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Design of a volume phase holographic grating for astronomical observation using the rigorous coupled wave analysis

When observing remote celestial bodies and examining the structure and evolution of space it is important to acquire and analyze the spectrum of stars by means of dispersion gratings. To do so, we developed for the Subaru telescope facility in Hawaii a highly dispersive volume phase holographic (VPH) grism with high efficiency of 80% or more and high resolution from 2500-8000 covering the visible wavelength region. The VPH grism consists of two prisms and a refractive index modulation grating which satisfies the Bragg diffraction condition. To achieve highest performance we employed the rigorous coupled wave analysis (RCWA) in designing the VPH grating made from photopolymer. The grating was fabricated with grating periods ranging from of 0.68 to 1 μm for design wavelengths of 400nm, 520nm and 680nm. We further worked out the relationship between the thickness of the grating and the refractive index modulation of the polymer, rigorously analyzed the gradient of the interference fringes and multi-layers structure using glass substrates. The design and evaluation based on RCWA is effective for the fabrication of high-performance VPH grisms and will be thoroughly presented in this talk.