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Storing Light

A new optical device could make high-speed computing and communications possible.
June 20, 2007

A microscopic device for storing light developed by researchers at Cornell University could help free up bottlenecks in optical communications and computing. This could potentially improve computer and communications speeds by an order of magnitude.

The new device relies on an optically controlled “gate” that can be opened and closed to trap and release light. Temporarily storing light pulses could make it possible to control the order in which bits of information are sent, as well as the timing, both of which are essential for routing communications via fiber optics. Today, such routing is done, for the most part, electronically, a slow and inefficient process that requires converting light pulses into electrons and back again. In computers, optical memory could also make possible optical communication between devices on computer chips.

Switching to optical routing has been a challenge because pulses of light, unlike electrons, are difficult to control. One way to slow down the pulses and control their movement would be to temporarily confine them to a small continuous loop. (See “Tiny Device Stores Light.”) But the problem with this approach is getting the light in and out of such a trap, since any entry point will also serve as an exit that would allow light to escape. What’s needed is a way to close the entryway once the light has entered, and to do so very quickly–in less time than it takes for the light to circle around the loop and escape. Later, when the light pulse is needed, the entryway could be opened again.

The Cornell researchers, led by Michal Lipson, a professor of electrical and computer engineering at the university, use a very fast, 1.5-picosecond pulse of light to open and close the entryway. The Cornell device includes two parallel silicon tracks, each 560 nanometers wide. Between these two tracks, and nearly touching them, are two silicon rings spaced a fraction of the width of a hair apart. To trap the light in these rings, the researchers turned to some of their earlier work, in which they found that the rings can be tuned to detour different colors by shining a brief pulse of light on them.

Light of a certain color passes along the silicon track, takes a detour through one of the rings, and then rejoins the silicon track and continues on its way. However, if the rings are retuned to the same frequency the moment after a light pulse enters a ring, the light pulse will circulate between the rings in a continuous loop rather than rejoin the silicon track and escape. Tuning the rings to different frequencies again, such as by shining another pulse on one of the rings, allows the light to escape this circuit and continue on to its destination.

Work remains to be done before such a device will function in a commercial system. So far, the rings only capture part of a pulse of light. As a result, any information encoded in the shape of the overall pulse is lost. This can be solved by compressing the pulse and using a cascade of rings, says Mehmet Yanik, a professor of electrical engineering and computer science at MIT.

The other issue is that the length of time a light pulse can be stored is relatively short, Lipson says. If the light stays in the ring for too long, it will be too weak to use. Lipson says it might be possible to make up for light losses by amplifying the light signal after it leaves the rings to restore any lost power.

Other schemes for storing light have been demonstrated in the past, but these were impractical, requiring carefully controlled conditions, for example, or a large, complicated system. The new approach is an important step forward because it makes it possible to store light in ambient conditions and in a very small device, says Marin Soljacic, a professor of physics at MIT. Once you’ve done that, he says, “then it becomes interesting to industry.”

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