Patent Publication Number: US-7903982-B2

Title: Optical receiver

Description:
TECHNICAL FIELD 
     The present invention relates to an optical receiver, and, more particularly to an optical receiver technology applied to very-high-speed/very-long-distance optical transmission systems and the like. 
     BACKGROUND ART 
     In the very-high-speed/very-long-distance optical transmission systems by an Intensity Modulation/Direct Detection (IM/DD) method, an error correction code has been mainly used as means for improvement of reception sensitivity of the optical receiver. However, in the recent rapid increase in a transmission volume and a transmission distance, application of a technology for further improvement of reception sensitivity of the optical receiver is indispensable. There is a demand for a simple technology for improving reception sensitivity other than the error correction code. 
     Conventionally, as the technology for improving reception sensitivity of this type, there is an example disclosed in a patent document 1 described below. 
     In the patent document 1, improvement of reception sensitivity is attained by providing k sets (n×k) discriminators that have different thresholds from one another with n discriminators having an identical threshold as one set, an optical distributor that distributes a reception signal to the respective discriminators, and a selector that selects one of discrimination results of the respective discriminators and outputs the discrimination result, performing majority judgment using the selector, and selecting a discrimination result with higher accuracy from the discrimination results of the respective discriminators. 
     In a nonpatent literature 1 described below, for example, as an application of the reception technology described in the patent document 1, improvement of sensitivity of a receiver is realized by subjecting one optical signal to wavelength division at a reception stage, providing discriminators for the respective optical signals divided, and adopting more probable data from discrimination results in the respective discriminators. 
     Patent Document 1 
     Japanese Patent Application Laid-Open No. 2003-234699 
     Nonpatent Literature 1 
     Masahito Tomizawa, Yoshiaki Kisaka, Akira Hirano, and Yutaka Miyamoto, “Error correction without additional redundancy by novel optical receiver with diverse detection,” in OSA Trends in Optics and Photonics (TOPS) vol. 70, Optical Fiber Communication Conference, Technical Digest, Post-conference Edition (Optical Society of America, Washington D.C., 2002), WX7, pp. 368-370. 
     In the realization of the very-high-speed/very-long-distance optical transmission systems, improvement of sensitivity of the optical receiver is an important element and a technology that is simple and has an easy configuration is indispensable. 
     However, the invention described in the nonpatent document 1 uses a method of dividing a spectrum of an optical signal using an optical filter. Thus, the invention has a problem in that a division penalty in the optical filter tends to occur and sufficient improvement of sensitivity cannot be expected. 
     Since the invention also uses the optical filter in implementation, there is also a problem in that limitation on a configuration occurs in terms of dependency on a signal modulation system, stability with respect to temperature, and the like. 
     Moreover, even if improvement of sensitivity of the optical receiver is realized by a minimum number of discriminators, it is necessary to control a discrimination threshold by preparing two sets of two discriminators (four discriminators in total). Thus, the method is not a simple and easy method from the viewpoint of control of the discriminators or implementation. 
     In view of such circumstances, it is an object of the present invention to provide an optical receiver that prevents occurrence of a division penalty due to an optical divider, eliminates dependency on a modulation system, and secures easiness of implementation. 
     DISCLOSURE OF INVENTION 
     An optical receiver according to one aspect of the present invention includes an optical divider that divides an optical input signal into a plurality of paths; an optical-to-electrical converter that converts the divided optical input signal into an electrical signal; a discriminator that outputs a discrimination result obtained by discriminating the electrical signal output from the optical-to-electrical converter based on a predetermined threshold; and an operational circuit that performs a predetermined operation based on the discrimination result output from the discriminator. 
     According to the present invention, the optical divider divides an optical input signal into a plurality of paths, the OE converters convert the optical input signals after division into electrical signals, the discriminators output discrimination results obtained by discriminating, based on predetermined thresholds, the electrical signals output from the OE converters, and the operational circuit performs a predetermined operation based on the discrimination results output from the discriminators. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of a structure of an optical receiver according to a first embodiment of the present invention; 
         FIG. 2A  is a diagram for explaining a flow of a discrimination result of the optical receiver shown in  FIG. 1 ; 
         FIG. 2B  is a diagram of a truth table of an OR circuit; 
         FIG. 2C  is a diagram of a truth table of an AND circuit; 
         FIG. 3  is a graph of a bit error rate characteristic with respect to a change in a threshold; 
         FIG. 4  is a block diagram of a structure of an optical receiver according to a second embodiment of the present invention; 
         FIG. 5  is a block diagram of a structure of an optical receiver according to a third embodiment of the present invention; 
         FIG. 6  is a block diagram of a structure of an optical receiver according to a fourth embodiment of the present invention; and 
         FIG. 7  is a block diagram of a structure of an optical receiver according to a fifth embodiment of the present invention. 
     
