Abstract:
An optical receiving apparatus includes a signal brancher for branching an optical input signal received from an optical transmission line into a first signal component and a second signal component, a clock extractor for extracting a clock having an amplitude from the second signal component, and a discriminator for discriminating the first signal component. The apparatus also includes a threshold controller for generating a discrimination threshold for the discriminator according to a relationship between the extracted clock amplitude and a bit error rate to which the extracted clock amplitude corresponds.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
   This application is a divisional of U.S. application Ser. No. 10/460,895, filed Jun. 12, 2003, now abandoned, which is a divisional of U.S. application Ser. No. 09/546,917, filed Apr. 11, 2000, now abandoned, which claims priority of Japanese patent Application No. Heisei 11 (1999) 145416, filed May 25, 1999. 

   FIELD OF THE INVENTION 
   This invention relates to an optical receiving apparatus and method. 
   BACKGROUND OF THE INVENTION 
   In an optical transmission system, transmission characteristics of the transmission system are measured at the time of installation and a discrimination threshold of a received signal light at a receiving terminal is determined according to the obtained result. Thereafter, a signal value of the received signal light is discriminated with the discrimination threshold. 
   As mentioned above, in conventional systems, the discrimination thresholds are fixed. However, it has been understood that the transmission characteristics of the optical transmission line fluctuate with time and, consequently, the optimum discrimination threshold of the received signal light varies as well.  FIG. 18  shows a measured result of a time variation of an optimum discrimination threshold. The vertical line and horizontal line show the optimum discrimination threshold and elapsed time respectively. 
   If the discrimination threshold of the received signal light is remained at the fixed value in spite of such variation, a bit error rate (Q value) becomes deteriorated. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide an optical receiving apparatus and method for stably discriminating a received signal regardless of a time variation of transmission characteristics. 
   Another object of the present invention is to provide an optical receiving apparatus and method for adaptively adjusting a discrimination threshold of a received signal light according to a variation of transmission characteristics. In order to achieve the above-mentioned objects, in the invention, transmission characteristics of an optical transmission line are evaluated according to an input signal from an optical transmission line and then a discrimination threshold of the received signal is controlled to become an optimum value according to the evaluated result. Therefore, the reception characteristics can be controlled to keep the optimum state always or practically all the time responding to the variation of the transmission characteristics of the optical transmission line. 
   The transmission characteristics of the optical transmission line can be evaluated, for example, from the number of errors of the received signal. The discrimination threshold of the signal is varied within a predetermined range, then an optimum discrimination threshold is determined from the evaluated results of the transmission characteristics at the respective discrimination thresholds, and the optimum discrimination threshold is generated for a predetermined period thereafter. Consequently, the satisfactory reception characteristics can be automatically selected responding to the variation of the transmission characteristics. 
   The transmission characteristics of the optical transmission line also can be evaluated using another method in which the received signal is discriminated by a plurality of fixed thresholds different from one another and then the error numbers among the obtained respective results are compared to calculate a standard deviation of at least one of mark and space sides. Since it is unnecessary to scan the discrimination thresholds, the optimum value can be rapidly determined. 
   Also, the transmission characteristics of the optical transmission line can be evaluated using the other method in which the received signal is discriminated by a plurality of fixed thresholds different from one another and then the error numbers among the obtained respective results are compared to estimate a distribution of the error numbers corresponding to the discrimination thresholds. In this case, the discrimination threshold having the minimum error number in the estimated error number distribution is determined as the optimum value. Since it is unnecessary to scan the discrimination value, the optimum value can be rapidly determined. 
