  Optical frequency combs are indispensable links between the optical and
microwave domains, enabling a wide range of applications including precision
spectroscopy, ultrastable frequency generation, and timekeeping. Chip-scale
integration miniaturizes bulk implementations onto photonic chips, offering
highly compact, stable, and power-efficient frequency comb sources. State of
the art integrated frequency comb sources are based on resonantly-enhanced Kerr
effect and, more recently, on electro-optic effect. While the former can
routinely reach octave-spanning bandwidths and the latter feature
microwave-rate spacings, achieving both in the same material platform has been
challenging. Here, we leverage both strong Kerr nonlinearity and efficient
electro-optic phase modulation available in the ultralow-loss thin-film lithium
niobate photonic platform, to demonstrate a hybrid Kerr-electro-optic frequency
comb with stabilized spacing. In our approach, a dissipative Kerr soliton is
first generated, and then electro-optic division is used to realize a frequency
comb with 2,589 comb lines spaced by 29.308 GHz and spanning 75.9 THz (588 nm)
end-to-end. Further, we demonstrate electronic stabilization and control of the
soliton spacing, naturally facilitated by our approach. The broadband,
microwave-rate comb in this work overcomes the spacing-span tradeoff that
exists in all integrated frequency comb sources, and paves the way towards
chip-scale solutions for complex tasks such as laser spectroscopy covering
multiple bands, micro- and millimeter-wave generation, and massively parallel
optical communications.
