Joerg Schille

Micro processing of metals using a high repetition rate femto second laser: from laser process parameter study to machining examples

The paper presents a study of laser micro processing of metals by using a high repetition rate femto second laser. On stainless steel (AISI 304), copper and aluminium the impact of the significant laser processing parameters onto the machining process was investigated, such as laser fluence, repetition rate, lateral pulse distance and polarisation. The machining results were evaluated by the ablation rate, surface roughness, process efficiency, material removal rate and the wall-angle. For complementary discussions the experimental data were compared with results achieved in theoretical analysis. Outgoing from the results appropriate laser processing parameters were derived in order to optimise the machining process. With the application of ultra short laser pulses high-quality machining results with a minimal thermal load and a roughness Ra of the laser processed surface of only some hundreds nano meter were obtained. On other hand high machining throughputs were achieved due to application of high repetition rates. Finally, the possibilities and the limits of the high repetition rate femto second laser technology in laser micro processing are demonstrated by means of three-dimensional micro structured machining examples.The paper presents a study of laser micro processing of metals by using a high repetition rate femto second laser. On stainless steel (AISI 304), copper and aluminium the impact of the significant laser processing parameters onto the machining process was investigated, such as laser fluence, repetition rate, lateral pulse distance and polarisation. The machining results were evaluated by the ablation rate, surface roughness, process efficiency, material removal rate and the wall-angle. For complementary discussions the experimental data were compared with results achieved in theoretical analysis. Outgoing from the results appropriate laser processing parameters were derived in order to optimise the machining process. With the application of ultra short laser pulses high-quality machining results with a minimal thermal load and a roughness Ra of the laser processed surface of only some hundreds nano meter were obtained. On other hand high machining throughputs were achieved due to application of high repet...

High-rate laser microprocessing using a polygon scanner system

This paper discusses results obtained in high-rate laser microprocessing by using a high average power high-pulse repetition frequency ultrashort pulse laser source in combination with an in-house developed polygon scanner system. With the recent development of ultrashort pulse laser systems supplying high average power of hundreds watts and megahertz pulse repetition rates, a significant increase of the productivity can potentially be achieved in micromachining. This permits upscaling of the ablation rates and large-area processing, gaining increased interest of the ultrashort pulse laser technology for a large variety of industrial processes. However, effective implementation of high average power lasers in microprocessing requires fast deflection of the laser beam. For this, high-rate laser processing by using polygon scanner systems provide a sustainable technological solution. In this study, a picosecond laser system with a maximum average power of 100 W and a repetition rate up to 20 MHz was used. I...

High repetition rate femtosecond laser processing of metals

Previously, in high repetition rate femto second laser processing novel laser matter interacting effects were reported, such as heat accumulation and particle shielding. In this study, high repetition rate laser processing was investigated to discuss and understand the impact of laser repetition rate and accompanied accumulative laser material interacting effects. Therefore, a high repetition rate femto second fibre laser setup joint together with galvo scanner technology was applied in laser micro machining of metals (copper, stainless steel, aluminium). High repetition rate laser processing of aluminium and stainless steel lead to considerably lowered ablation thresholds accompanied with higher ablation rates. Laser ablation behaviour of copper was almost independent of the repetition rate with neither considerable lower ablation thresholds nor higher ablation rates. For explanation, heat accumulation caused by higher repetition rates were assumed as mainly ablation behaviour influencing effect, but thermal material properties have to be considered. Furthermore laser machining examples demonstrate the possibilities and limits of high repetition rate laser processing in 3d micro structuring. Thus, by using innovative scanning systems and machining strategies very short processing times were achieved, which lead to high machining throughputs and attract interest of the innovative laser technology in Rapid Micro Tooling. For discussion, high repetition rate processing results are evaluated by means of comparative machining examples obtained with 1 kHz femto second laser system.

