Stimulated Brillouin scattering induced mode degradation in high-power narrow-linewidth linearly polarized fiber amplifiers

•Present the first experimental results about the relationship between the Modal degradation (MD) and stimulated Brillouin scattering (SBS) in high-power continuous-wave (CW) narrow-linewidth (NL) linearly polarized fiber amplifiers.•For the first time, the time-frequency domain information analysis is added to the SBS and MD phenomena in the experiment, and the relationship between SBS and MD is completely studied in combination with the beam quality analyzer.•A near-diffraction-limited output beam without MI is achieved up to 760 W with M2x = 1.09 and M2y = 1.08, and polarization extinction ratio (PER) > 19 dB. In our high-power continuous-wave (CW) narrow-linewidth linearly polarized master oscillator power amplifier (MOPA) system, the mode degradation (MD) induced by stimulated Brillouin scattering (SBS) was observed. In the experiments, the beam quality maintained LP01 mode until the onset of SBS, and significantly deteriorated with the surging of SBS. Once the method of suppressing SBS was implemented, such as linewidth broadening and polarization extinction ratio (PER) weakening of the seed laser, the output laser mode recovered to fundamental mode, revealing the MD effect was evidently mitigated. Meanwhile, the associated relationship between MD and SBS was also investigated based on the analysis of spectrum, oscilloscope, and beam quality.