  Optical frequency combs play a crucial role in optical communications,
time-frequency metrology, precise ranging, and sensing. Among various
generation schemes, resonant electro-optic combs are particularly attractive
for its excellent stability, flexibility and broad bandwidths. In this
approach, an optical pump undergoes multiple electro-optic modulation processes
in a high-Q optical resonator, resulting in cascaded spectral sidebands.
However, most resonant electro-optic combs to date make use of lumped-capacitor
electrodes with relatively inefficient utilization of the input electrical
power. This design also reflects most electrical power back to the driving
circuits and necessitates costly RF isolators in between, presenting
substantial challenges in practical applications. To address these issues, we
present an RF circuit friendly electro-optic frequency comb generator
incorporated with on-chip coplanar microwave resonator electrodes, based on a
thin-film lithium niobate platform. Our design achieves more than three times
electrical power reduction with minimal reflection at the designed comb
repetition rate of ~ 25 GHz. We experimentally demonstrate broadband
electro-optic frequency comb generation with a comb span of >85 nm at a
moderate electrical driving power of 740 mW (28.7 dBm). Our power-efficient and
isolator-free electro-optic comb source could offer compact, low-cost and
simple-to-design solution for applications in spectroscopy, high-precise
metrology, optical communications.