   Furthermore, the transmission characteristics of the optical transmission line can be evaluated with amplitude of a clock extracted from the received signal. In this case, the optimum discrimination threshold is determined according to the amplitude of the extracted clock. Since it is unnecessary to scan the discrimination thresholds, the optimum value can be rapidly determined. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other, objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a schematic block diagram according to a first embodiment of the invention; 
       FIG. 2  is an operation flow chart of a threshold control circuit  28 ; 
       FIG. 3  is a schematic diagram showing a relation between thresholds and the error numbers; 
       FIG. 4  is a measured example of a Q value variation relative to a threshold variation; 
       FIG. 5  is an example of measured results of the Q value when a discrimination threshold is suitably controlled according to the embodiment; 
       FIG. 6  is a measured example showing a relation between the optimum thresholds and the distribution of signal mark levels; 
       FIG. 7  is a schematic diagram showing a variation of a bit error rate relative to the discrimination thresholds; 
       FIG. 8  is a schematic block diagram according to a second embodiment of the invention; 
       FIG. 9  is a schematic block diagram according to a third embodiment of the invention; 
       FIG. 10  is a schematic block diagram according to a fourth embodiment of the invention; 
       FIG. 11  is a schematic block diagram according to a fifth embodiment of the invention; 
       FIG. 12  is a schematic diagram showing a variation of the bit error rate relative to the discrimination thresholds; 
       FIG. 13  is a schematic diagram showing a variation of the bit error rate relative to the discrimination thresholds in a state that the bit error rate on a space side is increased; 
       FIG. 14  is a schematic diagram showing a variation of the bit error rate relative to the discrimination thresholds in a state that the bit error rate on a mark side is increased; 
       FIG. 15  is a schematic block diagram according to a sixth embodiment of the invention; 
       FIG. 16  is a schematic block diagram according to a seventh embodiment of the invention; 
       FIG. 17  is a schematic block diagram of an optical transmission system in which the optical receiving apparatus of the above-mentioned respective embodiments is disposed at a receiving station; 
       FIG. 18  is a measured result of a time variation of the optimum discrimination threshold; and 
       FIG. 19  is a schematic block diagram of an embodiment to evaluate transmission characteristics in an optical circuit. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Embodiments of the invention are explained below in detail with reference to the drawings. 
     FIG. 1  is a schematic block diagram of an optical reception terminal in which a first embodiment of the invention is installed. 
   A signal light enters from an optical transmission line  10  to an optical reception terminal  12  of the embodiment. A photodetecting element  14  in the optical reception terminal  12  converts the signal from the optical transmission line  10  into an electric signal and applies it to one input of a comparator  16 . A threshold generating circuit  18  generates a threshold Vth for binarizing the output of the photodetecting element  14  and applies it to the other input of the comparator  16 . The comparator  16  compares the output of the photodetector  14  and the output Vth of the threshold generating circuit  18  to binarize the output of the photodetecting element  14 . A demultiplexing circuit  20  demultiplexes the output of the comparator  16  into n (e.g. n=16) channels and applies binary signals of the respective channels to error correcting circuits  22 - 1 ˜ 22 - n  respectively. The error correcting circuits  22 - 1 ˜ 22 - n  correct errors of the signals from the demultiplexing circuit  20  and apply them to a multiplexer  24  as well as send the number of the errors to a counter  26 . Most of the existing error correcting circuits comprise such function for outputting the error number and therefore it is not necessary to provide a particular error correcting circuit for the embodiment. The multiplexer  24  multiplexes the n signals from the error correcting circuits  22 - 1 ˜ 22 - n  on the time domain and supplies them as STM signal to the following circuit (when the optical transmission line  10  is, for instance, an international optical fiber transmission line, the following circuit is a domestic communication network). 
   The counter  26  sums up the number of the errors from the error correcting circuits  22 - 1 ˜ 22 - n  and applies the total result to a threshold control circuit  28 . The output value of the counter  26  represents the Q value of the optical transmission line  10 . The threshold control circuit  28  can apply a threshold control signal to the threshold generating circuit  18  in order to change its generating threshold Vth. The threshold control circuit  28  controls the threshold generating circuit  18  to generate a plurality of thresholds one after another, determines an optimum discrimination threshold for the present transmission condition of the optical transmission line  10  from the outputs of the counter  26  corresponding to the respective thresholds, and directs the threshold generating circuit  18  to generate the determined discrimination threshold until the following discrimination threshold is optimized thereafter. 
     FIG. 2  shows an operation flow chart of the threshold control circuit  28  and  FIG. 3  is a schematic diagram showing the relation between the thresholds and the number of the errors. In  FIG. 3 , the horizontal axis shows the discrimination thresholds and the vertical axis shows the number of the errors as a bar graph. 
