mixer  112 , a photoelectric converter  113 , a variable gain amplifier  115 , and a gain control signal generating circuit  116 . The variable gain amplifier  115  and the gain control signal generating circuit  116  configure an amplifier circuit  114 . Note that, in the optical reception apparatus  100  shown in  FIG. 9 , constituent elements identical to those of the optical reception apparatus  1  shown in  FIG. 1  are denoted by the reference numerals in the 100s. 
     In the optical reception apparatus  100  shown in  FIG. 9 , the multiplexed optical signal  121  input to the optical reception apparatus  100  is branched by the optical coupler  101 , and one multiplexed optical signal  102  is input to the optical mixer  112  while other multiplexed optical signal  103  is input to the photoelectric converter  104 . Then, by converting the branched multiplexed optical signal  103  to an electric signal by the photoelectric converter  104 , the power of the multiplexed optical signal  121  input to the optical reception apparatus  100  can be monitored. 
     However, since the multiplexed optical signal  121  is an optical signal in which optical signals of a plurality of wavelengths are multiplexed, when the multiplexed optical signal  121  input to the optical reception apparatus  100  is monitored, all the optical signals input to the optical reception apparatus  100  are monitored. Hence, in this case, the power of an optical signal of a particular wavelength  123  cannot be measured solely. 
     Accordingly, in the optical reception apparatus  1  according to the present embodiment, as shown in  FIG. 1 , using the gain control signal  26  for controlling the gain of the variable gain amplifier  15 , the monitor signal  27  corresponding to the power of the optical signal  23  output from the optical mixer  12  is generated. That is, the variable gain amplifier  15  amplifies solely the electric signal  24  corresponding to the optical signal  23  selected from the multiplexed optical signal  21 . Further, the gain control signal  26  is a signal for controlling the gain of the variable gain amplifier  15 , and varies in accordance with the power of the optical signal  23 . Hence, by generating the monitor signal  27  using the gain control signal  26 , the power of the optical signal  23  can be monitored. 
     Further, with the optical reception apparatus  1  according to the present embodiment, since the power of the optical signal  23  is monitored using the gain control signal  26 , it is not necessary to provide the optical coupler  101  for branching the multiplexed optical signal or the monitor-purpose photoelectric converter  104  (see  FIG. 9 ). Further, with the optical reception apparatus  1  of the present embodiment, by causing the multiplexed optical signal  21  and the local oscillation light  22  to interfere with each other and performing a coherent detection, the optical signal  23  corresponding to the wavelength of the local oscillation light  22  is selectively extracted from the multiplexed optical signal  21 . Hence, it is not necessary to provide an arrayed waveguide grating (AWG) or an optical filter for extracting an optical signal from a multiplexed optical signal. Accordingly, the optical reception apparatus can be reduced in size, and the manufacturing costs of the optical reception apparatus can be reduced. 
     By the invention according to the present embodiment described above, the optical reception apparatus and the monitor signal generating method with which the power of an optical signal of a particular wavelength can be monitored can be provided. 
     Second Embodiment 
     Next, a description will be given of a second embodiment of the present invention. In the present embodiment, a description will be given of the case where the optical reception apparatus described in the first embodiment is applied to the dual polarization quadrature phase shift keying (DP-QPSK) scheme. 
       FIG. 3  is a block diagram showing an optical reception apparatus  2  according to the present embodiment. As shown in  FIG. 3 , the optical reception apparatus  2  according to the present embodiment includes a local oscillator (LO)  31 , a polarization beam splitter (PBS)  32 , a 90-degree optical hybrid circuit  34 _ 1 ,  34 _ 2 , a photoelectric converter  35 , amplifier circuits  36 _ 1  to  36 _ 4 , monitor signal generating units  37 _ 1  to  37 _ 4 , analog-digital converter circuits  38 _ 1  to  38 _ 4 , and a digital signal processing circuit  39 . 
