## 2 Experimental Setup  
The upper section of Figure 1 illustrates the potential applications of utilizing FBG subsidence sensors in conjunction with FSOs to measure water levels and tilt. Such applications are pertinent in various contexts, including dams, offshore regions, and sea islands, among others. The optical coupler is vital in enhancing the multi-sensory pathways, allowing for the strategic distribution of multiple FBG sensing networks across various locations within measurement applications. When the density of FBG sensors increases, challenges may arise due to wavelength overlap or interference, which can adversely impact the accuracy of wavelength interrogation results. To address this issue, implementing machine learning techniques is recommended to effectively resolve the wavelength interrogation challenges [40]. The primary framework of the experiment is illustrated in the lower section of Figure 1. For real applications, the sensing signal transceiver must be strategically positioned at the sensing center. Through FSO, the optical signal can be captured by the FBG sensor located at the measurement point. To facilitate experimental procedures and enhance operational efficiency, the broadband light source (BLS) emits the optical signal via an optical circulator (Cir.) to the 2 m FSO link in the lab instead of the outdoor corresponding scene. This is subsequently connected to the FBG array sensors for measurement purposes. The sensed signal is then routed back to the optical circulator and directed to the optical spectrum analyzer (OSA) for comprehensive signal observation. The FBG sensing module comprises 3 FBGs; two FBG subsidence sensors are used to measure the water level and tilt simultaneously, and the remaining FBG is used to correct air temperature change. The FBG subsidence sensor consists of a conventional FBG that supports a buoy [41]. Both the FBG and the buoy are contained within a hollow cylindrical bottle, which is designed with an opening to facilitate water flow in and out. Consequently, variations in water level, or changes induced by the tilting of the cylindrical bottle, will affect the buoy's positioning. This, in turn, will stretch the FBG, leading to alterations in the center wavelength of the FBG. For the measurement conditions, FBG subsidence sensors are placed in the temperature bath (TB) to control the water level and temperature for customization of measurement conditions. The tilted measurement setup is made by placing a gasket under the leftmost or rightmost side of the rectangular TB. The tilting of the TB tilts the FBG subsidence sensors, but the water surface remains flat. The length of the TB is 32 cm, and the thickness of the gasket is 2.4 mm, so the calculation shows that each additional gasket used will increase the tilt angle by 0.424\({}^{\circ}\). To enhance measurement sensitivity, a gasket with a thickness of 1.6 mm has been positioned beneath the FBG subsidence sensors. Furthermore, the two FBG subsidence sensors have been strategically placed at opposing ends of the TB apparatus. This arrangement ensures that, in instances of TB tilt, there is a pronounced water level difference between the two sensors, thereby optimizing the accuracy of the measurements. Incidentally, the equipment's brand and model are respectively affiliated with UNICE Inc. (Taoyuan, Taiwan) NA0101 for BLS; Thorlabs Inc. (Newton, NJ, USA) F810 FC-1550 for FSO; Citro Technologies Inc. (Taipei, Taiwan) for the FBG subsidence sensor; and Anritsu Inc. (Atsugi, Japan) MS9740A for OSA.  
Figure 1: The application vision and experimental setup of using FBG module to integrate with FSO to simultaneously measure inclination angle and water level.