## 1 Introduction  
High-performance carbon fiber-reinforced plastic (CFRP) composite materials are well known for their high strength to weight ratio, being light in weight, and resistance to corrosion [1; 2]. However, aging-related damage and low-velocity impact damage in composites, such as fatigue cracks and delamination, can significantly reduce their structural integrity and durability. In addition, manufacturing imperfections can result in embedded defects, including voids, cracks, and inclusions [3; 4]. Since the size, location, and properties of embedded defects in composites are generally unknown and difficult to detect, there is an urgent need to develop new non-destructive evaluation (NDE) and structural health monitoring (SHM) technologies to help assess the quality of composite products and to help provide accurate inspections throughout a composite's service life. Early detection of embedded and barely visible damage in composites is imperative for long-term operation, risk management, and prognostics of complex composite structures. Recent advance of NDE technologies has led to efficient damage detection in composites. Currently well-accepted NDE technologies include acoustic emission, infrared thermography, and ultrasonic testing. Acoustic emission identifies damage initiation and tracks damage growth by continuously analyzing elastic waves generated via energy release from localized sources within the tested structures [5]. This method can be potentially used for in situ damage characterization, since the failure events are detected as they occur. Infrared thermography has been used to detect subsurface cracking and embedded delamination in composites [6,7]. The obtained thermal patterns induced, either by directly heating the sample or applying a mechanical oscillatory load, have been analyzed to study embedded damage in composites. However, it is difficult to measure through-thickness locations of the damage using infrared thermography. Ultrasonic techniques are some of the most popular NDE methods for damage detection in composite structures and have been extensively reported in the literature [8, 9, 10]. For example, [PERSON] et al. used Lamb wave methods to detect delamination, transverse ply cracks and through-holes in quasi-isotropic graphite/epoxy composites [11]. [PERSON] et al. developed a tomographic damage imaging approach by combining inverse acoustic wave propagation by combining the k-space method with the adjoint method [12]. Although NDE technologies have been used for a broad range of engineering applications, most of the NDE equipment cannot detect micro-scale damage initiation in composites in depth. High-frequency ultrasound can detect micro-scale damage. However, the detectable depth is limited. Real-time NDE has been under investigation as a method to monitor the integrity of materials and structures over the past two decades. Progress has been made in developing and improving real-time NDE, which allows early detection of material defects, providing timely warning to those at stake [13, 14, 15, 16, 17]. Real-time NDE technologies utilizing advanced sensors (i.e., piezoelectric ceramic sensors [18, 19], impedance-based sensors [20, 21], piezoresistive sensors [22, 23], and fiber Brag grating sensors [24, 25]) have been referred to as structural health monitoring (SMH) and prognostics. In order to detect structural defects under regular load conditions, innovative signal processing and pattern recognition algorithms have been developed [26, 27, 28]. While progress has been made in the development of SHM and prognostics, this technology has not been implemented in industries that require large-scale applications, especially in the aerospace industry, due to limitations pertaining to the sensors, the power supplies, and real-time data processing. Laser-induced ultrasound has been recognized as a promising technical solution for NDE and SHM of CFRP composites.