Subwavelength Grating-Assisted Contra-Directional Couplers in Lithium Niobate on Insulator

Grating-assisted contra-directional couplers are important optical components in photonic integrated circuits (PICs), which can serve as fundamental building blocks for optical (de)multiplexers, switches, filters, and splitters, among others. Recently, lithium niobate on insulator (LNOI) has emerged as an attractive PIC platform with many important active photonic circuit components demonstrated, including electro-optic modulators and second-harmonic wavelength converters. Nevertheless, the LNOI component toolbox still lacks some passive photonic circuit components to fulfill the requirements of future high-performance PICs. In this contribution, a subwavelength grating-assisted contra-directional coupler (GACDC) is proposed, designed, and experimentally demonstrated in a silicon nitride-loaded LNOI waveguide platform. As an example of its applications, a three-channel wavelength-division (de)multiplexer is fabricated and experimentally demonstrated by cascading several GACDCs, with a narrow wavelength spacing of 3.2 nm (400 GHz). The measured device insertion loss of each channel is about 2.5 dB with low interchannel crosstalk below -20 dB. A fabrication tolerance analysis is also carried out, indicating that the proposed GACDCs are tolerant to waveguide width variations.