Patent Publication Number: US-2017373785-A1

Title: Receiving device and local light control method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-127025, filed on Jun. 27, 2016, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a receiving device and a local light control method. 
     BACKGROUND 
     There are recently known 40 Gbps/100 Gbps optical transmission systems that include a digital coherent receiving device employing a DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) modulation method, for example. 
     The receiving device acquires a main light signal by causing local light (locally-generated light) to interfere with a received signal, and performs digital signal processing on the acquired main light signal. When performing the digital signal processing, the receiving device adjusts the power of the main light signal so as to obtain a gain satisfying the performance requirement of the main light signal in accordance with the resolving power of an analog/digital converter (ADC). Moreover, the receiving device is designed in consideration of device environment, a span loss on a transmission line, and the like in order to maintain the steady output power of local light used for coherent reception. 
     Patent Literature 1: Japanese Laid-open Patent Publication No. 2010-245772 
     Patent Literature 2: Japanese Laid-open Patent Publication No. 2014-123905 
     Patent Literature 3: Japanese Laid-open Patent Publication No. 2014-168176 
     For example, when a span loss on a transmission line is small, because the receiving device can perform demodulation by using only the power of a main light signal, the receiving device can perform communication in some cases even if the output power of local light is lowered. 
     However, in the receiving device, because the output power of local light is constantly output even when communication can be performed after the output power of local light is lowered, the output power of local light becomes excessive and thus power consumption used for local light is wasted. 
     SUMMARY 
     According to an aspect of an embodiment, a receiving device includes a light source, a wave multiplexer, a converter, a demodulator and a processor. The light source outputs local light. The wave multiplexer causes the local light to interfere with a received signal to acquire an optical signal. The converter converts the optical signal into an electrical signal. The demodulator demodulates the electrical signal to acquire a demodulated signal. The processor is configured to correct an error of the demodulated signal. The processor is configured to acquire a signal correction amount and/or an error rate. The processor is configured to control the light source in order to adjust an output intensity of the local light based on the signal correction amount and/or the error rate. 
     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. 1  is a block diagram illustrating a hardware configuration example of a receiving device according to embodiments; 
         FIG. 2  is a block diagram illustrating a functional configuration example of a receiving device according to a first embodiment; 
         FIG. 3  is a diagram explaining an example of a correspondence relationship between the number of correction bits and the reception power of a main light signal that are associated with variations in the output power of local light; 
         FIG. 4  is a flowchart illustrating an example of processing operations of a controller in the receiving device that are associated with a first local light control process; 
         FIG. 5  is a block diagram illustrating a functional configuration example of a receiving device according to a second embodiment; 
         FIG. 6  is a flowchart illustrating an example of processing operations of a controller in the receiving device that are associated with a second local light control process; 
         FIG. 7  is a block diagram illustrating a functional configuration example of a receiving device according to a third embodiment; 
         FIG. 8  is a flowchart illustrating an example of processing operations of a controller in the receiving device that are associated with a third local light control process; 
         FIG. 9  is a block diagram illustrating a functional configuration example of a receiving device according to a fourth embodiment; 
         FIG. 10  is a flowchart illustrating an example of processing operations of a controller in the receiving device that are associated with a fourth local light control process; 
         FIG. 11  is a block diagram illustrating a functional configuration example of a receiving device according to a fifth embodiment; and 
         FIGS. 12 and 13  are flowcharts illustrating an example of processing operations of a controller in the receiving device that are associated with a fifth local light control process. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the present invention will be explained with reference to accompanying drawings. The disclosed technology is not limited to the embodiments explained below. The embodiments explained below may be appropriately combined within a scope in which the combined embodiments do not contradict each other. 
     [a] First Embodiment 
       FIG. 1  is a block diagram illustrating a hardware configuration example of a receiving device  1  according to embodiments.  FIG. 2  is a diagram explaining a functional configuration example of the receiving device  1  according to the first embodiment. The receiving device  1  illustrated in  FIG. 1  is, for example, a digital-coherent light receiving device. The receiving device  1  includes a local light source  2 , an optical hybrid circuit  3 , an OE (Optical/Electrical converter)  4 , an ADC (Analog/Digital Converter)  5 , and a DSP (Digital Signal Processor)  6 . The receiving device  1  further includes a field programmable gate array (FPGA)  7  and an integrated circuit (IC)  8 . Moreover, the local light source  2 , the optical hybrid circuit  3 , the OE  4 , the ADC  5 , and the DSP  6  have a module configuration, for example. The local light source  2  includes, for example, a laser (not illustrated) that is a light source for emitting local light and a driving circuit  21  that controls the drive of the laser. 
     The optical hybrid circuit  3  includes a wave multiplexer  31 . The wave multiplexer  31  causes local light to interfere with a received signal to acquire a main light signal. For example, the wave multiplexer  31  mixes a received signal with local light without delaying the phase of the local light to obtain an X-polarized and Y-polarized I-component main light signal. Moreover, the wave multiplexer  31  delays the phase of local light and mixes the received signal and the local light to obtain an X-polarized and Y-polarized Q-component main light signal. 
     The OE  4  performs electric conversion of the X-polarized and Y-polarized I-component main light signal to adjust a gain and also performs electric conversion of the X-polarized and Y-polarized Q-component main light signal to adjust a gain. The OE  4  includes an OE converter  41  and a gain control unit  42 , for example. The OE converter  41  is a converter that performs electric conversion of the X-polarized and Y-polarized I-component main light signal from the optical hybrid circuit  3  and also performs electric conversion of the X-polarized and Y-polarized Q-component main light signal from the optical hybrid circuit  3 . The gain control unit  42  adjusts gains of an X-polarized and Y-polarized I-component electrical signal and an X-polarized and Y-polarized Q-component electrical signal that are electrically converted by the OE converter  41 . 
