Study Of Effect On The Optical Field Mode Induced By Thermal Deformation Of Cavity Mirror Under Laser Irradiation

In this paper, we focus on the positive-branch confocal unstable resonator that commonly used in high power lasers, and analyze the influence of cavity mirror thermal deformation on the optical field mode. In our opinion, the results of the study can provide a reference for the characteristics analysis and control of the beam output from high energy laser resonator which has been working for long time.The main research contents include: analysis of optical field mode in the resonant cavity based on the angular spectrum propagation method, thermal deformation calculations of cavity mirror under laser irradiation, influence of thermal deformation on the optical field mode, and impact study of different beams on the focusing distance of launch system. The main research content is as follows:1.We establish a calculation model of optical field mode in the resonant cavity based on the angular spectrum propagation method. By using "soften" treatment of the cavity mirror aperture edge and thin layer distribution of the gain in active cavity, we derive the numerical calculations of optical field mode. We also complete the programming of optical field mode in the resonant cavity based on MATLAB software. And the optical field mode of positive branch unstable cavity calculated by the program has been verified by comparing with the reported results in the literature. Three kinds of the optical field model in active cavity are analyzed and calculated, the results show that the vibration amplitude of the optical field distribution is closely relative to the small signal gain distribution. Excluding phase perturbation, phase distribution is mainly decided by the structure of resonant cavity, and the output wave is approximate to the plane wave.2.The analysis of temperature rise and thermal deformation of cavity mirror under laser irradiation is carried out. O n the basis equations of thermal deformation controlling of cavity mirror and the finite element analysis, we complete the command flow control programming based on ANSYS software, and obtain the numerical calculations of the temperature and deformation. Results show that the temperature rise and the deformation of cavity mirrors increase with the increase of laser power and irradiation time. In an unstable high-energy resonant cavity, the temperature rising and deformation of the secondary mirror is larger than the primary mirror. And greater thermal deformation of the cavity mirror has an impact on laser output optical field.3. By using the methods of time discretization and sequence coupling of optical field mode and cavity mirror deformation, we study the influence of thermal deformation on the optical field mode. Then we calculated the amplitude and phase distributions and far field beam spot at the time of 4 seconds since the moment of laser beam outputting. Calculation results show that the optical filed changing with the thermal deformation of cavity mirror, and the wave surface spread gradually along the laser beam outputting. Under the conditions in the paper, the approximate plane wave at the initial moment turn out to be approximate divergent sphere at the end. And the far-field beam spot splits into two beams in the y direction. The experiments verify the influence of the cavity mirror thermal deformation on laser output optical field. Furthermore, the influences of the temperature rising and deformation by utilizing cooling method on the back of cavity mirrors on the optical field are discussed and analyzed.4. We analytically achieved the relationships between focus distance which we calculated also and focusing amount of plane wave, spherical wave and Gaussian beam outputting from a laser launch system. Results show that at the same focusing amount of the telescope, the focus position of plane wave and spherical wave are different from the beam waist position of Gaussian beam. By increasing the beam waist of Gaussian beam and enlarging the beam spreading ratio of the launching system, the difference of focus distance between Gaussian beam and plane wave can be decreased. The difference of focus distance between spherical wave and pla ne wave can be decreased by increasing the curvature radius of spherical wave. Also we find that when the focus distance is required unchanged; the focusing amount should be adjusted due to the characteristics of output beam. If we launching the spherical wave by using the method of plane wave, the focus position must be diverged from the anticipated position.