Research On Generation Mechanisms Of Soliton Rain And Output Performances In Thulium Doped Fiber Laser

Tm-doped fiber laser(TDFL)has high conversion efficiency,good beam quality,and good heat dissipation.Its output wavelength is in the atmospheric window,and it is safe for human eyes.It can be widely used in medical diagnosis and treatment,ultra-fine material processing,and mid-infrared pump source,lidar and free space communications.In recent years,TDFL has made rapid progress in multi-wavelength,ultra-high power,and ultra-short pulses.However,there is few studies on the dynamic behavior and output characteristics of the soliton,and the exploration of the nonlinear dynamic mechanisms in its formation is not completed.In the research work,by using the nonlinear polarization rotation(NPR)technology,the fundamental frequency mode-locked pulses,multiple soliton and strange soliton rain pulses are obtained in the passively mode-locked TDFL.Combining theoretical simulation and experiment,the dynamic characteristics of soliton rain formation are explored.With the help of the composite filtering mechanism of NPR and polarization controller-thulium doped fiber(PC-TDF)structure,the mode competition in the homogeneous broadening gain medium is effectively suppressed,and the multi-wavelength output is achieved.The multi-wavelength output performances are investigated in detail,and the experimental results and simulation results are also analyzed and discussed.The research results in this thesis are helpful to understand the dynamic evolution and generation mechanisms of passive mode-locked fiber lasers.The main research contents of the thesis are as follows:1.The theoretical model of thulium-doped fiber laser based on nonlinear polarization rotation.Using the coupled Ginzburg-Landau equation and setting appropriate simulation parameters,the different types of soliton pulses,including single soliton,multiple soliton,soliton molecule and noise-like soliton,etc.are obtained.The soliton rain phenomenon is a weak mode-locking state,which often appears in the gradual transition process from continuous wave to mode-locking.By optimizing the simulation parameters,various singular soliton rain modes are achieved.The influence of polarization orientation on the evolution of soliton rain and its spectral characteristics are analyzed.The data information related to the internal structure and dynamic evolution of the soliton rain are used to carry out the theoretical simulation,the movement speed and direction of the floating soliton are obtained,and the behavior of the floating soliton is analyzed.2.Based on the NPR technology,the passive mode-locked TDFL laser is built to study the output performances of soliton rain pulse.By optimizing the cavity parameters,stable continuous wave mode-locking pulse,constant-amplitude pulse and non-constant-amplitude pulse are observed,and the pulse output characteristics and the pulse splitting are analyzed.After adjusting and optimizing the cavity parameters,the soliton rain patterns of different shapes are obtained,and the pulse behavior characteristics and evolution process are analyzed.The fundamental frequency,second and third harmonic soliton rain pulses are realized by appropriately increasing the pump power.The influences of pump power,cavity length and polarization orientation on the output characteristics of soliton rain are detailedly studied,and the physical mechanism is given.3.A multi-wavelength TDFL based on NPR effect and PC-TDF structure are designed.The PC-TDF structure is constructed by using the fiber birefringence effect and the polarization hole-burning effect,the mode competition in the homogeneous broadening gain medium is effectively suppressed.The composite filter is composed of NPR and PC-TDF structure,and its filtering mechanism is analyzed theoretically.With the help of theoretical model,the factors of affecting the filtering period bandwidth and tunability are investigated.The multi-wavelength output characteristics of the non-PC-TDF structure and the PC-TDF structure are analyzed,and stable single-wavelength,dual-wavelength,and three-wavelength laser are obtained in the composite filter mechanism TDFL,and the tuning operation of the channel spacing and wavelength difference are realized.