Editor's note: We are posting the text of this article from the August 1977 issue of Scientific American for all our readers because the author has won the 2009 Nobel Prize in Physics.

Three months ago the Bell System began the, commercial evaluation of a light-wave communication system in which messages are coded into pulses of light transmitted through hairthin glass fibers. The new system carries voice. data and video signals over one and a half miles of underground cable interconnecting two switching offices of the Illinois Bell Telephone Company and a large commercial building in Chicago's business center. The light-guide cable. only half an inch in diameter. contains 24 fibers in two ribbons of 12 fibers each. The information capacity of each fiber is 44.7 megabits per second, meaning that the light source feeding into the fiber is turned on and off 44.7 million times per second. At this pulse rate a single fiber can carry 672 one-way voice signals; thus the 24 fibers have a capacity of 12 X 672, or 8,064. two-way conversations. To match this capacity with conventional pairs of copper wires would require a cable many times larger. Apart from such technological advantages. the light-guide system will save copper and greatly increase the potential capacity of existing underground duct systems.

There is nothing particularly new in using light for communication. After all, the American Indians sent up smoke signals and the English built bonfires to warn of the approach of the Spanish Armada. In the 1790's Claude Chappe built an optical telegraph system consisting of semaphore stations on hilltops throughout France. The system. which reputedly could transmit messages a distance of 200 kilometers in 15 minutes, remained in service until it was superseded by the electric telegraph. In 1880 Alexander Graham Bell invented the "photophone," with which he demonstrated that speech could be transmitted on a beam of light. In one system Bell focused a narrow beam of sunlight onto a thin mirror. When the sound waves of human speech caused the mirror to vibrate, the amount of light energy transmitted to a selenium detector varied correspondingly. The light reaching the detector caused the resistance of the selenium, and therefore the intensity of the current in a telephone receiver, to vary, setting up speech waves at the receiving end. And at least until World War II it was common for naval vessels to exchange messages with Morse-coded light signals.

What is new today are the techniques available for generating a light beam that can be modulated at extremely high rates and, equally important, for transmitting the resulting signals through a glass fiber several miles long with an acceptably low loss of energy. The modern interest in light-wave communications dates from the first demonstration of the laser in 1960. This device, which can emit a nearly monochromatic beam of intense visible, or infrared radiation, opened up a region of the electromagnetic spectrum whose frequencies were 10,000 times higher than the highest frequencies then in service for radio communication systems. Since potential informationcarrying capacity increases directly with frequency, communication engineers had expended great ingenuity over many decades developing systems of ever higher frequency. From the early days of radio they had pushed useful frequencies gradually upward by about five orders of magnitude, from about 100 kilohertz (100,000 cycles per second) to about 10 gigahertz (10 billion cycles per second). Now the laser provided an increase of four more orders of magnitude to 100 terahertz (100 trillion cycles per second). By utilizing only a small part of the full range of light frequencies generated by the laser a single light-wave system could in principle simultaneously carry the telephone conversations of every person living in North America.

Comments