2?m Pulsed Fiber Laser Based On Multimode Interference Effect

In recent years,with the rapid development of 2?m mode-locked fiber lasers,it has great application potential in the fields of biomedicine,laser ranging,non-metallic material processing,infrared optical frequency synthesis and high-speed optical communications.The research of 2?m mode-locked fiber lasers has attracted great interest because of the advantages of good power expansibility and high efficiency in the wide spectral laser gain range of 1.8?m to 2.1?m.However,in the resonant cavity of the all-fiber-based 2?m laser,there are relatively few studies on the mode-locking operation of the saturable absorber based on the multimode interference effect of the fiber.Therefore,this article has carried out the research of 2?m pulsed fiber laser based on multimode interference effect,and conducted experimental analysis on the generation methods of various ultrashort pulses.The main research contents include:1.A new passive mode locking method-multimode interference mode locking is studied.A single-mode-multimode-single-mode(SMS)structure is constructed by welding a short gradient refractive index multimode fiber in a single-mode fiber laser.The multimode interference effect of the structure is used to realize a saturable absorption mechanism and finally complete mode locking pulse operation.2.2?m thulium-doped fiber laser based on a single SMS structure is built to realize the generation of traditional soliton pulse and soliton molecular pulse.Mode locking requires precise control of the length of the multimode fiber.By winding the multimode fiber into the polarization controller,it can achieve a function similar to a saturable absorber.By studying the influence of the length of the multimode fiber on the mode-locking performance,it is shown that the SMS mode-locking structure can effectively reduce the influence of the length of the multimode fiber on the mode-locking performance.Finally,a stable traditional soliton pulse and soliton molecular pulse are realized in a single SMS structure.When the pump power is 256 m W,the center wavelength of the traditional soliton spectrum is 1887.60 nm,the 3 d B bandwidth is 3.83 nm,the pulse width is 3.18 ps,and the tunable range of 25 nm is realized at the same time.When the pump power is 389 m W,a soliton molecule with the center wavelength of 1888.12 nm and a modulation period of 2.72 nm is generated.3.2?m thulium-doped fiber laser based on a dual SMS structure is built,and the influence of a polarization controller wrapped with a graded index multimode fiber on the mode-locking performance of the SMS structure is studied.Combining theoretical analysis and experimentation,the annular cavity is improved to suppress the phenomenon of pulse instability.Finally,stable traditional soliton and bound state pulse are generated in the dual SMS structure.When the pump power is 390 m W,the center wavelength of the traditional soliton pulse is 1987 nm,the pulse width is 3.39 ps,and the 3d B bandwidth is 4.24 nm.When the pump power is 497 m W,the center wavelength of the double pulse bound state is1984.28 nm,and the modulation period is 1.75 nm.