  Microcombs - optical frequency combs generated in microresonators - have
advanced tremendously in the last decade, and are advantageous for applications
in frequency metrology, navigation, spectroscopy, telecommunications, and
microwave photonics. Crucially, microcombs offer the prospect of fully
integrated miniaturized optical systems with unprecedented reductions in cost,
size, weight, and power. However, this goal has been consistently hindered by
the use of bulk free-space and fiber-optic components to process microcombs,
limiting form factors to the table-top. Here, we address this challenge by
introducing an integrated photonics interposer architecture to process
microcombs and replace discrete components. Taking microcomb-based optical
frequency synthesis in the telecom C-band around 1550 nm as our target
application, we develop an interposer architecture that collects, routes, and
interfaces octave-wide optical signals between photonic chiplets and
heterogeneously integrated devices that constitute the synthesizer. We have
implemented the octave spanning spectral filtering of a microcomb, central to
the interposer, in the popular silicon nitride photonic platform, and have
confirmed the requisite performance of the individual elements of the
interposer. Moreover, we show that the thick silicon nitride needed for bright
dissipative Kerr soliton generation can be integrated with the comparatively
thin silicon nitride interposer layer through octave-bandwidth adiabatic
evanescent coupling, indicating a path towards future system-level
consolidation. Our interposer architecture addresses the immediate need for
on-chip microcomb processing to successfully miniaturize microcomb systems. As
microcombs and integrated devices evolve, our approach can be readily adapted
to other metrology-grade applications based on optical atomic clocks and
high-precision navigation and spectroscopy.

Applied Physics;Optics