Anomaly of Thermal Stress in Optical Fiber Preforms Caused by Aluminum and Phosphorous Doping

It is well known that thermal stress influences the properties of optical fibers. These stresses are caused by variations in the thermal expansion coefficient (TEC) of the differently doped areas, like the core and the cladding, in the fiber. Simple models of changes in the TEC for samples doped with multiple elements assume an additive superposition of the changes caused by each dopant for its own. Results of non-destructive polarimetric stress measurements on fiber preforms doped with aluminum as well as phosphorous are presented. We show, that the induced stress cannot be explained by an additive change in the TEC of the glass material. The occurring stresses are smaller than the sum of the ones generated by only one dopant. This result is in agreement with measurements of the refractive index profile of these preforms. The changes in the index are also not additively for doping with both aluminum and phosphorous.