Development and performance testing of OTDR-based twisted fiber optic strain sensors

To accurately monitor the deformation patterns of rock masses in high-stress soft rock tunnels, a coiled optical fiber strain sensor was developed using optical time-domain reflectometry. The effects of spring diameter, pitch, number of turns, and compression displacement on fiber optic loss and sensor sensitivity were analyzed, and the sensor structure was optimized through performance testing. The results indicate that the bending loss of the optical fiber within the sensor is significantly influenced by the geometric dimensions of its helical structure. Key factors affecting this include the fiber’s unit-length bending loss, bending radius, winding pitch, and the number of turns, while the spring wire diameter has minimal effect. The initial loss of the optical fiber decreases as the bending radius and pitch increase, but increases with the number of turns and compression displacement. Initial sensitivity decreases as the bending radius and compression displacement increase, while showing a trend of first increasing and then decreasing with the winding pitch. When the spring wire diameter is 1.2 mm, the diameter is 14 mm, the pitch is 14 mm, and the effective number of turns is 4, the maximum range of the spring-optical fiber structure can reach 50 mm. The structure was enhanced and encapsulated into a sensing section, which, when connected end-to-end with the delay section, forms a complete sensor. The individual sensor demonstrated low error and exhibited a linear relationship between “optical loss and compression displacement,” enabling displacement monitoring up to 100 mm, with the possibility of achieving larger ranges through further refinement. Additionally, multiple sensors can be connected in series to create a quasi-distributed measurement system, allowing for precise monitoring of deformation in the deep rock mass and surrounding areas of the tunnel.