Precise length measurements in multimode optical fibers

Selective optical excitation permits both the group index and the group delay of on-axis modes of multimode fibers to be determined with high precision. The group index of several types of fiber was measured at 1310 nm in a fiber Michelson interferometer, and the values were tabulated. Group delays were obtained from the transit time of short-duration optical pulses. From these data the length of reference fibers approximately 2 km long was calculated. Length-measurement accuracy was limited by group-index uncertainties to approximately 0.04%. Also, a technique that uses these reference fibers to minimize uncertainties in distance measurements made with multimode optical-time-domain reflectometers is described.