  Optical frequency combs have revolutionized precision measurement,
time-keeping, and molecular spectroscopy. A substantial effort has developed
around "microcombs": integrating comb-generating technologies into compact,
reliable photonic platforms. Current approaches for generating these microcombs
involve either the electro-optic (EO) or Kerr mechanisms. Despite rapid
progress, maintaining high efficiency and wide bandwidth remains challenging.
Here, we introduce a new class of microcomb -- an integrated optical frequency
comb generator that combines electro-optics and parametric amplification to
yield a frequency-modulated optical parametric oscillator (FM-OPO). In stark
contrast to EO and Kerr combs, the FM-OPO microcomb does not form pulses but
maintains operational simplicity and highly efficient pump power utilization
with an output resembling a frequency-modulated laser. We outline the working
principles of FM-OPO and demonstrate them by fabricating the complete optical
system in thin-film lithium niobate (LNOI). We measure pump to comb internal
conversion efficiency exceeding 93% (34% out-coupled) over a nearly flat-top
spectral distribution spanning approximately 1,000 modes (approximately 6 THz).
Compared to an EO comb, the cavity dispersion rather than loss determines the
FM-OPO bandwidth, enabling broadband combs with a smaller RF modulation power.
The FM-OPO microcomb, with its robust operational dynamics, high efficiency,
and large bandwidth, contributes a new approach to the field of microcombs and
promises to herald an era of miniaturized precision measurement, and
spectroscopy tools to accelerate advancements in metrology, spectroscopy,
telecommunications, sensing, and computing.

Optics;Quantum Physics