Abstract: This paper discusses the research and application of fiber optic sensing technology in bridge inspection at home and abroad; and takes strain testing as an example to introduce the detection principle, method and latest research results of fiber optic sensing technology in bridge inspection. Finally, the paper points out the development direction of fiber optic sensing technology in bridge inspection. Keywords: fiber optic sensing, bridge, inspection I. Introduction The construction and maintenance of bridges is an important part of a country's infrastructure construction. Therefore, the safety and reliability of bridges has become a major issue related to the national economy and people's livelihood. For a long time, people have mainly used electrical detection methods for bridge safety inspection. Due to electromagnetic interference and moisture corrosion, this method cannot be placed for a long time. A large number of sensors need to be placed temporarily during the inspection. This not only requires a lot of manpower and material resources, but also requires specially trained engineers. At the same time, since the measured results are instantaneous, they cannot accurately and timely predict the working status of the bridge. Therefore, the results still cannot meet the current safety needs. From the mid-1970s to the present, fiber optic sensing technology has made great progress after more than 20 years of rapid development. Its reach has been involved in various fields such as national defense, aerospace, industrial and mining enterprises, energy and environmental protection, biomedicine, metrology and testing and household appliances [2-3]. Fiber optic sensors have become an effective method for bridge inspection due to their small size (diameter of only 125mm, and the length of the fiber optic sensor can be as short as a few centimeters), light weight, non-conductive, fast response, corrosion resistance, and immunity to electromagnetic, radio frequency, and lightning current interference, as well as their unique advantages of integrating sensing and transmission. Embedding fiber optic sensors in bridges to measure internal stress, strain, and structural damage has become an effective detection technology in bridge inspection. In recent years, research institutions at home and abroad have invested a lot of human and material resources in the research of the application of fiber optic sensing technology in bridge inspection and have achieved certain results. II. Principle of Fiber Optic Detection Technology (1) Overview of Bridge Inspection Methods The traditional bridge inspection method is the electrical detection method, which is a method of measuring strain by externally attaching resistance strain gauges to a certain part of the bridge. The principle it is based on is to form a bridge structure with strain gauges to sense the change in strain of the measured body and convert it into the required electrical charge, so as to use the relationship between the strain change De and the resistance change DR of the strain gauge DR=aDe (a is the strain rate) for detection. For example, the portable dynamic strain gauge DY-3 and portable super strain gauge YD-88 developed by Anshan Electric Measurement Technology Research Institute, which are widely used in my country, are such products. This measurement method is based on strain-electricity, uses electrical signals as the carrier for conversion and transmission, and uses wires to transmit electrical signals. Therefore, its use is limited by the environment. For example, high humidity may cause short circuits, especially in high temperature and flammable and explosive environments, which can easily cause accidents. (2) Technical principle of bridge detection using optical fiber. Optical fiber sensing technology is a technology that uses the characteristic of optical fiber to be sensitive to certain specific physical quantities to convert external physical quantities into signals that can be directly measured. Since optical fiber can not only be used as a medium for the propagation of light waves, but also the characteristic parameters (vibration, phase, polarization state, wavelength, etc.) of light waves when propagating in optical fiber change indirectly or directly due to the action of external factors (such as temperature, pressure, strain, magnetic field, electric field, displacement, rotation, etc.), optical fiber can be used as a sensing element to detect various physical quantities. This is the basic principle of optical fiber sensors. As shown in Figure 1. Figure 1 Schematic diagram of fiber optic sensing principle. Fiber optic sensors can be classified from different perspectives, such as the function of the fiber, the signal modulation method, and the object being measured. From the perspective of the function of the fiber, they can be divided into non-functional sensors and functional sensors [5-7]. In non-functional sensors, the fiber only plays the role of transmitting light; while in functional sensors, the fiber plays both the role of transmitting light and the role of sensing. Currently, most of the high-precision, high-resolution, and miniaturized sensors developed are functional sensors. If classified from the perspective of optical signal modulation method, there are light intensity modulation type, phase modulation type, and polarization modulation type. Among them, light intensity modulation type is widely used in general engineering measurement because of its simple structure and large measurement range. In places where the measurement accuracy requirement is high, phase and polarization modulation are used. With the rapid development of science and technology, the requirements for the accuracy, stability, and miniaturization of sensors are getting higher and higher. Therefore, phase modulation type and polarization modulation type sensors are the main objects of current research and development. Currently, the fiber optic sensors used in bridge detection are mainly phase modulation type. (3) Advantages of fiber optic bridge detection method Fiber optic sensors use optical signals as the carrier for transformation and transmission and utilize optical fibers to transmit signals. Compared with traditional bridge inspection sensors, fiber optic sensors have many unique advantages: (1) anti-electromagnetic interference, electrical insulation, corrosion resistance, and intrinsic safety; (2) light weight, small size, and variable shape; (3) minimal impact on the measured medium; (4) extremely high sensitivity and resolution; (5) easy to reuse and network, and conducive to forming telemetry networks and fiber optic sensing networks with existing optical communication technologies; (6) low cost. III. Development of Fiber Optic Sensing Technology in Bridge Inspection Abroad In 1989, Mendez et al. of Brown University in the United States [8] first proposed the possibility of using fiber optic sensors for