Design for mode field coupling of active and passive waveguide

The process where a laser and a Si waveguide were coupled was simulated by MATLAB. In order to obtain the coupling efficiency accurately and quickly, an improved finite-difference beam-propagation method(FD-BPM) and the simple transparent boundary condition(STBC) were selected. Several spot-size convertors (SSCs) of different structure with tapered Si waveguide, square SiON waveguide and SiO₂ layer were designed, simulated and analyzed. On the basis of the results, the effects of the structure of the tapered Si, the shape of the SiON waveguide and the overall SSC length on the coupling efficiency were analyzed. On the other hand, the influence of the refractive index of the SiON waveguide on the coupling efficiency was discussed. The optimum refractive index range with the best coupling efficiency was obtained. They had an instructive significance for the design of the SSC.