High power ultrashort pulsed lasers are an ultimate manufacturing tool for a large variety of materials and provide outstanding properties for high precision manufacturing with almost no thermal ...effects and numerous new processing possibilities. However, using high power ultrashort pulsed lasers with high pulse repetition frequencies in the MHz region can cause thermal issues like overheating, melt generation and low ablation quality. High ablation quality only can be achieved, if all process parameters are carefully matched, which requires a deep understanding of the process, intensive simulation for selecting processing strategies and innovative system components. Beside ultra high speed scanning using polygon scanners the use of multiple laser beams provide the best and most versatile high power ablation solution. With switchable single beams using parallel acoustooptic and phase modulating beam steering systems together with diffractive optical beam splitter, high ablation rates can be achieved while maintaining the high processing quality. However, using multiple laser beams each single beam can influence the adjacent beam either by heat accumulation or by plasma and vapour emission. Distance of the single beams, pulse repetition frequency and scanning strategy have to be matched to the material and ablation geometry. With a careful adaption of all parameters highly accurate and fast processing can be achieved. With this approach a next step up to an all optical manufacturing system can be provided. DOI: 10.2961/jlmn.2019.02.0003 Keywords: ultrafast laser processing, multi beam processing, diffractive optical elements, beam splitting, high speed scanning, laser ablation, laser drilling, simulation
The North Sea and its coastal zones are heavily impacted by anthropogenic activities, which has resulted in significant chemical pollution ever since the beginning of the industrialization in Europe ...during the 19th century. In order to assess the chemical Anthropocene, natural archives, such as sediment cores, can serve as a valuable data source to reconstruct historical emission trends and to verify the effectiveness of changing environmental legislation. In this study, we investigated 90 contaminants covering inorganic and organic pollutant groups analyzed in a set of sediment cores taken in the North Seas' main sedimentation area (Skagerrak). We thereby develop a chemical pollution fingerprint that records the constant input of pollutants over time and illustrates their continued great relevance for the present. Additionally, samples were radiometrically dated and PAH and PCB levels in porewater were determined using equilibrium passive sampling. Furthermore, we elucidated the origin of lead (Pb) contamination utilizing non-traditional stable isotopic analysis. Our results reveal three main findings: 1. for all organic contaminant groups covered (PAHs, OCPs, PCBs, PBDEs and PFASs) as well as the elements lead (Pb) and titanium (Ti), determined concentrations decreased towards more recent deposited sediment. These decreasing trends could be linked to the time of introductions of restrictions and bans and therefor our results confirm, amongst possible other factors, the effectiveness of environmental legislation by revealing a successive change in contamination levels over the decades. 2. concentration trends for ΣPAH and ΣPCB measured in porewater correspond well with the ones found in sediment which suggests that this method can be a useful expansion to traditional bulk sediment analysis to determine the biologically available pollutant fraction. 3. Arsenic (As) concentrations were higher in younger sediment layers, potentially caused by emissions of corroded warfare material disposed in the study area after WW II.
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•Comprehensive pollution fingerprint for the North Sea covering the last 100 years.•Quantification of 90 contaminants in sediment cores covering 6 contaminant groups.•Concentration declines of pollutants could be linked to restrictions and bans.•As levels observed in recent years were linked to discarded ammunition from WW II.•Concentration trends in sediment and porewater were comparable for PAHs and PCBs.
We report on investigations of the energy deposition in the volume of thin glass during an ablation cutting process with pulsed femtosecond laser radiation by time-resolved pump-probe shadowgraphy. ...For a single laser pulse, the temporal evolution of the transient electronic excitation of the glass volume is imaged up to 10 ps after initial excitation. For an increasing number of laser pulses, the spatial excitation of the glass volume significantly changes compared to single pulse irradiation. Sharp spikes are observed, which reduce the transmission of the illuminating probe pulse. This indicates local maxima of the absorption and, therefore, energy deposition of the pump pulse energy in the glass volume. Furthermore, for an increasing number of pulses, different shapes of the surface ablation crater are observed. To study the correlation between the shape of the surface ablation crater and the energy deposition in the glass volume, simulations of the spatial intensity distribution of the pump pulse are executed by means of linear beam propagation method. We show that the transient excitation spikes observed by pump-probe shadowgraphy can be explained by refraction and diffraction of the laser radiation at the surface ablation crater. Our results provide an experimental validation for the physical reason of an ablation stop for an ablation cutting process. Moreover, the simulations allow for the prediction of damage inside the glass volume.
We report on the dependence of a laser radiation induced ablation process of graphite on the applied pulse duration of ultrashort pulsed laser radiation smaller than 4 ps. The emerging so-called ...non-thermal ablation process of graphite has been confirmed to be capable to physically separate ultrathin graphitic layers from the surface of pristine graphite bulk crystal. This allows the deposition of ablated graphitic flakes on a substrate in the vicinity of the target. The observed ablation threshold determined at different pulse durations shows a modulation, which we ascribe to lattice motions along the
c
axis that are theoretically predicted to induce the non-thermal ablation process. In a simple approach, the ablation threshold can be described as a function of the energy penetration depth and the absorption of the applied ultrashort pulsed laser radiation. Based on the analysis of the pulse duration dependence of those two determining factors and the assumption of an invariant ablation process, we are able to reproduce the pulse duration dependence of the ablation threshold. Furthermore, the observed pulse duration dependences confirm the assumption of a fast material specific response of graphite target subsequent to optical excitation within the first 2 ps.
Laser ablation using ultra-short pulsed laser radiation with pulse durations below 10ps is known to be capable of high-precision machining with very small thermal load for the processed workpiece. ...Although the processing quality is excellent, the productivity is too small for many applications. Therefore, an upscaling of the achievable ablation rate is one major research topic in the field of ultra-short pulse laser ablation. In this study, we identify and investigate a novel high-speed ablation process for the ablation of Inconel 718 using ultra-short pulsed laser radiation. The formation of a thin layer of molten material avoids the formation of rough microstructures on the workpiece surface that usually appear for the processing with high average powers and high spatial pulse overlaps. Thus, processing with high average power of more than 50W using conventional, high efficient and high flexible beam deflection technology becomes applicable. The presented process is approximately 20 times faster than conventional ultra-short pulse laser ablation with average powers of less than 5W.
The influence of the repetition rate, average power, pulse duration and hatch distance on the high-speed ablation process is investigated. The potential of an optional finishing process to remove the molten layer by melt-free ablation with the same laser system but different processing parameters is demonstrated.
The present white paper is submitted as part of the “Snowmass” process to help inform the long-term plans of the United States Department of Energy and the National Science Foundation for high-energy ...physics. It summarizes the science questions driving the Ultra-High-Energy Cosmic-Ray (UHECR) community and provides recommendations on the strategy to answer them in the next two decades.
The present white paper is submitted as part of the "Snowmass" process to help inform the long-term plans of the United States Department of Energy and the National Science Foundation for high-energy ...physics. Further, it summarizes the science questions driving the Ultra-High-Energy Cosmic-Ray (UHECR) community and provides recommendations on the strategy to answer them in the next two decades.
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