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Bent fibers put networks at risk

01 Aug 2003

Research suggests that bent optical fibers can be permanently damaged at power levels of just a few hundred milliwatts.

The combination of moderate optical powers and tight bends can prove catastrophic for optical fibers, according to research carried out by BT Exact in the UK.

Although the effect is unlikely to cause problems in current networks, it means that designers may need to think carefully before scaling up the power in their systems or deploying Raman amplifiers with pump powers of several hundred milliwatts or more.

In the July 10th issue of Electronics Letters, Ed Sikora and his colleagues report that powers as low as 500 mW can induce permanent damage in singlemode fiber that is bent (13 mm bend diameter or less).

"These bends could be found in exchange racks or splice trays, for example, especially if a fiber is tugged or pulled," Sikora told Optics.org.

The BT researchers carried out tests on four types of fiber subjected to a range of bend diameters (5 to 15 mm) and optical powers of up to a few watts. In all cases the fibers fail within 53 hours.

"What was unexpected was that the catastrophic failure can occur in 90° bends at fairly low powers of less than 1 W or so," said Sikora. "It's important to understand that we're not saying that networks are going to fall over tomorrow, but as powers go up you have to aware this effect could occur under certain circumstances."

According to the BT researchers, the damage is caused by an increase in temperature that occurs when the power leaks out of the fiber at a bend and is absorbed by its coating. This either causes the fiber coating to burn off leaving the silica beneath exposed or if the temperature is high enough (around 1100°C) the fiber itself deforms giving rise to a large permanent optical loss. The failure occurs more rapidly as the power level rises and the fiber diameter shrinks.

"A fairly small percentage of the power is absorbed but as it is absorbed it changes the structure of the coating causing some more absorption until there is a run away effect," said Sikora. "Depending on the input power the temperature can easily go up to 1000°C or more."

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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