     The optical reception apparatus  2  receives a multiplexed optical signal  51  generated on the transmission apparatus side (not shown). The multiplexed optical signal  51  is an optical signal in which optical signals being different in wavelength from each other are multiplexed. Further, in the present embodiment, in the multiplexed optical signal  51 , X polarized light (first polarized light) and Y polarized light (second polarized light) being orthogonal to each other are multiplexed. The X polarized light and the Y polarized light are modulated independently of each other, and capable of independently transmitting information. Further, the X polarized light and the Y polarized light are each modulated by four different phases. 
     The polarization beam splitter  32  receives the multiplexed optical signal  51 , and splits the multiplexed optical signal  51  into the X polarized light  52  and the Y polarized light  53  being orthogonal to each other. Then, the polarization beam splitter  32  outputs the split X polarized light  52  to the 90-degree optical hybrid circuit  34 _ 1  (a first optical hybrid circuit), and outputs the split Y polarized light to the 90-degree optical hybrid circuit  34 _ 2  (a second optical hybrid circuit). 
     The local oscillator  31  outputs local oscillation light  54  having a prescribed wavelength to each of the 90-degree optical hybrid circuits  34 _ 1 ,  34 _ 2 . That is, the local oscillator  31  outputs, to 90-degree optical hybrid circuits  34 _ 1 ,  34 _ 2 , the local oscillation light  54  having the wavelength corresponding to the wavelength of an optical signal to be extracted from the multiplexed optical signal  51 . For example, the local oscillator  31  is configured to include a wavelength variable laser, and capable of varying the wavelength of the local oscillation light  54  output from the local oscillator  31  so as to correspond to the wavelength of the optical signal to be extracted from the multiplexed optical signal  51 . 
     The 90-degree optical hybrid circuit  34 _ 1  includes an optical mixer (a first optical mixer). The 90-degree optical hybrid circuit  34 _ 1  receives the X polarized light  52  and the local oscillation light  54  and causes the X polarized light  52  and the local oscillation light  54  to interfere with each other, thereby separating an optical signal corresponding to the wavelength of the local oscillation light  31  from the X polarized light  52 . Further, the 90-degree optical hybrid circuit  34 _ 1  splits the X polarized light  52  into an in-phase component (the I component) and a quadrature component (the Q component). Then, the 90-degree optical hybrid circuit  34 _ 1  outputs two optical signals included in the in-phase component as first differential signals, and outputs two optical signals included in the quadrature component as second differential signals. 
       FIG. 4  is a diagram showing one example of the 90-degree optical hybrid circuit  34 _ 1 . As shown in  FIG. 4 , the 90-degree optical hybrid circuit  34 _ 1  includes optical couplers  61 _ 1  to  61 _ 3 ,  62 _ 1  to  62 _ 3 , a  7   r/ 2 phase shifter  63 ,  7 E phase shifters  64 _ 1 ,  64 _ 2 , and optical mixers  65 _ 1  to  65 _ 4  (the first optical mixer). 
     The X polarized light  52  input to the 90-degree optical hybrid circuit  34 _ 1  is branched by the optical couplers  61 _ 1  to  61 _ 3 , and introduced to the optical mixers  65 _ 1  to  65 _ 4 . The local oscillation light  54  input to the 90-degree optical hybrid circuit  34 _ 1  is branched by the optical coupler  62 _ 1  and the optical coupler  62 _ 3 , and thereafter introduced to the optical mixer  65 _ 1 . The local oscillation light  54  input to the 90-degree optical hybrid circuit  34 _ 1  is branched by the optical coupler  62 _ 1  and the optical coupler  62 _ 3 , and thereafter has its phase shifted by π by the π phase shifter  64 _ 1 , to be introduced to the optical mixer  65 _ 2 . 
