Experimental Observation of of Complex Dark-Bright Soliton Coexisting Regimes in Mode-Locked Fiber Lasers

Fiber lasers have emerged as an attractive platform for investigating ultrafast dissipative complex nonlinear dynamics. Observation of bright and dark soliton and their interaction is a fascinating area of research. However, the combination of bright and dark solitons coexistence has been limited to a few works. In [1], [2], by employing a saturable absorber (SA) such as 2D materials and using the cavity's inherent nonlinear effect,Dark-Bright-Dark soliton (DBDs) was reported in a passive mode-locked laser. However, no work has been reported on generating DBDs using the artificial SAs-based mode-locked fiber lasers. Artificial SAs are attractive alternate options for material-based SAs, owing to their broad bandwidth and easy implementation. As one would know, in fiber lasers, the bright soliton develops when the effective nonlinearity is attractive, where self-phase modulation (SPM) and dispersion balance, while dark soliton manifests repulsive nonlinearity regions due to a phase inversion and energy redistribution within the cavity. To understand the generation of multiple bright-dark soliton (BDs) pairs, Manakov had theoretically pointed out that a dark pulse on the slow axis and a bright pulse on the fast axis could simultaneously propagate without any distortion. So,the interaction between these two orthogonal polarization states enables coupling, facilitating the coexistence of dark and bright soliton pairs, and cross-phase modulation (XPM) enhances the contrast between solitons and maintains stability. Interestingly, dark-dark-bright (DDB), bright-bright-dark (BBD), and even dark-bright-dark (DBD) complex soliton pairs may generate due to the cross-polarization phase modulation and SPM.