Lasers, Light and Communications
by “Awake!” correspondent in the British Isles
LIGHT—how precious and vital it is to man! Our very lives depend on it, for with no light from the solar system’s great powerhouse, the sun, all life on earth would eventually cease. Owing, no doubt, to the great beauty of light with its endless variety in colour and form, man has pursued from earliest times a deeper understanding of its nature. Along with this he has sought ways of producing and using light for his greater benefit.
Of all his ideas one of the most fascinating came to reality in the 1960’s, catching the imagination of many even outside the world of science. It was the invention of the laser. The first successfully operated laser, in 1960, employed ruby as the working material and produced a red beam, but today many different materials can be used: carbon dioxide, water, helium, argon—each producing its own characteristic colour of light.
How does the light produced by a laser differ from that from other sources? And what practical applications do lasers now have?
Essentially, lasers possess two properties that no other light sources have to the same extent. First of all, the laser does not spew out its light in many directions as an electric light bulb does, but confines it to a narrow, intense, pencillike beam. Secondly, the light itself is extremely pure or “coherent”—like sounding a single pure note on a musical instrument rather than many notes simultaneously.
Because of these special properties, lasers have found many applications in diverse fields. The laser’s directional property has been used to measure the distance of the moon from the earth by directing a beam from a 60-inch (152-cm) telescope. The distance was measured to within one inch (25 mm)! The high intensity of the light makes lasers useful for cutting and welding. Paper, cloth and even diamonds can be cut, and thick steel plates can be welded together very fast by powerful carbon-dioxide lasers. In the field of medicine, laser scalpels are now available. They can be manipulated more accurately than a surgical knife and have the added advantage of the beam itself coagulating the blood, thereby making clamps for blood vessels unnecessary. In the eye, welding of a detached retina is now routinely performed by an argon-gas laser, and delicate vocal-cord operations have been experimentally performed in the throat.
But perhaps one of the most exciting and widespread uses of lasers and certain other types of light sources is now opening up. Scientists have already achieved light-wave communication systems. Already prototypes are in operation in which telephone or television signals can be sent on light travelling along glass fibres instead of electricity along wires. It is expected, in fact, that in the early 1980’s widespread application of light-wave communication systems may be made to telephone transmission.
How is it possible to communicate by means of light? What advantages does this method offer and how will it affect our everyday lives? Let us examine in detail how light-wave systems have been developed. First, we need to consider briefly the physical nature of light itself to see that, in certain respects, it is very similar to the waves already commonly used for communication purposes.
The Nature of Light
In 1864, James Clerk Maxwell, a Scottish physicist, succeeded in combining the laws of electricity and magnetism. He found that, when so combined, they predicted the existence of waves of various types. One of these was identified as being light, but others, then unknown, were later discovered and are now known as radio waves, radar waves and X rays, all of which are invisible.
Maxwell’s theory proved that all the different kinds of waves, including light, are similar in nature; they all consist of electric and magnetic forces that vibrate or oscillate. What makes the difference between, say, a light wave and a radio wave is only the speed or “frequency” of the oscillations. In a light wave the forces vibrate about 100 million times faster than in a typical radio wave.
So, just as a radio wave can carry the music and picture signals for radio and television, a light wave can be made to do the same by using somewhat similar principles and techniques. But because its frequency is so fast, light, when coherent, is theoretically much superior. It has the potential ability to carry a vast amount of information, much more than a radio wave can. It was the hope of realising this possibility that prompted scientists to investigate light-wave communication systems soon after the invention of the laser.
Transmitting the Light
One of the first major problems encountered in the development of a practical system was that of transmission from the source to the receivers. It was soon realised that sending a laser beam directly through the open atmosphere (as is done with radio waves) was neither reliable nor practical. Over long distances, fog, rain, clouds or snow can scatter or block the beam, but even in clear weather temperature variations in the atmosphere can refract or bend the beam off course. In addition, precisely aligned mirrors would be necessary to turn the beam around corners and for it to enter and leave buildings.
In 1966, two British engineers, K. C. Kao and G. A. Hockham, working at Standard Telecommunications Laboratories in England, suggested a better solution to the problem. For many years it had been known that light could be “conducted” or guided by flexible glass fibres as thin as a human hair, like an electric current conducted by a wire. At that time, however, the glass of which the fibres were made was poor. It scattered and absorbed the light to such an extent that half the power was lost after the light had travelled only 10 feet (3 m) along the fibre. Kao and Hockham suggested that if an enormous improvement of the glass quality could be achieved, glass fibres could be used to carry light over many miles.
Based on this idea, Corning Glass Works and Bell Laboratories in the U.S.A., the Nippon Sheet Glass Company of Japan and various research groups in Britain turned their attention in parallel to the methods of manufacturing glass fibre. The first breakthrough came in 1970 when Corning announced a new low-loss fibre made from almost pure silica glasses. Soon the other research groups made further advances, exploring new kinds of glasses and developing new methods of fibre manufacture. Today, glass fibres are routinely produced that can guide light for a mile (1.6 km) before losing half the power; some of the best fibres currently produced lose only a third of the light over this length!
