Experimental study on the evolution of bright-dark pulses in a 2 µm mode-locked thulium/holmium co-doped fiber laser

We experimentally demonstrate the effects of pump power and polarization state on the evolution of bright-dark pulses (BDPs) in a figure-eight passively mode-locked thulium/holmium co-doped fiber laser (THDFL) using nonlinear amplifying loop mirror (NALM) technology. As the pump power increases from 0.5 W to 1 W, the BDPs are separated and exhibit a linear increase in the time interval between the bright pulse (BP) and dark pulse (DP), which roughly corresponds to the reciprocal of the modulation frequency in the radio frequency (RF) spectrum. Additionally, the polarization state in the cavity is altered by adjusting the polarization controller (PC), and the central wavelength of spectrum, pulse waveform, and full width at half maximum (FWHM) of BDPs in different states are presented.