Investigation of index change in compression molding of AsSeS chalcogenide glass.

Chalcogenide glasses are emerging as alternative materials for low-cost and high-volume glass molding processes for infrared optics. In precision glass molding, it is well documented that the refractive index variation in the molded elements can lead to substantial amounts of aberrations. The variation has such a significant effect that the optical designs with molded lenses need to be carefully considered and compensated for index variation to achieve targeted optical performance. This research is aimed to evaluate the refractive index change of a chalcogenide glass during the molding process by both finite element method-based simulation and optical experiment. First, a set of mold inserts was designed and machined by high-speed single-point diamond milling. The structure of the lower mold insert was semiclosed and detachable, which facilitated the molded infrared prisms' release from the mold. Second, finite element method simulation was implemented to predict the refractive index change during the cooling phase by using the Tool-Narayanaswamy-Moynihan model for structural relaxation behavior. It was confirmed that refractive index variation occurred inside the molded wedge due to rapid thermal cycling. However, the amount of variation in the molded element indicates that the refractive index change during the molding process was not uniform. Finally, the refractive index of the molded wedge was measured by an optical setup. The results showed that the index shift is approximately -0.0226 for AsSeS, which matched the numerical result by simulation. Compared with oxide glass materials, the index drop of AsSeS has a significant effect on optical performance of molded optics, and the postmolding refractive index should be taken into account in the optical design. In summary, the results presented in this article provided reliable references for refractive index change of AsSeS glass, crucial for precision glass molding or similar applications.