Abstract:
There is provided an optical receiver including a variable-ratio splitter to split an input signal light into a plurality of signal lights, based on a variable ratio, a plurality of photo detectors to receive the plurality of signal lights respectively, an operation circuit to output a reception electrical signal, based on a reception processing on one of the plurality of signal lights, a calculation circuit to calculate a total power of the plurality of signal lights received by the plurality of photo detectors, and an output unit to output a signal regarding the total power.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-082415, filed on Mar. 31, 2010, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an optical receiver and a communication system which includes the optical receiver. 
     BACKGROUND 
     When the signal light input to an optical receiving module applied in an optical communication system has large power, a large amplitude signal is input to a receiving circuit after a light receiving element such as an Avalanche Photo Diode (APD). As a result, the duty variation due to the amplitude saturation may deteriorate the received signal characteristic. In a long distance optical communication, since many optical amplifier may be provided at some points of an optical transmission line, a wide optical power range (dynamic range) is requested which can be received by an optical receiving module. On the other hand, for example, a technology has been known which has a variable optical attenuator before an internal light receiving element of an optical receiving module and attenuates signal light when input signal light has large power. 
     In recent years, an input-power monitoring function which monitors the input power of a signal light to an optical receiving module may be requested in an optical receiving module. (Refer to Japanese Laid-open Patent Publication No. 2006-203179, for example). The input-power monitoring may be required to indicate the input power of a signal light before attenuated by a variable optical attenuator even while the signal light is being attenuated. On the other hand, for example, a technology has been known in which an optical coupler before the variable optical attenuator partially splits signal light, and the split signal light is monitored for implementing the input power monitoring. 
     However, the technology in the past may not monitor input power with high precision. For example, a branch ratio of an optical coupler may depend on the wavelength of signal light or temperature. Thus, the technology in which signal light is partially split by an optical coupler before a variable optical attenuator for monitoring may provide input-power monitoring results which vary in accordance with the changes in branch ratio of the optical coupler even for constant input power of signal light. 
     On the other hand, a look-up table may be created for wavelengths or temperatures in advance, and split signal light and the look-up table may be used to estimate the input power. However, creating a look-up table with high precision for correcting monitoring result variations due to the wavelength or temperatures may require many experiments and/or simulations, for example, which may increase the manufacturing costs for the optical receiving module. 
     The technology which partially splits and monitors signal light with an optical coupler before a variable optical attenuator may unconditionally split a part of signal light, which may cause signal light loss. Therefore, when signal light has small input power, the minimum receiving sensitivity may deteriorate. 
     SUMMARY 
     According to an aspect of the embodiment, there is provided an optical receiver including a variable-ratio splitter to split an input signal light into a plurality of signal lights, based on a variable ratio, a plurality of photo detectors to receive the plurality of signal lights respectively, an operation circuit to output a reception electrical signal, based on a reception processing on one of the plurality of signal lights, a calculation circuit to calculate a total power of the plurality of signal lights received by the plurality of photo detectors, and an output unit to output a signal regarding the total power. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  illustrates a first configuration example of an optical receiver; 
         FIG. 1B  illustrates a first configuration example of an operation circuit; 
         FIG. 2A  illustrates optical paths of signal light in variable-ratio splitter (φ=π); 
         FIG. 2B  illustrates an optical path of signal light in a variable-ratio splitter (φ=0 or 2π); 
         FIG. 2C  illustrates an optical path for signal light in a variable-ratio splitter (φ≠0, π, or 2π); 
         FIG. 3  is a graph illustrating changes in main signal and monitor signal corresponding to control voltage; 
         FIG. 4  is a flowchart illustrating an example of control for attenuating a main signal; 
         FIG. 5  illustrates a second configuration example of an operation circuit; 
         FIG. 6  is a flowchart of an example of control for minimizing the amount of attenuation on a main signal; 
         FIG. 7  is a flowchart describing an example of switching processing between control operations; 
         FIG. 8  illustrates an example of changing the amount of attenuation by the switching processing in a control operation; 
         FIG. 9  illustrates a second configuration example of an optical receiver; 
         FIG. 10  illustrates a third configuration example of an optical receiver; 
         FIG. 11A  illustrates a fourth configuration example of an optical receiver; 
         FIG. 11B  illustrates a third configuration example of an operation circuit; 
         FIG. 12A  illustrates a fifth configuration example of an optical receiver; 
         FIG. 12B  illustrates a fifth configuration example of an operation circuit; and 
         FIG. 13  illustrates a configuration example of a communication system applying an optical receiver. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     With reference to the drawings, preferred embodiments of the disclosed technology will be described in detail. In the disclosed technology, signal light is split on the basis of a variable ratio, and the split signal light is received. Thus, the power of the received signal light can be attenuated, and the total power of the received signal light can be calculated so that the input power before the signal light is attenuated may be monitored with high precision. 
