Raman Oscillation Induced by Weak Internal Reflections of Fiber Devices in High-Power Fiber Amplifiers

Stimulated Raman scattering (SRS) is one of the dominant restrictions in power scaling of high-power fiber lasers with diffraction-limited beam quality. In this work, the phenomenon of Raman oscillation induced by weak internal reflections of fiber devices in high-power fiber amplifiers are demonstrated experimentally and theoretically. The experimental results show that layouts of the two fiber devices impact SRS in a high-power fiber amplifier. Further experimental measurements reveal that weak internal reflections of fiber devices are important sources of end-facet reflections. A spectral evolution model is proposed and verifies the impact of weak end-facet reflections on SRS in high-power fiber amplifiers. Further theoretical analysis confirms the phenomenon of Raman oscillation, which is a practical Raman threshold of high-power fiber amplifiers. The overall experimental and theoretical results could provide a well reference for the optimal design of SRS-limited high-power fiber amplifiers.