Investigation on single to twin pulse transition in all-normal-dispersion mode-locked fiber oscillator

Abstract Mode-locked fiber oscillators operating in the all-normal dispersion (ANDi) regime support a range of dissipative soliton structures, including twin-pulse states. In this study, we experimentally investigated the influence of gain dynamics on the transition from single to twin ultrashort pulses in an ytterbium-doped mode-locked fiber oscillator. The system comprises of two independently pumped ytterbium-doped fiber segments, separated by a bandpass filter and a nonlinear polarization rotation-based saturable absorber within a unidirectional ring cavity. Stable single-pulse operation is achieved by continuously pumping one segment, while selective activation of the second segment induces pulse splitting. This controlled modulation of gain enables reversible and reproducible transition from single to twin-pulse operation. The sequential evolution from single to twin pulses is recorded for the first time, providing insight into the intermediate states in the splitting process. Numerical simulations support the experimental observations and further map the pump power parameter space that supports stable single and twin pulse regimes. We believe that these findings will advance the understanding of pulse splitting dynamics in ANDi mode-locked fiber laser.