Recently, degree of polarization (DOP) of light has been utilized as a parameter to monitor optical signal-to-noise-ratio (OSNR). However, in the presence of polarization-mode dispersion (PMD), when using DOP to determine OSNR, OSNR is always underestimated due to the depolarization effect induced by PMD. In this paper, we propose and experimentally demonstrate a simple PMD-insensitive OSNR monitoring technique based on DOP measurement. By the assistance of the transmitter-side polarization scrambling, the in-service OSNR parameter can be accurately derived from the measured maximum DOP value within the polarization scrambling period, which is immune to PMD effect. The monitoring performance is experimentally evaluated by statistical method at OSNR of 25 dB. Experimental results show that OSNR monitoring with about 1-dB standard deviation can be achieved in a 10-Gb/s NRZ-OOK system with DGD varying from 0 to 80 ps.
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