Research Of Passively Mode-locked Fiber Laser With Low Repetition Rate And High Energy

Low-repetition-rate and high-energy pulses have important applications in areas such as micromachining,biological sample detection,eye surgery and lidar,etc.One of the traditional ways to obtain low-repetition-rate and high-energy pulses is using chirped pulse amplification technology.This method needs a complicated structure and has low energy utilization.Long cavity,passively mode-locked fiber laser can directly output low-repetition-rate and high-energy pulses with a simple structure.It has great research value and has become one of the research hotspots in the field of fiber lasers in recent years.In this paper,based on the nonlinear optical loop mirror(NOLM)mode-locking technology,a long cavity,all-normal-dispersion ytterbium-doped fiber laser and a long cavity,negative-dispersion erbium-doped fiber laser are studied respectively.Low-repetition-rate and high-energy nanosecond pulses are achieved,which can be used as ideal seed light sources in multi-stage amplification systems and have broad application prospects in the fields of machining,fiber sensing and so on.The main contents of this paper are as follows:1.The influences of the dispersion and nonlinear effects on the NOLM mode-locked pulses are separately studied through numerical simulation.The functions of the parameters of fiber laser resonator are analyzed theoretically.Based on the cavity structure and the coupled nonlinear Schr?dinger equations(CNLSE),the mathematical model of the NOLM passively mode-locked fiber laser is established,and the numerical solution is realized by the split-step Fourier method as well as the Fourier spectral method in MATLAB.2.An NOLM-based all-fiber passively mode-locked ytterbium-doped fiber laser with low-repetition-rate and all-normal-dispersion cavity is built.The total length of the laser cavity is about 1502 m.A train of mode-locked pulses with an ultralow-repetition-rate of 133.18 k Hz is observed.The laser operates in dissipative soliton resonance(DSR)regime.Both the duration and single-pulse energy of output DSR pulse increase linearly with the increase of pump power.The maximum output pulse duration reaches 761.6 ns at the pump power of 414.47 m W,while the single-pulse energy reaches 60.2 n J.In addition,by changing the length of single mode fiber(SMF)in the NOLM,the influences of the cavity length on the duration and single-pulse energy are investigated.The result shows that the longer the cavity is,the larger duration the pulse has,while the lower the peak power is.3.An L-band passively mode-locked erbium-doped fiber laser with low-repetition-rate and negative-dispersion cavity is built,which operates in the DSR regime.The DSR pulse with a low-repetition-rate of 48.48 k Hz is obtained when the total cavity length is 4125 m.Under the pump power of 280.96 m W,the output pulse duration is574 ns and the single-pulse energy is 170.08 n J.The characteristics of harmonic DSR pulses and amplification stability of DSR pulses are also studied.The single-pulse energy is improved to 618 n J by amplification.After changing the cavity length by using the SMF with different length,the output DSR pulses characteristics are compared and the influences of cavity length on pulse output characteristics are verified.