Efficient processing with removal of modification in ultrashort pulse laser processing of diamond

In the field of micro and nano structuring ultrashort pulsed (USP) laser processing attracts increasing attention due to its ability to generate high-precision structures. A simulation of the USP process can lead to a reduced process development time and help to achieve a better geometrical quality of the manufactured microstructures. To predict the ablation shape, temperature distribution and distortion in a USP ablation process, a detailed simulation of the physical processes during and after the laser ablation is required. Different simulation tools such as multiscale simulations are already established but still need different and accurate input parameters regarding the material properties of the workpiece to be machined 1. A material characterization procedure that can be used in a standardized way for different materials and processing stations needs to be developed. The procedure determines the absorption coefficient, penetration depth and ablation threshold precisely matched to a USP machine and the material used. Based on the material characterization procedure a calibration of the used simulation has to be carried out as precisely as possible. The simulation can then be applied for a digital process development and subsequently validated with specific experiments1.

Laser-induced stress waves play a crucial role in ultrashort pulse laser (USPL) processing such as laser shock peening. However, previous studies have primarily relied on numerical simulations or two-dimensional measurements, lacking experimental validation of the three-dimensional stress distribution. This study presents what we believe to be a novel optical measurement technique to quantify the three-dimensional axisymmetric shear stress distribution of laser-induced stress waves using time-resolved circular polariscope imaging and integrated photoelasticity. Femtosecond laser pulses were used to generate stress waves in silica glass, and the induced birefringence was captured using a polariscope. The slow-axis angle and the optical retardance were extracted and used to reconstruct three-dimensional distribution of the shear stress component. The reconstructed stress distribution agrees with theoretical predictions and numerical simulations of a previous study. Additionally, we found that higher pulse energies resulted in broader stress wavefronts. This behavior is attributed to the expansion of the ablation region, which alters the wavefront shape. The ability to visualize and quantify the shear stress in three dimensions offers a more comprehensive understanding of stress wave interactions with materials. This study lays the foundation for further advancements in USPL processing and experiments utilizing USPL-induced high pressure.

In the field of micro- and nanostructuring multibeam ultra short pulsed (USP) laser processing attracts increasing attention due to its ability to generate periodic pattern with high throughput. For the generated structures, there is a wide range of applications including e.g. functional or design structures for the automotive industry, consumer electronics or filtration technology. Compared to state-of-the-art single-beam laser drilling by means of pulsed femto- and picosecond laser radiation, the multibeam approach allows for the application of high pulse energies maintaining the surface quality of a typical USP laser process. On the one hand the homogenous distribution of the applied pulse energy across the surface of the processed workpiece by a multitude of beamlets results in a reduced process time and a more economic laser process. On the other hand fundamental aspects of process strategies and thermal management have to be reconsidered. Due to the number (>100) of beamlets focused onto a relatively small scan field area of less than 4x4 mm, the relevance of contamination, microscopic and macroscopic heat accumulation become increasingly relevant. Therefore, specially designed scanning strategies, suction units and additional surface treatments have to be applied to generate hole pattern with packing density of more than 30 percent and reduced surface contamination.

A critical review on the simulation of ultra-short pulse laser-metal interactions based on a two-temperature model (TTM)

Over the last ten years, the need for organ donors for transplants became critical due to the increased incidence of organ failure [1]. Today, the development of tissue engineering (TE) appears as the best opportunity to overcome this shortage. TE is an interdisciplinary emerging field that aims to restore and maintain human tissue functions by applying engineering and live science principles [2]. However, one of its greatest challenges is the vascularization of tissue for the transport of oxygen and nutrients to prevent cell death. Here an innovative method is proposed to answer vascularization issues and the difficulty to create blood microcapillaries constructs, with a special interest to renal microcapillaries, which allow blood filtration. A cell-bilayer covering a tubular collagen I matrix with a diameter of about 150μm was developed and treated by ultra-short pulse (USP) laser processing in order to selectively remove the collagen core to create a capillary. The precise laser treatment allows indeed for the creation of voids in the fibre-shaped construct which results in the final formation of the capillary. Firstly, experiments were carried on a 2D model of gelatine hydrogel. The hydrogel-laser interaction was parametrically investigated in order to define a window of laser process parameters allowing the creation of voids within the hydrogel. The best window of laser process parameters was then applied to the 3D cell bilayer microfibres. Confocal microscopy examination demonstrated the presence of a lumen through the collagen I matrix without extended damage to surrounding cells. Live/Dead assays were also carried to assess cell viability.

