Based on the steady state rate equation and heat conduction equation,a finite element model of thermo optic coupling in high power fiber amplifier is established. This model is used to numerically study the mode instability induced by stimulated Raman scattering. For a forward pumped laser amplifier with a gain fiber of 25/400μm,the higher order modes of the laser signal light increases proportionally due to stimulated Raman scattering. When the proportion of higher order modes reaches about 5%,the mode field is distorted and the mode coupling of the signal light in the order of 100 ms appears which is consistent with the phenomenon of light intensity fluctuation that observed in the experiment. The simulation model provides an analytical method for studying the relationship between stimulated Raman scattering and mode instability in high power fiber amplifier.
LIANG Hui-sheng, YAN Ming-jian, ZHENG Yun-han, GAO Jun-feng, LI Yong-long, LI Fang-xin, ZHANG Xiao-yu, HAN Zhi-gang. Mode instability induced by stimulated Raman scattering in high power fiber laser[J]. LASER & INFRARED,2021,51(11):1440~1445