    
    
     BEST MODE(S) FOR CARRYING OUT THE INVENTION 
     Exemplary embodiments of an optical receiver according to the present invention are explained in detail below with reference to the accompanying drawings. The present invention is not limited by the embodiments. 
     First Embodiment 
       FIG. 1  is a block diagram of a structure of an optical receiver according to a first embodiment of the present invention. The optical receiver shown in the figure includes an optical amplifier  10  that amplifies an optical input signal, an optical divider  11  that divides the optical input signal into two path, that is, a path  1  and a path  2 , optical-to electrical (OE) converters  12   a  and  12   b  that convert the optical input signals after division into electrical signals, discriminators  13   a  and  13   b  that output discrimination results (digital data signals) obtained by discriminating, based on predetermined thresholds V th1  and V th2 , the analog electrical signals output from the OE converters  12   a  and  12   b , and an operational circuit  14  that performs a logical operation based on digital signals on the respective paths. The operational circuit  14  can be constituted by, for example, an OR circuit or an AND circuit. 
     Operations of the optical receiver shown in  FIG. 1  are explained. In the figure, an optical signal transmitted from a not-shown optical transmitter or the like is input to this optical receiver. The optical input signal received is amplified by the optical amplifier  10  and, then, divided into the two paths, that is, the path  1  and the path  2 , by the optical divider  11 . The optical input signals divided into two paths are converted into electrical signals by the OE converters  12   a  and  12   b  and output to the discriminators  13   a  and  13   b . The discriminators  13   a  and  13   b  output discrimination results (digital data signals) obtained by discriminating the analog electrical signals based on the predetermined thresholds V th1  and V th2  to the operational circuit  14 . The operational circuit  14  outputs a processing result obtained by performing predetermined processing. 
     An output (a discrimination result) of the optical receiver at the time when the optical signals divided into the two paths have intersymbol interferences of the same degree and Gaussian noises at output levels of the same degree, respectively, is examined below. 
     In this examination, it is assumed that characteristics, fluctuation in discrimination sensitivity, and the like of the discriminators  13   a  and  13   b  can be neglected. It is assumed that, as to probability that a discrimination result concerning an arbitrary analog electrical signal is wrong, the Gaussian noises included in the optical signals on the respective paths are uncorrelated to each other such that mutual independency between the discriminators  13   a  and  13   b  is maintained. 
     First, an output of the operational circuit  14  constituted by an OR circuit is examined. 
     Under these conditions, an error occurs in an output result of the operational circuit  14  in the following cases. 
     (a) In the case where logical “0” is input to an optical signal
         When one of discrimination results of the discriminators  13   a  and  13   b  is wrong   When both discrimination results of the discriminators  13   a  and  13   b  are wrong       

     (b) In the case where logical “1” is input to an optical signal
         When both discrimination results of the discriminators  13   a  and  13   b  are wrong       

     An error rate BER OR  of an output result at the time when the operational circuit  14  is an OR circuit is considered using  FIGS. 2A and 2B .  FIG. 2A  is a diagram for explaining a flow of a discrimination result of the optical receiver shown in  FIG. 1 .  FIG. 2B  is a diagram of a truth table of the OR circuit. 
     In  FIG. 2A , P 1 (1/0) shown at an output terminal of the discriminator  13   a  is probability that a discrimination result is misjudged as logical “1” when logical “0” is input to the discriminator  13   a . P 1 (0/1) is probability that a discrimination result is misjudged as logical “0” when logical “1” is input to the discriminator  13   a.    
     Similarly, P 2 (1/0) shown at an output terminal of the discriminator  13   b  is probability that a discrimination result is misjudged as logical “1” when logical “0” is input to the discriminator  13   b . P 2 (0/1) is probability that a discrimination result is misjudged as logical “0” when logical “1” is input to the discriminator  13   b.    
     In  FIG. 2A , P out (1/0) shown at an output terminal of the operational circuit  14  is probability that a discrimination result among discrimination results processed by the operational circuit  14  based on the discrimination results of the discriminators  13   a  and  13   b  is misjudged as logical “1” when logical “0” is input to an input terminal of the optical receiver. Similarly, P out (0/1) is probability that the operational circuit  14  misjudges a discrimination result as logical “1” when logical “0” is input to the input terminal of the optical receiver. 
     Therefore, an error rate BER OR  at the time when the operational circuit  14  is an OR circuit can be represented by 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     In Equation (1), it is assumed that a mark rate, which is an occupied time ratio of the input signals “ 0 ” and “ 1 ” in a predetermined transmission time, is 50% (½). 
     Moreover, for easiness of examination, it is assumed  13   b  are equal. In this case, it is possible to set V th1 , V th2 , and V th  as V th1 =V th2 =V th . The following relations are established among P 1 (0/1), P 1 (1/0), P 2 (0/1), and P 2 (1/0)
 
