# FBG-Based Monitoring of Geohazards: Current Status and Trends
## 2 Basics of Fiber Bragg Grating (FBG)
### Demodulation and Multiplexing Schemes  
According to the sensing principle, FBG can be used as a strain sensor or a temperature sensor. To measure wavelength shifts that result from variations in temperature or strain, an FBG sensing system should have an optical source that continuously interrogates the reflection spectrum, and a detection module that records the shifts in the peak reflectivity versus wavelength. Several interrogation configurations have been developed for reading out the Bragg wavelength shifts experienced by FBG, from which strain and temperature are calculated.  
Primarily, there are two techniques for detecting the precise wavelength of the peak in the FBG reflected spectrum. The first method, as shown in Figure 2a, is to send light from a broadband source into the optical fiber and analyze the spectrum of the reflected light. In this system, an optical circulator is used to transmit the outgoing light into the transmission fiber and redirects the back-reflected light to an optical spectrum analyzer (OSA). Michelson interferometry-based OSA can provide most accurate wavelength measurements [10]. This method has been used for many years in wavelength meters to determine the wavelength of laser sources with a high degree of accuracy.  
Figure 2: _Cont._  
The second method is based on a narrowband tunable laser source, as shown in Figure 2b. The tunable laser source sweeps over the spectral range including the FBG spectral peak. When a peak in the reflected intensity is detected by a power meter, the measurement module of laser wavelength records the accurate wavelength of the scanning laser. Using this configuration, the measuring accuracy of wavelength can be as high as 1 pm (1 pm = 10\({}^{-12}\) m) or, equivalently, less than 1 \(\upmu\)e or 0.1 \({}^{\circ}\)C [12].  
A most important attribute of FBG is that the sensors can be multiplexed in parallel or in series [12]. In this way, a quasi-distributed strain or temperature sensing array can be established. Currently, there are two mature schemes to interrogate such a string of sensors. The first one is called time division multiplexing (TDM), in which the time of flight using short duration light pulses is used to discern which signal is reflected from which grating along the fiber path. In theory, over 100 FBG sensors with the same Bragg wavelength can be on an optical fiber if the sensor spacing is large enough.  
The second and most utilized approach is wavelength division multiplexing (WDM). This approach requires all the serially connected FBG sensors should pose sufficient spacing of the Bragg wavelengths (e.g., 10 nm = 10\({}^{-8}\) m) to avoid spectral overlap during measurement. The interrogator system uses these unique Bragg wavelengths to keep track of the sensor locations. Sensor capacity on each optical fiber is dependent on the measurement range of each FBG sensor and the total spectral range of the interrogator. The typical range of a commercially available interrogator is 1510 to 1590 nm or 1525 to 1565 nm. Sometimes, WDM and TDM can be combined to form an FBG sensing network.  
Recently, [PERSON] et al. introduced an FDM technology, which can simplify the set up and installation of complex WDM scheme [13]. This method requires a tunable laser and limited number of sensors because of wavelength constraint in the communication band.