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Dry Process Creates Clearer Fibers

Photonics Spectra
Jul 2000
Aaron J. Hand

Although today's fiber optic cables may be able to handle today's telecommunications traffic, the future will demand considerably more bandwidth. Trying to resolve an argument about how contamination sneaks into the silica rods that make up the optical fibers, researchers have pointed to a solution that increases fiber transparency to the theoretical clarity limit, expanding bandwidth by a factor of five.

The researchers knew water was the main contaminant that decreased signal power, caused by the IR light exciting the OH molecules, but they didn't know how the water was being introduced into the manufacturing process. One theory was that it was leaking in during chemical binding. However, a team from Lucent Technologies' Bell Laboratories in Murray Hill, N.J., its plant in Norcross, Ga., and Rutgers State University of New Jersey in New Brunswick found that the chemists were not at fault.

Instead, water forms when OH torches are used to heat the silica rods and draw them out into fibers. Other researchers had estimated the water effect of OH torches and decided that there was no way they could have anything to do with the contamination, said Gordon A. Thomas, one of the Bell Labs researchers, who is now at the Massachusetts Institute of Technology. But the experiments were not done at the right temperature range or with the right sort of glass, he said.

The latest experiments, described in the March 16 issue of Nature, relied on an actual fiber manufacturing process, heating a SiO2 glass tube at 2300 °C with a torch that burned H2 with O2 along the 1-m length of the tube. Thomas and his fellow researchers stopped midprocess, removing a slice of the rod to examine it. What they found was that the previous extrapolations had been way off. Rather than just sitting in the outer part of the silica rod, as previously believed, the water quickly diffuses into the glass core, where it can degrade IR signals.

Between the low- and high-temperature regions, the glass core goes through a peculiar phase transition, turning from hard to very gooey, Thomas said. "Anytime you go through one of these phase transitions, the material behaves very differently," he said. "At the gooey, fluid state, the water moves very rapidly."

Choosing a water-free heat source is a simple solution to fiber contamination. Lucent, for one, already has an optical fiber line (called AllWaves) on the market that takes advantage of the new dry process.

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