Optical frequency combs have recently attracted a lot of attention due to their various potential applications in spectroscopy, metrology, sensing, microwave and terahertz generation, etc. While generally created in femtosecond mode-blocked lasers, frequency combs can be generated by cascaded four-wave-mixing (FWM) in optical resonators through the Kerr nonlinearity. In particular, whispering-gallery mode (WGM) resonators provide a compact and efficient solution to create this set of equidistant spectral lines with 1–100 GHz frequency intervals. For instance, magnesium and calcium fluoride (MgF2 and CaF2) WGM resonator with Q-factor in the order of 109 have already been used to produce variously shaped frequency combs [1]. From a theoretical point of view, different models have recently been developed to describe the formation of the combs, using for example a modal description [2, 3, 4].
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