A waveguide substrate configured includes a first surface, a second surface opposite the first surface, and a communication side having at least one projecting boss that at least partially defines a bore for receiving a ferrule of an optical connector. Each projecting boss includes an outboard end from which the bore extends into the waveguide substrate, and an end of the bore within the waveguide substrate defines an optical interface surface. At least one waveguide within the waveguide substrate extends from the optical interface surface. A first slot is formed in each projecting boss between the associated bore and the first surface, with the first slot extending from the outboard end of the projecting boss and along a majority of the bore.