Suppressing Nonlinear Refraction Effects In High-power Fiber Laser Based On Phase-shifted Long-period Fiber Grating

Recently,narrow-linewidth fiber lasers have attracted more and more attention due to their good temporal coherence,high beam quality,compact structure and low noise.However,with the increase of laser power,nonlinear refraction effect(such as SPM,self phase modulation)has become one of the main factors hindering the development of narrow-linewidth fiber lasers to narrower linewidth.Therefore,suppressing SPM in narrow-linewidth fiber lasers has become an important issue to be solved.Therefore,aiming at this issue,a method of utilizing phaseshifted long-period fiber gratings(PS-LPFGs)was proposed to suppress the SPM in fiber lasers.In this paper,the influence of SPM on output spectrum of the fiber laser was analyzed based on the structure of the fiber laser and the generation mechanism of the SPM.In order to design a PS-LPFG with nonlinear effect suppression function,the mathematical model of LPFG was analyzed based on mode-coupled theory,and then the influence of phase shift on PS-LPFG spectrum was analyzed.Further,based on mode-coupled theory,the influence of parameters such as phase shift size,phase shift position,periods number,index modulation amplitude and period on the transmission spectrum of PS-LPFG was simulated.Finally,based on theoretical model and simulation results,a PS-LPFG with nonlinear effect suppression function was designed.The UV point-by-point inscription system with online monitor system was designed and built in this paper to realize the lithography of the PS-LPFG.In the process of PS-LPFG inscription,the influence of period number,index modulation amplitude and period on the transmission spectrum of PS-LPFG was studied.Simultaneously,a pcocess combining constant-low-temperature and dynamic-high-temperature was applied on the PS-LPFG to enhance the power durability.Experimental results showed that the thermal slope of PS-LPFG decreased from 0.29℃/W to 0.03℃/W,which greatly improved power durability of PS-LPFG.To verify the suppression ratio of PS-LPFG in continuous wave(CW)laser,the test systems of oscillator and Master Oscillator Power Amplifier(MOPA)fiber laser were built respectively.PS-LPFG was applied to two kinds of fiber lasers for experimental verification.Experimental results showed that the SPM was effectively suppressed by PS-LPFG,and the output linewidth of the fiber laser was optimized.The 3d B bandwidth,13 d B bandwidth and30 d B bandwidth of the oscillator are narrowed by 2.0%,12.8% and 19.7% respectively.The3 d B bandwidth,13 d B bandwidth and 30 d B bandwidth of the output spectrum of MOPA laser are narrowed by 2.0%,25.5% and 38.0% respectively by PS-LPFG.