One offspring of the marriage of electricity and light is better technology for probing the world. Researchers are developing optoelectronic sensors that can record stresses in airplane wings or buildings, peer into living cells, or measure levels of air pollution. "No other technology gives the same capabilities," says physicist Alan D. Kersey, a sensor expert at the Naval Research Laboratory in Washington, D.C.

Sensor technology mostly exploits the fact that light travels in waves. Take the fiber-optic gyroscope now used in some planes. Laser light is split into two beams that travel in opposite directions through a coil of fiber. When the plane turns--and the gyroscope with it--light traveling one way around the coil will have to go slightly farther to reach the end of the fiber. The other path will be a bit shorter. So the two streams of light waves will arrive back at the starting point slightly out of sync, allowing a processor to calculate the change in direction. Built by Honeywell Inc. and others, these gyroscopes are simpler, lighter, and less costly than previous ones.

Light waves can also help peer into solid materials. University of Vermont engineers Peter L. Fuhr and Dryver R. Huston have embedded miles of optical fibers in concrete in a university building and other structures. Light waves shining down the fiber bounce off its internal walls like errant bobsleds, interfering with each other and forming a distinctive light pattern at the end of the fiber. The pattern changes as the fiber is bent or stretched by cracking deep within the concrete--and the change is analyzed by a computer. "This technology shows engineers what is going on inside a structure," says Fuhr. Aerospace companies envision using similar sensors to build "smart" airplane skin, says Kersey, that can "provide real-time analysis of what's going on in the wings."

Other scientists are making sensors by adding impurities to optical fibers that cause the glass to reflect, scatter, or emit light differently when a variable, such as temperature or pressure, changes. A single such fiber can be strung through a factory to monitor the temperature of dozens of machines and furnaces.

SWEET SPOT. University of Michigan chemist Raoul Kopelman has built a microscopic fiber-optic sensor that enables biologists to measure the pH level of a single living cell. Similar devices will monitor everything from calcium to sugar levels inside cells--a boon for biomedical research.

Optoelectronic sensors don't always involve optical fibers. Pollution detectors use laser beams that are reflected off tiny particles in the air. And Hewlett-Packard Co. has developed LED sensors capable of detecting even the slightest turn of a car's steering wheel. The information could be used with a microprocessor to adjust an active suspension system so it banks the vehicle into a turn. Mark Chandler, optoelectronics marketing manager at HP, predicts that such sensors will be a $100 million-a-year market by 2005.

So far, much of the cutting-edge work in sensors is being done for defense applications. The Navy is building cheap but sensitive fiber-optic sensors to pick up the faintest ripples created by lurking submarines. And Hughes Aircraft Co. and others are developing phased-array aircraft and satellite radars. Instead of large radar domes, these planes and satellites would use scores of tiny radar-beam emitters mounted on the plane's or satellite's skin. "A fiber-optic connection to each group of emitters makes it possible to steer the resulting radar beam," says Richard Lind, manager of the optical physics lab at Hughes. Such a system, experts say, could see a license plate from orbit. Not much, it seems, will be hidden from optoelectronic sensors.

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