Radiation multimode fibers

When light propagates along a fiber and impinges on nonuniformities due to imperfections in the fiber, some of its power is scattered, as shown schematically in Fig. 22-1 (a). Part of the scattered power is distributed into forward-and backward-propagating modes, while the remainder is radiated. For multimode fibers, the distribution of scattered power is best treated by the ray methods of Chapter 5. Here we are primarily interested in fibers that propagate only one or a few modes. We treat the nonuniformities of the perturbed fiber as induced current sources within the unperturbed fiber. The results of the previous chapter can then be used to describe excitation of bound modes and the radiation field [1-3]. 

This chapter is mainly concerned with the power radiated from single-mode fibers due to bending. We first consider losses from bends of constant radius, and then examine losses due to random, but slight, curvature along the fiber, commonly known as microbends. Bending losses for multimode fibers are described by the ray methods of Chapter 9. 

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