Apparatus including a waveguide segment having a cladding that surrounds an optically-transmissive core extending along a path; a heater in thermal communication with the waveguide segment along at least a part of the path; a thermally-conductive substrate; a thermally-conductive base in fixed alignment with the waveguide segment and the thermally-conductive substrate, the thermally-conductive base being discontinuously on the waveguide segment along the path; and a rib in fixed alignment with the waveguide segment and the thermally-conductive substrate, the rib transversely extending in a direction away from the waveguide segment at a point along the path. Process utilizing apparatus for generating a phase shift in light.