Research On Key Technologies Of All-fiber Spatiotemporal Mode-locked Laser With Multimode Interference Effect

Over the past several years,mode-locked fiber lasers have very important application prospects in optical communication,material processing,optical measurement,and other fields.As we put forward new requirements for the output energy,dimension of mode and nonlinear suppression of lasers,single transverse mode mode-locked fiber lasers cannot meet the existing requirements.The mode-locked laser based on multimode fiber has large mode field area and supports multiple transverse modes,which can acquire high-power laser output and achieve mode multiplexing.Its complex nonlinear dynamics provide the possibility for the discovery of new physical phenomena.Moreover,the pulse energy of spatiotemporal mode-locked laser is two orders of magnitude higher than that of traditional laser,and has higher performance.In recent years,people have refocused their eyes on multimode fiber lasers.Because the complexity of nonlinear effects in multimode fiber is far more than that in single-mode fiber,balancing the complex nonlinear effects and linear effects has become a difficult problem.This requires a spatiotemporal saturable absorber capable of coupling both longitudinal and transverse modes to control the net balance of group velocity and mode dispersion to establish the intrinsic pulse.In addition,multimode interference effect is common in all-fiber space-time mode-locked fiber lasers,and it plays an important role in multimode systems.The main research contents of this dissertation are as follows:1.The generation and transmission mechanism of solitons in multimode fibers are studied and analyzed.Starting from Maxwell equation,the distribution of transverse mode field of optical fibers and the generalized multimode nonlinear Schrodinger equation that describes the propagation of pulses in multimode optical fibers are derived through spatiotemporal separation;The distributed Fourier method for solving this equation and a more efficient massive parallel algorithm are introduced.Finally,the mode-locking principle of fiber lasers and the influence of multimode interference on spatiotemporal mode locking are introduced.2.Mode filtering is an important method to suppress the walk-off of modes.Research was conducted on how multimode interference effects affect the establishment of spatiotemporal mode-locked pulse.The effects of spectral filtering and spatial filtering in multimode interference effects on the establishment of spatiotemporal mode-locked pulses were analyzed.Based on the filtering mechanism of multimode interference effects,the multimode interference structure was optimized to improve the opticaloptical conversion efficiency of the system.The experiment was conducted to achieve stable and efficient spatiotemporal dissipative soliton pulse.In addition,by precisely tuning the polarization controller,second-order harmonic spatiotemporal dissipative solitons can be obtained.The experimental results show that the optimization of multimode interference effects is of great significance for the formation of spatiotemporal solitons and the improvement of the efficiency of system.It provides the direction and principle of optimization for large energy spatiotemporal mode-locking.3.The high modulation depth of the Mamyshev mechanism is significant for achieving mode locking.The influence of the Mamyshev mechanism on the realization of modelocking in multimode fiber lasers has been studied.By modeling the hybrid cavity structure composed of single mode and multimode optical fibers,the feasibility of the scheme is analyzed and confirmed.The spatiotemporal mode locking based on Mamyshev regenerative oscillation is realized at the 1.55 band.Injecting different degrees of tightness of bound state seed light into a Mamyshev fiber laser can obtain bound state spatiotemporal solitons,which indicates that single transverse mode bound state solitons can excite spatiotemporal solitons in multimode fiber lasers without changing the original soliton state.This work can avoid the loss of mode-locking caused by environmental instability,and has potential applications in the fields of communication and mode division multiplexing.