Switching and transformation of multi-state solitons in thulium-doped fiber laser based on nonlinear polarization rotation

Thulium-doped fiber (TDF) lasers are considered ideal platforms for studying 2 µm passively mode-locked fiber lasers. However, previous research primarily focused on soliton generation in TDF lasers, overlooking the switching and transformation between solitons in various states. This paper investigates the switching and transformation of multi-state solitons in TDF lasers based on nonlinear polarization rotation. By properly adjusting the pump power and polarization controller, we observe the switching and transformation between conventional soliton and bound state soliton, between soliton clusters and dual-wavelength soliton. The results were achieved using a simple structure (utilizing only one polarization controller) that exhibited superior repeatability and simplicity compared to a complex configuration (two polarization controllers). The experimental observation of the switching and transformation of multi-state solitons not only provides a deeper understanding of the dynamics of solitons in TDF lasers but also contributes to the design of multifunctional fiber lasers. This is significant for promoting the application of 2 µm fiber lasers.