Intermittent “Invisible” Soliton Pulsations Induced by the Temporal-Gain Dynamics in Passively Mode-Locked Fiber Lasers

Gain, as one of the critical parameters in passively mode-locked fiber lasers (PMLFLs), fluctuates periodically in the time domain, including the fast depletion and slow recovery processes. Unfortunately, since there are few soliton dynamics phenomena that can properly reflect its effect, time-varying gain is rarely considered a research topic in studying the nonlinear dynamics of fiber lasers. Here, based on the transformation of soliton states, the effects of the temporal-gain dynamics (TGD) on soliton evolutions are theoretically speculated and experimentally demonstrated, and a novel nonlinear dynamics phenomenon - intermittent "invisible" pulsation (IIP) is reported. The IIP is ultimately generated by the accumulation and consumption of gain residues. In addition to the fundamental frequency regime, the generation of IIP is also observed in the harmonic mode-locking regime, and the evolution characteristics of each pulse train are very similar to those in the fundamental frequency regime, demonstrating the independence of the evolution of each pulse train during harmonic mode-locking IIP and verifying the universality of IIP. Apart from providing new insights into the study of intermittent dynamics in ultrafast lasers, this research will also provide new ideas for the study of TGD in ultrafast fiber lasers.