     First Embodiment 
       FIG. 1A  illustrates a first configuration example of an optical receiver. An optical receiver  100  receives signal light transmitted from an optical transmitter through an optical fiber  10 . The optical receiver  100  includes a variable-ratio splitter  110 , photo detectors (PDs)  121  to  123 , I/V converters  131  to  133 , an operation circuit  140 , a receive signal output unit  150 , an input power output unit  160 , and a control voltage generator  170 . 
     On  FIG. 1A , the double arrows on arrows indicating signal light indicate that the signal light contains an x-polarized component (horizontal polarized component). The dots on the arrows indicating signal light indicate that the signal light contains a y-polarized component (perpendicular polarized component). The y-polarized component and x-polarized component may be polarized components which are orthogonal to each other, for example. The signal light output from the optical fiber  10  contains the y-polarized component and x-polarized component. 
     The variable-ratio splitter  110  splits signal light output from the optical fiber  10  on the basis of a variable ratio (power ratio). The variable-ratio splitter  110  splits signal light into three beams in accordance with the control voltage output from the control voltage generator  170 . The variable-ratio splitter  110  outputs partial signal light of the split signal light beams as a main signal to the PD  122 . This allows the variable-ratio splitter  110  to attenuate the signal light input from the optical fiber  10  by a variable amount of attenuation and cause the PD  122  to receive the resulting light. The variable-ratio splitter  110  outputs another signal light of the split signal light beams as a monitor signal to the PDs  121  and  123 . 
     More specifically, the variable-ratio splitter  110  includes a polarizer  111 , a variable phase plate  112 , and a polarizer  113 . The polarizer  111  (second polarizer) outputs the signal light output from the optical fiber  10  through an optical path which is different for each polarized component. More specifically, the polarizer  111  outputs the y-polarized component contained in the signal light output from the optical fiber  10  through an optical path r 1  to the variable phase plate  112 . The polarizer  111  outputs the x-polarized component contained in the signal light output from the optical fiber  10  through an optical path r 2  to the variable phase plate  112 . 
     The variable phase plate  112  is a polarization adjusting unit which changes the ratio of polarized components contained in signal light in accordance with the control voltage to be applied. More specifically, the variable phase plate  112  causes the signal light on the optical paths r 1  and r 2  output from the polarizer  111  to pass through with an index of refraction which is different for each polarized component so as to give a phase difference between polarized components of signal light beams. 
     More specifically, the variable phase plate  112  gives the phase difference according to the control voltage to be applied from the control voltage generator  170  to between the polarized components. This can change the direction of polarization of the signal light in accordance with the control voltage to be applied to the variable phase plate  112 . The variable phase plate  112  outputs signal light on the optical paths r 1  and r 2  in the changed directions of polarization to the polarizer  113 . 
     The polarizer  113  (first polarizer) splits a polarized component contained in a signal light. More specifically, the polarizer  113  outputs the signal light beams output from the variable phase plate  112  through an optical path which is different for each polarized component. More specifically, the polarizer  113  outputs the y-polarized component contained in the signal light on the optical path r 1  to the PD  121 . The polarizer  113  outputs the x-polarized component contained in the signal light on the optical path r 1  to the PD  122 . The polarizer  113  outputs the y-polarized component contained in the signal light on the optical path r 2  to the PD  122 . The polarizer  113  outputs the x-polarized component contained in the signal light on the optical path r 2  to the PD  123 . 
     The polarizer  111  and polarizer  113  may be implemented by a birefringent plate and/or a polarization beam splitter, for example. The variable phase plate  112  may be implemented by an element having an electro-optical effect, an element having a magneto-optical effect, or a liquid crystal element which changes the index of refraction in accordance with the applied control voltage, for example. 
     For example, when the variable phase plate  112  is an element of LN crystal (lithium niobate crystal) having an electro-optical effect, the control voltage applied to the LN crystal induces the change of the index of refraction, which allows periodical change of the phase difference φ from 0 to 2π between orthogonal polarized component. The polarizer  111 , variable phase plate  112  and polarizer  113  may be adjacent and integral to each other or may be provided with some spaces between them. 