     The ADC  5  performs digital conversion on the gain-adjusted I-component and Q-component electrical signals. The DSP  6  is a demodulator that performs digital signal processing on the digitally-converted I-component and Q-component electrical signals to demodulate the I-component and Q-component electrical signals into a demodulated signal. The FPGA  7  includes an FEC (Forward Error Correction)  71 , for example. The FEC  71  is a correction unit that performs an FEC process on the demodulated signal. The IC  8  controls the whole of the receiving device  1 . The IC  8  includes a first monitoring unit  81  and a controller  82 . The first monitoring unit  81  acquires the number of correction bits and/or an error rate of the demodulated signal from the FEC  71 . The controller  82  controls the whole of the IC  8 . 
       FIG. 3  is a diagram explaining an example of a correspondence relationship between the number of correction bits and the reception power of a main light signal that are associated with variations in the output power of local light. A first threshold α 1  is the number of correction bits and/or an error rate corresponding to an upper limit of an allowable range in which the signal quality of the main light signal can secure stable signal quality. A second threshold α 2  is the number of correction bits and/or an error rate corresponding to a lower limit of the allowable range in which the signal quality of the main light signal can secure stable signal quality. In case of the number of correction bits and/or the error rate in the allowable range between the first threshold α 1  and the second threshold α 2 , the output power of the main light signal is secured in a signal quality margin without adjusting the output power of local light. Moreover, the number of correction bits and/or an error rate are/is increased along with the degradation of the signal quality of the main light signal. 
     On the contrary, when the number of correction bits and/or the error rate exceed(s) the first threshold α 1 , the signal quality of the main light signal exceeds the lower limit of the signal quality margin. Therefore, the output power of the main light signal is adjusted to fall within the signal quality margin by increasing the output power of local light. When the number of correction bits and/or the error rate are/is less than the second threshold α 2 , the signal quality of the main light signal exceeds the upper limit of the signal quality margin. Therefore, it is determined that signal quality is sufficiently secured. Then, the output power of the main light signal is adjusted to fall within the signal quality margin by decreasing the output power of local light. 
     The first monitoring unit  81  acquires the number of correction bits and/or the error rate with respect to the demodulated signal after demodulation through the FEC  71 . When receiving the main light signal, the controller  82  acquires the number of correction bits and/or the error rate of the main light signal from the first monitoring unit  81 . When the acquired number of correction bits and/or error rate exceed(s) the first threshold α 1 , the controller  82  controls the driving circuit  21  of the local light source  2  in order to increase the output power of local light. When the acquired number of correction bits and/or error rate are/is less than the second threshold α 2 , the controller  82  controls the driving circuit  21  of the local light source  2  in order to decrease the output power of local light. 
     Next, operations of the receiving device  1  according to the first embodiment will be explained.  FIG. 4  is a flowchart illustrating an example of processing operations of the controller  82  in the receiving device  1  that are associated with a first local light control process. 
     The controller  82  determines whether a main light signal is received (Step S 11 ). When receiving the main light signal (Step S 11 : YES), the controller  82  controls the driving circuit  21  of the local light source  2  in order to set the output power of local light to an initial value (Step S 12 ). 
     The controller  82  clears the number of correction bits and/or the error rate of the FEC  71  (Step S 13 ), and acquires the number of correction bits and/or an error rate from the FEC  71  through the first monitoring unit  81  (Step S 14 ). The controller  82  determines whether an uncorrectable error is detected from the FEC  71  (Step S 15 ). When the uncorrectable error is detected (Step S 15 : YES), the controller  82  terminates the processing operations illustrated in  FIG. 4 . 
     When the uncorrectable error is not detected (Step S 15 : NO), the controller  82  determines whether the number of correction bits and/or the error rate acquired in Step S 14  exceed(s) the first threshold α 1  (Step S 16 ). When the number of correction bits and/or the error rate exceed(s) the first threshold α 1  (Step S 16 : YES), the controller  82  determines whether the output power of local light is the maximum (Step S 17 ). 
     When the output power of local light is not the maximum (Step S 17 : NO), the controller  82  controls the driving circuit  21  of the local light source  2  in order to increase the output power of local light (Step S 18 ). In other words, by increasing the output power of local light, the output power of the main light signal falls within the signal quality margin. As a result, the signal quality of the main light signal can be improved. Then, after controlling the driving circuit  21  of the local light source  2 , the controller  82  determines whether the uncorrectable error is detected from the FEC  71  (Step S 19 ). 
     When the uncorrectable error is detected (Step S 19 : YES), the controller  82  controls the driving circuit  21  of the local light source  2  in order to set the output power of local light to an initial value (Step S 20 ), and terminates the processing operations illustrated in  FIG. 4 . When the uncorrectable error is not detected (Step S 19 : NO), the controller  82  terminates the processing operations illustrated in  FIG. 4 . When the output power of local light is the maximum (Step S 17 : YES), the controller  82  moves to Step S 19  in order to determine whether the uncorrectable error is detected. 
     When the number of correction bits and/or the error rate do(does) not exceed the first threshold α 1  (Step S 16 : NO), the controller  82  determines whether the number of correction bits and/or the error rate are(is) less than the second threshold α 2  (Step S 21 ). When the number of correction bits and/or the error rate are(is) less than the second threshold α 2  (Step S 21 : YES), the controller  82  determines whether the output power of local light is the minimum (Step S 22 ). 
     When the output power of local light is not the minimum (Step S 22 : NO), the controller  82  controls the driving circuit  21  of the local light source  2  in order to decrease the output power of local light (Step S 23 ). In other words, by decreasing the output power of local light, the output power of the main light signal falls within the signal quality margin. As a result, while securing the stable signal quality of the main light signal, it is possible to suppress wasteful power consumption used for local light. After controlling the driving circuit  21  of the local light source  2 , the controller  82  moves to Step S 19  in order to determine whether the uncorrectable error is detected. 
     When the output power of local light is the minimum (Step S 22 : YES), the controller  82  moves to Step S 19  in order to determine whether the uncorrectable error is detected. When the number of correction bits and/or the error rate are(is) not less than the second threshold α 2  (Step S 21 : NO), the controller  82  terminates the processing operations illustrated in  FIG. 4 . Moreover, when the main light signal is not received (Step S 11 : NO), the controller  82  terminates the processing operations illustrated in  FIG. 4 . 
