Faserphotonik

The properties of optical fibers can significantly be influenced by intrinsic stress. It is well known that these stresses are caused by various reasons, e.g. the variations in the thermal expansion coefficient (CTE) of the differently doped regions in the fiber. The so called thermal stresses are only dependent on the composition of the fiber and not on its preparation history. One other main reason for stress in the final fiber is the mechanical force that is applied during the drawing process of the fiber. It generates so called mechanical stresses that depend on the composition of the fiber and the thermal history the fiber passed through. Using a non-destructive polarimetric system, we are able to measure the intrinsic stress state in optical fibers as well as in their preforms. Knowing on the one hand the thermal induced stresses in the preform of a fiber and on the other hand the final stress state in the fiber itself, we are able to differentiate between the two kinds of stress. So, it is possible to calculate the stress that is induced by the process of fiber drawing. We can show that the applied force during the fiber drawing significantly influences the stress in the fiber. We find that for high drawing forces, the stress state can be turned upside down in comparison to the thermal stresses that are induced by the material composition. Due to the fact that stress on the one hand has a strong effect on the mechanical properties of glass and modifies the refractive index, this can lead to significant effects on the fiber stability and modal behaviour. Finally we compare the mechanical stress states in polarization maintaining fibers after their preparation and after an additional high temperature step. We clearly find that it is possible to improve the birefringence of these fibers using appropriate preparation steps.