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.