Fig. 1. Experimental setup for micro-milling processing with a UV nanosecond laser

Fig. 2. Schematic cross-section of a Al₂O₃ ceramic blind hole

Fig. 3. Scanning diagram of the laser beam

Fig. 4. Results of optimal processing quality(1: photos of the hole; 2: microscopic diagram; 3: section diagram; 4: 3D morphology diagram). (a1)‒(a4) f₁ =100 mm; (b1)‒(b4) f₂ =160 mm; (c1)‒(c4) f₃ =250 mm

Fig. 5. Schematic diagram of a field lens focusing

Fig. 6. Influence of focal length on taper angle

Fig. 7. Influence of laser parameters on the taper angle. (a) Variation of taper angle with laser energy density; (b) variation of taper angle with scanning speed

Fig. 8. Influence of the laser energy density. (a)Variation of taper angle with energy density; (b)variation of bottom roughness with energy density

Fig. 9. Energy density distribution of a Gaussian beam

Fig. 10. Morphology of the hole bottom at different focal lengths. (a) f₁ =100 mm; (b) f₂ =160 mm ;(c) f₃ =250 mm; (d) larger view of the circle in Fig. 10(c)

Fig. 11. Influence of scanning speed. (a) Variation of taper angle with scanning speed; (b)variation of the bottom surface roughness with scanning speed

Fig. 12. Influence of the number of repeat scanning. (a)Variation of taper angle with the number of repeat scanning; (b) variation of the bottom surface roughness with the number of repeat scanning