Investigation On SBS In Fiber Applied In High-power LD-pumped MOPA System

In the modern military field, the solid-state lasers have been widely used as the excellent direction, small divergence angle and high energy. Solid-state lasers have many advantages, such as high output energy, high peak power, compact structure, easy for coupling, long life, mature technology of microstructure and so on. There are usually a large crystal and high-power pump source which are used to increase energy storage in the high-power solid-state laser systems, but it causes that a lot of heat remain in the gain medium which can not be cooled down in time. The remained heat results severe thermal effects and the quality of output beam will become bad. SBS phase conjugation can be used to compensate for the phase distortion in the high-power solid-state lasers. As the long length of fiber and high energy density in the fiber, the SBS threshold can be significantly lower and beam can obtain high fidelity. Compared to traditional SBS media which liquid and high-pressure gas are easily volatilized and leaked, fiber is not only convenient, but also non-toxic and pollution-free, good flexibility in line with the miniaturization of laser design.In order to search the best SBS parameters of pump and fiber structure, we should study the output characteristics with the variation of the parameters, and the SBS threshold energy changes with the variation of the system parameters. This requires building a theoretical model of SBS in fiber and solving the SBS coupled-wave equations in numerical methods. This paper can be divided into two parts, firstly the theoretical research and numerical simulation of SBS in fiber; secondly the corresponding thermal affects theory and numerical simulation.Based on the coupled-wave equations of SBS, series of reasonable approximations and simplifications of the coupled wave equations of Stimulated Brillouin Scattering in fibers are carried out and a transient two-dimensional model is built. We get the discrete coupled-wave equations by implicit finite difference method in time domain and backward difference method in space. By c programming language and matlab, we get the numerical simulation of the equations and the output characteristic of SBS in common fiber, tapered fiber and extreme tapered fiber. In addition, the rationality of the model is verified by comparing simulation results with experimental results which was engaged by our group member, Dr. Zhao Zhigang, in Institute of Applied Electronics, China Academic of Engineering Physics.This paper also uses SBS coupled-wave equation to get the three-dimensional distribution of light filed in fiber. Based on the data, thermal effects are established and calculated. The temperature distributions in the three kinds of fiber have been analyzed respectively and we design the cooling correspondingly to expect a higher output power and higher beam quality.