This PDF file contains the front matter associated with SPIE Proceedings Volume 10525, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Industrial laser systems continue to evolve in terms of power levels, brightness and mode of operation. While continouswave (cw) laser systems drive to more efficiency, compactness and robustness, ultra-short pulse (USP) solid-state lasers (SSL) gain increasing attention from industry due to their high average power levels, increasing pulse energies and more flexible repetition rates, eventually enabling increased productivity and novel materials processing applications. TRUPMF’s laser systems portfolio starting in 1985 mirrors this general development and is therefore an interesting subject to reflect the path of industrial laser systems from early CO₂ lasers to the latest generation of diode pumped nanosecond-pulsed thin-disk lasers with up to 400W of average output power at 343nm. Today, our advanced laser families enable novel applications such as EUV light generation, laser lift-off and surface annealing.

The Laser MegaJoule (LMJ) is a 176-beam laser facility, located at the CEA CESTA Laboratory near Bordeaux (France). It is designed to deliver about 1.4 MJ of energy to targets, for high energy density physics experiments, including fusion experiments. The commissioning of the two first bundles of 8 beams was achieved in December 2016 and commissioning of next bundles is on the way.

A computational system, PARC has been developed and is under deployment to automate the laser setup process, and accurately predict laser energy and temporal shape. PARC is based on the computer simulation code MIRO. For each shot on LMJ, PARC determines the characteristics of the injection laser system required to achieve the desired main laser output and supplies post-shot data analysis and reporting.

The presentation compares energy end temporal shapes measured after amplification and after frequency conversion with results computed with PARC. For most of the LASER shots, both measurement and computed results agree within five percent accuracy.

At high average power, ultra-short laser pulses (<1 ps) are not only affected by dispersive effects but also by thermallyinduced effects in the optical elements. In order to study those effects, we have developed a multi-physical simulation approach. It enables the analysis of the propagation of ultra-short pulses in the spatial and temporal regime against the background of the temperature-dependent refractive index profile, surface deformations, and dispersive effects. The simulation couples a thermo-mechanical finite element analysis and an optical analysis, using both ray-tracing and waveoptical methods. As case example, the influence of thermal and dispersive effects in a non-achromatic singlet lens and an achromatic doublet lens are analyzed and compared.

Long UV laser lines find application in crystallization of Si, lift-off of flexible OLED displays from assistance glass and other surface processing technologies. With the appearance of powerful and reliable pulsed UV DPSS lasers in the last years, many efforts are directed to the implementation of such lasers in the line systems instead of excimer lasers. For the processing with a high energy density, several DPSS lasers have usually to be coupled in a homogeneous line focus with 100 - 1000 mm length and FWHM of 20 – 40 μm. We describe an advantageous line design approach based on anisotropic laser beam transformation, which improves beam quality for the narrow line axis and allows tight focusing with large depth of the focus. Solutions for tuning the line width are discussed.

Stability and performance of laser technologies based on high power lasers depend on possibility to control the thermally induced optical effects: thermal focus shift and thermally induced aberrations. Even if the thermal focus shift is compensated, for example by mechanical movement of a processing head, the thermally induced aberration can be a reason of essential variation of intensity distribution and, hence, degradation of a process performance. This aberration effect is especially important in applications presuming use of beam shaping optics and lasers of high optical quality, for example powerful TEM₀₀ lasers for Selective Laser Melting (SLM) or remote welding. Analysis of optical effects happening due to gradient heating of optical components shows that aberrations are mainly issued by change of geometrical shape of optical surfaces (bulge) and appearing the gradient of refractive index. Both effects thermal focus shift and thermally induced aberrations have the same nature. Therefore, to avoid appearing of those effects it is suggested to apply optical components made from special optical materials with athermal properties where geometrical and refractive optical effects compensate each other, then influence of unwished thermal effects can be strongly reduced. There will be presented analysis of thermally induced aberrations appearing in protective windows made from different optical materials: fused silica, N-BK7 glass, aThermoXX glass. Comparison of simulation results and experimental data confirm effectiveness of the suggested approach.

