## 3 Principle of Fiber Bragg Grating Sensing Technology  
Quasi-distributed fiber Bragg grating (FBG) sensing technology is used as the sensor equipment for a big data monitoring system for coal pillar-free longwall mining. Fiber grating strain gauges encapsulated in many different types of fiber grating sensors are placed on objects so as to measure certain parameters of those objects. When the temperature, stress or strain of the measured object changes, the fiber experiences axial strain, which enlarges the grating period and reduces the radius of the fiber core and cladding. The photoelastic effect causes the refractive index of the fiber to change, which results in a wavelength shift of the grating. The measured parameters can then be calculated based on the linear relationship between strain and the wavelength offset of the grating. The principle of FBG sensing is shown in Figure 3.  
Figure 2: Formation principle of the roadway.  
An FBG is formed by along-axis periodic change in the refractive index of the fiber core. When the incident laser wavelength and the period of the FBG satisfy Formula (1), the grating will reflect the laser.  
\[\lambda_{B}=2n_{eff}\Lambda \tag{1}\]  
In the formula, \(\lambda_{B}\) is the wavelength, \(n_{eff}\) is the effective index of refraction and \(\Lambda\) is the grating spacing.  
Formula (1) shows that the reflected wavelength of FBG is related to the grid spacing and the refractive index of the optical fiber. When the optical fiber is deformed axially, the grid spacing and the refractive index will drift and the reflected wavelength will drift accordingly. The amount of deformation of the optical fiber can be obtained by measuring the amount of drift. The experimental results show that strain and temperature have a good linear relationship with the central wavelength and are independent of each other. The correlation formula is shown in Formula (2).  
\[\Delta\lambda_{B}=\alpha_{\varepsilon}\varepsilon+\alpha_{T}\Delta T \tag{2}\]  
In the formula, \(\alpha_{\varepsilon}\) is the strain sensitivity coefficient of the fiber grating, \(\alpha T\) is the temperature sensitivity coefficient of the fiber grating, \(\Delta T\) is the change in temperature value and \(\varepsilon\) is the strain.  
Because an FBG can accurately measure the micro-deformation of materials, FBGs have been packaged and attached to elastic components such as pressure, displacement and temperature sensors to achieve multi-variable sensor testing.  
Incorporating optical fiber information acquisition and transmission into the sensor equipment of the monitoring system for a coal pillar-free self-made roadway offers the following unique advantages:  
(1) Non-live line working is intrinsically safe and suitable for the flammable and explosive environment in an underground coal mine.  
(2) Optical fiber transmission suffers little loss over long distances, is not affected by electromagnetic field interference, temperature and humidity and has high transmission reliability.  
(3) The optical fiber sensor monitoring system has a large capacity, is simple and easy to maintain and makes multi-point and multi-parameter online monitoring easy, greatly reducing the variety and quantity of equipment required.  
(4) Optical fiber sensors make it easy to realize multi-point distributed measurement and long-term online pressure monitoring.  
Figure 3: Fiber Bragg grating (FBG) sensing principle.