## 1 Introduction
### Methods for Detecting Damage to Civil Engineering Structures  
There is a variety of visual, mechanical, electrical, acoustical, computer vision, global dynamic behavior, and other methods that are used for damage detection in civil engineering structures [12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. Nevertheless, many are susceptible to the adverse effects of moisture, chemical corrosion, electromagnetic interference, and lightning discharges. These factors make it rather difficult to use these tools on large structures, especially outdoors. Over the past few decades, optical fiber sensors have reached a high level of sophistication and performance, and they are increasingly used for structural health monitoring (SHM). The advantages of using optical fiber sensors in civil engineering are numerous. Firstly, the optical fibers (shown schematically in Figure 1) are made of dielectric materials, and, therefore, they do not conduct electricity and are immune to electromagnetic interference. Secondly, they are chemically inert, and therefore immune to adverse climatic factors. The optical-fiber sensor systems that are most commonly used for SHM employ Optical fiber Bragg gratings (FBGs), optical frequency domain reflectometry (OFDR), Brillouin scattering optical time-domain analysis (BOTDA), and optical fiber Fabry-Perot resonators (FPRs). These optical-fiber sensor systems are highly sensitive to strain and have a distributed sensing capability (for details, see the reviews [22; 23; 24; 25; 26; 27; 28] and references therein). Monitoring strain in structural elements is a useful way to confirm and ensure that bridges, buildings, etc., are indeed behaving as expected. Even in cases of seemingly proper structural behavior, these sensors provide useful information about the degree of coincidence between the predicted and observed behavior of the structure. Moreover, monitoring strain in existing buildings makes it possible to ensure their safety during nearby construction work or other hazardous activities. In addition, monitoring strain in buildings threatened by environmental or man-made factors helps specialists to take timely remedial measures. Some buildings from the past centuries are already equipped with distributed optical fiber sensor systems that provide useful information about the structural behavior of the buildings [29; 30; 31; 32; 33]. Moreover, distributed optical fiber sensors provide indirect information about structural damage. Anomalous strain concentrations in the vicinity of cracks indicate the cracking of structural elements [34; 35; 36; 37; 38; 39; 40]. This allows specialists to locate cracks and estimate their width using strain measurements and indirect numerical methods developed for this purpose. Such estimations have been carried out on reinforced concrete (RC) beams [34; 35; 36; 37; 38]. However, the maximum width of the cracks evaluated using this technique did not exceed 0.4 mm. In a previous study, structural cracks were detected in masonry, but the relationship between an anomalous strain and the respective crack width was not established [40]. Figure 1: Schematic drawings of ordinary communication-grade silica-core silica-clad optical fibers: (**a**) multimode; (**b**) single mode. Despite the impressive achievements in assessing the strain and temperature distribution in structures with the help of FBG, BOTDA, OFDR, and FPR-based sensors, the drawbacks of these sensors include the high cost of optical signal analyzers (also called interrogators) and the FBG, OFDR, and FPR gauges (the estimated cost of a high-resolution OTDR is USD 30,000-40,000; the cost of an FBG interrogator is USD 10,000-50,000; the cost of an OFDR interrogator is USD 60,000-100,000; and the cost of a high-performance BOTDA analyzer reaches USD 100,000, with the cost being highly dependent on the specific configuration and technical specifications.