Synthetic single-crystal sapphire (Al2O3) is the leading material for highly advanced optical applications due to a unique combination of excellent optical, physical and chemical properties. It is transparent over a wide range of wavelengths from vacuum ultraviolet to infrared regions (from 0.18 µm to 5.5 µm). Due to its hexagonal crystalline structure, sapphire is anisotropic in many optical and physical properties. Therefore the exact characteristics of optical components made from sapphire depend on crystallographic direction relative to the optical axis (C-axis, 0001). Sapphire possesses birefringence on all directions except for c-axis. Sapphire is the hardest of the oxide crystals. Due to its extreme surface hardness, sapphire can be scratched by only a few substances (for example, diamond and boron nitride). Thanks to its great strength, windows made from sapphire can be much thinner than windows of other optical materials. That's why they are useful even at wavelengths that are very close to their transmission limits.