X-ray Emission Hazards from Ultrashort Pulsed Laser Material Processing in an Industrial Setting

Interactions between ultrashort laser pulses with intensities larger than 1013 W/cm2 and solids during material processing can lead to the emission of X-rays with photon energies above 5 keV, causing radiation hazards to operators. A framework for inspecting X-ray emission hazards during laser material processing has yet to be developed. One requirement for conducting radiation protection inspections is using a reference scenario, i.e., laser settings and process parameters that will lead to an almost constant and high level of X-ray emissions. To study the feasibility of setting up a reference scenario in practice, ambient dose rates and photon energies were measured using traceable measurement equipment in an industrial setting at SCHOTT AG. Ultrashort pulsed (USP) lasers with a maximum average power of 220 W provided the opportunity to measure X-ray emissions at laser peak intensities of up to 3.3 × 1015 W/cm2 at pulse durations of ~1 ps. The results indicate that increasing the laser peak intensity is insufficient to generate high dose rates. The investigations were affected by various constraints which prevented measuring high ambient dose rates. In this work, a list of issues which may be encountered when performing measurements at USP-laser machines in industrial settings is identified.

Preface of the 18th NOLAMP conference

About the conference: The first conference on Nordic Laser Material Processing, NOLAMP was held in Oslo in 1987. Then the laser material processing was a rather new and revolutionary way to manufacture products with high quality and high efficiency. The laser research in the Nordic countries was in its pioneering age and the enthusiasm among researchers for the new possibilities was unrivalled. The intention for creating this conference, which was originally suggested by Dr. Bernt Thorstensen at SI in Oslo, Norway, was to establish a fruitful cooperation among Nordic laser researchers. Also NOLAMP intended to provide a forum for young researchers to present on-going works and to establish contacts in the scientific community as well as in industry. List of Conference date and location, Conference topics, Keynote speakers, Sponsors, Committee members, Editors, Reviewers are available in this pdf.

Artifact-free holographic light shaping through moving acousto-optic holograms

Holographic light modulation is the most efficient method to shape laser light into well-defined patterns and is therefore the means of choice for many intensity demanding applications. During the last two decades, spatial light modulators based on liquid crystals prevailed among several technologies and became the standard tool to shape light holographically. But in the near future, this status might be challenged by acousto-optic deflectors. These devices are well known for their excelling modulation rates and high optical power resilience. But only few scattered precedents exist that demonstrate their holographic capabilities, despite the many interesting properties that they provide. We implemented a holographic acousto-optic light modulation (HALM) system, that is based on displaying holograms on acousto-optic deflectors. We found that this system can eliminate the ubiquitous coherent artifacts that arise in holography through the inherent motion of acousto-optic holograms. That distinguishes our approach from any other holographic modulation technique and allows to reconstruct intensity patterns of the highest fidelity. A mathematical description of this effect is presented and experimentally confirmed by reconstructing images holographically with unprecedented quality. Our results suggest that HALM promotes acousto-optic deflectors from highly specialized devices to full-fledged spatial light modulators, that can compete in a multitude of applications with LC-SLMs. Especially applications that require large optical output powers, high modulation speeds or accurate gray-scale intensity patterns will profit from this technology. We foresee that HALM may play a major role in future laser projectors and displays, structured illumination microscopy, laser material processing and optical trapping.

Establishment of suitable parameters for laser machining based production of polymeric implants

Laser material processing enables precise machining of a wide variety of materials. In order to prevent an excessive heat input, which leads to irreversible material damage, the process parameters have to be adapted to the processed material. To keep the heat load as low as possible, the potential of femtosecond laser technology is exploited. The processing of semi-finished products using femtosecond laser technology is highly depending on the processed material. In this regard, we established a workflow and basic parameters for the processing of polymeric material. The performed parameter study varying cutting speed, cutting gas pressure and pulse energy to optimize the manufacturing process. Scanning electron microscopy (SEM) was utilized to analyze the cutting results, such as cut edge quality or possible melted areas. The established parameter set is also suitable for processing of very filigree material structures as used in innovative medical devices. The SEM analysis of the established parameter set showed that a homogeneous, nonwavy cut edge was created along the kerf.

