Optical mode coupler in integrated photonics

An optical device comprising:

• a photonic integrated circuit having an optical mode coupler, the optical mode coupler optically coupling a first planar optical waveguide having a first optical core at one horizontal plane of the photonic integrated circuit to a second planar optical waveguide having a second optical core at a different second horizontal plane of the photonic integrated circuit;
• wherein the optical mode coupler comprises two or more intermediate optical layers stacked vertically between the horizontal planes of the optical cores of the planar optical waveguides in the photonic integrated circuit, each of the intermediate optical layers comprising one or more optical rails, the optical mode coupler comprising optical cladding of a lower refractive index around the optical rails;
• and
• wherein the optical mode coupler is configured to cause light received from the first planar optical waveguide to excite an optical mode and guide the light of the optical mode such that the optical mode substantially overlaps the first planar optical waveguide and the optical rails of at least two of the intermediate optical layers in a vertical cross-section of the photonic integrated circuit.