In an MCM, an optical signal is conveyed by an optical waveguide disposed on a surface of a first substrate to an optical coupler having a vertical facet. This optical coupler has an optical mode that is different than the optical mode of the optical waveguide. For example, the spatial extent of the optical mode associated with the optical coupler may be larger, thereby reducing optical losses and sensitivity to alignment errors. Then, the optical signal is directly coupled from the vertical facet to a facing vertical facet of an identical optical coupler on another substrate, and the optical signal is conveyed in another optical waveguide disposed on the other substrate.