Multi-wavelength in-band OSNR monitor based on Lyot-Sagnac interferometer

A novel in-band OSNR monitor is proposed and experimentally demonstrated for WDM signal. By using a Lyot-Sagnac interferometer, the monitor realized OSNR measurement from 7.5~25 dB (within an accuracy of ± 0.5 dB) for 4-channel 40 Gbaud NRZ-QPSK signals, without requirement for prior knowledge of the noise-free coherence properties of signal. Further investigation proved that this OSNR monitor had high tolerance to chromatic dispersion (0~1152 ps/nm), first-order polarization mode dispersion (0~100 ps), and polarized noise. Moreover, the monitor worked very well even with input optical power as low as -8.24 dBm, and also worked in in C-band. Theoretical analysis and experimental results prove that it is capable of measuring OSNR for polarization-division-multiplexing signals.