  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.
