Research On High-Order Orbital Angular Momentum Passively Q-Switched Fiber Laser Based On Few-Mode Long-Period Fiber Grating

Orbital angular momentum（OAM）beams have attracted extensive attention in the field of atomic manipulation,data transmission and high-dimensional optical communication due to their unique properties of the spiral phase and intensity distribution.In recent years,OAM pulses generated by the combination of OAM mode and time-domain control technology have gradually shown potential application scenarios in ultrafast optics and material processing.OAM beams based on all-fiber laser have the advantages of easy integration,low cost and compact structure.Due to the difficulty in the fabrication of high-order mode converter and the limited selection of output mirror used to construct the cavity,the OAM modes generated and modulated by fiber lasers are mainly focus on the first and second-order,which limits the development and application of higher-order OAM modes.In this thesis,the mode coupling characteristics of few-mode long-period fiber grating（LPFG）are deeply studied and the conversion between the fundamental and third-order OAM modes is realized.The LPFG is applied to the proposed Q-switched fiber laser,the time-domain modulation of OAM mode is realized and the output of third-order OAM mode pulse is obtained.The main works are as follows:（1）A simulation model for analyzing the transmission of strong modulated few-mode LPFG is constructed,and the few-mode LPFG is fabricated by using CO₂ laser.By establishing the cross-section model of the grating region,the coupling process between the guided modes in the grating region is analyzed,and the asymmetric and azimuthal refractive index modulation are introduced into the fiber core region in each section.The transmission spectrum of LPFG used to realize the mode conversion between LP₀₁ and LP₃₁ modes is obtained by simulation,and the influence of grating parameters in the model on the transmission spectrum is further studied.In the experiment,by optimizing the simulation parameters and the preparation scheme,the multiple scanning method with gradual power is used to realize the fabrication of LPFG that can couple the LP₀₁ to LP₃₁ modes,and the output modes of the grating are detected,the generation of OAM_±3 modes are confirmed.（2）A Q-switched fiber laser which directly outputs the third-order OAM pulse is proposed and explored.The LPFG that overcomes the coupling issue of high-order mode is inserted into the cavity to realize the conversion between LP₀₁ and LP₃₁ modes.The OAM_±3 modes are generated by adjusting the phase shift of the odd and even modes of LP₃₁.Using a homemade saturable absorber based on graphene film,the transition of the laser from continuous wave to Q-switched state is realized.Determined by the operating wavelength of the combination of single-mode Bragg grating,LPFG and the seven-mode fiber output mirror,the center wavelength of the laser is located at 1548.2nm,and the corresponding 3d B bandwidth is 0.4nm.Within a certain range,the repetition rate and width of the output pulse can be adjusted by the pump power.The purity of OAM₊₃ and OAM_-3 are 90.0%and 90.2%,respectively.