Birefringence and internal stress in polystyrene optical fibers

The radial variations of the birefringence of the core of polystyrene optical fibers have been observed and characterized. Variations are much more important on the edge than along the axis, and they introduce a positive radial gradient of the ordinary refractive index that essentially governs light propagation. To explain the existence of such a birefringence, the conditions of the cooling of fibers during their drawing have been simulated. The radial variations of the temperature between the axis and the edge may increase by 10° and, because of the fast variation of the viscosity of polymers in the neighborhood of their glass transition, an important molecular orientation may result in the regions in which this transition first ocurs during the drawing. Comparison is performed between the behavior of polystyrene and poly(methyl methacrylate) core fibers.