## 3 Experimental Setup  
The local lateral stress device for the FBG is shown in Figure 3. It includes a test bench, a test fiber, a balance fiber, a pressure rod, a balance rod, an iron thin plate, and weights. The test fiber (with FBG) and the balance fiber (no FBG) are SMF-28e, placed parallel on the test bench and glued at both ends. The pressure rod and balance rod are two parallel hexagonal columns with a side length of 4 mm, perpendicular to both fibers. Notably, the pressure rod directly contacts the FBG area, while the balance rod does not. Figure 1: Relationship between change in refractive index and applied force under transverse stress. Figure 2: Schematic diagram of lateral stress: axial and cross-section of the grating. The purpose of this design is to ensure that the weights are placed stably on the iron plate, balancing the forces exerted on the two optical fibers. In this way, the gravity of the weight is applied evenly to the two fibers with a total of four stress points. Therefore, the force at each point in the experiment is a quarter of the gravity of the weight, iron plate, pressure rod and balance rod. During the experiment, all materials should be placed slowly and vertically to avoid grating fractures caused by pressure rod displacement or shear stress caused by fiber rotation. Figure 4 shows a schematic diagram of the experimental setup. The experimental system consists of an ASE light source, a spectral analyzer, an optical circulator and the FBG lateral stress device. Light emitted from the wideband light source (wavelength range 1510-1610 nm, output power \(-\)30 dBm) is input to circulator port 1 and is incident into the FBG from port 2 through the optical circulator. The light reflected from the FBG passes through the circulator to port 3 and then enters the spectral analyzer (Yokogawa AQ6375, minimum resolution of 5 pm) for spectral analysis. The spectra of the light source and the reflection spectra of the original FBG are shown in Figure 5. The light source is relatively flat, in the 1520-1610 nm range. The FBG used in this paper has a center wavelength of 1546.432 nm, a \(-\)3 dB bandwidth of 0.18 nm, and an FBG length of 10 mm. Figure 4: Schematic of the experimental setup. Figure 5: (**a**) Spectrum of the light source, (**b**) Reflection spectrum of FBG. Figure 3: FBG local lateral stress device: (**a**) schematic, (**b**) cross-sectional drawing. the reflected peak intensity is low, which is superimposed with the Bragg peak in the y-polarization direction, where the reflected peak intensity is high. Bragg peaks in the x direction are greatly influenced by those in the y direction, so the split appearance is not obvious. For ease of handling, we consider the split point display threshold for this FBG to be 4 N.  
The experiment included lateral stress loading and unloading in a range from 4.0 N to 7.0 N, with a step of 0.5 N. The spectra are shown in Figures 9 and 10, and the data are listed in Table 1. Let the left peak wavelength shift be \(\Delta\lambda_{\text{L}}\), the split point wavelength shift be \(\Delta\lambda_{\text{S}}\), and the right peak wavelength shift be \(\Delta\lambda_{\text{R}}\); the wavelength difference between the left and right peaks is \(\Delta\lambda_{\text{R}-\text{L}}=\lambda_{\text{R}}-\lambda_{\text{L}}\). Figure 8: Reflection spectra under different stress (a) 0 N (b) 1.5 N (c) 3.0 N (d) 4.0 N.  
Figure 10: Force dependence of the spectral characteristics of the FBG under pressure unloading. (a) Reflection spectra at different load pressures. (b) Response curve of wavelength variation of \(\Delta\lambda_{\text{L}}\), \(\Delta\lambda_{\text{S}}\) and \(\Delta\lambda_{\text{R}}\) of the FBG relative to force variation. (c) \(\Delta\lambda_{\text{R}-\text{L}}\) under different load pressures. Figure 1:0 Force dependence of the spectral characteristics of the FBG under pressure loading. (a) Reflection spectra at different load pressures.