Patent ID: 8405511

Claim:
An optical signal quality monitoring apparatus which evaluates the quality of an optical transmission signal by using a quality value estimated from a measurement value based on a value measured from the optical transmission signal, comprising: a trajectory length acquirer measuring a Stokes vector of the optical transmission signal over an optical signal modulation frequency band and acquiring the length of a trajectory in the optical signal modulation frequency band traced out on a Poincaresphere by the measured Stokes vector as the measurement value; a Differential Group Delay acquirer acquiring a Differential Group Delay value of the optical transmission signal the Stokes vector of which is measured by the trajectory length acquirer; and a quality value estimator estimating a quality value by using the length of the trajectory acquired by the trajectory length acquirer and the Differential Group Delay value acquired by the Differential Group Delay acquirer; the optical signal quality monitoring apparatus comprising: a relational expression setting section storing in a relational expression memory an approximate expression representing the relationship between a trajectory length and a quality value, the approximate expression being determined according to the value of Differential Group Delay and input from an external source beforehand; and a Degree of Polarization meter measuring a Degree of Polarization value in the optical signal modulation frequency band; wherein the Differential Group Delay acquirer comprises a Differential Group Delay estimator estimating a Differential Group Delay value from the Degree of Polarization value measured by the Degree of Polarization meter; and the quality value estimator comprises a selector selecting an approximate expression according to the Differential Group Delay value estimated by the Differential Group Delay estimator from the relational expression memory as a relational expression representing the relationship between the length of the trajectory and the quality value.