Cascaded Raman Fiber Laser Optimized Design

Raman fiber lasers (RFL) which base on the stimulated Raman scattering (SRS) in fibers, can generate laser over a wide range of wavelengths once appropriate pump sources are available. RFL can also be tuned in a wide-bandwidth to satisfy practical applications. Compared with laser diodes, as the best pump resources for raman fiber amplifiers ,cascaded Raman fiber lasers(CRFL) are more excellent due to theirs high output power and high coupling efficiency with fibers.This dissertation analyzes characteristics of the RFL numerically. Optimization configurations for different orders of CRFL have been found by solving the nonlinear coupled equations used to describe SRS in the fibers. The main contents of this paper are as following:Firstly, the basic theory of SRS is described and pump resources, gain mediums and types of cavity resonators for the CRFL are introduced.Second, a general structural model and nonlinear coupled equations of multi-order CRFL are presented, and the characteristics of the lasers are also analyzed by solving the coupled equations. The fourth-order Runge-Kutta method in combination with shooting method is put forward to deal with the coupled equations. A multi-dimension parallel shooting method is given to replace the general shooting method because of its better convergence and faster calculating speed. The simulating results demonstrate that the multi-dimension parallel shooting method is very suitable for solving the Raman coupled equations which are the representative of highly nonlinear problems.Third, for the coupled equations of Multi-order Raman fiber laser, the parallel shooting algorithms with faster speed of convergence may also make calculating processes divergent for the reason that the initial values for each Stokes power are all guessed arbitrarily. To overcome this problem, we propose that a method of adopting genetic algorithm (GA) which has great ability to find the optimal values in the global domain is employed to determine the initial values for each Stokes power. Then, the task of find more accurate values is finished by shooting methods since the searching ability of GA is weak in local region. Calculated results show that our scheme can solve the coupled equations effectively without the problem of divergence. The tolerance of our calculated values under the different genetic generation is given.At last, we simulated the characteristics of germanium-doped cascaded Raman fiber laser and phosphorus-doped Raman fiber laser, respectively, based on our proposed method. And the parameters of optimization of cascsded Raman fiber laser are presented.