Study On Thermal Deformation In High Energy Laser And Transmission System

High-energy laser system involves an optical train consisting of reflectors and windows, in which temperature increment, stress and deformation may happen because of the absorption of some fraction of the incident laser beam-energy so that the beam quality and transmission of energy to far-field will be affected. In this paper, on the basis of analysis about temperature increment, stress, deformation of optical components and phaseshifts, the influences of beam quality are studied theoretically and experimentally.The models of analyzing temperature and structure are established. Temperature increment and deformation are discussed with finite element method and ordered coupling method. Maximum tensile stress criterion is chosen as the destroying criterion of components.Laser-induced deformations of unstable resonator mirrors of high power laser result in some aberrations of resonator parameter, which distort laser modes and degrade beam quality. Based on the convergence criterion of root-mean-square (RMS) of phase and output power, the actual intensity on reflectors are solved with Fox-Li iteration so that the deformations of mirrors and one-way phaseshifts are calculated. By simulating one-way phaseshifts with Zernike polynomial, the laser modes are studied with method of iteration and geometriy. The evaluation factors of components influences on beam quality is discussed. The cooling methods and effects of mirrors and deformation of lateral coupling mirror are analyzed.The choice of laser output window height is relation to many factors, such as pressure loading, thermal loading, yield stress, influence on wavefront, failure by fatigue. Spherical windows can endure higher laser intensity. However, it brings some phaseshifts, which connect with laser windows parameters. The best spherical windows parameters are chosen according to RMS of phase.Stress-birefringence of laser windows is deduced by using index ellipsoid.Absolute distribution of laser beam intensity is gotten from comparative distribution. The absorption coefficient of material is calculated by measuring transmissive ratio. The influences on temperature increment, stress and phase of laser beam are computed with different time, window cooling, window rotating, shape and material of window, distribution of intensity. The relations between Hartmann measuring data and calculating data are presented with stress-birefringence. Temperature increments and phaseshifts of working CaF2 window and sampling fluoride glass window are measured. During experiments, the simulating cavity is designed, on which the sampling plane and spherical window are fixed.The phaseshifts from deformation of reflector are discussed. Deformation of one reflector is measured. However, by using the lag of thermal deformation, the aberrations of wavefront are detected in actual optical train, which are compared with the results of calculation. The affection on wavefront is analyzed with different level degree and patch-up method of output window in launching system.