Abstract
Photo‐thermo‐refractive (PTR) glass undergoes a refractive index change after UV exposure followed by heat treatment for nanocrystallization of NaF, allowing phase holograms to be permanently recorded in the glass. PTR glass was invented 60 years ago, and optical elements based on PTR glass have been produced for the past decade, but its detailed crystallization mechanism is largely unknown. Since solubility and composition of the parent glass determine the supersaturation and hence the thermodynamic driving force for crystallization, the present paper was aimed at measuring the solubility of NaF in PTR glass via estimating the equilibrium volume fraction of crystallized NaF as a function of temperature. The temperature above which all NaF originally contained in the parent glass remains dissolved in the melt, and that below which all NaF could crystallize, were also estimated. Redesigning of thermal processing parameters and composition of PTR glass, aiming at the improvement of optical elements based on this glass, can be performed in light of the results achieved.
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