Patent ID: 7200328

Claim:
A method for determining the origin of optical signal quality degradation comprising: an optical signal amplitude histogram measurement step of obtaining an optical signal amplitude histogram from an optical signal under measurement; an averaged Q-factor parameter measurement step of obtaining an averaged Q-factor parameter, which is an optical signal quality parameter calculated from the optical signal amplitude histogram obtained in said optical signal amplitude histogram measurement step; a waveform distortion parameter measurement step of obtaining a waveform distortion parameter, which is another optical signal quality parameter calculated from the optical signal amplitude histogram obtained in said optical signal amplitude histogram measurement step; and an optical signal quality evaluation step of evaluating an optical signal quality degradation factor using a measurement value of the averaged Q-factor parameter and a measurement value of the waveform distortion parameter obtained in said averaged Q-factor parameter measurement step and said waveform distortion parameter measurement step, wherein said optical signal quality evaluation step makes a decision that a main factor of the optical signal quality degradation is the waveform distortion in the optical signal under measurement when the averaged Q-factor parameter is reduced and the waveform distortion parameter is increased, and makes a decision that the main factor of the optical signal quality degradation is a factor other than waveform distortion when the waveform distortion parameter is not increased.