Optical Fibers

Type of physical science: Optical Fibers, Fiber optics, Telecommunications, Classical physics

Field of study: Optics

Durable, flexible, transparent glass fibers, impervious to rust and lightning strikes, have replaced metallic wire as the principal vehicle for transmitting sound and images over great distances.

Overview

Optical fibers have created a revolution in communication. Communication in its most elementary form involves the transmission of information from a sender to a receiver. The human voice is the medium of most human communication. Even in the earliest times, however, the voice proved an imperfect means of communicating because it could not be transmitted over great distances.

Prehistoric beings presumably broadened the scope of their communication by using smoke signals and other visual forms of communicating in order to expand the distances over which they could send information. Watchtowers were probably the earliest relays. They could receive messages from as far as smoke and fire could be seen, then replicate them and send them to other watchtowers. Drums and other auditory means were also used for transmitting data among the earliest humans.

Obviously, early communication was slow and inefficient when viewed from the perspective of modern communicative standards. The Greeks and Romans sent signals of reasonable sophistication from flaring towers in which the color of the smoke transmitted information by means of prearranged codes. As late as the seventeenth century, people communicated in this way, often using telescopes to increase the distances over which messages thus transmitted could be detected.

By the late eighteenth century, a quite sophisticated semaphore signaling system had been devised in France that, before the middle of the nineteenth century, was capable of transmitting messages, through relays, over distances in excess of three thousand miles.

A turning point in human communication came in 1845, however, when Samuel F. B. Morse invented the telegraph and devised the code through which messages were transmitted over great distances by metallic cables. Eventually, such cables were laid between the United States and Europe, enabling telegraph messages to pass quickly between the two continents.

Guglielmo Marconi's invention of wireless telegraphy in 1896 marked another significant advance in the ability of humans to communicate over substantial distances. Marconi's wireless, depending on strategically placed relays, eventually was capable of transmitting messages across the Atlantic Ocean.

Alexander Graham Bell, who invented the telephone, thought that communication by light waves was possible, but this possibility remained purely theoretical until 1960, when the invention of the laser brought about dependable methods of generating and reamplifying pulses of light for the transmission of information. The solid-state semiconductor laser, easily modulated, had such a short life at normal temperatures that its use was impractical for mass communication.

In 1966, Charles K. Kao and G. A. Hockham became the first researchers to propose the transmission of information by optical fibers, very thin glass filaments capable of carrying millions of bits of information on fibers the width of a human hair. Such fibers had been used in medical procedures since 1951. Up until this time, however, no one had suggested their use as a medium of communication.

By 1970, the Corning Glass company had developed an optical fiber that had a higher transmission loss than was desirable but that offered considerable promise. The transmission of information by copper coaxial cables was widely used at this time, but copper could rust, it was bulky compared to optical fibers, and its transmission loss presented problems.

Subsequently, as corporations involved in communications--notably telephone and computer companies and radio and television networks--have come to realize the value and potential of fiber optics, research in the field has been extensive. By the late 1980's, optical fibers were the preferred medium for the transfer of information.

Whereas previous technologies had involved the movement of electrons over cables made of metal, the new technology moved much greater quantities of information more quickly over fibers made of silicon. An initial advantage of this, aside from the quantitative advantages and advantages of speed, was that silicon is more abundant on Earth than the copper from which most metallic cables were made. Over and above this practical advantage, however, optical fibers were some ten times smaller and lighter than their metallic counterparts.

Because the use of optical fibers permits information to travel at the speed of light, such fibers could transmit considerably more information per minute than previous cables had. They were immune from interference from other electrical fields, thereby assuring clear transmission. Transmission losses from optical fiber cables are about one-tenth of those experienced when metallic cables are used.

Techniques for purifying the glass used to make optical fibers was so highly advanced by the late 1970's that interoffice lightwave communications became common. By the mid-1980's, intercity systems were operational, and by 1990, undersea fiber-optical cables had been strung across the Atlantic and Pacific Oceans. Whereas metallic cables beneath the ocean had a limited lifespan because of the motion of the sea and because of rusting, fiber-optic cables were virtually immune from these factors.

