| Spectroscopic measurements of extreme ultraviolet radiation (EUV) and soft x-ray (SXR), which contain a wealth of spectral lines for numerous elements, has become an important research method for nuclear fusion, astrophysics and characteristics of materials. And the grazing incidence flat-field spectrographs, which could easily couple plane detectors, are widely used for spectral analysis in EUV and SXR. As the core optical element of spectrograph, the EUV/SXR flat-field grating has two types of mechanical ruled grating and laminar-type holographic grating. At present, the mechanical ruled grating could be replaced by holographic grating to build spectrograph, and the holographic grating has the advantages of suppression of overlapping higher orders, low stray light and low scattered light.This thesis focuses on design and fabrication of flat-filed grating used in the wavelength range of0.8-6nm. The main works of this thesis are as follows:1. Based on the aberration theory of concave aberration-corrected grating, the groove density parameters of flat-field grating used in0.8-6nm are optimized with the average measured radius of curvature of grating blanks. The aspheric wave-front recording optics, which is feasible in the process of fabrication, are designed using light path theory. The parameters of laminar groove and fabrication tolerances are given by means of rigorous coupled wave analysis (RCWA).2. Different spectral image properties of different areas cause the low spectral resolution at wavelength below1.3nm. To improve the spectral resolution at wavelength below1.3nm, the diffraction efficiencies of different areas are designed respectively to adjust the proportion of regional image in the full spectral image. Otherwise the effect of curve groove on spectral resolution is presented, and different curve grooves could be obtained with the different parameters of the mirror in the recording optical path. Using cylinder mirror, the grating with almost straight groove can be obtained, and its spectral resolutions at wavelength over2nm are obviously improved.3. Fabrication of flat-field grating:the influence of adjustment error in recording optics path on groove density distribution are systemically analyzed. The grating patterns are recorded into photoresist using aspheric wave-front recording optics. Then the groove duty cycle of photoresist grating are adjusted through RIE with O2. At last, the photoresist patterns are transferred into fused silica substrate by reactive ion beam etching, and the laminar-type holographic grating with duty cycle of0.35±0.1and depth of9±1nm is obtained.4. A groove density measurement system is established with phase plate base on diffraction method. The influence of eccentricity errors on the groove density distribution measurement are analyzed. And the method to decrease the eccentricity errors with high rate of groove density variation is proposed. The fabricated holographic grating is measured, and the groove density error is within±0.5%, which meets design requirement.5. Near field holography is proposed to fabricate EUV/SXR flat-field grating, and the feasibility is analyzed theoretically. Under different exposure configuration, design and transfer error analysis of groove density distribution are presented. Meanwhile, the grating mask made by electron beam lithography equipment, which is used in near field holography, are discussed. And the discussed content are the approximation of groove density distribution and its impact on the quality of spectral image. |
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