Towards low-frequency acoustic sensing using antiresonant hollow-core fibers

Low-frequency acoustic waves exhibit low attenuation and strong penetration, enabling propagation over long distances in water and the solid Earth. This capability underpins applications in natural-hazard monitoring and early warning (earthquakes, tsunamis, volcanic activity), long-range marine sensing and surveillance, and structural and equipment-health monitoring in harsh environments. Traditional piezoelectric hydrophones suffer from high self-noise and electro-magnetic interference, motivating the exploration of fiber optic solutions. However, the operation of fiber solutions at low frequencies suffers from temperature cross-sensitivity due to the inherent thermal sensitivity of conventional optical fibers. Hollow-core fibers (HCFs), which guide light in air/vacuum, provide lower thermal and higher acoustic sensitivity than conventional solid core fibers; however, the extent to which they could address this limitation has never been studied. Here, we present a comprehensive study of the acoustic and thermal sensitivities of HCFs, combining theoretical modeling and experimental validation, aiming for high acoustic sensitivity while keeping thermal sensitivity low, hence reducing unwanted cross-sensitivity. We validate its results experimentally on three HCFs with different structural compositions, quantifying the trade-off between acoustic and temperature sensitivities. Besides, we demonstrate a substantial improvement in temperature–acoustic decoupling by employing an HCF made from Ti-doped ultralow-expansion (ULE) glass, achieving a reduction in thermal sensitivity by more than three orders of magnitude compared with standard single-mode fiber while maintaining comparable acoustic sensitivity. Further, based on the theoretical model we have developed, we show that more optimal coating material could improve the performance of ULE-HCF by another two orders of magnitude. This work thus guides comprehensive design of thermally stable and acoustically sensitive HCF sensors for low-frequency acoustic sensing.