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 3D simulations of the designed mode converter and de/multiplexer show insertion losses 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 devices is only 36 μm × 3.7 μm.