Lighting the way to speedier circuits.

The rate at which an electronic switch turns on and off is limited by the speed at which electrons travel through a semiconductor. Now, researchers have circumvented that limitation with the first switch controlled entirely by light. The new optical switch turns on or off at least 20 times faster than the maximum rate possible for an electronic system, they report.

"We use light to control light," says Alan Huang of AT&T Bell Laboratories in Murray Hill, N.J. "There are no semiconductors."

Previously reported "optical" switches incorporated semiconductor components and usually involved converting a signal from light to electrons, then back to light.

The new switch, together with recently developed optical amplifiers, completes the list of components necessary for building circuits in which light does all the work. "We have the makings of a new technology in optics that can go faster than electronics," Huang says. He described the switch's development and its implications for the design of optical computers at last week's Physics Computing '91 conference, held in San Jose, Calif.

Light travels through a medium such as glass considerably more slowly than it does in a vacuum. The new switch takes advantage of an extremely weak, barely discernible optical effect -- namely, that bright light travels through an optical fiber slightly faster than dim light.

The Bell Labs researchers send a laser beam into a 100-meter loop of optical fiber, splitting the beam into two components that travel through the loop in opposite directions. The two separate beams produce an interference pattern where they meet again and recombine. Any slight change that disturbs one beam but not the other alters the interference pattern.

Injecting a hosrt light pulse into the loop so that it travels in the same direction as one of the two interfering light beams makes this beam slightly brighter and speeds it up. The temporarily brightened segment of the beam arrives at the meeting place a fraction of a second earlier than it would otherwise, shifting the interference pattern.

The researchers use this brief, rapid shift in the pattern to control the exit of light from the device. Whereas a purely electronic switch takes at least 10 picoseconds to turn on and off, the optical switch needs only 0.5 picosecond. "We think we can push that even lower," Huang says.

Although this technology can't yet be used to construct a supercomputer, scientists now have all the ingredients they need to create a rudimentary all-optical computer. Huang and his co-workers have already used their optical switches, along with optical amplifiers, to construct simple circuits. They are investigating ways of building more complicated circuits and have taken the first steps toward miniaturizing the optical components.

"There's a lot of work that still must be done," Huang says. But because the components needed to build optical circuits are relatively easy to obtain, "my prediction is that within a couple of years, all the major universities are going to be doing experiments like ours."