The influence of the drawing process on the intrinsic stress in optical fibers and the arising possibility to optimize the birefringence of PM fibers

Abstract

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 coecient 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. Another main reason for stress in the final fiber is the mechanical force that is applied during the fiber drawing process. It generates so called mechanical stress that depends on the composition of the fiber and the thermal history of the fiber fabrication process. 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. In this paper we present results of stress measurements on optical ber preforms and fibers. We show, that the measured stress profile in the preform matches the theoretically assumed stress profile for thermal stress very well. Moreover we used this preform to draw fibers under different drawing conditions represented in a large difference in the applied force during the fiber drawing. We present the stress results for these differently fabricated fibers and show how huge the effect of the drawing tension can be. We find that for high drawing forces, the stress state can be reversed 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 signicant effects on the fiber stability and modal behaviour. Finally, we present a way to compensate the additionally induced mechanical stress, which is for example a very good possibility to increase the stress birefringence in polarization maintaining (PM) fibers with panda structure. We compare the mechanical stress states of such Panda Fibers after their fabrication with the state after an additional high temperature step. We clearly find that it is possible to improve the birefringence of these fibers using appropriate preparation steps.