Coupling to nanophotonic waveguides using a dual grating-assisted directional coupler

An analysis of a device we have called a dual grating-assisted directional coupler (DGADC) in silicon-on-insulator technology is presented. The device enables efficient coupling from optical fibres to small waveguides, typically <1 μm in cross-sectional dimensions. Coupling efficiency and device length are determined as functions of layer thicknesses and refractive indices, grating periods, depths and duty ratios, and finally wavelength. For our example calculation of coupling to a 250 nm silicon waveguide, this efficient and robust coupler can have coupling efficiency as high as 90%, while the wavelength range can have an FWHM of ∼1.6 nm at resonance. Fabrication tolerances have also been analysed and they are up to two orders of magnitude larger than those of a GADC with only one grating.