Study On Intermodal Stimulated Brillouin Amplification In Multimode Fibers

Multimode fiber(MMF) has a wide application prospect, as transmitting media, for high power laser and heavy data flow transmitting. Among these application fields, stimulated Brillouin scattering is an important effect that worth attention.SBS in multimode fiber involves interactions between multiple modes, including light modes and acoustics modes, which has leaded to a more complex situation than single mode fiber. The research on SBS in multimode fiber can be mainly divided into two respects, the one is focus in the variation of Stokes pattern during SBS, for example, mode clean effect. The other one is focus on calculation of SBS threshold and its depression. Hitherto, the former has neglected the influence of acoustic mode property on energy coupling of light, when the later has been concentrated in Brillouin gain spectrum but neglected the dynamic evolving of light mode patterns.In fact, the difference of Brillouin gain spectrum for different pairs of modes could have influence on the variation of Stokes patterns, which, I believe, could explain the mode clean or phase conjugation phenomenon in multimode fiber.In this article, the amplification property of SBS for inter-mode and intra-mode effect is studied, which the influence of light and acoustic modes properties has been considered. Based on theory for spatial situation, the spatial-temporal evolving equations of SBS for different pairs of modes is deduced, by which the process of SBS can be described comprehensively.First, based on finite differential method and waveguide boundary condition, the numerical method for mode properties for arbitrary refractive-index distribution fiber is proposed, which has been examined by the phenomenon of offset launching the two-mode fiber. The waveguide equations of acoustic field has been converted and calculated in similar way. The calculation of mode properties is necessary for the analysis of SBS.Then, according to the mode properties and phase-match condition, the relation governing the interaction of light and acoustic field is analyzed, and the spatial-temporal evolving equations for SBS between arbitrary pairs of modes is proposed. The description of Brillouin gain spectrum for specific pair of modes is given and the effect of mode properties on frequency shift and Brillouin gain can be determinedFinally, the SBS in multimode fibers with core diameter of 25μm and 50μm is calculated as examples, BGS of different pairs of light modes is given and compared with the existing result published, by which the accuracy of the established SBS modal can be verified.In conclusion, a simple and feasible numerical calculated method for light and acoustic mode properties for arbitrary index-distribution fiber is achieved, and the evolving equations and Brillouin gain spectrum for difference pairs of modes are given, which would act as reference for the further study of SBS in multimode fiber.