Passively Mode-Locked Fiber Lasers Based on Nonlinearity at 2-μm Band

2-μm fiber lasers are of wide potential applications in the fields of radar, sensing, and free-space communications. We introduced a simple approach to generate passively mode-locked pulse in thulium-doped fiber lasers based on nonlinear polarization rotation (NPR) in our previous work. The high repetition rate of 1.78 GHz is obtained by both NPR and semiconductor saturable absorption mirror. By using a polarization-maintaining fiber in the experiment, a birefringence Lyot filter as a fiber comb filter can be constructed, and then the tuning range of 94 nm can be obtained for the mode-locked laser. In this paper, we also demonstrated our recent research work, a square-wave mode-locked fiber laser at 2-μm band based on nonlinear amplifying loop mirror. The square-wave noiselike pulse (NLP) at longer wavelength of 2-μ m band can be observed. With the increase of the pump power, pulse width can be increased from 0.75 to 1.3 ns. The polarization vector characteristics of the square-wave NLP were analyzed experimentally. In addition, with the increase of the pump power, a kind of square-wave mode-locked pulse with low-intensity multipulse bunches can be observed, and these multipulse bunches can be operated at the multipulse mode-locked state. The side-mode suppression ratio decreased from 50 to 30 dB.