Transient Regime of Kerr Frequency Comb Formation
  Temporal growth of an optical Kerr frequency comb generated in a microresonator is studied both experimentally and numerically. We find that the comb emerges from vacuum fluctuations of the electromagnetic field on timescales significantly exceeding the ringdown time of the resonator modes. The frequency harmonics of the comb spread starting from the optically pumped mode if the microresonator is characterized with anomalous group velocity dispersion. The harmonics have different growth rates resulting from sequential four-wave mixing process that explains intrinsic modelocking of the comb. 
Anatoliy A. Savchenkov, Andrey B. Matsko, Wei Liang, Vladimir S.   Ilchenko, David Seidel, and Lute Maleki
physics.optics