Optical combs with a crystalline whispering gallery mode resonator
  We report on the experimental demonstration of a tunable monolithic optical frequency comb generator. The device is based on the four-wave mixing in a crystalline calcium fluoride whispering gallery mode resonator. The frequency spacing of the comb is given by an integer number of the free spectral range of the resonator. We select the desired number by tuning the pumping laser frequency with respect to the corresponding resonator mode. We also observe interacting optical combs and high-frequency hyperparametric oscillation, depending on the experimental conditions. A potential application of the comb for generating narrowband frequency microwave signals is demonstrated. 
Anatoliy A. Savchenkov, Andrey B. Matsko, Vladimir S. Ilchenko, Iouri   Solomatine, David Seidel, Lute Maleki