Carbon fiber reinforced plastics (CFRP) are of high interest for lightweight construction within many industrial sectors. Although knowledge about the laser processing of CFRP is continuously increasing, deeper process knowledge is of importance for the development of automatable and controllable industrial processes. Furthermore, new laser sources are developed continuously, providing, for example, high pulse energies for short and ultrashort pulse laser processing. In order to develop a fast and high quality cutting process, the lifetime of potentially shielding plasmas and particles is a key factor. For this purpose, time resolved investigations of pulsed laser processing were conducted. A fiber guided nanosecond pulsed laser source with an average laser power of PL = 1.5 kW and a maximum pulse energy of EP = 80 mJ was used for cutting. The setup for the time resolved investigation consisted mainly of an industrial camera, enabling a minimum exposure time of tB = 13 μs and a light source with a pulse duration of tP = 500 ns in order to realize pictures of the process with high spatial and temporal resolutions. An optical setup with a light source and a camera on opposite sides of the processing zone was chosen for the investigation. In order to ensure distortion-free imaging of the processing zone, the camera was equipped with telecentric lenses. The time resolved analysis is realized by a laser signal as the master clock and a delay generator, enabling the activation of the camera and light source in a defined delay in relation to the laser pulse. With the described setup, cutting processes with different laser parameters were investigated. The focus of the analysis was the plasma and particle formation depending on two main factors for the observed process parameters. One factor is the time delay between the laser pulse and taking the picture, and the other factor is the cutting kerf depth. The investigation revealed the lifetime of the plasma as well as the changes in the plasma between two consecutive pulses. The results indicate a plasma lifetime in the range of t = 20 μs to t = 200 μs. Consequently, the optimum repetition rate for the chosen laser system is expected to be in the range of f = 5 kHz to f = 50 kHz. For deeper cutting kerfs and smaller ablation rates, the area of plasma plumes reduces and the lifetime of the plasma seems to reduce in an analogous manner.

High-speed imaging is a valuable tool for investigations on laser processes. The high temporal resolution of high-speed imaging allows a detailed observation of different laser processes which helps to understand process mechanisms like e.g. the formation of spatters during laser welding or the formation of a heat affected zone during laser cutting of carbon fiber reinforced plastics (CFRP). In the following the potential of high-speed imaging as a tool for laser process development is shown using different applications as an example. Initially it is described how high-quality images can be achieved during laser processing although the laser beam itself and process emissions make this a challenging task. Subsequently different applications like laser welding, laser drilling of metals with ultra-short pulsed lasers and laser processing of CFRP are introduced. During laser welding, the formation of spatters and hot cracks can be observed. Furthermore the influence of spatial beam modulation on the welding process can be investigated by means of high-speed imaging. The capillary dynamics during laser welding of metals can be studied using high-speed X-rays imaging while in transparent materials the capillary dynamics can be directly observed. During laser drilling the drilling process can be immediately seen using a suitable experimental setup. With the help of high-speed imaging it was revealed that the formation of a heat affected zone during laser processing of CFRP with ultra-short pulses is caused by heat accumulation. Furthermore the dynamics of process emissions like particles or hot vapor generated during laser processing of CFRP was investigated.

Ultrashort pulse laser processing can result in the secondary generation of unwanted X-rays if a critical laser irradiance of about 1013 W cm-2 is exceeded. Spectral X-ray emissions were investigated during the processing of tungsten and steel using three complementary spectrometers (based on CdTe and silicon drift detectors) simultaneously for the identification of a worst-case spectral scenario. Therefore, maximum X-ray photon energies were determined, and corresponding dose equivalent rates were calculated. An ultrashort pulse laser workstation with a pulse duration of 274 fs, a center wavelength of 1030 nm, pulse repetition rates between 50 kHz and 200 kHz, and a Gaussian laser beam focused to a spot diameter of 33 μm was employed in a single pulse and burst laser operation mode. Different combinations of laser pulse energy and repetition rate were utilized, keeping the average laser power constant close to the maximum power of 20 W. Peak irradiances I0 ranging from 7.3 × 1013 W cm-2 up to 3.0 × 1014 W cm-2 were used. The X-ray dose equivalent rate increases for lower repetition rates and higher pulse energy if a constant average power is used. Laser processing with burst mode significantly increases the dose rates and the X-ray photon energies. A maximum X-ray photon energy of about 40 keV was observed for burst mode processing of tungsten with a repetition rate of 50 kHz and a peak irradiance of 3 × 1014 W cm-2.

A fractional dual-phase-lag generalized thermoelastic model of ultrashort pulse laser ablation with variable thermal material properties, vaporization and plasma shielding

The ongoing trend in the development of powerful ultrashort pulse lasers has attracted increasing attention for this technology to be applied in large-scale surface engineering and modern microfabrication. However, the emission of undesired X-ray photon radiation was recently reported even for industrially relevant laser irradiation regimes, causing serious health risks for laser operators. In the meantime, more than twenty influencing factors have been identified with substantial effects on X-ray photon emission released by ultrashort pulse laser processes. The presented study on enhanced X-ray emission arising from high pulse repetition frequency ultrashort pulse laser processing provides new insights into the interrelation of the highest-contributing parameters. It is verified by the example of AISI 304 substrates that X-ray photon emission can considerably exceed the legal dose rate limit when ultrashort laser pulses with peak intensities below 1 × 1013 W/cm² irradiate at a 0.5 MHz pulse repetition frequency. The peak intensity threshold value for X-ray emissions decreases with larger laser spot sizes and longer pulse durations. Another key finding of this study is that the suction flow conditions in the laser processing area can affect the released X-ray emission dose rate. The presented results support the development of effective X-ray protection strategies for safe and risk-free ultrashort pulse laser operation in industrial and academic research applications.

