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Description
Optical frequency combs in microresonators, often termed 'microcombs', are optical sources made up of a series of equally spaced frequency lines. These lines are typically produced in nonlinear microcavities through the Kerr nonlinearity. The discovery of dissipative temporal cavity solitons marked a significant breakthrough in the field, enabling the generation of broad and smooth spectra particularly suited to metrological frequency-comb applications[1].
We demonstrated the ability to generate localized pulses when a nonlinear microresonator is integrated within a fibre laser loop [2]. This led to our observation of laser cavity solitons. By combining the attributes of microresonators and multimode laser systems, this scheme provides an approach for the generation, stabilization and control of solitary optical pulses in microcavities. A distinctive feature of this architecture is the decoupling of the optical path in the gain medium from that of the parametric resonator [5]. Consequently, while the gain limits the pulse circulating in the amplifying cavity, the bandwidth generated within the nonlinear Kerr cavity is ultimately determined by its energy, dispersion and nonlinear interaction. This architecture can therefore overcome the gain-bandwidth limitation and has demonstrated high spectral efficiency, with parametric conversion efficiencies exceeding 50% [6].
Within this context, it is crucial to highlight the primary physical characteristics of these waves. These include their energy efficiency and dynamic properties, both of which are essential for initiating and restoring the system. We have shown that these states can emerge spontaneously and recover with resilience [3], including when interacting with other states within the system [4].
Lasing spontaneously emerges within the resonant lines of the nonlinear microresonator and leverages intrinsic stabilising feedback effects enabled by slow, energy-dependent nonlinearities, such as thermal effects and gain-induced refractive-index changes, in both the amplifying and nonlinear cavities [3,7,8]. This provides distinctive self-starting and self-recovery features to the architecture without external control. Similar stabilisation mechanisms are well explored in continuous-wave configurations, for example through thermal and self-injection locking [9,10], but here they act on the full pulse. The balance of these nonlinearities can be controlled through external global parameters, such as pump power and cavity length, allowing the system to naturally evolve into soliton, Turing or crystal states without resonance scanning or complex 'writing' procedures. The result is a highly versatile and robust system that can switch between different nonlinear states, including single-soliton operation, simply by tuning static global parameters [3]. The coexistence of different nonlinear states does not disrupt soliton formation; on the contrary, it can provide further robustness mediated by nonlocal nonlinearities [4].
Finally, the multi-modal nature of microresonator-filtered lasers fundamentally allows the lasing modes, and therefore the accessible nonlinear states, to be controlled with some degree of independence. In particular, the ability to select which modes reach threshold is enabled by the microresonator acting simultaneously as a nonlinear element and as a spectral filter. Very recent results show the capability to independently control the lasing threshold of different soliton families and access molecules and topological states [11], opening perspectives for topologically enabled robustness, ultra-low-phase-noise metrological sources and new regimes of photonic dynamics.
Microcombs are also increasingly considered as key sources for millimetre-wave and terahertz systems. The combination of favourable noise properties and high spectral efficiency can enable the direct driving of broadband emitters for communications, positioning and time-domain spectroscopy. We have recently demonstrated direct photoconductive conversion of a 50 GHz laser-cavity-soliton microcomb into a millimetre-wave baseband comb covering the sub-THz region [12]. The microresonator-filtered source combines low free-running phase noise and high spectral efficiency, enabling coherent conversion.
Notes and References
1 Herr, T. et al. Nature Photonics 2014, 8, 145-152.
2 Bao, H. et al. Nature Photonics 2019, 13, 384-389.
3 Rowley, M. et al. Nature 2022, 608, 303-309.
4 Cutrona, A. et al. Communications Physics 2023, 6, 259.
5 Peccianti, M. et al. Nature Communications 2012, 3, 765.
6 Cutrona, A. et al. Optics Express 2022, 30, 39816-39825.
7 Carmon, T.; Yang, L.; Vahala, K. J. Optics Express 2004, 12, 4742-4750.
8 Rowley, M. et al. Optics Express 2019, 27, 19242-19256.
9 Liang, W. et al. Nature Communications 2015, 6, 7957.
10 Shen, B. et al. Nature 2020, 582, 365-369.
11 Das, D. et al. Quenching of Noise in a Free-Running Möbius Microcomb, arXiv:2505.18911 (2025).
12 Peters, L. et al. Millimetre-wave comb generated by an optical microcomb. Nature Communications 2026. DOI: 10.1038/s41467-026-76747-2.