     When the number of correction bits and/or the error rate exceed(s) the first threshold α 1  and the output power of local light is not the maximum, the controller  82  that performs the first local light control process illustrated in  FIG. 4  increases the output power of local light. As a result, the stable signal quality of the main light signal can be secured by causing the output power of the main light signal to fall within the signal quality margin. 
     When the number of correction bits and/or the error rate are(is) less than the second threshold α 2  and the output power of local light is not the minimum, the controller  82  decreases the output power of local light. As a result, by causing the output power of the main light signal to fall within the signal quality margin, it is possible to suppress wasteful power consumption used for local light while securing the stable signal quality of the main light signal. 
     The receiving device  1  according to the first embodiment controls the driving circuit  21  in order to adjust the output power of local light on the basis of the number of correction bits and/or the error rate acquired by the first monitoring unit  81 . For example, when the number of correction bits and/or the error rate are(is) less than the second threshold α 2 , the receiving device  1  controls the driving circuit  21  in order to decrease the output power of local light. As a result, while securing the stable signal quality of the main light signal, it is possible to suppress wasteful power consumption used for local light. 
     In particular, because the receiving device  1  uninterruptedly operates 24 hours 365 days, for example, when entering an operating state once, the power consumption has large effects if the wasteful power consumption used for local light is suppressed. 
     It has been explained that the receiving device  1  of the first embodiment controls the output power of local light on the basis of the number of correction bits and/or the error rate. However, the first embodiment is not limited to the number of correction bits and/or the error rate. For example, the receiving device may control the output power of local light by using a signal amplitude value of a multiplexed signal before the digital conversion. An embodiment for this case will be explained below as a second embodiment.  FIG. 5  is a diagram explaining a functional configuration example of a receiving device  1 A according to the second embodiment. Moreover, because the same components as those of the receiving device  1  of the first embodiment have the same reference numbers, the explanations of the same configuration and operation are omitted. 
     [b] Second Embodiment 
     A difference between the receiving device  1  and the receiving device  1 A illustrated in  FIG. 5  is the point that the receiving device  1 A embeds therein, instead of the first monitoring unit  81 , a second monitoring unit  83  that acquires a signal amplitude value of a multiplexed signal before digital conversion, and adjusts the output power of local light on the basis of the monitoring result of the second monitoring unit  83 . 
     The gain control unit  42  in the receiving device  1 A adjusts an amplitude gain of a multiplexed signal that is the main light signal acquired by the wave multiplexer  31  in accordance with the resolving power of the ADC  5 . In this case, when a span loss of a transmission line connected to the receiving device  1 A is small, for example, because demodulation can be performed with only the output power of the main light signal, communications can be performed in some cases even if the output power of local light is lowered. The second monitoring unit  83  acquires the signal amplitude value of the electrical signal of the multiplexed signal from the gain control unit  42 . 
     When the signal amplitude value of the multiplexed signal before digital conversion is less than a first amplitude threshold and the output power of local light is not the maximum, a controller  82 A adjusts the output power of local light in an increasing direction. Moreover, the first amplitude threshold is the minimum signal amplitude value of an allowable range in which the main light signal can secure stable signal quality. 
     When the signal amplitude value of the multiplexed signal before digital conversion exceeds a second amplitude threshold and the output power of local light is not the minimum, the controller  82 A adjusts the output power of local light in a decreasing direction. Moreover, the second amplitude threshold is the maximum signal amplitude value of the allowable range in which the main light signal can secure stable signal quality. 
     Next, operations of the receiving device  1 A according to the second embodiment will be explained.  FIG. 6  is a flowchart illustrating an example of processing operations of the controller  82 A in the receiving device  1 A that are associated with a second local light control process. 
     The controller  82 A determines whether a main light signal is received (Step S 31 ). When receiving the main light signal (Step S 31 : YES), the controller  82 A controls the driving circuit  21  of the local light source  2  in order to set the output power of local light to an initial value (Step S 32 ). 
     The controller  82 A acquires a signal amplitude value of a multiplexed signal before digital conversion through the second monitoring unit  83  (Step S 33 ). The controller  82 A determines whether an uncorrectable error is detected from the FEC  71  (Step S 34 ). When the uncorrectable error is detected (Step S 34 : YES), the controller  82 A terminates processing operations illustrated in  FIG. 6 . 
     When the uncorrectable error is not detected (Step S 34 : NO), the controller  82 A determines whether the signal amplitude value of the multiplexed signal before digital conversion is less than the first amplitude threshold (Step S 35 ). When the signal amplitude value of the multiplexed signal before digital conversion is less than the first amplitude threshold (Step S 35 : YES), the controller  82 A determines whether the output power of local light is the maximum (Step S 36 ). 
     When the output power of local light is not the maximum (Step S 36 : NO), the controller  82 A controls the driving circuit  21  of the local light source  2  in order to increase the output power of local light (Step S 37 ). In other words, by increasing the output power of local light, the output power of the main light signal falls within the signal quality margin. As a result, the stable signal quality of the main light signal can be improved. After controlling the driving circuit  21  of the local light source  2 , the controller  82 A determines whether the uncorrectable error is detected (Step S 38 ). When the uncorrectable error is detected (Step S 38  YES), the controller  82 A controls the driving circuit  21  of the local light source  2  in order to set the output power of local light to the initial value (Step S 39 ), and terminates processing operations illustrated in  FIG. 6 . When the uncorrectable error is not detected (Step S 38 : NO), the controller  82 A terminates processing operations illustrated in  FIG. 6 . When the output power of local light is the maximum (Step S 36 : YES), the controller  82 A moves to Step S 38  in order to determine whether the uncorrectable error is detected. 
     When the signal amplitude value of the multiplexed signal before digital conversion is not less than the first amplitude threshold (Step S 35 : NO), the controller  82 A determines whether the signal amplitude value exceeds the second amplitude threshold (Step S 40 ). When the signal amplitude value exceeds the second amplitude threshold (Step S 40 : YES), the controller  82 A determines whether the output power of local light is the minimum (Step S 41 ). 