Characterisation of interaction phenomena in high repetition rate femtosecond laser ablation of metals

The paper discusses results obtained in ultrashort pulse laser irradiation of metals in order to characterise interaction phenomena occurring in highly repetitive laser processing, such as heat accumulation and particle shielding. The impact of the temporal pulse-to-pulse distance on the ablation process was investigated using repetition rates ranging between 25.8 kHz and 2.05 MHz. Interacting effects were studied by means of industrial grade metal sheets with various thermo-physical characteristics. The experimental results obtained were evaluated by theoretical calculations of both the ablation rate and surface temperature. Furthermore ultra high speed camera images were taken into discussion.Ablation rates obtained empirically for stainless steel and aluminium indicate increasing material removal at higher repetition rates and, hence, heat accumulation is proven as influencing effect. Thus in case of stainless steel and shorter pulse-to-pulse distances, temperature calculation yields the rise of the surface temperature. Additionally, ultra high speed camera images give evidence of more voluminous ablation plumes at shorter pulse-to-pulse distances, induced by intense laser matter interaction.In contrast, for copper only a marginal impact of the repetition rate on the material removal was found. Thus for highly heat-conductive materials the ablation rate is assumed almost independent from the temporal pulse-to-pulse distance. Even high speed camera images show minor impact of the repetition rate on the ablation process.Finally the application of the laser micro machining technology in micro-mould manufacturing is presented. As a result micro-featured plastic demonstrators were produced by micro injection moulding, offering a wide range of sensor applications, for example in microfluidic systems.The paper discusses results obtained in ultrashort pulse laser irradiation of metals in order to characterise interaction phenomena occurring in highly repetitive laser processing, such as heat accumulation and particle shielding. The impact of the temporal pulse-to-pulse distance on the ablation process was investigated using repetition rates ranging between 25.8 kHz and 2.05 MHz. Interacting effects were studied by means of industrial grade metal sheets with various thermo-physical characteristics. The experimental results obtained were evaluated by theoretical calculations of both the ablation rate and surface temperature. Furthermore ultra high speed camera images were taken into discussion.Ablation rates obtained empirically for stainless steel and aluminium indicate increasing material removal at higher repetition rates and, hence, heat accumulation is proven as influencing effect. Thus in case of stainless steel and shorter pulse-to-pulse distances, temperature calculation yields the rise of the su...

Laser microsintering of tungsten in vacuum

Laser microsintering of tungsten powder is investigated as a function of laser output power, pulse interval and vacuum level. The intensities are calculated for the evaporation thresholds of tungsten powder particles of various sizes. In addition, the powder layer generation and the resulting layer thicknesses are calculated. The powder abrasion occurring during the process was taken into consideration. Polished sections and REM images were prepared in order to analyse the experimental outcomes. The dependence of sinter density on the parameters is discussed.

High-pulse repetition frequency ultrashort pulse laser processing of copper

This paper presents results obtained in high-pulse repetition frequency ultrashort pulse laser microprocessing of copper. In the study, a variety of ultrashort pulse laser systems supplying high average laser power were applied in order to investigate the influence of the laser parameters on copper ablation. For this, laser pulses of different wavelengths (515 nm, 1030 nm) and pulse durations, ranging between 200 fs and 10 ps, were irradiated to the sample surface by raster scanning of the laser beam. The dependencies of average laser power, pulse energy, and the pulse repetition rate on the ablation rate, the ablation efficiency, and the productivity were studied. A maximum average laser power of 31.7 W was applied in this work. The pulse repetition rate was varied in the rage between 0.2 and 19.3 MHz. Finally, the machining qualities obtained were evaluated by means of surface roughness measurements and scanning electron microscope micrograph analysis.

Femtosecond laser induced refractive index structures in polymer optical fibre (POF) for sensing

Techniques to directly write localised refractive index structures in polymer optical fibres (POF) are presented, using UV (400nm) ultrafast laser with pulse lengths of 100 fs to create in-fibre gratings for sensing. No doping is necessary for photosensitisation so commercially available POF is used. An in-fibre grating consisting of a 1.8 μm wide refractive index structure with a periodicity of 189 nm was demonstrated in single mode polymer fibre with optimised laser processing parameters.

