  Microresonator Kerr frequency combs, which rely on third-order nonlinearity
($\chi^{(3)}$), are of great interest for a wide range of applications
including optical clocks, pulse shaping, spectroscopy, telecommunications,
light detection and ranging (LiDAR) and quantum information processing. Many of
these applications require further spectral and temporal control of the
generated frequency comb signal, which is typically accomplished using
additional photonic elements with strong second-order nonlinearity
($\chi^{(2)}$). To date these functionalities have largely been implemented as
discrete off-chip components due to material limitations, which come at the
expense of extra system complexity and increased optical losses. Here we
demonstrate the generation, filtering and electro-optic modulation of a
frequency comb on a single monolithic integrated chip, using a thin-film
lithium niobate (LN) photonic platform that simultaneously possesses large
$\chi^{(2)}$ and $\chi^{(3)}$ nonlinearities and low optical losses. We
generate broadband Kerr frequency combs using a dispersion-engineered high
quality factor LN microresonator, select a single comb line using an
electrically programmable add-drop filter, and modulate the intensity of the
selected line. Our results pave the way towards monolithic integrated frequency
comb solutions for spectroscopy data communication, ranging and quantum
photonics.
