Ultra-compact and robust mode division multiplexer based on a subwavelength grating structure

Mode-division multiplexing (MDM) increases optical-interconnect capacity by transmitting multiple orthogonal modes through a single multimode waveguide at a fixed wavelength. Compact, low-loss, and fabrication-friendly mode multiplexers are therefore essential for dense photonic integration. Here, we propose and experimentally demonstrate an ultra-compact four-channel (TE 0 –TE 3 ) mode multiplexer on a silicon-on-insulator (SOI) platform using a subwavelength-grating (SWG)-assisted asymmetric directional coupler. By optimizing SWG parameters to tailor modal effective indices and to align the wavelength dispersion of the phase-matched modes, robust phase matching and enhanced evanescent coupling are achieved without requiring sub-100-nm critical features. The device is designed for the 1500–1600 nm band and experimentally characterized over 1525–1575 nm. The device achieves coupling lengths of 5.25 μm, 4.00 μm, and 3.75 μm for TE 0 –TE 1 , TE 0 –TE 2 , and TE 0 –TE 3 conversion, respectively, with tapered bridge lengths of 5 μm, 5 μm, and 7.5 μm. All critical dimensions exceed 100 nm, enabling fabrication using standard electron-beam lithography (EBL). Across 1525–1575 nm, the measured insertion loss is