     When the output power of local light is not the minimum (Step S 41 : NO), the controller  82 A controls the driving circuit  21  of the local light source  2  in order to decrease the output power of local light (Step S 42 ). In other words, by decreasing the output power of local light, the output power of the main light signal falls within the signal quality margin. As a result, it is possible to suppress wasteful power consumption used for local light while securing the stable signal quality of the main light signal. After controlling the driving circuit  21  of the local light source  2 , the controller  82 A moves to Step S 38  in order to determine whether the uncorrectable error is detected. 
     When the output power of local light is the minimum (Step S 41 : YES), the controller  82 A moves to Step S 38  in order to determine whether the uncorrectable error is detected. When the signal amplitude value does not exceed the second amplitude threshold (Step S 40 : NO), the controller  82 A terminates processing operations illustrated in  FIG. 6 . Moreover, when the main light signal is not received (Step S 31 : NO), the controller  82 A terminates processing operations illustrated in  FIG. 6 . 
     When the signal amplitude value of the multiplexed signal before digital conversion is less than the first amplitude threshold and the output power of local light is not the maximum, the controller  82 A that performs the second local light control process illustrated in  FIG. 6  increases the output power of local light. As a result, by causing the output power of the main light signal to fall within the signal quality margin, the stable signal quality of the main light signal can be secured. 
     When the signal amplitude value of the multiplexed signal before digital conversion exceeds the second amplitude threshold and the output power of local light is not the minimum, the controller  82 A decreases the output power of local light. As a result, by causing the output power of the main light signal to fall within the signal quality margin, it is possible to suppress wasteful power consumption used for local light while securing the stable signal quality of the main light signal. 
     The receiving device  1 A according to the second embodiment controls the driving circuit  21  in order to adjust the output power of local light on the basis of the signal amplitude value of the multiplexed signal before digital conversion acquired by the second monitoring unit  83 . For example, when the signal amplitude value of the multiplexed signal before digital conversion exceeds the second amplitude threshold, the receiving device  1 A controls the driving circuit  21  in order to decrease the output power of local light. As a result, it is possible to suppress wasteful power consumption used for local light while securing the stable signal quality of the main light signal. Furthermore, it is possible to control the output power of local light with high accuracy in accordance with a receiving level of the received main light signal. 
     It has been explained that the receiving device  1 A of the second embodiment controls the output power of local light on the basis of the signal amplitude value of the multiplexed signal before digital conversion. However, the receiving device  1 A may control the output power of local light based on the signal amplitude value of the multiplexed signal before digital conversion together with the control of the output power of local light based on the number of correction bits and/or the error rate associated with the receiving device  1  of the first embodiment. In other words, because the output power of local light is previously adjusted by using the signal amplitude value of the multiplexed signal before digital conversion, the adjustment process of the output power of local light can be speedily performed by using the subsequent number of correction bits and/or error rate. 
     When the control of the output power of local light based on the number of correction bits and/or the error rate is together used, the receiving device  1 A may predict the output power of local light from the signal amplitude value of the multiplexed signal before digital conversion. In this case, the receiving device  1 A controls the driving circuit  21  in order to adjust the output power of local light on the basis of the prediction result and the number of correction bits and/or the error rate. As a result, the adjustment process of the output power of local light can be speedily performed by using the number of correction bits and/or the error rate. 
     It has been explained that the receiving device  1  of the first embodiment controls the output power of local light on the basis of the number of correction bits and/or the error rate. However, the embodiment is not limited to the number of correction bits and/or the error rate. For example, the output power of local light may be controlled by using a signal amplitude value of a multiplexed signal after digital conversion. An embodiment for this case will be explained below as a third embodiment.  FIG. 7  is a diagram explaining a functional configuration example of a receiving device  1 B according to the third embodiment. Moreover, because the same components as those of the receiving device  1  of the first embodiment have the same reference numbers, the explanations of the same configuration and operation are omitted. 
     [c] Third Embodiment 
     A difference between the receiving device  1 B illustrated in  FIG. 7  and the receiving device  1  is the point that the receiving device  1 B embeds therein, instead of the first monitoring unit  81 , a third monitoring unit  84  that acquires a signal amplitude value of a multiplexed signal after digital conversion, and adjusts the output power of local light on the basis of a monitoring result of the third monitoring unit  84 . The third monitoring unit  84  acquires, from the DSP  6 , a signal amplitude value of a multiplexed signal after demodulation, namely, a signal amplitude value of a multiplexed signal after digital conversion. 
     When the signal amplitude value of the multiplexed signal after digital conversion is less than a third amplitude threshold and the output power of local light is not the maximum, a controller  82 B adjusts the output power of local light in an increasing direction. Moreover, the third amplitude threshold is the minimum signal amplitude value of an allowable range in which a main light signal can secure stable signal quality. 
     When the signal amplitude value of the multiplexed signal after digital conversion exceeds a fourth amplitude threshold and the output power of local light is not the minimum, the controller  82 B adjusts the output power of local light in a decreasing direction. Moreover, the fourth amplitude threshold is the maximum signal amplitude value of the allowable range in which the main light signal can secure stable signal quality. 
     Next, operations of the receiving device according to the third embodiment will be explained.  FIG. 8  is a flowchart illustrating an example of processing operations of the controller  82 B in the receiving device that are associated with a third local light control process. 
     The controller  82 B determines whether a main light signal is received (Step S 51 ). When receiving the main light signal (Step S 51 : YES), the controller  82 B controls the driving circuit  21  of the local light source  2  in order to set the output power of local light to an initial value (Step S 52 ). 
     The controller  82 B acquires a signal amplitude value of a multiplexed signal after digital conversion through the third monitoring unit  84  (Step S 53 ). The controller  82 B determines whether an uncorrectable error is detected from the FEC  71  (Step S 54 ). When the uncorrectable error is detected (Step S 54 : YES), the controller  82 B terminates processing operations illustrated in  FIG. 8 . 
     When the uncorrectable error is not detected (Step S 54 : NO), the controller  82 B determines whether the signal amplitude value of the multiplexed signal after digital conversion acquired in Step S 53  is less than the third amplitude threshold (Step S 55 ). When the signal amplitude value of the multiplexed signal after digital conversion is less than the third amplitude threshold (Step S 55 : YES), the controller  82 B determines whether the output power of local light is the maximum (Step S 56 ). 