     In this way, the variable-ratio splitter  110  changes the direction of polarization of signal light by changed index of refraction, and splits the signal light having the changed directions of polarization on the basis of the polarized components. Thus, for example, compared with the configuration in which light absorption may be used to change the direction of polarization, signal light may be split with minimum signal light loss to branch the signal light. Thus, the total power of signal light split by the variable-ratio splitter  110  may be substantially equal to the power (input power) of the signal light input to the optical receiver  100 . 
     The polarized signal light beams split by the polarizer  111  are input to the variable phase plate  112 . Thus, independent of the polarized state of the signal light input to the optical receiver  100 , the direction of polarization of signal light on the variable phase plate  112  can be changed. It is assumed here that the optical axial direction of the variable phase plate  112  is the direction resulting from the synthesis of a Y-axis direction and an X-axis direction. In other words, the optical axial direction of the variable phase plate  112  tilts 45 degrees from the Y-axis direction to the X-axis direction. 
     The PDs  121  to  123  are provided at positions where signal light output from the variable branches  110  can be received. The PDs  121  to  123  perform photoelectric conversion on the received signal light and output them to the I/V converters  131  to  133 . The I/V converters  131  to  133  convert the current of signals output from the PDs  121  to  123  to voltage. The I/V converters  131  to  133  output the signals converted to voltage to the operation circuit  140 . The operation circuit  140  performs operations based on the signals output from the I/V converters  131  to  133 . 
       FIG. 1B  illustrates a first configuration example of the operation circuit. As illustrated in  FIG. 1B , the operation circuit  140  includes a reception processor  141 , a calculation circuit  142 , and an amount-of-attenuation controller  143 . The reception processor  141  performs reception processing on a signal (main signal) output from the I/V converter  132 . The reception processing by the reception processor  141  may include identification of signals. The reception processor  141  outputs the result of the reception processing as a receive signal to the receive signal output unit  150 . 
     The calculation circuit  142  calculates the total power of signals output from the I/V converters  131  to  133 . The total power of signals output from the I/V converters  131  to  133  corresponds to the power (input power) before attenuated by the variable-ratio splitter  110  of the signal light input from the optical fiber  10  to the optical receiver  100 . The calculation circuit  142  outputs the calculated total power to the input power output unit  160  as the input power to the optical receiver  100 . 
     The amount-of-attenuation controller  143  controls the amount of attenuation on the main signal received by the PD  122  on the basis of at least one of the signals output from the I/V converters  131  to  133 . More specifically, the amount-of-attenuation controller  143  outputs the voltage signal corresponding to control voltage applied to the variable phase plate  112  to the control voltage generator  170 . 
     Referring back to  FIG. 1A , the receive signal output unit  150  outputs the receive signal which is output from the operation circuit  140 . The input power output unit  160  outputs the total power (input power) calculated by the operation circuit  140 . The input power which is output unit  160  may include a user interface which notifies a user of input power. For example, the input power output unit  160  may include a monitor which displays input power or a monitor controller which causes a monitor to display input power. 
     The input power output unit  160  may include a speaker which audibly outputs input power or a speaker controller which causes a speaker to audibly output input power. The input power output unit  160  may include a printer which prints input power or a printer controller which causes a printer to print input power. The input power output unit  160  may include a communication interface which transmits input power to a different communication apparatus. 
     The control voltage generator  170  generates voltage corresponding to a voltage signal output from the operation circuit  140  as control voltage and applies the generated control voltage to the variable phase plate  112 . For example, the control voltage generator  170  may include a digital/analog converter (DAC) which converts a digital voltage signal output from the operation circuit  140  to an analog signal. The control voltage generator  170  applies the voltage signal converted to an analog signal as the control voltage to the variable phase plate  112 . 
       FIG. 2A  illustrates optical paths for signal light in the variable-ratio splitter (φ=π).  FIG. 2A  to  FIG. 2C  illustrate the polarizer  111 , variable phase plate  112 , polarizer  113  and PDs  121  to  123  among the components of the optical receiver  100 . In  FIG. 2A  to  FIG. 2C , the polarizer  111 , variable phase plate  112 , polarizer  113  and PDs  121  to  123  are provided with spaces between them. 
     When the phase difference φ given between the polarized components which are orthogonal in the variable phase plate  112  is π, as illustrated in  FIG. 2A , the signal light of the y-polarized light input from the polarizer  111  through the optical path r 1  to the variable phase plate  112  becomes x-polarized light which then is input to the polarizer  113 . Then, the signal light of the x-polarized light input from the variable phase plate  112  to the polarizer  113  is output to the PD  122 . 
