Study On Passively Mode-locked Fiber Lasers Based On High Modulation Depth Of The Saturable Absorber

Passively mode-locked fiber lasers is currently one of the hot topics in the study of ultrafast laser for their widespread applications in optical fiber communication,remote sensing, laser radar, spectral analysis and the military, due to there good stability, high efficiency and compactness. In order to achieve high peak power and ultrafast mode-locked laser, the laser cavity and optical element requires careful considerations. Among them, the saturable absorber is the key device in passively mode-locked fiber laser, which requires that the nonlinear materials with controllable nonlinear absorption ability, fast response and low loss. In addition, the modulation depth is also one of the key parameters of saturable absorber. The high modulation depth can enhance the filtering effect on the pulse edge, which is beneficial to obtain ultrashort pulse, but also can improve the stability of the ultrafast lasers. This thesis focuses on how to obtain a short pulse and stable passively mode-locked fiber laser.The influences of the high modulation depth of saturable absorber of passively mode-locked fiber laser on the output characteristics have been studied theoretically and experimentally. The main work is as follows:First, numerical simulations of the effect of saturable absorber parameters on passively Er-doped mode-locked fiber laser output characteristics have been carried out, and ultrafast picosecond laser output has been obtained based on topological insulators saturable absorber. Based on Ginzburg-Landau equation, the influences of the saturable absorber on the output characteristics of passively mode-locked fiber laser were investigated numerically. Based on Bi2Te3 saturable absorber, a passively mode-locked Er-doped fiber laser was designed and built to realize the stable soliton pulse output. When the pump power to reach 85.4 m W, the fiber lasers can self-start,and when the pump power was adjusted to 500 m W, the output pulse with repetition rate of 13.14 MHz, pulse width of about 505 fs can be obtained. By adjusting the polarization controller, it can be observed the soliton pulse laser output central wavelength can be continuously tuned between 1570.45 to 1572.32 nm.Second, the laser output characteristics of the passively mode-locked thulium doped fiber with nearly zero dispersion have been investigated numerically, which provides the guidance to optimize the design of mid-infra red ultrafast fiber laser.Based on Ginzburg-Landau equation, the effect of the saturable absorber parameters(modulation depth, saturation power and non-saturable loss, etc.) on passively mode-locked fiber laser output pulse characteristics has been studied. With the net cavity dispersion of-0.008 ps2, the stable soliton pulse output can be obtained. The numerical simulation results show that the high modulation depth, low saturation power of saturable absorber will be helpful to get narrower soliton pulse.