Filter-driven control of harmonic mode locking in PM fiber soliton laser

We report on the control of multi-pulse soliton dynamics and the stabilization of high-order harmonic mode-locking (HML) in an all-polarization-maintaining (PM) fiber laser. Comparative numerical simulations of long-range soliton interactions are presented for two cavity architectures, contrasting a filterless system with a spectrally tailored setup. The analysis unveils that introducing an asymmetric spectral constraint via a flat-top filter forces co-drifting solitons onto a steeper gain slope, magnifying their frequency divergence and, consequently, their inverse group velocity mismatch. Such enhanced gain-mediated repulsion acts as a strong restoring force that suppresses multi-pulse jitter and locks the periodic soliton train. Appropriate adjustment of the filter detuning and bandwidth allows stable HML operation to be experimentally achieved up to a 6.16 GHz repetition rate. Compared to the reference filterless regime, the stabilized configuration utilizing the asymmetric filter exhibits a 13–27 dB improvement in supermode noise suppression, forcing the RMS (root-mean-square) timing jitter deep into the sub-picosecond domain. The convergence between experimental observations and numerical models confirms that filter-driven frequency shifting offers a scalable approach for suppressing instabilities in complex multi-soliton systems converging to the HML state.