the inspection of reinforced concrete structures and buildings. Subsequently, developed countries such as the United States, Canada, the United Kingdom, Germany, Japan, and Switzerland have applied fiber optic sensing technology to the safety monitoring of large-scale civil infrastructure such as bridges and dams, and have made encouraging progress. One example is the white-light Fabry-Perot fiber optic sensor developed by Rotest, a Canadian company. They have used this sensor to detect internal conditions in bridge structures, such as stress, strain, structural vibration, degree of structural damage, and the occurrence and development of cracks, achieving good test results. This white-light interferometry-based fiber optic sensor has high accuracy and repeatability. It can be installed on the surface of materials or buildings, or embedded within them, to continuously monitor conditions such as strain, stress, displacement, cracks, pore pressure, and temperature. The Fabry-Perot sensor creates a vacuum cavity within the optical fiber (see Figure 2). When a light beam passes through the sensing fiber and enters the cavity, it is reflected at both ends of the vacuum cavity and returns along its original path. This vacuum cavity is called a fiber optic Fabry-Perot cavity (FP cavity). If the light intensity incident on the FP cavity is I0, the center wavelength of the incident beam is I[sub]0[/sub], and the cavity length of the FP cavity is L, the output light intensity IR after the two reflected beams meet and interfere is approximately: If the FP cavity is connected to the light source and photodetector by an optical fiber, the detection system shown in Figure 3 can be formed. When the optical fiber sensor is installed on the object under test, the internal strain of the object under test causes the cavity length L of the optical fiber FP cavity sensor to change synchronously, thereby changing the output light intensity IR. The cavity length of the FP cavity and even the deformation of the object under test can be derived from equation (1). Figure 2 Fabry-Perot optical fiber sensor for strain Figure 3 Schematic diagram of the sensing system In addition to Rotest, many other companies and laboratories around the world are engaged in this research. In 1993, Canada pre-installed fiber optic sensors on a carbon fiber prestressed concrete highway bridge. After the bridge opened, it continuously monitored the bridge for eight months, measuring the overall distributed strain within the concrete and processing the data using dynamic programming theory to accurately and quickly assess the bridge's service condition and lifespan. Meanwhile, Alavie et al. at the University of Toronto's Smart Structures Laboratory used Bragg fiber grating sensors to measure the stress on the Beddington Bridge in Canada. In 1996, the U.S. Naval Research Laboratory developed a health monitoring system for the I-10 bridge in New Mexico, consisting of 60 FBG (fiber Bragg grating) sensors capable of dynamic and static strain measurements. In 1997, Forster Miller in the United States also completed a health monitoring system for highway bridges in Butler County, Ohio, using FBG sensors. The University of Vermont, in collaboration with Electro-Optics, developed fiber optic corrosion sensing technology for detecting corrosion on bridges and highways, and first applied it to three bridges north of Vermont in the summer of 1997, achieving good measurement results. The Swiss Federal Institute of Technology (BIT) and Smart Structures GmbH have developed fiber optic strain/deformation sensors based on the principle of quasi-coherent optical interferometry. The sensor head can be easily embedded inside concrete structures or fixed to the exterior of any structure. To compare with traditional electrical testing techniques, Smart Structures GmbH installed fiber optic sensors along with traditional strain gauges and thermocouple strain sensors on a highway bridge near Geneva in 1995. However, only the fiber optic sensor successfully completed tests of concrete curing thermal shrinkage strain, load testing, and long-term application throughout the entire process from construction, final inspection, and opening to traffic. IV. Current Status of Fiber Optic Sensing Technology in Bridge Inspection in China Currently, bridge inspections in China are conducted at fixed points after design, construction, installation, and commissioning. The method used is electrical testing, i.e., as mentioned earlier, externally attaching strain gauges to a specific location on the beam. This method has significant limitations; it is time-consuming and labor-intensive; the data obtained is only at a specific point in time; and due to factors such as construction quality and installation, there is a certain degree of error compared to the original design parameters. Since the 1990s, China has begun research and application of fiber optic sensing technology. Tsinghua University, Tongji University, Chongqing University, Harbin Engineering University, and other institutions have conducted theoretical research on the application of fiber optic sensors in bridge inspection and have carried out prototype experiments in the laboratory, achieving good results. Figure 4 shows a white-light Michelson interferometer implemented with fiber optics. Professor Yuan Libo of Harbin Engineering University designed a fiber optic sensor based on the principle of white-light interference. Unlike Retest, his design uses a white-light interferometer with a fiber optic Michelson structure as shown in Figure 4, indirectly measuring the sensor length variation characteristics with bridge cable stress and strain by comparing optical path differences. Figure 5 shows a wavelength scanning interferometer with a dual FP structure. Professor Liao Yanbiao of the Department of Electronic Engineering at Tsinghua University established a new wavelength interferometer experimental system, as shown in Figure 5. The system uses a wavelength scanning light source and two collimated Fabry-Perot interferometers, one as a reference interferometer and the other as a sensing interferometer. This enables a large range of absolute distance measurements and relaxes the requirements for light source stability and scanning repeatability, giving the system a greater advantage over existing measurement methods in long-term distance monitoring. V. Conclusion Applying fiber optic sensing technology to bridge inspection has injected new vitality into bridge health monitoring and safety assessment. After nearly a decade of research abroad, relatively mature technologies have been developed, and related products are favored by users due to their high accuracy and practicality. However, their high price hinders their application in China. Therefore, future research needs to focus on developing high-performance, low-cost sensors based on principles and manufacturing processes to meet domestic market demands.