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.

The continuous increase of the average laser power of ultrafast lasers is a challenge with respect to the thermal load of the processing optics. The power which is absorbed in an optical element leads to a temperature increase, temperature gradients, changing refractive index and shape, and finally causes distortions of the transmitted beam. In a first-order approximation this results in a change of the focal position, which may lead to an uncontrolled change of the laser machining process. The present study reports on investigations on the focal shift induced in thin plano-convex lenses by a high-power ultra-short pulsed laser with an average laser power of up to 525 W. The focal shift was determined for lenses made of different materials (N-BK7, fused silica) and with different coatings (un-coated, broadband coating, specific wavelength coating). = ⋅ ⋅is the irradiated area given by the diameter d b of the beam on the workpiece. Processing in the focal plane of the focusing optics is the most common approach, since the highest fluence can be achieved at this position. When the position of the beam waist shifts by one Rayleigh length due to thermally induced effects, the fluence on the workpiece at the original position of the nominal focal plane decreases by a factor of 2 [4]. As the applied fluence significantly influences the efficiency of e.g. surface ablation [5], the thermally induced focal shift can dramatically reduce the processing efficacy. When working close to the ablation threshold, as in the case of surface structuring [6], the ablation process can stop completely even for small changes of the focal position. During laser drilling, the

Most metal forming processes use lubricants based on mineral oils as an intermediate medium to reduce friction and wear. To avoid the well-known drawbacks of oil lubrication, a novel and environment friendly lubrication system for deep-drawing processes was demonstrated at the University of Stuttgart. Liquid carbon dioxide and gaseous nitrogen are being used as volatile lubrication during the deep-drawing process, locally injected at high pressure through laser-drilled microholes. This new tribological system provides a significantly enlarged working range and at least 15% larger drawing depths compared to conventional oil lubrication.

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