Study On The Characteristics Of Nonlinear Polarization Rotation Mode-locked Pulse In 2?m Thulium Fiber Laser

Thulium-doped fiber lasers work in the 2 ?m mid-infrared band,and their emission spectrum contains two atmospheric windows and a strong water absorption peak,so they are widely used in military and civilian fields such as infrared guidance,optoelectronic countermeasure,biomedicine,and remote sensing detection.With the continuous expansion of the application fields in the mid-infrared band and the rapid development of 2 ?m-band fiber devices,the thulium-doped fiber laser has also become a leading research focus at home and abroad.We have studied the pulse output characteristics of a thulium-doped fiber laser,mode locked by the nonlinear polarization rotation method.Based on square wave pulses,the mechanism of pulse formation is explored from positive and negative dispersion media,and then the generations of dissipative soliton resonance pulses,noise-like pulses,h-type pulses and multi-wavelength pulses are analyzed.First,a summary is taken for the development of fiber lasers and the latest research status of dissipative soliton lasers,based on thulium-doped fiber lasers.Second,the thulium ion's level structure and its laser pumping method are introduced.In addition,we deduced the nonlinear Schr?dinger equation from Maxwell's equation,and introduced the split-step Fourier numerical method in detail.Then,through the established numerical model,the generating processes of square wave pulses in the positive and negative dispersion regions are simulated and the morphological characteristics of the pulses of dissipative soliton resonance and noise-like square wave are studied.Finally,a bidirectionally pumped thulium-doped all-fiber laser is designed and built to achieve the output of square wave pulses and h-shaped pulses.The mechanisms of forming the h-shaped and the multi-wavelength pulses are analyzed.And some special pulse states,obtained in experiment,are explained from two aspects: higher-order dispersion and birefringence.In this dissertation,the modulation effect of the nonlinear polarization rotation for mode-locked pulses is found through numerical simulations in the positive and negative dispersion regions.From the commonality of the two kinds of square wave pulses,it is concluded that the balance mechanism of gain and loss in the dissipative system is dominant,and the negative feedback in the saturation-like absorption mechanism plays a key role in the formation of square wave pulses.From the difference between the two square wave pulses,it is found that the relationship between the self-phase modulation and the dispersion determines the internal structure of a pulse.A series of nanosecond square wave pulses are obtained in experiment.The obtained square wave pulse resonance phenomenon indicates that the laser works in the negative feedback region.Through an analysis,it is found that the formation of htype noise pulses originates from the higher-order dispersion,which results in asymmetric pulse shapes in some polarization states,and the fiber birefringence results in a multiple peak structure in the spectrum of square wave pulses.The higher-order dispersion will not only affect the pulse shape in the time domain,but also lead to the asymmetry(multiple peaks)in the frequency domain.The above results are of great significance in further studying the generation of square-wave-like pulses,improving the quality of pulses,enhancing the modulation of pulse signals,and expanding their applications.