Influence of seed instability on the stimulated Raman scattering of high power narrow linewidth fiber amplifier

In this work, the effect of the temporal instability on stimulated Raman scattering (SRS) characteristics in a high power Yb-doped fiber amplifier is studied experimentally and theoretically. It is found that the SRS intensity is strongly influenced by the type of the seed source. When the power amplifier is seeded by a normal fiber oscillator laser, the SRS threshold is much lower than the value estimated by Smith threshold formula. The maximum output power in the experiment is 1910 W, which is limited by SRS effect. While the power amplifier is seeded by a phase modulated single frequency laser, the maximum output power is 2400 W without observation of any SRS. Such phenomenon can be explained by the different time-domain characteristics of the two different types of seed sources. On the one hand, the detailed measurements are performed experimentally. On the other hand, a numerical model is proposed. Both the experiment and the model reveal that strong intensity fluctuations exist in fiber amplifier seeded by a fiber oscillator laser. These intensity fluctuations lead to a much higher SRS output. With these considerations in mind, the model predictions are in good qualitative agreement with the experimental data.