This paper describes a multimode optical sensor that is deployed on or near railroad tracks, and detects the presence of flat wheels rolling on the rails, based on spectral characteristics of their induced vibrations. The sensing mechanism uses optical interferometry in which modally dispersive coherent light traveling through the multimode fiber mixes at the fiber's terminus, resulting in a characteristic pattern of light and dark splotches called speckle. The laser speckle is stable as long as the fiber remains immobile, but flickers when the fiber is vibrated. The system works by measuring the time dependence of this speckle pattern and applying digital signal processing to the fast Fourier transform (FFT) of the temporal data.
