Micromachining of Invar Foils with GHz, MHz and kHz Femtosecond Burst Modes

Abstract

In this work, a burst mode laser is used for micromachining of 20 µm–250 µm thick Invar (Fe64/Ni36) foils. Holes were drilled by firing multiple pulses transversely onto the sample without moving the beam (percussion drilling). The utilized laser system generates a burst of a controllable number of pulses (at 1030 nm) with tunable pulse-to-pulse time spacing ranging from 200 ps to 16 ns. The sub-pulses within the burst have equal amplitudes and a constant duration of 300 fs that do not change regardless of the spacing in time between them. In such a way, the laser generates GHz to MHz repetition rate pulse bursts with a burst repetition rate ranging from 100 kHz to a single shot. Drilling of the material is compared with the non-burst mode of kHz repetition rate. In addition, we analyze the drilling speed and the resulting dependence of the quality of the holes on the number of pulses per burst as well as the average laser power to find the optimal micromachining parameters for percussion drilling. We demonstrate that the micromachining throughput can be of an order of magnitude higher when using the burst mode as compared to the best results of the conventional kHz case; however, excess thermal damage was also evident in some cases.

Review on Experimental and Theoretical Investigations of Ultra-Short Pulsed Laser Ablation of Metals with Burst Pulses

“…It has been reported by several groups that both the ablated volume per time and the energy specific volume can be increased by several factors when processing material with ultra-short burst pulses with intra-burst repetition rates in the GHz regime compared to single pulse operation. For the milling of copper, it was reported that the maximum energy specific volume can increase by a factor of three [ 135 ], whereas for steel it can increase by a factor of two when using GHz burst pulses instead of single pulses [ 112 , 135 , 137 ]. For the drilling of copper, it was reported that the maximum energy specific volume can increase by a factor of 3.5; for steel, it can increase by a factor of 4.6; and, for aluminum, it can increase by a factor of 5.8 when using GHz burst pulses instead of single pulses [ 136 ].…”

“…A study about laser drilling of invar with ultra-short MHz and GHz bursts has shown an ambiguous picture. Depending on the sample thickness and intra-burst delay, slightly shorter drilling durations or even longer drilling durations were reported compared to drilling without bursts [ 137 ].…”

“…In general, it can be concluded that drilling processes are always more efficient than milling processes in this processing regime [135,136], which was explained by different melt flows in those two regimes [133]. Furthermore, the increase in energy specific volume is highly dependent on the time span of the used burst train and increases with longer time spans [22,[136][137][138][139]. Moreover, the used inter-burst delay plays a crucial role.…”

Micromachining of Transparent Biocompatible Polymers Applied in Medicine Using Bursts of Femtosecond Laser Pulses

“…We have used bursts of femtosecond pulses where the sub-pulses had a temporal separation of 400 ps. From previous investigations [ 21 ] it was known, that the heat input on the surface would be much more pronounced in this time scale as compared to a larger temporal separation (nanoseconds) setting. However, it was unknown what the best parameter combination that produces the highest heat flux onto the surface is.…”

“…Moreover, it was seen that implants with higher surface roughness values increase the risk of post–surgical complications (e.g., posterior capsular opacification) that lead to decrease in vision [ 15 ]. Recently it was discovered that using bursts of femtosecond laser pulses, when each pulse is divided into a sequence of sub-pulses with a temporal separation of few tens of nanoseconds or hundreds picoseconds, improves laser material processing by boosting the ablation efficiency [ 16 , 17 , 18 , 19 , 20 , 21 ]. It was also demonstrated, that by using bursts of pulses it is possible to further decrease the surface roughness compared to using the conventional single femtosecond pulse regimes [ 22 ].…”