Patent ID: 7391977

Claim:
A device, comprising: a polarization device to change polarization of received light to produce output light to find a maximum power level and a minimum power level of each WDM channel of different WDM channels in said received light; a WDM demultiplexer to receive said output light to separate different WDM channels in said output light; a plurality of polarization beam splitters in optical paths of said different WDM channels, respectively, wherein each polarization beam splitter splits a corresponding WDM channel into two monitor beams that are orthogonally polarized; a plurality of filters disposed in one of said two monitor beams for said different WDM channels, respectively, each operable to produce a difference in power between noise power levels in said two monitor beams for each WDM channel without affecting signal power levels in said each WDM channel in said two monitor beams; a plurality of pairs of optical detectors provided for different WDM channels, wherein two optical detectors in each pair are positioned to respectively receive said two monitor beams for a corresponding WDM channel to produce two detector outputs for said WDM channel; and a control unit to produce said control signal and to process detector signals to produce an output for each WDM channel indicative of a signal-to-noise ratio or a degree of polarization in said each WDM channel.