Theoretical And Experimental Study On Tunable Ytterbium-Doped Fiber Laser

Ytterbium-doped fiber lasers(YDFL)with the operation wavelength of 1μm have been widely used in optical communications,industrial processing and medical care,etc.With the wide application of laser in various fields,the demand for wavelength tunable and high-energy ultrashort pulse fiber laser is increasing rapidly.Due to its broad gain bandwidth,high optical conversion efficiency,and large saturation fluence of Ytterbium ion,YDFL can form a wavelength tunable pulse with wide range and high energy.In this thesis,wavelength-tunable continuous wave and dissipative soliton Yb-doped fiber lasers have been theoretically and experimentally studied.In addition,the parameters of the laser have been optimized by the numerical simulation method.The specific research contents and results are as follows:1.We introduced the development and classification of fiber lasers,reviewed the research progresses of wavelength-tunable,multi-wavelength continuous wave and mode-locked YDFLs,summarized several wavelength tuning methods and analyzed their merits and demerits.2.From Maxwell’s equations,we derived the nonlinear Schr?dinger equation for the optical pulse propagating in the single mode fiber and the Ginzburg-Landau equation for the optical pulse propagating in the gain fiber,and introduced a program solving the optical pulse propagation equation by using the split-step Fourier transform method.3.An all-fiber comb filter with precisely controlled channel spacing was proposed and demonstrated theoretically and experimentally.The fiber in one arm of a Mach-Zehnder interferometer(MZI)was drawn with a fiber flame-heated taper-drawing device,which has a precision of 1 μm.Therefore,the channel spacing was precisely controlled.Furthermore,the implementation conditions of the fusedtapering technique were analyzed,and the upper limit of single stretching length was obtained.4.A multifunctional Yb-doped fiber laser was demonstrated experimentally based on a tapered fiber variable attenuator and the MZI with precisely controllable channel spacing.When the pump power was relatively low,a single-wavelength lasing output with controllable tuning step was enabled,and the tunable range was 1026.7 to 1046.7nm.A laser output of multi-wavelength and various lasing lines was also made possible by increasing the pump power.It is found that the peak locations of the multi-wavelength laser can be shifted by simply bending the fiber taper.5.A nonlinear-polarization-rotation(NPR)mode-locked all-normal-dispersion YDFL was demonstrated.In the laser,by letting the MZI play as both the band-pass and the comb filter,we obtained a stable dissipative soliton.Furthermore,the wavelength of the dissipative soliton could be tuned from 1027.4 nm to 1044.8 nm by adjusting the polarization controller.Combined with the transmission function of NPR,the mechanism of wavelength-tunable was analyzed.6.By solving the Ginzburg-Landau equation,and then analyzing the graphene saturable absorbers with different layers and various surface properties numerically,we investigated the required minimum cavity dispersion(RMCD)and the output performance of the laser for the mode-locked YDFL,and the range of second-order group velocity dispersion(GVD)to form self-similar pulses.The results indicate that by using pristine monolayer graphene as the saturable absorber,the RMCD value of the laser was the smallest.In addition,the peak-power and pulse-width of the pulse and the dechirped pulse was the highest and the narrowest,respectively;the second-order GVD range of the self-similar pulses was the widest.Moreover,it is found that by reducing net cavity dispersion or increasing nonlinear effect,pulses with narrower width and higher peak power could be generated.