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10 July 2018 Photopolymer-based VPHGs for astronomy: update and new possibilities
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Abstract
Volume Phase Holographic Gratings (VPHGs) are diffractive elements widely employed in the field of astronomical spectrographs. Photosensitive materials are used for the production of such elements and photopolymers represent a very interesting possibility. In particular, Bayfol® HX solid photopolymers are high performance holographic materials that have been already used for the realization of VPHGs working in the visible for small spectrographs. Recently, a new set of GRISMs have been commissioned at BFOSC spectrograph in order to replace worn or outperforming ones and improve the instrument throughput. The first dispersing element covers the Hα band, while the second one is designed to work in the UV down to 330 nm. Issues related to the material absorption and to the light scattering were faced at short wavelengths. A step forward in the implementation of this class of holographic materials is the design of VPHGs working in the infrared. Two gratings were designed, covering the ZJ band (0.8 – 1.35 μm) and the JH band (1.05 – 1.9 μm). RCWA simulations were performed to find the parameters (refractive index modulation and thickness) required to obtain high efficiency in the target spectral ranges. Material absorptions are not negligible in the NIR and have to be taken into account during the design phase. Preliminary writing tests were performed giving interesting results. In order to make the design phase more reliable, a study of the dependence of the refractive index modulation on wavelength was performed.
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Paola Galli, Alessio Zanutta, Chiara Righi, and Andrea Bianco "Photopolymer-based VPHGs for astronomy: update and new possibilities", Proc. SPIE 10706, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation III, 1070651 (10 July 2018); https://doi.org/10.1117/12.2312253
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