Finite Element Calculation of Refractive Index in Optical Glass Undergoing Viscous Relaxation and Analysis of the Effects of Cooling Rate and Material Properties

Abstract

In this research, a methodology based on a numerical simulation model is presented to predict refractive index change introduced to two low Tg optical glasses, namely, P‐SK57 and P‐LASF47, by cooling. To model the structural relaxation behavior of glass around glass transition temperature, the Tool–Narayanaswamy–Moynihan (TNM) model is used. In addition, the fictive temperature of the glass samples during cooling is discussed. The effect of cooling rate on the fictive temperature of the glass samples is also presented. This study demonstrated that finite element method is capable of predicting refractive index of optical glass undergoing viscous relaxation. The simulated results in this study confirm that a higher cooling rate leads to a lower refractive index and a larger variation of refractive index in glass optics. The results also suggest that for glass, materials with high thermal conductivity and low heat capacity are preferred for compression molding process.