Patent Publication Number: US-7711218-B2

Title: Optical reception apparatus and controlling method thereof

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical reception apparatus that receives an optical signal of a multivalue phase modulating format used for an optical transmission system, and a controlling method thereof. 
   2. Description of the Related Art 
   Recently, a need for introduction of an optical transmission system corresponding to next-generation 40 gigabit per second (Gbps) is increasing, while transmission distance and frequency availability equivalent to those of an existing 10 Gbps system are required. As a means for realizing this, for example, there has been developed an optical transmission system that applies a multivalue phase modulating format such as Return to Zero-Differential Quadrature Phase Shift Keying (RZ-DQPSK) having excellent Optical Signal Noise Ratio (OSNR) tolerance and nonlinearity tolerance as compared with a Non Return to Zero (NRZ) modulating format, which has been applied in a conventional system corresponding to 10 Gbps or less. Moreover, in addition to the application of the above multivalue phase modulating format, there has been also adopted a technique for improving performance such as long distance transmission and high noise tolerance by performing error correction in an electric stage after an optical signal having an error due to transmission deterioration is photoelectrically converted according to a conventional error correction method by Reed-Solomon code or a new error correction coding method. 
     FIG. 6  is a diagram showing a configuration example of a known 40 Gbps optical transmission system. In this optical transmission system, a plurality of base stations  110  are connected to each other via an optical transmission line  100 , and each base station  110  includes a 40 Gbps router  111  connected with a client (not shown), and an optical transmission device  112  connected to the optical transmission line  100 . Between the router  111  and the optical transmission device  112  in each base station  110 , a 40 Gbps optical signal having a relatively wide optical spectrum width is transmitted in both directions. Moreover, between the optical transmission devices  112  in opposite base stations  110 , a 43 Gbps optical signal having a narrow optical spectrum width with an error correction code is transmitted for a long distance in both directions. 
     FIG. 7  is a diagram showing a configuration example of the optical transmission device  112  in  FIG. 6 . In this configuration example, a known framing process by a framer LSI  122  and an error correction code-adding process are executed with respect to the signal that has been photoelectrically converted by a 40 Gbps broadband module  121  that transmits and receives the optical signal to and from the router  111  on the client side. Moreover, a narrow-band optical signal for long distance transmission generated by a 43 Gbps RZ-DQPSK module  123  that performs RZ-DQPSK modulation processing according to the signal processed by the framer LSI  122 , is amplified to a required level by an optical amplifier  124  and then transmitted to the optical transmission line  100 . The narrow-band optical signal propagated through the optical transmission line  100  and received by the optical transmission device  112  is amplified to the required level by the optical amplifier  124 , and is then input to the 43 Gbps RZ-DQPSK module  123  and demodulated, and the error correction process of the received signal is performed by the framer LSI  122 . A broadband optical signal generated by the 40 Gbps broadband module  121  according to the signal processed by the framer LSI  122  is output to the router  111  on the client side. 
     FIG. 8  is a diagram showing a configuration example of a transmission unit in the RZ-DQPSK module  123  in  FIG. 7 . In the transmission unit, a continuous wave (CW) output from a light source  131  is provided to a phase modulator  132  and an intensity modulator  133 , and the phase modulator  132  and the intensity modulator  133  are driven based on an electric signal output from the framer LSI  122 , to thereby output an optical signal of an RZ-DQPSK modulating format. Specifically, parallel electric signals output from the framer LSI  122  are subjected to serial signal processing by a serializer  134 , and then separated into two flows of data signals D A  and D B  in a separation circuit  135 . By driving the phase modulator  132  by a drive signal generated by driving circuits  136 A and  136 B according to the respective data signals D A  and D B , a DQPSK modulated optical signal is output from the phase modulator  132 . Moreover, a clock signal CLK having a frequency corresponding to the data signals D A  and D B  is output from the serializer  134 , and the intensity modulator  133  is driven by a drive signal generated by a driving circuit  137  according to the clock signal CLK, to thereby output the RZ-DQPSK modulated optical signal from the intensity modulator  133 . 
