Tunable Brillouin multi-wavelength mode-locked fiber laser dominated by nonlinear optical loop mirror and assisted by nonlinear polarization rotation

A tunable Brillouin multi-wavelength mode-locked fiber laser (BMMFL) is proposed, predominantly based on a nonlinear optical loop mirror (NOLM) and assisted by nonlinear polarization rotation (NPR). The system employs 10 km of dispersion-shifted fiber (DSF) as the Brillouin gain medium to generate multiple wavelengths. The DSF, together with the polarization controller (PC1) and the optical coupler, forms the NOLM; the combined NOLM-NPR action functions as an equivalent saturable absorber to enable mode-locked pulse generation. By adjusting the PCs, the BMMFL generated 69 Brillouin multi-wavelength lines (BMLs) with a 10.9 GHz Brillouin frequency shift, when the pump power, Brillouin pump (BP) power, and BP central wavelength were fixed at 250 mW, 6 dBm, and 1546.5 nm, respectively. Under these conditions, no stably repeating mode-locked pulses were observed. A multi-wavelength tuning range of 30 nm was also obtained at different BP central wavelengths. Subsequently, by further adjusting the PCs, the BMMFL achieved a mode-locked pulse train with a pulse width of 236 ns and a fundamental frequency repetition rate of 19.92 kHz, with only 13 BMLs retained. At a pump power of 350 mW, the maximum average output power of the mode-locked pulse reached 1.29 mW. Moreover, harmonic mode locking up to the fifth order was achieved. This BMMFL offers broad application prospects in dense wavelength division multiplexing systems, laser processing and optical fiber communications.