Single mode grating coupler with vertical coupling direction and small back reflection

An optical apparatus, comprising:

• an optical device having an optical input-output face;
• at least two planar waveguide arms being located on a substrate;
• an optical splitter being located on the substrate and having an optical input and a plurality of optical outputs, each optical output being optically connected to a corresponding one of the planar waveguide arms;
• and
• an optical grating coupler being located on the substrate such that the planar waveguide arms transmit light to different end portions of the optical grating coupler such that the optical grating coupler forms diffraction patterns from the light transmitted by different ones of the waveguide arms such that a principal maximum of one of the diffraction patterns overlaps with a principal maximum of another of the diffraction patterns on the optical input-output face of the optical device, the principal maxima of the one and another of the diffraction patterns being directed in different directions.