Dependence of error rate degradation on graded-index-fiber excitation state

Degradation of error-rate characteristics has been observed in practical high-bit-rate digital systems using graded-index multimode fibers with center dips in the refractive index profiles. This degradation is caused by substantial intersymbol interference. To understand this phenomenon better, the relationship between the excitation state of a fiber and its pulse response has been studied both in theory and through experiments. It has been found that this phenomenon results from a center dip in the refractive index profile of a fiber, and that core center excitation brings about a fatal degradation in the error rate characteristics. Solutions to this problem are also discussed.