Crystallization heat treatments for the fabrication of volume Bragg gratings based on photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a functional optical material with broad applications in holographic optics. In this work, we examine the effect of crystallization heat treatment on the growth and distribution of sodium fluoride (NaF) nano-crystals in PTR glass using DSC, XRD, SEM, and TEM. The optical properties of the crystallized PTR glass are analyzed using a spectrophotometer. The results indicate that the NaF nano-crystals precipitated in PTR glass matrix are in the form of polycrystalline, and the growth of which is highly dependent on temperature. The growth of NaF nano-crystals saturates during the crystalline phase transition, resulting in decreased distribution uniformity. Higher crystallization temperature and longer crystallization time lead to an increase in absorption and scattering losses in the visible band. However, the crystallized PTR glass exhibits favorable absorption and scattering loss characteristics in the near-infrared band. By adjusting the crystallization treatments within the temperature range of 530 degrees C-570 degrees C for 10-120 min, refractive index modulation (RIM) values of the transmitting volume Bragg gratings (TVBG) ranging from 50 to 350 ppm can be achieved, which allows for maximum relative diffraction efficiencies exceeding 90% at 532 nm and 1053 nm using different crystallization processes. This research enhances the understanding of NaF nano-crystal growth mechanisms and provides insights into the application of holographic optical elements based on PTR glass in both the nearinfrared and visible bands.