This PDF file contains the front matter associated with SPIE Proceedings Volume 12414, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Laser Shock Processing (LSP) has been demonstrated as an effective technology for improving surface and mechanical properties of metals. The main recognized advantages of the technique consist in its capability of inducing a relatively deep compression residual stresses field allowing an improved mechanical behaviour against fatigue crack initiation and growth, mechanical wear and stress corrosion. Although significant experimental work has been performed in order to explore the optimum conditions for the application of the treatments and to assess their capability to provide enhanced mechanical properties, only limited attempts have been developed in the way of physical understanding of the characteristic processes and transformations taking place under the LSP regime. In the present paper, the integrated numerical-experimental approach to LSP processes design developed by the authors is presented, the incorporation of increasingly more accurate models for the characterization of metallic materials behaviour under LSP conditions being an always present objective. Different practical results at laboratory scale on the application of the LSP technique to different materials with different irradiation parameters are presented along with physical interpretations of the induced mechanical effects. Concrete issues as laser-plasma interaction in the ns, GW/cm² regime, material behaviour description for cyclically compressed matter, numerical simulation methods for the coupled plasma-thermomechanic analysis, practical implementation of the technique according to different approaches, etc. are discussed in the view of the last developments contributed by the authors and, finally, a tentative summary of the still open questions for the better knowledge and control of the process is presented.

Avoiding Pile-Up during spectral X-ray measurements during ultrafast laser processing with a single detector requires long measurement times at large distances from the processing area due to the short pulse duration and high photon fluxes. To enable fast measurements, an algorithm is presented which calculates the underlying Pile-Up free spectrum of any measured spectrum. Therefore, a statistical approach was used to describe the mean number of photons ⟨𝑛⟩ and their corresponding photon energies E_ph hitting the detector at each pulse. This number of photons hitting the detector each pulse was assumed to be geometrically distributed, whereas the photon energies follows a modified Maxwell-Boltzmann distribution, mainly defined by the temperature T_hot of the hot electrons in the laser plasma. An initial guess of ⟨𝑛⟩ and T_hot was used to calculate an expected Pile-Up spectrum at the detectors position. Comparing the calculated Pile-Up spectrum with the measured one and iteratively adjusting ⟨𝑛⟩ and T_hot results in the underlying Pile-Up free spectrum.

Hollow-core fibers (HCFs) have been under intense research interest thanks to their many advantages including low latency, low nonlinearity, and temperature insensitivity. The most recent progress on the double nested antiresonant nodeless fiber (DNANF) demonstrated fiber losses of only 0.174 dB/km. Transmission of ultra-short, high-peak-power pulses can greatly benefit from low nonlinearity of HCFs. However, the waveguide dispersion in HCFs such as DNANF is typically 2-3 ps/(nm·km) in the low-loss transmission region, still causing unwanted pulses broadening. Here, we demonstrate a low-loss interconnection between HCF and a dispersion-compensating fiber (DCF), enabling to obtain HCF+DCF link with zero-net dispersion. To adapt the relatively small mode-filed diameter (MFD) of DCF (4.9 μm) to the MFD of the HCF, we first splice a short segment of graded-index (GRIN) multi-mode fiber on the DCF. The GRIN fiber is then polished to a specific length to obtain an optimal MFD adaptation to our HCF, which was a nested antiresonant nodeless fiber (NANF) with 26.3 μm MFD at 1550 nm. We obtained a loss of only 0.55 dB for the whole DCF-GRIN-NANF component. By depositing an anti-reflective coating on the mode-field adapter end-face, the interconnection loss can be further reduced to 0.39 dB.

Modern ultrashort pulsed high power (USP) lasers enable advanced and highly parallel micromachining processes. Due to constraints of the system technology, parallelization and beam shaping is mostly static during the process, which limits many interesting applications. The increasing power limits of liquid crystal on silicon (LCoS) spatial light modulators (SLMs) now open the door to dynamic beam shaping and splitting in the machining process. Although the frame rates of high-performance LCoS displays are typically in the range of 60 Hz and above, these devices are still mainly used as programmable diffractive optical elements (DOEs) and usually remain static during the fabrication process due to the high computational cost of conventional computer-generated hologram (CGH) algorithms. Herein, we report on the application of real-time CGH algorithms for dynamically generating and modifying 3D spot-distributions for advanced micromachining processes with high power USP lasers. We use a recently demonstrated algorithm based on compressive sensing combined with a highly parallelized computation on graphics processing units (GPUs). This allows for the calculation of phase holograms on the timescale of typical SLM response time and enables an online adjustment of the beam shape and spot distribution. In a detailed investigation of different CGH generation strategies, the impact of hardware response times, calculation speed and heat accumulations on the parallelized multi-spot laser process is experimentally evaluated on silicon. The presented results reveal the challenges and limits of on-the-fly CGH computation in USP laser material processing with SLMs and assesses its gain in processing flexibility for versatile USP micromachining applications.

