Hysteresis in Laser Cavity-Solitons

A laser cavity-soliton (LCS) state can form in a system where a nonlinear Kerr microresonator is embedded in a fibre laser cavity, to produce an optical frequency comb in a microresonator, [1]–[3], resulting in a self-emergent, stable, and efficient microcomb [4], [5]. In general, the system can generate various states depending on specific parameter settings, with two critical parameters for achieving self-emerging solitary oscillations: gain, which controls the soliton's energy, and main cavity length, which governs the group velocity mismatch between the two nested cavities. Hysteresis is a phenomenon in which a system exhibits memory-like behaviour, allowing it to remain in one of two stable states under identical external conditions. This property is associated with bistability. A fundamental aspect of microresonator fibre laser systems is that the emergence of LCS states is intrinsically linked to slow, energy-dependent nonlinearities within the laser cavity [4]. While bistability due to fast Kerr nonlinearity has been extensively studied in soliton formation, it is less understood how the slow energy-dependent nonlinear processes affect it. Investigating this connection is crucial for optimizing the stability and tunability of microcomb states. To demonstrate hysteresis in LCS, we conducted experiments using a dedicated optical setup (Fig. 1a). By sweeping the pump power in opposite directions, we observed the formation of distinct states with different powers (Fig. 1b). Our focus was on the transition between single-soliton and two-soliton states, where we identified a bistable region exhibiting consistent hysteresis (Fig. 1b).