OFDR enabled high spatial resolution distributed twist measurement using spun fiber

We proposed and demonstrated a high spatial resolution distributed twist sensor based on optical frequency-domain reflectometry (OFDR) in spun single-mode fiber (SMF). Due to the unique helical structure of the stress rods in spun SMF, external twist causes axial stress in the fiber, which leads to a frequency shift in the Rayleigh backscattering signal (RBS) spectrum. Distributed twist sensing with a spatial resolution of 1.56 cm is demonstrated. The measured frequency shift exhibits a cubic fitting relationship with the twist angle within the range of ±180°. Moreover, the proposed sensor is able to identify the twist direction, as twist in the opposite direction will cause a frequency shift in the opposite direction of the cross-correlation spectrum. The proposed sensor offers advantages such as high spatial resolution, distributed twist measurement, and twist direction identification, which is very promising in applications such as structural safety monitoring, medical intervention, and biomimetic design, etc.