Light transmission through photo-sensitive multi-mode fibers (MMF) with controlled volume disorder is investigated. Experiment shows that a segment of disordered MMF as short as 10 cm is sufficient to distribute power uniformly over all co-propagating modes and the intensity at the output surface of the fiber follows the Rayleigh negative exponential function. To explain the experimental findings, a comprehensive theoretical model is developed with three main results. First, statistical properties of all components of the dielectric tensor are obtained and analyzed in the framework a microscopical model of photo-sensitivity in a germano-silicate glasses. Secondly, it is shown that induced birefringence is insufficient to explain mode mixing, and that cross-polarization mode coupling is essential. Such a coupling is shown to originate from the spatial correlation in the off-diagonal elements of the dielectric tensor. Third, a hybrid theory to describe propagation in a fiber with a spatially correlated disorder is developed. The proposed theory treats the deterministic part of the light via coupled-amplitude equations, and the randomly-phased component with coupled-power equations. The complete theory developed in this work has a predictive power -- it can guide the design of an artificial disorder based on the desired transmission properties of the fiber. Experiment shows that mixing all co-propagating modes can, indeed, be attained in a short segment of a suitably designed disordered MMF without a prohibitive loss. Such fibers can be useful for e.g. maximizing the information capacity multi-mode fiber links --Abstract, page v
