Investigation of thermoforming mechanism and optical properties' change of chalcogenide glass in precision glass molding.

Chalcogenide glasses are emerging as enabling materials for low-cost infrared optics due to their transparency in shortwave-to-longwave infrared bands and the possibility to be mass produced by precision glass molding (PGM), a near net-shape process. This paper aims to evaluate the thermoforming mechanism of As40 S60 glass around its glass transition temperature (Tg ) and investigate its refractive index change and residual stresses in a molded lens during and after PGM. First, a constitutive model was introduced to precisely predict the material behavior in PGM by integrating subroutines into a commercial finite element analysis (FEA) software. This modeling approach utilizes the Williams-Landel-Ferry equation and Tool-Narayanaswamy-Moynihan model to describe stress relaxation and structural relaxation behaviors, respectively. The numerical simulation revealed that the cooling rate above glass transition temperature (Tg ) can introduce large geometry deviations to the molded optical lens. The residual stresses in a molded lens are generated mainly at the temperature around Tg due to the heterogeneity of thermal expansion from viscoelastic to solid state, while structural relaxation occurs during the entire cooling process. The refractive index variations inside molded lenses were predicted by performing finite element method simulation and further evaluated by measuring wavefront changes using an infrared Shack-Hartmann wavefront sensor, while the residual stresses trapped inside the molded lenses were obtained by using a birefringence method. A combination of measurements of the molded infrared lenses and numerical simulation results provided an opportunity for optical manufacturers to better understand the mechanism and optical performance of chalcogenide glasses during and after PGM.