     FIG. 9  is a diagram showing a configuration example of a reception unit in the RZ-DQPSK module  123  in  FIG. 7 . In the reception unit, the RZ-DQPSK optical signal received from the optical transmission line  100  via the optical amplifier  124  is branched into two, and respectively transmitted to an arm A where a delay interferometer  141 A is formed and an arm B where a delay interferometer  141 B is formed. The delay interferometer  141 A makes a 1-bit time delay component and a π/4 rad phase-controlled component interfere with each other (delay interference), and outputs the interference result as two outputs. Moreover, the delay interferometer  141 B makes a 1-bit time delay component and a −π/4 rad phase-controlled component (the phase is shifted from that of the component in the delay interferometer  141 A by π/2 rad) interfere with each other (delay interference), and outputs the interference result as two outputs. Output beams from the respective delay interferometers  141 A and  141 B are received by photoelectric conversion circuits  142 A and  142 B having a pair of a photodiode and an amplifier, to thereby perform differential photoelectric conversion detection. Then, after output signals from the photoelectric conversion circuits  142 A and  142 B are provided to a multiplex circuit  143  and multiplexed, the multiplexed signals are provided to a deserializer  144  and subjected to parallel signal processing, and signal-processed signals are transmitted to the framer LSI  122  in the subsequent stage. Moreover, the output signals from the photoelectric conversion circuits  142 A and  142 B are also respectively provided to mixers  145 A and  145 B, and phase shift amounts in the respective delay interferometers  141 A and  141 B are controlled by control circuits  146 A and  146 B, respectively, so that an opening of an eye pattern in an output waveform of the respective mixers  145 A and  145 B becomes an optimum state. 
   As a technique associated with the optical reception apparatus corresponding to the reception unit shown in  FIG. 9 , for example, Japanese Unexamined Patent Publication No. 2005-80304 is known. In Japanese Unexamined Patent Publication No. 2005-80304, it is proposed, as one method for adjusting a relative delay in the delay interferometer, to monitor a bit error rate (BER) of the signal based on an interference signal generated by the delay interferometer, and adjust the relative delay based on the BER. 
   However, in the conventional technique for receiving the multivalue phase modulated optical signal such as the above RZ-DQPSK optical signal, there is a problem in that it becomes difficult to perform the error correction precisely by the framer LSI  122  in the subsequent stage, due to a burst error occurring in the reception unit in the RZ-DQPSK module  123 . 
   In other words, the optical signal received by the reception unit in the RZ-DQPSK module  123  is amplified by the optical amplifier  124  ( FIG. 7 ) for compensating a loss caused by the long distance transmission. Therefore, amplified spontaneous emission (ASE) occurring in the optical amplifier  124  is added as broadband optical noise. In the RZ-DQPSK signal added with the optical noise, for example, as shown in a conceptual diagram of  FIG. 10 , noise is carried on a light emission side corresponding to level “1”, and hence, the signal waveform largely collapses. 
   As in the configuration example shown in  FIG. 9 , when the mixers  145 A and  145 B are used to control the phase shift amount in the delay interferometers  141 A and  141 B, the control largely depends on the signal waveform, and there is an influence of manufacturing variations of the delay interferometers  141 A and  141 B. Therefore, the relative delay added to between the optical signals propagating through the arms A and B is not in an optimum state. Accordingly, for example, as shown in a conceptual diagram of  FIG. 11 , an error rate of the signal before error correction corresponding to the optical signal on the arm A side and an error rate of the signal before error correction corresponding to the optical signal on the arm B side are largely different from each other. The broken line in  FIG. 11  indicates an error rate (ideal value) when the relative delay is controlled in the optimum state, and the error rates on the arm A side and on the arm B side agree with each other. 
   Since the error rates on the arm A side and on the arm B side are different, a burst error in which frequent errors arise intermittently, occurs in the signal multiplexed in the multiplex circuit  143  ( FIG. 9 ). In a general error correction method it is difficult to handle such a burst error, and as a result, for example, as shown by the solid line in  FIG. 12 , error correction cannot be performed precisely by the framer LSI  122  in the subsequent stage, thereby causing degradation of reception performance. 
   SUMMARY 
   The present invention addresses the above points, with an object of providing an optical reception apparatus corresponding to the multivalue phase modulating format that can realize excellent reception performance by suppressing the occurrence of the burst error, and a controlling method thereof. 
