Mode-locked dark pulse Kerr combs in normal-dispersion microresonators

The generation of Kerr frequency combs in a coherently driven nonlinear microresonator is now extensively investigated more generally by the research community as a potentially portable technology for a variety of applications. Here, we report experiments in which dark pulse combs are formed in normal-dispersion microresonators with mode-interaction-assisted excitation, and mode-locking transitions are observed in the normal-dispersion regime. The mode-interaction-aided excitation of dark pulses appears to occur through a deterministic pathway, in sharp contrast to the situation for bright pulses in the anomalous dispersion region. The ability to mode-lock in the normal-dispersion regime increases the freedom in the microresonator design and may make it possible to extend Kerr comb generation into the visible, where material dispersion is likely to dominate.