Optimisation of a grating device for coupling radiation in and out of a thin-film waveguide on a silicon substrate

An analysis is reported of the operation and optimisation of the parameters of a grating device for coupling radiation in and out of a thin-film waveguide formed on a silicon substrate. The reflection of light at the interface between a buffer layer and silicon significantly alters the operation of such a grating coupler. The conditions for maximum coupling out of radiation into air or into the substrate are obtained for the TE and TM modes of the thin-film waveguide.