   In order to achieve the above object, the optical reception apparatus of one aspect of an embodiment comprises: a branching unit that branches an input optical signal of a multivalue phase modulating format into two; a first route through which one of the optical signals branched by the branching unit propagates; a second route through which the other of the optical signals branched by the branching unit propagates; a first delay interferometer arranged on the first route; a second delay interferometer arranged on the second route; a first photoelectric converter that receives an optical signal output from the first delay interferometer and converts it into an electric signal; a second photoelectric converter that receives an optical signal output from the second delay interferometer and converts it into an electric signal; a multiplexer that multiplexes a signal output from the first photoelectric converter and a signal output from the second photoelectric converter; and an error correction unit that performs error correction processing for a signal output from the multiplexer, wherein the optical reception apparatus further comprises: an error-number detector that detects a number of generated errors in a signal propagating on the first route side and also detects a number of generated errors in a signal propagating on the second route side; an error-number comparator that obtains a difference in a number of generated errors on the first and second route sides detected by the error-number detector; and a controller that controls at least one of the first and second delay interferometers and the first and second photoelectric converters so that the difference in the number of generated errors obtained by the error-number comparator is within a preset tolerance. 
   In the optical reception apparatus having the above configuration, the number of generated errors in the signal propagating in the first route and the number of generated errors in the signal propagating in the second route are independently detected, and control of the devices on the first route and the second route is performed so that the difference in the number of generations of errors is within the tolerance. As a result, unbalance in the error occurrence state between the first route and the second route is reduced, and the occurrence of the burst error is suppressed. 
   Therefore according to the optical reception apparatus of one aspect of an embodiment, since error correction can be performed precisely by the error correction unit with respect to the signal multiplexed by the multiplexer, reception characteristics of an optical signal of a multivalue phase modulating format can be improved. By applying such an optical reception apparatus to construct the optical transmission system, a super high-speed optical signal, for example, a 40 Gbps optical signal, can be transmitted for a long distance, while realizing high noise tolerance. 
   Other objects, features, and advantages of the present invention will become apparent from the following description of the embodiments in conjunction with the appended drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a block diagram showing a configuration of an optical reception apparatus according to a first embodiment. 
       FIG. 2  is a flowchart for explaining an operation in the first embodiment. 
       FIG. 3  is a block diagram showing a configuration of an optical reception apparatus according to a second embodiment. 
       FIG. 4  is a flowchart ( 1 ) for explaining the operation in the second embodiment. 
       FIG. 5  is a flowchart ( 2 ) for explaining the operation in the second embodiment. 
       FIG. 6  is a diagram showing a configuration example of a known 40 Gbps optical transmission system. 
       FIG. 7  is a diagram showing a configuration example of an optical transmission device in  FIG. 6 . 
       FIG. 8  is a diagram showing a configuration example of a transmission unit in an RZ-DQPSK module in  FIG. 7 . 
       FIG. 9  is a diagram showing a configuration example of a reception unit in the RZ-DQPSK module in  FIG. 7 . 
       FIG. 10  is a conceptual diagram of an optical signal waveform input to the reception unit in  FIG. 9 . 
       FIG. 11  is a conceptual diagram of an error rate of an optical signal corresponding to each arm in the reception unit in  FIG. 9 . 
       FIG. 12  is a diagram showing deterioration in an error rate due to an occurrence of a burst error. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereunder is a description of a best mode for carrying out the present invention, with reference to the accompanying drawings. Throughout the figures, the same reference symbols denote the same or corresponding parts. 
     FIG. 1  is a block diagram showing a configuration of an optical reception apparatus according to a first embodiment. 
   In  FIG. 1 , the optical reception apparatus in the first embodiment includes for example: an optical amplifier  10  to which an optical signal of an RZ-DQPSK modulating format transmitted through an optical transmission line  100  is input, which amplifies the optical signal to a required level; an RZ-DQPSK reception unit  20  that receives and processes the RZ-DQPSK optical signal output from the optical amplifier  10 ; and an error correction unit  30  that performs error correction processing of a reception signal output from the RZ-DQPSK reception unit  20 . 