     The local oscillation light  54  input to the 90-degree optical hybrid circuit  34 _ 1  is branched by the optical coupler  62 _ 1 , and thereafter has its phase shifted by π/2 by the π/2 phase shifter  63 . The local oscillation light  54  is further branched by the optical coupler  62 _ 2  and thereafter introduced to optical mixer  65 _ 3 . The local oscillation light  54  input to the 90-degree optical hybrid circuit  34 _ 1  is branched by the optical coupler  62 _ 1 , and thereafter has its phase shifted by π/2 by the π/2 phase shifter  63 . The local oscillation light  54  is further branched by the optical coupler  62 _ 2 , and thereafter has its phase shifted by π by the π phase shifter  64 _ 2 , to be introduced to the optical mixer  65 _ 4 . 
     That is, the optical mixer  65 _ 1  receives the local oscillation light  54  which is in-phase; the optical mixer  65 _ 2  receives the local oscillation light  54  which is out of phase by π; the optical mixer  65 _ 3  receives the local oscillation light  54  which is out of phase by π/2; and the optical mixer  65 _ 4  receives the local oscillation light  54  which is out of phase by 3π/2. 
     Therefore, the optical mixer  65 _ 1  outputs an optical signal Ip_ 1  which is in-phase; the optical mixer  65 _ 2  outputs an optical signal In_ 1  which is out of phase by π; the optical mixer  65 _ 3  outputs an optical signal Qp_ 1  which is out of phase by π/2; and the optical mixer  65 _ 4  outputs an optical signal Qn_ 1  which is out of phase by 3π/2. The optical signal Ip_ 1  and the optical signal In_ 1  are output as the first differential signals (differential signals of the in-phase component), and the optical signal Qp_ 1  and the optical signal Qn_ 1  are output as the second differential signals (the differential signals of the quadrature component). 
     The 90-degree optical hybrid circuit  34 _ 2  operates similarly to the 90-degree optical hybrid circuit  34 _ 1 . That is, the 90-degree optical hybrid circuit  34 _ 2  includes an optical mixer (a second optical mixer). The 90-degree optical hybrid circuit  34 _ 2  receives the Y polarized light  53  and the local oscillation light  54  and causes the Y polarized light  53  and the local oscillation light  54  to interfere with each other, thereby separating an optical signal corresponding to the wavelength of the local oscillation light  31  from the Y polarized light  53 . Further, the 90-degree optical hybrid circuit  34 _ 2  splits the Y polarized light  53  into the in-phase component (the I component) and the quadrature component (the Q component). Then, the 90-degree optical hybrid circuit  34 _ 2  outputs two optical signals (Ip_ 2 , In_ 2 ) included in the in-phase component as third differential signals, and outputs two optical signals (Qp_ 2 , Qn_ 2 ) included in the quadrature component as fourth differential signals. 
     As shown in  FIG. 3 , the optical signals Ip_ 1 , In_ 1  (the first differential signals), the optical signals Qp_ 1 , Qn_ 1  (the second differential signals), the optical signals Ip_ 2 , In_ 2  (the third differential signals), and the optical signals Qp_ 2 , Qn_ 2  (the fourth differential signal) output from the 90-degree optical hybrid circuits  34 _ 1 ,  34 _ 2  are respectively converted by photoelectric converters PD #1 to PD #8 to electric signals. 
     The amplifier circuit  36 _ 1  amplifies the electric signals corresponding to the first differential signals (Ip_ 1 , In_ 1 ) output from the photoelectric converters PD #1, PD #2, and generates output signals whose output amplitude is amplified to a certain level. The generated output signals are output to the analog-digital converter circuit  38 _ 1 . The analog-digital converter circuit  38 _ 1  converts the output signals from analog signals to digital signals, and outputs the digital signals to the digital signal processing circuit  39 . The monitor signal generating unit  37 _ 1  generates a monitor signal  60 _ 1  corresponding to the first differential signals (Ip_ 1 , In_ 1 ). 
     The amplifier circuit  36 _ 2  amplifies the electric signals corresponding to the second differential signals (Qp_ 1 , Qn_ 1 ) output from the photoelectric converters PD #3, PD #4, and generates output signals whose output amplitude is amplified to a certain level. The generated output signals are output to the analog-digital converter circuit  38 _ 2 . The analog-digital converter circuit  38 _ 2  converts the output signals from analog signals to digital signals, and outputs the digital signals to the digital signal processing circuit  39 . The monitor signal generating unit  37 _ 2  generates a monitor signal  60 _ 2  corresponding to the second differential signals (Qp_ 1 , Qn_ 1 ). 
