light guide

a structure for the directed transmission of luminous energy. Transmission by beams of light in air is often inefficient or impossible. The use of light beams over long distances is hampered mainly by the presence in the atmosphere of randomly distributed inhomogeneities, which cause a beam to be deflected and to spread. Various types of light guides are therefore employed. One type consists of a pipe in which glass lenses are placed at regular intervals—usually 50 to 100 m from each other. The lenses provide a periodic correction of the wavefront of the light beam. Gas lenses or mirrors of a certain shape can also be used as correctors.

Figure 1. Cross section of a cylindrical optical fiber with cladding

The most promising type of light guide is the glass optical fiber. It is a fine filament consisting of a core surrounded by a cladding (Figure 1). The core’s radius is a₁ and its refractive index is the outer radius of the cladding is a₂ and the cladding’s refractive index is n₂ < n₁. When light travels through the fiber, the rays undergo total internal reflection at the interface between the core and the cladding, and they propagate only along the core, although both the core and the cladding are made of optically transparent materials. Depending on the purpose of the fiber, the diameter 2a₁ can range from several micrometers to several tens of micrometers, and the diameter 2a₂ can range from several tens to several hundreds of micrometers. The values of 2a₁ and n₁/n₂ determine the number of types of waves (modes) that can propagate along the fiber for a given wavelength of light. If 2a₁ is small enough and the ratio n₁/n₂ is fairly close to 1, the fiber can transmit a single mode.

Optical fibers have found extensive application in technology (seeFIBER OPTICS). The outlook for the use of optical fibers in optical communications systems is very promising. The employment of fibers can sharply increase the transmission capacity of such systems to a level beyond that of any other known communications system. Lasers must be used as the light sources in these optical systems. The most important measure of the performance of optical fibers used in such systems is the loss of light owing to absorption and scattering in the fibers. Optical fibers with small light losses were developed by the 1970’s; over a distance of 1 km the transmission factor was 50 percent. Such light guides are made of quartz glass. The difference in the refractive indexes of the core and the cladding is achieved by doping the quartz glass with, for example, boron, titanium, or germanium.

Optical fibers with very low losses are made in the following manner. The materials for the cladding and the core—pure quartz glass and doped quartz glass—are obtained by oxidizing gaseous compounds of silicon and a doping element, for example, SiCl₄ and SiCl₄ + BCl₃. The compounds are then deposited from the gaseous phase in a certain sequence (with concurrent fusion) on the inner surface of a quartz tube. The quartz tube is then compressed, and the fiber is drawn from it.

Very promising optical-fiber wave guides with more complicated configurations have been developed. Examples are multiple-fiber guides and fibers whose refractive index varies continuously throughout the fiber’s cross section. In the Selfoc fiber, the refractive index decreases parabolically with the distance from the fiber’s axis.