Asymmetric spectral evolution dynamics in soliton fiber lasers under weak fourth-order dispersion

While soliton dynamics under dominant fourth-order dispersion (FOD) have been extensively studied, their behavior in the presence of weak FOD remains largely unexplored. This paper presents a numerical investigation of soliton evolution in a mode-locked fiber laser operating in a weak FOD regime, where the second-order dispersion is neglected. We demonstrate that stable pure-quartic soliton (PQS) propagation is achievable even under weak FOD conditions. As nonlinearity increases via the small-signal gain g0, the soliton spectrum undergoes asymmetric evolution, with the energy imbalance stochastically favoring either the short- or long-wavelength side. A further increase in g0 leads to periodic soliton pulsations and eventually triggers complex multi-soliton dynamics, including splitting, collisions, and energy transfer via annihilation. These findings reveal a rich phenomenon of nonlinear dynamics accessible in weak FOD regimes, distinct from both conventional and strong FOD systems.