 P   1 (1/0)= P   2 (1/0)= P (1/0)
 
 P   1 (0/1)= P   2 (0/1)= P (0/1)
 
Thus, it is possible to simplify Equation (1) as
 
     
       
         
           
             
               
                 
                   
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     As it is seen from Equation (2), when the OR circuit is used, if discrimination thresholds are adjusted to reduce P(1/0), P(0/1) is a main factor of an error. However, as indicated by Equation (2), since P(0/1) has effect on BER OR  only in the order of a square, BER OR ˜P(0/1) 2 ˜BER RAW   2 . BER RAW  is an error rate at the time when usual optical reception is performed using a single discriminator. 
     An output of the operational circuit  14  constituted by an AND circuit is examined.  FIG. 2C  is a diagram of a truth table of the AND circuit. 
     Under conditions identical with those for the operational circuit  14  constituted by the OR circuit, an error occurs in an output result of the operational circuit  14  constituted by the AND circuit in the following cases. 
     (a) In the case where logical “0” is input to an optical signal
         When both discrimination results of the discriminators  13   a  and  13   b  are wrong       

     (b) In the case where logical “1” is input to an optical signal
         When one of discrimination results of the discriminators  13   a  and  13   b  is wrong   When both discrimination results of the discriminators  13   a  and  13   b  are wrong       

     In these cases, if an approach same as that for deriving Equation (1) when the operational circuit  14  is the OR circuit is performed, an error rate BER AND  corresponding to Equation (2) at the time when the operational circuit  14  is the AND circuit can be represented by 
     
       
         
           
             
               
                 
