Coupled modes of anisotropic polarization maintaining fibers

Wave coupling between the fundamental vector modal fields of two identical, polarization-maintaining, weakly guiding fibers is analyzed. An improved coupled-mode theory is used that also applies to anisotropic materials. Coupling coefficients that are the elements of a coupling matrix are calculated and presented as functions of the relative tilt angle between the two fiber principal axes. Exact analytical expressions for the eigenvalues and their associated eigenvectors are given in terms of the elements of this coupling matrix, for any relative tilt angle. Symmetry relations that are found between elements of the coupling matrix much simplify the presentation of the eigenvectors. A weak-coupling approximation of the eigenvector expressions, which is also given, retains their qualitative properties.>