Compact Multimode Interference Coupler Based on Hybrid Silicon Rich Nitride-Thin Film Lithium Niobate Platform

On-chip multimode couplers are essential optical components in photonic integrated circuits (PIC). Recently, optical waveguides and devices based on lithium niobate on insulator (LNOI), also known as thin-film lithium niobate (TFLN), have gained significant attention due to their high refractive index contrast between the core and the cladding, enabling strong optical confinement [1]. However, CMOS compatible hybrid platform such as silicon rich nitride-thin film lithium niobate (SRN-LN) address existing challenges faced by monolithic TFLN platforms. These includes slanted waveguide sidewall, LiF diffusion, and LN redeposition during dry etching processes, which stem from the strong physical and chemical stability of LN material [2]. In this paper, we present the design and demonstration of a compact 1×2 MMI compact multimode interference (MMI) coupler coupler based on the hybrid SRN-LN platform. Furthermore, we analyze the MMI's performance in terms of loss, and uniform power splitting ratio.