Some fiber-optic cables are composed of more than one hundred pairs of fiber filaments. Sounds and visual images are coded into a series of light impulses, each one of which is referred to as a "bit." The transformation of sounds and visual images into bits is achieved through the use of a light-emitting diode or a semiconductor laser. These bits are capable of moving at rates of 3.4 billion per second, which permits up to 50,000 voice circuits for each pair of fibers.

It is necessary to reamplify these light pulses every 20 to 30 miles until they reach a receiver that will amplify and decode the impulses prior to regenerating the information as it originally existed. It is merely a matter of time before fiber-optic transmission is available residentially throughout much of the United States, thereby permitting people to have high-resolution television, digital sound systems, and electronic online facilities in their homes.

The average optical fiber is small, close to .125 millimeters. Even when the fiber is coated with plastic, its diameter is about 1 millimeter, whereas the average metallic cable pair ranges from 1 to 10 millimeters in diameter. The optical fiber cables, which are easy to install, seldom break even if they are bent or come under extreme pressure.

Because silicon has a specific gravity of 2.2 compared to copper's specific gravity of 8.9, fibers made from silicon are much lighter and easier to handle than those made from copper, the most common metal used in metallic cables. An optical fiber cable with the same capacity as a corresponding metallic cable will weigh between one-third and one-tenth what a conventional cable weighs.

The chemical stability of silicon products makes them corrosion resistant; therefore, fiber-optic cables have a considerably longer useful life than those made from other materials. In addition, the transmission loss of cables made from silicon is about one-fortieth that of coaxial cables or broadband city cables.

The capacity of fiber-optic cables is remarkably great. Despite their lightness and small size, optical fibers can transmit about one hundred times the information of comparable metallic cables and about ten times the amount of coaxial cables. Because glass is dielectric, electromagnetic induction and lightning do not affect it. It is also resistant to high temperatures up to the melting point of glass, about 1,900 degrees Celsius, so that it can be used under extreme conditions and resists damage by fire.

The glass fibers currently used in fiber-optic technology are made from ultrapure silica that has been fused. Were ordinary glass to be used, the impurities in it would reduce the intensity of the signal. It is, therefore, imperative to remove impurities before optical fibers are manufactured.

The minimum attenuation or the maximum transparency for silica occurs in wavelengths that approach infrared, about 1.5 micrometers. Most land-based systems throughout the world operate at about 1.3 micrometers, although long-range and transoceanic fiber-optic routes operate above that, close to the wavelength of maximum transparency, 1.5.

When light enters a fiber, it can take any of several irregular courses, called "modes." As the diameter of the fiber decreases, so do the number of modes, making for a better-focused result. The aim, therefore, is to make optical fibers as thin as possible, with the inner core having an index value about 1 percent larger than the surrounding area has. Single monomode fibers work well and are used extensively in most high-capacity optical-fiber systems.

The growth in fiber optics has been exponential, with the capacity of new trunk systems doubling every eighteen to twenty-four months during the 1990's. This growth is likely to continue as fiber-optic systems are made available to individual residences in more areas. At present, some planned residential communities are installing such systems and making them available to all residents in the community.

Applications

The three characteristics that make fiber-optic technology a highly promising communications development are compactness, durability, and speed. Small, light, and impervious to damage by high temperatures and chemical immersion, these fibers transmit data at the speed of light, thus allowing users in a broad range of settings a flexibility in accessing data that they never had previously.

The most obvious applications of fiber-optic technology are concerned with the transmission of telephone messages, the sending of television signals, and the promotion of computer networking. The technology has grown quickly during the last years of the twentieth century and is continually becoming more sophisticated and more reasonably priced.

New uses are being uncovered for this technology. Because optical fibers do not create sparks, points that are spliced do not generate a discharge. Therefore, optical fibers can be used safely in situations that have a potential for explosion or fire. Optical fibers can be manufactured so that they are either flexible or rigid. The flexible fibers are often used as sensors of electrical current, temperature, rotation, flow of fluids, or pressure. They are particularly useful along oil or natural-gas pipelines. Work is being done on interferometers that, attached to sensors, will be able to interfere with a process such as leakage of fluids, thereby limiting loss and damage.

Optical fibers have made possible the kinds of sensors without which the exploration of outer space would not be feasible. They also assure safe working conditions in factories and mines. The fiber optics used in these applications are highly sensitive and reliable, making their measurements with no electrical connections.