                   
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     A condition of a mark rate and conditions of a threshold level and the like are identical with those in the case of the OR circuit. 
       FIG. 3  is a graph of a bit error rate characteristic with respect to a change in a threshold. More specifically,  FIG. 3  is a graph of a result obtained by calculating a bit error rate of a discrimination output when a signal amplitude of electrical signals input to the discriminators  13   a  and  13   b  is set to “1” and standard deviations (equivalent to noise components) σ 0  and σ 1  of logical “0” and logical “1” are set as σ 0 =σ 1 =0.1. In the figure, the threshold is on the abscissa and the bit error rate is on the ordinate. 
     In the figure, when usual optical reception is performed using a single discriminator, the bit error rate takes a minimum value when the threshold is a standard value (0.5). On the other hand, when the OR circuit is used as the operational circuit  14 , the bit error rate takes a minimum value when the threshold is a value slightly higher than the standard value (in the figure, around 0.6). Moreover, the minimum value is reduced in the order of 10 −2  compared with the time when the single discriminator is used. Thus, it is evident that sensitivity of discrimination is improved. 
     This sensitivity improvement effect is attained by setting the threshold of discrimination slightly higher than the standard value, or reducing P(1/0), which is the probability that logical “0” is misjudged as logical “1”. 
     Similarly, when the AND circuit is used as the operational circuit  14 , the bit error rate takes a minimum value when the threshold is slightly higher than the standard value (in the figure, around 0.4). The minimum value is reduced in the order of 10 −2  compared with the time when the single discriminator is used. Sensitivity of discrimination is improved. 
     The sensitivity improvement effect in the case of the AND circuit is opposite to that in the case of the OR circuit. The sensitivity improvement effect is attained by setting the threshold of discrimination slightly lower than the standard value, or reducing P(0/1), which is the probability that logical “1” is misjudged as logical “0”. 
     The characteristic shown in  FIG. 3  indicates an example of a calculation result. For example, even when a noise distribution is different from the calculation result in  FIG. 3  or even when a noise distribution is an asymmetric distribution (σ 0 ≠σ 1 ), by selecting predetermined values as V th1  and V th2 , it is possible to improve reception sensitivity compared with that at the time when usual optical reception is performed using the single discriminator. 
     As explained above, according to the present embodiment, an optical input signal is divided into a plurality of paths, the optical input signals after division are converted into electrical signals, a predetermined operation is performed by the OR circuit, the AND circuit, or the like based on discrimination results obtained by discriminating, based on predetermined thresholds, the electrical signals output from the OE converters. Thus, it is possible to improve reception sensitivity of the optical receiver. 
     According to the present embodiment, an optical input signal is divided into two paths by the optical divider. However, the number of paths is not limited to two. The optical input signal may be divided into three or more arbitrary paths. 
     Second Embodiment 
       FIG. 4  is a block diagram of a structure of an optical receiver according to a second embodiment of the present invention. The optical receiver shown in the figure includes optical amplifiers  40   a  and  40   b  for compensating for a fall in an optical level between the optical divider  11  and the OE converters  12   a  and  12   b , respectively, in addition to the components of the optical receiver shown in  FIG. 1 . The other components are identical with or equivalent to those in the first embodiment. Thus, the components are denoted by the identical reference numerals and signs. 
     Operations of the optical receiver shown in  FIG. 4  are explained below. In the figure, an optical signal transmitted from a not-shown optical transmitter or the like is input to the optical receiver. The optical input signal received is amplified by the optical amplifier  10  and, then, divided into the two paths, that is, the path  1  and the path  2 , by the optical divider  11 . The optical signals divided into the two paths are amplified by the optical amplifiers  40   a  and  40   b , respectively. In this case, since optical power is sufficiently amplified by the optical amplifier  10 , if an optical loss due to the optical divider  11  is small, deterioration in an optical signal to noise ratio (SNR) due to the optical amplifiers  40   a  and  40   b  with the operations in the first embodiment, explanation of the operations are omitted. 
     Since noises imparted by the optical amplifiers  40   a  and  40   b  are uncorrelated with each other, the optical signals divided into the two paths form an uncorrelated noise distribution. 
     Therefore, even when the optical amplifiers  40   a  and  40   b  for compensating for a fall in a level of the optical signals divided into the two paths are present between the optical divider  11  and the OE converters  12   a  and  12   b , it is possible to improve reception sensitivity of the optical receiver as in the case of the first embodiment using the OE converters  12   a  and  12   b , the discriminators  13   a  and  13   b , and the operational circuit  14 . 
     As explained above, according to the present embodiment, even if the optical receiver includes optical amplifiers for compensating for a fall in a level of optical signals divided into two paths, it is possible to improve reception sensitivity of the optical receiver without deteriorating an optical SNR. 
     As the optical amplifiers  10 ,  40   a , and  40   b , any optical amplifiers such as a rare-earth-doped fiber amplifier and a semiconductor optical amplifier (SOA) may be used. 
     According to the present embodiment, an optical input signal is divided into two paths by the optical divider. However, as in the first embodiment, the number of paths is not limited to two and the optical input signal may be divided into three or more arbitrary paths. 