Performance of various laser technologies like welding, metal sheet cutting based on the use of powerful multimode fiber lasers, fiber-coupled solid-state and diode lasers can be improved by manipulation of energy distribution perpendicular and along optical axis. Welding, cladding with 0.5-2 mm size working spots benefit from “inverse-Gauss” intensity profiles, and doubled or tripled spots perpendicular to axis are good solutions to provide more uniform temperature profiles on a workpiece. Thick metal sheet cutting, some types of welding get benefits from distributing the laser energy along the optical axis resulting in more efficient usage of laser energy, higher and more stable cutting edge quality, faster processing. Since the radiation of multimode lasers is of low spatial coherence, characterized by big Beam Parameter Products (BPP) or M² values, it is difficult to control the intensity distribution by methods other than imaging of a fiber end using a collimator and focusing objective. A promising suggested solution is combining of imaging the fiber end and geometrical separation of focused spots either perpendicular to or along the optical axis using dedicated optical components. Thus, energy of high power lasers is distributed among multiple foci. To provide reliable operation with multi-kW lasers and avoid damages the multi-focus optical devices are designed as refractive elements with smooth optical surfaces. The paper presents descriptions of multi-focus optics as well as examples of intensity profile measurements of beam caustics and application results.

A double-beam deflection unit (scanner) with integrated on-axis sensors for distance and temperature measurement is developed for the production of structural hybrid composite parts. This robot-guided 3d laser scanner is integrated in a composite production cell for in-mold laser pretreatment for subsequent joining. A basic challenge of industrial laser applications - the placement problem of workpiece in workspace of robot - was solved using on-axis low-coherence interferometry. Varying positions of workpiece for remote laser processes can in the future be compensated in seconds, depending on beam characteristic, here with a theoretical accuracy of 60 μm (X,Y), 1 μm (Z), 0,003° (B,C) and 0,174° (A). Two issues reduce this theoretical accuracy in practice. However, the improvement of daily process preparation is significant and validated by measurement of accuracy of laser path image at a close-to-series carbon fiber reinforced plastic (CFRP) demonstrator of BMW. The optimal position of scanner relative to part is a fundamental condition for ablative 3d surface processing. Driven by the necessity of quality assurance within automotive industry the inherent potential of sensor technology is shown, additionally. This potential study gives impulses for further developments of multifunctional laser scanners.

When a serious motor vehicle accident happens, persons with severe injuries may be trapped inside damaged vehicles. In these cases, it is necessary to rescue the injured persons as fast as possible to increase their chance to survive. Nevertheless, secondary injuries due to the rescue procedures must be avoided. In this scenario, structural parts typically have to be cut to create rescue openings. Therefore, several high-tech rescue systems are available, most common hydraulic apparatus. Alternatively, reciprocating saws, angle grinders or plasma cutters are used, depending on the specific on-scene conditions.

Lately, there has been tremendous progress regarding the developments on vehicle safety. One example is the integration of super high-strength steels and carbon fiber reinforced plastics, concurrently meeting the requirements of weight reduction. As a result, mechanical rescue systems like hydraulic shears reach their performance limits.

The main goal of this work is the development of a mobile laser cutting device for rescue operations. The focus is put on high flexibility concerning the processing of high-strength materials and multilayer structures. Moreover, robustness, easy handling and system weight shall be optimized, as rescuers often work under harsh conditions concerning temperature, humidity, dirt and stress. Crucial aspect of laser rescuing is safety which must be guaranteed for all persons involved at any time. Here, results of laser cutting experiments, using materials and structures relevant for rescue situations, are presented.

End-caps of different types are widely used in fiber laser systems to reduce the fiber facet damage threshold and the back reflection. Most commercial end-caps consist of a cylindrical rod of fused silica with a flat or angle polished end-surface. By fusion splicing the end cap to the output fiber, the laser beam diverges and the spot size enlarges inside the end-cap. Thus, the power density at the air-silica interface is much lower compared to the case without end-capping. However, with the traditional cylindrical rod shape, the divergent beam angle exiting from the end-cap may not be desired, but will be the result due to the flat (or angled) end surface of the end-cap. For many applications (such as medical surgery, metal cutting, etc.), less divergent, collimated, or focused beams are required. Spherical shaped end-caps are developed to meet the requirements for lower back-reflection, higher reliability, and flexible beam (convergent or divergent) angle. In this paper, general ball shaped end-caps are studied and illustrated. Different ball shaped end-caps were tested and measured for a variety of applications. For example, a 2.2 mm diameter ball shaped end-cap was manufactured on the end of the output LMA fiber and tested with a 1.7 kW fiber laser system.

