Improved performance of a silicon-on-insulator-based multimode interference coupler by using taper structures

Multimode effects in a multimode interference (MMI) coupler based on silicon-on-insulator (SOI) are analyzed. Multimode interference in the vertical direction of an SOI-based MMI device degrades the self-imaging quality. Taper structures are introduced at the junction between input/output waveguides and multimode region to suppress the excitation of the undesired vertical modes. A 1 × 4 MMI coupler of low insertion loss (0.101 dB) and small non-uniformity (0.102 dB) is designed as a numerical example.