an apparatus and method for simultaneous channel and optical signal - to - noise ratio monitoring is adapted to function in multi - channel wavelength - division - multiplexed optical communication systems . a polarization controller is sequentially adjusted to perform the sequential polarization control with an optical signal . a polarization - selective electro - optic modulator acts as a polarizer and provides a signal dithering to improve the detection sensitivity . a beam splitter splits the dithered optical signal into two clusters . a photodetector receives the first cluster and measures the osnr . an optical element receives the second cluster and recognizes the channel wavelength of multiple channels . the apparatus can be packaged into a compact module and integrated on a chip . the channel monitoring covers a wide wavelength range and is tunable . the osnr monitoring can be accurate over a wide dynamic range .