Experimental Modal Identification Using Interferometric Sensing for Dual Use of Telecom Fiber Infrastructure in Structural Health Monitoring

Fiber-optic (FO) sensing adopting Laser Interferometry (LI) promises high sensitivity and distributed measurements for structural health monitoring (SHM), on the other hand, practical performance depends critically on how fibers are coupled to the host structure. In this study, a coherent LI technique, recently developed for underground seismicity detection, is applied for SHM and a systematic laboratory investigation is reported on a simply supported timber beam. Natural frequencies and damping ratios identified from coherent LI-based FO strain measurements are compared with conventional accelerometers under hammer-impact excitation. The comparison covers total of nine attachment strategies: the three configurations include the bare beam and a PVC conduit mounted on the beam with either weak (clamped) or strong (screwed) coupling. Each of the three configurations is instrumented with a single coupled, partially coupled, and uncoupled fiber. Across all attachment configurations, LI-based FO and accelerometer modal frequencies agree within ≲1% for the first three modes; however, FO-derived damping ratios are generally lower and more variable, with the largest scatter in weakly coupled (clamped) cases. Under ambient vibration, LI-based FO robustly identified the first mode even when the accelerometers used here were not sufficiently sensitive. The results confirm that the proposed methodology can track frequency shifts induced by attachment-dependent stiffness changes, similar to those induced by structural damage, suggesting feasibility for SHM and offering practical guidance on coupling considerations for field applications. In the longer term, the main purpose is to enable dual use of telecommunication fiber infrastructure in buildings by employing the coherent LI technique for SHM