The Study On Dynamics Of Stimulated Btillouin Scattering In Fiber Lasers

In the fiber lasers,stimulated Brillouin scattering has presented various dynamics and hence attracted comprehensive attention.Based on these dynamical characteristics,stimulated Brillouin scattering has been employed to realize Brillouin amplifiers in optic-fiber communication systems,Brillouin optical fiber sensing,pulse compression,signal-processing techniques(including stored light and slow and fast light,etc),and various kinds of new light sources.However,in these application areas where SBS is relevant,there still exist a series of problems needed to be studied further,such as the influence of active gain on SBS process in fiber lasers,influence of frequency detuning from resonance on the SBS dynamics in fiber,transient energy convection between the pump and Stokes waves in the SBS process,and dynamics of SBS in multimode fibers and so on.Moreover,Moreover,in the high-power narrow-linewidth pulsed fiber lasers,stimulated Brillouin scattering accompanied by other different nonlinear effects has evolved as the main factor which limits the power scaling of the fiber lasers and hence limits the application potential of narrow-linewidth fiber lasers in the areas of earth science,atomic and molecular physics,frequency convertion,and beam combinations.On the basis of the aforementioned cases,this paper carries through corresponding theoretical and experimental study and the main contents are listed as follows:The paper first reviews the study progress of the stimulated Brillouin scattering in the fiber.We present in detail various kinds of dynamics in single-mode and multimode fibers,where stimulated Brillouin scattering is relevant,and clarify the important meaning of expanding and deepening further the study on the dynamics of stimulated Brillouin scattering in the fiber lasers.We have demonstrated experimentally and theoretically that hybrid ytterbium/Brillouin gain can contribute to the partial mode locking in the high-loss ytterbium-doped fiber laser.With only few cavity modes contained within the Brillouin gain curve,the dynamics of self-pulsing was not observed for the stimulated Brillouin scattering signal generated by each pump mode in the experiment.The results from two different experimental schemes show that both the Brillouin and ytterbium gains are required to support enough Brillouin pump and Stokes cavity modes for the partial mode locking.We also present that it is the coupling between the stimulated Brillouin scattering of the neighboring Brillouin pump modes that plays a direct role in the partial phase synchronization between the different Stokes modes.Furthermore,we proposed a theoretical model about the partially mode-locked laser dynamics.Based on the model,the numerical results agree quantitatively with the experimental ones.It is also found numerically that the extent of the phase synchronization can be enhanced by the cascaded stimulated Brillouin scattering according to the same physical mechanism.We discover that without participation of Kerr effect frequency detuning from resonance can give rise to rich dynamical behaviors for stimulated Brillouin scattering in optical fibers.Distinct from the dynamics presented in the conventional Brillouin lasers,this kind of phenomena does not need external feedback at all but also presents a variety of classifiable dynamical features for continuous-wave pumping,including steady state,periodic state and chaos.We analyze that the main mechanisms responsible for these dynamical behaviors include the transient response of acoustic wave,relaxation oscillation,frequency mixing effect induced by three-wave coherent coupling and Brillouin gain-induced group velocity change.We prove theoretically that there exists transient energy convection between the pump and Stokes light waves in the stimulated Brillouin scattering process.We analyze that the transient energy reflux from the frequency-downshifted Stokes wave to the pump wave is determined by the reverse process of the stimulated Stokes scattering,namely the stimulated anti-Stokes scattering.Thus,we verify that both the stimulated Stokes scattering and its reverse process can occur in the transient stimulated Brillouin scattering process,and it is the stimulated Stokes scattering that dominates in the whole process of stimulated Brillouin scattering because of the finite lifetime of acoustical phonons.Furthermore,it is pointed that the direction of energy flow between the pump and Stokes waves will turn to the opposite side when any one of the three waves,including the pump,Stokes and acoustic waves,experiences an additional phase change of π.We have built the general theoretical model for the dynamics of stimulated Brillouin scattering in multimode fibers.Both the effect of optical and acoustic mode field profiles on the Brillouin gain spectrums and the coupling among all optical modes are considered in this model.Moreover,besides the stimulated Brillouin scattering,the group-velocity dispersion,self-phase modulation and cross-phase modulation are also taken into account in our model.Furthermore,in order to deal with problems of stimulated Brillouin scattering in fiber lasers and amplifiers,we generalize the model to multimode gain fibers with model competition induced by spatial gain saturation in the fiber.In addition,based on the aforementioned model,we have studied theoretically and numerically the thresholds of stimulated Brillouin scattering in multimode fibers.We deduce the corresponding transcendental equations for the Brillouin thresholds of continuous-wave signal under two different typical conditions,namely the long fiber(αL?1)case and short fiber(αL?1)case.Moreover,the theoretical predictions are compared with the results obtained by numerically integrating the Stokes power equations,and good agreements are achieved between them.We also carry through the theoretical study on the nonlinear dynamics in high-power narrow-linewidh pulsed fiber lasers and amplifiers,and build a quite comprehensive model to describe the output characterstics of the laser with the group-velocity dispersion,self-phase modulation,cross-phase modulation,four-wave mixing,stimulated Brillouin scattering,Brillouin-enhanced four-wave mixing and stimulated Raman scattering and mode competition in the multimode gain fiber taken into account.Based on this general theoretical model,its two applications are implemented for two different circumstances by employing appropriate simplification,and are also verified through the comparisons with the experimental results.