A Novel Design of a Wideband Digital Vertical Multimode Interference Coupler

A novel wideband vertical multimode interference coupler (MMIC) is proposed. The proposed coupler replaces the conventional core of MMICs with a multilayer waveguide made of silicon and silicon dioxide. The thickness of the silicon layers of this waveguide is modulated to get a linear relation between the propagation constants of the guided modes and their mode number. This modulation suppresses the wavelength dependence of the coupling length, which maximizes the coupler bandwidth. Since the refractive index profile of the multilayer waveguide of the coupler consists of a group of binary refractive index pulses, it is referred to as a digital vertical (DV)-MMIC. Numerical computations demonstrate the ability to design a DV-MMIC which couples more than 75% of optical power between 470 nm × 220 nm SOI nanowires separated by a vertical height of 3.3 μm over an ultrawide bandwidth of 825 nm.