The Study Of Large-mode-area Yb-doped Fibers With High Transverse Mode Instability Thresholds For High-Power Fiber Lasers

Due to the high conversion efficiency,good beam quality and compact stru cture,high-power single-mode Yb-doped fiber lasers have been widely applied in fields of industrial processing,military and national defense,etc.However,with the continuous improvement of output power,nonlinear effect（NLE）and transverse mode instab ility（TMI）have become the main factors restricting the power scaling.Large-mode-area（LMA）Yb-doped fibers based on the mode control technology can fundamentally overcome the NLE and TMI effects simultaneously.Nevertheless,domestic high-performance Yb-doped fibers still need to be imported from abroad,and LMA Yb-doped fibers with special structures have been embargoed abroad due to military reasons.This thesis focuses on LMA Yb-doped fibers based on mode control technology.Starting with the optimization of fiber fabrication process,the NLE and TMI thresholds improved by various mode control technologies were verified in the self-built high-power fiber laser test platforms and the LMA Yb-doped fiber with high transverse mode instability threshold suitable for high-power fiber lasers was obtained.Firstly,the fabrication processes of LMA Yb-doped fibers via modified chemical vapor deposition（MCVD）method and preform drawing were introduced.The key points to obtain Yb-doped fibers with high Yb³⁺-concentration and low background loss were then analyzed.Next,the basic parameters and their testing technology were introduced systematically.Based on the fiber fabrication and testing technology,a high-performance 20/400 double-clad Yb-doped fiber was developed.A 2kW all-fiber laser oscillator was built,of which the optical to optical efficiency was 82.5%,the beam quality M²～1.39 and the Raman suppression ratio as high as 46 d B.The mode control technology of bending Yb-doped fiber was studied.The typical features and monitoring methods of the TMI effect in all-fiber MOPA structures based on large-core-diameter（LCD）Yb-doped fibers operating in quasi-single mode and few mode were described respectively.The sharp deterioration of M² factors and the sudden change of time-frequency information were determined as the criterions concerning TMI threshold.The correlation between TMI threshold and the bending diameter of the Yb-doped fiber in the LCD Yb-doped fiber amplifier was explored.In the experiments,the TMI threshold was the lowest when the bending diameter range of t he Yb-doped fiber was 11-17.5 cm.Increasing the bending diameter can increase the threshold by51%,and reducing that could also achieve a 33%threshold increase.Based on the experimental results,it was proposed that the reason why bending affects TMI threshold lies in the joint action of mode coupling effect and mode bending losses.The mode control technology of reducing the core numerical aperture（NA）was studied.Simulation results revealed that refractive index fluctuations in the core region would make low-NA intrinsically single-mode fiber generate multimode behaviors.Low-refractive-index trenches were introduced in the cladding and delocalization of high order mode was created to increase the tolerance of low-NA Yb-doped fibers to the refractive index fluctuations of the core region,thereby the difficulty of fiber fabrication was lowered.Based on a self-developed 20/400 trench-assisted low-NA Yb-doped fiber,a laser output of 1.3 kW M²～1.04 with TMI threshold being 1.35 kW was obtained in an all-fiber MOPA.A linearly-and an exponentially-Yb-doped fiber were designed to balance the thermal load distribution,which reduced the maximum temperature by over50%.The mode control technology of reducing Yb³⁺doping region was studied.Some key points in fabricating confined-doped fibers were described and the optimized scheme was proposed considering the parameters of MCVD process and the doping composition.A 30/400 confined Yb/Ce co-doped fiber with optimized refractive index profile was obtained.Compared with the conventional Yb-doped fiber,the TMI threshold of the all-fiber MOPA employing the confined Yb/Ce co-doped fiber was increased by 74%.Thanks to the confined doping,the output beam quality of the amplifier was improved to M²～1.43 as the output power increased to 1.2 kW.A15-hour online operation experiment proved that the confined-doped fiber shown strong resistance to photodarkening.Notably,mode field distortion caused by bending would interfere the improvement of TMI threshold.After designing the fiber length and bending diameter,a 3.14-kW output was achieved in the backward-pumped all-fiber laser oscillator based on the confined Yb/Ce co-doped fiber.Neither stimulated Raman scattering nor TMI was observed.