Subharmonic Instabilities in Kerr Microcombs

We report experimental observation of subharmonic mode excitation in primary Kerr optical frequency
combs generated using crystalline whispering-gallery mode resonators. We show that the subcombs can
be controlled and span a single or multiple free-spectral ranges around the primary comb modes. In the
spatial domain, the resulting multiscale combs correspond to an amplitude modulation of intracavity
roll patterns. We perform a theoretical analysis based on eigenvalue decomposition that evidences the
mechanism leading to the excitation of these combs. The theoretical results are shown to be in good
agreement with the experimental measurements.