Optical coupling device having diffraction gratings for coupling light with a light guide and fabrication method thereof

An optical coupler device for coupling light with a light guide is provided. The device includes a first layer having a plurality of first diffraction gratings spaced apart via first trenches, the first diffraction gratings and the first trenches forming first periodic units. The device also includes a second layer having a plurality of second diffraction gratings spaced apart via second trenches, the second diffraction gratings and the second trenches forming second periodic units. Additionally, the second periodic units are offset in a lateral axis of the optical coupler device relative to the first periodic units by a relative shift distance S2 that is in a range from about 10 nm to about 600 nm.