Pure-quartic soliton molecules in normal fourth-order dispersion regimes based on spectral filtering effect

Recent theoretical verification of self-similar and dissipative pure-quartic solitons (PQSs) emphasized the similarity between PQS lasers and conventional fiber lasers, but the unique equilibrium mechanism hinders the formation of PQS molecules in normal fourth-order dispersion (FOD) regimes. In this paper, we investigated the effect of filters on shaping PQSs in normal FOD based on a passively mode-locked fiber laser model. A bandpass filter eliminates the time pedestal of dissipative PQSs, thus realizing a multi-pulsing state. When the filter bandwidth is appropriate, the effective spectral filtering effect can lower the pulse splitting threshold and enable the coherent restoration from chaotic PQSs to PQS molecules. Additionally, changing the central wavelength of the filter can generate PQSs and PQS molecules with asymmetric intensity distributions. These results are important guides for the manipulation of PQSs and the construction of high repetition-frequency fiber lasers.