     The amplifier circuit  36 _ 3  amplifies the electric signals corresponding to the third differential signals (Ip_ 2 , In_ 2 ) output from the photoelectric converters PD #5, PD #6, and generates output signals whose output amplitude is amplified to a certain level. The generated output signals are output to the analog-digital converter circuit  38 _ 3 . The analog-digital converter circuit  38 _ 3  converts the output signals from analog signals to digital signals, and outputs the digital signals to the digital signal processing circuit  39 . The monitor signal generating unit  37 _ 3  generates a monitor signal  60 _ 3  corresponding to the third differential signals (Ip_ 2 , In_ 2 ). 
     The amplifier circuit  36 _ 4  amplifies the electric signals corresponding to the fourth differential signals (Qp_ 2 , Qn_ 2 ) output from the photoelectric converters PD #7, PD #8, and generates output signals whose output amplitude is amplified to a certain level. The generated output signals are output to the analog-digital converter circuit  38 _ 4 . The analog-digital converter circuit  38 _ 4  converts the output signals from analog signals to digital signals, and outputs the digital signals to the digital signal processing circuit  39 . The monitor signal generating unit  37 _ 4  generates a monitor signal  60 _ 4  corresponding to the fourth differential signals (Qp_ 2 , Qn_ 2 ). 
       FIG. 5  is a block diagram for describing details of the amplifier circuit  36 _ 1  included in the optical reception apparatus  2  according to the present embodiment. While a description will be given of the amplifier circuit  36 _ 1  in the following, the same holds true for other amplifier circuits  36 _ 2  to  36 _ 4 . 
     The amplifier circuit  36 _ 1  includes a transimpedance amplifier  42 , a variable gain amplifier  43 , and a gain control signal generating circuit  44 . The amplifier circuit  36 _ 1  configures an AGC circuit. The differential signals (Ip_ 1 , In_ 1 ) output from the 90-degree optical hybrid circuit  34 _ 1  are converted to differential signals  56 _ 1 ,  56 _ 2  by the photoelectric converters PD #1, PD #2, and supplied to the transimpedance amplifier  42 . The transimpedance amplifier  42  converts the differential signals  56 _ 1 ,  56 _ 2  from current signals to voltage signals, and outputs differential signals  57 _ 1 ,  57 _ 2  to the variable gain amplifier  43 . Note that, the transimpedance amplifier  42  may be omitted. 
     The variable gain amplifier  43  amplifies the differential signals  57 _ 1 ,  57 _ 2 , and generates differential output signals  58 _ 1 ,  58 _ 2  whose output amplitude is amplified to a certain level. At this time, the variable gain amplifier  43  adjusts the gain of the variable gain amplifier  43  in accordance with a gain control signal  59  generated by the gain control signal generating circuit  44 . 
     The gain control signal generating circuit  44  generates the gain control signal  59  for controlling the gain of the variable gain amplifier  43 . For example, the gain control signal generating circuit  44  generates the gain control signal  59  for feedback-controlling the variable gain amplifier  43 , based on the amplitude voltage of the differential output signals  58 _ 1 ,  58 _ 2  output from the variable gain amplifier  43  and the preset target voltage Vt. 
     For example, the gain control signal generating circuit  44  generates the gain control signal  59  with which the amplitude voltage of the differential output signals  58 _ 1 ,  58 _ 2  output from the variable gain amplifier  43  (in other words, the absolute value of the amplitude voltage of the differential output signals  58 _ 1 ,  58 _ 2 ) and the target voltage Vt become equal to each other. Specifically, when the amplitude voltage of the differential output signals  58 _ 1 ,  58 _ 2  is higher than the target voltage Vt, the gain control signal generating circuit  44  generates the gain control signal  59  with which the gain of the variable gain amplifier  43  reduces. Conversely, when the amplitude voltage of the differential output signals  58 _ 1 ,  58 _ 2  is lower than the target voltage Vt, the gain control signal generating circuit  44  generates the gain control signal  59  with which the gain of the variable gain amplifier  43  increases. 