   The threshold control circuit  28  fetches the output (the total of the errors of the whole channels) of the counter  26  at the present threshold and stores its average value (S 1 ). Also, the threshold control circuit  28  controls the threshold generating circuit  18  to shift its discrimination threshold toward the minus side by a predetermined value (S 2 ) and fetches the output of the counter  26  at the threshold (S 3 ). The shifting amount of the discrimination threshold at a time can be rough to a certain extent.  FIG. 4  is a measured example showing a variation of the Q value relative to a variation of the threshold. In  FIG. 4 , the horizontal axis shows deviations from the optimum threshold with a volt unit, and the vertical axis shows a deteriorated amount (dB) from the Q value at the optimum threshold. When the discrimination threshold is varied within the width of 10 mV, the deteriorated amount of the Q value becomes no more than 0.1 dB. Therefore, the threshold should be varied every 10 mV to count the number of the errors. After that the shift of the threshold Vth toward the minus side (S 2 ) and the fetch of the output of the counter  26  at the threshold (S 3 ) are repeated until the threshold reaches the limit threshold on the minus side (S 4 ). 
   When the threshold reaches the limit threshold on the minus side (S 4 ), the threshold control circuit  28  adjusts the threshold to the initial value at S 1  (S 5 ), fetches again the output of the counter  26  for a predetermined period and stores its average value (S 6 ). Then, the threshold control circuit  28  stepwisely shifts the threshold Vth toward the plus side (S 7 ), fetches and stores the output of the counter  26  at the respective thresholds (S 8 ) until the threshold Vth reaches the limit threshold on the plus side (S 9 ). 
   At the point that the threshold Vth on the plus side reaches the limit threshold (S 9 ), the whole information is obtained that contains the number of the errors at the respective thresholds within the range from the limit threshold on the minus side to the limit threshold on the plus side. From the obtained result, the threshold control circuit  28  determines an optimum discrimination threshold to make the number of the errors minimum and controls the threshold generating circuit  18  to generate the determined threshold thereafter (S 10 ). 
   After the threshold determined at S 10  is used for a predetermined period, the flow shown in  FIG. 2  is again executed so as to optimize the discrimination threshold. 
   In  FIG. 2 , the information of the error number relative to the threshold is measured between both limit thresholds on the minus side and plus side. However, when the error number reaches over the limit value, it is meaningless to vary the threshold any further in the direction to increase the number of the errors. From this point of view, it is obvious that the threshold can be varied within the range in which the number of the errors reaches no more than the limit value at the steps from S 4  to S 9 . 
   In  FIG. 1 , to make it easily understandable, the comparator  16  binarizes the signal (the output of the photodetecting element  14 ) with one threshold. It is obvious, however, that this embodiment is applicable to the case in which thresholds for mark signal and space signal are separately provided. In this case, each threshold should be optimized respectively following the process shown in  FIG. 2 . 
     FIG. 5  shows a measured result of Q values when a discrimination threshold is adaptively controlled according to the embodiment. The solid line shows the Q values of the embodiment. As a comparative object, the broken line shows Q values when the discrimination threshold is fixed. By comparison between both lines, it is clear that, according to the embodiment, the average Q value can be kept in the higher range. 
   In the above embodiment, the discrimination threshold is determined so as to minimize the number of the errors. It is possible that an standard deviation of a mark level is measured and then an optimum threshold is determined according to the measured result. This method is also applicable to optimize the discrimination threshold.  FIG. 6  illustrates a measured example showing the relation between the optimum thresholds and a distribution of the mark level (standard deviation of the mark level) of the signal light. In  FIG. 6 , the vertical axis shows the standard deviations of the mark level and the horizontal axis shows the optimum thresholds. In this measured result, the correlation coefficient between the standard deviations of the mark level and the optimum thresholds is 0.82. It is understood from the measured result that the discrimination threshold can be dynamically optimized by feedback-controlling the discrimination threshold according to the measured result of the standard deviations of the mark level. 
