Gain Depletion and Recovery as a Key Mechanism of Long-Range Pulse Interactions in Soliton Fiber Laser

Soliton interaction through gain depletion and recovery (GDR) is one of the known mechanisms of long-range pulse interactions in soliton fiber lasers. It has been commonly assumed that the GDR mechanism produces only repulsive interactions, leading to an evenly spaced pulse configuration of harmonic mode-locking. However, our theoretical investigations and numerical simulations of the GDR-induced soliton interactions in fiber lasers, mode-locked by nonlinear polarization evolution, reveal a different perspective. We have found that the GDR soliton interaction is sensitive to the generation of dispersive waves and the formation of intense soliton pedestals. These can be controlled by altering the laser polarization settings. Interestingly, under certain conditions, we observed that the GDR interaction facilitates mutual long-range attraction between solitons. Our direct numerical simulations demonstrate that depending on the polarization settings, the initial soliton configuration in the laser cavity can evolve into various multisoliton ensembles. For instance, in the final stage, a soliton group may transform into a stationary harmonic mode-locking state or evolve into a complex of bound solitons. These findings suggest a new basis for explaining the effects observed during transitions between multisoliton formations, which occur due to changes in the polarization settings of the fiber soliton laser.