Suppressing artifacts and quantitative evaluation of internal defects in laser ultrasonic multi-mode time-domain synthetic aperture focusing technique via mode separation.

To address the issues of low utilization rate of multi-mode signals and artifacts caused by wave velocity mismatch in laser ultrasonic imaging, this paper proposes a multi-mode time-domain synthetic aperture focusing technique (MMT-SAFT) imaging method based on mode separation. The interaction mechanism between bulk waves and internal hole defects is first theoretically analyzed, and a mathematical model for quantifying defect depth and size is established. The time-domain average signal preprocessing method is introduced to enhance the defect echo signals, and preliminary MMT-SAFT imaging is performed to localize potential defect regions and extract corresponding spatial coordinates. Subsequently, wave velocity back-projection combined with an autocorrelation-based adaptive weighted fusion strategy achieves the separation of four wave modes, avoiding low amplitude signal loss caused by threshold screening. Finally, through multi-mode fusion superimposition imaging, the focusing effect of the defect regions are optimized. Experimental results demonstrate that the proposed method significantly improves reconstructed image quality, with average signal-to-noise ratio (SNR) and artifact suppression ratio (AR) enhanced by approximately 7.6 dB and 8.4 dB, respectively, effectively suppressing artifacts caused by mode crosstalk and velocity mismatch. Moreover, the time-of-flight-based quantification model achieves the estimation of defect depth and diameter with maximum relative errors of 2.4% and 11.5%, respectively. This method provides an effective approach for the collaborative utilization of multi-mode ultrasonic signals and the precise quantification of defects.