The invention provides a system and method for measuring optical signal-to-noise-ratio (OSNR) in an optical communication system. A channel filter is adapted to select one specific optical communication channel from a wavelength-division-multiplexing (WDM) optical communication system, wherein the channel comprises an optical signal carrying digital bit information and noise from associated optical power amplifiers in the system. At least one optical delay interferometer is adapted to measure at least two interferograms of the noisy signal. The invention provides a mechanism for calculating the in-band OSNR from extinctions of the interferograms measured at different optical delays by referring to each other, wherein said optical delays are selected to be substantially less than a bit period of the optical channel. Because of the selection of the optical delays and/or the self-reference between the two measurements, the system can follow any changes happening to the signal such as additional filtering, self (cross)-phase modulation, the bias and drive signal change of the modulator used to generate the optical signal.