An assembly includes optical fibers each having a waveguide core, a photonic integrated circuit (IC) that includes in-plane waveguides corresponding to the optical fibers, and a substrate bonded to the photonic IC with grooves that support the optical fibers. The substrate and photonic IC can have metal bumps that cooperate to provide mechanical bonding and electrical connections between the substrate and photonic IC. Portions of the optical fibers supported by the substrate grooves can define flat surfaces spaced from the optical fiber cores. The photonic IC can include passive waveguide structures with a first coupling section that interfaces to the flat surface of a corresponding optical fiber (for evanescent coupling of optical signals) and a second coupling section that interfaces to a corresponding in-plane waveguide (for adiabatic spot-size conversion of optical signals).