     The monitor signal generating unit  37 _ 1  generates the monitor signal  60 _ 1  using the gain control signal  59 . The monitor signal  60 _ 1  is a signal corresponding to the power of the differential signals  55 _ 1 ,  55 _ 2  (Ip_ 1 , In_ 1 ) output from the 90-degree optical hybrid circuit  34 _ 1 . 
     That is, as described in the first embodiment, the gain control signal  59  varies in accordance with the power of the differential signals  55 _ 1 ,  55 _ 2  (Ip_ 1 , In_ 1 ). The monitor signal generating unit  37 _ 1  can generate the monitor signal  60 _ 1  corresponding to the power of the differential signals  55 _ 1 ,  55 _ 2  (Ip_ 1 , In_ 1 ) using the gain control signal  59  varying in this manner. For example, the monitor signal generating unit  37 _ 1  may include an analog-digital converter circuit. In this case, the gain control signal  59  being an analog signal can be converted to a digital signal. 
     In the optical reception apparatus  2  according to the present embodiment also, the power of the optical signals output from the 90-degree optical hybrid circuits  34 _ 1 ,  34 _ 2  is monitored using the gain control signal  59  for controlling the gain of the variable gain amplifier  43 . Hence, the power of an optical signal of a particular wavelength can be monitored. 
     Note that, while the description has been given of the case where four monitor signal generating units  37 _ 1  to  37 _ 4  are included with reference to  FIG. 3 , at least one monitor signal generating unit will suffice. That is, the monitor signal generating unit may be provided for only the differential signals that must be monitored, out of the first differential signals (Ip_ 1 , In_ 1 ), the second differential signals (Qp_ 1 , Qn_ 1 ), the third differential signals (Ip_ 2 , In_ 2 ), and the fourth differential signals (Qp_ 2 , Qn_ 2 ). 
     Further, in the foregoing, the description has been given of the case where the multiplexed optical signal  51  includes the X polarized light and the Y polarized light. However, the invention according to the present embodiment may be applied also to the quadrature phase shift keying (QPSK) scheme in which no polarized light is used. In this case, the polarization beam splitter  32 , the 90-degree optical hybrid circuit  34 _ 2 , the photoelectric converters PD #5 to PD #8, the amplifier circuits  36 _ 3 ,  36 _ 4 , the monitor signal generating units  37 _ 3 ,  37 _ 4 , and the analog-digital converter circuits  38 _ 3 ,  38 _ 4  can be omitted. In the case where the invention according to the present embodiment is applied to the quadrature phase shift keying scheme, the 90-degree optical hybrid circuit  34 _ 1  receives a multiplexed optical signal and local oscillation light, and the 90-degree optical hybrid circuit  34 _ 1  outputs four optical signals Ip_ 1 , In_ 1 , Qp_ 1 , Qn_ 1  (in other words, two types of differential signals). The optical signals Ip_ 1 , In_ 1 , Qp_ 1 , and Qn_ 1  are processed similarly to the manner described above. 
     Third Embodiment 
     Next, a description will be given of a third embodiment of the present invention.  FIG. 6  is a block diagram showing an optical reception apparatus according to the third embodiment. The optical reception apparatus  3  according to the third embodiment is different from the optical reception apparatus  1  described in the first embodiment in that the power of the local oscillation light  22  output from a local oscillator  71  is controlled in accordance with the monitor signal  27  generated by the monitor signal generating unit  17 . Other configuration is similar to that of the optical reception apparatus  1  described in the first embodiment, and therefore identical constituent elements are denoted by identical reference numerals, and repetitive descriptions are omitted. 
