All-Fiber Vector Bending Sensor Based on a Hole-Assisted Fiber

In this paper, a novel hole-assisted fiber (HAF) composite structure-based optical fiber sensor for directional bending detection is proposed. The sensor is fabricated by splicing multimode fiber (MMF) to HAF, creating a hybrid structure capable of vectorial bending through its unique air-hole and multi-core assisted architecture. The distinctive cladding geometry of HAF enables precise determination of bending direction through differential sensitivity patterns. Experimental characterization across four orthogonal bending orientations revealed three directions with comparable negative sensitivities (-5.493 nm/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>, -7.153 nm/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>, and -5.284 nm/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> respectively). Significantly, when bending occurred with air holes positioned on the inner curvature, the sensor exhibited a distinct positive sensitivity of 1.095 nm/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>. Temperature testing demonstrated stable performance with the sensitivity of 9.2 pm/°C across 25-65°C. The combination of these features suggests strong potential for practical applications in high-precision directional bending sensing systems requiring both magnitude and orientation detection.