Theoretical And Experimental Study Of Thulium-doped Fiber Laser Based On MoS₂

Lasers emitted near 2?m have attracted much attention for versatile applications in military,sensing and surgery etc.Due to the advantage of high efficiency,cavity stability,easily pump,high power laser near 2?m can be obtained by thulium-doped fiber laser.With the development of passive mode-locking techniques and new-type two-dimension materials,mode-locked thulium-doped fiber has been studied widely.Compared with traditional single-mode fiber,large-mode-area double-cladding fiber can reduce stimulated Raman scattering and obtain high power output.This thesis focuses on the large-mode-area double-cladding thulium-doped fiber laser in theory and experiment.The main research works are summarized as follows:?1?The energy level and main pump methods are introduced,and advantages and disadvantages of three pump methods are compared and discussed.The theoretical model based on the 793nm pumped mode is established.The nonlinear Schrodinger equation in mode-locked thulium-doped fiber laser based on the saturable absorber is given and solved with split-step Fourier method.?2?The continuous-wave output characteristics in thulium-doped fiber laser are simulated base on the Rate Equation of Tm³⁺.The output parameters of three pump methods including backward-pumped,forward-pumped and double-end-pumped are compared.The results show that double-end pump is the best pumped method.Then the relationship between small signal gain and doped-fiber parameters with output is analyzed,which shows that there exists an optimum length of doped-fiber and doping concentration for maximum output.The processing of mode-locked pulse generation is simulated based on the nonlinear Schrodinger equation.The impact of saturable absorber parameters and dope-fiber parameters on average output power,pulse width,peak power is discussed.?3?A large-mode-area double-cladding thulium doped fiber laser is setup in experiment with pump of 793 nm laser diode.With the pump power of 8.32 W,the maximum continuous wave output power of 3.17 W is obtained which corresponds to a slope efficiency of 48%,and the central wavelength is 2037 nm.Thin film of MoS₂is prepared.Different MoS₂ based on different dispersant of solution dispersion uniformity is compared,then 1-methyl-2-pyrrolidone?NMP?is selected as the solvent.Using liquid chromatography preparation of MoS₂ saturable absorber,the optical properties of MoS₂ with different ultrasonic time is tested.The MoS₂ saturable absorber is inserted into the cavity for mode-locking.However,only continuous-wave operation is observed instead of Q-switching or mode-locking,which is probably due to that the parameters of doped fiber are not proper,which leads to a lower gain in cavity and does not reach the required fluence of MoS₂ saturable absorber thus Q-switching or mode locking cannot be stared.Finally maximum output power of2.84 W is obtained with pump power of 8.32 W with a slope efficiency of 41.8%,the dual-wavelength of 2048 nm and 2067 nm is observed in experiment.