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

Abstract

Kerr frequency combs from microresonators are now extensively investigated as a potentially portable technology for a variety of applications. Most studies employ anomalous dispersion microresonators that support modulational instability for comb initiation, and mode-locking transitions resulting in coherent bright soliton-like pulse generation have been reported. However, some experiments show comb generation in normal dispersion microresonators; simulations suggest the formation of dark pulse temporal profiles. Excitation of dark pulse solutions is difficult due to the lack of modulational instability in the effective blue-detuned pumping region; an excitation pathway has been demonstrated neither in experiment nor in simulation. Here we report experiments in which dark pulse combs are formed by mode-interaction-aided excitation; for the first time, a mode-locking transition is observed in the normal dispersion regime. The excitation pathway proposed is also supported by simulations.Microresonator-based optical frequency combs, also termed Kerr combs, are generated through conversion of a single pump frequency to a broadband frequency comb inside a high-quality-factor (Q) microresonator via the third-order Kerr nonlinearity [1]- [10]. The advantages of Kerr combs include very compact size, high repetition rate, and capability of generating ultra-broad combs.The dynamics of Kerr comb generation have attracted intense investigations since the first demonstration of the method [11]-[28]. It has been found that Kerr combs are not always coherent [11]-[12] and may be characterized by high intensity noise [13]-[14]; furthermore, lack of coherence and high intensity noise are generally correlated. Experiments have revealed transitions from low coherence, high noise states to highly coherent mode-locked states accompanied by a sudden drop in the comb noise [14]- [18]. It has been found in simulations and experiments that the mode locking of broadband Kerr combs is usually related to soliton formation in the cavity [15], [17]-[28]. These dissipative cavity solitons are localized structures stabilized by a balance between Kerr nonlinearity and dispersion. In time domain they exist as bright or dark pulses, depending on whether the cavity dispersion is anomalous or normal, respectively. Bright microresonator solitons in the anomalous dispersion region have been observed in experiments and well studied through simulations [15], [17]-[27]. Reference [17] reported a method of tuning the pump laser frequency to an effectively red-detuned regime (pump laser wavelength longer than resonant wavelength) which is typically difficult to achieve due to thermal instability [29]. Mode-locking transitions yielding bright solitons were observed after passage through a broadband chaotic state [17]. In contrast, although dark solitons have been predicted in normal dispersion microresonators in 2 / 32 2 / 32 theory and simulation [27]-[28], investigating dark solitons experimentally is extremely difficult and no time-domain char...

On Frequency Combs in Monolithic Resonators

“…This phenomenon was predicted [121,184,205,206] and demonstrated experimentally [92]. Both narrow (hyperparametric oscillation like) [207,208] and broad Kerr frequency combs were demonstrated under conditions of normal GVD [92,205] and no visible interaction of the resonator spectrum due to mode crossings.…”

“…This ratio is usually large in fiber loops but can be small (nearly one) in microresonators. This is true for modulation instability (Turing rolls [91]) in anomalous GVD case and for generation of high order dark solitons in normal GVD case [92] observed in microresonators (these regimes are discussed in what follows).…”

Coexistence of stable dark- and bright-soliton Kerr combs in normal-dispersion resonators

“…Such dissipative localized structures have also been studied in spatial systems, both in one and two dimensions, with different nonlinearities [19,20]. The generation of dark pulse Kerr frequency combs has been achieved experimentally by several groups [21][22][23].…”

Weak Langmuir optical turbulence in a fiber cavity

“…It is thus of fundamental different nature than the conventional dispersive shocks that develop either in the spatial or the temporal domain from coherent disturbances, which have been experimentally observed in ion-acoustic waves [100], water surface gravity waves [101], and fiber optics [102], and have recently regained great interest in optics [103][104][105][106][107][108][109][110][111][112]. Coherent dispersive shocks and their stationary analogues have shown to play a role also in passive cavity configurations [113][114][115], where one can envisage that they can impact the generation of combs in the normal dispersion regime [116,117]. Note that, in the incoherent case examined here, the incoherent shock singularity develops in the cavity well before that it reaches the statistically stationary steady state.…”