Distributional measurement of effective refractive index differences of multimode fibers by Brillouin spectrum decomposition

A novel method for distributionally evaluating the effective refractive index differences, Δn eff , in multimode optical fibers is presented based on the Brillouin spectrum analysis. The proposed method is successfully demonstrated in GeO 2 -doped step-index 2-mode fibers, for evaluating Δn eff between LP 01 and LP 11 modes, and the method could be straightforwardly expanded to fibers containing higher-order modes, by using a mode demultiplexer supporting the higher-order mode. To the authors' best knowledge, this is the first report of distributional measurements of the effective refractive index differences in multimode optical fibers.