     When the output power of local light is not the maximum (Step S 56 : NO), the controller  82 B controls the driving circuit  21  of the local light source  2  in order to increase the output power of local light (Step S 57 ). In other words, by increasing the output power of local light, the output power of the main light signal falls within the signal quality margin. As a result, the stable signal quality of the main light signal can be improved. After controlling the driving circuit  21  of the local light source  2 , the controller  82 B determines whether the uncorrectable error is detected (Step S 58 ). 
     When the uncorrectable error is detected (Step S 58 : YES), the controller  82 B controls the driving circuit  21  of the local light source  2  in order to set the output power of local light to the initial value (Step S 59 ), and terminates processing operations illustrated in  FIG. 8 . When the uncorrectable error is not detected (Step S 58 : NO), the controller  82 B terminates processing operations illustrated in  FIG. 8 . When the output power of local light is the maximum (Step S 56 : YES), the controller  82 B moves to Step S 58  in order to determine whether the uncorrectable error is detected. 
     When the signal amplitude value of the multiplexed signal after digital conversion is not less than the third amplitude threshold (Step S 55 : NO), the controller  82 B determines whether the signal amplitude value of the multiplexed signal after digital conversion exceeds the fourth amplitude threshold (Step S 60 ). When the signal amplitude value of the multiplexed signal after digital conversion exceeds the fourth amplitude threshold (Step S 60 : YES), the controller  82 B determines whether the output power of local light is the minimum (Step S 61 ). 
     When the output power of local light is not the minimum (Step S 61 : NO), the controller  82 B controls the driving circuit  21  of the local light source  2  in order to decrease the output power of local light (Step S 62 ). In other words, the output power of the main light signal falls within the signal quality margin by decreasing the output power of local light. As a result, it is possible to suppress wasteful power consumption used for local light while securing the stable signal quality of the main light signal. After controlling the driving circuit  21  of the local light source  2 , the controller  82 B moves to Step S 58  in order to determine whether the uncorrectable error is detected. 
     When the output power of local light is the minimum (Step S 61 : YES), the controller  82 B moves to Step S 58  in order to determine whether the uncorrectable error is detected. When the signal amplitude value of the multiplexed signal after digital conversion does not exceed the fourth amplitude threshold (Step S 60 : NO), the controller  82 B terminates processing operations illustrated in  FIG. 8 . Moreover, when the main light signal is not received (Step S 51 : NO), the controller  82 B terminates processing operations illustrated in  FIG. 8 . 
     When the signal amplitude value of the multiplexed signal after digital conversion is less than the third amplitude threshold and the output power of local light is not the maximum, the controller  82 B that performs the third local light control process illustrated in  FIG. 8  increases the output power of local light. As a result, the stable signal quality of the main light signal can be secured by causing the output power of the main light signal to fall within the signal quality margin. 
     When the signal amplitude value of the multiplexed signal after digital conversion exceeds the fourth amplitude threshold and the output power of local light is not the minimum, the controller  82 B decreases the output power of local light. As a result, by causing the output power of the main light signal to fall within the signal quality margin, it is possible to suppress wasteful power consumption used for local light while securing the stable signal quality of the main light signal. 
     The receiving device  1 B according to the third embodiment controls the driving circuit  21  in order to adjust the output power of local light on the basis of the signal amplitude value of the multiplexed signal after digital conversion acquired by the third monitoring unit  84 . For example, when the signal amplitude value of the multiplexed signal after digital conversion exceeds the fourth amplitude threshold, the receiving device  1 B controls the driving circuit  21  in order to decrease the output power of local light. As a result, it is possible to suppress wasteful power consumption used for local light while securing the stable signal quality of the main light signal. Furthermore, because the output power of local light is controlled finally by using amplitude information after digital conversion, errors in internal processing are hard to occur. 
     It has been explained that the receiving device  1 B of the third embodiment controls the output power of local light on the basis of the signal amplitude value of the multiplexed signal after digital conversion. However, the receiving device  1 B may control the output power of local light based on the signal amplitude value of the multiplexed signal after digital conversion together with the control of the output power of local light based on the number of correction bits and/or the error rate associated with the receiving device  1  according to the first embodiment. In other words, because the output power of local light is previously adjusted by using the signal amplitude value of the multiplexed signal after digital conversion, the adjustment process of the output power of local light can be speedily performed by using the subsequent number of correction bits and/or error rate. 
     When the control of the output power of local light based on the number of correction bits and/or the error rate is used together, the receiving device  1 B may predict the output power of local light from the signal amplitude value of the multiplexed signal after digital conversion. In this case, the receiving device  1 B controls the driving circuit  21  in order to adjust the output power of local light on the basis of the prediction result and the number of correction bits and/or the error rate. As a result, the adjustment process of the output power of local light can be speedily performed by using the number of correction bits and/or the error rate. 
     It has been explained that the receiving device  1  of the first embodiment controls the output power of local light on the basis of the number of correction bits and/or the error rate. However, the embodiment is not limited to the number of correction bits and/or the error rate. For example, the output power of local light may be controlled by using the output power of a received main light signal. An embodiment for this case will be explained below as a fourth embodiment.  FIG. 9  is a diagram explaining a functional configuration example of a receiving device  1 C according to the fourth embodiment. Moreover, because the same components as those of the receiving device  1  of the first embodiment have the same reference numbers, the explanations of the same configuration and operation are omitted. 
     [d] Fourth Embodiment 
     A difference between the receiving device  1 C illustrated in  FIG. 9  and the receiving device  1  illustrated in  FIG. 1  is a point that the receiving device  1 C includes TAP-PD  91  and a fourth monitoring unit  85  instead of the first monitoring unit  81 . The TAP-PD  91  is placed in the previous stage of the wave multiplexer  31 . When receiving a main light signal, the main light signal is transmitted to the wave multiplexer  31  and the fourth monitoring unit  85  by branching by the TAP-PD  91 . The fourth monitoring unit  85  acquires the output power of the main light signal branched at the TAP-PD  91 . A controller  82 C adjusts the output power of local light on the basis of a monitoring result of the fourth monitoring unit  85 . 
