Formation of Various Soliton Molecules in a 2- $\mu$ m Anomalous-Dispersion Mode-Locked Fiber Laser

We report on the experimental observations of stable single soliton and soliton molecules in an anomalous-dispersion thulium-doped passively mode-locked fiber laser by using the nonlinear polarization rotation technology. The stable single soliton operation can convert to the soliton pair operation by cautiously adjusting the PCs, at the pump power of 350 mW. It is experimentally demonstrated that the formation of stationary soliton pairs is related to the interaction between solitons and dispersive waves. By appropriately adjusting the cavity parameters, more complex soliton molecules composed of three, four, and five bound solitons can be observed. Meanwhile, we experimentally analyze complex nonlinear dynamics of soliton molecules around 2-μm.