| Ultrashort pulse fiber lasers are widely used in various fields,such as optical communication,precision processing and medicine.Nowadays,it is still a hot topic to further improve the pulse energy and peak power of pulse in a fiber laser.All-normaldispersion fiber lasers have been widely studied with the higher energy pulse.The allnormal-dispersion fiber laser only contains positive second-order dispersion but no negative dispersion.It can output dissipative solitons and self-similar solitons with high energy in proper parameters.Self-similar pulses(“similaritons”)are parabolic pulses that can avoid the deleterious effect of the optical wave breaking and tolerate strong nonlinearity,and these advantages attracted to a lot of attentions.As has been studied before,similaritons are solutions of the nonlinear Schrodinger equation with gain.Many researches have shown that self-similar pulses can be generated in optical fiber amplifiers,dispersion-decreasing fibers,optical fiber lasers and so on.In this paper,the evolution and characteristics of self-similar parabolic pulses in a all-normal-dispersion passively mode-locked fiber laser are studied by numerical simulation,so as to find out the cavity conditions suitable for the evolution of parabolic pulses.The main research contents include the following three aspects:1.The model of all-normal-dispersion fiber laser is established,and the nonlinear schrodinger equation is solved.The parabolic pulse evolution in an all-normaldispersion cavity is simulated and the effects of the cavity length and output coupling ratio are analyzed.And then the effects of the filter on the output parabolic pulse in the all-normal-dispersion fiber laser are studied.The results show that the filters of different shapes have important effects on the parabolic pulse output of the all-normal-dispersion passively mode-locked fiber laser.The pulse generated by the laser under the supergaussian filter with order 4 can form a parabolic shape in the maximum range of gain saturable energy,and the parabolic fitting factor of the pulse of order 6 and order 8 can reach a lower value under the appropriate gain saturation energy.2.The effects of different pumping schemes on the parabolic pulse are numerically studied in all-normal-dispersion fiber lasers.And the effects of different gain saturable energy and gain bandwidth on the fitting factor,pulse width,spectrum width and pulse energy of the output parabolic pulse are studied.The results show that the laser can output parabolic pulse with higher energy in the backward-pumping scheme,while producing parabolic pulse with lower gain saturable energy in the forward-pumping scheme.The gain bandwidth of gain fiber in all-normal-dispersion cavity has little effect on the output characteristics of parabolic pulse.3.The influence of the parameters of the saturable absorber on the output pulse characteristics of the all-normal-dispersion fiber laser is researched.The fitting factor of the output pulse,the time-domain pulse width and the peak power of the all-normaldispersion fiber laser are obtained by solving the nonlinear schrodinger equation.Output pulse parameters of fiber laser with different modulation depth,saturation intensity and nonsaturable absorption are investigated.The simulation results show that the parameters of the saturable absorber affect the pulse duration,pulse peak power and pulse shape.The parabolic pulse with narrow pulse width and high power can be generated in the all-normal-dispersion cavity by optimizing the parameters of saturable absorber.The research results further enrich the research on the evolution of parabolic pulses in the all normal dispersion laser cavity and have important theoretical reference value for the design of the all-normal-dispersion self-similar fiber lasers. |
