Broadband 2x2 multimode-interference coupler on a silicon-nitride platform

In this paper, we present the design, optimization, and implementation of a sub-wavelength grating (SWG) multi-mode interference coupler (MMI) on a silicon nitride photonic integrated circuit (PIC) platform with a significantly enhanced bandwidth compared to the conventional MMI. We demonstrate the extension of the SWG MMI theory, previously presented for the silicon-on-insulator platform, to the Si3N4/SiO2 platform. Our approach involves an initial parameter optimization for a non-paired design, followed by a shift to a paired design that offers enhanced bandwidth and a smaller footprint. The optimized SWG MMI exhibits a 1 dB bandwidth of 300nm for both the insertion loss and power imbalance, making it a significant addition to silicon nitride photonics.