   The RZ-DQPSK reception unit  20  branches the RZ-DQPSK optical signal received via the optical amplifier  10  into two, and transmits one of the branched optical signals to an arm A serving as a first route, and the other optical signal to an arm B serving as a second route. Delay interferometers  21 A and  21 B are respectively provided on the arms A and B. The delay interferometer  21 A makes a 1-bit time delay component and a π/4 rad phase-controlled component interfere with each other (delay interference), and outputs the interference result as two outputs. Moreover the delay interferometer  21 B makes a 1-bit time delay component and a −π/4 rad phase-controlled component (the phase is shifted from that of the same component in the delay interferometer  21 A by π/2 rad) interfere with each other (delay interference), and outputs the interference result as two outputs. Here, the delay interferometers  21 A and  21 B are formed by a Mach-Zehnder interferometer, respectively, and respective Mach-Zehnder interferometers are formed so that one arm becomes longer than the other arm by a propagation length corresponding to 1-bit time, and an electrode for phase-shifting the optical signal propagating through the other arm is formed therein. A bias voltage for phase shift to be applied to the electrodes in respective delay interferometers  21 A and  21 B is controlled by control circuits  25 A and  25 B. 
   Output beams from the respective delay interferometers  21 A and  21 B are respectively input to photoelectric conversion circuits  22 A and  22 B serving as first and the second photoelectric converters. The respective photoelectric conversion circuits  22 A and  22 B have, for example, a pair of a photodiode and an amplifier, and receive the output beams from the corresponding delay interferometer  21 A or  21 B, to perform differential photoelectric conversion detection. The output signals from the photoelectric conversion circuits  22 A and  22 B are transmitted to a multiplex circuit  23 , and a part of the respective output signals is also transmitted to the error correction unit  30 . The multiplex circuit  23  multiplexes the output signals from the photoelectric conversion circuits  22 A and  22 B and outputs a multiplexed signal to a deserializer  24 . The deserializer  24  performs parallel signal processing of the output signal from the multiplex circuit  23 . 
   The error correction unit  30  includes for example; an error correction circuit  32 , to which the output signal from the deserializer  24  is input via an error-number detection circuit  31 , an error-number detection circuit  33 A to which an output signal from the photoelectric conversion circuit  22 A on the arm A side is input, an error-number detection circuit  33 B to which an output signal from the photoelectric conversion circuit  22 B on the arm B side is input, and an error-number comparison circuit  34  for comparing the number of generations of errors detected by the respective error-number detection circuits  33 A and  33 B and obtaining the difference therebetween. 
   The respective error-number detection circuits  31 ,  33 A, and  33 B are circuits that count the number of generated errors in the signals respectively input thereto, based on an error correction code attached to the received optical signal. The error correction circuit  32  is a known circuit for performing error correction processing with respect to the reception signal that has passed through the error-number detection circuit  31 . The error-number comparison circuit  34  obtains the difference in the number of generations of errors detected by the error-number detection circuits  33 A and  33 B corresponding to the respective arms A and B, generates information required for feed-back controlling the respective delay interferometers  21 A and  21 B in the RZ-DQPSK reception unit  20  so that the difference in the number of generations of errors decreases (ideally, the difference in the numbers of errors becomes zero), and transmits the generated information to the respective control circuits  25 A and  25 B. The control circuits  25 A and  25 B having received the information from the error-number comparison circuit  34  control, for example, the bias voltage applied to the electrode for phase shift, to thereby reduce the unbalance in the error occurrence state between the respective arms A and B. 
   Here the error-number comparison circuit  34  calculates the number of generated errors per unit time based on the number of generations of errors detected by the error-number detection circuits  33 A and  33 B, and calculates a difference in these numbers of generations of errors. 
   The function realized by the error-number detection circuit  31  and the error correction circuit  32  is the same as an FEC function of the aforementioned framer LSI used in the conventional beam transmission device shown in  FIG. 7 . On the other hand, the function realized by the error-number detection circuits  33 A and  33 B and the error-number comparison circuit  34  is not included in the conventional framer LSI with the FEC function, and is a function newly added in the first embodiment. The function can be incorporated by changing the design of the conventional framer LSI. 
   Next is a description of an operation in the first embodiment, with reference to the flowchart in  FIG. 2 . 
   In the optical reception apparatus having the above configuration, at first in an input-blocked reception state in which the optical signal is not transmitted from the optical transmission line  100  to the optical amplifier  10 , the delay interferometers  21 A and  21 B corresponding to the respective arms A and B in the RZ-DQPSK reception unit  20  are controlled to a standby state waiting for an input of the optical signal, by setting the respective bias voltages for phase shift to a center value of a variable range. 
