Since the late 1950s, optical fibers have emerged as revolutionary tools in the fields of medicine and telecommunications. They are capable of transmitting light pulses containing data up to 13,000 miles (20,917 km) without significant distortion. They also permit the "piping" of light into otherwise inaccessible locations, making them useful in diagnostic procedures which would have previously required invasive surgery.
Optical fibers operate on the principle of total internal reflection. Every medium through which light can pass possesses a certain refractive index, the amount by which a beam of light is bent as it enters the medium. As the angle at which the light strikes the medium is decreased from the perpendicular, a point is reached at which the light is bent so much at the surface that it reflects completely back into the medium from which it originated; thus, the light will bounce back rather than escape. In an optical fiber, total internal reflection is accomplished by a layer of material, known as cladding, with a lower refractive index. Once light enters the fiber it is internally reflected by the cladding; this prevents light loss by keeping the beam of light zigzagging inside the glass core. The manufacturing of optical fibers consists of coating the inner wall of a silica glass tube with a hundred or more successive layers of thin glass. The tube is then heated to 2,000 degrees Celsius and stretched into a strand of thin, flexible fiber. The result is a clad fiber, approximately 0.0005 in. (0.0127mm) in diameter. By comparison, a human hair measures 0.002 in. (0.0508 mm).
Fiber optics received its first application in medicine. In the late 1950s, Dr. Narinder S. Kapany (b. 1927) hit upon the idea of building an endoscope capable of seeing around twists and turns in a patient's body by using fiber optic bundles. His device, which came to be called the fiberscope, consists of two bundles of fiber: one incoherent bundle, in which there is no relationship between the order of fibers from one end of the bundle to the other, to transmit light into the body of the patient, and one coherent bundle, in which the individual fibers have the same position at both ends of the bundle, to carry a color image back to the physician. Because of its small size and flexibility, the fiberscope can be used to view many areas inside the body, such as the cardiovascular and digestive systems, that physicians cannot otherwise see without performing surgery.
Optical fibers were first used in the field of telecommunications in 1966, when it became apparent that data transmitted by a laser, however bright, could be broken up and absorbed by uncontrollable elements such as fog and snow. The first optical fibers produced contained flaws that resulted in significant amounts of light loss. To boost the range of the light signal, energized atoms of the rare element erbium were used to amplify the signal at 1.54 micrometers, the wavelength at which the fibers are able to transmit light the farthest. This replaced the more costly method of converting weak light signals to electronic form and back to light again before sending them through the next segment. A telephone conversation is carried over optical fibers by a method called digital transmission. This is achieved by first converting sound waves into electrical signals, each of which are then assigned a digital code of 1 or 0. The light carries the digitally encoded information by emitting a series of pulses: a 1 would be represented by a light pulse, while a 0 would be represented by the absence of a pulse. Upon reception, the light waves are converted back into electronic data, which are then converted back into sound waves. By utilizing digital transmission, telecommunications systems carry more information farther, over a smaller cable system than its copper wire predecessor. A typical copper bundle measuring 3 in. (7.62 cm) in diameter can be replaced by a 0.25 in.(0.635 cm) wide optical fiber carrying the same amount of data. This improvement becomes important in areas where telephone cables must be placed underground, in which space is so highly limited.
The minute size of optical fibers also allows for a significant reduction in the weight of a particular system. The reduced weight is beneficial in systems that require rapid deployment of information, such as in military communications and in aircraft instrument wiring. By replacing the copper wiring on a jet aircraft, up to 1,000 lbs. (454 kg) may be saved, allowing for more economical fuel consumption. Optical fibers are also immune to electromagnetic interference, making them roughly one hundred times more accurate than copper; they typically allow only one error in one hundred million bits of data transmitted.
Optical fibers have been demonstrated as an ideal method of transmitting high-definition television ( HDTV) signals. Because its transmissions contain twice as much information as those of conventional television, HDTV allows for much greater clarity and definition in its picture; however, standard transmission technology is not capable of transmitting so much information at once. Using optical fibers, the HDTV signal can be transmitted as a digital light-pulse, providing a near-flawless image reproduction that is far superior to broadcast transmission, just as music from a digital CD is superior to that broadcast over FM radio. More commonly in the late 1990s though, cable television providers as well as telecommunications companies are laying hybrid cable combining fiber optic with coaxial, as a cost effective approach of bringing fiber optic technology within service of their customers. As of 1997, 25 million miles of fiber optic strands had been laid in the United States.
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