Fiber Optics

I

Introduction

Fiber Optics, a branch of optics dealing with the transmission of light through hair-thin, transparent fibers. Light signals that enter at one end of a fiber travel through the fiber with very low loss of light, even if the fiber is curved. A basic fiber-optic system consists of a transmitting device (which generates the light signal), an optical-fiber cable (which carries the light), and a receiver (which accepts the transmitted light signal and converts it to an electrical signal).

II

Operation

Fiber-optic transmission of light depends on preventing light from escaping from the fiber. When a beam of light encounters a boundary between two transparent substances, some of the light is normally reflected, while the rest passes into the new substance. How much of the beam is reflected, and how much enters the second substance, depends on the angle at which the light strikes the boundary. When the Sun shines down on the ocean from directly overhead, for example, much of its light penetrates the water. When the Sun is setting, however, its light strikes the surface of the water at a shallow angle, and most of it is reflected. Fiber optics makes use of certain special conditions, under which all of the light encountering the surface between two materials is reflected, to reduce loss.

A principle called total internal reflection allows optical fibers to retain the light they carry. When light passes from a dense substance into a less dense substance, there is an angle, called the critical angle, beyond which 100 percent of the light is reflected from the surface between substances. Total internal reflection occurs when light strikes the boundary between substances at an angle greater than the critical angle. An optical-fiber core is clad (coated) by a lower density glass layer. Light traveling inside the core of an optical fiber strikes the outside surface at an angle of incidence greater than the critical angle so that all the light is reflected toward the inside of the fiber without loss. As long as the fiber is not curved too sharply, light traveling inside cannot strike the outer surface at less than the critical angle. Thus, light can be transmitted over long distances by being reflected inward thousands of times with no loss (see Optics; Reflection).

III

Applications

The most widespread use of fiber optics is in communications. But optical fibers can carry light for illumination, to convey images, and even to transmit laser beams.

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A

Communications

Use of fiber optics in communications is growing. Fiber-optic communications systems have key advantages over older types of communication. They offer vastly increased bandwidths, allowing tremendous amounts of information to be carried quickly from place to place. They also allow signals to travel for long distances without repeaters, which are needed to compensate for reductions in signal strength. Fiber-optic repeaters are currently about 100 km (about 62 mi) apart, compared to about 1.5 km (about 1 mi) for electrical systems.

Many long-distance fiber-optic communications networks for both transcontinental connections and undersea fiber cables for international connections are in operation. Companies such as AT&T, MCI WorldCom, and Sprint have virtually replaced their long-distance copper lines with optical-fiber cables. Local telephone service providers use fiber-optic cables between central office switches and sometimes extend it into neighborhoods and even individual homes. Cable television companies transmit high-bandwidth TV signals to subscribers via fiber-optic cable.

Local area networks (LANs) are another growing application for fiber optics. Unlike long-distance communications, LANs connect many local computers to shared equipment such as printers and servers. LANs readily expand to accommodate additional equipment and users. Private companies also use fiber optics and its inherent security to send and receive data. Such firms and institutions as IBM, Wall Street brokerages, banks, and universities transfer computer and monetary information between buildings and around the world via optical fibers.

One of the fastest growing fiber-optic markets is transmitting information for so-called intelligent transportation systems: “smart” highways and streets with traffic lights that respond to changing traffic patterns, automated toll booths, and changeable message signs that give motorists information about delays and emergencies.

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