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linked by optical fibers, process radar information from around the globe. Army field communications systems also depend on optical fibers. Optical fibers are being used by the University of Pittsburgh to connect school computers. A college student or teacher will be able to get information from any connected computer, library, or classroom on campus. Other schools are installing similar networks. The new technology of fiber optics has grown quickly in the past decade. In the next ten to fifteen years, the copper wire telephone trunk lines in most of the world will be replaced with glass "wires." These slender strands will harness pulses of light to transmit the human voice and vast amounts of information in a twinkling. More and more, people will use beams of light to communicate with each other. Imagine how excited Alexander Graham Bell would be to know that his dream has come true.

3 How Are Optical Fibers Made? The glass used to make optical fibers must be very pure. Light must be able to pass through the length of the fiber without being scattered, or losing brightness. Though the glass in an eyeglass lens looks perfect, a three-foot-thick piece of this kind of glass would stop a beam of ordinary light. Tiny particles of iron, chromium, copper, and cobalt absorb or scatter the light. The glass in an optical fiber is nearly free of impurities and so flawless that light travels through it for many miles. If ocean water were as pure, we would be able to see the bottom of the Mariana Trench, over thirty-two thousand feet down, from the surface of the Pacific. An optical fiber has a glass inner core. Light travels through this highly transparent part of the fiber. The core of an optical fiber is surrounded by an outer covering called the cladding. The cladding is made of a different type of glass from the core of the fiber. For this reason, the cladding acts like a mirror. Light traveling through the core of the fiber is reflected back into the core by the claddingmuch like a ball bouncing off the inside wall of a long pipe. In this way, light entering one end of an optical fiber is trapped inside the core until it comes to the other end. How do people make these gossamer threads of glass that can carry light around curves and corners and over long distances? Optical fibers are manufactured in "clean rooms." The air in these rooms is filtered to keep out the tiniest particles of dust. Even the smallest specks of dirt could ruin the fiber as it is made. Workers in these areas usually wear jump suits or lab coats and caps made from lint-free fabric. An optical fiber starts out as a hollow glass tube. The tube is mounted on a machine that rotates it. A special gas is fed into the tube. A naming torch moves back and forth along the tube, heating it to nearly 1,600° С. With each pass of the torch, some of the hot gas inside forms a fine layer of glass on the inner wall of the tube. A series of different gases can be fed into the tube. With this method, layers of several different kinds of glass are added to the inside wall. When the addition of glass is complete, gas still inside the tube is gently sucked out. Now, the heat from the torch is increased to 200U° C-The hollow tube collapses into a solid glass rod called a preform. The preform is the size of a broomstickabout as big around as a fifty-cent piece and a yard long. The preform is cooled and carefully inspected. Light from a laser is used to make sure the core
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