Dynamic characteristics of tunable Brillouin fiber lasers

This study investigates the laser dynamics of tunable Brillouin fiber lasers (BFLs). Experimentally, a system analogous to a Brillouin optical time-domain reflectometry (BOTDR) was employed to convert the BFL's Brillouin frequency shift into an intermediate frequency signal. The operation state of the BFL was controlled via external-frequency modulation applied to a single-frequency pump laser. The transient establishment and dynamic tuning process of the BFL were then investigated through time-frequency analysis of the intermediate frequency signals. The results reveal that the BFL resonance evolves through three distinct stages, including the spontaneous Brillouin scattering, the multi-mode competition, and the final single longitudinal mode operation. The laser establishment time was on the order of several hundred nanoseconds and exhibited a negative correlation with pump power. The multi-mode competition takes tens of microseconds. The dynamics tuning characteristics of the BFL were compared at different tuning steps. When the tuning step is greater than the longitudinal mode separation and less than the bandwidth of the Brillouin gain spectrum, a new laser line is established through multi-mode competition by strong mode hopping, resulting from the overlap of the Brillouin gain spectra. When the tuning step exceeded the Brillouin gain bandwidth, the new laser line was independently reconstructed while the previous one was annihilated. These findings on BFL dynamics pave the way for achieving widely tunable and even mode-hopping -free scanning of BFLs.