Patent ID: 8121475

Claim:
An optical signal quality monitor comprising: a splitter for splitting an input optical signal into two split signals, the input optical signal having an input optical signal frequency; a converter for producing a converted signal, the converter including an electro-absorption (EA) modulator that modulates one of the split optical signals based on a signal determined by adding a predetermined offset frequency to 1/N of the frequency of the input optical signal (where N is a natural number), the converted signal having a frequency that is N times the offset frequency; a low-frequency photoelectric converter for changing the converted signal into an electrical signal and limiting it to a predetermined bandwidth to provide a bandwidth-limited electrical signal; a reference signal generator for generating an electrical reference signal, the reference signal converter either converting the other split optical signal into an electrical signal and filtering low-frequency components from it to form the electrical reference signal or filtering low-frequency components from the other split optical signal and converting the filtered signal into the electrical reference signal; an intensity ratio calculator for calculating an intensity ratio of the bandwidth-limited electrical signal to the electrical reference signal; and a quality monitoring section for monitoring the intensity ratio together with pre-stored characteristic information or checking the intensity ratio against the characteristic information to obtain a quality parameter of the input optical signal.