## 2 Research Progress in In Situ Road Health Monitoring Technology
### Fiber-Optic Grating Sensors  
In 1978, [PERSON] [17] successfully invented the world's first fiber-optic grating (FBG) technology. The working principle of the optical fiber grating strain gauge involves the deformation of an optical fiber when the load changes, which leads to the intensity, amplitude, phase, frequency, or polarization state of the light. Then, the demodulated optical fiber grating is used to detect the strain of the measured object. The central wavelength of a fiber grating is affected by strain and temperature conditions [18]:  
\[\Delta\lambda_{B}=2n_{eff}\Lambda\Bigg{(}1-\frac{n_{eff}^{2}}{2}(p_{12}+\mu(p_ {11}+p_{12}))\Bigg{)}\epsilon+(\alpha_{\delta}+\xi_{\delta})\Delta T \tag{1}\]  
in which \(\Delta\lambda_{\text{B}}\) is changes in the fiber grating; \(n_{\text{eff}}\) is the effective refractive index; \(\Lambda\) is the fiber grating period; \(\varepsilon\) and \(\Delta T\) are strain and temperature changes, respectively; and \(\mu\), \(\alpha_{\delta}\), \(\zeta_{\delta}\), \(p_{11}\), and \(p_{12}\) represent the Poisson ratio, thermal expansion coefficient, and thermal light coefficient of the optical fiber and the bounce light coefficient of the optical fiber grating, respectively. Optical fiber grating strain gauges have been widely used in civil engineering structures, the aerospace industry, the shipping industry, the petrochemical industry, the electric power industry, the nuclear industry, medicine, and other aspects of civil engineering structures due to their advantages of high strain-detection accuracy, strong anti-environmental interference ability, and small size [19]. Fiber grating sensors have been applied in the field of civil engineering safety monitoring, especially in the field of bridge engineering monitoring [20; 21]. Generally, according to the configuration and structure of fiber monitoring, fiber sensors can be divided into three types: discrete, quasi-distributed, and distributed. Generally, the sensor is embedded in the road surface for monitoring, which can be realized in two ways. One is to place the processed optical fiber sensor in the core sample or in the groove and then backfill it; the other is to bury and compact it simultaneously with the pavement construction process. But the latter is extremely prone to cause sensor damage [22]. [PERSON] et al. [23; 24] embedded distributed fiber-optic sensors in experimental rutting boards and existing road surfaces using the slotting and embedding method to identify external load information. They conducted experiments by comparing three embedding depths (10, 30, and 40 mm) and three moving loads (pedestrians, cars, and trucks) to study the applicability and sensitivity of distributed fiber-optic sensors in obtaining moving load information on road surfaces at different depths. However, it must be noted that due to the fragility of fiber grating sensors and the complexity of road construction environments and service conditions, fiber grating sensors are difficult to be directly used for field monitoring and are extremely easily damaged under load conditions. In order to improve their survival rate, it is necessary to carry out encapsulation research and research on installation protection methods. In 2018, for the first time in Flanders, [PERSON] et al. [25] conducted a pre-construction monitoring study on the bottom of asphalt pavements with different layers (surface and base). By embedding the fiber grating and installing it in the prefabricated base and asphalt layer groove, a cross-sectional strain measurement was obtained, as shown in Figure 2.  
With the gradual maturity and stability of optical fiber communication technology and optical fiber sensing technology, research has gradually shifted from the development of new optical fiber demodulation principles and structures to the practical packaging of existing optical fiber sensors to meet different monitoring needs, as shown in Figure 3. [PERSON] et al.