Compact CMOS-compatible 1 × 2 MMI coupler based on a hybrid silicon-rich nitride-thin-film lithium niobate platform

This paper presents the design and demonstration of a compact CMOS-compatible 1 × 2 multimode interference (MMI) coupler based on a hybrid silicon-rich nitride-thin-film lithium niobate (SRN-LNOI) platform. The MMI coupler design is optimized to ensure low-loss and uniform power splitting. Simulation results indicate excess losses of around -0.16 dB and -0.22 dB for the fundamental TE and TM modes, respectively, at an operation wavelength of 1550 nm. In comparison, the fabricated SRN-LNOI MMI coupler exhibits measured excess losses of -0.36 dB and -0.89 dB for the same modes. Additionally, the proposed MMI coupler demonstrates measured transmittance values ranging from -3.2 dB to -3.77 dB for the TE₀₀ mode and from -3.55 dB to -4.3 dB for the TM₀₀ mode across the wavelength range from 1520 nm to 1580 nm. This hybrid SRN-LNOI MMI coupler features low loss, compact design, and CMOS compatibility, making it an excellent platform for the development of efficient next-generation photonic devices for photonic integrated circuits (PICs).