   In order to find the standard deviation of the mark level, for instance, a method is applicable in which bit error rates are measured while varying discrimination thresholds and then Q values are obtained from the measured result (e.g. N. S. Bergano et al., IEEE Photonics Technology Letters, Vol. 5, pp. 304–306, 1993). When transmission characteristics such as Q value and the like are measured on a mark side (or a space side), bit error rates at respective threshold levels are measured while the discrimination threshold is shifted toward the mark side (or the space side). An optimum threshold can be determined from the variation of the measured result relative to the thresholds.  FIG. 7  illustrates a schematic diagram of the variation of the bit error rate relative to the discrimination threshold. The horizontal axis and vertical axis show the discrimination threshold and bit error rate respectively. The crosses show measured points. The inclination of the interpolation line connecting the measured points on the mark side shows the standard deviation of the mark level. Accordingly, when the bit error rates on the mark side corresponding to at least two discrimination thresholds are measured, the standard deviation on the mark side is obtained and thus the discrimination threshold can be optimized. 
   Each of  FIGS. 8 ,  9  and  10  shows a schematic block diagram of an embodiment in which the standard deviation on a mark side is measured and the discrimination threshold is optimized according to the measured result. Each of the embodiments in  FIGS. 8 ,  9  and  10  has fundamentally the same operation and function except for a branching step of a received signal. 
     FIG. 8  is explained first. A signal light enters an optical receiving apparatus  32  according to the invention from an optical transmission line  30 . A photodetecting element  34  in the optical receiving apparatus  32  converts the signal light from the optical transmission line  30  into an electrical signal, and a linear amplifier  36  linearly amplifies the output from the photodetector  34 . An electric signal branching circuit  38  branches the output of the amplifier  36  into a discriminating circuit  40  having a variable threshold and discriminating circuits  42 ,  44  respectively having fixed thresholds Va, Vb. The branching circuit  38  can be either one that simultaneously applies the output of the amplifier  36  to the discriminating circuits  40 ,  42  and  44  or that applies the output of the amplifier  36  to the discriminating circuits  42  and  44  when an optimum threshold is determined and applies the output of the amplifier  36  to the discriminating circuit  40  for the rest of the period. From the point of view of constant signal reception, the former configuration is obviously preferable. 
   The discriminating circuits  42  and  44  discriminate the input signals according to the fixed thresholds Va and Vb respectively. Error rate measuring circuits  46  and  48  measure bit error rates of the outputs from the discriminating circuits  42  and  44  and apply the measured results to a standard deviation calculating circuit  50 . The values of the thresholds Va and Vb are respectively preset so as to be able to measure bit error rates of two points required for calculating a standard deviation on the mark side. It is also applicable to calculate a standard deviation on the space side instead of that on the mark side. In optical pulse transmission, however, the standard deviation on the mark side can grasp the condition of the transmission line more accurately. 
   The standard deviation calculating circuit  50  calculates the standard deviation on the mark side from the measured results of the error rate measuring circuits  46  and  48 . A threshold generating circuit  52  determines an optimum discrimination threshold by comparing the standard deviation calculated by the standard deviation calculating circuit  50  with the premeasured relation between the standard deviation and optimum threshold, and applies the optimum threshold Vx to the discriminating circuit  40 . The discriminating circuit  40  discriminates the signal from the electric signal branching circuit  38  according to the threshold Vx from the threshold generating circuit  52 . The signal discriminated at the discriminating circuit  40  is applied to the following circuit as a received signal. 
   The part consisting of the error rate measuring circuits  46  and  48 , standard deviation calculating circuit  50  and threshold generating circuit  52  can be realized with digital arithmetic circuits such as a microcomputer and the like. The discriminating circuits  42  and  44  also can be included in the digital arithmetic circuit. 
   As readily understandable from the above description, the branching circuit  38  usually applies the output of the amplifier  36  to the discriminating circuit  40  and applies to the discriminating circuits  42  and  44  only when a new optimum discrimination threshold is to be determined. Needless to say, the branching circuit  38  can steadily apply the output of the amplifier  36  to all of the discriminating circuits  40 ,  42  and  44 . 
   As discussed above, in the embodiment shown in  FIG. 8 , the error rate, namely the standard deviation on the mark side is measured intermittently or constantly according to more than one fixed threshold. Then, the optimum discrimination threshold is determined from the measured result and the received signal is discriminated according to the optimum discrimination threshold. Therefore, in the embodiment, since the discrimination threshold of the received signal is varied according to the variation of the transmission condition, the receiving condition is always maintained to be most suitable. 
