Vertically-integrated multimode interferometer coupler for 3D photonic circuits in SOI

Vertical optical coupling is demonstrated in a multimode interferometer structure fabricated using silicon-on-insulator (SOI) material. These CMOS-compatible couplers are suitable to be used to transfer optical power between stacked waveguide layers of a three-dimensional photonic circuit. Coupling between these layers can be restricted to certain regions by selectively fabricating a silicon channel between them, resulting in an isolated multimode waveguide section. Standard photolithography and etching techniques were used to fabricate proof-of-concept devices consisting of a channel waveguide coupling into a silicon waveguide that is vertically multimode. An optical coupling ratio of 93±4% between the upper and lower waveguide regions of the device was achieved with a coupler length of 241 µm.