Picosecond-pulsed laser cutting of lithium metal substrates for post-lithium-ion battery production
Johannes Kriegler, Lucas Hille, Michael F. Zaeh

The battery industry is targeting the usage of lithium metal anodes in next-generation battery types due to their more than tenfold energy density compared to the currently dominating graphite anodes. This work explores ultrashort-pulsed laser cutting for shaping lithium metal foils to defined anode geometries. An encapsulated process environment, including a picosecond laser system, was developed, allowing the highly reactive lithium metal to be processed in an argon atmosphere. Process studies were performed, focusing on characterizing the process behavior for laser cutting of freestanding lithium metal foils and lithium metal / copper double-layers. The influence of the pulse fluence, the pulse repetition rate, and the pulse overlap on the cut edge quality was evaluated using confocal laser scanning microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results support the design of laser cutting processes for post-lithium-ion battery production on an academic and industrial scale by comparing various processing strategies.

Keywords: laser cutting; picosecond; battery production; lithium metal

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Influence of beam oscillation on the melt flow during laser beam fusion cutting and the resulting cut quality
Michael Sawannia, Madlen Borkmann, Patrick Herwig, Andreas Wetzig, Christian Hagenlocher, Thomas Graf

Laser fusion cutting was performed of 10 mm stainless steel (AISI 304) at a power of 8 kW and a feed rate of 4.5m/min, with and without beam oscillation. To analyze the influence of the beam oscillation on the melt flow, the 3D-geometry of the cutting front was determined with a polarization goniometer at a framerate of 75000 Hz and a local resolution < 50μm. Additionally, the melt ejection was recorded with a second camera horizontal to the sample at a frame rate of 8000 Hz. During beam oscillation with a frequency of 600 Hz and an amplitude of 0.15 mm a big step is visible on the cutting front, which probably redirects the gas and hence the melt flow. The angle of the big step matches excellent to the striation pattern in the lower half of the surface of the cut edges. Additionally, the melt flow behavior inside the kerf can be estimated, considering the periodic burst like melt ejection at the bottom.

Keywords: laser beam cutting; beam oscillation; cutting front; 3D-measurement; melt flow

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Process monitoring during ultrafast laser processing by means of soft X-ray emission
Julian Holland, Rudolf Weber, Christian Hagenlocher, Thomas Graf

Soft X-ray emission is a well-established phenomenon that occurs during laser material processing with ultrafast lasers at high irradiances. The emitted radiation spectrum provides details about the irradiance at the interaction zone of the laser beam and the processed material, making it a valuable beam diagnostic tool. However, quick measurements of the X-ray emission are necessary to make use of the spectral information. Unfortunately, these measurements are frequently hindered by energy pile-up because of the short emission times and high photon fluxes that occur during each laser pulse.
A spectrometer was employed to measure the spectral X-ray emission produced by laser processing at irradiances of up to 1.6.1014 W/cm² and a de-piling algorithm was used to derive the underlying pile-up free spectrum. These spectra were obtained at various focal positions to determine the corresponding local irradiance and thereby analyze the focusing properties of the laser beam. Utilizing the spectral X-ray emission allows for the measurement of beam properties at the highest average power and pulse energy attainable with modern laser systems.

Keywords: X-ray; ultrafast laser; diagnostics; spectroscopy; process-control

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High precision drilling with aspect ratios of 1:40: From laser source to application
Florian Lendner, Roswitha Giedl-Wagner, Steffen Rübling, Marc Sailer, Axel Fehrenbacher

In laser drilling, one challenge is to achieve a high drilling quality in high aspect ratio drilling. Ultra-short pulsed lasers use different concepts like thin disks, fibers and rods. The slab technology is implemented because of their flexibility and characteristics. They bring together both advantages and deliver high pulse energies at high repetition rates. Materials with a thickness > 1.5 mm demand specialized optics handling the high power and pulse energies with adapted processing strategies, integrated in a machine setup. In this contribution, we focus on all the necessary components and strategies for drilling high precision holes with aspect ratios up to 1:40.

