Flexible Smart Sensing Skin Based on Optical Microfiber Knot Resonators for Underwater Acoustic Wave Detection

Flexible smart sensing skins have attracted remarkable attention for their potential applications in human-machine interfaces and target detection. Here, a flexible smart sensing skin based on optical microfiber knot resonator (MKR) is proposed for underwater acoustic wave detection. By optimizing the fabrication processes, we have produced MKRs with high Q-factors (1×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup>) and high coupling depths (0.95), which can ensure the flexible skin has extremely high sensitivity. By designing the optical and mechanical resonant dual resonant structures, the proposed sensors have an underwater acoustic pressure sensitivity level as high as -145.6 dB re 1V/μPa at 1250 Hz, which is ~ 40.9 dB higher than a commercial hydrophone. The NEP is as low as 58.8 dB re 1 μPa/√Hz, which is equivalently 0.9 mPa/√Hz. A piece of smart sensing skin consisting of a quartet of MKR sensors is demonstrated to determine the spatial position of an underwater acoustic source in the three-dimensional (3D) space. The experimental results indicate that the average errors for the three orthogonal directions are -3.3%, 0.0%, and 2.4%, respectively. This study offers new insights into the accurate underwater target detection and positioning using flexible MKR sensors, which are promising for the future smart sensing skin technology.