Patent ID: 7756369

Claim:
An apparatus for monitoring an optical signal-to-noise ratio (OSNR) of an optical signal having a state of polarization (SOP) in a wavelength division multiplexed (WDM) transmission system, comprising: a variable polarization controller for rotating the SOP of the optical signal; a polarization beam splitter for splitting the optical signal at each SOP into first and second signal components having orthogonal polarizations; an optical power meter for simultaneously measuring a first optical power in a filter passband of the first signal component and a second optical power in the filter passband of the second signal component for each SOP, wherein the filter passband is centered on a measurement wavelength and has a filter bandwidth that is narrower than a bandwidth of the optical signal; a control unit for scanning the measurement wavelength over a predetermined wavelength range, and for setting a polarisation rotation of the polarization controller; and a signal processing unit for recording values of the first and second optical power, for accumulating a running minima and sum of the first and second optical power, and for calculating the OSNR from the accumulated running minima and sum.