     As shown in  FIG. 6 , the optical reception apparatus  3  according to the present embodiment includes the local oscillator (LO)  71 , the optical mixer  12 , the photoelectric converter  13 , the variable gain amplifier  15 , the gain control signal generating circuit  16 , the monitor signal generating unit  17 , and a local oscillation light control unit  72 . 
     The optical mixer  12  receives the multiplexed optical signal  21  and the local oscillation light  22 , and selects the optical signal  23  corresponding to the wavelength of the local oscillation light  22  from the multiplexed optical signal  21 . Then, the optical mixer  12  outputs the selected optical signal  23  to the photoelectric converter  13 . At this time, the optical mixer  12  causes the multiplexed optical signal  21  and the local oscillation light  22  to interfere with each other and performs a coherent detection, thereby selectively extracting the optical signal corresponding to the wavelength λ m  of the local oscillation light  22  from the multiplexed optical signal  21 . Hence, in order to properly extract the optical signal  23  of a particular wavelength from the multiplexed optical signal  21 , it is necessary to adjust the power of the local oscillation light  22  input to the optical mixer  12  to a proper value. 
     Accordingly, with the optical reception apparatus  3  according to the present embodiment, the power of the local oscillation light  22  output from the local oscillator  71  is controlled in accordance with the monitor signal  27  generated by the monitor signal generating unit  17 . That is, the local oscillation light control unit  72  generates a control signal  73  for controlling the local oscillator  71  in accordance with the monitor signal  27 , and outputs the control signal  73  to the local oscillator  71 . The local oscillator  71  adjusts the power of the local oscillation light  22  in accordance with the control signal  73 . 
     For example, when the power of the local oscillation light  22  is excessively small, the power of the optical signal  23  output from the optical mixer  12  also becomes small. At this time, since the monitor signal  27  indicates that the power of the optical signal  23  is excessively small, the local oscillation light control unit  72  controls the local oscillator  71  to increase the power of the local oscillation light  22 . 
     Further, for example when the power of the local oscillation light  22  is excessively great, the power of the optical signal  23  output from the optical mixer  12  also becomes great. At this time, since the monitor signal  27  indicates that the power of the optical signal  23  is excessively great, the local oscillation light control unit  72  controls the local oscillator  71  to reduce the power of the local oscillation light  22 . 
     For example, the local oscillation light control unit  72  may control the power of the local oscillation light  22  such that the value of the monitor signal  27  (that is, the power value of the optical signal  23 ) attains a prescribed value. Here, such a prescribed value can be arbitrarily determined. 
     In this manner, since the optical reception apparatus  3  according to the present embodiment can control the power of the local oscillation light  22  in accordance with the monitor signal  27 , the optical signal  23  having prescribed power can be extracted from the multiplexed optical signal  21 . 
     Fourth Embodiment 
     Next, a description will be given of a fourth embodiment of the present invention.  FIG. 7  is a block diagram showing an optical reception apparatus according to the fourth embodiment. The optical reception apparatus  4  according to the fourth embodiment is different from the optical reception apparatus  1  described in the first embodiment in that the power of a multiplexed optical signal  84  supplied to the optical mixer  12  is adjusted in accordance with the monitor signal  27  generated by the monitor signal generating unit  17 . Other configuration is similar to that of the optical reception apparatus  1  described in the first embodiment, and therefore identical constituent elements are denoted by identical reference numerals, and repetitive descriptions are omitted. 
     As shown in  FIG. 7 , the optical reception apparatus  4  according to the present embodiment includes the local oscillator (LO)  11 , a multiplexed optical signal adjusting unit  81 , the optical mixer  12 , the photoelectric converter  13 , the variable gain amplifier  15 , the gain control signal generating circuit  16 , the monitor signal generating unit  17 , and a multiplexed optical signal control unit  82 . 
     The multiplexed optical signal adjusting unit  81  adjusts the power of the multiplexed optical signal  21 , and outputs the adjusted multiplexed optical signal  84  to the optical mixer  12 . The multiplexed optical signal control unit  82  controls the multiplexed optical signal adjusting unit  81  in accordance with the monitor signal  27 . The multiplexed optical signal adjusting unit  81  can be configured, for example, using an attenuator (attenuator) that attenuates the multiplexed optical signal  21  in accordance with a control signal  83  output from the multiplexed optical signal control unit  82 . 