Keywords: Ultrafast micro drilling; industrial lasers; temporal energy deposition; burst optimization

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Holographic tuning of physical axicons
Jan Marx, Cemal Esen, Christian Lutz, Ralf Hellmann, Andreas Ostendorf

Axicon generated Bessel beams are a popular tool for high aspect ratio precision laser drilling. Spot diameter and working distance are given by the geometric parameters of the axicon and the wavelength used. Thus, it is difficult to manipulate the beam shape of a Bessel beam for a given setup. Spatial light modulators (SLMs) overcome limitations in flexibility. However, due to the limited phase shift of SLMs, only Bessel beams with flat cone angles and large focal length can be generated. In this contribution, an approach for generating Bessel beams with a shorter, but tunable focal length is presented. A physical axicon was combined with an SLM. A holographic image of a negative axicon is put on the SLM to generate a ring beam, which is focused by a subsequent physical axicon to get a small focal diameter. Thus, different sized high aspect ratio micro holes can be drilled without using any moving components.

Keywords: Bessel beam; micro drilling; axicon; spatial light modulator; beam shaping

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Effect of femtosecond pulsed laser micromachining parameters on acoustic emission process monitoring signals
Kerim Yildirim, Joselito Yam Alcaraz II, Balasubramanian Nagarajan, Tegoeh Tjahjowidodo, Sylvie Castagne

Micromachining with a ultrashort pulse (USP) laser is a widely-employed manufacturing process for numerous applications such as biomedical devices, micro-components and electronics. During USP ablation, shock waves are associated with the ablation mechanism. Acoustic emission (AE) monitoring is a non-destructive approach for monitoring shock wave behavior during laser micromachining. The produced AE signals are significantly influenced by USP laser parameters such as pulse energy, pulse repetition rate and number of pulses. Using time- and frequency-based analysis techniques, the correlation between these laser characteristics and the AE signals generated by the femtosecond laser pulses are investigated. The results indicate that AE signal intensity increases as pulse energy and number of pulses increase, indicating a greater degree of material removal. These findings can be utilized to optimize AE-based monitoring methodology for laser micromachining conditions for specific materials and applications.

Keywords: Ultra-short pulsed laser; femtosecond laser micromachining; real-time monitoring; acoustic emission sensing

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Femtosecond laser ablation of titanium foils in liquid environments
Christian R. Günther, Philipp L. Maack, Jan S. Hoppius, Cemal Esen, Andreas Ostendorf

The increased demand of high-tech components with smaller structures and advanced materials is pushing ultrashort laser ablation processes to their limits. Thermal stress and debris are typical defects when exceeding stable process parameters. A key to overcome these limitations will be the transfer of the process from gaseous to liquid environments.
The much more efficient heat transfer reduces the heat affected zone and recondensation of ablated matter on the material’s surface is prevented. As a result, a clean surface is achieved, and even brittle materials can be processed with a high energy input. In this study, the ablation rate, surface roughness and surface oxidation for titanium foils in both liquid and gaseous environment, are presented and compared respectively. The evaluation is based on scanning electron microscopy, laser scanning microscopy, micro-Raman spectroscopy and energy-dispersive X-ray spectroscopy.

Keywords: laser ablation; ablation rate; surface roughness; Raman spectroscopy; EDX

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Plasma-assisted laser cutting of stainless steel: An analysis of a first prototypical setup
Franz Urlau, Achim Mahrle, Sebastian Manzke, Moritz Krümmer, Christoph Leyens, Uwe Füssel

A new concept of solid-state laser fusion cutting is presented. The suggested approach applies a non-transferred hot argon plasma jet as auxiliary gas instead of the commonly used cold high-pressure nitrogen jet. Cutting experiments on 6 mm thick AISI 304 stainless steel samples were performed for both variants. Based on a two-level factorial design, the influence of geometrical and process parameters on the cutting performance, the kerf geometry and the cut edge quality was analyzed. With the hot plasma jet, the delivery pressure and the nozzle diameter of the auxiliary gas supply could be drastically reduced while maintaining a good cut edge quality compared to cutting with the cold high-pressure gas jet. As a result, a more than tenfold decrease in gas consumption was achieved.

