Comparative Drilling of Materials Using UV and IR picosecond lasers ablation with single and double-pulses emission

A method for precision drilling of stainless steel and aluminum plates using single and double picosecond pulses (8 ps at 1064 nm, 6 ps at 355 nm, and 5 ps at 266 nm), with a 25 ns interval between sub-pulses in the double-pulse mode, has been investigated. It was found that for each material, characterized by specific optical properties such as skin depth, there exists a critical thickness (CT), below which drilling with a pair of pulses (hereafter referred to as a doublet) can be performed using approximately 1.5–2 times fewer pulses than in the single-pulse drilling regime to achieve comparable through-holes. Plates with thicknesses below the CT are drilled more efficiently, and the inner surface quality of the resulting holes is improved compared to that produced by single-pulse drilling.For stainless steel, high-quality through-hole drilling is achievable when the plate thickness is less than 125 µm at 1064 nm, 200 µm at 355 nm, or 250 µm at 266 nm. For aluminum, the CT is 2–3 times higher than that for stainless steel. This finding suggests that thicker plates can be drilled without a significant decrease in ablation efficiency by increasing the sub-pulse delay within the doublet.Specifically, the delay time should exceed the time of flight of the plasma generated by the first sub-pulse in the doublet at the bottom of the “blind” hole, so that the second sub-pulse in the doublet interacts with the plasma cloud near or above the material surface. This time of flight is determined by the wavelength, optical penetration depth (skin depth), laser fluence and material properties.