Influence of polarization on pump-signal RIN transfer and cross-phase modulation in copumped Raman amplifiers

Pump-to-signal relative intensity noise (RIN) transfer in copumped Raman amplifiers is analyzed. By means of simulations and experiments, the authors call attention to the impact of the polarized nature of stimulated Raman scattering on RIN transfer. Although depolarizing the Raman pump enables polarization-independent signal gain, the authors demonstrate that proper modeling of RIN transfer requires including the polarization dependence of the Raman effect and the polarization mode dispersion of the fiber. A second key phenomenon is emphasized. Besides the RIN transfer induced by Raman gain, pump RIN is also transferred to the signal via nonlinear gain. This phenomenon can be addressed as pump-signal cross-phase modulation since it essentially generates phase noise on the signal. For the first time, in this paper, a model of pump-to-signal RIN transfer that includes polarization and nonlinear gain is presented. The authors reveal the key influence of these factors that had been uncared for so far. Their analysis is supported by experimental data. Signal noise characteristics (RIN spectrum and high-resolution optical power spectrum) measured at the output of a copumped Raman amplifier were presented. The Raman pump is a pair of semiconductor diodes in the first two experiments and then a Raman fiber laser. For these two kinds of depolarized or unpolarized Raman pumps that are commonly used in the field, the authors emphasize the necessity of considering polarization for the good assessment of RIN transfer.