   In the embodiment shown in  FIG. 8 , although the received signal is branched in the electric stage by the electric signal branching circuit  38 , it is also applicable to branch the received signal in the optical stage.  FIG. 9  illustrates a schematic block diagram of such embodiment for branching the signal in the optical stage. 
   A signal light inputs to an optical receiving apparatus  62  according to the invention from an optical transmission line  60 . An optical signal branching circuit  64  in the optical receiving apparatus  62  branches (switches or divides) the signal light from the optical transmission line  60  and applies it to photodetecting elements  66 ,  68  and  70 . The branching function of the optical signal branching circuit  64  can be the same with that of the electric signal branching circuit  38 . The photodetecting elements  66 ,  68  and  70  respectively convert the signals from the branching circuit  64  into electric signals. Linear amplifiers  72 ,  74  and  76  respectively linearly amplify the outputs from the photodetectors  66 ,  68  and  70 . 
   Discriminating circuits  78  and  80  respectively discriminate the output signals from the amplifiers  74  and  76  according to fixed thresholds Va and Vb and apply the results to a threshold control circuit  82 . The threshold control circuit  82  comprises the same configuration with the part consisted of the error rate measuring circuits  46  and  48 , standard deviation arithmetic circuit  50  and threshold generating circuit  52  of the embodiment shown in  FIG. 8 . That is, the threshold control circuit  82  calculates bit error rates from the outputs (the signal discriminated results according to the two different thresholds Va and Vb) of the discriminating circuits  78  and  80 , calculates a standard deviation on the mark side from the obtained bit error rates, and determines an optimum discrimination threshold from the standard deviation on the mark side. The threshold control circuit  82  then applies the determined optimum discrimination threshold Vx to a discriminating circuit  84 . 
   The discriminating circuit  84  discriminates the output signal of the linear amplifier  72  according to the discrimination threshold Vx from the threshold control circuit  82 . The signal discriminated at the discriminating circuit  84  is applied to the following circuit as a received signal. 
     FIG. 10  is a schematic block diagram of an embodiment combining the branching in the optical stage and in the electric stage. 
   A signal light enters an optical receiving apparatus  92  according to the invention from an optical transmission line  90 . An optical signal branching circuit  94  in the optical receiving apparatus  92  branches (switches or divides) the signal light from the optical transmission line  90  and applies it to photodetecting elements  96  and  98 . The optical signal branching circuit  94  can be either one that selectively applies the signal light from the optical transmission line  90  to the photodetecting element  96  or  98  or that divides the signal light into two portions and applies them to the photodetecting elements  96  and  98  simultaneously. From the point of view of continuous signal reception, the latter is more preferable. The photodetecting elements  96  and  98  respectively convert the signal lights from the branching circuit  94  into electric signals. Linear amplifiers  100  and  102  linearly amplify the outputs from the photodetecting elements  96  and  98  respectively. 
   An electric signal branching circuit  104  simultaneously applies the output signal from the linear amplifier  102  to discriminating circuits  106  and  108  respectively having fixed thresholds Va and Vb. The discriminating circuits  106  and  108  respectively discriminate the signals from the electric signal branching circuit  104  according to the fixed thresholds Va and Vb, and apply the results to a threshold control circuit  110 . The threshold control circuit  110  has the same configuration and operation with the threshold control circuit  82 . Namely, the threshold control circuit  110  calculates bit error rates from the outputs (the signal discriminated results according to the two different thresholds Va and Vb) of the discriminating circuits  106  and  108 , calculates a standard deviation on a mark side from the obtained bit error rates, and determines an optimum discrimination threshold from the standard deviation on the mark side. Then, the threshold control circuit  110  applies the determined optimum discrimination threshold Vx to a discriminating circuit  112 . 
   The discriminating circuit  112  discriminates the output signal from the linear amplifier  100  according to the discrimination threshold Vx from the threshold control circuit  110 . The signal discriminated at the discriminating circuit  112  is applied to the following circuit as a received signal. 