Quality of remote cutting

Remote cutting is stated to be a very fast laser cutting process for the cutting of thin materials. As the main characteristics of remote cutting have been studied only in past couple years, the quality of the cut is still not well compared and specified against other processes. Process parameters of remote cutting differ from the traditional laser cutting because of the multi-pass and vaporizing characteristics of the process. Process parameters and performance also vary depending on the used equipment and processed material. Parameters have to be optimized for each set up to achieve the best cut quality.Quality of laser cut kerf can be measured in various ways. In this study, various metals were processed using remote cutting. The quality of the cut kerfs were measured by burr height, kerf width, perpendicularity tolerance, surface roughness, 90 degree corner sharpness, and heat affected zone. The quality of the cut kerf is also compared to those of conventional CO2 laser cutting and remote fusion cutting with multimode fiber laser. When possible the comparison was carried out according to valid standard of thermal cutting.Remote cutting is stated to be a very fast laser cutting process for the cutting of thin materials. As the main characteristics of remote cutting have been studied only in past couple years, the quality of the cut is still not well compared and specified against other processes. Process parameters of remote cutting differ from the traditional laser cutting because of the multi-pass and vaporizing characteristics of the process. Process parameters and performance also vary depending on the used equipment and processed material. Parameters have to be optimized for each set up to achieve the best cut quality.Quality of laser cut kerf can be measured in various ways. In this study, various metals were processed using remote cutting. The quality of the cut kerfs were measured by burr height, kerf width, perpendicularity tolerance, surface roughness, 90 degree corner sharpness, and heat affected zone. The quality of the cut kerf is also compared to those of conventional CO2 laser cutting and remote fusion cutti...

High repetition frequency micro hole drilling of metal foils using ultrashort pulse laser radiation

This paper discusses latest results obtained in high-pulse repetition frequency micro hole percussion drilling of metal foils ranging between 25 μm and 100 μm thickness. In the investigations a high-repetition frequency, high average power ultrashort pulse laser source was applied, providing a near-infrared laser beam of 1.03 μm wavelength, 87.5 W maximum average laser power (at the processing plane), 51.48 MHz maximum pulse repetition frequency, 650 fs pulse duration, and 30 μm focal spot size. In the experiments, the process parameters pulse energy, pulse repetition frequency, and the pulse number were varied thus in order to evaluate their influence on both micro hole geometry (hole diameter, roundness), and drilling quality (thermal modification, melting residues). The minimum pulse number required to drill through the metal foils was detected by an in-house developed pulse counter. A significant lower pulse number to drill through the material was observed when irradiating ultrashort pulses of megahertz pulse repetition rates. On the one hand, this effect might be induced by thermal accumulation thus increasing the substrate temperature which, in turn, enhances the laser beam absorption or rather lowers the energy consumption needed for evaporation. On the other hand, pulse interactions with the still existing plasma at these very short pulse intervals cannot be neglected, so a higher amount of material might be ejected assisted by the higher plasma pressure. As another detrimental effect for megahertz pulses, a significantly lower processing quality was obtained that was due to intensive melting affected by heat accumulation, as the melt re-solidified within and/or surrounding the drilling holes. Surprisingly, by irradiating pulses at 20 MHz and above, the melt was completely ejected out from the drilling holes as the drilling efficiency was even the highest. Finally, based on the findings of this study, optimum parameter settings are presented with regard to highest machining quality and drilling throughput.

Laser micro processing using a high repetition rate femto second laser

The paper presents an overview about high repetition rate femto second laser machining and its applications. Novel phenomena in laser matter interaction with the repetition rate as one of the mainly influencing parameter are discussed. Depending on temporal distances between consecutive femto second laser pulses, either heat accumulation or particle shielding effects were detected. Heat accumulation enhanced the ablation behavior and higher ablation rates were found. On other hand energy losses due to particle shielding caused lowered ablation rates. Furthermore laser induced periodical surface structures, such as ripple formations and conical micro structures, were observed due to the high average laser energy input and accumulative effects. Formation und shape properties of these structures depend on laser processing parameter.Joining high repetition rate laser technology together with high speed beam deflection systems, significantly higher ablation rates and short processing times were achieved. Exemplarily demonstrated in 3D micro structuring, processing times were reduced more than 40 times compared to fs laser processing using repetition rates of some kHz. In consequence, prospectively high machining throughputs were reached, which attract increasing interest of the novel technology in industrial applications.The paper presents an overview about high repetition rate femto second laser machining and its applications. Novel phenomena in laser matter interaction with the repetition rate as one of the mainly influencing parameter are discussed. Depending on temporal distances between consecutive femto second laser pulses, either heat accumulation or particle shielding effects were detected. Heat accumulation enhanced the ablation behavior and higher ablation rates were found. On other hand energy losses due to particle shielding caused lowered ablation rates. Furthermore laser induced periodical surface structures, such as ripple formations and conical micro structures, were observed due to the high average laser energy input and accumulative effects. Formation und shape properties of these structures depend on laser processing parameter.Joining high repetition rate laser technology together with high speed beam deflection systems, significantly higher ablation rates and short processing times were achieved. Exemp...