A device structure includes: an interposer including metal wiring structures and optical waveguides that are embedded in interlayer dielectric layers, wherein the interposer includes a stepped outer sidewall including an outermost vertical surface segment, a laterally-recessed sidewall segment that is laterally recessed relative to the outermost vertical surface segment, and a connecting horizontal surface segment that connects the outermost vertical surface segment and the laterally-recessed vertical sidewall segment; and a fiber access unit having a first end that is optically coupled to a subset of the optical waveguides through at least one optical glue portion that is interposed between the fiber access unit and the laterally-recessed sidewall segment of the stepped outer sidewall.