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 side-exposure techniques. The results reveal that the birefringence is a quadratic function of the effective index change. An asymmetry coefficient near 0.4 μm^-1 maximizes the birefringence. The calculated photo-induced birefringence is negligible if the index change is lower than 5×10^-4. However, the birefringence can reach 5×10^-6 for large values of index change. The numerical method presented could be applied to the modeling of other asymmetric index profiles.