Fig. 2: Envisioned role of passive components within a photonic interposer for a dual-microcomb-based optical frequency synthesizer.

a Conceptual schematic showing how the passive components demonstrated in this work (highlighted in light gray), octave-wide dichroics, tunable ring filters for microcomb pump extinction, and multimode interferometers, could fit into the proposed interposer and system architecture to form an integrated dual-microcomb-based frequency synthesizer. The interposer is interfaced with THz repetition rate silicon nitride (also highlighted in light gray) and 20 GHz repetition rate silicon nitride or silica microcombs and a tunable laser via facet coupling, and with photodetectors and a second harmonic frequency doubler via heterogeneous integration. Dichroic directional couplers spectrally filter the silicon nitride microcomb in preparation for self-referencing and interference with the 20 GHz microcomb for repetition rate stabilization. In turn, the output tunable frequency synthesis laser is referenced to the 20 GHz microcomb. Multimode interferometers are utilized to generate these stabilization beat notes via balanced detection, and power monitors are used for additional system-level monitoring. Metal traces are not shown in this schematic. A detailed discussion showing the feasibility of such a system using only integrated components is included in the Supplementary information (Notes 7 and 8). b Micrographs of the individual interposer components shown in this work, dichroic directional couplers, ring resonator tunable filters, and multimode interferometers