Kinetics of degradation during fatigue and aging of fused silica optical fiber

Fused silica optical fiber tested in aggressive environments can exhibit a 'knee' in both the zero-stress aging and the fatigue under stress; degradation proceeds at an accelerated rate beyond the knee. This behavior leads to shorter lifetimes than predicted from short term data and to strength degradation even in the absence of an applied stress which can result in handleability problems. While the first observation of this behavior was for a humid environment, later work only reported the knee in liquid aqueous environments. This paper reports the observation of a pronounced fatigue and aging knee for a fiber tested in 85 degree(s)C, 85% relative humidity, clearly indicting this phenomenon can occur in more benign environments. Surface roughness measurements using atomic force microscopy also show an abrupt increase in roughness indicting that, for this fiber at least, the development of surface roughness before the knee can not be used as a precursor for predicting the position of the knee.