Passively Q-switched Fiber Laser Based On Two-dimensional Atomic Crystal

Passively Q-switched fiber lasers have been widely used in optical communications,optical sensors,biological technology,industry processing,and military for its high beam quality,high stability,high energy,and simple structures.Among the passively Q-switched fiber lasers,the saturable absorber is the most important component.It is indispensable for the saturable absorber to have broadband operating wavelength,fast response and low loss.At present,the most commonly used saturable absorber is the semiconductor saturable absorber mirror(SESAM),which has adjustable modulation depth and saturable energy by optimizing the structures.However,the operating wavelength of SESAM is limited and the complicated preparation makes them expensive.With novel nanomaterial emerging,the excellent nonlinear properties of carbon nanotubes have aroused researchers'interest.By the following investigations,it is found that carbon nanotube has low modulation depth and low damage threshold.The carbon nanotube can have broadband absorption by introducing nanotubes with different diameters and chirality,which determines the band gap of the nanomaterial.Two dimensional materials,a novel kind of material,have excellent electronics,optical,mechanical and nonlinear optical characteristics.By introducing them into the fiber laser,Q-switched or mode-locked fiber laser could be obtained.Two dimensional atomic crystals are the material with single atomic layer,like graphene,or few atomic layers.In the thesis,the nonlinear optical properties of graphene and topological insulator have been investigated.Based on the unique nonlinear saturable absorption,the passively Q-switched fiber lasers have been investigated.The experimental results are summarized as follows.Firstly,the dual wavelength Q-switched Er-doped fiber laser with wavelength spacing 0.8 nm has been realized based on graphene saturable absorber and cascaded fiber Bragg gratings.By adjusting the polarization controller,the dual wavelength operating regime can be switched to single wavelength 1542.9 nm or 1543.7 nm.The method provides an easy method to obtain Q-switched fiber laser with narrow wavelength spacing.The cascaded fiber Bragg grating can make the multi-wavelength operation easier,and the graphene can make the pulsed laser more stable.Secondly,the nanosecond Q-switched fiber laser has been obtained by using topological insulator as saturable absorber.By shortening the cavity length,and optimizing the preparation method of saturable absorber,the Q-switched pulse with minimum pulse width 217.6 ns,repetition rate 177.7 kHz,average power 1.33 mW can be obtained.As we known,it is the narrowest Q-switched pulse obtained based on topological insulator saturable absorber.Thirdly,the all fiber passively Q-switched fiber laser has been obtained by evanescent field coupling mechanism.By optimizing the tapering process and light deposition method,the saturable absorber can be prepared.By nonlinear measurements,it is found that the saturable absorber has excellent nonlinear absorption ability.By integrating the saturable absorber into the fiber cavity,the Q-switched pulse with wavelength 1543.2 nm,repetition rate 2.6 kHz?12kHz,and pulse duration 9 ?s?50 ?s can be obtained.At last,the narrow linewidth Q-switched fiber laser with topological insulator saturable absorber has been realized.By optimizing the linear laser cavity and nonlinear absorption parameters of the saturable absorber,the spectrum linewidth less than 0.06 nm,repetition rate 28.5 kHz,pulse duration 562.6 ns can be obtained.