Intrinsic mechanism for spectral evolution in single-frequency Raman fiber amplifier

In this work, the spectral evolution properties in single-frequency Raman fiber amplifier (RFA) with different pump manners are analyzed theoretically for the first time based on the gain dynamics. The analysis of gain dynamics reveals that the walk-off effect in counter-pumped manner produces a natural low-pass filter in single-frequency RFA. When applying rare-earth doped fiber lasers as the pump source, the strong temporal fluctuations in the pump source lead to spectral broadening in co-pumped manner, while the natural low-pass filter in countered-pumped case can still ensure single-frequency operation. Furthermore, applying temporal stable laser, such as single-frequency fiber laser or narrow band fiber laser spectral broadened by phase modulation technique, as the pump source would be superior for high-performance single-frequency RFA for the both two pump manners.