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the branch of optics that deals with the transmission of light and images through light conductors and waveguides for the visible wavelength range, particularly through multistrand light conductors and flexible fiber bundles. Fiber optics came into being as recently as the 1950’s.
In fiber-optical devices the light signals are transmitted through light conductors from one surface (the input face of the light conductor) to another (the output face) as a set of image elements, each of which is transmitted through its own light-conducting strand. The glass fibers that are usually used have a light-conducting strand (core) with a high index of refraction surrounded by a glass sheath with a lower index of refraction. Consequently, the rays undergo total internal reflection at the boundary between the core and the sheath and propagate only along the strand of the light conductor. In spite of a multitude of such reflections, losses of light in the conductor are mainly due to light absorption by the glass of the strand. The transmission coefficient of light conductors in the visible spectral region is 30-70 percent for a conductor length of 1 m. The diameter of the light-conductor strands in devices designed for various purposes varies from several microns to 1 cm. The propagation of light in light conductors with diameters that are large compared to the wavelength takes place according to the laws of geometric optics. How-ever, only certain types of waves, or combinations of waves, propagate in thinner fibers (of the order of a wavelength); this subject is treated within the framework of wave optics.
Rigid, multistrand, and bunched light conductors with a regular fiber arrangement are used for the transmission of an image. The image is projected onto the input face by a lens and is observed at the output through an eyepiece. The image quality in such devices is determined by the diameter of the strands, their total number, and their quality. The resolving power of such bunched conductors is 10-50 lines per mm; in rigid, multistrand light conductors and in devices produced by the fusion of such conductors it is up to 100 lines per mm. The defects in such devices, regardless of their location on the strands of the light conductor, are transmitted along the strands to the outlet end, distorting the image. This makes the production of high-quality devices difficult.
Plates cut across the grain from tightly fused fibers are used for the frontal glass of kinescopes; they transfer the image onto their external surface, which makes contact photography of the image possible. In this case, the main portion of the light emitted by the phosphor reaches the film, and the illumination of the film is dozens of times greater than in the case of photography using a camera with a lens.
The numerical aperture of the fiber devices is usually within the range of 0.4 to 1.0. Tapering light-conductor bundles (focusing cones) collect on the narrow end face the light flux incident to the wide end face. In this case, the illumination and the inclination of the rays increase at the outlet. An increase in concentration is possible to the point at which the numerical aperture of the cone of light at the outlet face equals the numerical aperture of the light conductor. A further decrease in the diameter of the outlet leads to the escape of a portion of the rays through the side of the light conductor or to the return of the rays to the wide end face.
Fiber optics is used in almost all areas of scientific re-search. Hundreds of kinds of optical and electronic-optical instruments are manufactured with fiber-optical parts. Rigid straight or prebent single-strand light conductors and bundles of fibers 15-50 microns in diameter are being used in medical cold-light instrumentation for illuminating the nasopharynx, the stomach, and other organs. In such instruments the light from an electric lamp is collected by a condenser on the input face of the light conductor or bunch, through which it is conducted to the cavity to be illuminated. This makes it possible to keep the lamp (a heat source) away from the cavity. Light conductors with fixed degrees of interweaving are used in high-speed cinematography, for recording nuclear particle tracks, as scanning converters in picture transmission and television measuring technology, as code converters, and as encoding devices. Active (laser) fibers have been developed; they perform as quantum amplifiers and quantum generators of light and are designed for high-speed computers and for performing the functions of logic elements or memory storage cells. Fibers attached at one end, resembling a skewed brush (septrons), make possible the analysis of sound spectra, the separation of voices from crowd noise, and the development of devices for controlling machines by voice signals.
Fiber components are produced from particularly pure materials. Light conductors and fibers are drawn from melts of the required type of glass. A new optical material, crystal fiber (which is grown from a melt), has been developed. In this material the threadlike crystals are the light conductors, and additives introduced into the melt are the interlayers.
REFERENCESKapany, N. S. Volokonnaia optika. Moscow, 1969. (Translated from English.)
Veinberg, V. B., and D. K. Sattarov. Optika svetovodov. Moscow, 1969.
V. B. VEINBERG
fiber optics[′fī·bər ‚äp·tiks]
fiber opticsThe use of optical fibers. Fiber-optic networks transformed the world of communications. Starting in the late 1960s and gaining momentum in the 1980s, the phone companies replaced their copper long distance trunks with fiber cable. Fiber transmission also reaches many commercial buildings and even to the home in certain neighborhoods. In time, the electronic circuits in computers may be replaced with light, in which case fiber pathways would be used throughout the system. See FTTP, optical fiber and fiber optics glossary.
|Is Freedman Having Fun?|
|It doesn't look that way. Alan Freedman, author of this encyclopedia, is always reading. More than two decades ago, he was learning about fiber optics even while on vacation, and although Jeff Hecht's books are great, the concepts are mind boggling.|