Design of a compact multimode interference coupler based on deeply-etched SiO2 ridge waveguides

A compact multimode interference (MMI) coupler is realized by using deeply-etched SiO2 ridge waveguides, which have a strong confinement in the lateral direction and improve the quality of self-images in the MMI section. A 1 × 4 MMI coupler is designed as a numerical example and demonstrates good performances such as a low insertion (0.0275 dB) and small non-uniformity (0.0507 dB). The total device size of the present MMI coupler is reduced to about 1/16 of that based on SiO2 buried waveguides.