     The signal light of the x-polarized light input from the polarizer  111  through the optical path r 2  to the variable phase plate  112  becomes y-polarized light which then is input to the polarizer  113 . The signal light of the y-polarized light input from the variable phase plate  112  to the polarizer  113  is output to the PD  122 . Thus, when the phase difference φ is π in the variable phase plate  112 , the split signal light beams are received by the PD  122 . In this case, no signal light is received by the PDs  121  and  123 . Therefore, the amount of attenuation on the main signal received by the PD  122  is equal to a minimum (0). 
     In this way, the variable-ratio splitter  110  in a specific state (φ=π) causes all of signal light input from the optical fiber  10  to be received by the PD  122  (predetermined light receiving portion). The reception processor  141  then performs reception processing on the signal light received by the PD  122  as a main signal. This allows the amount of attenuation on the main signal which undergoes the reception processing by the reception processor  141  to be equal to 0 by changing the variable-ratio splitter  110  to the specific state (φ=π). Thus, even when the signal light has low input power, the deterioration of the minimum receiving sensitivity can be prevented. 
       FIG. 2B  illustrates an optical path for signal light in a variable branch (φ=0 or 2π). When the phase difference φ given between the polarized components which are orthogonal in the variable phase plate  112  is equal to 0 or 2π, as illustrated in  FIG. 2B , the signal light input from the polarizer  111  through the optical path r 1  to the variable phase plate  112  is input as y-polarized light to the polarizer  113 . The signal light of the y-polarized light input from the variable phase plate  112  to the polarizer  113  is output to the PD  121 . 
     The signal light input from the polarizer  111  through the optical path r 2  to the variable phase plate  112  in input as x-polarized light to the polarizer  113 . The signal light of the x-polarized light input from the variable phase plate  112  to the polarizer  113  is output to the PD  123 . Thus, when the phase difference φ is 0 or 2π in the variable phase plate  112 , the split signal light beams are received by the PD  121  and PD  123 . In this case, no signal light is received by the PD  122 . Thus, the amount of attenuation on a main signal received by the PD  122  becomes a maximum. 
       FIG. 2C  illustrates an optical path for signal light in a variable-ratio splitter (φ≠0, π, or 2π). When the phase difference φ given between orthogonal polarized components in the signal light variable phase plate  112  is a value excluding 0, π, and 2π, as illustrated in  FIG. 2C , the signal light input from the polarizer  111  through the optical path r 1  to the variable phase plate  112  contains the x-polarized component and the y-polarized component and is input to the polarizer  113 . The ratio between the x-polarized component and the y-polarized component input to the polarizer  113  depends on the phase difference φ. The y-polarized component input from the variable phase plate  112  through the optical path r 1  to the polarizer  113  is output to the PD  121 . The x-polarized component input from the variable phase plate  112  through the optical path r 1  to the polarizer  113  is output to the PD  122 . 
     The signal light input from the polarizer  111  through the optical path r 2  to the variable phase plate  112  contains an x-polarized component and a y-polarized component and is input to the polarizer  113 . The ratio between the x-polarized component and y-polarized component input to the polarizer  113  depends on the phase difference φ. The y-polarized component input from the variable phase plate  112  through the optical path r 2  to the polarizer  113  is output to the PD  122 . The x-polarized component input from the variable phase plate  112  through the optical path r 2  to the polarizer  113  is output to the PD  123 . Thus, when the phase difference φ is a value excluding 0, π, and 2π in the variable phase plate  112 , the split signal light beams are received by the PDs  121  to  123 . Thus, the amount of attenuation on a main signal received by the PD  122  becomes equal to the amount of attenuation based on the phase difference φ. 
       FIG. 3  is a graph illustrating changes in main signal and monitor signal corresponding to control voltage. In  FIG. 3 , the horizontal axis indicates the magnitude of control voltage to be applied to the variable phase plate  112 . The vertical axis indicates the power of a main signal received by the PD  122 , and PD  121  and  123  and total power of a monitor signal. A characteristic  310  indicates changes in power of a main signal corresponding to the control voltage. A characteristic  320  indicates changes in total power of a monitor signal corresponding to control voltage. 
     As exhibited by the characteristic  310 , changing control voltage can change the power of a main signal periodically. Thus, the amount-of-attenuation controller  143  changes the control voltage to be applied to the variable phase plate  112  so that the power of the main signal output from the I/V converter  132  can be equal to a desirable value. Thus, the power (the amount of attenuation) of the main signal can be controlled. For example, when the control voltage to be applied to the variable phase plate  112  is equal to voltage V 1 , the power of the main signal becomes a minimum (while the amount of attenuation is a maximum). When the control voltage to be applied to the variable phase plate  112  is equal to voltage V 2 , the power of the main signal is equal to a maximum (while the amount of attenuation is equal to a minimum). 
