Research On Multimode Pulsed Fiber Lasers Based On Few Mode Erbium Doped Fibers

Pulsed fiber lasers are widely used in communication,material processing,measurement,and medical fields,large pulse energy and high peak power have always been an important research direction for pulsed fiber lasers.However,the core diameter and modal field aera of single mode fiber are small,which lead to the high nonlinear coefficient of the fiber.To obtain greater pulse energy and peak power,it is a good choice to use multimode fibers or few mode fibers with larger mode field areas,and the mode selective output of fiber laser will provide more flexible choices for the mode division multiplexing system in the future.Therefore,it is necessary to study multimode pulsed fiber lasers and mode control techniques for high-power lasers,mode division multiplexing communication and other needs.Based on research and summary of the current research status at home and abroad,a few mode Er-doped fiber and a Er/Yb co-doped fiber were used as the gain medium,a multimode Q-switched mode-locked fiber laser and a multimode mode-locked（spatiotemporal mode locked）fiber laser are designed,and the output characteristics of the lasers were experimentally studied.Finally,the wavefront shaping mode selection method was simulated and studied based on the diffraction theory and genetic algorithm,and the laser mode selection method based on wavefront shaping was experimentally studied.The specific research contents are as follows:1.A multimode Q-switched mode-locked fiber laser based on a few mode Erbium-doped fiber with a ring cavity was designed,and the characteristics of the laser was experimentally studied.The few mode erbium-doped fiber acted as both the gain medium and the saturable absorber,and multimode Q-switched mode-locked pulses output were obtained experimentally.The envelope repetition frequency of the multimode Q-switched pulse was 7.41 k Hz.The central wavelength of the output spectrum was 1565.65 nm,and the side mode suppression ratio was 36.1 d B.The output beam profile showed multiple transverse modes.Then,the variation of Q-switched pulse energy and Q-switched repetition frequency versus the pump power was studied.Finally,the wavelength adjustment of the multimode Q-switched mode-locked pulse was achieved by utilizing the multimode interference filtering,and the adjustment range of single wavelength was 1561.8nm～1575.1 nm.2.Two kind of spatiotemporal mode locked multimode fiber lasers were designed,and their output characteristics were experimentally studied.1)A partly multimode spatiotemporal mode-locked fiber laser with a ring cavity was designed,the gain medium was a single mode Erbium doped fiber,the laser was mode locked by nonlinear polarization rotation,and the influence of the length of multimode fiber on spatiotemporal mode locking was studied.Multimode solitons with a fundamental repetition rate of 11.57 MHz and a pulse width of 1.23 ps was obtained When the length of the multimode fiber in the cavity was 2 meters.Unstable pulse sequences were obtained when the length of the multimode fiber in the cavity was 17 meters.2)A figure"8"spatiotemporal mode-locked multimode fiber laser was designed,and the laser was mode locked by the nonlinear amplifying loop mirror,the active fiber was a few mode erbium/ytterbium co-doped fiber,the passive fiber was graded index multimode fibers.Noise-like pulses with a repetition rate of 2.78 MHz and a single pulse energy of 6.86 n J were obtained when the pump power was 2 W.And the central wavelength of the mode-locked spectrum was 1563.96 nm with a 3 d B band width of approximately 5.98 nm.Spatial beam self cleaning occurred due to the large pulse energy,and a near gaussian light field was obtained.The beam quality was measured to be 2.41.In addition,spatiotemporal mode-locked pulses,including single pulse,double pulse and three pulse,were also obtained by adjusting the polarization controllers,and the output beam profile were multi-transverse modes.3.The multimode fiber laser mode selection technique based on wavefront shaping was simulated and experimentally studied.Firstly,the wavefront shaping mode selection was simulated based on the diffraction theory and genetic algorithm,the selection of LP₁₁and LP₂₁modes was achievedd after 200 generations of iterative optimization,the simulated optimal fitness of the two modes was 0.934 and 0.932 respectively,which was close to the fitness“1”of the ideal light field.The fitness of the optimized LP₂₁mode is0.811 under the condition of multimode light source.The experiment of the mode seclection based on wavefront shaping outside the cavity was carried out then,and the experimental results were basically consistent with the simulation results.The research results of multimode pulsed fiber lasers and their mode control techniques would provide a reference for the improvement of pulse energy and peak power of the pulsed fiber lasers and the mode control of multimode fiber lasers.