     Third Embodiment 
       FIG. 5  is a block diagram of a structure of an optical receiver according to a third embodiment of the present includes an operational circuit  50  having a function different from that of the operational circuit  14  in the structure of the optical receiver shown in  FIG. 1 . The optical receiver also includes a bit-error-rate (BER) monitoring unit  51  that monitors an output of the operational circuit  50  and a discrimination-threshold control circuit  52  that controls discrimination thresholds of the discriminators  13   a  and  13   b  based on an output of the BER monitoring unit  51 . The other components are identical with or equivalent to those in the first embodiment. Thus, the components are denoted by the identical reference numerals and signs. 
     Operations of the optical receiver shown in  FIG. 5  are explained below. It is assumed that conditions such as characteristics and fluctuation in discrimination sensitivity of the discriminators  13   a  and  13   b , uncorrelation of Gaussian noises included in optical signals on respective paths, and mutual independency on discrimination results between the discriminators  13   a  and  13   b  are identical with those in the first embodiment. 
     In  FIG. 5 , an optical signal transmitted from a not-shown optical transmitter or the like is input to the optical receiver. The optical input signal received is amplified by the optical amplifier  10  and, then, divided into the two paths, that is, the path  1  and the path  2 , by the optical divider  11 . The optical input signals divided into the two paths are converted into electrical signals by the OE converters  12   a  and  12   b  and output to the discriminators  13   a  and  13   b . The discriminators  13   a  and  13   b  output discrimination results (digital data signals) obtained by discriminating the analog electrical signals based on the predetermined thresholds V th1  and V th2  to the operational circuit  50 . The operational circuit  50  outputs an operation result obtained by performing a predetermined operation based on the discrimination results of the discriminators  13   a  and  13   b . On the other hand, the discrimination-threshold control circuit  52  monitors a bit error rate based on the output of the operational circuit  50  and outputs a monitoring result to the BER monitoring unit  51 . The discrimination-threshold control circuit  52  controls discrimination thresholds of the discriminators  13   a  and  13   b  to reduce the bit error rate. 
     In the first and the second embodiments, thresholds imparted to the discriminators  13   a  and  13   b  are determined according to whether the operational circuit  14  is the OR circuit or the AND circuit as described above. On the other hand, according to the present embodiment, thresholds imparted to the discriminators  13   a  and  13   b  are determined based on a bit error rate. In other words, thresholds imparted to the discriminators  13   a  and  13   b  are determined first and the operational circuit  50  applies an optimum operational processing based on the thresholds determined. 
     As a simple example, when the operational circuit  50  has a function of an OR circuit and a function of an And circuit, if the thresholds imparted to the discriminators  13   a  and  13   b  are set smaller than a standard value, a function of the operational circuit  50  only has to be switched to the function of the AND circuit. On the other hand, if the thresholds imparted to the discriminators  13   a  and  13   b  are set larger than the standard value, a function of the operational circuit  50  only has to be switched to the function of the OR circuit. 
     Besides the method described above, for example, a method of sweeping discrimination thresholds by several points to determine a point at which an error rate is minimized may be adopted. Alternatively, discrimination thresholds may be swept in a try and error manner. Thresholds imparted to the discriminators  13   a  and  13   b  do not always have to be set equal. 
     These methods are particularly effective when a noise distribution of a signal is not Gaussian noise, when the noise distribution is asymmetric noise, or when a discrimination sensitivity difference between discriminators is large. 
     As explained above, according to the present embodiment, a function of the operational circuit is switched to, for example, the OR circuit or the AND circuit. Thus, even when an input condition of an optical signal changes, it is possible to maintain predetermined reception sensitivity. 
     According to the present embodiment, an optical input signal is divided into two paths by the optical divider. However, as in the other embodiments, the number of paths is not limited to two. The optical input signal may be divided into three or more arbitrary paths. 
     Fourth Embodiment 
       FIG. 6  is a block diagram of a structure of an optical receiver according to a fourth embodiment of the present invention. The optical receiver shown in the figure includes an optical polarization divider  61  instead of the optical divider  11  in the structure of the optical receiver shown in  FIG. 1 . The optical receiver also includes, between the optical polarization divider  61  and the optical amplifier  10 , an optical polarization controller  60  that controls a plane of polarization of an optical signal. Moreover, the optical receiver includes power monitoring units  63   a  and  63   b  that monitor optical power of an optical signal via optical couplers  62   a  and  62   b  provided in respective paths between the optical polarization divider  61  and the OE converters  12   a  and  12   b . The other components are identical with or equivalent to those in the first embodiment. Thus, the components are denoted by the identical reference numerals and signs. 
     Operations of an optical receiver shown in  FIG. 6  are explained below. In the figure, an optical signal transmitted from a not-shown optical transmitter is input to the optical receiver. The optical input signal received is amplified by the optical amplifier  10  and, then, subjected to polarization control by the optical polarization controller  60 , and divided into the two paths, that is, the path  1  and the path  2 , by the optical polarization divider  61 . 
