Role of bromine on the thermal and optical properties of photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a photosensitive silicate glass doped with cerium, silver, and fluorine. The precipitation of a minor crystalline phase after UV-exposure and thermal treatment induces a refractive index change, which is large enough to create diffractive optical elements. In this work we present a first attempt to understand the role of bromine on thermal and optical properties of PTR glass. We reveal that at least 75% of the concentration of bromine of commercial PTR glass is necessary to produce photo-induced crystallization. We also show that an increase of the bromine concentration will increase the mean refractive index of virgin PTR glass. Further thermal treatments induce a decrease of the mean refractive index of PTR glass. This decrease is finally shown to be larger in UV-exposed glass if photo-induced crystallization is present. We also show that bromine concentration will decrease water concentration in PTR glass and have an impact on the losses in visible. We demonstrate that the higher the bromine concentration, the higher the shift of the silver containing particles absorption band. In addition, if bromine concentration is high enough to produce photosensitivity, an increase of scattering due to the crystallization process is observed.