Numerical study of multi-phase flow in ultrashort pulse laser processing

In the field of microstructuring and nanostructuring ultrashort pulsed (USP) laser processing attracts increasing attention due to its ability to generate high-precision structures. A simulation of the USP process can lead to a reduced process development time and help to achieve a better geometrical quality of the manufactured microstructures. To predict the ablation shape, temperature distribution, and distortion in a USP ablation process, a detailed simulation of the physical processes during and after the laser ablation is required. Different simulation tools such as multiscale simulations are already established but still need different and accurate input parameters regarding the material properties of the workpiece to be machined. A material characterization procedure that can be used in a standardized way for different materials and processing stations needs to be developed. The procedure determines the absorption coefficient, penetration depth, and ablation threshold precisely matched to a USP machine and the material used. Based on the material characterization procedure a calibration of the used simulation has to be carried out as precisely as possible. The simulation can then be applied for a digital process development and subsequently validated with specific experiments.

Material removal rates as well as surface and subsurface quality are key aspects for the industrial application of ultrashort pulse (USP) laser machining. However, revealing so-called subsurface damage (SSD) is challenging. The presented study visualizes and quantifies subsurface damage patterns in areal USP laser ablation of fused silica (FS) and glass N-BK7 (BK). For the first time, using high-resolution optical coherence tomography (OCT) as non-destructive and three-dimensional (3D) evaluation method, SSD morphologies of areal laser machining induced damages are analysed. Influences of laser wavelength, beam geometry and processed material are investigated. Discovered differences of damage morphologies and depth in FS and BK point out the relevance of selecting suitable process parameters. Based on the evaluation of volumetric OCT data, the authors were able to quantify damage morphologies using the surface texture ratio as well as power spectral density functions. One important finding for the quantification and comparability of damage depths in USP laser processing is the influence of applicable evaluation thresholds. In comparison to area thresholds of 0.001% being applicable to OCT measurements, more lenient thresholds of e.g. 1% commonly applied in destructive SSD measurement methods in average result in a reduction of measured damage depths by a factor of ~ 2. This potentially leads to an underestimation of damage depths depending on methods on thresholds used. The presented measurement and evaluation methods as well as gained process insights are important assets for the future optimization of low-damage USP laser micromachining of brittle materials. Moreover, the general applicability and relevance of OCT-based morphological damage analysis in laser material processing is shown.

The industrial use of ultrashort laser pulses has made considerable progress in recent years. The reasons for this lie in the availability of high average powers at pulse repetition rates in the several 100 kHz range. The advantages of using ultrashort laser pulses in terms of processing precision can thus be fully exploited. However, high laser intensities on the workpiece can also lead to the generation of unwanted X-rays. Even if the emitted X-ray dose per pulse is low, the accumulated X-ray dose can become significant for high-repetition-rate laser systems so that X-ray exposure safety limits must be considered. The X-ray emission during ultrashort pulse laser processing was investigated for a pulse duration of 925 fs at 1030 nm wavelength and 400 kHz repetition rate. Industrially relevant materials such as steel, aluminum and glass were treated. Tungsten served as reference. X-ray spectra were recorded, and X-ray dose measurements were performed for laser treatment in air. For laser intensities > 2 × 1013 W/cm2 , X-ray doses exceeding the regulatory exposure limits for members of the public were found. Suitable X-ray protection strategies are proposed.

An Ultra Short Pulse (USP) laser has been used for surface milling of dielectric elastomer actuator (DEA) polymer (3M VHB tape). DEA’s, artificial analogues to natural muscle, act as flexible capacitors and consist of a polymeric dielectric membrane material sandwiched between flexible electrodes. Actuation is in part determined by the voltage difference across the electrodes and the dielectric membrane thickness. Prior to the present report, pre-stretching the membrane provided the sole means for controlling membrane thickness. Good quality ablation, without grossly observable thermal damage, was achieved at removal rates of 0.0047 mm3/s using a 250 kHz repetition rate and an energy delivery of 2 μjoules/pulse. The surface milling operation was performed as a set of parallel passes produced a series of small shallow trenches, the result of overlap between subsequent passes of the laser. These features did not significantly compromise performance of the test actuators and the USP machined polymer could be pre-stretched at least five times the original size without rupture. When placed in a support frame and subjected to high electric field strength the stretched membrane also maintained its relatively high breakdown strength. A proof of concept two-thickness actuator, one half of it milled, was produced and successfully actuated. USP laser processing has provided a design freedom, beyond pre-stretching, and has opened the way to novel variable thickness micro-actuators.