   Then, the RZ-DQPSK optical signal that has propagated through the optical transmission line  100  is input to the optical amplifier  10 . When the optical signal amplified by the optical amplifier  10  (including ASE noise) is branched into two by the RZ-DQPSK reception unit  20  and input to the delay interferometers  21 A and  21 B on the respective arms A and B, then in step  11  in  FIG. 2  (shown by S 11  in the figure, and similarly hereunder), the bias voltage for phase shift is swept in the variable range with respect to the delay interferometer  21 A on the arm A side, to detect the number of generated errors in the signal on the arm A side at a plurality of preset phase points by the error-number detection circuit  33 A, and the detection result is recorded in the error-number comparison circuit  34 . Then in step  12 , it is determined whether detection of the number of generated errors has been completed for all the phase points on the arm A side. When completion is confirmed, control proceeds to steps  13 A and  13 B. 
   At this time, detection of the number of generated errors at a certain phase point is complete, for example, with the lapse of predetermined time determined beforehand, and the bias voltage for phase shift is controlled so as to proceed to the next phase point. 
   In step  13 A, the bias voltage for phase shift on the arm A side is set to the center value to return the delay interferometer  21 A to the standby state. Then in step  13 B, the bias voltage for phase shift is swept in the variable range with respect to the delay interferometer  21 B on the arm B side, to detect the number of generated errors in the signal on the arm B side at a plurality of preset phase points by the error-number detection circuit  33 B, and the detection result is recorded in the error-number comparison circuit  34 . Then in step  14 , it is determined whether detection of the number of generated errors has been completed for all the phase points on the arm B side. When completion is confirmed, control proceeds to steps  15 A and  15 B. 
   In steps  15 A and  15 B, in the error-number comparison circuit  34 , the respective numbers of generations of errors on the arm A side and the arm B side detected in steps  11  and  14 , are compared to determine phase points corresponding to the respective arms A and B, at which the difference in the number of generated errors between the arms A and B is within a preset tolerance α. Information of the phase points is transmitted to the respective control circuits  25 A and  25 B. 
   Here, there could be a plurality of pairs of phase points corresponding to the respective arms A and B where the difference in the number of generated errors is within α. In such a case, for example, a pair of phase points having the smallest difference in the number of generated errors can be designated as a determination result, or a pair of phase points having the smallest difference in the number of generated errors at a phase point corresponding to the arm A can be designated as the determination result. 
   Next, in the respective control circuits  25 A and  25 B, the bias voltage for phase shift to be applied to the respective delay interferometers  21 A and  21 B is adjusted in accordance with the information from the error-number comparison circuit  34 . Then the number of generated errors in the signals on the respective arm A and B sides at the phase points after adjustment is respectively detected by the error-number detection circuits  33 A and  33 B, and respective detection results are transmitted to the error-number comparison circuit  34 . 
   In step  16 , in the error-number comparison circuit  34 , the respective numbers of generations of errors on the arm A side and the arm B side, detected in steps  15 A and  15 B, are compared to determine whether the difference in the number of generated errors between the arms A and B is within the tolerance α. When the difference is within the tolerance α, control of the delay interferometers  21 A and  21 B finishes. On the other hand, if the difference in the number of generated errors becomes larger than the tolerance a due to an influence of thermal interference or the like between the arms A and B, control proceeds to step  17 . 
   In step  17 , the bias voltage for phase shift on the arm A side or the arm B side is finely adjusted, and the number of generated errors corresponding to each arm A and B after the fine adjustment is detected again by the error-number detection circuits  33 A and  33 B. Then in step  18 , the respective numbers of generations of errors on the arms A and B detected in step  17  are compared in the error-number comparison circuit  34 , and the fine adjustment in step  17  is repeated until the difference in the number of generated errors between the arms A and B becomes within the tolerance α. 
   The number of generated errors in the signal corresponding to each arm A and B is individually detected in the above manner, and the phase shift amount in the delay interferometers  21 A and  21 B is feed-back controlled so that the difference in the number of generated errors between the arms A and B becomes within the tolerance, based on the respective detection results. By so doing, even if the broadband ASE noise is added to the optical signal by the optical amplifier  10  in an input stage, occurrence of the burst error can be effectively suppressed. Accordingly, the reception signal output from the photoelectric conversion circuits  22 A and  22 B corresponding to the respective arms A and B, then multiplexed by the multiplex circuit  23 , and further subjected to the parallel signal processing by the deserializer  24  can be corrected by the error correction circuit  32  precisely, thereby enabling improvement in the reception characteristics of the RZ-DQPSK optical signal. If the configuration of such an optical reception apparatus is applied to the reception unit of the RZ-DQPSK module and the FEC function of the framer LSI shown in  FIG. 7 , an optical transmission system corresponding to the next generation 40 Gbps, capable of long-distance transmission, and having high noise tolerance can be realized. 
