  Optical frequency comb (OFC) technology has been the cornerstone for
scientific breakthroughs such as precision frequency metrology, redefinition of
time, extreme light-matter interaction, and attosecond sciences. While the
current mode-locked laser-based OFC has had great success in extending the
scientific frontier, its use in real-world applications beyond the laboratory
setting remains an unsolved challenge. Microresonator-based OFCs, or Kerr
frequency comb, have recently emerged as a candidate solution to the challenge
because of their preferable size, weight, and power consumption (SWaP). On the
other hand, the current phase stabilization technology requires either external
optical references or power-demanding nonlinear processes, overturning the SWaP
benefit of Kerr frequency combs. Introducing a new concept in phase control,
here we report an internally phase stabilized Kerr frequency comb without the
need of any optical references or nonlinear processes. We describe the comb
generation analytically with the theory of cavity induced modulation
instability, and demonstrate for the first time that the optical frequency can
be stabilized by control of two internally accessible parameters: an intrinsic
comb offset and the comb spacing. Both parameters are phase locked to microwave
references, with 55 mrad and 20 mrad residual phase noises, and the resulting
comb-to-comb frequency uncertainty is 0.08 Hz or less. Out-of-loop measurements
confirm good coherence and stability across the comb, with measured optical
frequency fractional instabilities of 5x10^-11/sqrt(t). The new phase
stabilization method preserves the Kerr frequency comb's key advantages and
potential for chip-scale electronic and photonic integration.
