Origin of SBS-induced mode distortion in high power narrow linewidth fiber amplifiers

Stimulated Brillouin scattering (SBS)-induced mode distortion (MD) in high power narrow linewidth fiber amplifiers has been implemented, and the origin has been investigated from the aspect of the evolution of the optical spectrum, spatial beam profiles, and temporal-frequency domain characteristics. It is shown that, following the onset of the backward giant pulses generated by SBS, forward giant pulses were generated, which reached multi-kilowatt level peak power and triggered the onset of stimulated Raman scattering (SRS). After the onset of SRS, the beam quality starts to degrade, and the beam profiles deteriorate obviously. It reveals that the SBS-induced MD is a two-stage physical process: SBS-induced forward giant pulses trigger the SRS effect, and then the SRS effect causes the beam deterioration of the signal laser, which means that SRS is the origin of the MD observed after the onset of SBS. To the best of our knowledge, this is the first revelation of SBS-induced mode distortion in high power narrow linewidth fiber amplifiers, which can facilitate the in-depth understanding and effective suppression of the complicated mode evolution phenomena.