   As a simpler method, bit error rates on both mark side and space side are measured, and an optimum discrimination threshold is estimated from the variation of the measured values. On the assumption that variation slopes of the bit error rates on the mark and space sides relative to the discrimination thresholds are constant respectively, the optimum discrimination threshold can be determined with a simpler configuration since it is sufficient if only one bit error rate is measured on each of the mark and space sides. When the bit error rates are measured according to a plurality of discrimination thresholds on the mark and space sides respectively, variation slopes of the bit error rates on the mark and space sides relative to the discrimination thresholds can be measured dynamically. Therefore, it is obvious that the discrimination threshold can be optimized more accurately. 
     FIG. 11  illustrates a schematic block diagram of an embodiment for optimizing a discrimination threshold according to bit error rates on the mark and space sides. 
   A signal light enters an optical receiving apparatus  122  according to the invention from an optical transmission line  120 . A photodetecting element  124  in the optical receiving apparatus  122  converts the signal light from the optical transmission line  120  into an electric signal, and a linear amplifier  126  linearly amplifies the output from the photodetecting element  124 . An electric signal branching circuit  128  branches an output from the amplifier  126  to a discriminating circuit  130  with a variable threshold, and discriminating circuits  132 ,  134  with fixed thresholds Vc, Vd respectively. The branching circuit  128  comprises the same function with the branching circuit  38 . 
   The discriminating circuit  132  discriminates marks in the input signal according to the fixed threshold Vc for the mark. The discriminating circuit  134  discriminates spaces of the input signal according to the fixed threshold Vd for the space. The threshold Vc is set higher than a standard discrimination threshold for discriminating a binary signal, and the threshold Vd is set, in reverse, lower than the standard discrimination threshold. An error rate measuring circuit  136  calculates the bit error rate on the mark side from the output of the discriminating circuit  132 , and an error rate measuring circuit  138  calculates the bit error rate on the space side from the output of the discriminating circuit  134 . The measured results of the error rate measuring circuits  136  and  138  are applied to a threshold control circuit  140 . The threshold control circuit  140  determines an optimum discrimination threshold Vx from the bit error rates on the mark and space sides measured by the error rate measuring circuits  136  and  138 , and applies it to the discriminating circuit  130 . 
   The discriminating circuit  130  discriminates the signal from the electric signal branching circuit  128  according to the threshold Vx from the threshold control circuit  140 . The signal discriminated at the discriminating circuit  130  is applied to the following circuit as a received signal. 
   The decision mechanism of the optimum threshold Vx at the threshold control circuit  140  is explained below referring to  FIGS. 12 ,  13  and  14 .  FIGS. 12 ,  13  and  14  show variations of the bit error rate relative to the discrimination thresholds.  FIG. 12  shows an initial state,  FIG. 13  shows a state in which the bit error rate on the space side is increased compared to the initial state shown in  FIG. 12 , and  FIG. 14  shows, inversely, a state in which the bit error rate on the mark side is increased compared to the initial state shown in  FIG. 12  respectively. In  FIGS. 12 ,  13  and  14 , the horizontal axis shows the discrimination thresholds and the vertical axis shows the bit error rates. 
   In the initial state shown in  FIG. 12 , the discrimination threshold V 1 , corresponding to the intersection point of the straight line representing the bit error rates on the mark side and that representing the bit error rates on the space side, indicates the optimum discrimination threshold Vx. When the inclinations of the two straight lines showing the bit error rates on the mark and space sides are already known, the discrimination threshold V 1  corresponding to the intersection point is easily calculated by measuring the bit error rate on the mark side according to the threshold Vc and the bit error rate on the space side according to the threshold Vd, as shown in the embodiment of  FIG. 11 . When the inclination variation of the bit error rate relative to the discrimination threshold is not negligible or a more precise optimum discrimination threshold is desired, it is obvious that the bit error rates on both mark and space sides should be measured according to a plurality of discrimination. 
   When the bit error rate on the space side is increased from the initial state shown in  FIG. 12 , the threshold V 2  corresponding to the intersection point of the two straight lines of the bit error rates on the mark and space sides moves to the right direction compared to the threshold V 1  as shown in  FIG. 13 . Accordingly, the threshold control circuit  140  applies the discrimination threshold V 2  as a new optimum discrimination threshold Vx to the discriminating circuit  130 . 