Fiber sensors can withstand chemical immersion as well as high temperatures. Radiation does not affect them, so they can be used in almost any setting, however hazardous. They continue to work in the harshest environments.

Flexible fibers make possible the kind of high-density illumination used in automobiles and airplanes. Through their use, the area of robotics has grown tremendously by exploiting the sensory sensitivity of flexible fibers. Surgery has been transformed through the use of instruments the functioning of which depends upon optical fibers. Procedures that were once invasive and life-threatening are now virtually noninvasive. Medical and dental patients currently endure less shock following procedures and recover from them more quickly that ever before, largely because of the sophisticated medical and dental advances that fiber optics have spawned.

By aligning bundles of optical fibers from end to end, instruments have been devised that enable medical personnel to look into areas of the body that were once inaccessible. Such medical equipment as endoscopes and sigmoidoscopes carry clearly focused light into the body's most arcane precincts, enabling physicians to make diagnoses that would in the past have required exploratory surgery.

Rigid optic fibers, usually numbering in the thousands, can be fused into a block to form optic plates. These are used to transfer images from a cathode-ray tube to such permanent photographs as are necessary in fax machines, phototypesetting, photocopying machines, computer-graphic displays, and many other applications.

Because of their low transmission loss and broad wavelengths, optical fibers are used for long-distance telephone cables, subscriber cables for television, telephone-switchboard cable, video-transmission cable, and submarine cable. Their light weight and compactness make them appropriate for various military purposes, most notably for use with aircraft, ships, guided missiles, and both manned and unmanned spacecraft.

The lack of electromagnetic induction in optical fibers makes them impervious to such cataclysmic events as nuclear blasts. Theoretically, they could well withstand such an event, thereby keeping communication channels operative in times of crisis. For the same reason, they are especially useful in areas where lightning is common and in monitoring and controlling power apparatus.

Light propagates in an optical fiber as a lightwave. Its propagation is largely confined to the fiber's core, although a minuscule amount of energy is also propagated around the core's surface. This propagation of light enables it to travel along the fibers to its destination, where it provides light.

The wide-band capacity of cables made from optical fibers has led to research that will eventually affect millions of people. For example, work is being done to link home computers over the Internet so that customers will be able to have a continually updated stock portfolio scroll across their computer screens. Negotiations are also in progress with multimedia firms that will permit subscribers to download compact disks into their computers directly rather than obtaining them through external outlets.

This technology has broad implications for the classroom as well as the home. In cable-modem trials being conducted in elementary and secondary schools, students and teachers have virtually instant access to graphics that, with older technologies, took so much time to retrieve that it was often not practical to use them.

It is estimated that only about 5 percent of the mid-1990's cable systems in the United States can accommodate two-way transmissions of data, but it is merely a matter of time before most cable systems move in this direction. When this occurs, interactive television will become a reality in homes that have the appropriate equipment. The popularity of the Internet, made possible by optical fibers, suggests that interactive television will be well accepted when it is readily available, although its cost may be considerable. Whereas past communications systems depended upon radio waves for the transmission of data, fiber-optic technology depends upon light waves, which have a much higher frequency than radio waves. They function at about 100 trillion hertz, enabling them to carry hundreds of millions of telephone conversations simultaneously, far outstripping the capacity of current telecommunication satellite and microwave links.

Context

Fiber-optic technology directly affects the lives of nearly every citizen of every country in the world. Because it is linked inextricably to nearly all communications systems, this technology is brought into play every time anyone anywhere turns on a television set, talks on the telephone, uses a computer, or deals with any agency that uses one.

Even in the most primitive and isolated settings, nearly everyone has some communication link with the outside world, and such links are increasingly dependent upon fiber-optic cables. Although the theoretical underpinnings of this technology were articulated by Alexander Graham Bell as early as the end of the nineteenth century, it was not until the development of the laser technology of the 1960's that practical steps toward the fiber-optic transmission of data were taken.

Technical impasses that delayed the development of this technology continued until a decade later; once they were overcome, however, industry moved quickly toward developing efficient ways of purifying glass and of creating the virtually indestructible fibers needed for the construction of fiber-optic cables that would carry messages across great distances at incredible speed. Lasers transform the information that travels over transparent optical fibers into light pulses. These light pulses are then sent down extremely thin fibers at the speed of light to photodetectors that transform the pulses back into electrical impulses that are transmitted locally.

