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Register a new userDissipative Kerr solitons in optical microresonators
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This chapter describes the discovery and stable generation of temporal dissipative Kerr solitons in continuous-wave (CW) laser driven optical microresonators. The experimental signatures as well as the temporal and spectral characteristics of this class of bright solitons are discussed. Moreover, analytical and numerical descriptions are presented that do not only reproduce qualitative features but can also be used to accurately model and predict the characteristics of experimental systems. Particular emphasis lies on temporal dissipative Kerr solitons with regard to optical frequency comb generation where they are of particular importance. Here, one example is spectral broadening and self-referencing enabled by the ultra-short pulsed nature of the solitons. Another example is dissipative Kerr soliton formation in integrated on-chip microresonators where the emission of a dispersive wave allows for the direct generation of unprecedentedly broadband and coherent soliton spectra with smooth spectral envelope.
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Dissipative solitons are self-localized structures resulting from a double balance between dispersion and nonlinearity as well as dissipation and a driving force. They occur in a wide variety of fields ranging from optics, hydrodynamics to chemistry and biology. Recently, significant interest has focused on their temporal realization in driven optical microresonators, known as dissipative Kerr solitons. They provide access to coherent, chip-scale optical frequency combs, which have already been employed in optical metrology, data communication and spectroscopy. Such Kerr resonator systems can exhibit numerous localized intracavity patterns and provide rich insights into nonlinear dynamics. A particular class of solutions consists of breathing dissipative solitons, representing pulses with oscillating amplitude and duration, for which no comprehensive understanding has been presented to date. Here, we observe and study single and multiple breathing dissipative solitons in two different microresonator platforms: crystalline $mathrm{MgF_2}$ resonator and $mathrm{Si_3N_4}$ integrated microring. We report a deterministic route to access the breathing state, which allowed for a detailed exploration of the breathing dynamics. In particular, we establish the link between the breathing frequency and two system control parameters - effective pump laser detuning and pump power. Using a fast detection, we present a direct observation of the spatiotemporal dynamics of individual solitons, revealing irregular oscillations and switching. An understanding of breathing solitons is not only of fundamental interest concerning nonlinear systems close to critical transition, but also relevant for applications to prevent breather-induced instabilities in soliton-based frequency combs.
Solitons are shape preserving waveforms that are ubiquitous across nonlinear dynamical systems and fall into two separate classes, that of bright solitons, formed in the anomalous group velocity dispersion regime, and `dark solitons in the normal dispersion regime. Both types of soliton have been observed in BEC, hydrodynamics, polaritons, and mode locked lasers, but have been particularly relevant to the generation of chipscale microresonator-based frequency combs (microcombs), used in numerous system level applications in timing, spectroscopy, and communications. For microcombs, both bright solitons, and alternatively dark pulses based on interlocking switching waves, have been studied. Yet, the existence of localized dissipative structures that fit between this dichotomy has been theoretically predicted, but proven experimentally elusive. Here we report the discovery of dissipative structures that embody a hybrid between switching waves and dissipative solitons, existing in the regime of (nearly) vanishing group velocity dispersion where third-order dispersion is dominant, hence termed as `zero-dispersion solitons. These dissipative structures are formed via collapsing switching wave fronts, forming clusters of quantized solitonic sub-structures. The switching waves are formed directly via synchronous pulse-driving of a photonic chip-based Si3N4 microresonator. The resulting frequency comb spectrum is extremely broad in both the switching wave and zero-dispersion soliton regime, reaching 136 THz or 97% of an octave. Fourth-order dispersion engineering results in dual-dispersive wave formation, and a novel quasi-phase matched wave related to Faraday instability. This exotic unanticipated dissipative structure expands the domain of Kerr cavity physics to the regime near zero-dispersion and could present a superior alternative to conventional solitons for broadband comb generation.
We present the results of asymptotic and numerical analysis of dissipative Kerr solitons in whispering gallery mode microresonators influenced by higher order dispersive terms leading to the appearance of a dispersive wave (Cherenkov radiation). Combining direct perturbation method with the method of moments we find expressions for the frequency, strength, spectral width of the dispersive wave and soliton velocity. Mutual influence of the soliton and dispersive wave was studied. The formation of the dispersive wave leads to a shift of the soliton spectrum maximum from the pump frequency (spectral recoil), while the soliton displaces the dispersive wave spectral peak from the zero dispersion point.
The capability to store light for extended periods of time enables optical cavities to act as narrow-band optical filters, whose linewidth corresponds to the cavitys inverse energy storage time. Here, we report on nonlinear filtering of an optical pulse train based on temporal dissipative Kerr solitons in microresonators. Our experimental results in combination with analytical and numerical modelling show that soliton dynamics enables storing information about the systems physical state longer than the cavitys energy storage time, thereby giving rise to a filter width that can be more than an order of magnitude below the cavitys intrinsic linewidth. Such nonlinear optical filtering can find immediate applications in optical metrology, low-timing jitter ultra-short optical pulse generation and potentially opens new avenues for microwave photonics.
We demonstrate stable microresonator Kerr soliton frequency combs in a III-V platform (AlGaAs on SiO$_2$) through quenching of thermorefractive effects by cryogenic cooling to temperatures between 4~K and 20~K. This cooling reduces the resonators thermorefractive coefficient, whose room-temperature value is an order of magnitude larger than that of other microcomb platforms like Si$_3$N$_4$, SiO$_2$, and AlN, by more than two orders of magnitude, and makes soliton states adiabatically accessible. Realizing such phase-stable soliton operation is critical for applications that fully exploit the ultra-high effective nonlinearity and high optical quality factors exhibited by this platform.
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