In this study, we have investigated the evolution process and dynamic characteristics of a multi-pulse regime in an erbium-doped fiber ring laser based on a single-mode fiber–graded-index multimode fiber–single-mode fiber (SMF-GIMF-SMF) structure as an optical modulator. By utilizing the excellent nonlinear optical absorption of the SMF-GIMF-SMF (SMS) device with a modulation depth of ~8.68%, stable single-pulse mode locking at the frequency of 9.84 MHz can be readily observed at low pump power. In addition, the single-pulse operation can evolve into a multiple-pulse regime on account of the peak-power-clamping effect via suitably raising the pump power and carefully regulating the polarization state. Further, the single-shot temporal evolution of multiple pulses is monitored, indicating that this state shows unique and interesting temporal characteristics with variable pulse separations and inconsistent pulse intensities, which, as far as we know, is the first such observation in ultrafast fiber lasers. Additionally, this study, based on the time-stretch dispersive Fourier transformation method, suggests that these multiple pulses consist of chaotic wave envelopes with erratic intensities and changeable pulse energy. We believe that these findings have profound implications for revealing fascinating nonlinear pulse dynamics in ultrafast fiber optics.

In this study, we have demonstrated an ultrafast Tm-doped fiber laser utilizing the nonlinear multimode interference (NL-MMI) effect, with a single mode fiber–grade index multimode fiber–single mode fiber (SMF-GIMF-SMF) structure serving as the saturable absorber (SA). In addition to stable pulses, mode-locked pulses with chaotic features can be obtained in this fiber laser, characterized by a high average output power and pulse energy, resembling noise-like pulses. By employing the time-stretch dispersive Fourier transform (TS-DFT) technology, it can be seen that the sub-pulses constituting these pulses exhibit noisy characteristics with random intensities and energies. Furthermore, the numerical simulations elucidate the corresponding generation mechanism and dynamic evolution. These findings significantly enhance the comprehension of pulse dynamics and offer novel insights into the technological development and application prospects of ultrafast fiber lasers.