Abstract: The invention relates to a grating coupler comprising: —an optical substrate arranged to transfer a light beam, and—a diffraction grating arranged on, or imbedded in, the surface of said optical substrate, said diffraction grating comprising diffraction grating elements comprising each a coating arranged asymmetrically on said diffraction grating elements. The grating coupler is further arranged to satisfy the condition: (n1×sin(I?I)+?2)/?×P>1, wherein n1 is the refractive index of the optical medium to the incident light side of the diffraction grating elements, n2 is the refractive index of the optical medium to the diffracted light side of the diffraction grating elements, lal the absolute value of the incident angle of the light beam incident on the grating coupler ? is the vacuum wavelength of the diffracted light, and P is the period of the diffraction grating elements.