Windows and Optical Elements

Materials for windows and optical elements must be made with sufficient purity and uniformity to have low absorption and scatter and good index of refraction homogeneity. For most low-power window applications, the absorption and scatter attenuation coefficients typically need to be less than 0.1 cm ^{? 1} (and known down to 10 ^?3 cm ^{? 1}). Transparency for long-length optical fibers (1-km length) and high-power laser windows requires low-level absorption coefficients, i.e., well below 10 ^{? 3} cm ^{? 1}. For low-power-loss applications, higher-order intrinsic processes and extrinsic impurities and defects become important.

Uniformity of the refractive index throughout an optical element is a prime consideration in selecting materials for high-performance lenses, elements for coherent optics, laser harmonic generation, and acoustooptical devices. In general, highly-pure single crystals achieve the best uniformity, followed by glasses (especially those selected by the manufacturer for homogeneity), and lastly polycrystalline materials.

Practical manufacturing techniques limit the size of optics of a given material (glasses are typically limited by the moduli, i.e., deformation caused by the element's weight). Some manufacturing methods, such as hot pressing, also produce significantly lower-quality material (especially when the thinnest dimension is significantly increased). Cost of finished optical elements is a function of size, raw material cost, and the difficulty of machining, polishing, and coating the material. Any one of these factors can dominate cost.

References 8 to 10 contain general information on the manufacturing methods for glasses and crystalline materials.