11.2 TYPES OF MATERIALS

Materials used for transparent elements (e.g., windows, lenses, beamsplitters, fibers, nonlinear crystals) are essentially fully dense and pure. Fully dense means the material is essentially at theoretical (or x-ray) density. Significant deviations from full density (e.g., <99.9 percent of theoretical density) mean that voids in the material are sufficiently large and numerous to make the material highly scattering, hence translucent. High purity is necessary to ensure low absorption throughout the intrinsic transmission region. (Intentional impurities are used to make devices such as lasers, or to add color to make broadband filters.)

Transparent materials may be classified in several ways. The materials may be crystalline or noncrystalline. Crystalline materials include both single crystals and polycrystalline forms. Single-crystal materials, particularly those with high isotopic purity, are used in applications requiring very low scatter, very low absorption (high purity), substrates for epitaxy, or in devices making use of birefringence (wave-mixing crystals, acoustooptical and polarization devices). Polycrystalline optical materials are used in applications requiring strength or the need to manufacture into near-final-form shape (e.g., by hot pressing or deposition).

Noncrystalline optical materials include glasses and plastics. Glass is the most commonly used optical material, accounting for over 90 percent of the optical elements produced. The term glass is applied to a material that retains an amorphous state upon solidification. More accurately, glass is an undercooled, inorganic liquid with a very high room-temperature viscosity. The gradual change of viscosity with temperature is characterized by several temperatures below the melting point,...