Low loss directional coupling between highly dissimilar optical waveguides for high refractive index integrated photonic circuits

An optical signal coupler for coupling at least one mode of an optical signal between at least one first Input/Output (I/O) waveguide and at least one second waveguide forming part of an integrated circuit, each of said at least one first I/O waveguide adapted to convey an external optical signal along an external signal path, said first I/O waveguide having a first effective refractive index and a first core having a first bulk refractive index, each of said at least one second waveguide adapted to convey an internal optical signal along an internal signal path, said at least one second waveguide having a second effective refractive index and a second core having a second bulk refractive index, said second bulk refractive index being higher than said first bulk refractive index, said second core having at least a portion with a reduced thickness to exhibit a constant effective refractive index matching said first effective refractive index of said first I/O waveguide over said at least a portion, said coupler comprising:

• at least one alignment structure for positioning said first core of at least one first I/O waveguide with respect to said second core of at least one corresponding second waveguide with an overlap therebetween and a separation distance there between over said at least a portion to permit tunnel coupling of said at least one mode of said optical signal between said external signal path and said internal signal path.