Compact broadband directional coupler

Directional couplers are key building blocks for network-on-chip devices such as wavelength demultiplexers and switches. The operational bandwidth of directional couplers is, however, limited, and existing techniques to extend their bandwidth result in significantly increased device footprints and degraded phase response. Here we present a novel approach to enhance the bandwidth of directional couplers without affecting their size or phase response. Using dispersion engineering of sub-wavelength gratings we show by 3D FDTD simulations a fivefold increase in coupler bandwidth from 20nm to 100 nm.