Structural health monitoring is essential for the lifecycle maintenance of tunnel infrastructure. Distributed fiber-optic sensor (DFOS) technology, which is capable of distributed strain measurement and long-range sensing, is an ideal nondestructive testing (NDT) approach for monitoring linear infrastructures. This research aims to develop a distributed sensing network utilizing DFOS for structural integrity assessment of concrete immersed tunnels. The primary innovations of this study lie in the development of a general flowchart for establishing a sensing network and obtaining reliable field data, as well as its subsequent validation through a detailed case study. Concentrated joint deformations in typical immersed tunnels, detectable by the DFOS, are key indicators of structural integrity. This study addresses crucial elements of field monitoring system design, including the selection of appropriate optical fibers or cables and the determination of vital interrogator system parameters. It also covers sensor parameter determination, installation techniques, field data collection, and postanalysis. Furthermore, this research is exemplified by a case study that illustrates the successful implementation of a distributed sensing network in an operational immersed tunnel, and monitoring data reveals cyclic structural deformations under impacts of daily tide and seasonal temperature variations. The data obtained from this network play a significant role in subsequent condition assessments of tunnel structures. The research findings contribute to the assessment of large-scale infrastructure health conditions through the application of DFOS monitoring.

A novel microelectromechanical system (MEMS) magnetic sensor based on fiber-optic detection is presented in this paper. The magnetic field is detected by measuring the tilting motion of a mechanical suspension with permanent magnet attached. When the magnet is magnetized in different directions, the magnetic sensor can detect the in-plane or out-of-plane magnetic field. The MEMS structure was designed analytically, fabricated by using a bulk micromachining process, and assembled with the permanent magnet manually. Finally, the magnetic-sensing capability of an as-prepared sensor was tested by assembling it into the fiber-optic detection system. The testing result shows a sensitivity of 2.86 mV/μT for the in-plane magnetic field and 6.57 mV/μT for the out-of-plane. According to the mechanical-thermal noise theory, estimated resolutions are, respectively, close to 142 and 62 nT for the in-plane and out-of-plane magnetic field, which suggest the adequacy for measurement in earth magnetic field range. The reported magnetic sensor shows the capability to fulfill the high-resolution detection of low-frequency signals, benefitted from the utilization of permanent magnet and fiber-optic detection. Moreover, the sensitive direction of reported magnetic sensor can be easily switched by varying its magnetized direction, which, therefore, paves the way to the integrated tri-axis magnetic sensor.