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.
The burst mode for ps and fs pulses for steel and copper is investigated. It is found that the reduction of the energy in a single pulse (in the burst) represents the main factor for the often reported gain in the removal rate using the burst mode e.g. for steel no investigated burst sequence lead to a higher removal rate compared to single pulses at higher repetition rate. But for copper a situation was found where the burst mode leads to a real increase of the removal rate in the range of 20%. Further the burst mode offers the possibility to generate slightly melted flat surfaces with good optical properties in the case of steel. Temperature simulations indicate that the surface state during the burst mode could be responsible for the melting effect or the formation of cavities in clusters which reduces the surface quality.
For most applications, the benefit of the burst mode can easily be explained: the energy of each pulse in an n-pulse burst is n times smaller compared to single pulses with identical average power and repetition rate. Thus, the peak fluence of each pulse is nearer the optimum value and the removal rate is therefore increased. It is generally not as high as it would be if single pulses with identical peak fluence but n times higher repetition rate could be applied. However, there are situations where the burst mode can lead to higher efficiencies, i.e., specific removal rates and a real increase in the removal rate can be obtained. For copper at 1064 nm and with a 3-pulse burst, the specific removal rate amounts to about 118% of a single pulse. For silicon, a huge increase from 1.62 to 4.92 μm3/ μJ was observed by applying an 8-pulse burst. Based on calorimetric measurements on copper and silicon, the increased absorptance resulting from a rougher surface is identified as an effect which could be responsible for this increase of the specific removal rate. Thus, the burst mode is expected to be able to influence surface parameters in a way that higher efficiencies of the ablation process can be realized.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.