# Study of Crack Growth of Transparent Materials Subjected to Laser Irradiation by Digital Holography
## 3 Experimental Setup  
In this experiment, an SC-PRO multispectral pulsed laser as the light source was used. The output wavelength was adjusted from 430 nm to 1450 nm, and the working power was 8 W. The frequency adjustment range was 0.1-80 MHz. DHC-MER-500-7 UM CCD pixel size 2.2 \(\upmu\)m \(\times\) 2.2 \(\upmu\)m; a resolution of 1544(H) \(\times\) 1944(V) was used to record the hologram. The calibration magnification of the digital holographic interference system was 1.976 [42].  
The schematic diagram of the optical system is shown in Figure 1. It is a Mach-Zehnder off-axis holographic interference system and the wavelength of the light source is 633 nm. The systems have two major components: the pump and the probe. The reflective mirror M1 can move in a direction perpendicular to the optical axis and is used as a switch between the irradiation optical path and the optical detection path. As shown in Figure 1, when the mirror M1 is placed at the position represented by the solid line, the optical path LASER-M1-M4-L3 would form an irradiation system for the sample. The focal length of the lens L3 is 150 mm. When the mirror M1 is placed at the position represented by the dotted line, a Mach-Zehnder off-axis holographic interference system is established to image the transparent sample. In this system, SF is the spatial filter, and the transmission-to-reflection ratio of the beam splitters BS1 and BS2 is kept at equal 50:50 intensities.  
For holographic detection, the laser light passes through the spatial filter SF and is divided into two beams through the beam-splitting prism BS1. First, the object light passes through the reflective mirror M2, the cracked sample, and the convex lens L1. Second, the reference light passes through the reflective M3 and the convex lens L2. Two beams are thus sent subsequently to interfere with the CCD plane using the beam splitter BS2. Here, the focal lengths of the lens L1, L2 is 120 mm, \(\mathbf{p}\) equal to the focal lengths of L1 and L2 and is called the object distance, and \(\mathbf{q}\) is called the image distance. According to the lens imaging principle, after passing through the lens, the actual size of the sample is magnified \(\mathbf{q/p}\) times.  
The experiment was run in two phases. The first phase examined which laser wavelength band has the greatest influence on crack by provoking crack growth. In the second phase, the maximum-impact-irradiation crack analysis was performed. Accordingly, the two groups of samples were made in the same way. To verify the significance of the presence of cracks on the phase transition in transparent media, a crack-free sample was used. The tested samples are shown in Figure 2. Figure 2a,b are produced from Dammar varnish coated on coverslips with known cracks, and Figure 2c is set as a crack-free sample.  
In Figure 2a,b, the area of the cracked sample inside the red rectangle is about 85 mm\({}^{2}\), and the white ellipse in Figure 2a-c shows the field captured by the CCD.  
Figure 1: Schematic diagram of the optical path system, both legs of the optical setup are seen LASER-M4-L3 as pumping and LASER-M1-BS1-CCD as probe.  
Figure 2: (**a**) Crack sample in the first phase, (**b**) crack sample in the second phase, and (**c**) crack-free sample.