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The Synthetic Aperture Focusing Technique (SAFT) is employed in laser ultrasonic non-destructive testing to image defects. However, during the laser ultrasonic imaging process, the influence of multi-mode waves often introduces various artifacts in the imaging results. We investigated a multi-iteration frequency-domain SAFT (MIF-SAFT) algorithm to tackle this challenge. Initially, a single low-precision F-SAFT imaging result is performed to identify potential defect locations. Subsequently, a back-projection method is applied to determine defect positions and separates acoustic waves of different modes in the B-scan. After that, a multi-mode F-SAFT imaging is performed for evaluation, and the above process is repeated until the imaging outcome stabilizes. The result shows that artifacts are removed when the image remains unchanged after multiple iterations, and the imaging quality is effectively improved by MIF-SAFT. Furthermore, including directivity patterns of laser ultrasound also enhances defect imaging capabilities within MIF-SAFT. Additionally, we examined the MIF-SAFT's capacity to detect multiple defects, which can image them simultaneously. • Multi-iteration in F-SAFT for artifact removal. • Different ultrasonic signals are separated by using the algorithm. • Directivity patterns are performed to enhance the imaging result. • Applied for multiple defects imaging simultaneously. [ABSTRACT FROM AUTHOR]

Different from the traditional frequency-mixing technique which employs a contacting transducer, the laser-induced acoustic nonlinear frequency-mixing detection technique utilizes a laser source to instigate crack motion and generate acoustic waves. Thus, apart from the temperature oscillation induced by the pump laser, the "basic temperature" originating from the probe laser can also influence the crack. This additional variable complicates the contact state of the crack, yielding a more diverse range of nonlinear acoustic signal attributes. In light of this, our study enhances the conventional opto-acoustic nonlinear frequency mixing experimental setup by integrating an independent heating laser beam. This modification isolates the impact of the "basic temperature" on crack width while also dialing down the probe laser power to mitigate its thermal effects. To amplify the sensitivity of crack detection, we deliberated on the optimal laser source parameters for this setup. Consequently, our revamped system, paired with fine-tuned parameters, captures nonlinear acoustic signals with an enriched feature set. This investigation can provide support for the non-contact opto-acoustic nonlinear frequency mixing technique in the detection and evaluation of micro-cracks. • A three-beam opto-acoustic nonlinear frequency-mixing technique is proposed. • The addition of the third cw laser enables finer adjustments to the crack. • Richer nonlinear acoustic features are obtained using the proposed technique. [ABSTRACT FROM AUTHOR]