Today a single fiber-optic cable barely wider than a human hair can carry half a million phone calls simultaneously. The same technology is soon expected to boost performance dramatically in the realm of computers and printers through the use of microlasers and optic-fiber interconnects.
Yale scientists have discovered a new way of manipulating laser light on the microscopic scale that could greatly increase the information-carrying capacity of today's fiber-optic cables. The finding was announced Jan. 2 in the journal Nature in a cover story by A. Douglas Stone, professor of applied physics and physics, and his former graduate student Jens Noeckel.
"All lasers require a component called an optical resonator, which traps light after it is generated, allows it to be amplified, and selects out a specific frequency of light to be emitted in a desired direction," explains Professor Stone, who proposes a radically new design for a resonator that could be more efficient, more powerful and cheaper to produce on a microscopic scale.
He and Mr. Noeckel, along with Yale physicist Richard Chang, have applied jointly with the University for a patent on the resonator design, called an asymmetric resonant cavity ARC. The discovery, which is based on computer simulations, has attracted the interest of fiber-optic giant Corning Inc. "The collaboration with Corning is a natural one because the accepted approach for sending even more information on fiber-optic networks is to pack the information into different frequencies, or colors, of light, all transmitted at the same time on the same cable," Professor Stone says.
The ARC resonator can be used to detect each of the arriving light frequencies, much like a radio receiver sorts out different radio frequencies. "This approach could save millions of dollars by making it possible to send high-quality video and audio signals at greater speeds on existing fiber-optic networks without installing new cable," he says, adding that a 10-fold increase in speed might be possible eventually.
The discovery is based on the principles of chaotic motion illustrated by the path of a billiard ball bouncing off the wall of a table that is round instead of rectangular. Professor Stone had been using computer models of such motion in his study of chaos theory and realized that rays of light bouncing inside an oval chamber, such as a pill box or glass bead, would behave the same way.
"Light can be thought of either as a ray or a wave, although most scientists today rely on James Clerk Maxwell's wave theory of light, developed at the end of 19th century, which led to the invention of radio," Professor Stone explains. "However, the wave theory of light is very difficult to use in this case. By retreating to the old-fashioned ray theory, which dates back to the 1600s, combined with new-fangled chaos theory, we were able to get an insight into this problem that no one else had."
Like sound in a "whispering gallery"
The Yale computer simulations showed that a light ray could be trapped inside a perfectly round optic resonator, where it would bounce along the perimeter. "This is analogous to sound trapped in a circular 'whispering gallery,' such as the famous one in St. Paul's Cathedral in London, where sound flows along the walls," Professor Stone says. "A whisper can be heard by someone standing against the opposite wall, but not by someone standing in the center of the room."
However, trapped light eventually escapes equally in all directions from a perfectly circular resonator, making it useless for practical applications, notes the physicist. A slightly asymmetrical resonator, on the other hand, traps light of a certain frequency and emits it in a specific direction, making "whispering gallery'" resonators -- like the ARC resonator -- useful as micro-lasers and as detectors of fiber-optic signals.
Because Professor Stone's theory can predict the directions that light will be emitted from an ARC detector of a given shape, he believes he can find the best shape for each application. Corning Inc. now is fabricating glass resonators only 80 millionths of a meter across that are formed to Professor Stone's specifications. They will be tested at Yale for suitability as ARC detectors in research funded by the National Science Foundation.
"We have to build up our understanding of these completely new devices very systematically so we know what to expect from different shapes," says Professor Stone, who joined the Yale faculty in 1986 and was named a Presidential Young Investigator the following year. "Scientists already know how to make microresonators, although no one had thought of the ARC design before. People are getting excited about it. Hopefully in the next five years there will be a lot of research at Yale and elsewhere to understand exactly how it works."