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Mechanical stress can significantly influence the optical and mechanical properties of fibers. We present results of
nondestructive measurements of thermal stress in rare-earth-doped optical fibers, which are related to the doping
variations across the fiber cross section and the preparation conditions. The samples are preforms and fibers with a large
ytterbium doped core that were prepared by an alternative glass forming process, which is capable to realize bigger ratios
of core to cladding diameter than the MCVD-method. To the best of our knowledge, this is the first time measurements
of stresses and birefringence were made on such active optical fibers and their preforms. The results are compared to
previous results on ytterbium-doped samples fabricated by the MCVD-technology. From the resultant stress profile, we
could calculate the stress induced index changes that are relevant for the light propagation in optical fibers, and discuss
the reasons for the observed mechanical stress. In addition, we could evaluate the samples regarding their changes in
thermal expansion coefficient induced by doping with one or more oxides in several concentrations and different ratios.
We confirmed our earlier result that the active doping component ytterbium generates much higher stress than other
common dopants like aluminum, phosphorus or boron.
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