In this paper, we introduce a simple technique to measure mirror dispersion at angles of incidence from 0 to 55 degrees, on up to 12” large aperture optics. A 160 mm diameter round mirror and a 142x100 mm rectangular-shaped mirror have been scanned over with micrometer spatial accuracy in order to investigate the dispersion properties across their surfaces. The measurement is based on spectrally resolved interferometry, consisting of a Michelson-interferometer and a combined visible and infrared spectrometer. A tungsten halogen lamp with 10 mW average power makes it possible to measure the dispersion properties over the 500-1300 nm spectral range. The mirror to be measured is part of sample arm of the Michelson-interferometer in a fixed position, while the interferometer can be translated along two axes. At the input of the interferometer we inserted a wire-grid polarizer, and sensitivity of the chirp mirrors to the polarization state have been measured at different incidence angles. We used the Fourier Transform method to process the recorded interference fringes. The group delay dispersion and the third order dispersion are obtained up to ±0.5 fs² and ±2 fs³ accuracy, respectively. To present the flexibility of the device we also scanned several different chirped mirrors including +135 fs₂ and -500 fs² at the 800 nm central wavelength.

Due to their numerous advantages in system technology, especially fiber optic beam delivery, solid state lasers have become state of the art for welding. Even so the obstacle of the so called "spatter regime" has been waiting for an industrial solution in order to gain high productivity.

In general the spatter regime occurs in a distinct welding speed range. With deep penetration welding applications like in power train the spatter regime shows up at welding speeds of 5 to 6 m/min. Beyond 6 m/min the mass loss due to weld spatter reaches a level resulting in undercuts of the weld seam and potentially internal porosity, both of which reduce the mechanical strength properties of the part. Moreover, if spatter particles are present or dislodged when welded parts are being assembled, the gear box itself may incur in-service damage.

This report presents recent results addressing this problem and demonstrates significantly reduced spatter when welding at high speed, which today has only been achievable with CO₂ laser beam welding. This new process approach is based on applying the disk laser with a special new beam forming method. The welding speed can be increased significantly and the energy efficiency of the process can be reduced dramatically.

In laser beam welding, excessive evaporation leads to bulging of the capillary tip which results in the generation of metal vapor-filled bubbles in the melt pool. These bubbles either collapse and dissolve, or solidify and remain as pores in the weld seam, thereby degrading the quality of the weld seam. We investigated the mechanism of bubble formation and collapse in detail for laser beam welding of aluminum by means of online X-ray videography. Capillary shapes were reconstructed from these high-speed videos and correlated to two kinds of processes either sensitive to pore formation or unsusceptible to pore formation. We show that the fluid dynamics in the melt pool is strongly influenced by subsequent bulging and collapsing of bubbles. Its influence on the melt flow was quantified by analyzing the trajectories of tracer particles in the melt pool. We found that the generation and collapse of bubbles is a major driver of the dynamics in the melt pool. The melt is accelerated to velocities of up to several hundreds of millimeters per second by collapsing bubbles. Similar effects were found in laser beam irradiation of transparent media, such as ice and water, which allows to resolve the generation and collapse of capillary bulging with higher temporal and spatial resolution.

New processes and lasers are required to optimize and improve processes for laser welding of batteries. Highly reflective materials cause problems due to lack of absorption, stability, spatter and brittle intermetallic phases. We present solutions for battery welding using pulsed green lasers and nanosecond pulsed IR lasers. Green laser improved process stability and spatter formation between 0.1-0.8 mm penetration depth could be avoided. Nanosecond welding process differs from normal keyhole welding and formation of intermetallic phases can be suppressed. Analysis is based on highspeed camera measurements and metallurgical studies, showing significant improvement in the process and weld quality.

Even after the 50th anniversary of the laser invention and about the same time this unique tool is used in industry the improvement of the beam material interaction is still ongoing, at universities, research institutes and inside the R and D facilities of companies. Sometimes the focus of this work is different. Whereas one side is trying to understand the science and physical coherences, the other hand side’s goal is to succeed with a unique selling point with a user benefit of this technology. But despite these differences, the ultimate goal is to approve, lasers to be conventional tools and remain competitive to other technologies or even extend or exceed the technological borders. This contribution of Precitec to the LASE 2018 will try to demonstrate the recent technical innovations developed inhouse. These new features, one in the area of laser cutting will be briefly described technically but the main part of the paper will show the benefits of these devices.