   Contrarily, when the bit error rate on the mark side is increased from the initial state shown in  FIG. 12 , the threshold V 3  corresponding to the intersection point of the two straight lines of the bit error rates on the mark and space sides moves to the left direction compared to the threshold V 1  as shown in  FIG. 14 . Accordingly, the threshold control circuit  140  applies the discrimination threshold V 3  as a new optimum discrimination threshold Vx to the discriminating circuit  130 . 
   As described above, in the embodiment shown in  FIG. 11 , the discrimination threshold is optimized adaptively according to the condition of the transmission line with the simple configuration, and therefore the receiving condition is maintained at the optimum state. 
   In the same way that the embodiment shown in  FIG. 8  is modified to the embodiments shown in  FIGS. 9 and 10 , the embodiment shown in  FIG. 11  also obtain the equivalent operating effect when it is modified to a configuration that the signal is branched in an optical stage and/or an electric stage. 
     FIG. 15  shows a schematic block diagram of an embodiment for optimizing a discrimination threshold with amplitude of a clock signal reproduced from a received signal. 
   A signal light enters an optical receiving apparatus  152  according to the invention from an optical transmission line  150 . A photodetecting element  154  in the optical receiving apparatus  152  converts the signal light from the optical transmission line  150  into an electric signal, and a linear amplifier  156  linearly amplifies the output from the photodetecting element  154 . An electric signal branching circuit  158  branches the output from the amplifier  156  to a discriminating circuit  160  with a variable threshold and clock extracting circuit  162 . The branching circuit  158 , similarly to the branching circuits  38  and  128 , can be either one that simultaneously applies the output of the amplifier  156  to the discriminating circuit  160  and clock extracting circuit  162  or that selectively applies the output to the discriminating circuit  160  or the clock extracting circuit  162 . From a viewpoint of continuity of signal receiving, the former function is more preferable. 
   The clock extracting circuit  162  extracts a clock out of the signal from the branching circuit  158 . In a standard optical receiving apparatus, a limiting amplifier is employed in order to control the amplitude of the clock signal to be constant. However, the embodiment uses the linear amplifier  156 , and therefore the clock extracting circuit  162  can obtain the clock signal having the amplitude according to a waveform of a received signal light. 
   The clock signal extracted at the clock extracting circuit  162  is linearly amplified by a linear amplifier  164  and applied to a threshold control circuit  166 . The threshold control circuit  166  controls the discrimination threshold of the discriminating circuit  160  at the optimum value Vx according to the amplitude of the clock signal from the linear amplifier  164 . That is, as shown in  FIG. 14 , when noise on the mark side is large, the optimum threshold moves to the space side and at the same time the amplitude of the clock decreases due to the influence of the noise. In reverse, when the noise on the mark side is small, the optimum threshold moves to the mark side and at the same time the amplitude of the clock increases due to the influence of the noise. The threshold control circuit  166  is preprogrammed with the information to indicate such relations between the clock amplitude and the optimum threshold, thus determining an optimum threshold Vx by comparing the amplitude (clock amplitude) of the output from the linear amplifier  164  with this information, and applies it to the discriminating circuit  160 . 
   The discriminating circuit  160  discriminates the signal from the electric signal branching circuit  158  according to the threshold Vx from the threshold control circuit  166 . The signal discriminated at the discriminating circuit  160  is applied to the following circuit as a received signal. 
     FIG. 16  shows a schematic block diagram of an embodiment in which the embodiment shown in  FIG. 15  is modified so that the signal is branched in the optical stage instead of in the electric stage. 
   A signal light enters an optical receiving apparatus  172  according to the invention from an optical transmission line  170 . An optical signal branching circuit  174  in the optical receiving apparatus  172  branches (switches or divides) the signal light from the optical transmission line  170  and applies it to photodetecting elements  176  and  178 . The branching function of the optical signal branching circuit  174  is similar to that of the electric signal branching circuit  158 . The photodetecting circuits  176  and  178  respectively convert the signal light from the branching circuit  174  into an electric signal. A linear amplifier  180  linearly amplifies the output from the photodetecting element  176  and applies it to a discriminating circuit  182 . 