     The controller  82 C totalizes the output power of the received main light signal and the output power of local light. When the totalized value is less than a first totalized threshold and the output power of local light is not the maximum, the controller  82 C adjusts the output power of local light in an increasing direction. Moreover, the first totalized threshold is the minimum signal amplitude value of an allowable range in which the main light signal can secure stable signal quality. 
     When the totalized value exceeds a second totalized threshold and the output power of local light is not the minimum, the controller  82 C adjusts the output power of local light in a decreasing direction. Moreover, the second totalized threshold is the maximum signal amplitude value of the allowable range in which the main light signal can secure stable signal quality. 
     Next, operations of the receiving device  1 C according to the fourth embodiment will be explained.  FIG. 10  is a flowchart illustrating an example of processing operations of the controller  82 C in the receiving device  1 C that are associated with a fourth local light control process. 
     The controller  82 C determines whether a main light signal is received (Step S 71 ). When the main light signal is received (Step S 71 : YES), the controller  82 C controls the driving circuit  21  of the local light source  2  in order to set the output power of local light to an initial value (Step S 72 ). 
     The controller  82 C acquires the output power of the main light signal from the TAP-PD  91  via the fourth monitoring unit  85  (Step S 73 ). The controller  82 C determines whether an uncorrectable error from the FEC  71  is detected (Step S 74 ). When the uncorrectable error is detected (Step S 74 : YES), the controller  82 C terminates processing operations illustrated in  FIG. 10 . 
     When the uncorrectable error is not detected (Step S 74 : NO), the controller  82 C computes a totalized value of the output power of the main light signal acquired in Step S 73  and the output power of local light (Step S 75 ). The controller  82 C determines whether the totalized value is less than the first totalized threshold (Step S 76 ). When the totalized value is less than the first totalized threshold (Step S 76 : YES), the controller  82 C determines whether the output power of local light is the maximum (Step S 77 ). 
     When the output power of local light is not the maximum (Step S 77 : NO), the controller  82 C controls the driving circuit  21  of the local light source  2  in order to increase the output power of local light (Step S 78 ). In other words, the output power of the main light signal falls within the signal quality margin by increasing the output power of local light. As a result, the stable signal quality of the main light signal can be improved. After controlling the driving circuit  21  of the local light source  2 , the controller  82 C determines whether the uncorrectable error is detected (Step S 79 ). 
     When the uncorrectable error is detected (Step S 79 : YES), the controller  82 C controls the driving circuit  21  of the local light source  2  in order to set the output power of local light to the initial value (Step S 80 ), and terminates processing operations illustrated in  FIG. 10 . When the uncorrectable error is not detected (Step S 79 : NO), the controller  82 C terminates processing operations illustrated in  FIG. 10 . When the output power of local light is the maximum (Step S 77 : YES), the controller  82 C moves to Step S 79  in order to determine whether the uncorrectable error is detected. 
     When the totalized value is not less than the first totalized threshold (Step S 76 : NO), the controller  82 C determines whether the totalized value exceeds the second totalized threshold (Step S 81 ). When the totalized value exceeds the second totalized threshold (Step S 81 : YES), the controller  82 C determines whether the output power of local light is the minimum (Step S 82 ). 
     When the output power of local light is not the minimum (Step S 82 : NO), the controller  82 C controls the driving circuit  21  of the local light source  2  in order to decrease the output power of local light (Step S 83 ). In other words, the output power of the main light signal falls within the signal quality margin by decreasing the output power of local light. As a result, it is possible to suppress wasteful power consumption used for local light while securing the stable signal quality of the main light signal. After controlling the driving circuit  21  of the local light source  2 , the controller  82 C moves to Step S 79  in order to determine whether the uncorrectable error is detected. 
     When the output power of local light is the minimum (Step S 82 : YES), the controller  82 C moves to Step S 79  in order to determine whether the uncorrectable error is detected. When the totalized value does not exceed the second totalized threshold (Step S 81 : NO), the controller  82 C terminates processing operations illustrated in  FIG. 10 . Moreover, when the main light signal is not received (Step S 71 : NO), the controller  82 C terminates processing operations illustrated in  FIG. 10 . 
     When the totalized value obtained by totalizing the output power of the main light signal and the output power of local light is less than the first totalized threshold and the output power of local light is not the maximum, the controller  82 C that performs the fourth local light control process illustrated in  FIG. 10  increases the output power of local light. As a result, the stable signal quality of the main light signal can be secured by causing the output power of the main light signal to fall within the signal quality margin. 
     When the totalized value obtained by totalizing the output power of the main light signal and the output power of local light exceeds the second totalized threshold and the output power of local light is not the minimum, the controller  82 C decreases the output power of local light. As a result, by causing the output power of the main light signal to fall within the signal quality margin, it is possible to suppress wasteful power consumption used for local light while securing the stable signal quality of the main light signal. 
     The receiving device  1 C according to the fourth embodiment controls the driving circuit  21  in order to adjust the output power of local light on the basis of the totalized value of the output power of the main light signal acquired by the fourth monitoring unit  85  and the output power of local light. For example, when the totalized value exceeds the second totalized threshold, the receiving device  1 C controls the driving circuit  21  in order to decrease the output power of local light. As a result, it is possible to suppress wasteful power consumption used for local light while securing the stable signal quality of the main light signal. 
     It has been explained that the receiving device  1 C of the fourth embodiment controls the output power of local light on the basis of the totalized value obtained by totalizing the output power of the main light signal and the output power of local light. However, the receiving device  1 C may control the output power of local light based on the totalized value together with the control of the output power of local light based on the number of correction bits and/or the error rate associated with the receiving device  1  according to the first embodiment. In other words, because the output power of local light is previously adjusted by using the totalized value, the adjustment process of the output power of local light can be speedily performed by using the subsequent number of correction bits and/or error rate. 
     When the control of the output power of local light based on the number of correction bits and/or the error rate is used together, the receiving device  1 C may predict the output power of local light from the totalized value of the output power of the main light signal and the output power of local light. In this case, the receiving device  1 C controls the driving circuit  21  in order to adjust the output power of local light on the basis of the prediction result and the number of correction bits and/or the error rate. As a result, the adjustment process of the output power of local light can be speedily performed by using the number of correction bits and/or the error rate. 
