Military Intelligence To Keep Burglars At Bay

It's a stormy night and your burglar alarm starts clanging. A break-in? More likely, lightning triggered your alarm. But in the next few years, nuisance alarms could become less common, thanks to advances in fiber optics, which are immune to electrical storms and other electromagnetic interference.

Military bases were the first to install loops of glass fibers to protect their peripheries. Inside a loop, lasers pump beams of light in opposite directions. When the fiber is disturbed by pressure or sound, the relationship of the two beams changes. This "phase shift" is picked up by a photodetector, which generates an electrical signal that a computer interprets as a drop of rain, a falling leaf--or a thief. Such systems used to cost more than $20,000 per loop. Thanks to the plummeting cost of electronic components, Mason & Hanger National Inc. in Huntsville, Ala., has brought the price of a loop down to $3,400. Even so, industry leader Fiber SenSys Inc. in Beaverton, Ore., predicts it will be five years before systems show up in middle-class suburbs.

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