ESTIMATION OF COUPLING LENGTH IN MODE-COUPLED MULTIMODE OPTICAL FIBERS.

Transmission characteristics such as the bandwidth and the loss of multimode optical fibers are very complicated due to mode coupling effects. It is important to know the coupling length of fibers for understanding the transmission characteristics. The coupling length is defined as the length at which the length-dependency of the pulse-width is changed from the linear to the square root. In this paper a method is given to estimate the coupling length from a comparison between the measured transfer function at one length of the fiber and a theoretical one, which is obtained using an algebraic method. Using this method, the coupling length of fibers can be estimated from relatively short fibers without any damages to fibers. As an example, the coupling lengths of graded-index and step-index multimode optical fibers are estimated using this method.