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Abstract

An apparatus and method for measuring distances along single or multiple paths using fiber optics. The system uses a broadband optical source and broadband sensors. The lengths of one or more measurement paths are obtained with respect to the known length of a reference path. In one embodiment of the invention, two oscillators are used, producing signals, e.g., pulse trains, at slightly different frequencies (F1 and F2). The pulse train at F1 propagates through the fiber optic paths, and then is multiplied by the signal at F2, producing a time-expanded version of the original pulse at a frequency .vertline.F1-F2.vertline.. In a second embodiment, the phase shift of the light signals is measured at multiple frequencies, rather than at just a single frequency. When the light signals from the different measurement optical fibers are combined, the resultant net phase shift is a nonlinear function of both the lengths of the measurement optical fibers and the modulation frequency. Thus, if there are P measurement optical fibers, by obtaining the net phase shift at P different frequencies, a system of P simultaneous equations with P unknown is obtained. These equations are solved to determine the time delay in each of the P different optical fibers.

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