Beam Quality Control For High Average Power Regenerative Amplifier

With the advantages of high gain,high beam quality and compactness,regenerative amplifier is an important means to realize ultrashort pulse laser amplification all the time.Larger pulse energy,wider gain bandwidth and higher average power are the main research directions of regenerative amplifiers.To achieve high-energy pulse output,the gain material needs a higher emission cross-section and a longer fluorescence lifetime,while a wider gain bandwidth requires a wider gain spectrum.A higher average power means better thermal performance.Limited by the material characteristics of the laser gain medium and the interaction between the above parameters,the output capacity of the regenerative amplifier is greatly limited.The regenerative amplifier technology with wide spectral gain and high average power output is one of the most potential and challenging research directions in this field.In this paper,a regenerative amplification scheme is proposed to achieve wide spectral gain by using multiple laser heads with multiple gain media.On this basis,4f system is introduced into the regenerative cavity to solve the thermal stability complexity of the regenerative cavity caused by multiple laser heads.Furthermore,the astigmatism caused by non ideal 4f system is analyzed and the corresponding solutions are given.The main work and innovation of this paper are as follows:1.Based on Yb:Lu2O3 and Yb:Sc2O3,a double head regeneration and amplification scheme is proposed to obtain a gain bandwidth greater than 20 nm.Based on the principle of stimulated radiation,the gain,spectrum and energy storage parameters of laser gain materials are analyzed.In order to solve the problem of mutual restriction among the three parameters mentioned above,the methods to break through the theoretical limitation are introduced,including the method of spectral compensation or using multiple gain materials at the same time to expand the spectrum of output laser.On this basis,aiming at the demand target of single pulse energy 100mJ,average power 200W and gain bandwidth greater than 20nm,a regeneration and amplification scheme based on Yb:Lu2O3 and Yb:Sc2O3 is proposed,and a preliminary design scheme is given.2.A method of introducing 4f system into the regenerative cavity to simplify the thermal complexity caused by multiple laser heads is proposed,and a thermal stable ring cavity is designed.In view of the complicated thermal lens of the regenerative amplifier caused by the simultaneous operation of multiple gain media in the above amplifier by the simultaneous operation of multiple gain media.scheme under the condition of high average power operation,a method of introducing 4f system into the regenerative cavity is proposed to simplify the thermal lens problem in the regenerative amplifier,and the feasibility of this method is proved by the theory of ABCD matrix.On this basis,based on the three-layer thin disk amplifier,the temperature distribution and deformation distribution of the thin disk laser head under the specific pump and heat dissipation conditions are calculated by using the finite element method,and then the specific value of the thermal lens of the thin disk laser head is calculated.By using the lens equivalent method,a simple design method of thermal stable ring cavity is proposed.Based on the results of the thermal lens calculation,the design of the ring cavity is given to meet the thermal stability conditions,so as to minimize the fluctuation of the beam size.3.The possible astigmatism problems in the above schemes are analyzed and the solutions are given.Aiming at the possible astigmatism problem in the design of regenerative cavity,the influence of astigmatism caused by oblique incidence on the beam quality is analyzed through theoretical calculation.Astigmatism will make the near-field profile of the beam become ellipse,and the fluctuation of beam size will increase.In order to solve this problem,this paper proposes a scheme of astigmatism compensation.Adding an additional convex mirror near the laser head in the cavity can provide the astigmatism with opposite sign.By controlling the incidence angle of each component,the astigmatism compensation can be effectively realized.Theoretical calculation shows that the astigmatism compensation scheme can basically eliminate the beam deformation caused by astigmatism and the increase of beam size fluctuation.