In an integrated optical waveguide device including a first top portion on the substrate, and a plurality of juxtaposed waveguide cores within cladding layers on a second top portion of the substrate, the waveguide cores having respective inner endfaces parallel to a longitudinal open slotway formed in the substrate between the first and second top portions, the substrate being undercut from the slotway to produce a cantilevered section of the waveguide cores and cladding layers, for permitting the plurality of waveguide cores and associated cladding layers to be simultaneously cleaved, thereby providing a smooth endface thereacross.