Using Visible Light

All forms of modern communication--radio and television signals, telephone conversation, computer data--rely on a carrier signal, a wavelike electromagnetic oscillation with a particular frequency.

Electro-magnetic signals are described in terms of their wavelength (the distance between the peaks of two waves) or their frequency (expressed in hertz, the number of wave cycles per second); the shorter the wavelength, the higher the frequency. By modulating the carrier, we can encode the information to be transmitted; the higher the carrier frequency, the more information a signal can hold.

Copper wire is limited to a frequency of only 1 megahertz, or 1 million cycles per second, enough to carry a few dozen voice channels; at higher frequencies, the wire's electrical resistance increases substantially. Coaxial cables--consisting of a solid conductor placed inside a hollow one to channel the signal between them and shield it from interference--became predominant after World War II and were used for trunk lines between cities; they can carry frequencies of up to 10 gigahertz, or 10 billion cycles per second. Unfortunately these coaxial cables are relatively expensive to lay over long distances, and even satellite and earthbound microwave systems, which operate at frequencies of up to 40 gigahertz, began to reach their practical limit in terms of information-carrying capacity per channel.

The idea of using visible light as a medium for communication had occurred to Alexander Graham Bell back in the late 1870s, but he did not have a way to generate a useful carrier frequency or to transmit the light from point to point. In 1960, an idea first introduced by Albert Einstein more than 40 years earlier bore practical fruit with the invention of the laser. This achievement prompted researchers to find a way to make visible light a communication medium--and a few years later fiber optics arrived.

August 19, 2011

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