Keywords: fibre laser cutting; plasma-assisted laser cutting; plasma jet; stainless steel

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Laser drilling of high-density micro-holes on metals using a 1064nm nanosecond pulsed fibre laser
Themistoklis Karkantonis, Etienne Pelletier, Toby Barnard, Simon Tuohy, Dimitris Karnakis

High-precision laser drilled micro-holes are ubiquitously found in industrial manufacturing, e.g. fuel-injectors, mesh filters, micro-dies, with a pressing need to cost-effectively scale up their production. For lasers to displace other established manufacturing technologies, high production throughput and yield are necessary which require the use of high-average-power lasers. Nanosecond pulsed fibre lasers offer an attractive choice to meet this challenge as they provide high-average-power without requiring capital intensive purchase or operation. However, unwanted thermal effects accompany their use, e.g. residual substrate heating, hole crater dross, hindering their wider adoption in high-precision applications. Here, we study high-density 1064nm fibre laser percussion drilling of 300 μm thick stainless-steel (exit hole diameters of 27 – 43 μm, pitch distance range 200 – 500 μm) by utilising the otherwise unwanted laser heat accumulation to improve drill speeds. Effective beam scanning strategies are coupled with thermal imaging to minimise any dimensional hole variations from residual heating, whilst aiming to monetise hole quality gains from progressively lower laser-induced temperature gradients in the gradually heated substrate. The results demonstrate the effectiveness of the proposed method in counteracting heat-induced side effects and consequently improving the overall laser drilling performance.

Keywords: Nanosecond fibre lasers; high-density holes; percussion laser drilling; heat accumulation

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Laser fusion cutting of ultra-thin glass (UTG) using a profile-controlled beam for residual stress reduction
Sho Itoh, Naoto Nagano, Souta Matsusaka, Hirofumi Hidai

We have developed a method to cut ultra-thin glass (UTG) via laser fusion cutting approach. This study presents a method to reduce residual thermal stress. A focused CO2 laser beam spot melted glass, and then the locally molten area was blown away via assisting air. Two laser beams, one circular and the other elliptical, were superposed to control the beam profile and illuminated. Photoelastic analysis and the sample fracture pattern proved that this method could reduce the residual thermal stress in glass caused by laser fusion cutting.

Keywords: Laser fusion cutting; multi-laser beam; flexible glass; ultra-thin glass (UTG); residual thermal stress

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High speed remote cutting of ultra-thin aluminum foils
Alessandro Ascari, Erica Liverani, Caterina Angeloni, Michele Francioso, Alessandro Fortunato

High speed on-the-fly remote cutting by means of a galvo scanner of ultra-thin (12 μm) aluminum and copper foils has become very attractive in many fields, such as production of batteries for e-mobility applications and packaging in general. The present paper investigates performance and limitations of a commercial galvo scanner in cutting 12 μm thick aluminum foils with a high brilliance single mode 1.2 kW laser source. The main goal is to achieve a good quality cut at a speed of at least 10 m/s guaranteeing straightness of the edges and sharpness of the corners. According to this, the role of dynamic performance of the scanner is investigated, with particular attention to the effect of accelerations and decelerations on the shape of the kerf. The results are characterized by means of optical microscopy in order to assess the overall cutting quality.

Keywords: Laser remote cutting; Batteries; E.mobility; High-speed cutting; Thin foils; Aluminum

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UV nanosecond laser pulse-on-demand operation for high-throughput microstructuring
Jaka Mur, Julien Didierjean, Jernej Jan Kočica, Arnaud Guillossou, Julien Saby, Jaka Petelin, Alexandra Bourtereau, Girolamo Mincuzzi, Rok Petkovšek

Industrial UV nanosecond lasers represent a relatively low cost and effective tool for machining a wide variety of materials, from metals to polymers, glass, and transparent dielectrics. Proven industrial applications sparked the ongoing trend to improve the laser stability, reduce the costs, and increase the throughput. The maximal scanner throughput can be achieved by controlling the laser pulse emission through the so-called pulse-on-demand (POD) laser operation. We present a novel POD seeding module jointly utilized with a high-power MHz-level rod-type UV nanosecond laser for microstructuring applications. We studied the effects of scanning speed on POD operation, using state-of-the-art galvanometric scanners, reaching 20 m/s scanning speeds with a 10 μm diameter laser beam. We analysed single- and multi-pass laser percussion drilling performance of metal and polymer materials to assess the setup accuracy and precision, surface structuring of ITO-glass material, and ablation efficiency for pulse durations in range of 1-4 ns.

