Distributed detection of microcracks by differential narrowing of FWHM in BFS

Brillouin scattering based optical fiber sensors provide the opportunity for distributed detection of multiple cracks in structures. Because of capability for making high spatial resolution measurements, pre-pump-pulse Brillouin optical time domain analysis (PPP-BOTDA) systems have been successful in detecting microcracks. Recent studies have shown that when the space between two neighboring microcracks is smaller than the full width at half maximum (FWHM) of the peaks in Brillouin frequency shift (BFS), the strain peaks in BFS interfere, and inhibit detection of multiple microcracks. The focus of the present study is to narrow the size of FWHM by computing the difference in BFS of the strain peaks at two slightly different spatial resolutions. The differential FWHM obtained in this manner is narrower than the original FWHM for single microcrack, as well as two neighboring microcracks. The efficiency of the proposed approach was tested through experiments. The results indicated that it was possible to reduce the FWHM for single cracks, and also to detect the presence of dual microcracks with crack spacing distances smaller than the FWHM. However, the effectiveness of the proposed approach is also dependent on the spatial resolution of measurements, and the signal to noise ratio of the measurement system. While higher spatial resolutions are required for effective narrowing of FWHM in differential approach, the measurement noise at higher spatial resolutions limited the applicability of the method for very small crack opening displacements.