Report looks at impact of moderate optical powers, tight bends

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

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.

The researchers state that powers as low as 500 milliwatts can induce permanent damage in singlemode fiber that is bent with a 13 millimeter bend diameter or less. Such bends can be found in exchange racks or splice trays, particularly in areas where a fiber is tugged or pulled.

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

The researchers said that, unexpectedly, the catastrophic failure can occur in 90 degree bends at fairly low powers of less than 1 watt. According to the 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 1100øC, 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.

The researchers state that a fairly small percentage of the power is absorbed. But as it is absorbed it changes the structure of the coating. This causes some more absorption until there is a run away effect. Depending on the input power the temperature can easily go up to 1000øC or more, the researchers state.

BT Exact is based in the United Kingdom. For more information visit www.btexact.com.