Keywords: pulse-on-demand; nanosecond pulses; UV laser; microstructuring

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Investigating cut quality degradation due to heat accumulation during the laser cutting of high thickness mild steel plates
M. Busatto, L. Caprio, M. Cazzador, M. Vanin, B. Previtali

Cut quality degradation due to heat accumulation is a phenomenon which can significantly hinder the performance and output of laser cutting systems. As the workpiece temperature increases, boundary conditions to the process are modified and the cut quality can be significantly affected. In the present investigation, the degradation of the cut quality is investigated through a methodological framework to relate it to the workpiece temperature, thus identifying critical conditions for the cutting of mild steel sheets of different thicknesses. An industrial 6 kW fiber laser cutting system is employed both to pre-heat under controlled conditions and cut the mild steel material. In-line measurement of the workpiece surface is achieved by means of a calibrated MWIR thermal camera whilst roughness measurements of the cut profile allowed to characterize the quality in accordance with ISO 9013. Finally, different mitigating strategies are presented to avoid or compensate defect formation.

Keywords: Laser cutting; Heat accumulation; Process development

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Current status of laser electrode structuring for enhanced lithium-ion batteries
Lucas Hille, Philipp Senft, Johannes Kriegler, Michael F. Zaeh

The trade-off between a high fast-charging capability and a high energy density is a central challenge for lithium-ion batteries. The conflict of objectives can be significantly alleviated by introducing microscopic channels into the electrode coatings facilitating lithium-ion diffusion through the porous electrode network. Short-pulsed and ultrashort-pulsed laser radiation are versatile tools to structure electrodes with micrometer precision at a low heat input. Hence, laser structuring of lithium-ion battery electrodes has seen widespread research attention recently. This publication examines current advances and challenges in the field through a comprehensive literature analysis. The state of research is clustered regarding processed electrode materials, created structure geometries, and laser material processing aspects, such as beam sources, process parameters, and process strategies. Further, laser structuring of graphite anodes is discussed in detail. Finally, future research directions are pointed out, especially regarding the industrialization of laser electrode structuring.

Keywords: Laser structuring; Electrode structuring; Lithium-ion batteries; Short-pulsed laser; Ultrashort-pulsed laser

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The progress of microhole formation during laser percussion drilling of steel observed by highspeed synchrotron x-ray imaging
Manuel Henn, Christian Hagenlocher, Daniel Holder, Daniel J. Förster, Rudolf Weber, Dennis Haasler, Marc Hummel, Alexander Olowinsky, Felix Beckmann, Julian Moosmann, Thomas Graf

The formation of microholes during the percussion drilling of metals using ultrashort laser pulses remains poorly understood. The quality of a microhole can be affected by heat accumulation, the development of side channels, and microhole bulging due to plasma formation. Highspeed synchrotron X-ray imaging was applied to capture the space- and time-resolved evolution of the microhole shape during laser percussion drilling of stainless steel. The recorded images show that heat accumulation leads to the formation of a melt film on microhole walls, which dynamically fluctuates during drilling. Additionally, the formation of side channels was observed in the region of the maximum drilling depth where the overall fluence on the microhole walls drops below the threshold fluence. The experimentally captured quantities agree well with existing models and will significantly improve their prediction accuracy.

Keywords: Microholes, Percussion drilling, Ultrashort laser pulses, Highspeed synchrotron X-ray imaging

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Defects formation in ultrafast laser machined glass
Jan Novotný, Libor Mrňa

Ultrafast lasers are a versatile tool for processing all materials, but especially very hard and brittle ones due to the non-contact nature of the process. Forming free-form structures, such as those needed for micro-optics or microfluidics is still a challenging task, where ultrafast lasers can be exploited conveniently. Although only minimal heat and mechanical stress are exerted during processing, defects that hinder intended use can still develop. Limiting these defects through using the right settings leads to faster and simpler finishing of fabricated parts. We present the results of using ≈ 1ps 1030nm laser to produce structures in fused silica. Formation of cracks, recast material, and excessive roughness is studied to set appropriate parameters of the ablation process.

