Theoretical Research On P-doped Cascaded Raman Fiber Lasers

With the rapid development of the global communications, the ultrahigh capacity of a new generation of optical fiber communication system is demanded urgently. Therefore, optical amplifiers are encountering a great challenge as one of the vital components of optical communication system. Currently, as a new generation of amplifier, Raman fiber amplifier has been thought as an ideal and potential solution. However, the key problem is how to find a suitable Raman pump source. Due to Raman fiber laser with high power output and the flexible lasing wavelength, it can perfectly meet the requirements of Raman pump sources.Based on the stimulated Raman scattering in fibers, Raman fiber laser can generate laser over a wide range of wavelength once appropriate pump sources are available. With the help of model building of cascaded Raman fiber laser, the nonlinear coupled equations which used to describe stimulated Raman scattering in fibers are analyzed and solved, also compared and contrasted the solution with the numerical methods and experimental results. The main contents of the paper are as follows:Firstly, the basic theory of stimulated Raman scattering is introduced in detail. Model building of cascaded Raman fiber laser and the nonlinear coupled equations with boundary conditions are given.Secondly, the approximate explicit solution of optical output powers for P-doped cascaded Raman fiber laser were acquired by linear model. With the aid of average powers and gain factors, the differential equations depicting the distribution of pump and Stokes radiations along Raman fiber were simplified to the algebraic equations. Therefore the approximate optical output powers were got by linear method and Lambert-W function. Then the output power curves in the whole range of input pump powers were assumed to be piecewise linear, so the relative error expression was obtained. The simulation showed that the assumption was reasonable in the large range of the fiber length and the reflectivity of fiber grating. The results provided better guidance to simplify the calculation and design on the cascade Raman lasers.Lastly, the second-order approximate solution of P-doped cascaded Raman fiber laser was obtained by nonlinear method. It was gained by adopting second-order nonlinear method in gain factor. Then a detailed introduction was given to numerical methods. Comparing second-order solution and linear explicit solution with numerical simulation and experimental data, the results showed that the second-order method was more accurate than the linear method. It could provide better guidance to the experimental design aiming at dual-wavelength outputs and the method effectively improved operation speed. It was superior to general numerical methods.