## 3 Results  
The MRF wedge area on the #1-#7 samples in Table 1 was first microscopically observed based on the traditional MRF spot method. Figure 3 shows a series of microscope images of subsurface cracks at different depths along the center line of the MRF wedge of sample 2#, which is a typical morphology of an ultra-precision grinding subsurface. The surface morphology after grinding mainly includes fracture pits, microcracks and grooves. Subsurface defects are mainly microcracks, and the quantity of cracks gradually decreases with the increase in depth. When the distance from the surface exceeds 3 um, the microcracks disappear completely. It should be noted that when the distance from the surface reaches 2.9 um, the quantity of subsurface cracks not only becomes scarce, but also its appearance size is further reduced, and it becomes increasingly difficult to judge subsurface defects through microscopic observations relying on human eyes. Moreover, in the entire measurement process, the characterization content of subsurface damage only includes the geometric size and distribution of cracks, and no information on the changes in subsurface laser damage performance is given, which cannot directly guide the subsequent polishing of ultra-violet laser-irradiated fused silica optics for improving laser damage performance. Based on the MRF spot method, a further photothermal weak absorption analysis was performed on the MRF wedge area of samples #1-#7, as shown in Figure 4, to obtain the laser damage performance of each sample at different depths on the subsurface. Then, the test areas were selected at different depths along the central axis of the MRF wedge for in situ fluorescence confocal imaging to investigate the evolution of subsurface damage distribution in the depth direction. It can be observed that, due to the clustering of cracks (Figure 4b), the surface absorption value after grinding is unstable, but the surface absorption rate is generally high as the order of magnitude remains at hundreds of ppm. When the test depth increases along the central axis, the cluster cracks disappear (Figure 4c) and the photothermal absorption rate drops drastically, suggesting that the absorption level has reached the base level. The subsurface cracks generated by grinding are usually densely distributed in shallow areas. With the increase in depth, crack density gradually decreases. After exceeding a certain depth, the cracks are only sporadically distributed, and their distribution is inconsistent in different areas. Then the depth at which this densely distributed crack disappears is the crack cluster depth, which corresponds to the subsurface damage cluster depth (subsurface defect cluster depth), SSD\({}_{\text{(cluster)}}\) in the photothermal signal. After exceeding the subsurface damage cluster depth, some sporadic peaks still appear in the photothermal signal. The fluorescence confocal test identified that not only individual cracks, but also other defects cause additional absorption (Figure 4d). This individual damage may be caused by uncontrollable vibrations, transient changes in the external environment, large abrasive particles, etc. When these sporadic peaks disappear, the subsurface absorption characteristics of the sample are consistent with those of the matrix material, and no damage is found in the  
Figure 2: MRF spot method to measure subsurface cracks after grinding: (**a**) sample surface and MRF wedges after HF shallow etching; (**b**) three-dimensional profilometer to measure the center line of the wedges. fluorescence confocal test (Figure 4f). The two results determine that the depth at which the absorption peak completely disappears is the maximum subsurface damage depth. With the guidance of this method, we compared the MRF wedge central axis profile curve to determine the subsurface damage performance distribution characteristics of each sample.