Air-Coupled Fiber-Optic Ultrasonic Sensor Based on an Open-Cavity and Optically Tuned Polymer Microbubble

An exceptionally sensitive and photothermally tunable polymer microbubble is fabricated at the fiber tip for enhanced ultrasonic detection. A modified acrylate adhesive is applied to the end face of a photonic crystal fiber, after which pressurized air is utilized to create an ultra-thin microbubble structure. Compared to silica-based counterparts, the polymer microbubble, with its lower Young’s modulus and open-cavity design, exhibits a significantly higher ultrasonic sensitivity of 60.9 mV/kPa in water. The direct detection of laser ultrasound in air is thus obtained with an air coupling distance of more than 3 cm. The microbubble sensor has a wideband frequency response exceeding 12 MHz, and its stability is demonstrated in different coupling media. Additionally, the photothermal effect of the acrylate adhesive facilitates optical tunability of the microbubble cavity length through controlled laser exposure time. The microbubble sensor is integrated into a laser ultrasound system to achieve the high signal-to-noise ratio evaluation of a defective aluminum plate. These findings highlight that the polymer microbubble exhibits advanced acoustic performance in air-coupled detection and flexible tunability, as well as its broadband response and compact fabrication. The proposed sensor provides an innovative and reliable solution for detecting weak strains under both dynamic and static conditions.