The characteristics of wavelength tuning and pulse separation changing of soliton pairs at 2 mu m are still largely unexploited although they are helpful for the application of bound states in the field of high-capacity soliton fiber communication systems. We report on the experimental observations of stable bound solitons in a thulium-holmium co-doped passively mode-locked fiber laser by using the nonlinear polarization rotation technology. At a fixed pump power, the stable single soliton operation is converted to the soliton pair operation by cautiously adjusting the polarization controllers. Tightly bound soliton pairs with continuously tunable wavelengths from 1960.48 nm to 1989.45 nm, with a soliton separation of ~4 ps, are observed. Meanwhile, several different pulse separations from 5.2 ps to 11.2 ps of soliton pairs are also realized. With the increasing pump power, a triple-soliton bound state is also observed. The results help in understanding the nonlinear dynamics of fiber lasers.
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