Hot-cavity spectroscopy of dark pulse Kerr combs in microresonators

Kerr frequency combs are generated through cascaded four-wave mixing in high-Q microresonators [1]. These devices are pumped with a continuous-wave laser and modulational instability (MI) is responsible for the growth of the initial comb lines. Since it is easier to satisfy the MI phase matching condition in the anomalous dispersion regime, most studies on Kerr combs have focused on anomalous dispersion microresonators. However, coherent microresonator combs can also take place in the normal dispersion regime. In these combs, phase matching is attained with the aid of the mode coupling between transverse modes of the microresonator [2]. One particularly interesting comb state that operates in the normal dispersion regime is the dark pulse Kerr comb [3]. The time domain pulses of these combs arise as interlocking switching waves that connect the upper and lower homogenous steady state solutions of the bi-stability curve in the continuous-wave-driven Kerr cavity [see Fig. (a)] [3]. These combs are of high interest as most nonlinear materials suitable for fabricating microresonators display normal dispersion in the visible and near infrared ranges. Moreover, these combs provide a much higher power conversion efficiency compared to bright-soliton combs, which makes them particularly useful for telecommunications [4]. Nevertheless, the formation of dark-pulse combs and their dynamics remain largely unexplored. In this work, we provide a better physical understanding of the formation of this type of combs by measuring the system's response as the pump laser is tuned into resonance. We discover that the formation of the dark pulse Kerr combs is associated with the appearance of an extra resonance, in striking similarity to recent observations made for bright temporal solitons in the anomalous dispersion regime [5], and explain the findings based on cross-phase modulation (XPM) induced by the two homogeneous steady state solutions of the Kerr cavity.