Theoretical analysis of the SRS-induced mode distortion in large-mode area fiber amplifiers.

In this paper, the spectral evolution properties of different transverse modes with the stimulated Raman scattering (SRS) effect are analyzed in large-mode-area (LMA) fiber amplifiers for the first time. Both the ratios of laser power in Raman Stokes light and high order modes (HOMs) can be calculated through the comprehensive analysis of transverse mode competition and nonlinear transverse mode coupling processes. The theoretical study reveals that SRS-induced inter-modal wave mixing (IM-WM) effect would transfer power from signal light in LP mode to Raman Stokes light in LP mode and lead to the onset of the mode distortion phenomenon in high-power LMA fiber amplifiers. Different from the traditional thermal-induced mode instability (MI) phenomenon, the SRS-induced mode distortion could occur by just with the contribution of quantum noise.