Water-free process makes high-bandwidth fiber

John Wallace

Fused-silica optical fibers are hindered from reaching their theoretical clarity limit by the very process used to make them, which contaminates them with water. The resulting OH absorption peaks-notably one at slightly shorter than 1400 nm-limit the fiber`s bandwidth and, thus, the number of WDM channels that can be transmitted. Researchers at Lucent Technologies (Murray Hill, NJ, and Norcross, GA), Rutgers University (New Brunswick, NJ), and the Massachusetts Institute of Technology (Cambridge, MA) have experimentally clarified the physics of the contamination process and used their results to make dry fiber that lacks the usual absorption peaks.

Following standard methods, the researchers created a fiber preform by coating the inside of a fused-silica tube with ultrapure glass and collapsing it using a torch that burned hydrogen with oxygen. Rather than drawing the preform into a fiber, the researchers cut and polished a slice of the preform and probed it with a 0.5-mm-diameter beam of light, observing the strength of an OH absorption line occurring at 2.72 µm. They found that OH produced by the torch passed through the cladding of the preform and infiltrated its core at significant concentrations.

Based on this discovery, the researchers have developed a manufacturing process that uses a dry heat source such as an oxygen plasma torch. The process has been used to make dry fiber that transmits at low loss in the 1400-nm region. Lucent has demonstrated simultaneous low-loss transmission of signals over this fiber at 1310, 1400, and 1550 nm. An impediment to wide use of this fiber is the current lack of EDFAs that perform in the 1400-nm region.