In Situ Collection of Nanoparticles during Femtosecond Laser Machining in Air

Nanoparticles generated during laser material processing are often seen as annoying side products, yet they might find useful application upon proper collection. We present a parametric study to identify the dominant factors in nanoparticle removal and collection with the goal of establishing an in situ removal method during femtosecond laser machining. Several target materials of different electrical resistivity, such as Cu, Ti, and Si were laser machined at a relatively high laser fluence. Machining was performed under three different charge conditions, i.e., machining without an externally applied charge (alike atmospheric pulsed laser deposition (PLD)) was compared to machining with a floating potential and with an applied field. Thereby, we investigated the influence of three different charge conditions on the behavior of laser-generated nanoparticles, in particular considering plume deflection, nanoparticle accumulation on a collector plate and their redeposition onto the target. We found that both strategies, machining under a floating potential or under an applied field, were effective for collecting laser-generated nanoparticles. The applied field condition led to the strongest confinement of the nanoparticle plume and tightest resulting nanoparticle collection pattern. Raster-scanning direction was found to influence the nanoparticle collection pattern and ablation depth. However, the laser-processed target surface remained unaffected by the chosen nanoparticle collection strategy. We conclude that machining under a floating potential or an applied field is a promising setup for removing and collecting nanoparticles during the machining process, and thus provides an outlook to circular waste-free laser process design.

Squared Focal Intensity Distributions for Applications in Laser Material Processing

Tailored intensity profiles within the focal spot of the laser beam offer great potential for a well-defined control of the interaction process between laser radiation and material, and thus for improving the processing results. The present paper discusses a novel refractive beam-shaping element that provides different squared intensity distributions converted from the Gaussian output beam of the utilized femtosecond (fs) laser. Using the examples of surface structuring of stainless-steel on the micro- and nano-scale, the suitability of the beam-shaping element for fs-laser material processing with a conventional f-Theta lens is demonstrated. In this context, it was shown that the experimental structuring results are in good agreement with beam profile measurements and numerical simulations of the beam-shaping unit. In addition, the experimental results reveal the improvement of laser processing in terms of a significantly reduced processing time during surface nano-structuring and the possibility to control the ablation geometry during the fabrication of micro-channels.

UV Durable LCOS for Laser Processing

Liquid-Crystal-On-Silicon (LCOS) Spatial Light Modulator (SLM) is widely used as a programmable adaptive optical element in many laser processing applications with various wavelength light sources. We report UV durable liquid-crystal-on-silicon spatial light modulators for one-shot laser material processing. Newly developed LCOS consists of UV transparent materials and shows a lifetime 480 times longer than the conventional one in 9.7 W/cm2 illumination at 355 nm. We investigated the durability of polymerization inhibitor mixed liquid crystal in order to extend its lifetime.

Dynamic, Adaptive Inline Process Monitoring for Laser Material Processing by Means of Low Coherence Interferometry

Surface laser structuring of electrical steel sheets can be used to manipulate their magnetic properties, such as energy losses and contribute to a more efficient use. This requires a technology such as low coherence interferometry, which makes it possible to be coupled directly into the existing beam path of the process laser and enables the possibility for an 100% inspection during the process. It opens the possibility of measuring directly in the machine, without removing the workpiece, as well as during the machining process. One of the biggest challenges in integrating an LCI measurement system into an existing machine is the need to use a different wavelength than the one for which the optical components were designed. This results in an offset between the measurement and processing spot. By integrating an additional scanning system exclusively for the measuring beam and developing a compensation model for the non-linear spot offset, this can be adaptively corrected by up to 98.9% so that the ablation point can be measured. The simulation model can also be easily applied to other systems with different components and at the same time allows further options for in-line quality assurance.