     As exhibited by the characteristic  310  and characteristic  320 , the sum of the power of the main signal corresponding to the control voltage and the total power of the monitor signal corresponding to the control voltage is constant. Thus, the amount-of-attenuation controller  143  may change the control voltage to be applied to the variable phase plate  112  so that the total power of the monitor signals to be output from the I/V converters  131  and  133  can be equal to desirable value. Thus, the power of the main signal (the amount of attenuation) can be controlled. 
       FIG. 4  is a flowchart illustrating an example of control for attenuating a main signal. As attenuation control for attenuating a main signal to be received by the PD  122 , the amount-of-attenuation controller  143  in the operation circuit  140  performs the following steps, for example. It is assumed here that memory in the optical receiver  100  prestores target power Target of a main signal. First of all, the power of a signal (main signal) output from the I/V converter  132  is stored in a variable field B 1  (step S 401 ). 
     Next, whether the power stored in the variable B 1  agrees with the target power Target or not is determined (step S 402 ). If the power stored in the variable B 1  agrees with the target power Target (Yes in step S 402 ), the processing exits the series of operations. If not (No in step S 402 ), the absolute value of the difference between the power stored in the variable B 1  and target power Target is stored in a variable C 1  (step S 403 ). 
     Next, the control voltage to be applied to the variable phase plate  112  is increased by a unit amount N step (step S 404 ). Next, the power of the signal (main signal) output from the I/V converter  132  is stored in a variable B 2  (step S 405 ). Next, the absolute value of the difference between the power stored in the variable B 2  and the target power Target is stored in a variable C 2  (step S 406 ). 
     Next, whether the value stored in the variable C 2  is smaller than the value stored in the variable C 1  or not is determined (step S 407 ). This allows the determination on whether the power of the main signal has approached to the target power Target by step S 404  or not. If the value stored in the variable C 2  is smaller than the value stored in the variable C 1  (Yes in step S 407 ), the control voltage to be applied to the variable phase plate  112  is increased by the unit amount N step (step S 408 ). 
     Next, the power of a signal (main signal) output from the I/V converter  132  is stored in a variable B 3  (step S 409 ). Next, the absolute value of the difference between the power stored in the variable B 3  and the target power Target is stored in a variable C 3  (step S 410 ). Next, whether the value stored in the variable C 3  is equal to 0 or not is determined (step S 411 ). If the value stored in the variable C 3  is not equal to 0 (No in step S 411 ), the processing returns to step S 408 . If the value stored in the variable C 3  is equal to 0 (Yes in step S 411 ), the processing exits the series of operations. 
     In step S 407 , if the value stored in the variable C 2  is not smaller than the value stored in the variable C 1  (No in step S 407 ), the control voltage to be applied to the variable phase plate  112  is reduced by the unit amount N step (step S 412 ). Next, the power of the signal (main signal) output from the I/V converter  132  is stored in a variable B 4  (step S 413 ). 
     Next, the absolute value of the difference between the power stored in the variable B 4  and the target power Target is stored in the variable C 4  (step S 414 ). Next, whether the value stored in the variable C 4  is equal to 0 or not is determined (step S 415 ). If the value stored in the variable C 4  is not equal to 0 (No in step S 415 ), the processing returns to step S 412 . If the value stored in the variable C 4  is equal to 0 (Yes in step S 415 ), the processing exits the series of operations. The variables B 1 -B 4  and C 1 -C 4  are provided, for example, in memory of the optical receiver  100 . 
     By repeating the steps above, the amount of attenuation on a main signal to be received by the PD  122  may be controlled so as to be equal to the target power Target. For example, if the power of a signal received by the PD  122  is equal to a desirable value (such as −10 [dBm]), the power of the signal output from the PD  122  may be prestored as a target power Target in memory of the optical receiver  100 . Thus, the power of the signal received by the PD  122  can be equal to the desirable value (such as −10 [dBm]). 
       FIG. 5  illustrates a second configuration example of the operation circuit. In  FIG. 5 , like numbers refer to like components to those illustrated in  FIG. 1B , and the description will be omitted. As illustrated in  FIG. 5 , the calculation circuit  142  in the operation circuit  140  may output the calculated input power to the input power output unit  160  and amount-of-attenuation controller  143 . The amount-of-attenuation controller  143  changes the control operation over the amount of attenuation on the basis of the input power output from the calculation circuit  142 . For example, the amount-of-attenuation controller  143  changes the control for attenuating the main signal as illustrated in  FIG. 4  and control for minimizing the amount of attenuation on the main signal as illustrated in  FIG. 6  on the basis of the input power. 