Neodymium phosphate glass (Nd:glass) is the typical gain medium in the large clear aperture slab amplifiers, and provide more than 99% energy of a high power laser facility. Cladding structure is employed for decreasing the amplified spontaneous emission (ASE) and parasitic oscillation (PO) which are the key limits for the gain of large clear aperture slab amplifier. Polymer cladding is generally used since it was proposed.

Polymer is an organic glue which has risk to decompose and fracture under high flash-lamp irradiation while the irradiation is a common condition in the operation of a high power laser facility. Once the polymer glue falls off, the suppression of ASE and PO of cladding structure would be damaged. In addition, the impurities existing in the glue may form the counteractive scattering sources and reduce the suppression of the cladding structure. Moreover, the decomposer of the organic glue may affect the cleanliness of the slab cavity. Thus, an inorganic edge cladding structure based on hydroxide-catalysis bonding (HCB) was proposed which can match the requirements of the cladding structure and evade the organic glue in the high flash-lamp irradiation slab cavity. An antireflective film was used as the medial material whose refractive index was matched with the cladding structure. Bonded samples were prepared based on HCB and confirmatory experiment under high flash-lamp irradiation was finished.

The backscattered light originated when machining with femtosecond laser pulses can be used to accurately measure the processed surface position through an interferometer, as recently demonstrated by our group, in a setup that uses the same laser beam for ablation and inspection. The present work explores the characteristics of the laser light reflected by the target and its interaction with the resulting plasma to better understand its propagation physics and to improve the dynamic focusing system. The origin of this returning radiation was studied and has been traced, mainly, from the peripheral area of the focal spot (doughnut-like). By means of a Mach-Zehnder setup, the interferometric pattern was measured and analyzed aiming to access the influences of the plasma on the laser beam properties, and therefore on the retrieved information. Finally, the wavefront of the laser that creates and propagates through the plasma was characterized using a Shack-Hartmann sensor.

Worthwhile applications for laser beam ablation in the macro range are found wherever hard materials, short process times and high precision are required. The laser beam ablation of thick-walled metals is gaining attention, especially in the field of industrial dismantling and the elimination of hazardous substances. This requires a high degree of flexibility, both in the machining process and in terms of mobility with regard to the place of use. Special attention is given to situations where sensitive, contaminated or explosive materials are behind the metal walls to be removed. It must be ensured that these substances are not influenced critically under any circumstances. In this context, a process of removing hazardous substances in closed cavities is developed. To prevent risks by initiating a reaction with these dangers, the thickness of the 10 mm plates is reduced without applying mechanical force and without exceeding a critical temperature. The volumetric rates of laser beam ablation have hitherto been severely limited by the requirement of high surface finishes. This article shows that the volumetric rates can be significantly increased by means of an adapted process strategy and an off-axis process gas control. Despite high volumetric ablation rates and the associated high laser energy, it is ensured that there is no danger to the hazardous substances. The critical temperature of 300°C which is several times lower than the melting temperature of the sheath, is not exceeded at the rear surface of the sheet metal. Results are presented which show how different process parameters affect the ablation depth and the workpiece temperature.

Boron thin films were produced using femtosecond pulsed laser deposition aiming at the development of a neutron detector. As neutrons have no charge, in order to detect this particle converter materials are applied, promoting nuclear reactions that result in the emission of charged particles, allowing the neutrons presence to be indirectly inferred. Among the possible conversion materials, ¹⁰B has a considerable cross section for thermal neutrons and accessible cost. Furthermore, the nuclear reaction produces easily detectable alpha particles, making it a rational option to develop a low cost and portable neutron detector. The boron ablation threshold fluence has been measured by the Diagonal Scan (Dscan) technique that resulted in the minimum laser energy value of 17.7 (6) μJ and fluence of 5.63(19) J/cm². Boron deposition was performed varying the pulse energy and deposition duration. The growth rate, morphological and physical aspects of the boron pulsed laser deposition were characterized by a Scanning Electron Microscope and an optical profilometer. The films surfaces have a flaky aspect with eventual droplets which had decayed overtime to a more smooth surface. The studied parameters allowed producing a boron coating with the optimal thickness in order to minimize self-absorption effect in the film, thus increasing efficiency.

