The present invention provides a method and an apparatus for monitoring
optical signal to noise ratio (OSNR) in a wavelength division
multiplexing optical transmission system. The present invention utilizes
a polarization nulling method and a tunable optical bandpass filter in
order to reliably monitor the OSNR by considering a finite polarization
nulling ratio, polarization mode dispersion and non-linear birefringence
of the optical system in real time measurement of the OSNR. Further, the
tunable optical bandpass filter is controlled to filter all wavelength
bands of wavelength division multiplexed signals. Since the invention may
monitor a plurality of demultiplexed optical signals with a single
apparatus, the overall cost of the OSNR monitoring equipment is
significantly reduced.