Optimization of the volume ablation rate for metals at different laser pulse-durations from ps to fs

Abstract

Ultra short laser pulses in the ps or fs regime are used, when high requirements concerning machining quality are demanded. However, beside the quality also the process efficiency denotes a key factor for the successful transfer of this technology into real industrial applications. Based on the ablation law, holding for ultra short pulses with moderate fluences, it has been shown that the volume ablation rate can be maximized with an optimum setting of the laser parameters. The value of this maximum depends on the threshold fluence and the energy penetration depth. Both measures themselves depend on the pulse duration. For metals the dependence of the threshold fluence is well known, it stays almost constant for pulse durations up to about 10 ps and begin then to slightly increase with the pulse duration. The contrary behavior is observed for the energy penetration depth, it decreases over the whole range when the pulse duration is raised from 500 fs to 50 ps. In this paper we will show that the maximum ablation rate can therefore be increased by a factor of 1.5 to 2 when the pulse duration is reduced from 10 ps down to 500 fs.

Influence of the Pulse Duration onto the Material Removal Rate and Machining Quality for Different Types of Steel

“…Similar calculations have been done for a Gaussian shaped beam as emitted by most ultra short pulsed systems (Neuenschwander et al (2012), Neuenschwander et al (2010, Raciukaitis et al (2009)). Again a maximum removal rate per average power (ablation efficiency) is observed:…”

“…Due to incubation effects the threshold fluence also strongly depends on the number of pulses applied, which is described for metals by Schmid et al (2011), Neuenschwander et al (2012, Jee et al (1988) and Mannion et al (2004). Additionally it was found that also the energy penetration depth shows an incubation effect of the same kind.…”

“…Neuenschwander et al (2012) show that for a top hat beam the efficiency of the ablation process depends on the ratio between the threshold fluence and the applied fluence th / . The efficiency shows a maximum value of 1/e, i.e.…”

“…Additionally it was found that also the energy penetration depth shows an incubation effect of the same kind. The maximum removal rate may therefore also strongly depend on the number of pulses applied; more details are given in Schmid et al (2011), Neuenschwander et al (2012.…”

Compact gigawatt-class sub-picosecond Yb:YAG thin-disk regenerative chirped-pulse amplifier with high average power at up to 800 kHz

“…However, sub-picosecond pulses can further increase the ablation efficiency for certain materials, depending on the available average power and peak fluence as well as the desired feature size. On the one hand, for practical situations the efficiency generally changes little from 1 ps to 500 fs [3,4]. On the other hand, the situation may be different at harmonic wavelengths, where a careful optimization at high average power is challenging, in particular as frequency conversion is typically optimized for certain pulse durations and energies.…”