A fiber Bragg grating-based inclinometer probe with enhanced sensitivity for a higher slope profiling resolution

A fiber Bragg grating (FBG)-based inclinometer probe with enhanced sensitivity has been developed for slope or ground movement monitoring. The inclinometer probe utilized six FBGs for the tilt measurement and a strain -free FBG that provided the temperature compensation factor. The inclinometer probe was fabricated entirely using a 3D printer and can fit into the standard inclinometer casing, which can be placed into the boreholes. The dimension of the probe is similar to the conventional inclinometer probe, with a total length of 70 cm. Additionally, this design was equipped with three highly compact tilt sensors within the same probe length, providing a better resolution of the inclination profile. Each tilt sensor possesses a flexible middle shaft fabricated using thermoplastic polyurethane (TPU) and was equipped with two FBGs for bi-directional tilt angle measurement (+x and-x). Initially, the tilt sensor was calibrated in the laboratory, which yielded a sensitivity value of 0.0215 nm/degrees. This value is higher than most previous designs by a factor of two because of the middle shaft's elasticity, which can induce a more significant strain on the FBG. The horizontal displacement of a conventional inclinometer casing could be observed during the field test, which proves the device's functionality. The results have indicated that the inclinometer can be applied in several geotechnical applications, particularly ground movement monitoring.