Method and apparatus to determine a discrimination threshold of a received signal, and an optical transmission system

An apparatus to determine a discrimination threshold of a received signal from an optical transmission line, comprising:

• a polarization controller disposed between an optical transmitter for outputting an optical signal and the optical transmission line to rotate polarization of the optical signal output from the optical transmitter;
• a photodetector to convert the optical signal propagated on the optical transmission line into an electric signal;
• a discrimination circuit to discriminate the output from the photodetector according to a discrimination threshold;
• an error rate measuring circuit to measure a bit error rate of the output from the discrimination circuit;
• and
• a control circuit which controls the polarization rotating amount of the polarization controller and the discrimination threshold of the discrimination circuit to search the worst bit error rate at each discrimination threshold by changing the polarization rotating amount of the polarization controller at each of a plurality of discrimination thresholds and to search a discrimination threshold having a predetermined bit error rate out of the worst bit error rates.

An apparatus to determine a discrimination threshold of a received signal input from an optical transmission line comprising:

• a polarization controller disposed between an optical transmitter for outputting an optical signal and the optical transmission line to rotate polarization of the optical signal output from the optical transmitter;
• a photodetector to convert the optical signal propagated on the optical transmission line into an electric signal;
• a discrimination circuit to discriminate the output from the photodetector according to a discrimination threshold;
• an error rate measuring circuit to measure a bit error rate of the output from the discrimination circuit;
• and
• a control circuit which controls the polarization rotating amount of the polarization controller and the discrimination threshold of the discrimination circuit to search a polarization rotating amount having the worst bit error rate by changing the polarization rotating amount of the polarization controller at an initial discrimination threshold, and to scan the discrimination threshold of the discrimination circuit keeping the polarization rotating amount to search a discrimination threshold having a predetermined bit error rate.

An optical transmission system comprising:

• an optical transmission line;
• an optical transmitter to output an optical signal;
• a polarization controller disposed between the optical transmitter and the optical transmission line to rotate polarization of the optical signal output from the optical transmitter;
• a photodetector to convert the optical signal propagated on the optical transmission line into an electric signal;
• a discrimination circuit to discriminate the output from the photodetector according to a discrimination threshold;
• an error rate measuring circuit to measure a bit error rate of the output from the discrimination circuit;
• and
• a control circuit which controls the polarization rotating amount of the polarization controller and the discrimination threshold of the discrimination circuit to search a discrimination threshold corresponding to a predetermined bit error rate out of the worst bit error rates relative to the variation of the polarization direction of the optical signal.