Accurate Finite Element Analysis of Polarization-Maintaining Optical Fibers,

Single-mode polarization-maintaining optical fibers that can maintain a state of polarization over a long length play an important role in coherent optical communications and optical fiber sensing systems. These fibers are realized by using axially nonsymmetrical refractive-index distribution or by a nonsymmetrical stress distribution to reduce mode coupling between two orthogonally polarized modes. Eigenmodes of these special fibers cannot be found analytically, they must be determined by approximate methods. So far some numerical techniques have been tried to analyze such fibers among them point-matching method, mode-matching method 6 and finite element method can be mentioned. In this paper some polarization-preserving optical fibers are investigated rigorously using an accurate vector H-field finite element method. The finite element method is particularly suitable for the analysis of arbitrarily shaped waveguides hence it is a very flexible analysis tool for fibers with axially nonsymmetrical cross-section or refractive-index distribution. The present finite element formulation is made via a full H-vector field, which is particularly suitable for optical waveguides as field continuity is automatically satisfied at dielectric interfaces. By contrast, in the previous EzHz formulation continuity conditions needed to be specifically imposed. The present H formulation is also valid for general anisotropic refractive indices lossless without destroying the canonical form of the eigenvalue matrix and this formulation is accurate for the analysis of high-birefringence fibers.