MANUFACTURING with short pulse lasers FOCUS into packets of several closely spaced ultrashort sub-pulses, known as bursts, which usually consist of two to several hundred or even thousand pulses. The burst is characterized by the number of pulses, their individual energy, and, in particular, the inter-pulse delay between successive pulses within the burst. Inter-pulse delays of a few tens of nanoseconds are usually referred to as MHz-bursts, while hundreds of picoseconds correspond to so called GHz-bursts( cf. Figure 1).
BURST MACHINING OF METALS When applied to metals, burst processing allows access to interaction regimes that differ fundamentally from those with single pulses. As the sub-pulses within a burst interact with a surface that is already electronically excited, melted, or partially shielded by plasma and vapor, both absorption and energy distribution can change significantly. Depending on the regime used, this can lead to increased ablation efficiency, altered surface morphologies, and improved surface quality at similar or even higher throughput compared to machining with single pulses [ 1 ].
Double-pulse experiments show that the second pulse can substantially modify the ablation dynamics by interacting with the transient plume and molten layer, which may either reduce or, for certain delay ranges and materials, partially recover the ablation efficiency compared to single-pulse irradiation. Fine tuning of melted layers can lead to very high surface qualities with Arithmetic Average Roughness of Ra ≈ 100 nm. In practical, in machining strategies such as for scribing and milling, the intra-burst structure determines how energy is distributed across successive phases of excitation, melting, and material removal. Precise polishing strategies can be applied, particularly using longer pulse packets in the GHz range, which consist of dozens or hundreds of sub-pulses and exhibit physical effects comparable to ns laser pulses. This results in smoother topographies
Figure 1. Schematic representation of ultra-short pulse and burst regimes in the time domain, illustrating the redistribution of the energy of an individual pulse into multiple closely spaced sub-pulses and distinguishing between MHz and GHz intra-burst delays. Combinations of both MHz- and GHz-burst pulses are called bi-bursts, which are not shown for the sake of simplicity.
and reduced heat-affected zones, but usually at the expense of ablation efficiency. MHz-bursts with intra-burst delays of 10 ns and longer, on the other hand, typically interact with expanded and hence more transparent ablation clouds and melt layers, which can promote high removal rates and efficient material ejection, but can also lead to remelting, resolidified edges, or increased micro-roughness if not carefully tuned. Selecting and switching between these modes and adjusting the number of pulses per burst provides a good way to balance throughput and surface quality for a given metal and target geometry.
Representative micromachining results achieved with metals using burst mode operation include deep grooves, cavities produced with
2.5D milling strategies( projection in a plane of the 3 rd dimension), and finely structured surfaces with customized roughness and morphology. In grooves and cavities, burst machining can significantly improve sidewall morphology and the amount of redeposited material with appropriate intra-burst parameters and materials, while maintaining or slightly increasing removal efficiency compared to single-pulse machining. On flat surfaces, burst processing can be used to create deterministic micro- and nanostructures or to control the transition between rough, highly light-absorbing textures and smoother functional surfaces. An example of a geometry after laser milling of copper with MHz-bursts is shown in Figure 2.
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