       FIG. 6  is a flowchart of an example of control for minimizing the amount of attenuation on a main signal. In order to control for minimizing the amount of attenuation on a main signal, the amount-of-attenuation controller  143  in the operation circuit  140  performs the following steps. First of all, the total power of signals (monitor signals) output from the I/V converters  131  and  133  is stored in a variable A 1  (step S 601 ). 
     Next, whether the value stored in the variable A 1  is equal to 0 or not is determined (step S 602 ). If the value stored in the variable A 1  is equal to 0 (Yes in step S 602 ), the processing exits the series of operations. If the value stored in the variable A 1  is not equal to 0 (No in step S 602 ), the control voltage to be applied to the variable phase plate  112  is increased by a unit amount N step (step S 603 ). Next, the total power of signals (monitor signals) output from the I/V converters  131  and  133  is stored in a variable A 2  (step S 604 ). 
     Whether the value stored in the variable A 2  is smaller than the value stored in the variable A 1  or not is determined next (step S 605 ). This allows determination of whether the amount of attenuation of the main signal is reduced by step S 603  or not. If the value stored in the variable A 2  is smaller than the value stored in the variable A 1  (Yes in step S 605 ), whether the value stored in the variable A 2  is equal to 0 or not is determined (step S 606 ). If the value stored in the variable A 2  is equal to 0 (Yes in step S 606 ), the processing exits the series of operations. 
     In step S 606 , if the value stored in the variable A 2  is not equal to 0 (No in step S 606 ), the control voltage to be applied to the variable phase plate  112  is increased by the unit amount N step (step S 607 ). Next, the total power of signals (monitor signals) output from the I/V converters  131  and  133  is stored in a variable A 3  (step S 608 ). 
     Whether the value stored in the variable A 3  is equal to 0 or not is determined next (step S 609 ). If the value stored in the variable A 3  is not equal to 0 (No in step S 609 ), the processing returns to step S 607 . If the value stored in the variable A 3  is equal to 0 (Yes in step S 609 ), the processing exits the series of operations. 
     In step S 605 , if the value stored in the variable A 2  is not smaller than the value stored in the variable A 1  (No in step S 605 ), the control voltage to be applied to the variable phase plate  112  is reduced by the unit amount N step (step S 610 ). Next, the total power of signals (monitor signals) output from the I/V converters  131  and  133  is stored in a variable A 4  (step S 611 ). 
     Then, whether the value stored in the variable A 4  is equal to 0 or not is determined (step S 612 ). If value stored in the variable A 4  is not equal to 0 (No in step S 612 ), the processing returns to step S 610 . If the value stored in the variable A 4  is equal to 0 (Yes in step S 612 ), the processing exits the series of operations. The variables A 1 -A 4  are provided, for example, in memory of the optical receiver  100 . 
     By repeating the steps above, the amount of attenuation by the variable-ratio splitter  110  on a main signal to be received by the PD  122  can be minimized (0 [dB]). Thus, even when the signal light from the optical fiber  10  to the optical receiver  100  has low input power, the deterioration of the minimum receiving sensitivity can be prevented. 
       FIG. 7  is a flowchart describing an example of switching processing between control operations. As switching processing between control operations, the amount-of-attenuation controller  143  in the operation circuit  140  performs the following steps. First of all, the input power output from the calculation circuit  142  is acquired (step S 701 ). Next, whether the input power acquired by step S 701  is larger than a threshold value or not is determined (step S 702 ). 
     If the input power is not larger than the threshold value in step S 702  (No in step S 702 ), whether a control operation for small signal input is being performed or not is determined (step S 703 ). The control operation for small signal input may be control for minimizing the amount of attenuation as described in  FIG. 6 , for example. If the control operation for small signal input is being performed (Yes in step S 703 ), the processing returns to step S 701 . If the control operation for small signal input is not being performed (No in step S 703 ), the control operation for small signal input is started (step S 704 ), and the processing returns to step S 701 . 
     If the input power is larger than the threshold value in step S 702  (Yes in step S 702 ), whether a control operation for large signal input is being performed or not is determined (step S 705 ). The control operation for large signal input may be control for attenuating a main signal as described in  FIG. 4 , for example. If the control operation for large signal input is being performed (Yes in step S 705 ), the processing returns to step S 701 . If the control operation for large signal input is not being performed (No in step S 705 ), the control operation for large signal input is started (step S 706 ), and the processing returns to step S 701 . 
