| With the development of large mode field double clad fiber and high-power semiconductor laser technology,the output power of single fiber of fiber laser has reached the level of 10000 watts.With the increase of output power,mode instability in fiber lasers has become one of the main factors affecting the safe operation of fiber lasers.Aiming at the mode instability of high-power fiber laser,the thermal effect,thermal mode instability and Raman mode instability of fiber amplifier active fiber are simulated and experimentally studied in this paper.The finite element model of active fiber thermal effect is established,and the influence of pump wavelength on active fiber thermal effect is simulated.Compared with the 976 nm pumping wavelength,the absorption coefficient of the active fiber pumped by 973 nm is lower,and the pump light distribution in the fiber is more uniform,which effectively reduces the local thermal load of the gain fiber.The finite element model of thermally induced mode instability of fiber amplifier is established.The effects of pump mode and seed power on the thermally induced mode instability are simulated.The results show that the effect of thermally induced mode instability can be suppressed by back pumping and increasing seed power.A finite element model of mode instability induced by stimulated Raman scattering(SRS)in fiber amplifier is proposed.Firstly,based on the assumption of steady-state periodic heating,according to the classical heat conduction and laser rate equation theory,considering the influence of the change of thermal refractive index,gain distribution and SRS on the proportion of higher-order modes,the model of SRS induced mode instability in fiber laser amplifier is established A three-dimensional finite element model is established.A forward pumped laser amplifier with 25 / 400 μ m gain fiber is simulated.When the ratio of high-order modes is about 5%,the mode field is distorted and the signal light is coupled in the order of 100 milliseconds.A kilowatt scale fiber amplifier system is built to study the thermal effect,thermal mode instability and Raman mode instability of active fiber.Firstly,it is verified that the thermal effect of active fiber can be improved by proper pump wavelength and bidirectional pump mode,and the thermal mode instability can be suppressed by using back pump and increasing seed power.The time and frequency domain characteristics of mode instability induced by stimulated Raman scattering are analyzed when the intensity of Raman spectrum is 24 times lower than that of signal light spectrum The period of fluctuation is about 200 ms,which proves the explanation of the physical mechanism of mode instability caused by stimulated Raman scattering. |
