Optical fibres constitute a unique ground to investigate one of the most prolific and controversial discovery of modern physics, namely the Fermi-Pasta-Ulam (FPU) recurrence, which becomes manifest as the growth and decay cycles of a comb of sideband pairs from a weakly modulated pump, occurring via modulational instability (MI). To date, however, the genuine spontaneous symmetry-breaking nature of the MI recurrence was never observed (in any area of physics) due to intrinsic limitations of the experimental realizations. In this work, we overcome such limitations by implementing a novel experimental technique which allows us to reconstruct the longitudinal evolution in amplitude and phase of frequency modes via heterodyne detection of the backscattered light. As a result, we clearly observe how the control of the input modulation seed results into qualitatively different types of FPU recursive behavior associated with the spontaneously broken symmetry.

The recent demonstration of dissipative Kerr solitons in microresonators has opened a new pathway for the generation of ultrashort pulses and low-noise frequency combs with gigahertz to terahertz repetition rates, enabling applications in frequency metrology, astronomy, optical coherent communications, and laser-based ranging. A main challenge for soliton generation, in particular in ultra-high-Q resonators, is the sudden change of circulating intracavity power during the onset of soliton generation. This sudden power change requires precise control of the seed laser frequency and power or fast control of the resonator temperature. Here, we report a robust and simple way to increase the stability range of the soliton regime by using an auxiliary laser that passively stabilizes the intracavity power. In our experiments with fused silica resonators, we are able to extend the pump laser frequency stability range of microresonator solitons by two orders of magnitude, which enables soliton generation by slow and manual tuning of the pump laser into resonance and at unprecedented low power levels. Both single-and multisoliton mode-locked states are generated in a 1.3-mm-diameter fused silica microrod resonator with a free spectral range of ~50.6 GHz, at a 1554 nm pump wavelength at threshold powers <3 mW. Moreover, with a smaller 230-μmdiameter microrod, we demonstrate soliton generation at 780 μW threshold power. The passive enhancement of the stability range of microresonator solitons paves the way for robust and low threshold microcomb systems with substantially relaxed stability requirements for the pump laser source. In addition, this method could be useful in a wider range of microresonator applications that require reduced sensitivity to external perturbations.

We experimentally investigate the interplay of Turing (modulational) and Faraday (parametric) instabilities in a bistable passive nonlinear resonator. The Faraday branch is induced via parametric resonance owing to a periodic modulation of the resonator dispersion. We show that the bistable switching dynamics is dramatically affected by the competition between the two instability mechanisms, which dictates two completely novel scenarios. At low detunings from resonance, switching occurs between the stable stationary lower branch and the Faraday-unstable upper branch, whereas at high detunings we observe the crossover between the Turing and Faraday periodic structures. The results are well explained in terms of the universal Lugiato-Lefever model.