Investigation Of 2 μm High Energy Fiber Laser Based On Nonlinear Optical Loop Mirror

High-energy fiber lasers are widely used in the fields of optical frequency combs,biomedicine,lidar,etc.Due to their narrow pulse width,high pulse energy,and wide spectrum width.At present,the related research of 2μm high-energy fiber lasers still exist some disadvantages such as high device cost,complex structure and not conducive to all fiberization.Therefore,this thesis makes full use of the advantages of the fiber waveguide to improve the laser’s key performance such as pulse energy and average output power.The main research contents of this thesis can be summarized as follows:Firstly,through the introduction of the research background and significance of this subject,the method of generating high-energy pulses and the application scenarios of high-energy fiber lasers are explained in detail.The research status of high-energy passive mode-locked fiber lasers in the past ten years is listed,and the mode-locking method and mode-locking structure are summarized.On this basis,this thesis establishes an overall scheme that uses the composite cavity as the resonant cavity,the793 nm semiconductor laser as the pump source,the double-clad Thulium-doped fiber as the gain medium,and the nonlinear fiber loop mirror as the mode-locking mechanism.Secondly,this thesis analyzes the pros and cons of these pumping schemes among the three common pumping methods of thulium-doped fiber lasers.Since multiple transition methods exist in the excitation and transition process of ³H₆-³H₅,there exist other light sources which will affect the quality of 2μm laser.And ³H₆-³F₄is a two-level system which the particle inversion ability is weak.³H₆-³H₄compared with others it has high utilization rate and strong particle inversion ability.Based on the above analysis,it is determined that ³H₆-³H₄is the pumping method in this thesis.In addition,based on ³H₆-³H₄,a mathematical theoretical model of the laser was established,and then the transmission characteristic curve of the pump power and signal power in the gain fiber was simulated according to the values of the parameters in this mathematical model.Numerical studies have shown that combining the various parameters of the laser and the actual situation in the laboratory,the optical gain fiber length in this thesis is determined to be 5 m.Finally,the principle of mode locking of the nonlinear fiber loop mirror is analyzed,and it is concluded that a single fiber loop mirror has a dilemma in the choice of parameters through simulation.Therefore,in view of the inherent shortcomings of traditional nonlinear fiber loop mirror lasers,a hybrid mode-locked fiber laser structure containing two fiber loop mirrors is proposed.This cavity type not only has the advantages of compact structure,high flexibility of parameters,and no use of optical isolators,but also be able to improves the pulse output performance.In this structure,only need to fine-tune the optical components of the laser,three kinds of high-energy mode-locked pulses can be directly output from the all-fiber hybrid cavity.In the net negative dispersion region,a high-energy broadband noise-like mode-locked pulse with a pulse energy of 46 n J,an average output power of 300.20 m W,and a full width at half maximum of 29.18 nm was realized respectively;ultra-large numerical aperture fiber was used to realize dispersion compensation in the cavity.Dispersion-managed soliton pulses with pulse energy of 52.80 n J and average output power of 343.30 m W;Gaussian pulses with pulse energy of 154.08 n J and average output power of 385.20m W are realized by using single-mode fiber as an extension fiber.Compared with the existing 2μm high-energy fiber laser,the experimental results of the three pulses have improved energy,and the parameters have been analyzed in detail.