Study On Output Characteristics Of Nonlinear Polarization Rotation Mode-locked Thulium Fiber Laser

The thulium-doped fiber laser operating in the 2 ?m band is located in the atmospheric transparent window and has a strong water absorption peak,which is widely used in industrial processing,laser medical treatment,military and defense and so on.It is also an ideal platform to study the dynamics of complex pulses in fiber lasers.Based on simulation and experiment,the output characteristics of the thuliumdoped fiber laser mode-locked by nonlinear polarization rotation are studied in this paper.The generation and properties of noise-like rectangular and h-shaped pulses are numerically simulated.The output characteristics of rectangular and h-shaped pulses are studied experimentally,on which,some special pulses such as chair-like pulses,step-like pulses,pulse bundles and bundle groups are obtained and studied.The main contents of this paper are as follows:(1)Based on the principle of nonlinear polarization rotational saturable absorber,the transfer matrix and transmittance curves are introduced,and the transmittance curves under different polarization states are analyzed in detail.The conditions for realizing mode locking and the principle of peak power clamping effect are given.At the same time,the coupled Ginzburg-Landau equation considering nonlinear polarization rotational mode-locking structure is presented with the split-step Fourier method.On this basis,the formation and evolution of noise-like rectangular and hshaped pulses are simulated.The formation mechanism of h-shaped pulse,specially the effects of pump power and polarization,is analyzed emphatically.(2)The properties of rectangular and h-shaped pulses generated by thulium-doped fiber laser are investigated experimentally.The effects of pump power,single-mode fiber's length and output coupling ratio on the output characteristics of rectangular pulse width,average output power,pulse energy and peak power are mainly studied,as well as the influences of pump power and polarization on h-shaped pulses,which is consistent with the simulation results.(3)Chair-like pulses,step-like pulses,pulse bundles and their bundle groups are generated in passively mode-locked thulium-doped fiber laser in the 2 ?m band,and the properties of these pulses are studied in detail.It is found in experiment that all these three special shaped pulses can be controlled by changing the pump power and adjusting the polarization controllers.