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It is well known that thermal stress can significantly influence the properties of optical fibers. These stresses are caused
by variations in the coefficient of thermal expansion (CTE) of the differently doped areas in the fiber, like the core and
the cladding. On the one hand, the stress has a strong effect on the mechanical stability of fibers. On the other hand, the
stress also modifies the most essential property of a fiber, the refractive index distribution, and therefore also the
propagation properties. Similar to the effect of generation of birefringence in polarisation maintaining fibers, thermal
stress also generates changes in the refractive index of the differently doped regions in the fiber.
We report on results of non-destructive polarimetric stress measurements in ytterbium doped fiber preforms, that are
codoped with aluminum as well as with phosphorus. Simple models of changes in the CTE for samples doped with
multiple elements assume an additive superposition of the changes caused by each dopant. In contrast to such simple
models, our investigations have shown that the induced stress cannot be explained by an additive change in the CTE of
the glass material. The occurring stresses turn out to be smaller than the simple sum of the effects generated by the
respective dopants. This result is also in agreement with measurements of the refractive index profile of these samples.
The changes in the index are again not additive for doping with both aluminum and phosphorus.
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