Optical Power Flow in Multimode Fibers

Loss, coupling, and delay differences among the modes of multimode fibers influence their transmission characteristic in a complicated way. An approximation of the modes by a continuum leads to a comprehensive description of these interrelations. We relate the mode power distribution to the far-field output and calculate these distributions as functions of the fiber length and the input. We report measurements of the far-field distributions at various lengths of a cladded low-loss multimode fiber. A comparison of theory and experiment yields a quantitative estimate of the mode coupling involved. We associate this coupling with random irregularities of the fiber configuration and straightness, and construct a quantitative model of such irregularities.