| Ultrafast pulse has been widely applicated in precision measurement and detection,communication and information processing,laser medical treatment and ultrafast optical diagnostics,due to its superior characteristics,such as high peak power,narrow pulse width and large pulse energy.Mode-locked fiber laser is an attractive platform for generating ultrafast pulses.In addition,it has been widely studied because of its simple structure,low cost,high reliability,good beam quality and easy miniaturization.The performance of saturable absorbers(SAs)has a direct influence on the output characters of mode-locked lasers.In recent years,relying on their superior performance,SAs based on single-welled carbon nanotubes(SWNT)and graphene have been widely used in passive mode-locked fiber lasers,which provides the guarantee for the development of ultrafast pulse lasers.Inspired by the research on graphene,the optical properties of layered metal dichalcogenides also attract much attention of researchers.In this dissertation,we focus on the preparation methods of SAs based on carbon nanotubes,graphene and serval layered metal dichalcogenides,and we experimentally study the output pulse characteristics of passively mode-locked lasers based on these SAs.Bound soliton fiber laser,switchable dual-wavelength soliton mode-locked fiber laser,near-and mid-infrared mode-locked fiber lasers,and mode-locked fiber laser of high-energy rectangular pulse have been realized by different cavity structures,SAs,and net-cavity dispersion.The main research contents of this dissertation are as follows:1.Firstly,switchable-wavelength passive mode-locked fiber laser was achieved by SWNT SA,with the help of chirp fiber Bragg grating(CFBG).Then,conventional solitons(CSs)and bound-state solitons in an anomalous dispersion linear cavity fiber laser were obtained based on the mixture of graphene and SWNT.By adjusting the polarization state of light wave and pump power,the evolution from CS to bound-state CS mode locking operation was studied.In addition,output characteristics of two and three bound-state CSs were also observed.Through theoretical model of passive mode-locked fiber laser,the numerical simulation of the evolution process was completed.2.By optimizing the preparation method of graphene SA,evanescent field interaction scheme of the propagating light with gaphene was proposed.The optical damage threshold of SA based on this scheme was greatly improved.Based on the prepared graphene SA,CSs were obtained in passive mode-locked fiber laser at 1.5 mm wavelength.In addition,switchable dual-wavelength passive mode-locked fiber laser was achieved because of the variation of the gain spectrum of gain fiber with pump power.3.With the help of micro-nano operating platform,Tungsten Disulfide(WS2)-clad microfiber SA was prepared under a microscope,and the SA was utilized to achieve passive mode-locked fiber laser.The CSs were obtained in a large negative dispersion erbium-doped fiber laser whose energy was limited to 0.1 n J.Increasing the pumping power,high-energy rectangular pulses were also achieved in this laser system.As the pump power further enhancement,the pulse width and energy of the rectangular pulses were increased linearly without pulse split or pulse shape distortion.The broadest pulse duration of about 130 ns and the maximum pulse energy of about 90 n J were obtained at the pump power of 800 mw.4.The nonlinear optical responses of SnS2 were investigated,and the SnS2–based SA was applied in fiber lasers at 1,1.5,and 2 mm wavelengths to generate pulse.Firstly,Sn S2 film made by Chemical vapor deposition(CVD)method was covered on D-shaped fiber,which realized the interaction between evanescent field of the propagating light and gaphene and improved the optical damage threshold of Sn S2 SA.CSs with Kelly sidebands were obtained in erbium-and thulium-doped fiber laser.And dissipative solitons(DSs)were also achieved in all-normal-dispersion ytterbium-doped fiber laser. |
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