Low-loss directional coupler for the C, L and U bands based on subwavelength gratings

Directional couplers are ubiquitous components for power distribution in silicon photonics integrated circuits. Despite significant advances in their performance, architectures providing tailorable coupling ratios over increasingly broad bandwidths are still sought after. Compact footprint and low losses are also essential features for circuits comprising multiple coupling stages. In this work, we propose a compact directional coupler with arbitrary coupling ratio, based on dispersion engineering through subwavelength metamaterials. Low losses and a flat spectral response over a broad bandwidth are experimentally demonstrated for multiple coupling ratios between 0.08 and 1. Results show average excess losses below 0.24 dB and coupling ratio deviations below $\pm$1 dB for a 170 nm bandwidth completely covering C, L and U telecommunication bands (1505 - 1675 nm).