Low-frequency fiber-optic optoacoustic transducer arrays for large-area high-precision online non-destructive testing

In Fiber-optic optoacoustic (FO-OA) non-destructive testing (NDT), expanding the ultrasonic detection range remains a critical challenge for large-area applications. While extending the detection range to larger scales necessitates addressing issues related to ultrasound intensity and propagation losses. Recent efforts have focused on enhancing ultrasound intensity, while methods to further extend the detection range by reducing ultrasound propagation losses are highly significant for ultrasonic NDT. This study introduces a frequency-selective functional layer in ultrasound transducers to effectively lower the ultrasound frequency. By incorporating polyethylene (PE) particles into the thermally expandable material to form the functional layer that enhances the attenuation of high-frequency components, the central ultrasound frequency is reduced from 1 MHz to 0.3 MHz. This approach expanded the detection area of the flat plate structure to 90 × 54 cm². A 6-element FO-OA transducer array was fabricated on a single optical fiber, enabling the successful localization of all defects on the specimen with a positioning error of <0.894 cm. These parameters demonstrate state-of-the-art performance in optoacoustic NDT for the tested configurations. This work demonstrates that the frequency reduction method effectively mitigates the missed detection issues arising from the limited inspection range. Further, testing on a thick specimen (30 × 30 × 1.5 cm³) also yielded a low positioning error of 0.781 cm, even with multiple ultrasound propagation modes, indicating that the ultrasound with PE particles offers superior immunity to multimodal aliasing, providing a robust solution for complex media.