Laser structuring of tool surfaces with circular spots and diameter below 30 μm as well as laser decoating with line spots above 50 mm are established in industry. For the fulfilment of processes with intermediate requirements, e.g. local laser pretreatment on 3d surfaces of carbon fiber-reinforced plastics (CFRP) for subsequently intrinsic joining, an established technology is missing. Attuned robot and scanner systems with high flexibility and sufficient accuracy, as well as application oriented path programming and a methodology for shorting cycle times are missing. A new integration approach enables for the first time an overlapping workspace of injection molding machine and robot-guided 3d laser scanner for in-mold processing, adaptable to a wide range of polymer processing machines worldwide.

Large-area surface processes at components with a production volume >30.000 pc./a require efficient path generation and processing. Supplementary to the 3d segment process developed by Fraunhofer Institute for Production Technology IPT tools are developed for scalable robot-guided decoating process. With respect to scan field size and angle of incidence a software is set up, which inherently allows a free implementation of genetic algorithms for finding a sufficient solution with the minimal number of scan fields and the corresponding optic positions. Furthermore, process optimization is done by a systematical analysis of resin ablation and heat affection using NIR short pulses at single spots, lines and rectangular. The conflicting objectives of increasing adhesive strength and decreasing cycle time for the specific FRP application is analyzed and quantified.

Conventional heat treatment of a component is performed in a furnace whereby the component is continuously heated to a high temperature and quenched to get a hardened material. The furnace heating and tempering process take several hours which is expensive over a long run and less flexible. Laser transformation hardening is an attractive heat treatment technology which can be used to enhance the surface properties of highly stressed components such as cams, gears, and bearings without altering its bulk properties. The highly intense laser beam rapidly heats up the irradiated surface above its austenitization temperature which cools down instantaneously (self-quenching) as the laser moves away from the spot producing a hardened surface. The fast heating and cooling generate a non-equilibrium phase transformation of which very little is understood. An attempt was made to improve the surface properties of steel through solid solution hardening and microstructure refinement using a 250 W fiber laser. To identify the effect of various parameters on laser hardening, scanning conditions such as beam spot size, scan rate, power input, surface condition and overlap ratio were controlled. The change in hardness and morphology of laser treated surface were carefully investigated. The results show the surface hardness increased above 800 HV after laser treatment compared to 260 HV of the as-received specimen. It is found that austenitization has the highest effect on hardness achieved and can be controlled by proper choice of laser parameters and scanning rates.

Diamond exhibits incredible chemical and mechanical properties. Utilization of diamond for industrial applications has universally spread thanks to the discovery of its industrial synthesis and the enhancement of its properties by its formation under a polycrystalline form, named polycrystalline diamond composite (PCD). For industrial production PCD bulk material has to pass through several complex processes from leaving the belt press down to be polished as a wafer with a mirror-finished surface. Especially the polishing process is an extremely laborious method originally discovered to polish diamond gemstones hundred years ago. Laser ablation has repeatedly been proven to be a cost efficient process for industrial purposes. Elaboration of new ablation processes for PCD manufacturing is increasing supported by the continuous improvement of ultra-short lasers and their high performances. The ambition of this project is to develop an alternative polishing process of PCD wafers by ultra-short laser ablation achieving better performances than traditional mechanical polishing. This paper presents a relationship between material related properties of various PCD grades and performance of ultra-short laser ablation process at low laser average power. Specifically, this paper demonstrates the high impact of the pulse duration on the ablation rate, the dependence of the optimal fluence for highest ablation rate on the PCD composition and the effect of fluence on diamond graphitization through Raman spectroscopy analysis.

We demonstrate the use of kW class, high energy pulsed nanosecond lasers for surface cleaning applications. A hybrid ablation-detachment process allows removal rates of semi-transparent paint from primer at 40,000 mm³ /kW.min. The process shows a threshold at which a nonlinear increase in removal rate is observed for energies above 100mJ for most paints. The impact of pulsed laser de-painting on substrates is examined.