We present a method to enhance the sensitivity of interferometer based techniques employed for the in-band measurement of optical signal-to-noise ratio (OSNR) in wavelength division multiplexed (WDM) optical transmission systems. With respect to conventional schemes, a narrow band filter (NBF) is added before the variable delay interferometer performing the autocorrelation measurement. The NBF is used to select within the signal spectrum only those frequencies where the signal power spectral density (PSD) is locally lower and more sensitive to noise effects. Performance monitoring of a 10 Gb/s on-off keying (OOK) non-return to zero (NRZ) signal is numerically and experimentally investigated for an OSNR level ranging from 5 dB to 25 dB. Design criteria are pointed out for the bandwidth and the detuning of the NBF with respect to the signal carrier wavelength, as well as for the delay of the interferometer, in order to maximize the system sensitivity to OSNR variations. Experimental results, achieved by using an integrated ring-resonator NBF, demonstrate that presented technique enables to effectively detect small noise variations at high OSNR levels (>15), where conventional interferometer based techniques exhibit a poor sensitivity. Moreover we demonstrate that the NBF can operate directly on the transmitted optical signal without introducing any significant perturbation, this making the presented technique suitable for in-line OSNR measurements.
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