Dynamic Curvature and Torsion Monitoring Using Quasi-Integer-Ratio Time-Expanded ΦOTDR

Shape sensing based on optical fiber sensing brings interesting advantages such as high sensitivity, lightweight, long-term reliability, and minimal intrusiveness, among others. However, the fiber interrogation methods employed to date, namely fiber Bragg gratings, OFDR or phase-sensitive (Phi)OTDR are either not entirely distributed, have poor performance in spatial resolution or dynamic sensing or are only valid for short (i.e., few meter-long) fibers. Here, we present a fully distributed optical fiber sensing system able to perform dynamic curvature and torsion sensing over a range of 125 m, with 10 cm resolution and at a sampling rate of 1 kHz. This performance is attained by interrogating three cores of a multi-core fiber (MCF) with time-expanded (TE-) Phi OTDR in its quasi-integer ratio (QIR) mode. With respect to the previously presented TE-Phi OTDR-based curvature sensing proof-of-concept, the use of QIR mode opens the possibility of effortlessly expanding range and/or sampling rate without a significant cost increase. Additionally, in this work the detection of fiber torsion is also addressed, offering for the first time to our knowledge the complete information towards fully distributed shape sensing at acoustic sampling rate with cm-scale resolutions over a record fiber length of >100 m. The unique sensing performance offered by the proposed system may become a valuable tool for a broad range of applications within civil and mechanical engineering, industrial manufacturing or seismology, among others.