Improvements in VPHGs for astronomy based on photopolymers

Volume Phase Holographic Gratings (VPHGs) are optical dispersing element widely used in astronomical spectrographs. In the last years, the availability of high performance photopolymers allowed for the development of innovative approaches to produce these dispersing elements. The key activities that have been carried out are: i) a production process based on photopolymeric holographic materials (in particular Bayfol((R))HX by COVESTRO AG) was defined; ii) high quality VPHGs > 170 mm in diameter were manufactured; iii) innovative configurations to increase the dispersion/spectral range were implemented. The effectiveness of these activities is confirmed by the fact that more than 10 devices are mounted on observing facilities and several more are in development or planned. In this paper, we present the VPH technology based on photopolymers.