Stress birefringence in fiber ribbons

Cables where several fibers (typically, 4-16) are collected in a ribbon allow cost reduction and mass splicing. Their wide use in the access network and, in perspective, in long-haul links, has drawn attention on all factors affecting their performance. Polarization mode dispersion (PMD) in fibers embedded in ribbons has already been investigated, but with contradictory results. All previous results appear to indicate that birefringence in fibers belonging to ribbons can be significantly affected by stress distribution in the ribbon itself, which depends on thermal and mechanical properties of the coating. We report new theoretical and experimental results, which reconfirm this conjecture. Birefringence is shown to depend strongly on the fiber position in the ribbon. Theoretical results are based on a finite-element-method numerical simulation, based on the theory of elasticity, and apply over the temperature range (15-31°C) where viscosity in all materials can be ignored. Central fibers are found to be under compression in the ribbon plane, while lateral fibers do not experience significant compression, as the ribbon coating is softer than silica. Birefringence can be estimated in a simple way from the stress distribution. To validate the predictions experimentally, PMD measurements were performed on a 4-fiber ribbon