Talking and Seeing Through Glass
LIGHT—that ancient, mysterious symbol of wisdom and intelligence—is now no longer just a symbol. In recent years it has swiftly and quietly assumed its rightful role and become the actual carrier of all kinds of information. For light to achieve its real potential for transmitting intelligence over very long distances, two developments were needed: (1) a special kind of light and (2) a special kind of light guide.
Recently, by means of a series of exciting new developments, we are now sending incredibly large masses of intelligence of all kinds over great distances and at tremendous speeds using beams of light. Yes, it is now possible to talk, see, and hear with remarkable speed and efficiency, by means of tiny beams of light traveling down hairlike threads of glass. Like gossamer spiderwebs, these threads of glass protected in cables, already run between cities in the United States, in Europe, and in Japan. They are now in process of spanning the oceans, going from continent to continent.
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. Let us consider one example, talking and hearing with light.
Talking and Hearing With Light
Talking and hearing, as well as seeing, by light involve some of the most complex technology of our day. Let us, however, go through just a few of the steps that take place in talking and hearing with light to get a little appreciation of the process.
Even though light is used in the transmission, the actual procedure starts just as before, by talking into the telephone. The sound waves from our voices are still converted into corresponding electrical signals in the telephone. Then samples of these electrical signals are “sliced” out at very high speed. This process is similar to a movie camera, which actually takes a series of still pictures, or slices, of the motion. These pictures are then projected, frame by frame, in rapid succession to give the viewer the impression of movement. Similarly, these electrical slices are removed and coded in a multistep process and then converted into light pulses. The coded light pulses then travel down the thread of glass to the receiving end. When they arrive at the receiving end, they are converted back by the reverse process into sound waves in the earpiece of the telephone. What are the present benefits to us? What are the prospects for the future?
Some of the Present Benefits
Just as we have begun to accept and appreciate our present worldwide communications network, a whole new system has appeared. Fiber optics promises to replace multiconductor telephone cables, microwave networks, and even some satellite stations, yet with a host of benefits added.
◼ Communication Without Interference. One of the most important advantages of fiber-optics transmission for the telephone customer is that it practically eliminates many of the kinds of interference to which we have become accustomed. Lightning, power lines, generators—all have annoyed us with static and noise. Even heavily shielded copper conductors cannot prevent some of these disturbances from coming through.
If your telephone conversation was transmitted part way by satellite, you may have noticed a fraction of a second communication delay or observed atmospheric effects. In the past, echoes may even have been present. Fiber optics tends to eliminate noticeable delays and provides clear, undisturbed reception.
◼ Communication With Security. Complete security is one of the outstanding advantages of fiber optics. Thus, cross talk is eliminated, and any illegal wire tapping is essentially impossible. No means has yet been devised to tap in on the light rays, at least not without greatly reducing the signal and thus providing a warning.
◼ Great Efficiency. The fantastic efficiency of information transmission by light can be understood when we consider that thousands of telephone conversations can be carried with just one pair of light fibers. The estimate is that the entire contents of Webster’s unabridged dictionary can be sent thousands of miles in six seconds over one single thread of glass.
◼ Minimum Space—Withstands Hostile Environment. Many places are already benefiting from this new development. Metropolitan regions gain from new, high-volume communications with greatly reduced equipment requirements. Whole rooms full of outdated switching equipment can now be replaced with fiber-optic equipment requiring only a small area. Also, remote areas such as the Florida Keys now enjoy noise-free service. The hostile saltwater environment in the Keys and like areas tends to cause electrical shorting and chemical deterioration. However, with fiber optics, there is minimum effect.
Looking Into the Future
The future for the new development appears to be most promising. Already the changeover is proceeding much more rapidly than was predicted by some. It is reported that one of the biggest problems is to select a system that will not be obsolete by the time it is installed.
◼ Voice, Video, and Computers From One Terminal. High Technology magazine in its February 1986 issue reports under “Business Outlook” that “Fiber Optics has quickly become the preferred medium for transmitting voice, data, and video in the U.S.—especially over long distances.” The article continues with the statement: “We are starting to deploy a fiber network that will extend into the home. Using one terminal that can handle voice, video, and . . . query a database for information.” This is opening up opportunities for at least some people to do shopping, banking, and buying of airline tickets, and have certain library privileges from their home. They should even be able to see their friends when they talk on the phone—all with light through those marvelous glass fibers.
[Pictures on page 20]
Light traveling down a glass fiber reflects internally and is not lost through the wall
High-strength fibers and wires give protection
Glass or plastic cladding lessens the amount of escaping light
[Picture on page 22]
This small fiber-optic cable carries as many or more telephone conversations than this large conventional cable