Numerical analysis of the contribution of the transverse asymmetry in the photo-induced index change profile to the birefringence of optical fiber

The birefringence of an optical fiber resulting from an asymmetry of the index profile is numerically evaluated using a finite element method with a full-vectorial formulation. Such asymmetric index profiles, assumed to vary exponentially across the fiber core, could be induced during the writing of fiber Bragg gratings with UV sideexposure techniques. The results reveal that the birefringence is a quadratic function of the effective index change. An asymmetry coefficient near 0.4 μm maximizes the birefringence. The calculated photo-induced birefringence is negligible if the index change is lower than 5x10. However, the birefringence can reach 5x10 for large values of index change. The numerical method presented could be applied to the modeling of other asymmetric index profiles.