An effective and accurate method for the design of directional couplers

A new method which combines the three-dimensional (3-D) beam propagation method with the overlap integral method is presented for the design of directional couplers. A backward beam propagation is calculated for the output region to reduce the computation time greatly. The design method is illustrated with buried silica-on-silicon waveguide couplers. Results obtained with the 3-D model are compared with those from the two-dimensional model and it is shown that the 3-D model should be used. The design results are verified by both 3-D simulations and experiments. The new design method is shown to be effective and accurate.