High-Performance Mode-Multiplexing Device with Anisotropic Lithium-Niobate-on-Insulator Waveguides

Thin-film lithium-niobate photonics has attracted intensive attention due to its superior material properties. In particular, multimode lithium-niobate photonics is becoming attractive for the applications of high-capacity optical interconnects and high-efficiency nonlinear optics. However, multimode manipulation in lithium-niobate-on-insulator (LNOI) waveguides is not easy because there might occur some mode hybridness due to the vertical asymmetry as well as the material anisotropy. In this paper, high-performance mode-multiplexing devices with anisotropic LNOI waveguides are proposed, consisting of a multi-channel mode (de)multiplexer and a sharp multimode waveguide bend (MWB). The mode (de)multiplexer is designed with Z-propagation LNOI waveguides to avoid the mode hybridness. Meanwhile, the sharp MWB is designed to connect with a Y-propagation waveguide, enabling access to the highest electro-optic coefficient and second-order nonlinear coefficient. The fabricated mode (de)multiplexer has a low excess loss less than 0.2 dB as well as low crosstalk less than -19 dB in the wavelength range of 1520-1600 nm. These high-performance multimode photonic devices are useful for developing multimode LN photonics.