Fig. 1. Schematic of laser helical drilling and in-situ imaging systems

Fig. 2. In-situ observations at different moments during machining of micro-holes on brass

Fig. 3. Hole depth versus time and drilling speed versus hole depth when machining micro-holes on brass. (a) Hole depth versus time; (b) drilling speed versus hole depth

Fig. 4. In-situ observations of change of hole depth at different moments and change curves of drilling speed and hole depth when machining micro-holes on single crystal diamond. (a) In-situ observations;(b) drilling speed versus hole depth; (c) hole depth versus time

Fig. 5. In-situ observation at different moments when machining micro-holes on titanium at different laser pulse frequencies

Fig. 6. Hole depth and drilling speed versus time when machining micro-holes on titanium under different pulse frequencies. (a) Hole depth; (b) drilling speed

Fig. 7. In-situ observation and final cross-sectional SEM images when machining micro-holes on single crystal diamond at different laser pulse frequencies

Fig. 8. Effect of laser focus position on maximum hole depth when machining micro-holes on titanium. (a) Schematic of lifting process of workpiece position; (b) in-situ observation of maximum hole depth changing with position of focal plane

Fig. 9. Effect of laser focus position on maximum hole depth when machining micro-holes on single crystal diamond

Fig. 10. In-situ observation of maximum hole depth at different pulse energy values when machining micro-holes on titanium

Fig. 11. Maximum hole depth and average drilling speed versus hole depth under different single pulse energy values when machining micro-holes on titanium. (a) Maximum hole depth; (b) average drilling speed