Generating narrow bandwidth pulses in a hybrid mode-locked fiber laser.

We demonstrate for the first time, to our knowledge, an all-fiber erbium-doped mode-locked laser in which mode-locking (ML) is realized by the combination of nonlinear polarization rotation and a saturable dynamic filtering effect, thereby generating nearly transform-limited ultrashort pulses with a pulse duration and spectral width of 45.2 ps and 0.0775 nm, respectively. The laser achieves both ML and harmonic ML by increasing the pump power. Simultaneously, the filtering function is maintained by the saturable dynamic induced grating (SDIG) throughout the power-modulation process. Furthermore, numerical simulations are used to analyze the pulse energy evolution in the cavity, revealing the advantages of hybrid ML in decreasing the pulse duration and time-bandwidth product under narrow filtering status. This work proposes a practical method to achieve ultrafast laser pulses with a narrow bandwidth, solving the problem that the SDIG has a hard time realizing a stable ML sequence.