Self-stabilized Q-switched Fiber Lasers Based On Stimulated Brillouin Scattering

Benefiting from the features of compact structure,stability and reliability,passive Q-switched fiber lasers play an important role in industrial processing,lidar,ranging and remote sensing.Among them,Self Q-switched fiber lasers based on stimulated Brillouin scattering(SBS)have attracted wide attention of researchers due to their advantages of no wavelength limitation,low cost,and all-fiber structure.However,the random output characteristics have seriously limited their development.In order to overcome the random output defect of SBS-based Q-switched fiber lasers,self-stabilized Q-switched lasers based on SBS were studied in simulations and experiments in this thesis.Firstly,the passively Q-switching technique and its implementation methods were introduced based on the basic principles.Through numerical simulations,the variation of the population inversion and the principle of pulse generation in passive Q-switched fiber lasers were discussed.In addition,the origin and process of the physical phenomena related to the SBS-based Q-switched lasers were discussed and analyzed.Then,a systematic numerical simulation model of Q-switched fiber lasers based on SBS was established.The Q-switching process induced by SBS was analyzed in detail through coupled partial differential equations.Traditional boundary conditions were deployed to simulate the Q-switching process of traditional SBS-based Q-switched fiber lasers.Random pulse output was successfully reproduced and their characteristics were detailedly analyzed.Finally,based on the analysis of simulation results,an improved boundary condition was proposed,and stable Q-switched pulses were successfully achieved with this improved boundary condition.According to the simulation results,the instability of the repetition frequency of the output pulse was reduced from 8.24% to 0.86%,and the instability of the peak power was reduced from 43.42% to 1.36%.With guidance of the simulation results of the numerical model,a method of generating a suitable boundary condition by using a Fabry-Perot interferometer composed of two uncontacted fiber connectors was proposed.The above-mentioned Fabry-Perot interferometer was added into the structure of the traditional SBS-based Q-switched fiber laser.During the experiment,the length of the Fabry-Perot interferometer was carefully adjusted and Q-switched pulses with quite stable repetition frequency and peak power were successfully realized.Compared with traditional stabilization methods of SBSbased self-Q-switched fiber lasers,adding an FP interferometer into cavity can efficiently stabilize the Q-switched pulses at a lower cost.In addition,the all-fiber structure improves the overall stability of the laser and is convenient for the integration with other systems.In the experiment,the signal-to-noise ratio of the radio frequency spectrum was measured to be 68.12 d B.Additionally,the advantages of high average output power and high single pulse energy based on this type of lasers were analyzed.In the experiment,under 600 mW pumping,the constructed laser achieved output with an average power of 55 mW and a single pulse energy of 3.5 μJ.Self-stabilized SBS-based Q-switched fiber lasers with a ring structure were designed and implemented,and the spectrum of the output pulses was analyzed.Finally,the tuning characteristics of a self-stabilized SBS-based Q-switched laser were discussed.At a fixed pump power,by continuously changing the cavity length of the Fabry-Perot interferometer while maintaining the Q-switching state of the laser,the repetition frequency of the output can be continuously and significantly changed.Experiments showed that the higher the pump power,the larger the tuning range of the cavity length of the Fabry-Perot interferometer,as well as the repetition frequency of the corresponding output pulse.With 600 mW pumping,the cavity length of the Fabry-Perot interferometer was adjusted in a narrow range which was less than 3 μm,while the repetition frequency was continuously changed from 12.38 kHz to 35.32 kHz.Such a significant variation of repetition frequency indicates that the self-stabilized SBS-based Q-switched fiber lasers can be used in the fields of stress sensing,environmental monitoring and gas detection.The relative stability of the repetition frequency and pulse width during the adjustment of cavity length of the Fabry-Perot interferometer was analyzed.The linear relationship between the energy of a single pulse and the pulse width was also discussed.