An apparatus includes an optically opaque substrate, which includes first and second opposite surfaces and has one or more openings traversing through the substrate between the first and second surfaces. One or more optical transducers are attached to the first surface of the substrate so as to emit or detect light via the respective openings. One or more lenses are positioned against the respective openings on the second surface of the substrate, and are configured to couple the light between the optical transducers and respective optical fibers.