Laser processing is widely used in the industrial field, nonetheless, there is a tremendous amount of time required for prototyping products using multiple parameters such as laser power and number of laser pulses, which is necessary to be adjusted based on targeted material, shape, and environment. Optical coherence tomography (OCT) using near-infrared light, which has the ability to measure depth of laser processed holes or grooves, is a promising imaging technique for reducing time for trial-and-error. We explored the possibilities of developing a predicting algorithm for laser processing parameters using a spectral-domain OCT (SD-OCT) with a center wavelength of 900 nm. Sapphire, copper, silica, aluminum, and brass were processed by our pulse laser with a wavelength of 1030 nm, pulse width of 290 femtoseconds (fs), and repetition rate of 100 kHz. Different holes with laser powers from 5 to 20 W and pulse cumulative numbers from 10 to 100 counts were fabricated. Diameters and depths of holes were measured by a SD-OCT as well as a laser scanning microscope as a reference. The measured results using a SD-OCT and a laser microscope were compared, and the averaged difference was less than 19.0 %. Next, a predicting plane, relating laser processing parameters consisting of laser power and cumulative laser pulses to the measured parameters consisting of diameter and depth, was studied. The predicting relational plane obtained by a Sapphire sample predicted the hole shapes of copper, silica, aluminum, and brass within 30.1% of coefficients of variation (cv). In addition, the SD-OCT system imaged clear microcracks in sapphire samples that were not captured by the laser microscope. These results indicate that an integrated system of the OCT and a laser processing machine has promise for predicting laser processing parameters and reducing time for trial-and-error.

In industrial high power laser welding and cutting applications there is an increasing need for high-speed process monitoring and control to meet industry demands on increased productivity, quality, traceability and reproductivity as Industry 4.0 standards are adapted. The latest generation of Coherent’s QD Fiber Optic Cables has the option for integrated process monitoring sensors and functions inside of the fiber cable connector (SmartQD). This provides a compact process monitoring solution, with no need for installation of additional equipment, where the sensors come prealigned out of the box for monitoring parallel to the optical axis. The water-cooled QD fiber connector provides protection from the industrial environment and keeps the sensors at a defined temperature. With carefully selected and developed optics and electronics the sensors inside the SmartQD connector captures process light at three different wavelength ranges: back-reflected laser light, near infrared (1200 – 1600 nm) and visible (300 – 700 nm). The SmartQD sensors have a maximum bandwidth of 500 kHz, > 12 ENOB. The connector itself is equipped with an industrial electrical connector with either analog or digital data interface. The process data is then evaluated with an external program. In this paper we present welding result on various materials, such as aluminum and mild steel using the SmartQD fiber cable. We present results on detecting weld defects and imperfections caused by for example loss of focal position, misalignment, and material surface defects. Defects and imperfections that are resolved include lack of penetration, incomplete fusion, spattering, undercut, and blow-out.

Powder-based laser metal deposition (LMD) represents a future-oriented additive manufacturing process given the large number of available materials as well as the high degree of geometric freedom. These potentials are further enhanced by high build rates and a better material utilization compared to turning or milling. However, the LMD process is very complex as a result of the large number of machine and process parameters. Different thermal conditions occur during layer-by-layer buildup. Newly supplied energy due to the laser-material interaction with the newest layer is supplemented by the heat energy still stored in the solidified component, which is distributed inhomogeneously depending on material and part geometry. In order to avoid dimensional deviations and microstructural defects, the machining parameters have to be adjusted during the LMD process. The following paper presents a strategy for the efficient and process-safe buildup of thin-walled VDM Alloy 780 geometries on 316L substrates using powder-based LMD. Adaptive adjustment of parameters and monitoring of cooling times improve the dimensional accuracy of the geometry while avoiding defects. The fabricated specimens are then analyzed using optical microscopy, SEM and EDX. Porosity analysis is performed by various methods. In addition, mechanical properties are determined by tensile tests in perpendicular and parallel directions as well as by macrohardness tests.

