Research On Pulsed Fiber Lasers And Their Nonlinear Dynamics

Pulsed fiber lasers are desirable light sources in a diverse range of applications such as laser processing,fiber optic communication,laser surgery,fiber sensing,and LIDAR due to their small size,strong heat dissipation,high beam quality,good stability,and high conversion efficiency.Among fiber lasers,ultra-short-pulse passive mode-locked fiber lasers have developed rapidly,and have made many advances in non-linear pulse dynamics,mid-infrared band fiber lasers,and new saturable absorbers,which have become hot spots for international research.This thesis focuses on the study of multi-pulse dissipative soliton resonance in fiber lasers,hybrid mode-locking technology and the application of carbon nanotubes in mid-infrared pulsed fiber lasers.The main research contents of this thesis are as follows:?1?The characteristics of dissipative soliton resonance pulses in passively mode-locked fiber lasers are theoretically and experimentally investigated.Through numerical simulations,the splitting mechanism of dissipative soliton resonance pulses in all-normal dispersion regime is discussed for the first time.Under certain system parameters,during the formation of dissipative soliton resonance pulses,many dark solitons are generated internally.The flat-topped nature of sissipative soliton resonance pulses makes it possible for dark solitons to collide with each other and generate a huge intensity dip,causing the pulse to split into two new dissipative soliton resonance pulses.The behavior of multiple dissipative soliton pulses in the cavity is also studied.It is found that changing the pump power of the laser do not vary the number of pulses in the cavity.In the experiment,a passively harmonic dissipative soliton resonance mode-locked erbium-doped fiber laser is built for the first time using a highly nonlinear fiber,and an output with a repetition frequency up to 972.8 MHz is achieved,and the repetition frequency does not change with the pump power,which verifies the results of the simulations.In addition,by inserting anomalous dispersion single-mode fibers of different lengths into the laser cavity,the harmonic dissipative soliton resonance pulses characteristics are investigated at different values of dispersion and cavity length.?2?The numerical simulations is used to study the hybrid mode-locking technology based on the interaction of the artifical saturable absorber and the material saturable absorber in 2?m erbium-doped fiber lasers.The simulation results show that the material saturable absorber provides auxiliary pulse shaping mechanism in the low power stage,which can make the laser enter the mode-locked regime quickly,while the artifical saturable absorber in the high power stage plays the main pulse shaping mechanism,which can dramatically shorten the pulse width and increase peak power.Therefore,the hybrid mode-locking technology makes up for the shortcomings of high mode-locking thresholds,difficulty in self-starting,poor ambient temperature,and long cavity length in the single artifical saturable absorber mode-locked laser.It is also not limited by the slow response time and low damage threshold of the material saturable absorber.?3?By using single-walled carbon nanotubes as saturable absorber,passively Q-switched and mode-locked 3?m Ho³⁺/Pr³⁺co-doped fiber lasers are achieved.Single-walled carbon nanotubes with diameters of up to 2-3?m made by CVD method are transferred to the surface of a gold mirror to form a carbon nanotube saturable absorption mirror.In the experiments,a typical power-dependent measurement setup based on a balanced twin detector system is used to characterise the saturable absorber mirror.The saturated light intensity,modulation depth and unsaturated absorption loss are 1.66 MW/cm2,16.5%,and 71.8%,respectively.The saturable absorption mirror is placed in a Ho³⁺/Pr³⁺co-doped fiber lasers to obtain stable Q-switched pulses with a slope efficiency of 12.8%and a center wavelength of 2865.6 nm.After carefully adjust the saturable absorption mirror,stable mode-locked pulses are also obtained,with a center wavelength of 2865.2 nm,a repetition frequency of 12.7 MHz,and an average power of 71.8 mW.?4?A broadband wavelength-tunable mid-infrared pulsed fiber laser is achieved by dding a blazed grating as a wavelength selection device.Single-walled carbon nanotubes usually have a larger diameter distribution,and their band gap is inversely proportional to their diameter,so they have a wider absorption bandwidth.Thanks to the wide bandwith of carbon nanotube,by continuously rotating the angle of the blazed grating,55 nm tunable Q-switched and 70 nm mode-locked output are successfully achieved.Experimental results prove that carbon nanotubes are one of the most promising saturable absorbers in the mid-infrared waveband.