Wavelength-selective coupling between vertically integrated thin-film waveguides via supermode by a pair of grating couplers

Grating coupling between vertically integrated waveguides via supermode was discussed and demonstrated. Two thin-film waveguides with grating couplers were stacked on a substrate. A guided wave in one waveguide is converted b y a grating coupler to a supermode propagating contradirectionally, and the n converted by another grating coupler to a guided wave in the other wavegu ide. The coupling shows high wavelength selectivity, and the wavelength-div ision add/drop multiplexing function can be combined. A demonstrator was de signed and fabricated. Theoretically predicted coupling efficiency was almo st 100% with wavelength selectivity of 2 nm, while the experimentally obtai ned value was 40% in the efficiency and 1 nm in the selectivity.