A fiber optic laser-ultrasonic transmitter based on collapsed photonic crystal fiber for ultrasonic testing

Ultrasonic testing technology is widely used for its exceptional accuracy and resolution. Commercial ultrasonic testing systems basically use piezoelectric transducers as ultrasonic excitation elements. However, the performance of piezoelectric transducers is not stable enough in high-temperature and high-humidity environments. Therefore, a fiber optic laser-ultrasonic transmitter based on collapsed photonic crystal fiber (CPCF) was proposed. The CPCF guides the laser in the core into the photoacoustic material and realizes the ultrasonic excitation on the side wall of the fiber based on the optical-thermal-acoustic energy conversion. Finite element analysis and experimental research on the photonic crystal fiber (PCF)-CPCF-PCF microstructure show that controlling the CPCF length precisely enables quantitative modulation of the optical coupling coefficient, reflecting the optical energy in photoacoustic energy conversion. Three PCF-CPCF-PCF microstructures with different optical coupling coefficients were fusion spliced onto an optical fiber, and three-point ultrasonic excitation was successfully achieved on aluminum plates. The peak-to-peak values are all around 1.5 V, indicating that the energy of ultrasonic waves is balanced, which helps realize the baseline-free and standardized application of transmitters. Furthermore, the detection effect of ultrasonic waves generated by the transmitter on the thickness and defects of the plate was studied. By analyzing the ultrasonic echoes in the aluminum and carbon fiber-reinforced polymer plates, the corresponding wave speeds were calculated to be 6030.52 and 2522.34 m/s, similar to the longitudinal wave speeds in the corresponding materials. The indicators of ultrasonic waves in the time domain and frequency domain are sensitive to the thickness and defects of the plate. The amplitude and time-of-flight of the ultrasonic wave change monotonically with increasing damage severity. This study elucidates the principle of the fiber optic ultrasonic transmitter and demonstrates its capability of quasi-distributed ultrasonic excitation. Furthermore, the feasibility of non-destructive testing based on this transmitter was also verified.