     It is assumed here, for example, that the threshold value to be compared in step S 702  is −10 [dBm]. In this case, if the input power to the optical receiver  100  is equal to or smaller than −10 [dBm], the amount-of-attenuation controller  143  performs the control for minimizing the amount of attenuation as described in  FIG. 6  so that the deterioration of the minimum receiving sensitivity can be prevented. If the input power to the optical receiver  100  is larger than −10 [dBm], the amount-of-attenuation controller  143  performs the control for attenuating a main signal as described in  FIG. 4  so that the main signal to be received by the PD  122  can be attenuated. 
       FIG. 8  illustrates an example of changing the amount of attenuation by the switching processing in the control operation. The table  800  in  FIG. 8  illustrates attenuation results of the switching processing (where the threshold value is equal to −10 [dBm]) described in  FIG. 7 . More specifically, the table  800  illustrates the power of a main signal received by the PD  122  and the amount of attenuation of the main signal received by the PD  122  when the input power of signal light from the optical fiber  10  to the optical receiver  100  changes. 
     As illustrated in the table  800 , when the input power is equal to or smaller than −10 [dBm], the control for minimizing the amount of attenuation as described in  FIG. 6  changes the amount of attenuation of the main signal received by the PD  122  to 0 [dB]. When the input power is larger than −10 [dBm], the control for attenuating a main signal as described in  FIG. 4  changes the power of the main signal received by the PD  122  to −10 [dBm] (target power Target). 
     Second Embodiment 
       FIG. 9  illustrates a second configuration example of a optical receiver. In  FIG. 9 , like numbers refer to like components to those illustrated in  FIG. 1A , and the description will be omitted. As illustrated in  FIG. 9 , the optical receiver  100  may include a reflector  911  and a reflector  912  in addition to the components illustrated in  FIG. 1A . In this case, the PD  123  and I/V converter  133  illustrated in  FIG. 1A  may be omitted. 
     The polarizer  113  outputs an x-polarized component contained in signal light on the optical path r 2  to the reflector  911 . The reflector  911  reflects the signal light output from the polarizer  113  to the reflector  912 . The reflector  912  reflects the signal light output from the reflector  911  to the PD  121 . The PD  121  receives the signal light output from the polarizer  113  and the signal light output from the reflector  912 . 
     Thus, the PD  121  receives a plurality of signal light beams which are different from the signal light which undergoes reception processing by the reception processor  141  among signal light beams split by the variable-ratio splitter  110 . Even without the PD  123  and I/V converter  133  (as in  FIG. 1A ), the signal light beams split by the variable-ratio splitter  110  can be received. This may reduce the element number for control and the power consumption. 
     Third Embodiment 
       FIG. 10  illustrates a third configuration example of a optical receiver. In  FIG. 10 , like numbers refer to like components to those illustrated in  FIG. 9 , and the description will be omitted. As illustrated in  FIG. 10 , the optical receiver  100  may include a beam splitter  1011  in the configuration illustrated in  FIG. 9  instead of the reflector  912 . 
     The polarizer  113  outputs a y-polarized component contained in signal light on the optical path r 1  to the beam splitter  1011 . The reflector  911  outputs the signal light output from the polarizer  113  to the beam splitter  1011 . The beam splitter  1011  multiplexes the signal light from the polarizer  113  and the signal light from the reflector  911  and outputs the multiplexed signal light to the PD  121 . The PD  121  receives the multiplexed signal light output from the beam splitter  1011 . 
     In this way, the PD  121  receives a plurality of signal light beams which are different from the signal light which undergoes reception processing by the reception processor  141  among signal light beams split by the variable-ratio splitter  110 . Thus, even without the PD  123  and I/V converter  133  (as in  FIG. 1A ), the signal light beams split by the variable-ratio splitter  110  can be received. This may reduce the element number for control and the power consumption. 
     Fourth Embodiment 
       FIG. 11A  illustrates a fourth configuration example of a optical receiver. In  FIG. 11A , like numbers refer to like components to those illustrated in  FIG. 1A , and the description will be omitted. As illustrated in  FIG. 11A , the variable phase plate  112  may give a phase difference according to the control voltage input from an external device to the optical receiver  100  to between polarized components. In this case, the control voltage generator  170  (as in  FIG. 1A ) may be omitted. 
       FIG. 11B  illustrates a third configuration example of the operation circuit. In  FIG. 11B , like numbers refer to like components to those illustrated in  FIG. 1B , and the description will be omitted. As illustrated in  FIG. 11B , the operation circuit  140  illustrated in  FIG. 11A  may not have the amount-of-attenuation controller  143 . As illustrated in  FIG. 11A  and  FIG. 11B , the optical receiver  100  may control the amount of attenuation on a main signal on the basis of externally input control voltage instead of performing feedback control with the power of the main signal received by the PD  122 . 
