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...

Kerr frequency combs generated from microresonators are the subject of intense study. Most research employs microresonators with anomalous dispersion, for which modulation instability is believed to play a key role in initiation of the comb. Comb generation in normal dispersion microresonators has also been reported but is less well understood. Here we report a detailed investigation of few-moded, normal dispersion silicon nitride microresonators, showing that mode coupling can strongly modify the local dispersion, even changing its sign. We demonstrate a link between mode coupling and initiation of comb generation by showing experimentally, for the first time to our knowledge, pinning of one of the initial comb sidebands near a mode crossing frequency. Associated with this route to comb formation, we observe direct generation of coherent, bandwidth-limited pulses at repetition rates down to 75 GHz, without the need to first pass through a chaotic state.Recently high quality factor (Q) microresonators have been intensively investigated for optical comb generation. Both whispering gallery mode resonators employing tapered fiber coupling and chip-scale microresonators employing monolithically fabricated coupling waveguides are popular. Tuning a continuous-wave (CW) laser into resonance leads to build-up of the intracavity power and enables additional cavity modes to oscillate through cascaded four-wave mixing (FWM) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Modulational instability (MI) of the CW pump mode is commonly cited as an important mechanism for comb generation [16][17][18]. According both to experiment and to theoretical analysis, comb generation preferably occurs in resonators with anomalous dispersion. However, comb generation in resonators characterized with normal dispersion has also been observed experimentally [5,8,[19][20][21][22][23][24]. Several models have been proposed to describe this phenomenon. Although MI gain is missing in fibers or waveguides with normal dispersion, when it comes to resonators, the detuning provides an extra degree of freedom which enables MI to take place in the normal dispersion regime, hence providing a route to comb generation [16,18,25]. However, this mechanism requires either a precise relationship between detuning and pump power, making it difficult to realize practically, or hard excitation, a nonadiabatic process under which pump photons must be initially present in the resonator [17].Mode coupling has also been suggested as a mechanism enabling comb generation in resonators with normal dispersion [26]. When resonances corresponding to different families of transverse modes approach each other in frequency, they may interact due to imperfections in the resonator. The theory of mode coupling in resonators has been well-established [27], and frequency shifts and avoided crossings have been observed [28][29][30][31][32][33]. In the anomalous dispersion regime, mode coupling has been reported to affect the bandwidth scaling of frequency combs [34] and the p...

Simultaneous Kerr comb formation and second-harmonic generation with on-chip microresonators can greatly facilitate comb self-referencing for optical clocks and frequency metrology. Moreover, the presence of both second- and third-order nonlinearities results in complex cavity dynamics that is of high scientific interest but is still far from being well-understood. Here, we demonstrate that the interaction between the fundamental and the second-harmonic waves can provide an entirely new way of phase matching for four-wave mixing in optical microresonators, enabling the generation of optical frequency combs in the normal dispersion regime under conditions where comb creation is ordinarily prohibited. We derive new coupled time-domain mean-field equations and obtain simulation results showing good qualitative agreement with our experimental observations. Our findings provide a novel way of overcoming the dispersion limit for simultaneous Kerr comb formation and second-harmonic generation, which might prove to be especially important in the near-visible to visible range where several atomic transitions commonly used for the stabilization of optical clocks are located and where the large normal material dispersion is likely to dominate.

Optical resonators with high quality factors (Qs) are promising for a variety of applications due to the enhanced nonlinearity and increased photonic density of states at resonances. In particular, frequency combs (FCs) can be generated through four-wave mixing in high-Q microresonators made from Kerr nonlinear materials such as silica, silicon nitride, magnesium fluoride, and calcium fluoride. These devices have potential for on-chip frequency metrology and high-resolution spectroscopy, high-bandwidth radiofrequency information processing, and high-data-rate telecommunications. Silicon nitride microresonators are attractive due to their compatibility with integrated circuit manufacturing; they can be cladded with silica for long-term stable yet tunable operation, and allow multiple resonators to be coupled together to achieve novel functionalities. Despite previous demonstrations of high-Q silicon nitride resonators, FC generation using silicon nitride microresonator chips still requires pump power significantly higher than those in whispering gallery mode resonators made from silica, magnesium, and calcium fluorides, which all have shown resonator Qs between 0.1 and 100 billion. Here, we report on a fabrication procedure that leads to the demonstration of "finger-shaped" Si 3 N 4 microresonators with intrinsic Qs up to 17 million at a free spectrum range (FSR) of 24.7 GHz that are suitable for telecommunication and microwave photonics applications. The frequency comb onset power can be as low as 2.36 mW and broad, single FSR combs can be generated at a low pump power of 24 mW, both within reach of on-chip semiconductor lasers. Our demonstration is an important step toward a fully integrated on-chip FC source. Kerr comb generation in microresonators starts when an external continuous-wave (CW) laser is tuned into a cavity resonance; this causes intracavity power to build, which enables additional cavity modes to oscillate through nonlinear wave mixing [10]. FC formation has now been demonstrated in a variety of Kerr nonlinear materials such as silica [9,14-18], silicon nitride (Si 3 N 4 ) [19-21], aluminum nitride [22], CaF 2 [23], and MgF 2 [24]. Recently, dissipative Kerr solitons have also been demonstrated in MgF 2 and Si 3 N 4 optical microresonators [25,26]. Out of these materials, stoichiometric Si 3 N 4 has distinctive 2334-2536/16/111171-10 Journal

Microresonator-based Kerr frequency comb (microcomb) generation can potentially revolutionize a variety of applications ranging from telecommunications to optical frequency synthesis. However, phase-locked microcombs have generally had low conversion efficiency limited to a few percent. Here we report experimental results that achieve ~30% conversion efficiency (~200 mW on-chip comb power excluding the pump) in the fiber telecommunication band with broadband mode-locked dark-pulse combs. We present a general analysis on the efficiency which is applicable to any phase-locked microcomb state. The effective coupling condition for the pump as well as the duty cycle of localized time-domain structures play a key role in determining the conversion efficiency. Our observation of high efficiency comb states is relevant for applications such as optical communications which require high power per comb line.Comment: 4 pages, 4 figures, typos correcte