Predicted stresses and strains in fused biconical taper couplers subjected to tension.

In fused biconical taper (FBT) couplers, the cores of the fibers are positioned close to each other. In order to bring the cores of the fibers in close proximity, the cladding in the fused midportion of the coupler has to be made thin. At the same time the coupler must be sufficiently strong, both on a short- and a long-time scale, and must be able to withstand appreciable axial deformations. These can be caused by the thermal contraction mismatch of the coupler with its substrate or can be due to a deliberate prestressing of the coupler structure in order to improve its dynamic stability. In this study an easy-to-apply analytical stress model is developed for the evaluation of stresses and strains in FBT couplers from the expected or measured total elongation. In the analysis the nonprismaticity of the coupler structure as well as the nonlinear behavior of the material was considered. The numerical example has been carried out for a coupler that is intended for undersea applications. The results obtained can be used in the stress-strain analyses and physical design of light-wave FBT couplers.