Numerical modeling of weakly fused fiber-optic polarization beamsplitters. Part II: the three-dimensional electromagnetic model

For part I see ibid., p. 685, 1998. We establish a three-dimensional (3-D) electromagnetic model for simulating the power transfer characteristics of fused tapered fiber-optic couplers by combining a vectorial modal calculation for the optical waveguide with dumbbell-shaped cross section, as reported in Part I of this paper, and the step-like approximation method for the tapered structure. The model is used to study weakly fused couplers functioning as polarization beamsplitters. The simulation results are compared to the measured ones and it is shown that good agreement has been achieved not only for the coupling power variation during the drawing process, but also for the spectral response of the coupler. We also study the dependence of the coupling behavior of the extremely weakly fused couplers on different coupler structure parameters. The dependence on the aspect ratio of the cross-sectional shape and the heated length is found to be sensitive. The model is a useful tool in helping design couplers for proper operating conditions