Patent ID: 7400830

Claim:
A quality monitoring method of wavelength division multiplexed signal light, for monitoring the quality of a wavelength division multiplexed signal light transmitted via an optical transmission path, comprising: branching a part of the wavelength division multiplexed signal light being propagated through said optical transmission path as a monitor light: selecting, as an object to be measured, a signal light of one wavelength from the signal lights of a plurality of wavelengths contained in said branched monitor light; repeatedly measuring for a plurality of times the frequency of occurrences of bit error in a previously set time for said selected signal light to be measured; judging based on said measurement results as to whether or not said signal light to be measured is deteriorated in the quality thereof, together with a deterioration factor; and outputting said judgment result as monitoring information, wherein, when it is judged that said signal light to be measured is deteriorated in the quality thereof, it is judged whether or not signal lights exist on wavelength grids adjacent to said signal light to be measured, and when the signal lights exist on the adjacent wavelength grids, the frequency of occurrences of bit error in the previously set time is repeatedly measured for the plurality of times for the signal lights on the adjacent grids, and the deterioration factor of said signal light to be measured is judged based on said measurement results.