The Impact Of Material Nonlinearity On The Performance Of Optical Elements Under High-intensity Lasers

In the indirect-drive laser inertial confinement fusion facilities,the uniform laser irradiation of hohlraum is one of the essential conditions for the successful ignition of ICF.There are many factors that affect the uniformity of the irradiation of the target surface,such as the spatial arrangement and intensity distribution of incident laser beams,the focus positions and the perfection of target surfaces.However,when the incident laser intensity is very high,the material's nonlinear Kerr effect can affect the intended nominal performance of the optical elements,which will cause the actual focal spot distribution on the target surface to be different from the theoretically calculated focal spot distribution during the experiment.Therefore,it is necessary to study and analyze the effect of nonlinear Kerr effect of materials on the performance of optical elements under the incidence of high-intensity lasers.In this thesis,based on the basic theory of beam transmission,numerical algorithms,and the theory of optical field regulation of optical elements,the research on the fundamental Gaussian beam is first carried out.At the same time,the influence of material nonlinear effect on the performance of optical elements including optical lenses and continuous phase plates is studied.The theory,model,algorithm and results are studied comprehensively,and the results with practical value are obtained.The main research contents and innovative work of this thesis include:1.In the theoretical study of the fundamental Gaussian beams,the derivation process from Helmholtz equation to several kinds of Gaussian beams is introduced.At the same time,the paraxial approximation error of the tightly focused fundamental Gaussian beam is analyzed when the size of beam waist is equal to or less than the optical wavelength of laser.It is found that the maximum error distribution has an annular structure and contains the largest error ring on the waist plane.The size of laser beam waist should be >1.2 wavelengths for a 1% permissible error,while the beam waist should be >3.5 wavelengths for a 0.1% permissible error.2.Based on the theory of nonlinear Kerr effect of optical materials,a new lens transmission model of lens under the incidence of high-intensity laser is established.The traditional phase transfer function of optical lens is modified,and a new numerical calculation method corresponding to the new transmission model is given.The two-dimensional Fast Fourier Transform algorithm for calculating Fresnel diffraction integral formula is used to simulate and compare the field distribution of focal spots for three different incident beams.At the same time,the focal spot field distribution on several planes adjacent to the nominal focal plane of optical lens was obtained using the spatial angular spectrum theory algorithm.The research results show that the Kerr effect of lens materials under the incidence of high-intensity laser beams has apparent impact on the focusing performance of optical lenses.This effect not only makes the actual focal length of the lens smaller than the nominal focal length,but also changes the spatial light field patters of focal spots on the focal plane,and this effect is even more pronounced for the incidence of Hermitian Gaussian beams.3.The improved Gerchberg-Saxton(GS)algorithm is applied to obtain the phase distribution function of a continuous phase plate under two incident light field and target light field shape distributions under nonlinear Kerr effect.Combined with the calculation results of optical lens,a complete beam transmission model and the final light field distribution under different incident light laser beams are finally given.At the same time,the influence of the non-linear Kerr effect of the phase plate on the target light field distribution under the incidence of high-intensity laser beam is analyzed in combination with the focusing performance of the lens.The results show that the nonlinear effect of material will lead to the final light field distributions of object focal spots different from the predetermined light field distributions.