## 3 Experimental Results  
As previously stated, variations in water levels or angular dip result in shifts in the position of the buoys on the FBG subsidence sensors. These shifts subsequently induce strain on the FBGs, leading to alterations in their center wavelengths. Consequently, this mechanism allows for the understanding that, at a consistent water level, variations in the inclination angle of FBG subsidence sensors can alter the buoy's submerged surface area. This, in turn, affects the buoyancy, leading to the FBG's elongation. To enhance measurement resolution, it is advisable to increase the immersion area variation of the buoy whenever the inclination angle varies. As previously indicated, incorporating a gasket at the base of the FBG subsidence sensor can augment its sensitivity. This enhancement is attributed to the buoy's rectangular design; when the FBG subsidence sensor is positioned at an angle, it results in a greater alteration of the buoy's immersed area with each change in tilt, in contrast to a vertically oriented FBG subsidence sensor. The experimental results are shown in Figure 2, in which the initial wavelength of FBG1 is 1548 nm; the initial wavelength of FBG2 is 1542 nm, so the overall tilting of the TB to the right, such as 0.424\({}^{\circ}\), 0.848\({}^{\circ}\), 1.272\({}^{\circ}\), and 1.696\({}^{\circ}\), will lead to a decrease in the water level at the FBG1, and thus the buoyancy force decreases, so that the FBG wavelength is subjected to long wavelength drift. Conversely, a leftward tilt of the TB will cause the water level at FBG1 to rise and increase the buoyancy, resulting in a reduction of the strain on the FBG and a drift to a shorter wavelength. On the other hand, FBG2 is located on the right side of TB, so when TB is tilted left or right, the water level at FBG2 changes in the opposite direction to that at FBG1, which makes the wavelength drift direction of FBG2 opposite to that of FBG1. Figure 2a-f sequentially presents the tilt measurements at water levels of 1 cm, 1.5 cm, 2 cm, 2.5 cm, 3 cm, and 3.5 cm. It is observed that the drift trends of the FBG wavelengths exhibit consistency across different water levels during the tilting process. A crucial aspect of Figure 2 is that the solid line indicates the FBG wavelength drift observed when the FBG subsidence sensors are installed at an oblique angle. In contrast, the drift represented by the dotted line corresponds to the FBG wavelength changes when the sensors are positioned normally. The measurement results affirm the initial hypothesis, demonstrating that the inclined placement of FBG subsidence sensors enhances measurement sensitivity. Furthermore, the drift in FBG wavelength appears to be a somewhat nonlinear pattern, especially at low water levels. This is because in low water, the buoys are in contact with less water, and the FBG subsidence sensors are placed on both sides of the TB rather than in the center; therefore, when the buoys are tilted left and right at the same water level, the buoys are buoyed differently, thus causing the drift of the FBG wavelength to be not so linear. The experimental results presented in Figure 2 indicate that the skewed FBG subsidence sensors exhibit greater wavelength variation and enhanced sensitivity. Consequently, subsequent experiments will measure skewed readings at various water levels and temperatures, adhering to the experimental configuration depicted in Figure 1. Figure 3 presents the drift results of the FBG wavelength at 1542 nm, evaluated under varying water temperatures and levels. Given that fluctuations in water temperature also influence the buoyancy of the buoys, it is essential to examine the measurements across different thermal conditions.