ACCURATE COUPLING-COEFFICIENTS FOR FIBER COUPLERS WITH WEAKLY FUSED CROSS-SECTIONS

For fused tapered fiber couplers with weakly fused cross sections, pow er coupling takes place between two touching tapered cladding cylinder s embedded in the air, and the cylinders then become a strongly guidin g structure. We provide accurate polarization-dependent coupling coeff icients for such a structure, on the basis of a rigorous vectorial num erical calculation. Our results for the corresponding weakly guiding s tructure are compared with previous vector perturbation methods base o n scaler fields. it is found that the scaler theories are not applicab le to those cases with large normalized frequencies or with small fibe r separations when the polarization-splitting effect is concerned.