Internal motion of the creeping soliton in mode-locked Er-doped fiber lasers

Creeping solitons offer a valuable platform to probe deeper dynamic processes of wave systems, and they are identified in mode-locked fiber lasers generally as a zig-zag trajectories of the pulses in their temporal evolution. In addition to exploring the fascinating characteristics in the time domain and spectrum, it is crucial to investigate the underlying mechanisms responsible for such behavior. In this paper, the internal motion of creeping solitons in a mode-locked fiber laser is presented. As the pulse propagates in the cavity, the peak power and energy of the two spectral sidebands vary synchronously yet inversely, while that of the spectral center remain nearly constant. Numerical simulations further reveal that the temporal creeping originates from a continuous energy exchange between spectral sidebands, driven by a pump‐induced imbalance between gain, loss, nonlinearity and dispersion. This imbalance leads to persistent spectral reshaping, with redistributed energy directly manifesting as temporal creeping. These findings provide a more profound explanation for the creeping behavior in mode-locked Er-doped fiber lasers, thereby advancing the fundamental understanding of pulse dynamics in nonlinear dissipative systems.