Waveguides of optically transparent interposers are adiabatically coupled to respective waveguides of photonic integrated circuits that are mounted to the optically transparent interposers. In particular, photonic integrated circuits can be mounted to an interposer that has both optical connections and electrical connections. The optical connections of the interposer can be adiabatically coupled to the photonic integrated circuit. The electrical connections can be connected to the photonic integrated circuit and to a host board that also supports an ASIC.