Analytical study on the stimulated Raman scattering threshold in distributed-pumped fiber amplifiers

In this paper, the stimulated Raman scattering (SRS) threshold of the distributed-pumped fiber amplifier consisting of multiple sub-amplifiers is analytically studied based on the rate-equation model. The SRS threshold formula is presented where the predictions agree well with the numerical results. With the help of the formula, the effect of the pumping scheme on the SRS suppression of the distributed-pumped fiber amplifier is systematically investigated. It is found that with a given number of pump injection ports, the SRS suppression can be optimized by making only some of sub-amplifiers counter-pumped with the others bi-directional-pumped. It is also suggested that the first sub-amplifier should be adopted to be counter-pumped for suppressing SRS if only one sub-amplifier can be single-directional-pumped. The variation of SRS threshold with the number of sub-amplifiers is also discussed in detail. We believe that these results can provide significant guidance on understanding and designing distributed-pumped fiber lasers and amplifiers.