Keywords: Ultrafast laser; laser ablation; glass, defects

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Laser micromachining of biomimetic structures with ultrashort laser pulses for passive transport of lubricants in milling tools
Kathrin Placzek, Daniel Holder, Oliver Schwarz, Christian Hagenlocher; Rudolf Weber; Thomas Graf

During the milling of metals, a minimum quantity of lubricant has to be guided to the cutting edge of the tool. In the framework of this work, this fluid transport is passively implemented by the capillary effect like it is present in the open microfluidic systems of various natural materials. One promising approach for the fabrication of microchannels is micromachining with ultrashort laser pulses. In the present work, using ultrashort laser pulses, such biomimetic microchannels for passive transport were micromachined on cemented tungsten carbide. The biomimetic microchannels provide a hierarchical structure consisting of a wide channel with a width of 450 μm and subchannels with a width of 50 μm. To evaluate the lubricant transport, flow velocity measurements of structures of different depths were performed. The biomimetic structure with a total depth of 200 μm shows very promising results with a maximum flow rate of 0.35 mm³/s.

Keywords: Laser micromachining; grooves; microchannels, ultrashort laser pulses; fluid transport

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Control of wall profile of trenches produced by femtosecond laser using a flat top triangular beam shape
Mikel Gomez-Aranzadi, Jorge Fantova, Oihane Beldarrain, Luis Omeñaca, Ainara Rodriguez, Enrique Castaño, Santiago Miguel Olaizola Izquierdo

In this work we demonstrate the possibilities that beam tailoring tools offer in terms of control of shapes in small sized geometries generated using femtosecond lasers. We focused in trench depths below 20 μm and down to 1 μm, as this type of geometries are normally generated using a gaussian beam, which results in a trench shape resembling the gaussian distribution of the beam, i.e., without any control of the inclination or shape of the walls. In order to overcome this, a 20 μm side flat top triangular laser spot has been used with the aim of controlling the inclination of both walls. The results confirm the possibility to generate trenches with wall inclinations from 12° to more than 50°, variating both the orientation of the triangular spot with respect to the scanning direction and the number of scans per trench.

Keywords: ultrafast lasers; beam shaping; top-hat; micro-structuring

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From jewels to quantum, challenges in Laser MicroJet® Cutting
Jeremie Diboine, Alexandre Schönhaus, Falk Braunmüller, Joerg Pausch

By cutting and sawing of natural diamonds into brilliants, Synova has been a significant provider of diamond cutting systems through its water jet-guided laser technology. In jewelry, the Laser MicroJet® (LMJ®) is allowing the cutting of natural stones of ever-growing thickness with low damage and increased yield. The increasing cost reduction and quality of lab-grown diamond (LGD) over recent decades has enabled an industrial exploitation of diamond’s outstanding hardness, thermal conductivity and very broadband transmission for industrial sectors such as wear resistant coatings, tool making, super abrasives, optics and sensors. Today, LMJ®’s unique value proposition and versatility as cutting tool serves as a catalyst for industrial breakthroughs with LGD diamonds, including cutting of quantum crystals, coring and slicing of LGD blocks, diamond turning and shaping of 3D-geometry. We present a retrospective of technical challenges for the LMJ® in the world of diamonds by showcasing the most challenging and most important machining applications.

Keywords: water jet guided laser; diamond; natural; synthetic; CVD; LGD; jewel; quantum; optics; processing

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Experimental study of laser drilling on LTCC with ultra-short pulsed laser
Lunzhen Hu, Qingchuan Guo, Yuqiang Hou, Yetao Liu, Liang Lü, Hao Wang, Lihui Zhou, Evgeny L Gurevich, Andreas Ostendorf

Low Temperature Co-Fired Ceramics (LTCC) is being widely used as a circuit substrate material for microwave devices and electronic equipment. This paper presents experimental results of the picosecond laser drilling technology for LTCC under different conditions. The laser has a maximum power of 30 W with a spot size of 3 mm, pulse width of 10 ps, maximum repetition of 1000 kHz, and wavelength of 355 nm. By optimizing the parameters, a series of drilled holes with an aperture of 200 μm and an accuracy of less than ±5 % are achieved. When the frequency, scanning speed, and pulse energy are 200 KHz, 1000 mm/s, and 15 μJ, respectively, the heat-affected zone is weakly visible and the hole edge has good surface finish. The experiments demonstrate that currently available ultra-short pulsed lasers are suitable for drilling LTCC materials which opens up new opportunities in this field.

