Pulse duration tunable Figure-9 laser enabled by a composite spectral mode of dissipative soliton

Dissipative soliton is a commonly employed operation regime for mode-locked lasers. In contrast to conventional soliton, a variety of stable spectral solutions, featuring wildly different spectral shapes and bandwidths, exist. While a few theoretical works have articulated the conditions necessary for different spectral solutions, experimentally, the transitions between these various dissipative soliton states are rarely investigated, and the physical knobs governing the state transition of dissipative soliton are still unclear. Here, in a Figure-9 laser, we observed a composite spectral mode of dissipative soliton, and demonstrate the detailed transition process between the two constituent states. Furthermore, by utilizing the dramatic change in spectral width during the mode transition, we realize wide tuning of the pulse duration from 230 fs to 13.7 ps, by only adjusting the pump power. Our results provide experimental support for understanding the various dissipative soliton working states and the transition process among them. In the meanwhile, the technical approach represents an effective way for swiftly tuning the pulse duration of mode-locked fiber lasers.