  The generation of ultra-low noise microwave and mmWave in miniaturized,
chip-based platforms can transform communication, radar, and sensing systems.
Optical frequency division that leverages optical references and optical
frequency combs has emerged as a powerful technique to generate microwaves with
superior spectral purity than any other approaches. We demonstrate a
miniaturized optical frequency division system that can potentially transfer
the approach to a CMOS-compatible integrated photonic platform. Phase stability
is provided by a large-mode-volume, planar-waveguide-based optical reference
coil cavity and is divided down from optical to mmWave frequency by using
soliton microcombs generated in a waveguide-coupled microresonator. Besides
achieving record-low phase noise for integrated photonic microwave/mmWave
oscillators, these devices can be heterogeneously integrated with semiconductor
lasers, amplifiers, and photodiodes, holding the potential of large-volume,
low-cost manufacturing for fundamental and mass-market applications.
