Environmental effects on the mechanical reliability of fused silica optical fibers

Fused silica optical fiber subjected to adverse environmental conditions exhibits a decrease in time of its strength, either under an applied stress below the initial strength ("fatigue"), or under zero-stress condition ("zero-stress aging"). These phenomena are governed by the environment in which the fiber operates and can determine the operative lifetime of the fiber. To have reliable lifetime predictions is a critical issue, especially for applications in which the fiber is working in harsh environment and in sites of difficult access (for example, undersea cables for telecommunication). Although the fiber reliability has been extensively studied, a complete understanding of the mechanisms responsible for the fatigue and aging behavior of fused silica fiber is still lacking.; The purpose of this work was to investigate the mechanisms causing fatigue and aging of the fiber and to develop methods to improve the resistance against these negative phenomena. The effects of different environments and of different experimental techniques on the mechanical reliability of the fiber were studied. A cation dependence of strength and short term fatigue was found on fiber immersed in alkali hydroxide solutions at high pH. This dependence is not explainable in terms of traditional crack growth models. The investigation was extended to the effects of other chemical species (alkali earths, halides) and some possible mechanisms were indicated. The effect of different loading modes in two-point bending dynamic fatigue tests was studied. Significant effects on the surface morphology were found by evaluating different sample preparation techniques for atomic force microscopy of aged fiber. Finally, a method was developed which dramatically improves the fatigue and aging resistance of the fiber in adverse environments at low applied stress: by mixing colloidal silica particles in the polymer coating of the fiber, lifetime increases up to a factor of 400 times were obtained in static fatigue experiments; zero-stress aging tests also showed substantial improvements. The silica dispersed in the coating is more reactive than the fiber surface; therefore, it reacts preferentially with corrosive species, namely water, diffusing through the coating. This reaction causes the partial inhibition of the roughening process responsible for the fiber strength degradation.