Research Of Pulse Dynamics Mode-locked Thulium-doped Fiber Laser Based On Nonlinear Polarization Rotation

In recent years,the research on fiber laser has gradually developed towards long wavelengths.Among them,thulium doped fiber laser can output lasers with wavelengths around 2 ?m,which has attracted widespread attention.The mode-locking technology in thulium doped fiber laser is an important condition for generating ultra-short pulses.Although passively mode-locked fiber laser can achieve different mode-locked pulses,under the drive of rich and complex nonlinear dynamics,some pulse of special shape or other characteristics of pulse can be discovered by controlling the dispersion,nonlinearity,fiber gain and component loss and other parameters in the fiber resonator cavity.This thesis uses Nonlinear Polarization Rotation(NPR)mode-locked thulium-doped fiber laser as the experimental platform and dispersion management as the basis.The dynamic characteristics of the pulses of passively mode-locked fiber lasers with different dispersion are systematically studied.The main work is as follows:1.First,a negative-dispersion NPR mode-locked thulium-doped fiber laser is constructed,and a special shape h-type pulse with a repetition frequency of 19.8 MHz,a signal-to-noise ratio of 56 d B,and center wavelength near 1984.74 nm has been obtained.Unlike the traditional soliton,this pulse has rarely been reported.The single pulse energy is 6.7 n J,which breaks the limit of the traditional soliton area theory.Pulses are similar in shape to the letter "h" and are called h-like pulses.Further evolutionary analysis found that the characteristics of h-like pulses are similar to dissipative soliton resonance pulses.As the pump power increases,this type of pulse has free from wave-breaking,large pulse duration,and high pulse energy.2.Second,based on the Ultra-High Numerical Aperture(UHNA7)fiber as a dispersion compensation fiber,a NPR mode-locked thulium-doped fiber laser with zero dispersion and positive dispersion is constructed.When the laser operates in an anomalous region near zero dispersion,the experiment has obtained noise-like mode-locked pulses with a spectral width of 24.3 nm,a signal-to-noise ratio of 43 d B,and a repetition frequency of 6.88 MHz.Through the evolution analysis of the pulses,it is found that noise-like mode-locked pulses can split to generate harmonic mode-locked pulses,which indicates that noise-like pulses can not increase energy infinitely,and the pulses also split under certain conditions.When the laser is operating in the positive dispersion region,the experimental output of the dissipative soliton resonance pulse is obtained.The pulse evolves from a Gaussian to a flat-topped rectangular pulse with a pulse repetition frequency of 5.16 MHz and a signal-to-noise ratio of 51 d B.Experiments have verified that the pulses broaden with the increase of pump power without pulse splitting,indicating that under positive dispersion,the pulse can tolerate higher nonlinear effects and can achieve higher energy pulse output.3.Finally,in order to further study the pulse output characteristics under large positive dispersion,based on the tapered micro-nano fiber as a dispersion compensation fiber,a NPR mode-locked thulium-doped fiber laser with large positive dispersion is constructed.The waveguide dispersion of tapered micro-nano optical fiber and the fiber core have a certain relationship,especially in the 2 ?m band,controlling the diameter of the fiber core can obtain the larger and positive dispersion coefficient tapered micro-nano fiber,which can make up the deficiency of UHNA7 fiber.Moreover,the tapered micro-nano fiber has strong nonlinear effects,and local nonlinear effects will also be conducive to the study of nonlinear pulse dynamics.In the experiment,Q-switched mode-locked pulses and dual-wavelength mode-locked pulses are obtained at different pump powers.The analysis of pulse evolution found that when Q-switched mode-locked pulses evolved dual-wavelength pulses,the intensity of the spectral doublet peaks of the dual-wavelength pulses were not the same.As the pump power was increased from 3.7 W to 7.0 W,it was found that the peak intensity of long wavelengths gradually increased.And the depth of the depression between the two peaks is also deepened.When the pump power reaches 7.0 W,the peak intensity of the two wavelengths approaches uniformity.