     It has been explained that the receiving device  1  of the first embodiment controls the output power of local light on the basis of the number of correction bits and/or the error rate. However, the embodiment is not limited to the number of correction bits and/or the error rate. For example, the output power of local light may be controlled by using signal amplitude values of multiplexed signals before and after digital conversion. An embodiment for this case will be explained below as a fifth embodiment.  FIG. 11  is a diagram explaining a functional configuration example of a receiving device  1 D according to the fifth embodiment. Moreover, because the same components as those of the receiving device  1  of the first embodiment have the same reference numbers, the explanations of the same configuration and operation are omitted. 
     [e] Fifth Embodiment 
     A difference between the receiving device  1 D illustrated in  FIG. 11  and the receiving device  1  is a point that the receiving device  1 D embeds therein, in addition to the first monitoring unit  81 , the second monitoring unit  83  that acquires a signal amplitude value of a multiplexed signal before digital conversion and the third monitoring unit  84  that acquires a signal amplitude value of a multiplexed signal after digital conversion. Furthermore, when the signal amplitude value of the multiplexed signal before digital conversion acquired by the second monitoring unit  83  exceeds the second amplitude threshold and the output power of local light is not the minimum, a controller  82 D adjusts the output power of local light in a decreasing direction. Moreover, the second amplitude threshold is the maximum signal amplitude value of an allowable range in which a main light signal can secure stable signal quality. 
     When the signal amplitude value of the multiplexed signal after digital conversion acquired by the third monitoring unit  84  is less than the third amplitude threshold and the output power of local light is not the maximum, the controller  82 D adjusts the output power of local light in an increasing direction. Moreover, the third amplitude threshold is the minimum signal amplitude value of the allowable range in which the main light signal can secure stable signal quality. 
     When the number of correction bits and/or the error rate acquired by the first monitoring unit  81  exceed(s) the first threshold α 1  and the output power of local light is not the maximum, the controller  82 D adjusts the output power of local light in an increasing direction. Furthermore, when the acquired number of correction bits and/or error rate are(is) less than the second threshold α 2  and the output power of local light is not the minimum, the controller  82 D adjusts the output power of local light in a decreasing direction. 
     Next, operations of the receiving device  1 D according to the fifth embodiment will be explained.  FIGS. 12 and 13  are flowcharts illustrating an example of processing operations of the controller  82 D in the receiving device  1 D that are associated with a fifth local light control process. 
     In  FIG. 12 , the controller  82 D determines whether a main light signal is received (Step S 91 ). When the main light signal is received (Step S 91 : YES), the controller  82 D controls the driving circuit  21  of the local light source  2  in order to set the output power of local light to an initial value (Step S 92 ). 
     The controller  82 D clears the number of correction bits and/or the error rate (Step S 93 ), and acquires a signal amplitude value of a multiplexed signal before digital conversion from the gain control unit  42  via the second monitoring unit  83  (Step S 94 ). Furthermore, the controller  82 D acquires a signal amplitude value of a multiplexed signal after digital conversion from the DSP  6  via the third monitoring unit  84  (Step S 95 ). The controller  82 D acquires the number of correction bits and/or the error rate from the FEC  71  via the first monitoring unit  81  (Step S 96 ). 
     The controller  82 D determines whether an uncorrectable error is detected from the FEC  71  (Step S 97 ). When the uncorrectable error is detected (Step S 97 : YES), the controller  82 D terminates processing operations illustrated in  FIG. 12 . 
     When the uncorrectable error is not detected (Step S 97 : NO), the controller  82 D determines whether the signal amplitude value of the multiplexed signal before digital conversion acquired in Step S 94  exceeds the second amplitude threshold (Step S 98 ). When the signal amplitude value exceeds the second amplitude threshold (Step S 98 : YES), the controller  82 D determines whether the output power of local light is the minimum (Step S 99 ). 
     When the output power of local light is not the minimum (Step S 99 : NO), the controller  82 D controls the driving circuit  21  of the local light source  2  in order to decrease the output power of local light (Step S 100 ). In other words, the output power of the main light signal falls within the signal quality margin by decreasing the output power of local light. As a result, it is possible to suppress wasteful power consumption used for local light while securing the stable signal quality of the main light signal. After controlling the driving circuit  21  of the local light source  2 , the controller  82 D determines whether the uncorrectable error is detected (Step S 101 ). 
     When the uncorrectable error is not detected (Step S 101 : NO), the controller  82 D determines whether the signal amplitude value of the multiplexed signal after digital conversion acquired in Step S 95  is less than the third amplitude threshold (Step S 102 ). When the signal amplitude value of the multiplexed signal after digital conversion is less than the third amplitude threshold (Step S 102 : YES), the controller  82 D determines whether the output power of local light is the maximum (Step S 103 ). 
     When the output power of local light is not the maximum (Step S 103 : NO), the controller  82 D controls the driving circuit  21  of the local light source  2  in order to increase the output power of local light (Step S 104 ), and moves to M 1  illustrated in  FIG. 13 . In other words, the output power of the main light signal falls within the signal quality margin by increasing the output power of local light. As a result, the stable signal quality of the main light signal can be improved. 
     When the signal amplitude value of the multiplexed signal before digital conversion does not exceed the second amplitude threshold (Step S 98 : NO), the controller  82 D moves to Step S 101  in order to determine whether the uncorrectable error is detected. Moreover, when the output power of local light is the minimum (Step S 99 : YES), the controller  82 D moves to Step S 101  in order to determine whether the uncorrectable error is detected. 
     When the signal amplitude value of the multiplexed signal after digital conversion is not less than the third amplitude threshold (Step S 102 : NO) or when the output power of local light is the maximum (Step S 103 : YES), the controller  82 D moves to M 1  illustrated in  FIG. 13 . When the uncorrectable error is detected (Step S 101 : YES), the controller  82 D moves to M 2  illustrated in  FIG. 13 . Moreover, when the main light signal is not received (Step S 91 : NO), the controller  82 D terminates processing operations illustrated in  FIG. 12 . 
