Generation and evolution of bright-dark pulses in a thulium/holmium co-doped fiber laser based on nonlinear polarization rotation

Abstract

We report an experimental phenomenon of the splitting and merging of the bright-dark pulses (BDPs) in a mode-locked thulium/holmium co-doped fiber laser (THDFL) based on nonlinear polarization rotation (NPR). By adding 100-m-long highly nonlinear fiber (HNLF) into a simple ring cavity, the BDPs can be generated. The time interval between the bright and dark pulses increases linearly with the pump power and approximately equals to the inverse of the modulation frequency of the radio frequency (RF) spectrum. Apart from that, the bright and dark pulses are shown not to be orthogonally polarized. The obtained results are valuable for the evolutionary mechanism of the BDPs in passively mode-locked fiber lasers (PMLFLs).

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