| In high-energy laser system, the positive-branch confocal unstable resonator is used as the laser oscillator usually. The various degree of deformation may be happened because of mighty density of laser power, which affects the beam quality of laser and transmission of energy to far-field. In this paper, according to the result of experiment, the influence of thermal deformation of the reflectors to beam quality and confocal unstable resonators' mode are studied by numeral simulation.On the basis of comparison of several algorithms of the calculation of unstable resonator's mode. Fast Fourier transform method(FFT) is chosen in this paper. The model of multi-layer distribution of gain is established. Sampling granularity in real space used in FFT, quotient of guard band and distance between subsections are made certain respectively. The convergence criterion of root-mean-square of plural y which is the quotient of the optical field's distributions separated by one pass is established. Under the circs of multi-kind of distribution of gain and different measurement of distortion, the output optical field distribution is calculated. Meanwhile, the far-field and near-field(transmission of a limited space) distribution of laser beam is given.The result shows that the thermal deformation of the mirror is one of the main important factors influencing beam quality. The degree of thermal distortion affects phase distribution of output laser beam and radius of far-field facula directly. On the other hand, gain distribution influences the distribution of thermal deformation. Strictly speaking, the later beam quality of the confocal unstable resonator is worse than the former one during output of laser beam.In this paper, the result of numeral simulation is consistent with that of experiments mainly. It has certain conduction meaning to the experiment. It indicates that appropriate cooling or other methods can improve beam quality efficiently.
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