   Next is a description of a second embodiment. 
     FIG. 3  is a block diagram showing a configuration of an optical reception apparatus according to the second embodiment. 
   In  FIG. 3 , the optical reception apparatus according to this embodiment is an application example for simplifying the configuration of the error correction unit  30  in the configuration of the first embodiment shown in  FIG. 1 , by having a configuration where optical switches (SW)  26 A and  26 B are respectively arranged on the arms A and B of the RZ-DQPSK reception unit  20  as a signal blocking unit, so that the optical signal output from the optical amplifier  10  can be selectively input to either one of the delay interferometers  21 A and  21 B. Here, the error correction unit  30  is formed from the error-number detection circuit  31  and the error correction circuit  32 , and a reception controller  40  is separately provided as a configuration for performing the feed-back control of the delay interferometers  21 A and  21 B based on the error-number detected by the error-number detection circuit  31 . The function of the error correction unit  30  is the same as the FEC function of the framer LSI used in the conventional beam transmission device shown in  FIG. 7 . 
   The reception controller  40  has, for example, an error-number comparison circuit  41  and a control circuit  42 . The error-number comparison circuit  41  has the same function as that of the error-number comparison circuit  34  in the first embodiment. The control circuit  42  controls ON/OFF of the optical switches  26 A and  26 B in the RZ-DQPSK reception unit  20  according to a signal output from the error-number comparison circuit  41 , and controls the bias voltage for phase shift applied to the delay interferometers  21 A and  21 B. 
   Next is a description of an operation of the second embodiment, with reference to the flowchart in  FIG. 4  and  FIG. 5 . 
   In the optical reception apparatus having the above configuration, at first in the input-blocked reception state in which the optical signal is not transmitted from the optical transmission line  100  to the optical amplifier  10 , the delay interferometers  21 A and  21 B corresponding to the respective arms A and B in the RZ-DQPSK reception unit  20  are controlled to the standby state waiting for an input of the optical signal, by setting the respective bias voltages for phase shift to a center value of the variable range, and here turning on the optical switch  26 A on the arm A side and turning off the optical switch  26 B on the arm B side. 
   Then, the RZ-DQPSK optical signal having propagated through the optical transmission line  100  is input to the optical amplifier  10 . The optical signal amplified by the optical amplifier  10  (including ASE noise) is branched into two by the RZ-DQPSK reception unit  20 , and while the optical signal transmitted to the arm A side passes through the optical switch  26 A in the on state and is input to the delay interferometer  21 A, the optical signal transmitted to the arm B side is blocked by the optical switch  26 B in the off state. Then, in step  21  in  FIG. 4 , the bias voltage for phase shift is swept in the variable range by the control circuit  42  with respect to the delay interferometer  21 A on the arm A side, to detect the number of generated errors in the signal on the arm A side at a plurality of preset phase points by the error-number detection circuit  31 , and the detection result is recorded in the error-number comparison circuit  41 . Then in step  22 , it is determined whether detection of the number of generated errors has been completed for all the phase points on the arm A side. When completion is confirmed, control proceeds to step  23 , where the control circuit  42  sets the bias voltage for phase shift on the arm A side to a center value. Then in step  24 A, the optical switch  26 A on the arm A side is switched from ON to OFF, and in step  24 B, the optical switch  26 B on the arm B side is switched from OFF to ON. 
   In step  25 , the bias voltage for phase shift is swept in the variable range with respect to the delay interferometer  21 B on the arm B side, to detect the number of generated errors in the signal on the arm B side at a plurality of preset phase points by the error-number detection circuit  31 , and the detection result is recorded in the error-number comparison circuit  41 . Then in step  26 , it is determined whether detection of the number of generated errors has been completed for all the phase points on the arm B side. When completion is confirmed, control proceeds to steps  27 A and  27 B. 
