  Optical frequency combs based on mode-locked lasers have proven to be
invaluable tools for a wide range of applications in precision spectroscopy and
metrology. A novel principle of optical frequency comb generation in
whispering-gallery mode microresonators ("microcombs") has been developed
recently, which represents a promising route towards chip-level integration and
out-of-the-lab use of these devices. Presently, two families of microcombs have
been demonstrated: combs with electronically detectable mode spacing that can
be directly stabilized, and broadband combs with up to octave-spanning spectra
but mode spacings beyond electronic detection limits. However, it has not yet
been possible to achieve these two key requirements simultaneously, as will be
critical for most microcomb applications. Here we present a key step to
overcome this problem by interleaving an electro-optic comb with the spectrum
from a parametric microcomb. This allows, for the first time, direct control
and stabilization of a microcomb spectrum with large mode spacing (>140 GHz)
with no need for an additional mode-locked laser frequency comb. The attained
residual 1-second-instability of the microcomb comb spacing is 10^-15, with a
microwave reference limited absolute instability of 10^-12 at a 140 GHz mode
spacing.

Optics