Theoretical Study Of Ultrashort Pulse Mode-locked Fiber Laser Based On All Normal Dispersion

Ultrashort pulse laser has obvious advantages,such as broad spectrum width,narrow pulse width and high peak power etc.It is particular importance for the research and application.Recently,the ultrashort pulse lasers have been widely applied in micro-processing,medicine,communications,microscopic detection etc.Compared with the solid-state laser,the fiber laser has the advantages of low cost,compact structure and good heat dissipation.Passively mode-locked lasers operating in anomalous dispersion region can provide stable mode-locked pulse train.But the accumulation of nonlinear phase shift in fiber limits the pulse energy.As a result,for the traditional soliton mode-locked fiber laser,the pulse energy is very low because increasing energy of pulse will lead to pulse breaking.Furthermore,the pulse energy can be increased for the mode-locked fiber lasers operating with normal dispersion.In this case,the peak power of pulse is reduced by pulse broadening,which can lead to a deceasing of the nonlinear phase shift.In this paper,we mainly study the passively mode-locked fiber laser operating in the normal dispersion region.The contents are as follows:First,in order to discuss the pulse characteristics of passively mode-locked fiber lasers both in normal and anomalous dispersion regions,the soliton mode-locked fiber laser,the dispersion managed soliton mode-locked fiber laser and the self-similar pulse mode-locked fiber laser operating at 1550nm are designed.In the above cases,the differences of pulse width,pulse energy and peak power of the output pulses are analyzed by numerical simulations.The results show that the self-similar pulse mode-locked fiber lasers with normal dispersion has advantage of increasing pulse energy.Second,we design the Yb doped mode-locked fiber laser operating at 1060nm with normal dispersion.Furthermore,the characteristics of output pulses from fiber lasers are detailedly analyzed by discussing the influences of dispersion,nonlinearity,small signal gain coefficient and cavity length on pulse width,pulse energy and peak power.Third,since the filter plays a special role in the process of pulse evolution.It is necessary to analyze the influences of the filter on the characteristics of normal dispersion mode-locked fiber laser.In this paper,by introducing band-pass gaussian filter and birefringence filter in the mode-locked fiber lasers,the pulse characteristics are discussed with initial signals of noise and smooth pulse,respectively.Furthermore,through optimizing cavity structure and adjusting relevant parameters,the stable pulse output can be achieved in all-normal mode-locked fiber lasers.