The purity of both substrate and coating materials has a significant impact on the final reflection/transmission and laser damage threshold characteristics of an optical component, especially in the deep ultraviolet. In order to consistently produce high efficiency, high damage threshold optics, ARO has worked extensively in qualifying its own materials vendors, and developed a sophisticated procurement methodology to maintain our supply of high purity materials.
The most commonly used substrate materials for ARO optics are either fused silica or calcium fluoride (CaF2). Fused silica is a robust material that is relatively easy to fabricate into a wide range of different shaped optical components making it a relatively economical choice of substrate material. Fused silica comes in variety of different grades, with the two major categories based on UV transmission. So called “UV grade” is useful for transmissive optics down to about 200 nm, while “standard grade” fused silica starts to attenuate significantly below 260 nm. It is difficult to obtain UV grade fused silica in diameters much above 100 mm, so components of this size and above are quite expensive. Within the UV and standard grade classifications, a number of different grades are available, based on parameters such as bubble and inclusion content, index of refraction homogeneity, striae, and birefringence.
CaF2 is a somewhat hygroscopic crystalline material. It typically demonstrates transmission down to about 170 nm, and is also available in different grades, based on UV transmission. The natural anisotropy and brittleness of CaF2 render it prone to chipping and fracturing during polishing, and make it necessary to control the orientation of the crystal’s axis with respect to the polished face. This makes it difficult for the fabricator to simultaneously meet tight surface quality (scratch and dig) and flatness (or surface figure) specifications. It is therefore important not to overspecify CaF2 components, otherwise they will be much more costly than necessary. Since CaF2 components are typically more expensive than equivalent fused silica optics, it is generally only used at wavelengths where fused silica is not transmissive, most notably at the 193 nm excimer wavelength.