## 2 Experimental Method
### Laser Ultrasonic Testing  
The proposed non-contact laser ultrasonic inspection system was utilized. The whole system was set in pulse-echo mode, which is an ultrasonic detection method. Pulse refers to the generation of ultrasonic waves, and echo refers to the signal received from the sample surface. Figure 2 shows the experimental setup of the LUT system. The LUT parameters used for the target sample are shown in Table 2.  
A 532 nm frequency-doubled Nd: YAG pulsed laser in Q-Switch mode (Q-smart 100, Quantel laser Co., Ltd., Les Ulis, France) was used for ultrasonic wave generation. Once the generation laser beam produced by the generation laser was irradiated on the top surface of the sample, an extremely small area was heated by the high laser power density, and ultrasonic waves were generated via thermal expansion due to thermoelastic mechanisms. The ablation threshold of Ti-6 Al-4V was observed at 708 mJ/cm\({}^{2}\) for the pulsed Nd: YAG laser in previous work [35]. In this work, the laser absorption coefficient A of the sample was around 0.1. The pulsed laser energy density E can be calculated by E = (A \(\times\) e)/s, where e is the laser energy per pulse (m)), and s is the laser heating area (cm\({}^{2}\)).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Generation Laser** & **Value** &
\begin{tabular}{c} **Detection Laser** \\ **Parameters (Unit)** \\ \end{tabular} & **Value** \\ \hline Wavelength (nm) & 532 & Wavelength (nm) & 532 \\ Laser diameter (mm) & 1 & Laser diameter (\(\mu\)m) & 25 \\ Laser energy (m/pulse) & 50 & Laser power (W) & 0.2 \\ Pulse width (ns) & 8 & Averaging number & 512 \\ Repetition frequency (Hz) & 20 & Scanning length (mm) & 40 \\  & & Scanning resolution (mm) & 0.1 \\  & & Index length (mm) & 8 \\  & & Index resolution (mm) & 0.2 \\ \hline \hline \end{tabular}
\end{table}
Table 2: LUT parameters used for the target sample.  
Figure 2: Setup for the laser ultrasonic testing (LUT) system: (**a**) photography and (**b**) principle scheme.  
The calculated pulsed laser energy density was 637 mJ/cm\({}^{2}\), which was lower than 708 mJ/cm\({}^{2}\) to avoid ablation on the top surface of the sample.  
A scanning LDV with a built-in galvanometer and auto-focal lenses was utilized as a detection device, with lens diameter of 25 mm and focal distance of 4 mm. A commercial CW diode pumped solid-state laser (Cobolt Samba-532, Hubner Cobolt Inc., San Jose, USA) with a wavelength of 532 nm was used as the laser source of the LDV. The other components of the LDV contained of a femto-amplifier (HSA-Y 200 MHz, Femto Easy Inc., Pessac, France), wave plates, and gratings. The LDV produced a detection laser beam to monitor the small displacement on the sample surface. The measurement range of the out-of-plane velocity of the LDV was from 0.01 \(\upmu\)m/s to 10 m/s, based on the Doppler Effect. The voltage/displacement signal received from the top surface of the sample by the LDV was stored on a computer and processed by MATLAB codes.  
As shown in Figure 2, the target sample was fixed in front of the focal lens. The generation laser beam was irradiated on the top surface of the target sample. The inspection was conducted in pulse-echo mode, i.e., the generation laser and detection laser were located on the same side of the sample. During inspection, the generation laser and detection laser were fixed. The spacing of laser irradiation point and detection point was set to 5 mm. The sample was fixed on the moving platform and moved along each scanning path driven by step-motor. The corresponding signals were received by the LDV at the target detection point, as shown in Figure 3. The LUT scanning area of the target sample was \(40\times 8\) mm\({}^{2}\), and the spatial resolution along each scanning path (scanning resolution) and between two adjacent scanning paths (index resolution) were 0.1 and 0.2 mm, respectively. The ultrasonic waveforms were averaged 512 times to improve the signal-to-noise ratio.