Generation of a coherent near-infrared Kerr frequency comb in a   monolithic microresonator with normal GVD
  We demonstrate experimentally, and explain theoretically, generation of a wide, fundamentally phase locked Kerr frequency comb in a nonlinear resonator with a normal group velocity dispersion. A magnesium fluoride whispering gallery resonator characterized with 10 GHz free spectral range and pumped either at 780 nm or 795 nm is used in the experiment. The envelope of the observed frequency comb differs significantly from the Kerr frequency comb spectra reported previously. We show via numerical simulation that, while the frequency comb does not correspond to generation of short optical pulses, the relative phases of the generated harmonics are fixed. 
Wei Liang, Anatoliy A. Savchenkov, Vladimir S. Ilchenko, Danny   Eliyahu, David Seidel, Andrey B. Matsko, and Lute Maleki
physics.optics