Spatiotemporal Mode-locked Fiber Laser Based On Multimode Nonlinearity

High integration fiber lasers have better performance in structure,anti-interference and heat dissipation,which promote the rapid development of microprocessing,optical spectroscopy,metrology and imaging,providing a platform for research and exploration of novel fields.In recent years,the processing and application of the strong nonlinearity in the fiber becomes the focus of research.How to make full use of the nonlinear effect to get the ultrafast laser with better performance has become a significant demand.Compared with single-mode fiber laser,multimode fiber has higher degree of freedom,higher energy tolerance and more complex nonlinear characteristics,which has potential application prospects.However,the research on passively mode-locked multimode fiber lasers is still in its infancy,and there are still many unknown and novel phenomena worth exploring.In this context,the nonlinear effects in multimode fiber mode-locked lasers are investigated in this paper.By adjusting the nonlinear parameters of the multimode fiber laser,the ultrashort pulse mode-locked multimode fiber laser with high energy and high repetition frequency has been realized.The main research contents are as follows:1、The mode transmission theory of multimode fiber is described.Characterization of modes,dispersion properties,propagation constants and linear dispersion effects of multimode fiber especially the graded-index multimode fiber are summarized.Based on the nonlinear Schrodinger equation,the generalized multimode nonlinear Schrodinger equation is deduced for pulse propagation in multimode fiber.The parametric characterization of linear and nonlinear propagation characteristics of multimode fiber is also illustrated,and the mode theory is claimed by theoretical analysis and experiment.2、The nonlinear effect of Kerr nonlinear beam self-cleaning in multimode fiber mode-locked lasers is studied systematically.The spatiotemporal mode-locking mechanism of multimode fiber is discussed by means of dislocation fusion of the gain multimode fiber and the graded-index multimode fiber to stimulate the high-order transverse modes.The mismatching between the fiber cores forms a virtual filter in the cavity,realizing the normal dispersion mode-locking spectrum with adjustable wavelength and the mode-locking pulse with high beam quality and high energy up to 30 n J.Furthermore,the threshold conditions of nonlinear beam self-cleaning effect at different wavelengths are analyzed.3、The realization of high-repetition-rate pulses in multimode fiber lasers is proposed and studied.The dissipative four-wave mixing effect is realized by using microfiber knot resonator and highly nonlinear fiber.Based on theoretical analysis,the fiber is tapered by the flame tapering technique,and then looped into a ring.By controlling the diameter of the microfiber knot resonator,the transmission spectrum with comb filter characteristic is fabricated.The mode-locked pulse with repetition frequency up to GHz is obtained by applying the device to the spatiotemporal mode-locked multimode fiber laser.The conditions under which nonlinear effects dominate the mode-locked cavity are analyzed.