Stress in optical waveguides 1: Preforms

The intrinsic stress properties of GeO2- and F-doped optical fiber preforms have been investigated in detail. The materials were prepared by low-pressure plasma-induced chemical vapor deposition (PCVD), and the dopant concentrations cover the range normally used in optical fiber manufacture (+1% > Δ > −1%). Homogeneously doped preform regions exhibit a constant stress level. This level is exclusively dependent on dopant concentrations. In GeO2-doped silica the stress increases linearly with the dopant concentration. For F-doped silica, however, this dependency is strongly nonlinear. A negative stress difference between undoped PCVD material and the substrate tube material can be explained by the reduced thermal expansion coefficient of PCVD-SiO2 caused by chlorine incorporated during the deposition step. The experiments agree excellently with theoretical predictions based on thermal expansion data.