Atomic force microscopy studies on optical fibers

The strength of brittle materials is controlled by the presence of stress concentrating defects which can be produced during manufacture or subsequent use. Surface defects, which dominate for glasses, can slowly grow under the combined influence of applied stress and environmental moisture, leading to delayed failure. However, the pristine as-drawn surface of optical fibers is preserved by the application of a polymer coating immediately after drawing so that defects are of atomic dimension. For such high strength material (1 GPa), the strength can be degraded in aggressive environments by surface corrosion. AFM has proved a valuable tool for examining changes in the surface morphology of optical fibers on the nanometer scale and relating them to strength degradation. The surface of fused silica fiber corrodes in aggressive (i.e., warm and wet) environments to form roughness which, while only a few nm root mean square (rms), can substantially degrade the strength leading to drastically foreshortened lifetimes. The incorporation of nanosized silica particles in the fiber coating extends the lifetime under stress by factors of up to 300 or more. AFM has verified that the particles operate by slowing the surface roughening process. Heavy metal fluoride glass fibers (based on ''ZBLAN'' compositions) have many potential applications but are considerably less durable than fused silica and can rapidly degrade in room temperature water. Surface analysis using AFM has shown that the fastest degradation is brought about by conditions that lead to the formation of crystals in the fiber surface.