   Similarly to the clock extracting circuit  162 , a clock extracting circuit  184  extracts a clock from the output of the photodetecting element  178 . Similarly to the case shown in  FIG. 15 , the amplitude of the clock output from the clock extracting circuit  184  reflects the noise condition of the optical transmission line  170 . 
   A linear amplifier  186  linearly amplifies the clock signal extracted at the clock extracting circuit  184  and applies it to a threshold control circuit  188 . The threshold control circuit  188 , similarly to the threshold control circuit  166 , controls the discrimination threshold of the discriminating circuit  182  to an optimum value Vx according to the amplitude of the clock signal from the linear amplifier  186 . 
   The discriminating circuit  182  discriminates the output from the linear amplifier  180  according to the threshold Vx from the threshold control circuit  188 . The signal discriminated at the discriminating circuit  182  is applied to the following circuit as a received signal. 
     FIG. 17  shows a schematic block diagram of an optical transmission system in which the optical receiving apparatus of the above-discussed embodiments is employed as a reception terminal. An optical transmission terminal  210  outputs an optical signal onto an optical transmission line  212 . The optical transmission line  212  comprises a number of optical fibers  214  and optical amplification repeaters  216  for connecting those optical fibers  214  in serial. The signal light propagated on the optical transmission line  212  enters an optical reception terminal  218 . The optical reception terminal  218  having the above-mentioned built-in optical receiving apparatus adaptively optimizes the discrimination threshold of the signal according to the transmission condition of the optical transmission line  212  and discriminates the received signal. Accordingly, the most suitable discrimination threshold is selected according to the time variation of the transmission characteristics on the optical transmission line and therefore the satisfactory signal receiving performance is also maintained. 
   In the above embodiment, although the transmission characteristics are finally evaluated with the electric signal, it is also applicable to evaluate them in the optical state.  FIG. 19  shows a schematic block diagram of such embodiment. 
   In  FIG. 19 , a signal light enters an optical receiving apparatus  192  according to the invention from an optical transmission line  190 . An optical signal branching circuit  194  in the optical receiving apparatus  192  branches (switches or divides) the signal light from the optical transmission line  190  and applies it to a characteristics-evaluating optical circuit  196  and photodetecting element  198 . The branching function of the optical signal branching circuit  194  may be the same as that of the signal branching circuits  158  and  174 . The characteristics-evaluating optical circuit  196  generates a discrimination threshold control signal for determining a discrimination threshold of a received signal out of the input signal light from the optical signal branching circuit  194 . The photodetecting element  198  converts the signal light from the branching circuit  194  into an electric signal. A linear amplifier  200  linearly amplifies the output of the photodetecting element  198  and applies it to a discriminating circuit  202  having a variable threshold. The discriminating circuit  202  discriminates the output signal from the linear amplifier  200  with a discrimination threshold according to the discrimination threshold control signal from the characteristics-evaluating optical circuit  196 . The signal discriminated at the discriminating circuit  202  is applied to the following circuit as a received signal. 
   The characteristics-evaluating optical circuit  196  comprises, for instance, a saturable absorber. The saturable absorber is an element that absorbs weak input light and also transmits intense input light without absorbing. Considering that the amplitude variation of the optical signal affects the optimum discrimination threshold, it is possible to obtain the information for determining the discrimination threshold from the optical signal transmitted through the saturable absorber. Namely, when the output light of the saturable absorber is weak, it is considered that the amplitude of the optical signal is small, and thus the discrimination threshold should be moved toward the space side. In reverse, when the output light of the saturable absorber is intense, it is considered that the amplitude of the optical signal is large, and thus the discrimination threshold should be moved toward the mark side. In this way, the discriminating threshold may be determined from the transmitted light out of the saturable absorber. Thus, the transmission characteristics are evaluated in the optical stage, and the discrimination threshold of the received signal can be feedforward-controlled according to the evaluated result. 
   As readily understandable from the above explanation, according to the invention, a signal can be received in an optimum state regardless of a variation of transmission characteristics. 
   While the invention has been described with reference to the specific embodiment, it will be apparent to those skilled in the art that various changes and modifications can be made to the specific embodiment without departing from the spirit and scope of the invention as defined in the claims.