A fine-drawn silica (glass) fiber or filament of exceptionally high purity and specific optical properties (refractive index) that transmits laser light impulses almost instantaneously with high fidelity. Such fibers are made from quartz coated with germanium-doped silica by vapor deposition; 100 or more filaments are assembled into a cable that has extremely high data-carrying capacity. These are applicable not only to telephonic communication systems, for which they are now being used, but also to remote-sensing devices that permit analysis of samples at widely separated locations. Thus, one of the most important developing uses of optical fibers is in analytical instrumentation. Because, as they are nonelectrical and noncorrosive, optical fiber cables are safe to use in highly toxic or explosive environments, e.g., radioactive separations and hazardous waste analyses. The laser beam is coupled to the end of the cable (which may be up to 1000 m long) by a device called an optrode; the light traverses the cable and interacts with the sample, eliciting a signal that is reflected back through the same cable to a spectrometer. Fiber optics are also used in other forms of instrumentation, e.g., radiation dosimeters and high-temperature thermometers. In the latter case, the fibers are made from single crystals of alumina.See Glass, Optical; Laser; Thermometer (5).
