The application of artificial light to carry information, that is, to transmit signals over longer distances, dates from the use of fire to send messages. In the 1880s, Bell demonstrated his system for transmitting speech using a modulated beam of light. This invention paved the way for more complex light communications over distances of several hundred meters. However, such techniques only became widespread with the invention and implementation of the optical laser. The availability of coherent (monochromatic) light sources has led to the development of modern light-based, dense data transmission. Since then, many methods for transmitting information optically have been proposed and investigated. For example, initially, laser beams were shone through free space (i.e., the atmosphere). This approach has some drawbacks, for instance, system unreliability due to natural atmospheric effects (rain, snow, fog, and turbulence). These difficulties indicate why the propagation of light waves within shielded environments, such as through gas-filled pipes and tubes containing sequential arrangements of optical lenses, have been investigated. Of such approaches, optical fibers provide the best method of channeling light over long distances. For instance, a 550,000-mile optic fiber cable is hidden under the ocean transmitting vast quantities of information at high speed across the globe. However, in the vacuum of outer space, wave guidance is necessary, and in 2013, as part of the first demonstration of free-space laser communication, NASA scientists beamed an image of the Mona Lisa to the Lunar Reconnaissance Orbiter, a satellite orbiting the moon, from Earth.
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