Soliton dynamics in fourth-order dispersion mode-locked fiber lasers with wavelength-tunable filters

Spectral filtering is a powerful mechanism for generating diverse soliton states in mode-locked fiber lasers, particularly in tunable systems where the center wavelength can be continuously adjusted. With the growing interest in pure quartic solitons, the corresponding pulse dynamics in fourth-order dispersion (FOD) fiber lasers become significant. In this work, we numerically investigate the effect of spectral filtering on pulse characteristics in fourth-order dispersion fiber lasers. By incorporating a tunable band-pass filter in the laser, both the central wavelength and spectral shape of pulses are altered, thereby affecting their temporal behaviors. The results demonstrate that the changes in central wavelength of the filter and saturation energy of the gain fiber drive the operating regime from single pulses to soliton molecules. The detuning between the filter and gain centers introduces equivalent group-velocity dispersion (GVD), which causes GVD offset and linear phase of the pulse. When the detuning is large, the pulse characteristics evolve from pure-quartic solitons toward conventional solitons, as confirmed by the energy-duration scaling law. These findings reveal the decisive role of spectral filtering in shaping soliton dynamics in FOD fiber lasers and provide theoretical guidance for the experimental realization of tunable pure-quartic dispersion fiber lasers.