Fused silica(FS, also known as quartz glass ) is a type of glass that is very hard and transparent. This material has excellent optical properties. It consists of almost 100% pure _SiO2 and becomes amorphous at high temperatures. All types of transparent FS have almost identical optical properties. Its optical and thermal properties are better than other types of glass due to its purity.

Fused silica is transparent over a wide wavelength range from the deep UV to the IR range (0.16µm – 3.5µm). The UV transmission is greater than with most other types of glass. The water content of the FS, on which its IR transmission depends, is determined during its manufacture. The UV-grade FS, which is thermally treated, has a higher water content and less metallic impurities, making it more transparent in the UV range. The electrically treated IR-grade FS has more metallic impurities, so that its UV transmission is limited to 250nm. On the other hand, it has an IR transmission of up to 3.6µm. Due to its low coefficient of thermal expansion (5.5 ∙ ^10-7 on average from 20°C to 320°C), FS can withstand large temperature differences without cracking. FS is chemically stable and is therefore resistant to most acids, with the exception of hydrofluoric acid (HF) and phosphoric acid (_H2PO4). Due to its high optical purity and high thermal, chemical and mechanical stability, FS is an excellent choice for many critical applications in high quality optical systems.

Fused Silica is used to manufacture windows, prisms, lenses and other optical materials in the IR to UV range. Due to its low thermal expansion, it is also used as a material for precision mirror substrates. Quartz glass has almost ideal properties for the production of surface mirrors for telescopes.