Optimization of a volume phase holographic grism for astronomical observation using a photopolymer

We have developed grisms, designed for astronomical observation with the 8.2m Subaru telescope on Mauna Kea, Hawaii. In order to improve both efficiency and cost, we proposed a new type of grism by sandwiching a volume phase holographic (VPH) grating between two prisms, dubbed 'VPH grism'. Photosensitive materials are required for fabrication of VPH grating with high groove density. In this paper, we employed the liquid photopolymer with high viscosity. Using the analysis program valid for the refractive index modulation grating of the rigorous coupled wave analysis (RCWA), we were able to design and fabricate a grism prototype with good performance. From the data we obtained, it was ascertained that the experimental VPH grism had excellent performance. We further aimed to develop a VPH grism with higher dispersion and efficiency by making use of a new photopolymer that has larger refractive index modulation than the present material.