Multipoint FBG Sensing System for Long-Range and Long-Term Structural Health Monitoring by Incoherent FMCW Optical Ranging System

Fiber Bragg gratings (FBGs) have been widely used for structural health monitoring (SHM) of large-scale infrastructures such as buildings and bridges. Although the wavelength-division multiplexing (WDM) technique has been used to interrogate FBG sensors, the scalability is limited by the requirement of different Bragg wavelengths for each sensor. In this study, we developed the FBG-based sensing system utilizing the distance-division multiplexing (DDM) technique realized by the incoherent frequency-modulated continuous-wave (I-FMCW) optical ranging system. Unlike the WDM technique, the proposed DDM technique allows multiple FBGs with identical Bragg wavelengths, which gives a significant increase in the number of multiplexed sensors without requiring additional wavelength allocation. Moreover, because the I-FMCW optical ranging system does not rely on optical interference, it enables long-range interrogation over several kilometers, making it more robust and practical for SHM applications. We successfully demonstrated the FBG sensing system consisting of ten multiplexed FBGs in the 500-m-long fiber, achieving interrogation of 1-m ranging resolution and automatic multipoint temperature sensing in the range of $5~^{\circ } $ C– $75~^{\circ } $ C. Furthermore, we validated the long-term stability of the developed system over 48 h with temperature measurement errors within a few degrees Celsius. Compared to the previous I-OFDR-based techniques, our work demonstrates a practical SHM solution. These results highlight the potential of the I-FMCW-based DDM technique for scalable, long-range, and automated SHM.