## 3 Innovative SHM Techniques
### Interferometric Sensors  
Interferometric sensors work by measuring interference between beams of light. Two types of sensors are generally used for SHM purposes: Fabry-Perot interferometers (FPI) and the SOFO (a French acronym for surveillance d'ouvrages par fibres optiques) sensors. FPIs are used in various sensing applications on civil infrastructures to measure strain, displacement, pressure, and temperature [95; 96; 97]. They are appreciated for their low cost, high sensitivity and ultra-compactness. Despite these advantages, to our knowledge, no applications of FPI sensors on CHS buildings have been found in the literature. The SOFO measurement system, developed at the Swiss Federal Institute of Technology in Lausanne (EPFL) [98], has mainly been used for the last 20 years for both dynamic and static monitoring of civil [99; 100; 101; 102], geotechnical [103; 104], and other structures. Some applications of SOFO sensors for the monitoring of CH buildings are depicted in [105; 106; 107], in particular for the monitoring of crack displacements and deformations. [PERSON] et al. [108] described the application of an SHM system based on the combined use of SOFO and conventional sensors for monitoring the Royal Villa in Monza, Italy. Optical fibre sensors were mainly used as extensometers for crack monitoring before, during, and after renovation. The monitoring system proved to be a valuable tool for the monitoring-based rehabilitation process. [PERSON] et al. [105] described the application of SOFO sensors on several heritage structures and historical monuments, including San Vigilio Church in Switzerland, where the long-gauge sensors were used to unobtrusively monitor the evolution of cracks and the curvature of the main vault, preserving the view of the frescos. [PERSON] et al. [106] analysed the results of a long-term SHM system installed on the facades of the bell tower of St. Anastasia Cathedral in Zadar, Croatia, to understand the behaviour of existing cracks. The crack displacements were measured via optical deforma \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{3}{*}{**Parameters**} & \multirow{3}{*}{\
\begin{tabular}{c} **Interferometric** \\ **Sensors** \\ \end{tabular} } & \multirow{3}{*}{
\begin{tabular}{c} **Fibre Bragg** \\ **Grating** \\ \end{tabular} } & \multicolumn{3}{c}{**Distributed Sensors**} \\ \cline{3-5}  & & & **Rayleigh** & **Raman** & **Brillouin** \\  & & & **Scattering** & **Scattering** & **Scattering** \\ \hline Strain & x & x & x & & x \\ \hline Temperature & x & x & x & x & x \\ \hline Pressure & x & * & & & \\ \hline Displacement & x & * & & & \\ \hline Deformation & x & & & & \\ \hline \hline \end{tabular}  
* indicates parameters that can be measured by easily adapting the conventional sensor configuration.  
\end{table} 
Table 2: FOS types and main measurement parameters. tion SOFO sensors. The authors observed a correlation between crack displacements and daily and seasonal temperature variations, without any cumulative trend showing that the variations would damage the structure permanently. Fibre Bragg Gratings Sensors  
FBGs are one of the most studied and widespread FOS sensing technology applied to SHM because of their great flexibility [107]. An FBG is a microstructure within the core of an optical fibre, which can detect physical parameters (strain, temperature, etc.) based on wavelength reflection. They consist of a glass core characterized by a periodic 'grating' of material with a modulated index of refraction. More technical details about this technology are illustrated in [109, 110]. A basic scheme of an FBG sensor is shown in Figure 6.  
Several works explored the use of FBG to monitor various static and dynamic parameters of historical buildings. [PERSON] et al. [111] discussed the development of a continuous monitoring system for the Cathedral of Como, Italy, which has always been subject to the risk of subsidence due to changes in the lake level.