## 1 Introduction  
With the increase in mining depth, coal mine disasters are increasing, such as coal and gas outburst [1; 2; 3], rock burst [4; 5; 6], and large deformation of surrounding rock [7; 8; 9; 10; 11; 12], which bring severe challenges to the safe and efficient mining of deep coal resources. The in-situ stress is one of the most critical factors, as it is not only the fundamental force leading to the disasters, but also has great impacts on gas pressure, rock burst degree, rock mass deformation, and strength characteristics. Therefore, in order to achieve the effective control of coal mine disasters, accurate and reliable in-situ stress data must be obtained through field measurement. Hydraulic fracturing and overcoming are the two most widely used in-situ stress measurement methods at present [13; 14; 15]. Both of them are suitable for rock masses of substantial hardness and integrity. However, due to the influence of the sedimentary environment and tectonic movements, the deep rock masses of coal mines are usually weak and/or broken, which often leads to measurement failure. For this reason, a novel in-situ stress measurement technique, specialized for deep fractured rock masses, was put forward as the rheological stress recovery (RSR) method [16; 17; 18]. It is based on the strong rheological characteristics of fractured rock masses under high stress, and carried out by embedding the stress sensors into the drilling holes filled with grout. Due to the strong rheology of rock masses, the stress near the sensors gradually recovers. Meanwhile, the measured stress of the sensors increases and eventually stabilizes. The initial rock stress was finally evaluated via the recovery data obtained from the stress sensors (Figure 1). According to the measuring principle of the RSR method, since the stress state of a point contains six stress components, the sensor needs to obtain six normal stresses in different directions to calculate the rock stress. However, in the field of geotechnical engineering, the commonly used pressure sensors are unidirectional and can only obtain one normal stress, such as the various pressure boxes used for measuring earth pressure, lining pressure, and pulling pressure of the anchor rod and cable [19; 20; 21]. In order to achieve multidirectional normal stress measurements, a three-direction vibrating wire pressure sensor was developed [22]. It resembles a cube, and contains three orthogonal pressure sensing units with a maximum range of 30 MPa (Figure 2). Based on the three-direction pressure sensor, the RSR method has been successfully applied to obtain in-situ stress measurements in Pingdingshan No.1 and No.11 coal mines [17; 22; 23]. At the same time, several problems have been found in the field. Firstly, the sensor is bulky and inconvenient to install. Secondly, it is difficult to determine the three-dimensional attitude of the sensor in the borehole. Thirdly, the proximity of the two sensors to each other can easily cause stress field disturbance, which is difficult to evaluate. In recent years, due to the outstanding advantages of anti-electromagnetic interference, electrical insulation, and intrinsic safety, fiber optic sensing technology has been widely applied in coal mines. Many coal mines in China have established a sound optical fiber communication system, laying the foundation for the monitoring and early warning of deep coal mine disasters and the construction of smart mines. Meanwhile, in the field of sensor development, which includes such instruments as the fiber Bragg grating (FBG) sensor [24; 25; 26], distributed fiber optic sensors and fiber optic gas sensors have been developed and widely applied due to their high measurement accuracy and stability. Based on the testing principle of the RSR method and fiber optic sensing technology, this paper developed a new FBG stress sensor to solve the problems existing in the current three-direction  
Figure 1: Schematic diagram of RSR method.