Modeling of optical frequency comb generation in whispering gallery mode resonators and limiting effects

We report a detailed study about the various physical mechanisms that may limit the spectral span of optical frequency combs generated with whispering gallery mode resonators. We developed a modal expansion model able to track the individual dynamics of the various modes, and identify the key parameters determining the comb spectral features. We show that, although the walk-off between the dispersive WGM eigenfrequency distribution and the ideal equidistant comb is the main limiting factor, other phenomena such as modal confinement, material absorption, and modal overlap may also play an important role in extending the comb span.