Study Of Raman Self-frequency Shift Of Femtosecond Solitons In Optical Communications

Propagation of picosecond solitons in fibers is well described by the nonlinear Schrodinger(NLS) equation.However, for sub-picosecond and femtosecond pluses,effects of the nonlinear dispersion is so significant that it can no longer be treated at a perturbation. The equation including the nonlinear dispersion, i.e. the modified nonlinear Schrodinger(MNLS) equation, is a better model.In this thesis,we take MNLS solitons as a model for femtosecond soltions.Then based on the recently developed direct perturbation theory for perturbed MNLS equation,we study the Raman self-frequency shift of femtosecond solitons and its suppression by the means of frequency filters.For this purpose,first,we define the physical parameters of MNLS solitons; next,through the direct perturbation theory for perturbed MNLS equation,under adiabatic approximation,we obtain the evolving parameters of MNLS soliton on our questions and then choose the parameters of frequency filters based on it;at last,we directly numerically simulated our questions by Split-step Fourier(SSF) Method after improving the arithmetic of the differentiation of the nonlinear item for satisfying MNLS equation.Our treatment shows in the Raman effect case,MNLS solitons,the same as NLS solitons,keeps its energy,initial center and initial phase unchanged.However,the Raman effect,not the same as NLS solitons.reduces its amplitude,widens its width and under the same conditions,the self-frequency shift of MNLS soltions is closer to the result of numerical simulation.In the Raman effect together with frequency filters case,MNLS solitons,the same as NLS solitons,keeps initial center and initial phase unchanged and bandwidth-limited frequency filters can make the mean frequency of MNLS soltion stand a steady value at the red side of the initial mean frequency, i.e. suppress the self-frequency shift of MNLS soltions.And that,the otherphysical parameters of MNLS soltion last stand a steady value.However under the same conditions,the steady value of the mean frequency of MNLS soltions is closer to the initial mean frequency and the result of numerical simulation.