Integrated stealth-sensing fiber-optic sensor via Dirac-cone phononic crystal

The integration of underwater acoustic stealth with high-sensitivity fiber-optic sensing remains fundamentally challenging, as conventional stealth materials attenuate or scatter incident sound waves required for signal detection. In this work, the non-double-blind hybrid acoustic sensing node is proposed and demonstrated, which simultaneously achieves platform stealth and enhanced acoustic signal acquisition. The device monolithically integrates a fiber laser hydrophone with a two-dimensional pillar-array phononic crystal engineered through Dirac-cone dispersion and quadrupole-mode coupling. The phononic crystal forms a perfect acoustic stealth region with an effective area of 60.0 mm × 60.0 mm at a frequency of 48.16 kHz, concealing both the sensor and its associated interrogation or power units. Unlike absorptive stealth structures, the crystal redirects and concentrates incident acoustic energy within the stealth band toward the embedded fiber sensing core. As a result, a significant reduction in acoustic backscattering is achieved while the local sound pressure detected by the fiber laser hydrophone is enhanced by approximately 6.18 dB. In addition, the anisotropic wave response of the phononic crystal introduces angular selectivity, enabling directional acoustic sensing from a single sensing element. These results demonstrate a practical strategy for integrating advanced fiber-optic hydrophones with acoustic stealth structures, offering a compact solution for simultaneous concealment, sensitivity enhancement, and directional underwater acoustic sensing.