## 3 Fibre-Optic-Based Smart Geosynthetics
### Types of Fibre-Optic Sensors
#### 3.2.3 Bragg Gratings or Wavelength-Based Sensors  
Fibre Bragg grating (FBG) sensors are intrinsic, multiplexable, local, absolute, and interruption-proof OF-based strain sensors that have attracted considerable attention since their invention in the early 1990s. The main concept is to periodically modulate the fibre core refractive index over a short distance (approximately 1 cm) to act as a collection of weakly uniformly spaced mirrors diffracting the incident light and reflecting the wavelength proportional to the refractive index and spacing. As shown in Figure 8, the Bragg grating behaves as an extremely thin optical filter. When a broadband light pulse is transferred through the fibre, most of the light passes through the FBG except for a specific frequency, which is reflected. The reflected wavelength peak shifts when the temperature is changed or the grating is subjected to a uniform axial strain owing to changes in the refractive index and spacing. These changes can be monitored using an optical spectrum analyser (OSA) and are consequently converted into usable data. Multiple Bragg gratings centred at various wavelengths can be written in the same OF and simultaneously interrogated because commercially available white light sources exhibit an \(\sim\)40-60 nm wide spectrum and because the maximum strain- or temperature-induced drift is approximately 5 nm [50].  
FBG multiplexing is simple to implement. Indeed, because information is encoded in the wavelength, the sensor is extremely resistant to aging, which enables long-term strain measurements without having to recalibrate the device.  
Brillouin distributed optical fibre sensors (DOFSs) are also among the wavelength-based OF sensors that have gained considerable attention for application to geotechnical health monitoring in recent decades [51; 52; 53; 50]. Because DOFSs are insensitive to external disturbances, small, corrosion resistant, and lightweight and because DOFSs exhibit fast data collection and low measurement cost, they are advantageous for application to geotechnical health monitoring. However, the high installation cost and complex geosynthetic encapsu  
Figure 7: Power output from OF-based interferometer sensor.  
lation remain some of the leading obstacles to DOFS field applications [54; 55; 56]. Because DOFSs were initially proposed based on distinct light wavelength effects, they can be categorised as Raman-, Rayleigh-, and Brillouin-scattering-based distributed sensors [57; 58; 59]. An overview of scattering-based distributed sensors is illustrated in Figure 9.  
Figure 10 depicts the primary OF sensors for field applications, including powerful temperature and strain sensors designed and adopted for monitoring tunnelling [60; 61].  
Polyurethane-sheathed OF sensors offer a lower elastic modulus and, hence, exhibit higher sensitivity toward soil deformation. However, metal-cable-reinforced OF sensors--e.g., glass-fibre-reinforced polymer (GFRP) reinforcers, aluminium pipes, and metal reinforcers--strongly couple with coarse grains and/or rocks, thereby providing more options for field strain, stress, displacement, and temperature sensing. [PERSON] et al. (2014) provided an overview of several encapsulated DOFSs applied to geosynthetics for slope monitoring and considered various material properties [62; 63]. Figure 11 illustrates the basic DOFS structures applied to slope monitoring.  
Figure 8: Basic FBG operating principle for detecting temperature and strain.  
Figure 9: Overview of scattering-based distributed sensors.