Design And Applications Of Large Aperture Diffractive Optical Elements In "Shen Guang" Device

The research devoted to this thesis focuses on the design and applications of large aperture diffractive optical elements (DOE) that will be used to realize uniform illumination on focal plane for inertial confinement fusion (ICF) system. The works are under the support of 863-804 project. The primary achievements obtained are as the following:1. A general iterative Fourier transform algorithm is widely used for designing DOE, because of its brief structure and high rapidity of convergence. The key of this kind of method is the efficiency of feedback obtained from output beam that controls the phase distribution of input beam in each iterative loop. The step iterative optimization (SIO) method is presented, which inserts a quasi-optimum process to optimize the value of feedback coefficients in every loop. The design results and performance of practical elements obtained by using SIO method both prove that the new method is more effective than other previous ones.2. The optimization algorithm can modulate the output results to an ideal profile precisely. However, this method is time-consuming because that lager numbers of configurations should be tried to find a local or global result. The key to the problem of optimization efficiency is the selection of a good searching path along which we can approach the extremum rapidly. A spatial spectrum optimization method for the design of DOE is presented based on the analysis of spatial frequency spectra of intensity distribution on the focal plane. In this new method, the goal and Cost Functions are chosen in spatial spectrum domain to determine the searching path. Compared with existing spatial optimization methods, not only the uniformity of intensity distribution on the focal plane will be improved, but also the modulations, especially low frequency components, will be well controlled by using the new method. The numerical simulation result shows that the new method will help improve the practical performance of DOE. 3. In practice, continuous phase plate (CPP) and random phase plate (RPP) have been tried for high power laser beam shaping. CPP realized by using ion beam rotation etching technique has a relatively high design index, but need for high matching accuracy. Though RPP can easily fabricated by using ion beam mask etching, the low performance index of RPP limits its field of application. The practical performance of CPP and RPP are not satisfied. So a scheme of multi-step phase plate that can be realized with four masks' etching facility is presented. Both theoretical analysis and experimental result show that this scheme is much better than that of CPP and RPP under the existing technological level.4. Two 100mm external diameter multi-step phase plates used for beam shaping were designed and manufactured. The test results show good performance of these two practical elements. Through the numerical simulation of etching tolerance, some schemes to improve design and processing method are presented.5. With the development of DOE technique, the practical performance of large aperture elements has been achieved or even exceeded the level of lens array. But some adverse factors, such as wave distortion and lateral misalignment of input laser beam, may affect the application of large aperture DOE. Based on the analysis of such factors, several improvement programs are given.6. Smoothing by spectral dispersion (SSD) is an efficient technique to reduce high frequency components in energy distribution. A DOE used with SSD technique will further improve the uniformity of focal spot. A new design method of DOE used with SSD is described. In this method, spatial spectrum optimization is used to design DOE that mainly control the low frequency components which the SSD is not able to smooth in main lobe of focal spot. Preliminary results show the method is effective.