Full length article Multi-mode high resolution TFM imaging of microdefects based on laser ultrasonic full matrix capture Zenghua Liu a,* , Long Chen b , Yanping Zhu a , Xiaoyu Liu b , Cunfu He a a School of Information Science and Technology, Beijing University of Technology, Beijing 100124, China b College of Mechanical and Energy Engineering, Beijing University of Technology, Beijing 100124, China A R T I C L E INFO Keywords: Laser ultrasonic Multi-mode imaging Microdefect Full matrix capture Multi-scale principal component analysis ABSTRACT The identification and visualization of submillimeter microdefects have long been a critical focus within the realm of ultrasonic testing. The utilization of high-frequency signals is imperative due to the small acoustic interaction structure. An additional benefit of using lasers is their non-contact nature and ability to utilize multiple imaging modes, effectively reducing false or missed detections and enhancing credibility. This study proposes an advanced optimization algorithm for imaging and quantitative evaluation of submillimeter micro- defects using laser ultrasonic full matrix capture (FMC) and total focusing method (TFM). The analysis of acoustic directionality under laser excitation and detection provides a foundation for mode selection. In preprocessing, the multi-scale principal component analysis (MSPCA) method is introduced to suppress coherent noise and blind spots. Moreover, phase coherence methods are implemented to further reduce mode artifacts, background noise, and residual surface acoustic wave (SAW). Additionally, multi-mode imaging and mode fusion are achieved using reflected longitudinal waves, reflected shear waves, converted longitudinal waves, and converted shear waves. Experiments were carried out on two different types of microdefects, namely side drilled holes (SDHs) and blind holes (BHs), resulting in high resolution TFM imaging with a minimum BH size of 0.3 mm. The outcomes demonstrate the efficacy of MSPCA in suppressing coherent noise, with further enhancement in imaging quality after phase coherence weighting. In addition, the  6 dB threshold of multi-mode images also provided the size information. This research is poised to contribute to the resolution of quality assessment challenges in powder metallurgy, welding, and additive manufacturing processes. 1. Introduction With the rapid development of high-end manufacturing and major equipment, such as in aerospace or power system applications, the minimum defect tolerance allowed in materials and structures continues to decrease. The detection of submillimeter microdefects has become particularly important for high-value manufactured components. At present, such defects are mainly detected using X-rays and ul- trasonics. X-rays have higher spatial resolution, but their practical ap- plications are often limited by penetration thickness, and the placement of detectors in the field. The latter relies on contact or immersion automated systems, using ultrasonic transducer arrays, or a single transducer driven by displacement control mechanisms for scanning [1]. Contact ultrasonic inspection is easily affected by the coupling state. When dealing with non-planar structures, wedges with specific geome- tries need to be designed. Immersion inspection is suitable for newly machined or disassembled parts. Neither X-ray nor conventional ultra- sonic methods are suitable for online inspection during processing or service. The use of optical methods to excite and detect ultrasonic waves makes it possible to detect submillimeter microdefects online. Non- contact optical methods will help overcome coupling problems caused by non-planar structures or high temperature conditions. On the other hand, the focused laser spot can excite high-frequency body wave signals under thermoelastic excitation, which also facilitates the detection of submillimeter defects [2]. By arranging multiple lasers or fiber delay groups, it is also possible to achieve beam focusing and deflection similar to piezoelectric trans- ducer arrays. However, such designs lack versatility and are difficult to achieve multi-point focusing or large angle deflection scanning as freely as piezoelectric transducers [3,4]. One approach that does not change the hardware configuration is to image defects using B-scan, C-scan, synthetic aperture focusing * Corresponding author at: Faculty of Information Technology, Beijing University of Technology, Beijing 100124, China. E-mail address: liuzenghua@bjut.edu.cn (Z. Liu). Contents lists available at ScienceDirect Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec https://doi.org/10.1016/j.optlastec.2024.111913 Received 24 April 2024; Received in revised form 23 September 2024; Accepted 28 September 2024 Optics & Laser Technology 181 (2025) 111913 Available online 2 October 2024 0030-3992/© 2024 Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.