Intrinsic Mechanism for Spectral Evolution in Single-Frequency Raman Fiber Amplifier

In this paper, the spectral evolution properties in single-frequency Raman fiber amplifier (RFA) based on different pump schemes are analyzed theoretically for the first time based on the gain dynamics. It reveals that the walk-off effect in counter-pumped scheme acts as a natural low-pass filter for single-frequency RFA. When multilongitudinal mode rare-earth-doped fiber laser is used as the pump source for RFA, strong temporal fluctuations of the pump source would lead to spectral broadening in copumped scheme, while single-frequency operation could be maintained in the countered-pumped case because of the natural low-pass filter.