System for monitoring quality of optical signals having different bit rates

Abstract

An optical signal quality monitoring system is provided, by which the quality of optical signals can be examined using a single monitoring system, not depending on the bit rate of each signal. In the system, an optical signal having a bit rate N·f , that is, N times as much as basic clock frequency f , is sampled by using a pulse repetition frequency f /n −&Dgr;f or f /n +&Dgr;f where n is a predetermined natural number and the pulse repetition frequency slightly differs from f /n by &Dgr;f, and an amplitude histogram of the optical signal is determined based on results of the sampling. Regarding the sampling points which constitute the histogram, a set of higher-level points and a set of lower-level points are extracted and a ratio of a difference between an average level of the set of higher-level points within a predetermined period and an average level of the set of lower-level points within a predetermined period, to the sum of standard deviations of both sets within each predetermined period is calculated as a coefficient of the S/N, and the quality of the optical signal is examined based on the coefficient. By performing optical sampling, quality of optical signals having bit rates of a few dozen Gbit/s or more can be monitored.