Single and multi-pulse regimes in hybrid mode-locked fiber lasers based on nonlinear polarization rotation and SMF-GIMF-SMF saturable absorber

We propose an all-fiber erbium laser that is mode-locked by integrating nonlinear polarization rotation (NPR) with a structure consisting of single-mode fiber, graded-index multimode fiber, and single-mode fiber (SMFGIMF-SMF). This mode-locking exhibits a self-starting feature and can be reinstated during the modulation of the polarization controller (PC). The pulses achieved have a 3 dB spectral bandwidth of 10 nm and a wavelength tunable range of 7 nm, which are superior to those obtained from a single NPR mode-locked laser within an identical cavity. The generation of multi-pulse regimes is demonstrated through manipulation of the pump power and adjustment of the PC. These findings highlight the superiority of hybrid mode-locking over single modelocking.