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 mu ch more important on the edge than along the axis, and they introduce a positive radial gradient of the ordinary refractive index that essen tially governs light propagation. To explain the existence of such a b irefringence, the conditions of the cooling of fibers during their dra wing have been simulated. The radial variations of the temperature bet ween the axis and the edge may increase by 10-degrees 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 drawin g. Comparison is performed between the behavior of polystyrene and pol y(methyl methacrylate) core fibers.