Keywords: picosecond UV laser; laser drilling; LTCC

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Laser cutting of natural fiber reinforced composites with high speed and minimum damage
Christian Strohl, Kathrin Placzek, Daniel Holder, Christian Hagenlocher, Johannes Baur, Thomas Graf

Flax fiber reinforced composites Plastik (FFRP) offer a sustainable alternative to synthetic reinforcement fibers like glass or carbon. However, processing of FFRP using conventional machining techniques results in low cut qualities due to fraying and delamination.
Laser cutting of FFRP can avoid these issues with the added benefit of FFRP having a similar decomposition temperature of matrix and fiber when compared to synthetic composites. Avoiding quality decreases from thermal influences while still providing high cutting speeds is a major challenge when cutting FFRP with lasers.
For this purpose, laser cutting of 2.5 mm FFRP plates was investigated using different laser systems. Very high cutting speeds of up to 8000 mm/min with minor thermal damage were achieved using a continuous wave CO2 laser. High cutting qualities with no thermal damage at cutting speeds of up to 32 mm/min were achieved using a femtosecond laser.

Keywords: natural fiber reinforced composite; laser cutting; laser ablation cutting; cut quality

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Study of the ablation efficiency for USP-processing of different materials in the GHz-Burst-Regime
J. Rücker, M. Kahle

We studied the ablation efficiency on crater formation, line scribing and cavity milling experiments on copper, silicon, stainless steel and ceramics with picosecond GHz burst pulses. The intra-burst repetition rate was 5.12 GHz, the number of pulses per bust was varied between 128, 512 and 1024 within a burst fluence of up to 19 J/cm². For the inter-burst repetition rate of 200 kHz, the used laser system had a maximum average power of 100 W and maximum pulse energy of 500 μJ. The high repetition rate was created through a pulse-divider-module outside of the laser system. The results show a growth of ablation efficiency compared to kHz processing by a minimum of two times for all processing technics on silicon and copper and for crater formation and line scribing on stainless steel. A comparable efficiency remains for ceramics. The resulting machining quality is affected by the heat input but remains comparable.

Keywords: GHz; burst mode; ultrashort pulse; ablation efficiency.

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3D microstructuring in metals using a UV laser microspot scanning system
Rose Mary, Stefan Remund, Beat Neuenschwander

High resolution UV laser micro-ablation and micro-drilling is investigated using a 10 ps, 355 nm ultrashort pulsed laser and a galvanometer scanner equipped with a microscan objective MSE-G2-UV. The microscan extension enables the laser beam to be very tightly focused down to a diameter of < 1.5 μm and thus converts a standard laser scanning system into a microspot scanning system. Laser ablation characteristics in steel, copper and monocrystalline silicon is reported for a repetition rate of 200 kHz. The usually observed cone-like protrusions in steel at higher peak fluences was found to be absent using the microscan. Micro-structuring of various designs that elucidate the resolutions achievable with such a small spot is also presented. Micro-drilling in Mylar film with an exit hole diameter of ~3 μm is also reported.

Keywords: laser micromachining; uv laser ablation; microscan; micro-ablation; micro-drilling

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Acousto-optic pulse-selective laser beam deflection for micromachining with ultrashort pulsed lasers
Jan Perwas, Matthias Springer, Jan Düsing, Jürgen Koch, Peter Jäschke, Stefan Kaierle

Acousto-optic deflection of laser beams promises the opportunity of fast pulse-by-pulse deflection without negative effects of inertia of traditional galvanometer scanner systems. Despite being known for quite some time, integration of acousto-optic deflectors (AODs) in conventional ultrashort pulsed laser machines still seems to be sparse.
Our work addresses possible opportunities that result from the combination of AODs optimized for wavelengths of 515 nm and 1064 nm with traditional galvanometer scanner systems as well as pico- and femtosecond lasers of average powers up to 10 W and 25 W. Therefore, the development of an elaborated FPGA-based controller as well as an appropriate optical setup for the implementation into an existing laser machining setup has been investigated. Key has been the realization of sufficient interfaces and data processing structures in combination with a commercial controller system for new and accelerated laser processing strategies such as pattern or multi spot deflection for improved material processing.

