Increase of the minor field component in stress-induced birefringent fibers, due to nonuniformity of stress

Analytical evaluations of the minor -component of a LPO1 mode polarized along -axis in strongly birefringent optical fibers are proposed, for weakly guiding step-index fibers. In the assumption where the -tensor is everywhere diagonalized in axes, it is shown that the anisotropy has little effect on the minor component and that the polarization ratio (Expower/Eypower) remains within a few decibels from its value in an isotropic fiber. A considerably larger change can be predicted from a more realistic analysis of real birefringent fibers, taking into account the nonuniformity of the stress which produces the birefringence; this nonuniformity makes off-diagonal coordinates-dependent terms to appear in the -tensor. A recent experimental report can thus be explained.