Grating coupler with high efficiency

An optical coupling device, comprising:

• a dielectric substrate layer;
• a first waveguide layer, comprising a first semiconductor material, which is disposed over the dielectric substrate layer;
• a dielectric intermediate layer overlying the first waveguide layer;
• a second waveguide layer, which comprises a second semiconductor material, different from the first semiconductor material, which is disposed over the dielectric intermediate layer and is patterned to define a waveguide;
• a first grating, which is formed in the second waveguide layer and is configured to diffract input light of a given wavelength from the waveguide into a specified diffraction order at a given coupling angle, whereby a first fraction of the input light propagates out of the optical coupling device into a transmitted beam at the given coupling angle while a second fraction of the input light is diffracted by the first grating into the dielectric intermediate layer in a conjugate diffraction order;
• and
• a second grating, which is formed in the first waveguide layer and is configured to diffract the second fraction of the input light into a second diffraction order, propagating out of the device at the given coupling angle, whereby the second grating acts as a reflector for the conjugate diffraction order to enhance coupling efficiency of the input light into the transmitted beam.