Ultra-Broadband Mode Converter and Multiplexer Based on Sub-Wavelength Structures

Current bandwidth capacity provided by wavelength-division multiplexing and polarization-division multiplexing is insufficient to keep up with the increasing bandwidth demand required for new services. Mode-division multiplexing technology paves the way to further increase transmission and bandwidth capacities in photonic interconnects. In this work, we propose an ultra-broadband two-mode converter and de/multiplexer based on a sub-wavelength engineered multimode interference coupler, a 90° phase shifter, and a symmetric Y-junction for the silicon-on-insulator platform. Sub-wavelength grating waveguides enable dispersion engineering to further increase the bandwidth operation of conventional multimode interference coupler and, subsequently, of mode de/multiplexer based on them. Full three-dimensional simulations of the designed mode converter and de/multiplexer show insertion loss below than 0.84 dB and crosstalk lower than -20 dB over an unprecedented bandwidth of 300 nm (1.4-1.7 μm). The overall footprint of the proposed device is only 36 μm × 3.7 μm.