Harmonic passive mode locking of bound-soliton structures in fiber lasers

•Operations with equidistant identical multisoliton structures are demonstrated.•Multisoliton structures are due to the attraction of closely spaced pulses.•The distantly spaced soliton structures are repulsed.•Investigated regimes are due to an asymmetry of wings of individual solitons.

It has been shown by numerical simulation that additional inertial nonlinearity of the refractive index results in repulsion of solitons in multi-pulse passive mode-locked fiber lasers if the intersoliton distance is sufficiently large. At the same time, closely spaced solitons are attracted to each other. It has been found that the combination of these properties of soliton interaction can lead to harmonic passive mode locking of identical structures of bound solitons (generation with an equidistant arrangement of soliton structures in the laser cavity). These properties of soliton interaction are caused by the asymmetry of the wings of an individual soliton, which manifests itself as the asymmetry of Kelly's spectral peaks and is due to the inertial nonlinearity of the refractive index. Other possible mechanisms of this asymmetry are also discussed.