External grating structures for interfacing wavelength-division-multiplexed optical sources with thin optical waveguides

An arrangement for providing optical coupling between an external light source generating optical signals at a plurality of different wavelengths and a relatively thin, planar silicon optical waveguide, the arrangement comprising

• an evanescent coupling layer disposed over at least a portion of the planar silicon optical waveguide, the evanescent coupling layer exhibiting a refractive index less than the refractive index of silicon;
• a prism coupling structure disposed over the evanescent coupling layer, the prism coupling structure including at least one coupling facet formed at a predetermined angle with respect to the waveguide surface, the at least one coupling facet for intercepting optical signals within a defined wavelength range, and a prism coupling surface disposed over the evanescent coupling layer;
• and
• at least one diffractive optical element including at least one volume phase grating with index modulation fringes slanted with respect to the normal vector associated with the input and output surfaces of the at least one diffractive optical element disposed beyond or on the at least one coupling facet of the prism coupling structure, the at least one diffractive optical element exhibiting a period, modulation index, and grating depth sufficient to modify the launch angle for each different wavelength of operation and improve the coupling efficiency between the prism coupling structure and the relatively thin, planar silicon optical waveguide.