Brillouin Interaction Between Individual Optical Modes Excited in Multimode Fibers

A multimode optical fiber supports the excitation and propagation of a singular, pure optical mode. This mode, characterized by a field pattern that adheres to the boundary conditions, remains constant throughout the length of the fiber. When two such pure optical modes, moving in opposite directions, are initiated, they could interact via the stimulated Brillouin scattering (SBS). In this study, we introduce an analytic theoretical framework to describe the SBS interactions between two counterpropagating optical modes, each selectively excited in an acoustically uniform multimode optical fiber. Using a weakly guiding step-index fiber model, we have formulated an analytical expression that maps the spatial distribution of sound field amplitude within the fiber core. Furthermore, we have investigated the characteristics of the SBS gain spectra, particularly focusing on the interactions between modes of varying orders. Through this approach, we aim to provide comprehensive insights into the sound propagation phenomena associated with SBS in multimode optical fibers, highlighting their unique influences on the SBS gain spectrum.