A rigorous comparison of the performance of directional couplers with multimode interference devices

The authors report, for the first time, a direct comparison between a directional coupler and a multimode interference-based device, in relation to their performance characteristics such as crosstalk, polarization dependence and the effect of fabrication tolerances. The coupling efficiency for a nonidentical coupler, produced inadvertently due to problems with fabrication tolerances, is also demonstrated. The vector finite element (FE) and the least squares boundary residual (LSBR) methods are employed as the numerical tools used in this simulation study.