     The optical mixer  12  receives the multiplexed optical signal  84  and the local oscillation light  22 , and selects the optical signal  23  corresponding to the wavelength of the local oscillation light  22  from the multiplexed optical signal  84 . Then, the optical mixer  12  outputs the selected optical signal  23  to the photoelectric converter  13 . At this time, the optical mixer  12  causes the multiplexed optical signal  84  and the local oscillation light  22  to interfere with each other and performs a coherent detection, thereby selectively extracting the optical signal corresponding to the wavelength λ m  of the local oscillation light  22  from the multiplexed optical signal  21 . Hence, in order to properly extract the optical signal  23  of a particular wavelength from the multiplexed optical signal  84 , it is necessary to adjust the power of the multiplexed optical signal  84  input to the optical mixer  12  to a proper value. 
     Accordingly, with the optical reception apparatus  4  according to the present embodiment, the power of the multiplexed optical signal  84  input to the optical mixer  12  is adjusted in accordance with the monitor signal  27  generated by the monitor signal generating unit  17 . The multiplexed optical signal control unit  82  generates the control signal  83  for controlling the multiplexed optical signal adjusting unit  81  in accordance with the monitor signal  27 , and outputs the control signal  83  to the multiplexed optical signal adjusting unit  81 . The multiplexed optical signal adjusting unit  81  adjusts the power of the multiplexed optical signal  21  in accordance with the control signal  83 , and outputs the adjusted multiplexed optical signal  84  to the optical mixer  12 . 
     For example, when the power of the multiplexed optical signal  84  is excessively great, the power of the optical signal  23  output from the optical mixer  12  also becomes great. At this time, since the monitor signal  27  indicates that the power of the optical signal  23  is excessively great, the multiplexed optical signal control unit  82  controls the multiplexed optical signal adjusting unit  81  to reduce the power of the multiplexed optical signal  84  input to the optical mixer  12 . 
     Further, for example when the power of the multiplexed optical signal  84  is excessively small, the power of the optical signal  23  output from the optical mixer  12  also becomes small. At this time, since the monitor signal  27  indicates that the power of the optical signal  23  is excessively small, the multiplexed optical signal control unit  82  controls the multiplexed optical signal adjusting unit  81  to increase the power of the multiplexed optical signal  84  input to the optical mixer  12 . 
     For example, the multiplexed optical signal control unit  82  may control the power of the multiplexed optical signal  84  such that the value of the monitor signal  27  (that is, the power value of the optical signal  23 ) attains a prescribed value. Here, such a prescribed value can be arbitrarily determined. 
     In this manner, since the optical reception apparatus  4  according to the present embodiment can control the power of the multiplexed optical signal  84  input to the optical mixer  12  in accordance with the monitor signal  27 , the optical signal  23  having prescribed power can be extracted. 
     Fifth Embodiment 
     Next, a description will be given of a fifth embodiment of the present invention.  FIG. 8  is a block diagram showing an optical reception apparatus according to the fifth embodiment. The optical reception apparatus  5  according to the fifth embodiment has a configuration in which the optical reception apparatus  3  according to the third embodiment and the optical reception apparatus  4  according to the fourth embodiment are combined. 
     That is, the optical reception apparatus  5  according to the present embodiment controls the power of the local oscillation light  22  in accordance with the monitor signal  27  generated by the monitor signal generating unit  17 , and further adjusts the power of the multiplexed optical signal  84  in accordance with the monitor signal  27 . 
     As shown in  FIG. 8 , the optical reception apparatus  5  according to the present embodiment includes the local oscillator (LO)  71 , the optical mixer  12 , the photoelectric converter  13 , the variable gain amplifier  15 , the gain control signal generating circuit  16 , the monitor signal generating unit  17 , the local oscillation light control unit  72 , the multiplexed optical signal adjusting unit  81 , and the multiplexed optical signal control unit  82 . Note that, these constituent elements are similar to those in the first, third and fourth embodiments, and therefore identical constituent elements are denoted by identical reference numerals, and repetitive descriptions are omitted. 
