Research On Characteristics Of Brillouin Scattering In Special Fibers

Stimulated Brillouin scattering（SBS）in optical fibers has become a research hotspot due to its rich properties and broad application prospects.SBS is an important nonlinear effect of acousto-optic coupling,which has been widely used in various important fields such as distributed optical fiber sensing,optical fiber amplifiers and lasers,slow light,optical frequency conversion,and radio frequency signal processing.Both theory and application of SBS in single-mode fibers（SMFs）have been widely reported.With the development of mode-division multiplexing technology,the research of SBS in few-mode fibers（FMFs）has attracted much attention.But so far,for the intra-mode and inter-mode SBS induced by vector higher-order optical modes（HOMs）or elastic waves of different polarization states,the mechanism and laws of acousto-optic coupling are still unclear.In this paper,during the processes of forward SBS（FSBS）and backward SBS（BSBS）,under the interaction of different vector optical modes and acoustic modes of different polarization states,the rich and complex acousto-optic coupling characteristics and Brillouin gain characteristics have been systematically studied and characterized.The main research contents and innovations are as follows:1.Different from the common SBS coupled wave equation and the solution that only considers the electrostrictive body effect,under the two mechanisms of photoelastic effect and moving boundary effect,a detailed derivation of acousto-optic coupling in the SBS process is carried out.The optical properties of vector optical mode and the acoustic properties of elastic waves are obtained,as well as the acousto-optic coupling coefficient and the resulting optical force.In a Brillouin gain spectrum （BGS）,each scattering peak corresponding to the different acoustic mode is characterized.The model is able to analyze the enhancement or weakening of SBS under the interaction of the two mechanisms,and has broad applicability.2.The mechanism of BSBS induced by shear acoustic modes（SAMs）with considerable transverse and longitudinal displacements is revealed in step-index fibers（SIFs）with high refractive index differences.The SAMs interact with the HE₁₁ mode,which ultimately generates a BGS with multi-peak structure in a new frequency range around 6GHz.Moreover,the effect of the core radius and refractive index of the SIF on the BGS contributed by the SAM is also investigated.3.The intra-mode FSBS induced by different optical modes of HE₁₁ mode,TE₀₁ mode, TM₀₁ mode,and HE₂₁ mode is studied and analyzed in few-mode micro-nano fibers （FMNFs）.And the HE₁₁ mode-induced FSBS and BSBS are compared in micro-nano fibers.The acousto-optic coupling mechanism is also analyzed in detail.One or several specific low-order transverse acoustic modes（TAMs）with GHz frequencies are excited and interact with different optical modes,which leads to intra-mode FSBS and dominates the BGS distribution.The distributions of the displacement fields and polarization states of the excited three types of TAMs are regular.During the different intra-mode FSBS processes,the frequencies and peaks of scattering peaks corresponding to different TAMs are compared with the change of FMNF radius. And several distinct maximum values of Brillouin gain exist at the FMNF radius of about 0.8μm.4.The influence of the polarization state of HE₁₁ mode on the FSBS and BSBS characteristics is investigated.In micro-nano fibers,when the polarization angleθof the HE₁₁ modes in the pump light and the scattered light varies from 0 toπ/2,the Brillouin peak corresponding to the excited acoustic mode with azimuth number M =2 remains basically unchanged.But for the excited acoustic mode with M=0,the Brillouin peak value andθhave a clear functional relationship of（1+cos（2θ））.5.The inter-mode FSBS induced by the different vector optical mode pairs in FMNFs is studied in detail and discussed separately.The distribution law of the acoustic mode field for selective excitation is summarized.During the inter-mode FSBS process of HE₁₁-TE₀₁,the Brillouin scattering peak corresponding to the R₁₁ acoustic mode is particularly prominent with an exceptionally large peak value of 260.5W^-1m^-1.As the FMNF radius changes,the Brillouin peak corresponding to the R₁₁ acoustic mode remains the highest in the forward BGS,at least an order of magnitude higher than the peaks contributed by other simultaneously excited acoustic modes.And meanwhile,the Brillouin peak by the R₁₁ acoustic mode can reach 16 times larger than the maximum Brillouin peak during the intra-mode FSBS process.As the FMNF radius changes,the acoustic modes TA₀₂,R₂₂,and R₂₁ consistently correspond to the highest Brillouin peaks in the BGSs of TE₀₁-TM₀₁,TE₀₁-HE₂₁,and TM₀₁-HE₂₁, respectively.Among them,considering the inter-mode FSBS induced by TE₀₁ mode, TM₀₁ mode and HE₂₁ mode,the Brillouin peak caused by the interaction between the R₂₁ acoustic mode and the TM₀₁-HE₂₁ optical mode pair is the highest.