An optical waveguide network (11) is formed in a substrate (78) of an electrooptically active material. The optical waveguide network 11 has input and output facets (50, 52) where optical signals may be input to and output from the integrated optics chip (82). At least one lateral trench (84) is formed in the substrate (78). The lateral trench (84) is arranged to prevent light rays incident thereon from inside the substrate (78) from propagating to the output facet (52). The lateral trench (84) may be formed as a slot that extends toward the surface of the substrate (78) where the optical waveguide network (11) is formed, or the trench (84) may be parallel to the plane of the optical waveguides (11). The trench (84) may be formed in a surface (71, 77) that is either parallel or perpendicular to the plane of the optical waveguide network (11).