     Fifth Embodiment 
       FIG. 12A  illustrates a fifth configuration example of a optical receiver. In  FIG. 12A , like numbers refer to like components to those illustrated in  FIG. 1A , and the description will be omitted. As illustrated in  FIG. 12A , the variable-ratio splitter  110  may not have the polarizer  111 , PD  123  and I/V converter  133 . 
     Even without the polarizer  111 , when the direction of polarization of signal light input from the optical fiber  10  to the optical receiver  100  is constant and is known, for example, the direction of polarization of the signal can be changed by correctly selecting the optical axial direction of the variable phase plate  112 . This allows splitting signal light by the ratio according to the control voltage to be applied to the variable phase plate  112  and causes the split signal light beams to be received by the corresponding PD  121  and  122 . 
       FIG. 12B  illustrates a fifth configuration example of the operation circuit. In  FIG. 12B , like numbers refer to like components to those illustrated in  FIG. 1B , and the description will be omitted. As illustrated in  FIG. 12B , in the operation circuit  140  illustrated in  FIG. 12A , the calculation circuit  142  calculates the total power of signals output from the I/V converters  131  and  132 . The total power of signal light output from the I/V converters  131  and  132  exhibits the power (input power) before the attenuation by the variable-ratio splitter  110  of the signal light input from the optical fiber  10  to the optical receiver  100 . The amount-of-attenuation controller  143  controls the amount of attenuation on a main signal received by the PD  122  on the basis of at least one of the signals output from the I/V converters  131  and  132 . 
     Sixth Embodiment 
       FIG. 13  illustrates a configuration example of a communication system applying the optical receiver. As illustrated in  FIG. 13 , a communication system  1300  includes an optical transmitter  1310 , an optical amplifier  1320 , a optical receiver  100 , and optical fibers  11  and  12 . The optical transmitter  1310  transmits signal light through the optical fiber  11  to the optical amplifier  1320 . 
     The optical amplifier  1320  amplifies the signal light transmitted from the optical transmitter  1310 . The optical amplifier  1320  transmits the amplified signal light through the optical fiber  12  to the optical receiver  100 . The optical receiver  100  receives the signal light transmitted from the optical amplifier  1320 . The optical receiver  100  may be any one of the aforementioned optical receivers  100 . 
     Since the optical receiver  100  attenuates the signal light and performs the reception processing of the attenuated signal light, the deterioration of characteristic of the received signal light can be prevented even without an optical attenuator on the optical fiber  12 , which can reduce the equipment costs. The dynamic range of power of signal light can be increased because the optical receiver  100  attenuates the signal light and performs the reception processing of the attenuated signal light. Thus, the margin for the system design of the communication system  1300  may be increased. 
     Others 
     In this way, in the optical receiver  100  of embodiments described above, signal light is split by a variable ratio, and the split signal light beams are received. Thus, the power of the received signal light can be attenuated by a variable amount of attenuation. This allows prevention of deterioration of the received signal characteristic even when signal light having large power is input to the optical receiver  100 . 
     In the optical receiver  100 , even when the amount of attenuation on the received signal light is changed, the total power of the received signal light is calculated so as to monitor the input power of the signal light before the attenuation. Even when light transmission characteristic of the polarizer  111 , variable phase plate  112  or polarizer  113  changes depending on the wavelength of signal light or temperature, the relationship that the total power of the split and received signal light are equal to the input power is typically satisfied. Therefore, with the optical receiver  100 , even when the wavelength of signal light or temperature changes, the input power can be monitored with high precision. 
     Furthermore, even without creating a look-up table illustrating input power for each wavelength of signal light or temperature, for example, the input power may be monitored with high precision. Thus, the input power monitoring may be implemented at low costs, and the manufacturing costs for the optical receiving module can be minimized. 
     Furthermore, the signal light loss due to unconditional splitting of a part of signal light does not occur, compared with the technology that partially splits and monitors signal light with an optical splitter before the variable optical attenuator, for example. Thus, even when the signal light has small input power, the deterioration of the minimum receiving sensitivity can be prevented. 
     The optical receiver  100  has been described in which the signal light received by the PD  122  undergoes reception processing by the reception processor  141  as a main signal. However, the signal light received by the PD  121  or PD  123  may undergo reception processing by the reception processor  141  as a main signal. 
     As described above, the optical receiver and communication system allows monitoring input power with high precision. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.