Modal analysis of Doppler shift and strain time histories extracted from distributed fiber optic sensing data

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Abstract While fiber optic sensing systems (FOSS) operate through a variety of technologies, optical frequency domain reflectometry (OFDR) has emerged as a means for distributed sensing, wherein a single fiber contains hundreds to thousands of sensing points along its length. Currently, most OFDR distributed sensing focuses on strain sensing; however, prior to deriving strain information, the changes in the carrier frequency of optical gratings can also be monitored. These changes in carrier frequency register as Doppler shifts, and this study explores treating these shifts like dynamic signals, e.g., time histories. Further, this study demonstrates that the Doppler shift time histories contain modal information distinct from strain measurements that can be similarly extracted through traditional modal analysis. Using experimental data from modal testing of a cantilevered plate, frequency-based modal identification is performed on both strain and Doppler shift time histories recovered from an OFDRbased distributed FOSS. The results reveal that Doppler shift time histories yield estimates of the slope mode shapes, as opposed to the strain mode shapes produced by the strain time histories, which can be used to recover displacement mode shapes through numerical integration. In addition, comparisons between the dynamic signals show that the Doppler shift time histories provide a greater number of identifiable modes with higher correlations to reference mode shapes than strain measurements.