Generating narrow bandwidth pulses in an all-fiber figure-9 mode-locked fiber laser

Narrow bandwidth mode-locked lasers with pulse duration over 100 ps have various applications in fields such as quantum communication and molecule excitation. Despite the increasing application demand, in comparison to the one with ultrashort pulse duration and broad spectral bandwidth, there is less attention given to the generation of narrowband spectra in mode-locked lasers. In this contribution, we demonstrate a passively mode-locked ytterbium-doped fiber laser that relies on a standard fiber Bragg grating (FBG) as a narrowband reflector and a nonlinear amplifier loop mirror (NALM) as a saturable absorber. This laser can generate stable pulses with a pulse duration of 124.3 ps and an ultranarrow 3 dB spectral bandwidth of 27 pm. Both numerical simulations and experiments have demonstrated that adjusting the fiber length bias within the NALM loop to suppress nonlinear-induced spectral broadening can effectively facilitate the generation of narrowband mode-locked pulses.