Keywords: Acousto-optic; laser beam deflection; FPGA; ultrashort laser pulse micro machining

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Ultrafast laser micromachining with GHz-bursts
Inka Manek-Hönninger, Pierre Balage, Théo Guilberteau, Manon Lafargue, Guillaume Bonamis, Martin Delaigue, Clemens Hönninger, and John Lopez

Micromachining with femtosecond lasers operating in the novel GHz-burst regime has recently attracted increasing attention. Indeed, very interesting results have been published on ablation and percussion drilling in this regime. In this contribution, we show our latest results for micromachining of different transparent dielectric materials. We report our latest results on top-down percussion drilling with a Gaussian beam as well as on cutting with a Bessel beam. The dependence on the burst parameters such as burst repetition rate, number of pulses per burst, and burst energy are discussed. The quality and aspect-ratios of the drilled holes will be presented. Concerning cutting with a non-diffractive beam, the surface quality of the cutting planes will be discussed in terms of roughness and straightness.

Keywords: Femtosecond laser processing; GHz-burst regime; Glasses; Drilling; Cutting; Bessel beam

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Unwanted X-ray emission in ultrashort pulse laser processing: from metallic to biological materials
Sebastian Kraft, Katrin Böttcher, Jörn Bonse, Jörg Schille, Udo Löschner, Jörg Krüger

X-rays can be generated as an unwanted side effect during ultrashort pulse laser material processing of technical work pieces and even biological samples with laser intensities above 1013 W/cm2. First studies demonstrate the need to address this effect in industrial as well as in medical applications. This secondary hazard should be considered in work safety and risk assessment.

Keywords: ultrashort pulse laser processing; laser-induced X-ray emission; secondary hazard

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Adjustment of the cutting front and kerf by means of beam shaping to increase the speed of laser cutting
Jannik Lind, Christian Hagenlocher, Niklas Weckenmann, David Blazquez-Sanchez, Clemens Ackermann, Rudolf Weber, Thomas Graf

The shape of the laser beam significantly influences the geometry of both the cutting front and the cutting kerf. The angle
of the cutting front impacts the absorptivity while the width of the kerf defines the amount of material, which must be
molten. The kerf´s geometry therefore determines the maximum possible cutting speed. These relations between
absorptivity, the width of the kerf, and the maximum cutting speed are described by a simple model in Lind et al., 2023.
In order to verify the prediction of the model, the geometry of the interaction zone was observed by means of X-ray
imaging. The results show an increase of absorptivity in case of an enlargement of the beam in the direction of the feed,
while the cross-sectional area of the kerf increases in case of an enlargement of the width of the beam in the transversal
direction.

Keywords: Laser beam cutting; beam shaping; online high-speed X-ray imaging; maximum cutting speed

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Femtosecond laser processing with GHz long burst and second harmonic generation
Eric Audouard, Marie Fleureau, Guillaume Bonamis and Eric Mottay

Femtosecond pulses with bursts of GHz repetition rate can significantly improve the ablation efficiency of femtosecond lasers. Freely adjustable bursts involve thermal and non-thermal ablation mechanisms. The long GHz bursts can remove very efficiently heated matter, with non-thermal ablation leading to a high-quality material processing. We report an experimental study on crystalline silicon line scribing with GHz burst regime of femtosecond pulses at 515 nm. The number of pulses per bursts varied from 50 ppb to 200 ppb. A second harmonic generation module was placed at the output of a 100 W femtosecond GHz laser to generate up to 50W of second harmonic (SHG,515nm) at 200 kHz. The results show the highest ablation efficiency obtained on silicon with fs pulses (more than 10 mm3/min/W), and the crucial role of the pulse overlap.

Keywords: femtosecond laser processing, GHz bursts, second harmonic generation

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Thermal focus shift avoidance strategies in high power laser cutting
Niklas Weckenmann, Mathias Bach, Simon Hensel, Lukas Höfflin, Simonas Kičas, David Blazquez-Sanchez, Marcel Schäfer

Laser powers above 20 kW are getting more important to significantly increase productivity in laser cutting, especially of sheet thicknesses beyond 10 mm. Furthermore, laser powers of >40 kW exhibit potential to compete with plasma cutting, providing better edge quality, less material consumption due to smaller cutting kerfs at higher energy efficiency, thus also lowering CO2 emissions. This market demand is challenged by the intrinsic thermal focus shift of each optical element within the cutting system. Today, optical coatings for up to 20 kW of laser power are often optimized to balance performance and economic aspects. Minimizing focus shift can be done by, first, reducing energy deposition within AR-coatings, second, pre-correcting actual shifts by means of approximation, and third, measuring the shift and correcting it in a closed loop control. A detailed analysis of the in-situ thermal focus shift in real applications will be presented, providing an optimal combination of listed measures.

