The discovery that light could be channeled along a thin glass fiber first led to the invention of the endoscope, allowing medical doctors to see inside their patients, and then led to fiber-optic communication circuits, the greatest revolution in communication technology since the introduction of microwave ground and satellite links.
The first major use of fiber optics was in medicine. It resulted from the basic understanding that glass can carry light. An optical fiber is a transparent substance, such as glass or plastic, that has been formed into a thread that makes a hair-thin light pipe. The term “light pipe” is used because light flows through the fiber just as water flows through a pipe; if the fiber bends, light flows around the bend.
Suppose that one hundred fibers are laid parallel to each other, forming a bundle, and the fibers at each end of the bundle are cemented together and polished to a smooth, flat surface. If objects at the far end are lighted, their image will immediately appear on the near end of the bundle. The bundle remains flexible except at the ends. If the bundle is placed inside a tight-fitting, flexible tube, the bundle can bend, allowing the operator to see around corners. This is the essence of the endoscope, used by doctors to inspect the gastrointestinal tract. Modern endoscopes also have a group of fibers to transmit bright light, a tube through which micromanipulators can be inserted and used, and a television camera on the near end to magnify the image.
The physical principle behind fiber optics is known as “total internal reflection.” When light traveling in glass strikes the boundary between the glass and the surrounding air, normally, some of the light is reflected back into the glass and some is transmitted into the air. However, if the light beam strikes the air-glass interface at an angle to the interface that is less than a certain critical angle, all of the light will be reflected back into the glass, and none will be transmitted into the air. This is total internal reflection. Light traveling a zigzag path down a thin optical fiber will nearly always be totally internally reflected.
The glass core of a modern optical fiber is very thin, only about 9 micrometers across (a micrometer is one-millionth of a meter), or about one-tenth the diameter of a human hair. The critical angle between the light path and the side of the fiber can be made substantially larger through the coating of the fiber with a substance in which the speed of light is less than it is in the glass. This coating, called cladding, is always used because it makes the fiber more efficient. Cladding was first proposed by American optical physicist Brian O’Brien in 1951 and first used in a fiber bundle by the Dutch professor A. C. S. van Heel in 1953.
An optical fiber is completed with the application of a coating over the cladding to provide physical strength and protection. Thus an optical fiber consists of a glass or plastic core, the cladding, and the protective coating. An optical cable consists of a bundle of optical fibers surrounded by strengthening fibers and a cable jacket. While plastic is lighter and more flexible than glass, it is not as transparent; it can be used only for shorter distances, such as within a building. For distances of more than a few hundred meters, glass fiber is used.
A conventional telephone circuit requires a telephone that converts sound to electrical signals.
Fiber-optics strands.
Developing practical fiber-optic circuits required the simultaneous development of optical fibers, light sources, light receivers, amplifiers, and associated electronics. In 1966, Charles K. Kao and Charles Hockham
In the early 1970’s, the military began installing some fiber-optic circuits in ships and airplanes. In 1977, several telephone companies—American Telephone and Telegraph (AT&T) and Bell Systems in Chicago; General Telephone and Electronics in Long Beach, California; and the British Post Office at Martlesham Heath, near Ipswich, England—began field-testing and sending live telephone traffic on fiber systems. In 1980, broadcasters of the Winter Olympics at Lake Placid, New York, asked for a fiber-optic video transmission system as a backup. It worked so well that they used it for their primary system. Long-distance fiber-optic cables were installed at an ever-increasing pace: in 1983, New York City to Washington, D.C.; in 1984, Boston to Washington, D.C.; in 1988, a transatlantic submarine cable; in 1989, a transpacific submarine cable; and by 1997, cables spanned the Earth.
In 1993, a single copper circuit could carry twenty-four simultaneous telephone conversations. The copper circuit accomplished this by adding the conversations to twenty-four different carrier tones. A scheme to combine communication signals and send them over a single circuit is referred to as multiplexing. A fiber-optic pair could carry 24,192 simultaneous conversations. Fiber optics did that by using several carrier frequencies, known as wavelength-division multiplexing (WDM). This process entailed compressing many seconds of conversation into a small fraction of a second, sending the compressed seconds, and then expanding the compressed seconds back to normal conversation at the receiving end. This last act is known as time-division multiplexing (TDM).
The development of fiber optics was pushed by the need for more carrying capacity for phone and data lines, especially for the Internet. More bandwidth was needed. Bandwidth refers to the range of frequencies required to send data at a particular rate. If the information is sent on a carrier signal, bandwidth refers to how far apart the frequency of a second carrier signal must be so that the two signals do not interfere. This is similar to the connection between two radio stations in an area where both stations can be heard. Frequency modulation (FM) stations are allowed a 200 kilohertz (kHz) bandwidth. For example, a station with a carrier frequency of 90.1 megahertz (MHz) is allowed to have some of its signal anywhere between 90.0 MHz and 90.2 MHz. Televison stations are allowed a 6 MHz bandwidth. A coaxial cable has a usable bandwidth of 750 MHz, allowing for about 112 television channels. However, a modern fiber-optic cable using multiplexing can carry data at a terahertz (1012 Hz). This is enough for more than 160,000 TV channels. It is also fast enough to send the contents of thirty thousand encyclopedia volumes in one second. Optical fibers have a potential bandwidth of 50 THz or more. Communication will continue to rely on fiber optics.
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Hecht, Jeff. City of Light: The Story of Fiber Optics. New York: Oxford University Press, 1999. Popular, level account of the birth and maturation of fiber optics. Includes a time line and a list of people, laboratories, and companies important to the development of optical fibers. -
_______. Understanding Fiber Optics. Upper Saddle River, N.J.: Pearson Prentice Hall, 2006. Avoids theory and math but is more technical than City of Light. Explanations are clear, and the book has a useful glossary. Probably of more use as a reference book than for reading straight through. -
Lacy, Edward A. Fiber Optics. Englewood Cliffs, N.J.: Prentice Hall, 1982. Nice account of the beginning of fiber optic communications, many helpful drawings, and a good glossary.
Direct Transoceanic Dialing Begins
Optical Pulses Shorter than One Trillionth of a Second Are Produced
First Commercial Test of Fiber-Optic Telecommunications
Rise of the Internet and the World Wide Web