Fibres are made by pulling them from glass fed through a furnace. By winding them onto a drum during the process, single fibres of up to several miles continuous length can be made. In practice, a protective plastic coating is put on the fibre and 100 or more individual fibres are placed side by side with suitable strength members and an outer sheathing to form a “fibre-optic cable.” Such cables are now the central component of light-wave communication systems, each fibre of the cable forming a separate and distinct channel.
How does a glass fibre guide light? The answer lies in a principle in physics known as “total internal reflection.” At a steep angle a beam of light strikes the boundary between two kinds of glass, the glass below the boundary being denser (optically). Some of the light is transmitted and some is reflected. (See diagram.) If, however, the angle is made sufficiently shallow, all the light is reflected as if the boundary were a mirror. This condition is called “total internal reflection.” The fibre has a core made of the denser glass and a cladding of the other glass. Light rays of suitably shallow angles are then guided within the core glass, the light being reflected back and forth along the fibre.
Parallel to the fibre research over the last decade, effort was also directed toward the development and improvement of the other components of the system. Early lasers were bulky and inefficient. There was a need to make new long-life lasers that would be compatible with the fibres. In addition, it was necessary to devise efficient methods of coding the light with the electrical signals at the transmitter and decoding at the receivers.
Today, tiny lasers smaller than a pinhead made from alloys of the elements aluminium, gallium and arsenic have lifetimes of over one year. They produce the light beam when an electric current is “injected” through the device and so are called “injection lasers.” Light-emitting diodes (LED’s), commonly used in electronic calculators, can be constructed in a simpler manner from the same elements. Although their light is not coherent, they still have great importance for lower-capacity light-wave systems.
In such lasers and LED’s, the light beam can be switched on and off electrically many millions of times per second! Thus, like an extremely fast Morse code, telephone or television signals are sent as a coded sequence of light flashes or “pulses” along a glass fibre. At the receiving end, special light detectors made of silicon convert the fast stream of light pulses back to electrical signals.
The stage to which research has progressed is evident from the fact that already several preliminary light-wave systems are in use and more advanced systems are presently undergoing tests in many countries—Britain, U.S.A., Germany, France and Japan leading among them.
Since March 1976, for example, television signals for some 34,000 viewers in the Hastings area of England have been sent via a 4,700-foot (1.4-km)-long fibre-optic cable. The electrical signals are carried on the light produced by a light-emitting diode.
Bell Laboratories have made extensive tests on a prototype system at their facility in Atlanta, U.S.A. The system employed an injection laser and two 2,100-foot (0.6-km)-long fibre-optic cables, each containing 144 individual glass fibres. With light sent along each fibre, a cable had the ability to carry more than 40,000 voices simultaneously! The cables were installed in underground ducts to simulate a typical city telephone system. No fibres were broken during installation.
In Germany, the Telecommunications Group at Munich have installed an experimental fibre-optic cable for the transmission of telephone and television signals. The system has been in successful operation for 12 hours a day since August 1976 without any disturbances.
Other early applications of similar systems have been made on aircraft, ships and in computer links. As more refinements are made in the new technology and in engineering skills required for splicing and connecting glass fibres and cables, it is expected that they will replace many metallic cables in the communications field.
What advantages will be gained from using light and fibre-optic cables? Additionally, what effect will all of this have on our everyday lives?
Advantages and the Future
The use of glass fibres for communications offers several advantages over conventional copper wires. There is no metal in fibres, so they are free from electrical interference. Fibres and fibre-optic cables are of relatively small diameter—a factor of great value in city telephone networks where underground ducts are often congested. They are lighter than copper wires—a great asset for aircraft and satellites where weight must be kept down. And finally, but most important, fibres are cheap to produce.
Initially, fibre-optic cables are seen as a means to accommodate the growth of the already existing communications networks. To the average person this could mean the slowing down of the rise in telephone costs and improving, perhaps, the ease of telephoning.
In the long term, though, the advantages are much more exciting. They rest on this huge information-carrying capacity possible with coherent light, not as yet fully exploited. In order to harness this potential, a new field called “integrated optics” has emerged since 1969. In it, lasers are completely miniaturised and tiny light circuits connect up optical components.
New and fascinating communication ideas are being envisaged. Private homes and offices, fitted with fibre-optic cables instead of telephone wires, could then have direct television access to new centralised services such as computerised libraries, educational centres, banks, medical centres, stores, and so on. With this facility a person could from his own home dial the computerised library for the book of his choice and then read it on his television screen, or call his bank to have his current financial statement displayed. A housewife, if confined to her house, could use a teletype to make up her shopping list on the television screen and then relay the order to a superstore at the push of a button. Video telephones may enable you to see the person you speak with on the telephone!
It is clear, then, that the powerful ability of light to communicate is opening up many new prospects for the future. As light-wave systems begin to move out of the laboratories and into practical use, many benefits may come. When we reflect on all of this we can well appreciate the marvellous and intricate nature of light itself. Truly, man’s inventiveness and his inward quest for knowledge are well provided for in the unending treasures of creation.—Ps. 145:16.
[Diagram on page 22]
(For fully formatted text, see publication)
PRINCIPLE OF TOTAL INTERNAL REFLECTION
light beam at steep angle
light beam at shallow angle
all light is reflected
HOW A GLASS FIBER GUIDES LIGHT
light rays at shallow angles zigzag along the core
denser glass core