We present a novel type of grating-assisted coupler for coupling light from an optical fibre to a thin semiconductor waveguide. The Dual Grating-Assisted Directional Coupler (DGADC) consists of two gratings, one thick waveguide with a refractive index similar to the refractive index of the fibre, a thin semiconductor waveguide and a layer with refractive index which is between that of the thick and thin waveguides. Specifically, a coupler in silicon on insulator (SO!) has been analysed. The maximum coupling efficiency for this coupler can approach 1 00%, while the minimum coupling length can be around 2mm.