Tunable and switchable multi-wavelength bright and dark pulse actively Q-switched random fiber laser based on NPR and EOM

In this paper, a tunable and switchable multi-wavelength bright and dark pulse actively Q-switched random fiber laser (RFL) based on nonlinear polarization rotation (NPR) structure and elector-optic modulator (EOM) is proposed. The threshold of the laser is 33 mW, and the light conversion efficiency is about 3.18 %. In the experiment, a 25 km long single-mode fiber (SMF) is used to provide random distribution feedback, and as the pump power increases, a single-channel to six-channel laser output can be obtained. When the pump power is fixed at 300 mW, switching between three, four, five, and six channels can be easily achieved by adjusting the two polarization controllers (PCs) in the ring cavity. Wavelength tuning of three and four channels is realized with a fixed wavelength interval of 1.5 nm. In addition, by changing the waveform of an external signal source, it is possible to switch between bright pulses, dark pulses, and light-dark pulse pairs. The proposed random fiber laser has broad application prospects in the fields of distributed sensing system and biomedical imaging.