Restabilization and spectrotemporal shaping of pure-quartic solitons by output coupling in fourth-order-dispersion mode-locked fiber lasers

Pure-quartic solitons (PQSs) in fourth-order-dispersion (FOD) mode-locked fiber lasers have attracted considerable attention as a route toward high-energy ultrafast pulse generation beyond the conventional soliton regime. However, the output coupler is generally treated as a fixed cavity loss, and its influence on PQS stability, dynamics, and pulse shaping remains insufficiently understood. In this work, we numerically investigate the role of the output coupling ratio in the evolution of PQSs in a mode-locked fiber laser with weak FOD. The results show that output coupling not only determines the temporal and spectral profiles of stable PQSs, but also plays a key role in reshaping the stability boundary and restabilizing unstable pulses. At relatively low gain saturation energy, stable PQSs exist only within a finite range of output coupling ratios; as the ratio increases, the pulse exhibits temporal broadening, spectral narrowing, and progressive suppression of temporal tails and spectral sidebands. At higher gain saturation energy, the PQS first becomes unstable and is then restabilized as the output coupling ratio is further increased. Moreover, the critical output coupling ratio required for restabilization increases monotonically with gain saturation energy. These findings demonstrate that the output coupler can serve as an effective and experimentally accessible control parameter for PQS stabilization and spectrotemporal tailoring in FOD fiber lasers, providing practical guidance for the design of ultrafast sources with controllable pulse characteristics.