OPTICAL FIBERS ARE BUILT TO last about 40 years, until some hypothetical technology comes along to replace them. But how long will they really stand up? No one knows for sure. Any accelerated-wear test runs the risk of failing to accurately duplicate the stresses that a fiber would undergo over four decades. That's a serious knowledge gap: Companies such as Corning Inc. and AT&T don't know whether it's necessary for them to spend extra money to lengthen fiber life with the special hermetic carbon coatings they have developed.

That's where Minoru Tomozawa comes in. The professor of materials engineering at Rensselaer Polytechnic Institute in Troy, N.Y., is trying to determine what makes fibers deteriorate. One finding: Water is deadly. It

can slowly seep right through the plastic coatings and, like an acid, dissolve the glass fibers.

Water will penetrate into the fibers most easily where fibers are bent. Tomozawa also showed that water does not just deepen existing cracks but can also initiate cracks even in perfectly formed fibers.

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