Research On The Key Techniques Of High-Power Confined-Doped Fiber Laser

The power scaling of high-brightness fiber lasers is limited by the emergence of nonlinear effects and transverse mode instability(TMI).In recent years,researchers have exploited large-mode-area(LMA)optical fibers to achieve high-power laser output.However,the employment of conventional LMA fibers often results in beam quality degradation due to the increased number of supported transverse modes.Therefore,it is difficult to achieve high output power and high beam quality simultaneously.This work focuses on the confined-doped fiber laser system and carries out theoretical analysis as well as experimental study on the mode evolution characteristics,mode control and nonlinear effects suppression methods in the confined-doped fiber laser system,and explores the feasibility of using LMA confined-doped fiber to achieve simultaneous high-power,high-beam-quality laser output.The research contents and results are as follows:1.A theoretical model with consideration of the transverse spatial hole burning effect is established for studying the mode evolution characteristics of confined-doped fiber laser systems.The effects of the doping radius,bending loss,and bending-induced mode distortion on the gain filtering effect are comprehensively studied.Moreover,the influences of the seed laser’s power,pump power,and the pump injection directions on the mode evolution characteristics are also comprehensively investigated.This work could provide a good reference for the optimization of confined-doped fiber and fiber laser systems.2.Comparative studies of the confined-doped fiber amplifiers and fiber oscillators are carried out both theoretically and experimentally.The differences in mode evolution characteristics between the confined-doped fiber amplifiers and the oscillators are theoretically compared.According to the simulation results,five kinds of optical fiber with the core/inner cladding diameter of 40/250μm are designed and fabricated,the relative doping ratios of which cover 0.5,0.75,and 1.The homemade fibers are used to build fiber laser amplifiers and oscillators with the output power of～1.5 k W,where the gain tailoring effect of the homemade confined-doped fiber is successfully verified.By characterizing and analyzing the parameters of the homemade confined-doped fibers,the performance of different fibers in amplifiers and oscillators is analyzed,and possible reasons for the differences between the experimental results and the theoretical simulation results are explained.This work provides a practical reference for selecting the power scaling scheme.3.The mode control characteristics and TMI threshold boosting capability of the high-power confined-doped fiber amplifier are carried out.Comparison experiments are carried out using the homemade conventional ytterbium-doped fiber and confined-doped fiber with a relative doping ratio of 0.75.As a result,the TMI threshold of the confined-doped fiber amplifier is 4.74 k W.The beam quality factor remains around M~2≈1.75 before the onset of TMI.While,the TMI threshold of the fiber amplifier employing the conventional fiber is～1.76 k W,and the beam quality factor constantly degrades from M~2≈1.5 to M~2≈2.3.Therefore,the advantages of the confined-doped fiber in achieving high power output,maintaining good beam quality,and mitigating the TMI effect are verified.4.The influences of the mode purity of the seed laser on the TMI threshold in the confined-doped fiber amplifier are experimentally investigated,and the possible mechanism is theoretically explained.The TMI threshold is significantly increased by introducing a certain amount of high-order mode contents into the seed laser,and high-brightness(M~2=1.83)fiber laser with an output power of 7.49 k W is experimentally achieved.In addition,in order to provide seed lasers with flexible and controllable mode content,mode controllable fiber lasers are explored by taking advantage of the acoustic-induced fiber grating,the feasibility of which in increasing the TMI threshold is successfully verified in a kilowatt level fiber amplifier.This work can contribute to further power scaling of high-brightness fiber laser systems.5.Experimental and theoretical study on the nonlinearity suppression method is carried out.First,by exploiting the backward tandem-pumped confined-doped fiber amplifiers,high-brightness fiber laser of 10.1 k W is successfully achieved.Furthermore,a bidirectional tandem-pumped confined-doped fiber amplifier is reported for the first time,and high-brightness fiber laser output of 7.88 k W is successfully achieved.Moreover,5.96 k W narrow-linewidth fiber laser output is achieved in a bidirectional tandem-pumped confined-doped fiber amplifier,which is the highest power level ever reported in a narrow-linewidth fiber laser.In addition,in order to increase the nonlinearity threshold of the fiber laser system,the feasibility of employing ultrashort-wavelength fiber laser(1007 nm)as the pump source for pump absorption enhancement is investigated.The feasibility of increasing the pump absorption by utilizing the 1007nm fiber laser in tandem pumping is successfully verified through theoretical simulation and comparative experiments,which would contribute to the nonlinearity suppression as well as further power scaling of the fiber laser system.