Efficient coupling of narrow beams into polyimide waveguides by means of grating couplers with high-index coating.

The effect of a thin high-index coating deposited on polyimide waveguide grating couplers was investigated. A comprehensive numerical study was performed using an efficient simulation tool based on a Floquet-Bloch algorithm, and the results of this study were compared with experimentally obtained values for input coupling efficiencies. The application of a high-index coating permits efficient coupling from narrow beams even in material systems with a low index difference. This not only facilitates a denser integration of grating couplers but also permits low-loss lateral tapering to single-mode waveguides.