Numerical modeling of weakly fused fiber-optic polarization beamsplitters. I. Accurate calculation of coupling coefficients and form birefringence

The weakly fused tapered fiber-optic coupler, when properly designed, can function as a polarization beamsplitter. Accurate electromagnetic modeling of the coupler performance is important in helping design the couplers for proper operating conditions. The form birefringence which corresponds to the difference in the coupling coefficients between orthogonal polarization states causes the polarization splitting in the coupler. The form birefringence is small, and we find that accurate calculation of such small quantities is crucial in the modeling. In this paper, we provide accurate polarization-dependent coupling coefficients and form birefringence for the weakly fused coupler structure by a vectorial numerical calculation based on the boundary integral approach. It is found that to obtain form birefringence values of enough accuracy, a large number of divisions along the coupler boundary are required. We propose a nonuniform scheme for the boundary division to save computing efforts. Form birefringence values for different coupler aspect ratios and normalized frequencies are presented.