Spectrally ultra-narrow soliton bunches based on a figure-9 mode-locked thulium-doped fiber laser

We experimentally demonstrate the formation of soliton bunches in a mode-locked thulium-doped fiber laser with an ultra-narrow spectral output. By adjusting the pump power from 2.4 to 6 W, soliton bunches are maintained at a central wavelength of 1951.2 nm, with a 3-dB spectral bandwidth of ∼50 pm and a pulse repetition rate of 1.64 MHz. Within this pump power range, the number of soliton pulses within the bunch increases from 4 to 19 while maintaining consistent intensity profiles. To the best of the authors' knowledge, this study presents the first experimental demonstration of long-picosecond soliton bunches with stable ultra-narrow spectral linewidths at a wavelength near 2 µm in a figure-9 all-fiber laser. These findings highlight the potential of the proposed laser source for applications in mid-infrared spectroscopy, nonlinear microspectroscopic imaging, and polymer processing.