A switchable fiber-optic ultrasonic transmitter array based on a ring-shaped fiber-optic structure for ultrasonic testing

This study investigates a switchable ring-shaped fiber ultrasonic transmitter (RFUT) array through theoretical modeling and experimental validation. By adjusting the fiber's bending curvature, the optical coupling ratio of RFUT units can be precisely tuned, enabling the series integration of multiple transmitters along a single fiber to form a scalable array. The use of optical flanges and patch cords facilitates a modular design, allowing for flexible reconfiguration within the array. Numerical simulations confirm effective acoustic superposition within the array configuration. Experimentally, compared with a reference single-point RFUT, the two-unit array broadened the 3-dB bandwidth from 9.4375 to 12.0300 MHz and increased the bandwidth-to-pulse-width ratio from 3.0882 to 5.8455, yielding improved temporal and frequency resolution. Furthermore, the acoustic velocity calculated from the array was 6309.1483 m/s, aligning closely with the theoretical longitudinal wave speed of aluminum (Al). The performance of the array was further validated through experiments using a three-unit array on a 1-mm CFRP plate and a four-unit array on a 4-mm Al plate, which achieved superposition efficiencies of 73.2539% and 75.2029%, respectively. Notably, the three- and four-unit arrays exhibited estimated signal-to-noise ratio gains of 6.6500 and 8.2700 dB, highlighting the broadband and efficient excitation capability of the proposed architecture. Compared to existing fiber-optic transmitter structures, the RFUT offers superior potential for time–frequency resolution while benefiting from a simplified fabrication process. These results demonstrate a compact and scalable route toward fiber-optic ultrasonic arrays, with significant potential for applications in non-destructive testing and structural health monitoring (SHM).