# Recent Advancements in Non-Destructive Testing Techniques for Structural Health Monitoring
## 5 Fiber Optics
### Distributed Optical Fiber Sensors  
Distributed Optical Fiber Sensors (DOFSs) provide the same benefits as FOSs, but additionally, they can monitor variations of one-dimensional structural physical fields along with the entire optical fiber in a truly distributed way. A single connection cable is necessary by DOFSs to communicate the data to the reading, compared to the large number of cables required by discrete sensors. This feature enables DOFSs to be very cost-effective and increases their potential use in SHM of large civil engineering infrastructure [32]. When this technique is used, fibers are mounted to the surface or embedded inside the material [44], and strain and temperature can be measured. When the variation of these monitored parameters is transmitted to the FOS, the signal in the fiber is modulated by these physical parameters. By measuring the variation of this modulated signal, distributed fiber sensing is achieved. The methods that have been employed can be subdivided in two main categories: optical time domain reflectometry (OTDR) and optical frequency domain reflectometry (OFDR) [32]. When the above techniques are applied, different types of scattering processes may occur in DOFSs, i.e., Rayleigh, Raman, and Brillouin scattering [45]. When OTDR is used, a short optical pulse is launched into the optic fiber, and the amount of light, which is backscattered, is processed by a photodetector. During this process, Rayleigh scattering is produced due to the microscopic and random variations of the fiber core refractive index. The spatial variations occurring in the fiber scattering coefficient are detected by measuring the intensity of the Rayleigh backscattered signal. These spatial variations in the fiber scattering coefficient are called attenuation [46]. The attenuation decays exponentially with time when the fiber is not subjected to any external perturbation, such as strain, temperature, etc. On the contrary, when the fiber is affected by external conditions and is subjected to an external perturbation, a sudden variation is exhibited in the attenuation, which is located at the point of the perturbation (Figure 7[32]). This variation of the attenuation is utilized to provide information about the fiber properties and the external environmental factors that affect the fiber. In addition, different types of scattering have been employed to OTDR, such as Raman and Brillouin scattering. The spatial resolution of an OTDR, \(\Delta Z_{\text{min}}\), is given by the following equation:  
\[\Delta Z_{\text{min}}\ =\ \frac{c\tau}{2n} \tag{4}\]  
where \(c\) is the velocity of light, \(n\) is the refractive index of the optic fiber, and \(\tau\) is the pulse width. In contrary to the OTDR method that measures the intensity of the Rayleigh backscattered signal, the OFDR method is used for reading the interference fringes of the Rayleigh scattered light in the frequency domain. Thus, the spectral response of the fiber is provided by the OFDR, which indicates the variation of the external conditions that affect the fiber, i.e., strain, temperature variations, etc. In the case of the OTDR method, the spatial resolution, \(\Delta Z\), is given by the following expression:  
\[\Delta Z\ =\ \frac{c}{2n\Delta\text{F}} \tag{5}\]  
where \(\Delta\)F is the tunable laser source. There are several recent applications of the DOFS method in SHM in Civil Engineering. DOFSs were used by [PERSON] et al. [PERSON] et al. (2018) to evaluate damage in a 13.5 m composite wind turbine blade, which was tested under static loading. Different sensing technologies were used to measure strain: FBGs, DOFSs using an Optical Backscatter Reflectometer (OBR) and strain gauges. Comparison of the methods showed that the FOSs effectively measured strain and correspondent cracks induced during the test. A novel method, based on distributed fiber optical Brillouin technique was employed by [PERSON] et al. [PERSON] et al. (2018) to monitor corrosion expansion of steel reinforcement in reinforced concrete structures.