Tunable sub-300  fs soliton and switchable dual-wavelength pulse generation from a mode-locked fiber oscillator...

In this Letter, we demonstrate, for the first time, to the best of our knowledge, tunable soliton and switchable dual-wavelength pulse generation from a nonlinear polarization rotation mode-locked Er³⁺-doped fluoride fiber oscillator around 2.8 µm, employing a LiNbO₃ birefringent plate (BFP)-based Lyot filter. In the single soliton state, the wavelength can be continuously tuned in the region of 2752.4∼2807.2nm (∼55nm) with a pulse width between 199 and 270 fs, by rotating the BFP with the aid of its spectral filtering effect. At higher pump levels, two kinds of dual-wavelength synchronous mode-locking states (i.e., 2746.4/2782.0 and 2787.2/2825.2 nm) are observed, and can be flexibly switched by adjusting the rotation angle of the BFP. This approach opens up new opportunities for the development of versatile mid-infrared ultrashort laser sources.