Dynamics of dissipative pure-quartic solitons and molecules in NPR mode-locked fiber lasers with positive fourth-order dispersion

Ultrafast laser technologies have greatly benefited from the unique properties of optical solitons, which enable stable pulse propagation and waveform control. While conventional solitons are well described by the balance between second-order group velocity dispersion (GVD) and Kerr nonlinearity, higher-order dispersion introduces richer dynamics. In the regime dominated by positive fourth-order dispersion (PFOD), the formation and evolution of dissipative pure-quartic solitons (DPQSs) and their molecular states in nonlinear polarization rotation (NPR) mode-locked fiber lasers remain insufficiently explored. Here, we numerically investigate the generation mechanisms and dynamic behaviors of DPQSs and bound-state molecules. The results reveal that pulse characteristics are determined by the interplay among FOD, nonlinearity, gain saturation, and spectral filtering. Increasing PFOD raises the pulse-splitting threshold, supporting higher-energy single pulses, whereas larger gain saturation energy facilitates soliton molecule formation. The spectral filtering bandwidth further governs the transition from single-pulse to multi-pulse regimes: a moderate bandwidth enhances pulse coupling and stabilizes bound states, while an excessively broad one weakens spectral confinement, leading to instability. These findings deepen the understanding of dissipative soliton dynamics in PFOD-dominated systems and provide theoretical guidance for designing high-energy, stable ultrafast fiber lasers.