Microbending Losses of Single-Mode, Step-Index and Multimode, Parabolic-Index Fibers

Two types of optical fibers appear to be promising for wideband communications, the monomode step-index fiber and a multimode fiber with parabolic (or nearly parabolic) refractive index distribution. 1 A third fiber type, the W-fiber,2 is so similar to the step-index fiber that its properties are easily included in the discussion of single-mode fibers. However, single and multimode fibers suffer radiation losses caused by unintentional random bends of the fiber axis.3-4 Thus, it is of interest to compare the two types of fibers and investigate which of them is better from the point of view of microbending losses. This loss comparison is the objective of this paper. Radiation losses of optical fibers are caused by coupling between the guided modes and radiation modes.5 Two modes with propagation constants /3i and 02 are coupled via a spatial frequency 6 of the coupling function when4 Pi-fr=0. (1) 937 A guided mode with propagation constant p 0 couples directly to the radiation modes using the entire spatial frequency spectrum in the range ' P,, ~