Stimulated Raman scattering suppression by intensified four-wave mixing process in narrow-linewidth fiber oscillator

Master-oscillator power-amplifier (MOPA) structure is a widely used construction for narrow-linewidth fiber lasers. However, stimulated Raman scattering (SRS) would be inevitably affected by the characteristic of pre-stage oscillator and enhanced during power scaling as well as long fiber transmission. In this paper, we demonstrated an effective method to mitigate the Raman noise induced after long fiber transmission of oscillator laser by intensifying four-wave mixing (FWM) process. A theoretical model is conducted based on nonlinear Schrodinger equation (NSE) and the power conversion equations. Numerical simulation results indicate that the oscillator with high reflection (HR) grating which owns a wider bandwidth is easier to inspire the FWM process during the laser transmission and tends to have a lower Raman ratio at the output, and in experiments a SRS suppression more than 15 dB is observed. This work should have significant reference for SRS suppression during the power scaling of high-power fiber amplifiers by master-oscillator power-amplifier (MOPA) structure.