Fig. 1: Concept of photonic interposers for integrated processing of microcombs.

Photonic interposers for fully integrated microcomb-based systems will need to interface multiple photonic devices, such as microcombs and other nonlinear elements, and lasers and photodetectors. The functions of such interposers can be broadly classified into two parts, first, the broadband spectral routing of microcombs, and second, the coherent mixing of specific filtered bands and teeth of the microcombs with additional external signals. The broadband spectral routing of microcombs includes separation of f and 2f components for self-referencing via second harmonic generation (SHG) and additional filtering for repetition rate detection, which together enable microcomb stabilization for metrological-grade applications. Depending on the application, further microcomb processing may be required, such as extraction of the pertinent comb reference band for optical frequency synthesis or of comb teeth matched to specific atomic transitions for optical clocks. These bands and teeth are subsequently mixed with tunable lasers and clock lasers for beat note detection for synthesis and timekeeping. In this work, we demonstrate individual passive components suitable for such an interposer for a dual-microcomb-based optical frequency synthesizer and implement the requisite spectral filtering of an octave-spanning microcomb