Theory of the backscattering process in multimode optical fibers.

A theory is presented which predicts the form of backscattered signatures obtained from multimode fibers in terms of the longitudinal variation of the fiber's characteristic parameters. Within a certain limit, the theory yields simple, analytical results; this limit requires one to employ a novel form of fiber excitation for which we adopt the term mode-filtered excitation. From backscattering signatures obtained using this form of excitation, it is possible to unambiguously separate intrinsic loss from fluctuations of the fiber's characteristic parameters. Various experimental verifications of the theory as well as demonstrations of the efficacy of the mode-filtering technique are presented.