A Design of a Vertical Multimode Interference Coupler With an Ultra-Wide Bandwidth

An ultra-wideband vertical multimode interference coupler (MMIC) is proposed. The proposed coupler replaces the conventional core of MMICs with a digital core of strongly coupled guiding layers. The thickness of the layers is modulated to fit the spectrum of their guided modes to a linear grid. This linearity suppresses the wavelength dependence of the coupling length, which maximizes the coupler bandwidth. Numerical computations demonstrate the ability to design a digital vertical-MMIC that couples more than 70% of optical power between 470 nm × 220 nm silicon-on-insulator nanowires separated by a vertical height of 3.1 μm over an ultra-wide bandwidth of 805 nm.