   In steps  27 A and  27 B, in the error-number comparison circuit  41 , the respective numbers of generations of errors on the arm A side and the arm B side detected in steps  21  and  25  are compared, to determine phase points corresponding to the respective arms A and B, at which the difference in the number of generated errors between the arms A and B is within the preset tolerance α. Information of the phase points is transmitted to the control circuit  42 . In the control circuit  42 , the bias voltage for phase shift to be applied to the respective delay interferometers  21 A and  21 B is adjusted in accordance with the information from the error-number comparison circuit  41 . 
   In step  28  in  FIG. 5 , the number of generated errors in the signal on the arm B side at the phase points after adjustment is detected by the error-number detection circuit  31 , and the detection result is transmitted to the error-number comparison circuit  41 . Then in step  29 B, the optical switch  26 B on the arm B side is switched from ON to OFF, and in step  29 A, the optical switch  26 A on the arm A side is switched from OFF to ON. In step  30 , the number of generated errors in the signal on the arm A side at the phase points after adjustment is detected by the error-number detection circuit  31 , and the detection result is transmitted to the error-number comparison circuit  41 . 
   In step  31 , in the error-number comparison circuit  41 , the respective numbers of generations of errors on the arm A side and the arm B side, detected in steps  28  and  30 , are compared to determine whether the difference in the number of generated errors between the arms A and B is within the tolerance α. When the difference is within the tolerance α, control proceeds to step  34 . On the other hand, if the difference in the number of generated errors becomes larger than the tolerance a due to an influence of thermal interference or the like between the arms A and B, control proceeds to step  32 . 
   In step  32 , the optical switch  26 A on the arm A side or the optical switch  26 B on the arm B side is turned on by the control circuit  42 , to perform fine adjustment of the bias voltage for phase shift, and the number of generated errors corresponding to each arm A and B after the fine adjustment is detected again by the error-number detection circuit  31 . Then in step  33 , the respective numbers of generations of errors on the arms A and B detected in step  32  are compared in the error-number comparison circuit  41 , and the fine adjustment in step  32  is repeated until the difference in the number of generated errors between the arms A and B becomes within the tolerance α. When the difference in the number of generated errors is within the tolerance α, then in step  34 , the optical switches  26 A and  26 B on the respective arms A and B are turned on by the control circuit  42 , to finish the control of the delay interferometers  21 A and  21 B. 
   By selectively switching the ON/OFF state of the optical switches  26 A and  26 B provided on the respective arms A and B in the above manner, the number of generated errors in the signal corresponding to the respective arms A and B is detected in a time-shared manner in the common error-number detection circuit  31 , and even though the phase shift amount in the respective delay interferometers  21 A and  21 B is feed-back controlled based on the detection result, so that the difference in the number of generated errors between the arms A and B is within the tolerance, the same effect as for the aforementioned case of the first embodiment can be obtained. Moreover, the function of the error correction unit  30  in this embodiment is the same as the FEC function of the conventional framer LSI, and the existing LSI can be directly used. Therefore a lower cost optical reception apparatus can be realized. 
   In the second embodiment, an example is shown in which the optical switches  26 A and  26 B are provided, respectively, on the arms A and B, so that the optical signal is selectively input to either one of the delay interferometers  21 A and  21 B. However, the present invention is not limited thereto, and for example, a signal corresponding to one of the arms A and B may be guided to the error-number detection circuit  31  by selectively driving the photoelectric conversion circuits  22 A and  22 B by the control circuit  42 . Moreover, a configuration example is shown in which the reception controller  40  is provided separately from the RZ-DQPSK reception unit  20  and the error correction unit  30 . However, as in the first embodiment, the control circuit may be provided in the RZ-DQPSK reception unit  20 , and the error-number detection circuit  31  may be provided in the error correction unit. 
   Furthermore in the first and the second embodiments, an optical reception apparatus corresponding to the RZ-DQPSK modulation method has been described. However, the present invention is not limited thereto, and the configuration of the present invention is effective for the multivalue phase modulating format in which the received optical signal is branched into a plurality of arms to perform a demodulation process, regardless of whether intensity modulation (RZ-pulsing) is performed. In addition, in the embodiments, the bias voltage for phase shift applied to the respective delay interferometers  21 A and  21 B is feed-back controlled according to the difference in the number of generated errors. However, for example, even if an output signal level of the respective photoelectric conversion circuits  22 A and  22 B is feed-back controlled according to the difference in the number of generated errors, it is possible to reduce the unbalance in the error occurrence state between the respective arms A and B.