Keywords: high-power laser cutting; thermal focus shift; focus correction

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Water jet guided laser cutting of carbon fibre reinforced polymer
Helen Elkington, Sundar Marimuthu, Kuda Chingwena, Bethan Smith

Across the last few years there has been a growing interest in the exploitation of carbon fibre reinforced polymer (CFRP). This composite material consists of a resin matrix and carbon fibre reinforcement. The differences present between the thermal and physical properties of the matrix and fibre, and the materials high melting temperature, can cause difficulties when laser machining. Laser machining is also typically associated with thermal damage, as it relies on a thermal based process to remove material, and taper, due to the convergent-divergent nature of the laser beam. These limitations can be overcome through utilisation of a water jet guided laser.
This paper investigates the basic characteristics of water jet guided laser cutting of CFRP, using a high-power laser with an average power of 400 W. Experimental trials were performed to understand the effect of different process parameters on the cut quality and overall cutting speed.

Keywords: Water jet guided laser; CFRP; cutting

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High-speed laser micro-hole drilling of 1 mm thick C263 Nickel alloy
Mitchell Leering, Chris Ellisa, Sundar Marimuthua, Bethan Smith

Over the last few years, there has been an accelerated push to advance hydrogen electrolysers and fuel cell technology, with green hydrogen identified as a viable means to contribute to the decarbonisation of many instrumental industries. Laser drilling is an enabling technology to produce porous structures and micro-holes in various critical components used throughout the construction of modern hydrogen electrolysers. To commercialise micro-hole drilling in this area it is necessary to develop processing capabilities that permit processing at high speeds and throughput rates in various materials.
This paper investigates the capabilities of creating micro-holes using a single mode fibre laser in 1 mm thick C263 Nickel alloy, a common material used in hydrogen electrolysers. A single-mode fibre laser is used in combination with a nozzle-based coaxial processing gas to produce the holes. Experimental trials were performed to understand the effect of peak power and pulse duration on the size and quality of single pulse laser drilled micro-holes. Micro-holes with average diameter less than 90 μm were produced at a rate of approximately 185 holes per second whilst achieving a high level of consistency.

Keywords: Laser micro-hole drilling, hydrogen, electrolyzer, fuel cell

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High-rate micro-drilling in large surfaces by means of a high-power femtosecond laser and multibeam delivery
Roberto Ocaña, Emeric Biver, Alexandre Vendramini, and Julien Pouysegur

We report on the development of a prototype for micro-drilling large Ti sheets used in the technology HLFC for reduction of friction in the leading edges of aircrafts. The system uses a custom kW femtosecond laser, an optical path compensator to account for the large surface processing requirement, and a two-stage multibeam generator based on polarization and diffraction beam splitting. The multibeam is then introduced into custom laser heads that exploit different ways to drill the workpiece and to apply the inert local gas protection environment necessary to avoid oxidation of the sample and the formation of unwanted metallographic phases. Preliminary results show an improvement of the overall quality with production rates in the order of magnitude of other conventional technologies using longer laser pulses.

Keywords: USP laser; high-power; micro-drilling; beam-splitting; DOE; hybrid laminar flow control; multibeam

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Fs-laser fabrication of PMN-PT piezo actuators
Sandra Stroj, Barbara Lehner, Julia Freund, Armando Rastelli, Fadi Dohnal

Due to its outstanding piezoelectrical properties, single-crystal PMN-PT has proven to be a very promising material for actuator applications. However, PMN-PT is very difficult to process by conventional technological methods since it is very sensitive to thermal and mechanical load. In this work we demonstrate that surface structuring as well as high aspect-ratio cutting is possible using a femtosecond laser. We realised a laser generated actuator platform for tuning the optical emission of an entangled photon source by introducing biaxial strain. The strain-tuning device with a size of 5x5 mm² is realized in a two-step laser process where the electrical contacts for individual control of six actuator legs are structured by selective laser removal of a thin gold layer, followed by the PMN-PT structuring and cutting process. This method is to our knowledge unique for realizing complex and miniaturized actuators based on single crystal piezoelectrical materials.

Keywords: femtosecond laser, cutting, actuator, strain-tuning