ANALYSIS: ULTRASHORT PULSED LASERS


Ultrashort pulse lasers: becoming a fully digitised production technology


Dr Arnold Gillner, of Fraunhofer ILT, discusses the growing application range of ultrashort pulse lasers in industrial manufacturing


Ultrashort pulse lasers present a class of laser beam sources for industrial applications that offer outstanding features with respect to intensity, photon density and pulse duration. Operating in either the picosecond or


femtosecond domain, the energy from USP lasers can be concentrated into a material with ultra-high precision in the range of a few hundred nanometres during processing. Moreover, in ablation applications, due to the absorbed energy being removed via the ablated material, in addition to the very low thermal penetration that ultrashort pulses offer, thermal damage can be avoided when processing materials using USP lasers.


From micro to macro Due to the power scaling developments currently ongoing for USP lasers in the kilowatt range – for example the Fraunhofer cluster of excellence ‘Advanced Photon Sources’ aims develop a generation of USP lasers with average powers ranging up to 20kW over the next three years – it is possible that these benefits could be brought to macro processing applications in the future. This would open up large markets for the technology outside the micro processing field, where many USP lasers are currently used. These macro processing applications


could include using high-power USP lasers to process fibre-reinforced composite materials at large scales without thermal influence, enabling large surfaces to be provided with microstructures that minimise friction.


One such material is carbon fibre, a high- strength composite playing an increasingly


34 LASER SYSTEMS EUROPE SUMMER 2019 Pyramidal structure created in tool steel using a picosecond laser @LASERSYSTEMSMAG | WWW.LASERSYSTEMSEUROPE.COM @researchinfo | www.researchinformation.info


important role in the automotive and aerospace industries. While processing carbon fibre using mechanical means leads to rapid tool wear, due to the high strength of the material, this is not an issue when using USP lasers, as they are contact- and therefore wear-free. Under optimum conditions, however,


USP lasers enable the almost non-thermal processing of carbon fibre, since despite the high thermal conductivity of the material, thermal input is minimised by the short duration of the pulses. This enables the resin matrix to remain intact and the structure of the processed part to be maintained. As an example, using a pulse duration of 15ps, pulse energy of 30µJ and a pulse repetition rate of 1MHz, a volume ablation rate of 80mm³/min can be currently achieved when processing carbon fibre, with the heat-affected zone being almost zero. Even higher ablation rates over larger areas can be expected in the future, with the increases in power that are coming. Other macro applications for USP lasers


can be found in the generation of functional surfaces. High-intensity and non-linear absorption effects can be achieved when using USP lasers, causing self-organising processes to occur in ablation that lead to micro and nanoscale structures being produced in materials. These ultra-small structures can be used to establish superhydrophobicity or micro optical functionalities on a surface. These surfaces can be used to join polymer components to metal parts in lightweight design in the automotive industry.


Wielding higher power Using increasingly higher-power USP lasers with high repetition rates in the megahertz region could cause thermal issues, such as overheating, melt production and low ablation quality if certain parameter sets and fluence ranges aren’t considered. High ablation quality can only be achieved when the processing fluence is close to the ablation threshold, which, going forward will require new processing strategies and innovative system components. As recently discussed at the fifth Ultra


Short Pulsed Laser Workshop at the Fraunhofer Institute for Laser Technology in Aachen, in order to use high-power USP lasers, the most important developments that need to take place have to be carried out in the field of system technology. Controlled laser fluence on the surface of the workpiece, fast beam manipulation and modulation, as well as flexible beam design, are all necessary to take advantage of the new developments of USP lasers. Besides using polygon scanners to


achieve ultra-high speed scanning, multiple laser beams provide the best and most versatile high-power ablation solution. With switchable single beams out of a spacial light modulator or a diffractive optical beam splitter, high ablation rates can be achieved, while maintaining the high processing quality of USP laser ablation. With this approach,


Fraunhofer ILT


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