The development of ultrashort pulsed laser systems actually goes far beyond the kW level. But e.g. for metals and single pulses todays standard methods like galvo scanners are not suited for higher average powers and alternative approaches have to be developed. We will get an insight into actual developments using multi-pulse strategies in temporal representation as pulse bursts and in spatial representation as multi-beams or with direct beam forming. A combination of these methods with synchronized scanning or real pulse on demand option could pave the way for using high average powers.
Machining of stainless steel with ultrashort laser pulses is often a challenging task due to heat accumulation problems leading to bumpy surfaces or due to the formation of cavities or cone-like protrusions (CLP) at high pulse energies. With a specific diffractive optical element (DOE) leading to a special beam shape and synchronized scanning a removal rate of 16 mm3/min was achieved on steel AISI 304 with an average power of 180 W and a repetition rate of 1 MHz. Flat and shiny surfaces without CLP's and bumps having a surface roughness of sa < 500 nm were achieved. In case of copper the maximum removal rate amounted 17 mm3/min with a surface roughness of sa < 400 nm at a repetition rate of 400 kHz and an average power of about 150 W. The experiments clearly show, that with beam forming high average powers can be used for high quality laser micromachining with ultrashort laser pulses and single beams at average powers exceeding 100 W.
The commercially available Synova Laser MicroJet® technology combines conventional laser capabilities with compressed water jet that precisely guide laser beam in a similar manner to optical fibers. Due to physical water breakdown, technology is typically focused on the nanosecond pulse duration range. A stable beam shaping setup with a diffusor and commercial fiber to couple into water jet, was developed allowing to test Laser MicroJet® at 100-300 ps pulse duration. The change to energy intensity profile with diffuser allowed to triple coupled energy without inducing the physical breakdown in water and could be further increased by implementing 2 and 3 pulse bursts into the setup. High quality scribing was achieved at Si wafer at high scanning speed. Preliminary results on multilayer Si-wafer have demonstrated that scribing quality is in this case more feedrate dependent, limited chipping occurring at speed of commercial interest. Cutting tests were performed on semiconductors as well as on metals. On both, it was possible to achieve high quality cuts with high feedrate up to 12 mm/s with Ra < 0,3 μm.
We investigated GHz pulse bursts ablation on metals, silicon, zirconium dioxide, soda-lime glass and sapphire for surface structuring applications with a commercial laser system providing a burst pulse frequency of 5.4 GHz and a maximum of 25 pulses per burst pulse train. The results on metal show dramatic decrease of the ablation efficiency and a reduction of the machining quality. For silicon we also observed a reduction of the ablation efficiency for GHz pulse bursts but found a strong increase for MHz pulse bursts using a 10 ps laser system. On glass an increase of the ablation rate for GHz pulse bursts was observed, however with pure machining quality indicated by crakes in the surface and boarders. Zirconium oxide was the only investigated material, where a GHz pulse bursts induces a moderate higher ablation efficiency with comparable surface qualities, however a 10% higher ablation rate was obtained with a 10 ps laser system.
[100], [110] and [111] oriented silicon shows different behavior when it is machined with 10 ps pulses in the NIR. For the [100] orientation the roughness increases to 2.8 µm when the peak fluence is raised to 1.6 J/cm2 then drops down to a value below 200 nm for a fluence of 2 J/cm2 and stays below 300nm for fluences up to 7.5 J/cm2. For the other orientations a completely different behavior is observed. The roughness constantly increases to 900 nm at 1.6 J/cm2 and then further to about 8 µm for a peak fluence of 7.5 J/cm2.
The upscaling of laser micromachining processes with ultrashort pulses is limited due to heat accumulation and shielding effects. Multibeam scanning represents one of the strategies to overcome this drawback. It is in general realized by combining a diffractive beam splitter with a galvanometer scanner. A full synchronization with the laser repetition rate offers new possibilities with minimum thermal impact. We will demonstrate this by means of a multipulse-drilling on the fly process with a regular 5x5 spot pattern having a spot to spot spacing of 160µm. With a constant speed (synchronized to the laser) this pattern can be moved by exactly this spacing between two laser pulses. At a repetition rate of 100 kHz and an average power of 16 W we were able to drill more than 1'500 holes/s in a 10µm thick steel foil without any thermal impact. In a next step we will extend this technology with an SLM to different periodic patterns.
Bursts of 230 fs pulses with up to 25 pulses having a time spacing of 180 ps were applied to steel AISI304, copper DHP, brass and silicon in real surface texturing (milling) application by machining squares. The previously reported very high removal rates for GHz bursts could not be confirmed, on the contrary, the specific removal rate tremendously drops down to less than 10% for the metals and 25% for silicon when the number of pulses per burst is increased. This drop is fully in line with shielding effects already observed in case of MHz pulses and double pulse experiments. The increase of the number of pulses per burst directly goes with strongly increased melting effects which are assumed to additionally re-fill the already machined areas in this milling application. Calorimetric experiments show an increasing residual heat with higher number of pulses per burst. Further the removal rates of the GHz bursts directly follow the tendency of single pulses of identical duration. This fosters the hypothesis that in case of metals and silicon only melting and melt ejection lead to the high reported removal rates for GHz bursts in punching applications and that no additional "ablation cooling" effect is taking place.
In ultra-short pulsed laser micromachining of metals shorter pulses generally lead to higher throughput e.g. for copper the specific removal rate drops to about 25 % if the pulse duration is raised from 10 ps to 50 ps and it is increases by about 35 % for a decreased pulse duration of 350 fs. In contrast, a significantly higher increase of 100 % was observed for stainless steel AISI 304 when the pulse duration is decreased from 10 ps to 350 fs, whereas the drop for the longer pulse duration of 50 ps was of the same order of magnitude as for copper. Recent results of a two-pulse burst experiment on copper with temporal varying intra-burst pulse distance have shown, that pulse distances from 10 ps to 100 fs cause only a slight increase in efficiency of approximately 10 % whereas for pulse distances from 10 ps to 500 ps the efficiency is reduced to nearly 50 %. Although, these factors differ from the well-studied pulse duration experiments, they show a similar tendency. Therefore, we assume a dominant common cause.
In literature the diminished efficiency is often referred to particle- or plasma shielding for tens of picoseconds pulse durations. In this sense, the ablation of the material during the pulse is faded by itself for longer pulses as well as the ablation of the material from the first pulse is restricted by the second pulse in the burst experiments. Additional investigations concerning reflection and calorimetric considerations will help to underline this hypothesis.
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