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, Δneff, in multimode optical fibers is presented based on the Brillouin spectrum analysis. The proposed method is successfully demonstrated in GeO2-doped step-index 2-mode fibers, for evaluating Δneff between LP01 and LP11 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.