Mode Coupling in an Optical Fiber with Core Distortions

Optical-fiber communication systems utilizing incoherent light sources such as light-emitting diodes require the use of multimode waveguides to insure efficient excitation of the guide. Such guides, however, suffer from multimode pulse dispersion, because modes with higher group velocity arrive at the receiver earlier than modes with lower group velocity, limiting the information-carrying capacity of the fiber. Thus, in a fiber of length L, uniform core of index n, uniform cladding of index n (1 -- A), and constant cross section, a short pulse feeding equal amounts of power to every mode at the input will arrive at the other ends with a width r = nAL/c, where c is the speed of light in free space.1 For example, a fiber with A = 0.01 and n2 = 1.5 will have a delay spread of T/L = 50 ns/km, a serious limitation on either highcapacity or long-distance transmission. Multimode pulse distortion can be reduced by introducing coupling between the guided modes.2 The reduction in the pulse length comes about because some power traveling in a fast mode is eventually transferred to a slow mode, while power starting out in a slow mode finds 3