## 4 Discussion  
Detection sensitivity is a major parameter in pNDE. Our theoretical model and calculations showed that the pulse width of the excitation laser beam was a crucial factor for the effective generation of photoacoustic waves. Mathematical analysis revealed that the resultant photoacoustic pressure was proportional to the time derivative of the excitation pulse [42; 43]. Compared to the typical laser-induced ultrasound system (nanoseconds pump laser) [44; 45], the implementation of the 7 picosecond pulsed laser can produce an increase in photoacoustic signal conversion efficiency, which will lead to improved detection sensitivity. Further, a higher frequency photoacoustic signal will lead to better depth resolution when paired with a high-frequency ultrasound transducer.  
The lateral resolution of pNDE was estimated by the diffraction-limited spot size of the optical focus [42]. For a wavelength of 532 nm and a numerical aperture (NA) of 0.10, the lateral resolution of the pNDE micro-scale scan was theoretically determined to be 2.71 \(\upmu\)m. This resolution was sufficient to resolve individual carbon fibers (diameter: 7-10 \(\upmu\)m) inside the CFRP composite in a non-invasive manner (Figure 3a). The axial resolution in the depth direction was jointly determined by the laser pulse width and detection bandwidth of the ultrasonic transducer. In the macro-scale damage detection experiment, a 20 MHz ultrasonic transducer was used, which resulted in an axial imaging resolution of approximately 0.30 mm in the depth direction. In future studies, lateral resolution of the pNDE scan can be further improved by (i) increasing the NA of the objective lens, and (ii) using a shorter excitation source wavelength with the maximum imaging depth scaled accordingly. If a higher frequency transducer is employed, submicrometer spatial resolution can be achievable. This analysis indicates a new approach for microstructural determination inside CFRP composites with photoacoustic imaging for future research.  
The current pNDE detection speed was limited by the single ultrasonic probe for signal collection during the scanning. However, implementing a pair of galvanometer mirrors with optical scanning can dramatically improve the imaging speed (up to 30 kHz, the limitation of a galvanometer) and should be much faster than any mechanical scanning imaging system. Further increase in laser PRR has been limited mostly by the time required to store raw data and the ultrasound propagation time inside the CFRP composites. Additionally, the system can be further optimized for real-time imaging in field applications by increasing the laser PRR of the developed pNDE system, and the goal is to reach the multi-kHz laser PRR range.  
Conventional ultrasound methods and thermography are only capable of providing a contour of the damaged area rather than the detailed layer-by-layer distribution information demonstrated in the resulted images of the pNDE test [46]. Although current ultrasound-based non-destructive evaluation technologies can detect barely visible and embedded geometries, they do not have the adequate lateral resolution to identify micro-scale damage initiation in composites, especially for complex layered materials like CFRP composites. Our proposed technology has the advantage of being able to focus the excitation source to achieve a much higher lateral resolution than the ultrasound-based imaging system. Thus, the pNDE scanning method described in this paper shows great potential compared to current methods for the characterization of impact damage at multiple length scales via in situ imaging within the upper part of an in-depth damage distribution. With future hardware modifications, pNDE scans can theoretically be completed within minutes.