     With the optical reception apparatus  5  according to the present embodiment, the power of the local oscillation light  22  can be controlled in accordance with the monitor signal  27 . Further, the power of the multiplexed optical signal  84  can be adjusted in accordance with the monitor signal  27 . Hence, since the power of the local oscillation light  22  and the power of the multiplexed optical signal  84  can be controlled independently of each other, as compared to the optical reception apparatus according to the third and fourth embodiments, the power of the optical signal  23  output from the optical mixer  12  can be precisely adjusted. 
     Note that, in the fourth and fifth embodiments, the description has been given of the case where the power of the multiplexed optical signal  84  input to the optical mixer  12  is adjusted using the multiplexed optical signal adjusting unit  81  and the multiplexed optical signal control unit  82 . However, the power of the multiplexed optical signal  21  may be adjusted on the transmission apparatus side transmitting the multiplexed optical signal  21 . In this case, the monitor signal  27  must be transmitted to the transmission apparatus side. 
     Further, the invention described in the third to fifth embodiments is also applicable to an optical reception apparatus of the dual polarization quadrature phase shift keying (DP-QPSK) scheme described in the second embodiment. 
     In the foregoing, though the present invention has been described with reference to the embodiments, the present invention is not limited thereby. Various modifications that can be understood by a person skilled in the art can be made to the configuration or details of the present invention within the scope of the invention. 
     The present application claims priority based on Japanese Patent Application No. 2013-145238 filed on Jul. 11, 2013, the entire disclosure of which is incorporated herein by reference. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  2 ,  3 ,  4 ,  5  OPTICAL RECEPTION APPARATUS 
           11  LOCAL OSCILLATOR (LO) 
           12  OPTICAL MIXER 
           13  PHOTOELECTRIC CONVERTER 
           14  AMPLIFIER CIRCUIT 
           15  VARIABLE GAIN AMPLIFIER 
           16  GAIN CONTROL SIGNAL GENERATING CIRCUIT 
           17  MONITOR SIGNAL GENERATING UNIT 
           18  TRANSIMPEDANCE AMPLIFIER 
           21  MULTIPLEXED OPTICAL SIGNAL 
           22  LOCAL OSCILLATION LIGHT 
           23  OPTICAL SIGNAL 
           24  ELECTRIC SIGNAL 
           25  OUTPUT SIGNAL 
           25  GAIN CONTROL SIGNAL 
           26  MONITOR SIGNAL 
           27  LOCAL OSCILLATOR (LO) 
           32  POLARIZATION BEAM SPLITTER 
           34 _ 1 ,  34 _ 2  90-DEGREE OPTICAL HYBRID CIRCUIT 
           35  PHOTOELECTRIC CONVERTER 
           36 _ 1  to  36 _ 4  AMPLIFIER CIRCUIT 
           37 _ 1  to  37 _ 4  MONITOR SIGNAL GENERATING UNIT 
           38 _ 1  to  38 _ 4  ANALOG-DIGITAL CONVERTER CIRCUIT 
           34  DIGITAL SIGNAL PROCESSING CIRCUIT 
           61 _ 1  to  61 _ 3 ,  62 _ 1  to  62 _ 3  OPTICAL COUPLER 
           63  π/2 PHASE SHIFTER 
           64 _ 1 ,  64 _ 2  π PHASE SHIFTER 
           65 _ 1  to  65 _ 4  OPTICAL MIXER 
           71  LOCAL OSCILLATOR (LO) 
           72  LOCAL OSCILLATION LIGHT CONTROL UNIT 
           81  MULTIPLEXED OPTICAL SIGNAL ADJUSTING UNIT 
           82  MULTIPLEXED OPTICAL SIGNAL CONTROL UNIT