Er-doped mode-locked fiber lasers based on nonlinear polarization rotation and nonlinear multimode interference

We report an Er-doped fiber laser based on a hybrid mode-locked mechanism, which include a graded-index multimode fiber as a saturable absorber (SA) and nonlinear polarization rotation (NPR). By adopting two distinct mechanisms simultaneously, the laser can output shorter pulses with a high signal-to-noise ratio (SNR) and low noise jitter. At a pump power of 110 mW, a mode-locked pulse train with a pulse duration of 1.8 ps and a repetition rate of 49.80 MHz is generated. Compared with the implementation of the NPR mode-locked mechanism alone, the implementation of the hybrid mode-locked mechanism increased the SNR of the laser from 44 dB to 70 dB. The standard deviation of the average power fluctuation is 1.53%. Besides, a stable harmonic mode-locked pulse train can be obtained and switched by merely adjusting the pump power.