Evaluation of residual curvature in two-point bent glass fibers

In the presence of water vapor, oxide glasses exhibit faster surface stress relaxation than bulk stress relaxation at a temperature below the glass transition temperature. When surface stress relaxation takes place in a glass under an applied stress, surface residual stress can develop upon the release of the applied stress. This phenomenon can be explored by studying the permanent bending of a fiber that is observed when a fiber is heat-treated in air under a bending load, and subsequently released from the bending load. Simple and accurate methods to evaluate the residual curvature in a bent fiber from two-point bending treatments were developed and applied to the bending of SiO2 glass optical fibers.