Generation of wavelength-tunable soliton molecules in a 2-μm ultrafast all-fiber laser based on nonlinear polarization evolution

We report on the experimental observation of stable single solitons and soliton molecules in a 2-mu m thulium-holmium-doped fiber laser mode-locked through the nonlinear polarization evolution technique within an anomalously dispersive cavity. Single 0.65 nJ solitons feature a 7.3 nm spectral FWHM and 540 fs temporal duration, yielding a time bandwidth product close to the Fourier-transform limitation. Under the same pumping power of 740 mW, stable out-of-phase twin-soliton molecules, featuring a temporal separation of 2.5 ps between the two similar to 700 fs pulses, are generated in a deterministic way, while the central wavelength of the soliton molecules can be tuned from 1920 to 1940 nm. Finally, we present strong experimental evidence of vibrating soliton molecules. (C) 2016 Optical Society of America