• Talking and Seeing Through Glass
    Awake!—1987 | May 22

    • How is this possible, since all of us know that light tends to travel in straight lines? What makes it possible for the tiny rays of light to stay in the glass threads as they bend around corners? How do these rays go so far and carry so much information? A special kind of light that makes it all possible​—coherent light.

      Efficient Coherent Light

      The advantage of a ray of coherent light over a ray of ordinary light for the transmission of intelligence can be illustrated by photons of light traveling down a glass fiber compared to men walking down a road. Let us think of a ray of ordinary light as though it were a crowd of men of all sizes, all walking out of step and interfering with one another as they walk. On the other hand, a ray of coherent light might be compared to soldiers all of the same size, all in even rows, and all walking in step. Walking in step without interference would obviously move more men greater distances with greater efficiency and less loss of energy. So it is with coherent light.

      At this point some might say: ‘Why has this use of light been so slow in coming? Why has no one thought of it before?’ Actually, it is not completely new. At least one person, Alexander Graham Bell, saw the advantage of talking by means of light and published a paper in 1880 entitled “Selenium and the Photophones.”

      This idea showed great foresight, but without coherent light his invention could have had only limited success. It was not, however, until the 1960’s with the development of the LASER (Light Amplification by Stimulated Emission of Radiation) that the necessary first requirement was met. Bell also lacked the other principal requirement, a highly efficient light guide to transmit the information.

      Those Ingenious Glass Light Guides​—How Do They Work?

      While work was continuing with the development of lasers, others were inventing and developing glass materials of great clarity and ingenious composition that allowed the coherent laser light to travel very long distances. These materials were then drawn down to hairlike fibers.

      Many of us may recall seeing illuminated glass fibers used in eye-catching, artistic table decorations. To make these decorations, bunches of glass or plastic fibers are fanned out like flower arrangements and illuminated from the bottom ends. In these displays just ordinary light is usually used for illumination of the fibers. This illustrates, at least, how light can be made to travel down the threads of glass and around bends instead of just going in straight lines as it usually does. In these displays the light travels over very short distances.

      To enable the light to travel much greater distances than is required in artistic displays, special coatings of glass or plastic have been devised. These special coatings cause any rays of light that may be tending to escape to bend back into the glass and thus prevent further light loss. There are a number of ingenious variations in composition and construction of these coatings. Nevertheless, these many variations, each in its own way and under its own special conditions, help to increase the distance the light travels.

      Although these glass threads, or fibers, have greatly improved our ability to transmit and guide the light, it is still necessary to inject the light into the threads at the critical angle or less. We can understand the principle of how this works when we recall that the smooth surface of a lake can act like a mirror. In fact, the trees along the lake can sometimes be seen mirrored on the surface. This mirror effect is possible because the light coming into our eyes is coming from a very low angle. At just this particular angle, called the critical angle, the surface of the water reflects the light like a mirror. In like manner, when the light is injected into the glass threads at the critical angle or less, it is internally reflected inside the fiber, mirrorlike, with very little light escaping.

      It is expected that these rays will be able to travel up to 25 miles (40 km) or more down those tiny threads without need to regenerate the light. Future prospects are even more encouraging. According to a recent report, ultralow-loss fibers have been developed “that can transmit data thousands of miles without the need for repeaters.”

      In order to protect these marvelous conductors of light, it is necessary to place around them layers and wrappings of protective materials. In addition, high-strength fibers and wires, as well as electrical conductors, are often added to form small cables. When they are protected inside cables, these glass fibers provide an efficiency of transmission of information so great that electrical currents traveling through ordinary copper wires can no longer begin to compete. This is especially true for long distances. But how are data, pictures, and human voices carried by this special kind of light over those tiny glass fibers?

      How the Tiny Fibers Carry Their Big Loads

      Although the special kinds of light rays and the ingenious glass fibers impress us, the way the rays actually carry their enormous loads of intelligence is equally impressive. One basic secret lies in the tremendous speed of light, approximately 186,000 miles per second (300,000 km/​sec). The other is the extremely high frequencies of light waves, amounting to billions of cycles per second. Because of these high frequencies, and by coding the light pulses, tremendous amounts of intelligence can be crowded into the rays of light traveling down the tiny fibers.