Proof of principle for large-aperture air-coupled laser ultrasound system with down-sampling

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As a hybrid method, air-coupled laser-ultrasound (ACLU) has demonstrated significant potential in non-destructive evaluation due to the advantages of laser-ultrasound and air-coupled ultrasonic inspection across a variety of laminated composite materials. But for fast laser scanning, the field-of-view (FOV) of the ACLU system was limited to a few millimeters by the finite aperture of the air-coupled detector. With mechanical scanning, the imaging FOV can be expanded at the expense of speed, stability, and applicability. Another effective scheme is to employ a large-area air-coupled detector array to capture the laser-induced ultrasonic signals, which will also dramatically increase the complexity of hardware channels and the data volume of the ACLU system. Here, a proof-of-principle ACLU system based on a large-aperture detector array was presented with down-sampling. In addition to multiplying the detector aperture, the sampling rate of each channel was also decreased from 3 MHz to 100 kHz, and the overall imaging data volume had fallen to no more than 1 %. Comparing with traditional methods, the proposed ACLU system can detect the interfacial microdefects located in the boned layer of laminated composite materials, and the average pixel difference in large-area C-scan imaging was less than 18 %. Our eventual goal is to offer a simple, fast, and affordable large-aperture ACLU approach for non-contact inspection of composite materials.