## 4 Results and Discussion
It is defined as:  
\[\xi_{Z}=\frac{\left|\overline{y}_{Li}-\overline{y}_{Uli}\right|_{\text{max}}} {\lambda_{f}}\times 100\% \tag{16}\]  
Sensitivity refers to the change in the FBG wavelength per MPa, and is defined as:  
\[\alpha=\lambda_{f}/P_{\text{max}} \tag{17}\]  
where \(P_{max}\) is the maximum working range. According to the above definitions and calculation formulae, the characteristic indexes of the sensor are calculated. As can be seen in Table 2, the maximum working range of the sensor calibration is 30 MPa, the output wavelength of the full range is 2688 to 2900 pm, and the sensitivity ranges from 89.600 to 96.656 pm/MPa. According to Equation (2), there is a linear relationship between wavelength drift and fiber strain. Considering the elastic-optic coefficient, \(P_{e}\), is 0.216 for common fiber materials, the strain of full-range fiber grating can be calculated as 2238.273 to 2390.323. Table 3 reveals that the zero drift index of the sensor ranges from 0.035 to 1.158%, the linearity index ranges from 0.392 to 1.225%, the repeatability index ranges from 0.139 to 0.833%, the hysteresis index ranges from 0.281 to 1.166%, and the error index is within the range of 1.5%. The stability test results for \(1^{\#}\), \(3^{\#}\), and \(5^{\#}\) pressure sensing units are shown in Figure 16. It indicates that there is a creep phenomenon in the sensing unit under the long-term load, and the creep drift changes rapidly during the first few days, which gradually slows down in the later period, and finally stabilizes. According to the calibration coefficient and wavelength variation, the long-term measurement errors of the \(1^{\#}\), \(3^{\#}\), and \(5^{\#}\) sensing units were calculated as 1.51, 1.50, and 1.45%, respectively. \begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Unit No.**} & \multicolumn{2}{c}{**Maximum Working**} & **Output Wavelength of** & **Sensitivity** \\  & **Range/MPa** & **the Full Range (nm)** & **(pm/MPa)** \\ \hline
1 & 30 & 2.688 & 89.600 \\
2 & 30 & 2.845 & 94.833 \\
3 & 30 & 2.890 & 96.333 \\
4 & 30 & 2.900 & 96.656 \\
5 & 30 & 2.764 & 92.144 \\
6 & 30 & 2.753 & 91.767 \\ \hline \end{tabular}
\end{table} 
Table 2: Characteristics index of sensing units. Figure 16: Calibration curves of the sensor. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Unit No.** & \begin{tabular}{c} **Zero Drift** \\ **(\%)** \\ \end{tabular} & \begin{tabular}{c} **Linearity** \\ **(\%)** \\ \end{tabular} & \
\begin{tabular}{c} **Repeatability** \\ **(\%)** \\ \end{tabular} &
\begin{tabular}{c} **Hysteresis** \\ **(\%)** \\ \end{tabular} \\ \hline \(1\) & 0.521 & 1.225 & 0.403 & 1.166 \\ \(2\) & 0.035 & 1.144 & 0.159 & 0.281 \\ \(3\) & 0.069 & 0.950 & 0.253 & 0.542 \\ \(4\) & 0.483 & 0.602 & 0.375 & 0.586 \\ \(5\) & 1.158 & 0.860 & 0.833 & 0.941 \\ \(6\) & 0.254 & 0.392 & 0.139 & 0.545 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Error index of sensing units. Fiber wavelength division multiplexing (WDM) technology was adopted for the sensor to ensure that only one channel of the optical fiber demodulation instrument was occupied for a single sensor. That is, each sensing unit took up a different wavelength range, and then the optical fibers for each sensor were welded into the coupler box. As a result, only one optical fiber connector of the sensor was outputted. In order to achieve the use of the sensor in the coal mine, an intrinsically safe optical fiber demodulator was developed. The wavelength demodulator range is 1,525 to 1,565 nm. Based on this, the wavelength range of each sensing unit was designed to be 5 nm (-1 to 4 nm), that is, the wavelength change of each sensing unit under tension was no more than 1 nm, and the wavelength change under pressure was no more than 4 nm. According to the calibration test data, the initial wavelength of sensing units 1\({}^{\#}\) to 6\({}^{\#}\) are: 1,531.80; 1,537.05; 1,542.44; 1,547.35; 1,552.51; 1,557.43 nm, respectively.