Effects of Defect Modification and Reduction Techniques on the Radiation Sensitivity of Optical Fibers

We have investigated the relationship of precursor defects in as-drawn optical fiber to glass composition and processing conditions in order to understand the radiation sensitivity of doped-core optical fiber. Techniques are reported for improving the radiation hardness of graded-index multimode fibers through reducing the concentration of doping- and processing-induced defects as well as modifying the residual defects in as-drawn fiber. Significant decreases in radiation-induced loss have been observed for fibers pretreated with hydrogen. An investigation of the role of drawing-induced defects indicates that a lower draw temperature produces slightly harder fiber. A study of core/clad interfacial stress revealed that such stress does not play a major role in radiation sensitivity. Measurement techniques included in situ loss measurements at 850 nm and spectral loss measurements before and after -γ irradiation. In addition, photoluminescence proved to be an effective tool for characterizing specific defect centers. It was found for Ge/P-codoped fibers that the luminescence band at 650 nm attributed to drawing/radiation induced centers has an inverse correlation with induced loss. Previously unreported emission bands have been observed, including one at 720 nm which may be related to fluorine doping.