Distributed directional torsion sensing based on an optical frequency domain reflectometer and a helical multicore fiber.

We propose and experimentally demonstrate a distributed directional torsion sensor based on an optical frequency domain reflectometer (OFDR) using a helical multicore fiber (MCF). A theoretical model is first established to reveal that the ability of the torsion direction discrimination stems from the fiber design of the central-offset cores with helical structure and the shorter helical pitch holds higher sensitivity. Such a distributed torsion sensor is then experimentally realized by using an OFDR system with an adjacent sensing distance of 9.4 mm. Comparative experiments with three different MCFs fully prove the theoretical predication. Finally, a distributed directional torsion sensor is realized with a linear sensitivity of 1.9 pm/(rad/m) by using the helical MCF with a helical pitch of 6 mm. Such a torsion sensing system would find potential applications in the fields of bionic robotics, 3-D shape sensing, oil drilling and so on.