     The optical signals divided into the path  1  and the path  2  are tapped by the optical couplers  62   a  and  62   b . Optical power P 1  of the path  1  and optical power P 2  of the path  2  are monitored by the power monitoring units  63   a  and  63   b . A control circuit  64  controls the optical polarization controller  60  based on information from the power monitoring such that P 1  is always equal to P 2 . 
     With the structure described above, it is possible to stably receive the optical signals on the two paths. 
     ASE (Amplified Spontaneous Emission) noise components included in the two paths subjected to polarization division are in a polarization state in which the ASE noise components are orthogonal to each other. The ASE noise components are amplified independently from each other when the ASE noise components are optically amplified. Thus, it is experimentally indicated that the ASE noise components forms an uncorrelated noise distribution. 
     Therefore, even when the optical polarization controller  60 , the optical polarization divider  61 , and the optical couplers  62   a  and  62   b  are present on paths of an optical signal, as in the first embodiment, there is an effect that reception sensitivity is improved by the OE converters  12   a  and  12   b , the discriminators  13   a  and  13   b , and the operational circuit  14 . 
     As explained above, according to the present embodiment, the optical polarization controller is controlled such that respective output values of optical signal powers on a plurality of paths monitored by optical monitors are substantially equal. This makes it possible to stably receive optical signals on the paths. 
     According to the present embodiment, an optical input signal is divided into two paths by the optical divider. However, as in the other embodiments, the number of paths is not limited to two. The optical input signal may be divided into three or more arbitrary paths. 
     Fifth Embodiment 
       FIG. 7  is a block diagram of a structure of an optical receiver according to a fifth embodiment of the present invention. The optical receiver shown in the figure includes soft decision discriminators  70   a  and  70   b  instead of the discriminators  13   a  and  13   b  in the structure of the optical receiver shown in  FIG. 5 . The optical receiver also includes a plurality of operational circuits  71 - 1 ,  71 - 2 , . . . ,  71 - n  in association with a plurality of threshold levels and a BER monitoring unit  72  that can monitor bit error rates of a plurality of operational circuit outputs. Moreover, the optical receiver includes a path selecting unit  73  that selects an optimum discrimination result out of output results of the operational circuits  71 - 1 ,  71 - 2 , . . . ,  71 - n . The other components are identical with or equivalent to those in the third embodiment. The components are denoted by the identical reference numerals and signs. 
     Operations of the optical receiver shown in  FIG. 7  are explained below. It is assumed that conditions such as characteristics and fluctuation in discrimination sensitivity of the discriminators  13   a  and  13   b , uncorrelation of Gaussian noises included in optical signals of respective paths, and mutual independency on discrimination results between the discriminators  13   a  and  13   b  are identical with those in the third embodiment. 
     In  FIG. 7 , an optical signal transmitted from a not-shown optical transmitter or the like is input to the optical receiver. The optical input signal received is amplified by the optical amplifier  10  and, then, divided into the two paths, that is, the path  1  and the path  2 , by the optical divider  11 . The optical input signals divided into the two paths are converted into electrical signals by the OE converters  12   a  and  12   b  and output to the soft decision discriminators  70   a  and  70   b . The soft decision discriminators  70   a  and  70   b  output discrimination results (digital data signals) obtained by discriminating the analog electrical signals based on predetermined thresholds V th1 , V th2 , . . . , V thn  to the operational circuits  71 - 1 ,  71 - 2 , . . . ,  71 - n . Each of the operational circuits  71 - 1 ,  71 - 2 , . . . ,  71 - n  outputs an operation result obtained by performing a predetermined operation based on the discrimination results of the soft decision discriminators  70   a  and  70   b.    
     On the other hand, the BER monitoring unit  72  monitors a bit error rate based on the respective outputs of the operational circuits  71 - 1 ,  71 - 2 , . . . ,  71 - n  and outputs monitoring results to the path selecting unit  73 . The path selecting unit  73  selects and outputs an operational circuit output having a smaller bit error rate. 
     According to the present embodiment, unlike the fourth embodiment, the soft decision discriminators are used as the discriminators and an optimum discrimination result is selected out of outputs of the operational circuits. This makes it possible to further reduce a bit error rate while improving reception sensitivity. 
     As explained above, according to the present embodiment, the soft decision discriminating means are used to discriminate electrical signals and an output result having a low error rate is selected based on monitoring information of a bit error rate. This makes it possible to further reduce the bit error rate while improving reception sensitivity. 
     According to the present embodiment, an optical input signal is divided into two paths by the optical divider. However, as in the other embodiments, the number of paths is not limited to two. The optical input signal may be divided into three or more arbitrary paths. 
     INDUSTRIAL APPLICABILITY 
     As described above, the optical receiver according to the present invention is useful as an optical receiver applicable to very-high-speed/very-long-distance optical transmission systems and particularly suitable, for example, when it is desired to improve reception sensitivity of these systems.