Hybrid Mode/Wavelength Multiplexer Based on Lithium Niobate on Insulator

Abstract

Multiplexing technology is crucial for boosting optical communication systems’ traffic capacity and has seen extensive development in integrated photonics. However, for the emerging lithium niobate on insulator (LNOI) platform, on-chip mode and wavelength manipulation remain challenging─due to the material’s inherent anisotropy and LNOI waveguides’ vertical asymmetry─hampering high-performance multiplexer development and its use in advanced photonic systems. This work presents a highly scalable hybrid mode/wavelength multiplexer for high-capacity on-chip optical communication compatible with dense wavelength division multiplexing (DWDM) systems. Its architecture integrates a 4-channel mode multiplexer and four wavelength division multiplexers. The mode multiplexer is designed along the Z-propagating axis of an x-cut LNOI wafer to minimize mode hybridness, enabling simultaneous multiplexing of TE₀, TE₁, TE₂, and TE₃ modes. The wavelength division multiplexer uses cascaded Fabry–Perot cavity filters along the wafer’s Y-propagating axis. Fabricated devices show excellent performance: a 4-mode × 12-wavelength (3.2 nm spacing) multiplexer has a 3 dB bandwidth of ∼1.3 nm, an extinction ratio of up to 38 dB, an excess loss of ∼0.5 dB, and interchannel crosstalk below −25 dB. A 4-mode × 8-wavelength (1.6 nm spacing) version also achieves an excess loss of ∼0.5 dB and crosstalk below −18 dB. These results highlight multidimensional multiplexing’s value for high-capacity optical interconnections, laying the groundwork for advanced optical communication systems.