Highly-birefringent elliptical structures.

An analysis of isotropic multi-step fiber structures with elliptical geometry, in particular, the fiber structures which provide the large value of the birefringence, is presented. The multi -layer confocal-elliptical fiber can be analyzed by the exact analytical method with Mathieu function expansion. In that group of fibers, W-profile elliptical fiber provides the largest value of the birefringence. In order to investigate the three-layer elliptical fibers with layers of any ellipticity we apply the improved point-matching method with Mathieu function expansions. It was found that the highly-birefringent fibers with hollow layers outside an elliptical core can be suitable for the single-mode single-polarization transmission. Numerical results are presented for the silica fibers doped GeO2 and F, respectively. The results obtained are of the practical value and can be used directly in designing the elliptical fibers with large of the birefringence.