Numerical Study On The Passive Mode-locking Of The Erbium-doped Fluoride Fiber Laser At ~3?m

³?m ultrashort pulse laser has wide application value and development prospects in military,medical,scientific research fields.Among them,³?m fiber laser has became an important development direction and research hotspot in the laser field owing to its high compactness,excellent heat dissipation capacity,high stability,high conversion efficiency,high beam quality and low system maintenance cost.At present,the research on passively mode-locked fiber lasers in the³?m is mainly focused on experiments,but the theoretical research on this aspect is still rare.In this paper,the theoretical research of passively mode-locked erbium-doped fluoride fiber lasers at³?m is carried out.The main contents include:?1?The Ginzburg-Landau?G-L?equation and coupled G-L equation describing the evolution of optical pulse propagation in gain fiber are derived.Then the mode-locking principle of passively mode-locked fiber laser based on saturable absorber?SA?and nonlinear polarization rotation?NPR?is analyzed,and the corresponding theoretical model of passively mode-locked fiber laser is established.?2?According to the theoretical model of passively mode-locked erbium-doped fluoride fiber laser based on SA,the passively mode-locked erbium-doped fluoride fiber laser based on SA at³?m is numerically studied.The effects of erbium-doped fluoride fiber length,small signal gain and SA saturation power on the output soliton pulse characteristics of SA passively mode-locked erbium-doped fluoride fiber lasers are investigated.The numerical simulation results show that a nonlinear Schr?dinger?NLS?soliton pulse with the pulse width of 1.0 ps and the peak power of 228 W can be obtained when the erbium-doped fluoride fiber length is 5.0 m,the small signal gain is 1.5 m^-1 and the SA saturation power is 400 W.?3?On the basis of the theoretical model of passively mode-locked erbium-doped fluoride fiber laser based on NPR,a numerical study of passively mode-locked erbium-doped fluoride fiber laser at³?m is carried out.The effects of three important parameters of erbium-doped fluoride fiber length,small signal gain and linear phase delay on the output soliton pulse characteristics of NPR passively mode-locked erbium-doped fluoride fiber laser are studied.The numerical simulation results show that,under the condition of erbium-doped fluoride fiber length of 5.0 m,small signal gain of 1.1 m^-1 and linear phase delay of 1.40?,the fiber laser can generate NLS soliton pulses,whose narrowest pulse width is 500 fs,maximum pulse peak power is 1.5 kW,and maximum output pulse energy is 0.75 nJ.