Apparent activation energy of fused silica optical fibers in static fatigue in aqueous environments

Static fatigue of fused silica optical fiber is known to be caused by stress assisted reaction of silica with ambient moisture. Past studies have often been conducted in hot water to accelerate experiments compared with cooler vapor environment. However, changes in the pH of water with temperature has an important influence on the fatigue rate, but has been ignored in the literature. To study the effect of pH, static fatigue of fused silica fibers has been characterized over a range of temperatures in both distilled water and pH 7 buffer solution. The apparent activation energies were then obtained for the different environments. The silica fibers were found to fatigue faster in pH 7 buffer solution than in distilled water, and the apparent activation energy for fatigue was higher in pH 7 buffer. The difference in the results for the two environments can be understood in terms of the difference in their pH and how the pH varies with temperature.