     In M 1  illustrated in  FIG. 13 , the controller  82 D determines whether the uncorrectable error is detected from the FEC  71  (Step S 111 ). When the uncorrectable error is not detected (Step S 111 : NO), the controller  82 D determines whether the number of correction bits and/or the error rate exceeds the first threshold α 1  (Step S 112 ). When the number of correction bits and/or the error rate exceeds the first threshold α 1  (Step S 112 : YES), the controller  82 D determines whether the output power of local light is the maximum (Step S 113 ). 
     When the output power of local light is not the maximum (Step S 113 : NO), the controller  82 D controls the driving circuit  21  of the local light source  2  in order to increase the output power of local light (Step S 114 ). In other words, the output power of the main light signal falls within the signal quality margin by increasing the output power of local light. As a result, the stable signal quality of the main light signal can be improved. After controlling the driving circuit  21  of the local light source  2 , the controller  82 D determines whether the uncorrectable error is detected (Step S 115 ). 
     When the uncorrectable error is detected (Step S 115 : YES), the controller  82 D controls the driving circuit  21  of the local light source  2  in order to set the output power of local light to the initial value (Step S 116 ), and terminates processing operations illustrated in  FIG. 13 . When the uncorrectable error is not detected (Step S 115 : NO), the controller  82 D terminates processing operations illustrated in  FIG. 13 . When the output power of local light is the maximum (Step S 113 : YES), the controller  82 D moves to Step S 115  in order to determine whether the uncorrectable error is detected. 
     When the number of correction bits and/or the error rate do(does) not exceed the first threshold α 1  (Step S 112 : NO), the controller  82 D determines whether the number of correction bits and/or the error rate are(is) less than the second threshold α 2  (Step S 117 ). When the number of correction bits and/or the error rate are(is) less than the second threshold α 2  (Step S 117 : YES), the controller  82 D determines whether the output power of local light is the minimum (Step S 118 ). 
     When the output power of local light is not the minimum (Step S 118 : NO), the controller  82 D controls the driving circuit  21  of the local light source  2  in order to decrease the output power of local light (Step S 119 ). In other words, the output power of the main light signal falls within the signal quality margin by decreasing the output power of local light. As a result, it is possible to suppress wasteful power consumption used for local light while securing the stable signal quality of the main light signal. After controlling the driving circuit  21  of the local light source  2 , the controller  82 D moves to Step S 115  in order to determine whether the uncorrectable error is detected. 
     When the output power of local light is the minimum (Step S 118 : YES), the controller  82 D moves to Step S 115  in order to determine whether the uncorrectable error is detected. When the number of correction bits and/or the error rate are(is) not less than the second threshold α 2  (Step S 117 : NO), the controller  82 D terminates processing operations illustrated in  FIG. 13 . 
     In M 2  illustrated in  FIG. 13 , the controller  82 D moves to Step S 116  in order to set the output power of local light to the initial value. When the uncorrectable error is detected (Step S 111 : YES), the controller  82 D moves to M 3  illustrated in  FIG. 13 , namely, Step S 116  in order to set the local light to the initial value. 
     When the signal amplitude value of the multiplexed signal before digital conversion exceeds the second amplitude threshold and the output power of local light is not the minimum, the controller  82 D that performs the fifth local light control process decreases the output power of local light. As a result, by causing the output power of the main light signal to fall within the signal quality margin, it is possible to suppress wasteful power consumption used for local light while securing the stable signal quality of the main light signal. 
     When the signal amplitude value of the multiplexed signal after digital conversion is less than the third amplitude threshold and the output power of local light is not the maximum, the controller  82 D increases the output power of local light. As a result, the stable signal quality of the main light signal can be secured by causing the output power of the main light signal to fall within the signal quality margin. 
     When the number of correction bits and/or the error rate exceed(s) the first threshold α 1  and the output power of local light is not the maximum, the controller  82 D increases the output power of local light. As a result, the stable signal quality of the main light signal can be secured by causing the output power of the main light signal to fall within the signal quality margin. 
     When the number of correction bits and/or the error rate are(is) less than the second threshold α 2  and the output power of local light is not the minimum, the controller  82 D decreases the output power of local light. As a result, by causing the output power of the main light signal to fall within the signal quality margin, it is possible to suppress wasteful power consumption used for local light while securing the stable signal quality of the main light signal. 
     When the signal amplitude value of the multiplexed signal before digital conversion exceeds the second amplitude threshold and the output power of local light is not the minimum, the receiving device  1 D according to the fifth embodiment decreases the output power of local light. As a result, it is possible to suppress wasteful power consumption used for local light while securing the stable signal quality of the main light signal. 
     When the signal amplitude value of the multiplexed signal after digital conversion is less than the third amplitude threshold and the output power of local light is not the maximum, the controller  82 D increases the output power of local light. As a result, the stable signal quality of the main light signal can be secured. 
     When the number of correction bits and/or the error rate exceed(s) the first threshold α 1  and the output power of local light is not the maximum, the controller  82 D increases the output power of local light. As a result, the stable signal quality of the main light signal can be secured. 
     When the number of correction bits and/or the error rate are(is) less than the second threshold α 2  and the output power of local light is not the minimum, the controller  82 D decreases the output power of local light. As a result, it is possible to suppress wasteful power consumption used for local light while securing the stable signal quality of the main light signal. Also, the FEC  71  may be involved in the DSP  6 . 
     Components of each device illustrated in the drawings are not necessarily constituted physically as illustrated in the drawings. In other words, the specific configuration of dispersion/integration of each device is not limited to the illustrated configuration. Therefore, all or a part of each device can dispersed or integrated functionally or physically in an optional unit in accordance with various types of loads or operating conditions. 
     All or a part of the process functions performed by each device may be realized by a CPU (Central Processing Unit) (or microcomputer such as MPU (Micro Processing Unit) or MCU (Micro Controller Unit)) and a program that is analyzed and executed by the CPU (or microcomputer such as MPU or MCU), or may be realized by a hardware by wired logic. 
     According to an aspect of embodiments, it is possible to suppress power consumption used for local light. 
     All examples and conditional language recited herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations 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 invention 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.