# The Current Development of Structural Health Monitoring for Bridges: A Review
## 2 Monitoring Technology
### Sensor Monitoring Technology in BHM
#### 2.1.1 Fiber Optic Sensors  
In the early 1960s, fiber optics were studied and used in optical transmission systems [18]. With the wide application of information transmission technology based on optical transmission, people begin to focus on the study of material properties that affect optical transmission. In this case, optical measurement systems continue to make progress. A typical example in BHM is that FOS can be installed on the surface of rebars or embedded in pre-drilled holes to monitor the strain, temperature, and vibration of rebars. In 2019, [PERSON] et al. [19] proposed a method for monitoring prestress loss in prestressed concrete structures using FOS to provide a formal method for on-site evaluation. In the case of the Streicker Bridge strain sensor on Princeton University campus, they used the FOS strain measurement to study strain change at the centroid of the composite section. They reported that it has the advantage of being able to obtain accurate measurements when the temperature changes, and that it shows a wide range of applicability. The numerical results show the feasibility of FOS in the measurement of prestress damage.  
FOS can be divided into two types, distributed and discrete, based on whether they can monitor continuously with increasing distance. Distributed fiber optic sensors (DFOS) are a major research focus for achieving sensing measurements for thousands of measurement points using a single fiber optic cable [20]. According to different principles of light scattering, DFOS can be divided into the following three categories: DFOS based on Brillouin scattering, DFOS based on Raman scattering, and DFOS based on Rayleigh scattering [21; 22; 23]. The comparison of different types of DFOS is shown in Table 1. In the field of BHM, the sensing technology based on Brillouin scattering has been applied more widely.  
In 2017, [PERSON] et al. [24] proposed a structural health monitoring method for cable-stayed bridges based on the dynamic distributed sensing of bridge deck strains. To detect the location and size of the damaged cable, a formula was developed using the dynamic distributed sensing capability of Brillouin scattering optical time domain analysis (BOTDA), and the relationship between the strain redistribution on the deck of the cable-stayed bridge and the tension loss of the single cable was established. The applicability of this method was demonstrated in a scaled model test case of a cable-stayed bridge in the laboratory. In 2019, [PERSON] et al. [25] used FOS based on Brillouin scattering to monitor and normalize the distributed strain of trucks relative to the theoretical influence line of the bridge during multiple locations. In addition, they introduced the damage index in a method to identify microcracks. The author reports the test results on the concrete box girder bridge, which proves the effectiveness of this method. In 2022, [PERSON] et al. [26] proposed a hybrid structural health monitoring (SHM) system based on Brillouin DFOS. They introduced vibrating wire (VW) extensometers and temperature probes into the monitoring system and compared and corrected the data of the DFOS and extensometers according to the temperature effect. In the case study of a water pensstock bridge, it was confirmed that the strain time series collected by the two methods have a good match. The results showed that the application of Brillouin DFOS to the SHM system under complex conditions is a cost-effective and good performance scheme.