An optical fiber is basically a cylindrical dielectric waveguide with a circular cross section where a high-index waveguiding core is surrounded by a low-index cladding. Optical fibers are usually made of silica (SiO2) glass. The index step and profile are controlled by the concentration and distribution of dopants. For example, the core can be doped with germania (GeO2) or alumina (Al2O3) or other oxides, such as P2O5 or TiO2, for a slightly higher index than that of a silica cladding. Alternatively, to take advantage of low-loss pure silica, the cladding can be doped with fluorine for a slightly lower index while the core contains undoped pure silica. Silica fibers are ideal for light transmission in the visible and near-infrared regions because of their low loss and low dispersion in these spectral regions. They are therefore suitable for optical communications and most laser applications in this range of the spectrum. Optical fibers made of other materials are also developed for special applications. For example, low-cost plastic fibers can be used for short-distance interconnections between personal computers and printers in offices. Fibers composed of ZrF4, BaF2, AlF3, LiF3, and other fluorides have a low loss in the range of 2–4 μm in the mid infrared. They can be used for mid-infrared optical communication or medical applications. Fibers for other spectral regions, such as the 10-μm region of CO2 laser wavelengths, are also developed.

Optical fibers have a wide range of applications. Owing to their low losses and large bandwidths, their most important applications are fiber-optic communications and interconnections. Other important applications include fiber sensors, guided optical imaging, remote monitoring, and medical applications.