A device includes a grating coupler with a grating constant, two light sources, and a planar waveguide, which are configured to couple light with two different wavelengths λ1, λ2 into the waveguide. The waveguide has a waveguiding layer disposed adjacent to a substrate layer and a cover layer. The waveguiding layer has a thickness d and effective refractive indices of N(λk, jk), wherein λk is one of the wavelengths and jk is an order of a waveguide mode, wherein the coupled light of the wavelength λk has a coupling angle αk into the waveguide, and wherein an amount of difference between the coupling angles is a divergence angle Δα. Guiding of waveguide modes of the order jk>0 is possible for a wavelength of the coupled light. The waveguiding layer is arranged to couple the light via the grating coupler under a divergence angle of Δα