High-frequency Phase Distortion Characteristics Of High Power Laser Beam

It is well known through many finished physics experiments that the characteristic of focal spot is one of the most crucial factors which determine whether ignition can realize or not. Properties of focal spot are mainly determined by the quality of the beam before focused. In brief, the main of focal spot lies on the low frequency components of wavefront while the tail of it lies on the middle-high frequency components of wavefront. Many researches about the low frequency components of wavefront have come to very useful conclusions, however, for the middle-high components , relatively less work has been done and no useful results have been derived about its nonlinear propagation or its relation with the tail of the focal spot because of its complex transmission. The middle-high frequency components of wavefront in high-power laser system mainly come from hundreds of optics because even extremely small fluctuations on the surface of optics may become negligible by hundreds of addition and nonlinear transmission. The unwanted results from it include pinhole closure and nonlinear small-scale self-focusing which originate from high-intensity tail of the focal spot and nonlinear transmission of the middle-high frequency under high-intensity. So it is very useful to study the transmission of the middle-high frequency and its impact on the tail of the focal spot. The contents are mainly divided into five sections as follows:(1) Dividing the spatial frequency of the phase errors into several districts and making sure the district to study. First, the B-T theory, its assumptions and analysis of phase error's propagation are introduced. Then, basing on the nonlinear propagation of different spatial frequency and the requirements of ICF for the main focal spot, we divide the spatial frequency components of phase errors into several districts for the SG-Ⅲ system and make sure the emphasis to study.(2) Studying the transmission properties of the middle-high frequency components of phase error on the basis of B-T theory. We first study the transmission properties of a local phase error and draw a conclusion that none of the phase errors undergo apparent nonlinear increase except for those whose main spatial frequency components are in the high-gain district. Then, using numerical simulation, we study the impacts of phase error on the near field of the beam in practice and analyze the relation between the A B and the near-field contrast. Finally, we analyze the impacts of pinholes and different resources of phase error on the near field modulation.(3) Impacts of middle-high frequency components of phase error on the tail of focalspot. First, we discuss the relation between the tail of the focal spot and PSD, which describes themiddle-high frequency phase error, by both analytical and numerical ways. The results show that not spatial frequency but the amplitude of the phase error determines whether the PSD describes the tail of the focal spot completely or not and that the PSD can describe the tail of the focal spot only if the amplitude is small enough.(4) We discuss the regularities of linear and nonlinear transmission of PSD, and using B-T theory, we acquire the analytical expression of PSD after and before the transmission of the phase error.(5) Considering the method by which the low-frequency wavefront RMS is added, we believe that it is useful to add the PSD by the method of partly coherent addition. And basing on the above analysis, we put forward the basic way of decomposing the specifications of optics for the middle-high frequency phase errors.