Nonlinear power coupling in the birefringent fiber form birefringence effect

Nonlinear power coupling between two linearly polarized modes in birefringent fibers is examined. In asymmetrically shaped optical waveguides, such as rectangular and elliptical waveguides, which are typical models for polarization beam splitters making use of form birefringence, it is found that the intrinsic critical power of the antisymmetric mode, marking the nonlinear instability, approaches zero at a given aspect ratio; this aspect ratio is 1.587 for the rectangular shape and 1.376 for the elliptical shape.