Defects in optical fibres in regions of high stress gradients

Germanosilicate optical fibres produced by the modified chemical vapour deposition process are characterised by point defects which disrupt the tetrahedral network structure. It is shown that the GeE´ defect profiles in germanosilicate preforms and fibres, as measured by cathodoluminescence, do not follow the Ge composition profile, but are most intense in regions of high stress gradients, at the core-cladding interface and the Ge depletion region. The results suggest that the GeE´ centres arise from SiGe bond rupture during the draw of the fibre to reduce the thermoelastic stress.