Towards shape-sensing using time-expanded ΦOTDR

Optical fiber sensors are becoming a prominent solution to perform shape sensing thanks to their interesting and well-known advantages such as flexibility, lightweight, sensitivity, etc. However, the different approaches commonly used to date, which include fiber Bragg gratings, OFDR or OTDR architectures, are not able to achieve a fully distributed and fast performance over relatively long ranges (e.g., tens of meters) and with fine (cm-scale) resolution. Here, we present a novel scheme to perform curvature sensing attaining all the previously mentioned features by application of time-expanded phase-sensitive (TE-Φ)OTDR technology. TE-ΦOTDR is a promising distributed sensing technique that delivers a performance ranging between that of OFDR and ΦOTDR. As a proof-of-concept, we interrogate three cores of a multicore fiber (MCF) using TE-ΦOTDR, attaining curvature sensing with 10 cm resolution over a maximum measurable range of 125 m and a sampling rate of 50 Hz. The implementation of shape sensing schemes with the performance provided by TE-ΦOTDR technology may open the door to new and interesting applications in civil engineering, medicine and seismology.