CoBi₄Se₇-based mode-locked fiber laser generating dark pulses and its soliton dynamics analysis

In recent years, two-dimensional (2D) materials have been widely applied in the optoelectronic field, especially as saturable absorbers (SAs) in fiber lasers. CoBi₄Se₇, as a ferromagnetic topological insulator, possesses unique ferromagnetism and a relatively high bandgap, making it a promising candidate for use as an SA. This paper investigates the saturable absorption characteristics of CoBi₄Se₇ and develops a CoBi₄Se₇-D-type fiber SA for use in doped fiber lasers. Traditional bright pulses, dark pulses, bright-dark (B-D) pulses, and dark-bright (D-B) pulses are obtained, and the soliton dynamics of B-D pulses are analyzed. To the best of our knowledge, this is the first time that a CoBi₄Se₇-based SA has been used to realize a mode-locked fiber laser. It is observed and confirmed that in the process of generating B-D pulses, dark pulses are formed prior to bright pulses.