Mode Mixing With Reduced Losses in Parabolic-Index Fibers

In multimode optical fibers, light pulses are carried by many modes. Such multimode operation limits the information-carrying capacity of an optical fiber communications system because of pulse spreading. At the fiber input, all modes receive part of the energy of the light pulse simultaneously. However, at the fiber output, the light pulse is stretched out in time because each mode (or mode group) travels at a different group velocity. The length of the stretched pulse is proportional to the fiber length. The amount of pulse spreading in a multimode fiber depends on its construction. The dependence of the group velocity of the modes on the mode labels is influenced by the distribution of the refractive-index profile of the fiber in radial direction.1 Step-index fibers* typically exhibit more pulse spreading than fibers with graded refractive-index distributions. Fibers whose index profiles show a parabolic (or square law) dependence on the radial coordinate have the property that all modes have nearly the same group velocity so that pulse spreading on parabolic-index fibers is nearly minimized.1 Our remarks so far apply to multimode transmission in perfect fibers where each mode propagates independently of all the other modes. * Step-index fibers have a core with constant refractive index and a lower index cladding. 777