Although it was not until the mid-1970's that fiber-optic technology was developed sufficiently to reach mass markets, the growth that followed has been phenomenal. By 1990, fiber-optic cables had been laid across the floors of the Atlantic and Pacific Oceans, thereby improving the quality and speed of transmitting information from continent to continent.

The technology is now moving rapidly in the direction of completely transforming individual residences by linking them to huge worldwide information networks that will enable people to order specific films for showing on their television sets by merely punching in the appropriate codes. Compact disks that carry enormous quantities of information--the whole of the Encyclopaedia Brittanica, for example, is already available on a single disk--will be available for downloading by any subscriber to cable systems that offer such services.

Cost may retard the spread of such services to individuals. They will likely cost considerably more than current subscriptions to Internet links, but they will make much of the information in the best research libraries available to people in the remotest parts of the world. As with all such services, the cost is likely to drop substantially as the roster of subscribers lengthens.

The United States has already moved from being essentially a society economically based upon the manufacture of goods to being a service-oriented society the chief commodity of which is information. This change has enabled many people to work from their homes rather than in offices or factories. The whole texture of American society has been altered greatly by the information explosion that has typified the last half of the twentieth century and that will continue and probably accelerate in the twenty-first.

Principal terms

BIT: A single light pulse within a series that codes sounds and visual images

DIELECTRIC: A nonconductor of electricity

DIODE: An electronic device with two electrodes or terminals

ELECTROMAGNETISM: Magnetism developed by a current of electricity

LASER: A device that produces an intense monochromatic beam of light

MODE: The zigzag course light takes upon entering an optical fiber

RELAY: A system of transmitting information by stages; also, an electromagnetic device in which the opening and closing of one circuit activates another

SEMAPHORE: A visual signaling apparatus with moveable arms

SEMICONDUCTOR: A substance with a conductivity between that of a conductor and an insulator

Bibliography

Allard, Frederick C. Fiber Optics Handbooks for Engineers and Scientists. New York: McGraw-Hill, 1990. An eminently practical book that requires some background in physics for reasonable comprehension. Useful, but not for the beginner.

Li, Tingye, ed. Topics in Lightwave Transmission Systems. New York: Academic Press, 1991. The contributions to this extensive volume range from the theoretical to the practical. The prognostications made by some of the contributors about the future of fiber-optic technology and its implications for society are worthwhile.

Meardon, Susan L. Wymer. The Elements of Fiber Optics. Englewood Cliffs, N.J.: Prentice Hall, 1993. A reasonably clear discussion of fiber optics. Many portions of the book are accessible to those lacking a specific background in physics, although such readers will need to read the book selectively.

Murate, Hiroshi. Handbook of Optical Fibers and Cables. 2d ed. New York: Marcel Dekker, 1996. Although this book is accurate and comprehensive, it requires of its readers a considerable background in physics. The final chapter, "Applications of Optical Fiber," is particularly useful to those who can decipher it.

Okoshe, Takanori. Fiber Optics. New York: Academic Press, 1982. It is interesting to read this early book in the field in the light of more recent developments in fiber-optic technology. Okoshe writes quite technically, but the information he presents offers a useful historical perspective to those interested in the field.

Pal, Bishnu P., ed. Fundamentals of Fibre Optics in Telecommunication and Sensor Systems. New York: John Wiley & Sons, 1992. The entire subject of optical fibers in their application as sensors is explored thoroughly by the contributors to this volume. The essays vary in the technical demands they make upon readers. Much of the writing is highly technical and is peppered with physical formulae.

Senior, John M. Optical Fiber Communications: Principles and Practice. Englewood Cliffs, N.J.: Prentice-Hall, 1985. Despite its age, this book is more accessible to the nonspecialist than many in the field. Because the field has grown so rapidly, the age of this book makes it somewhat less useful than a number of later, more technical volumes.

Smith, D. W., ed. Optical Network Technology. London: Chapman & Hall, 1995. Of the sixteen essays in this volume in the BT Telecommunications Series, "Nerves of Glass," by D. W. Smith, and "New Applications of Optics from Modern Computer Design Methods," by P. F. McKee et al., are perhaps the most useful, although technical.