## 1 Introduction  
In the field of civil engineering, ground anchor systems are widely used to stabilize a soil slope and, thus, prevent slope failure. Since the Cheurfas dam in Algeria utilized pre-stressing technology in 1935, various anchor systems have been developed and utilized in constructions such as bridges, tunnels, and slopes [1, 2]. The anchor system aims to transmit the tensile force of the anchor to the compressive force of the ground. For this purpose, the ground anchor consists of three parts: (1) ground anchor body, (2) anchor head, and (3) anchor head-related part [3]. The ground anchor body consists of two parts. One is a fixed anchor section, which is fixed under the ground and resist the tensile force of the anchor (or tendon) by using friction and compression in the ground. The other is a free anchor section, which transfers the tensile force to the anchor head on the ground. They consist of stranded cables or rods and are covered by a sheath (sleeve). The anchor head on the ground changes the tensile force of the anchor to the compressive load on the anchored structure. A bearing plate is placed in between the anchor head and the anchored structure. A relatively wide bearing plate distributes the compressive load and, thus, prevents stress concentration on the anchored structure. Also, various accessories, e.g., nuts and wedges of saddles, are used together for easy operation. Soil slopes are influenced by extreme weather conditions such as freezing, thawing, heavy rains, and vibrations induced by construction processes or earthquakes. Severe deformation of slopes may cause slope failure. Thus, monitoring the condition of the slope is important to prevent disasters due to slope failure. The slope condition can be indirectly monitored by sensing the tensile force applied to the anchor because the slope deformation directly changes the tensile force of the anchor. If one identifies abnormal tensile force changes, an administration must initiate a follow-up to identify the exact reason and ensure appropriate maintenance. In addition, if the monitoring system expects a rapid slope failure, an emergency warning can be issued. The research studies on anchor tensile force measurements have been performed using wireless force sensors [4] and fiber optic FBG (fiber Bragg grating) sensors [5; 6]. In Reference [4], the wireless sensor modules were installed near each anchor, and then these electronics should be carefully protected from harsh environments. In the cases of using FBG sensors, References [5; 6], these studies have difficulties on the optical fiber fabrication for embedding in the rock bolt or the anchor cable. Thus, it is required to devise more practical and reliable ways to monitor the anchor force. One possible way to monitor the tensile force of the anchor is to measure the strain field of the bearing plate and convert it to the tensile force of the anchor. Anchor force is equal to the compressive force of the anchor head, and the strain field on the bearing plate changes depending on the compression of the anchor head. Thus, in this study, by simply attaching the sensing optical fiber on the bearing plate, we try to indirectly monitor the anchor force from the measured strain of the bearing plate. However, the strain field of the bearing plate is usually not uniform through the circumferential direction. Therefore, it is necessary to measure the strain field. Various optical fiber sensor techniques have been developing for a distributed strain field measurement, such as OFDR (optical frequency domain reflectometry) sensors using Rayleigh scattering, and BOCDA (Brillouin optical correlation domain analysis), BOTDA (Brillouin optical time domain analysis), and BOTDR (Brillouin optical time domain reflectometry) sensors using Brillouin scattering [7; 8]. Among these techniques, OFDR and BOCDA sensors can achieve a spatial resolution of less than 1 m [9; 10; 11]. Our previous study [12] demonstrated that the distributed strain field can determine the anchor force.