Patent ID: 8294896

Claim:
A method for measuring the optical signal-to-noise ratio of an optical signal containing a plurality of wavelength channels having time-varying polarization states, comprising: (a) filtering the optical signal to form a test signal comprising one of the wavelength channels or a selected optical frequency in one of the selected wavelength channels; (b) transforming the polarization state of the test signal into a random or pseudo-random sequence of different transformed polarization states; (c) analyzing the transformed polarization states of the test signal with an optical spectrum analyzer and a polarization filter at a predetermined fixed orientation; (d) measuring optical power in the test signal transmitted through the polarization filter at each of the transformed polarization states in order to determine a maximum and a minimum measured optical signal power; and (e) calculating a polarization extinction ratio from the measurements of the maximum and minimum received signals in order to obtain an optical signal-to-noise ratio for the test signal; wherein the measurement time in step (d) is sufficiently short so that the polarization state of the optical light signal in the selected wavelength channel is substantially constant during the measurement of the optical power, thereby limiting a degradation in the calculated polarization extinction ratio; and wherein step (b) comprises: i) randomly or pseudo-randomly modulating the state of polarization of the test signal at a first rate; and ii) transforming the modulated polarization state of the test signal into the random or pseudo-random sequence of different polarization states at a second rate slower than the first rate.