Experimental Verification of Relative Phase Noise in Raman Amplified Coherent Optical Communication System

The intensity fluctuation of a Raman pump laser source will not only transfer to the signal's intensity through stimulated Raman scattering effect, which has been investigated comprehensively from both theoretical and experimental perspective, but also affect signal's phase through cross-phase modulation (XPM) effect. The phase noise is characterized by relative phase noise (RPN) parameter. Although the analytical model of RPN was proposed and the simulations were performed, no experimental verification has been carried on so far. In order to verify the statistical property of RPN, we conducted an experiment to measure the RPN transfer function using a continuous-wave signal and a modulated Raman pump laser diode. The measurement result for the co-pumping configuration has achieved good agreement with the theoretical calculation, validating the proposed theory. In addition to the measurement, we found clear Raman pump induced phase modulation in the experiment using a CO-OFDM signal, demonstrating that RPN will occur in a practical transmission system. With the measured RPN transfer function and the prior knowledge of Raman pump relative intensity noise, we can predict the RPN induced system performance degradation in a theoretical manner.