  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.
