Rigorous analysis of form birefringence of weakly fused fiber-optic couplers

Based on a rigorous vectorial formulation, the surface integral equation method is used to solve the vectorial mode patterns and the coupling characteristics of weakly fused fiber-optic couplers. Form birefringence versus aspect ratio of the coupler cross-section for different normalized frequencies V are calculated. A dramatic feature-that there is a local maximum of form birefringence for the weakly fused couplers with large V-values-is found. The dependence of the aspect ratio at which the coupler is isotropic on the V-value is also presented. The fused coupler with large V-value is isotropic as the aspect ratio is close to 1.8, but its polarization effect is still significant when the aspect ratio is 2.0