Radiation build-up and dissipation in Raman random fiber laser

Raman random fiber laser (RRFL) is a complex physical system that arises from the distributed Raman amplification and the intrinsic stochasticity of fiber scattering. The analysis of the underlying lightwave kinetics at steady state has been the focus of considerable interest. However, the transient state, such as the RRFL build-up and dissipation, is particularly important for revealing the lightwave interaction process. Here, we investigate for the first time the RRFL dynamics at the transient state and track the RRFL temporal and spectral evolution theoretically and experimentally. Particularly, with the contribution of random distributed feedback, RRFL build-up exhibits continuous Verhulst logistic growth curves without cavity-related features, which is significantly different from the step-like growth curve of conventional fiber lasers. Furthermore, the radiation build-up duration is inversely related to the pump power, and the spectral evolution of the RRFL undergoes two phases, i.e., from spectral density increase to spectral broadening. From the steady state to the pump switch-off state, the RRFL output power dissipates immediately, and the remaining Stokes lightwave from the Rayleigh scattering will gradually disappear after one round trip. This work provides new insights into the transient dynamic features of the RRFL.