## 3 Innovative SHM Techniques
### Smart Sensing Technologies
#### 3.1.1 Fibre Optic Sensors  
FOS are some of the most promising and fastest growing technologies [83] due to several advantages, such as immunity to electric and electromagnetic interference, compatibility with ordinary telecommunication fibres, good resistance to high temperatures, chemical inertia, embedding capability, the low cost of the sensors, and a low size weight. The fibres consist of a flexible strand commonly composed of several layers: a central core, a cladding, and occasionally, an external coating to provide mechanical and environmental protection (Figure 5). External perturbations produce geometrical and optical changes in the guided light, which is translated into a corresponding change in the monitored parameter. The principles and detailed descriptions of fibre-optic technology are out of the scope of this paper. The reader is referred to several relevant articles, e.g., [84; 85; 86; 87; 88].  
Many FOS technologies have been developed during the last decades [88]. Several classifications can be found in the literature according to different criteria (e.g., sensing location, operating principle, or application), as shown in Table 1. The most commonly used types of FOS for the SHM of civil structures are interferometric sensors, fibre Bragg gratings (FBGs), and distributed sensors. The significant parameters detected by each sensor type are given in Table 2.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Criteria** & **Types** \\ \hline \hline \multicolumn{2}{c}{Single point sensors} \\ Sensing location & Quasi-distributed sensors \\  & Distributed sensors \\ \hline \hline \multicolumn{2}{c}{Intensity sensors} \\ Operating principle & Phase sensors \\  & Frequency sensors \\  & Polarization sensors \\ \hline \hline \multicolumn{2}{c}{Physical sensors} \\ Application & Chemical sensors \\  & Biomedical sensors \\ \hline \hline \end{tabular}
\end{table}
Table 1: Main criteria for FOS classification.  
Figure 5: Typical optical fibre cross–section.  
The last two decades have seen an increasing number of FOS applications for civil engineering monitoring (during construction and in-service stages), such as on bridges [89; 90; 91; 92], geotechnical structures [93], pipelines [94], and high-rise buildings [94]. Although FOS technologies offer several advantages suitable for CHS, such applications are still relatively limited. The benefits that FOS may offer to CHS monitoring comprise their low invasiveness, the potential of integration with textile reinforcements, and the possibility to perform truly distributed measurements because of their sensing capacity at any point along their length. In addition, FOS technologies exhibit high flexibility and versatility, allowing for the measurement of different types of parameters, such as strain, temperature, vibration, humidity, and chemicals (e.g., pH, analyte), even on the same fibre. Another important advantage is that, if appropriately designed and packaged, FOS can be very robust and long lasting, even in harsh environments, allowing for long-term monitoring applications. Finally, optical sensors offer higher sensitivity in detecting movement faster than traditional SHM and allow for remote data acquisition, namely without the presence of technicians in the field, which reduces labour time and costs.