Undoped YAG is a new substrate and window material that can be used for both UV and IR optics. It is particularly useful for high temperature and high-energy applications. YAG shows no trace absorption in the 2-3mm region where glasses tend to be highly absorbent due to the strong H2O band. Sapphire is also slightly birefringence. The mechanical and chemical stability of YAG is similar to Sapphire, but YAG is not birefringence and is available with high optical homogeneity.