There are many distinct commercial sensor systems for monitoring or controlling laser processes. Most of them are based on camera or photodiode technology. Furthermore, the introduction of real in-process measuring systems in laser material processing like OCT has significantly increased the safety of both defect detection and process control [5][6][7][8][9]. The focus of this contribution, however, relates to the use of artificial intelligence algorithms to "See New Things". We will discuss how classified, physical properties can be derived from already reliable process information - "seeing the unseen", so to speak. Instead of defining complex rules for algorithms, the use of Data Science and Machine Learning methods reveals hidden structures in noisy unstructured data and make it possible to find the relationships of the data to the physical measurement. There are several systems available in the market which capture or measure and analyze physical effects of the process zone and their properties during the laser processing, but none of these effects and properties stand for “seam quality” for themselves, which is typically defined by mechanical, geometrical and metallurgical properties of the solidified seam. The process properties like emitted visible and thermal radiation or geometrical values of the melt pool or the penetration depth of the laser forming the keyhole (penetration depth) only provide indications of how the desired quality might be achieved. When welding thousands of seams per shift during serial production for Li-Ion batteries in e-mobiles, very often users would like to stop their fully automatic machines once the laser process is suddenly running out of previously set boundaries. Such an approach allows users to find systematic faults where and when they occur, rather than at the end of the line where quality inspection takes place and the line is still running, producing more and more parts with the same faults. When it comes to the use of artificial intelligence algorithms the following questions must be answered: ▪ Is it possible to map an input time series to a real value? ▪ Can we get out the strength of the weld from the process emissions? ▪ Is the information buried in the signal? The vision, which can be represented by Figure 1, is that by using the data from process emissions and specific data models, a relationship to physical quantities can be established. Thus, AI goes a significant step beyond the simple "good-bad" statement.

Laser welding as a tool for manufacturing highly precise parts for electronic and electro mobility components is gaining worldwide importance. The precise control of energy input is one paramount challenge of welding highly reflective materials with high thermal conductivity such as copper and its alloys. Laser beam wavelengths in the visible range show an increase in absorptivity from < 5% (1030 nm) to ≥ 40% (515 nm) on copper at room temperature and open new options in material processing technologies. This paper presents the in situ observation of laser welding processes on Cu-ETP and CuSn6 with laser beam sources of 1030 nm and 515 nm wavelength using synchrotron radiation at DESY Petra 3 Beamline P07 EH4. The influence of laser power from 1 kW up to 4 kW and feed rates from 50 mm/s up to 500 mm/s on vapor capillary geometry and dynamics with same focal diameters is compared. For the investigations, a synchrotron beam of 2x2 mm² in size with a photon energy of 89 keV is used for investigation. The material samples are analyzed by means of material phase contrast method to show boundaries between solid, liquid, and gaseous material phases. It is found, that both welding processes show a different geometry of the vapor capillary. A different sensitivity to changes of the feed rate of the welding process is observed. The vapor capillary of the 1030 nm welding process tends to be more sensitive on feed rate changes while showing an overall better weld seam quality. When welding with 515 nm, comparatively higher feed rates lead to better welding results.

New lightweight construction concepts should make it possible to reduce the weight of aircrafts and vehicles. This has an effect on energy consumption, in order to reduce CO₂ emissions. New material combinations are used to implement these concepts, or in some cases, plastics substitute metals. The production of single-variety plastic components is particularly advantageous, as these can be returned into a recycling cycle. Thermoplastics are suitable for the production of such composites. If such parts are joined by means of adhesive bonding, the adhesive would remain on the components and prevent sorted recycling. Welding does not have this disadvantage. Wide weld seams are required for the transmission of high forces. Applying laser welding, wide weld seams could be generated using conventional beam shaping techniques using a homogeneous intensity distribution. However, such an intensity distribution is critical if the component geometry has small radii. To solve this problem, a welding system was developed as part of the MultiSpot project, which makes it possible to adapt the intensity distribution to the weld path. For the evaluation of the new welding system, different intensity distribution profiles were developed based on bead on plate welds and then transferred to the demonstrator components.

The ongoing development of lasers with integrated beam-shaping features offers new opportunities for research in the field of laser beam welding. Main challenges for laser welding of aluminum alloys, such as AW-5083 or AW-6082, are pore formation and cracking. The locally adapted power distribution has a major influence on the keyhole required for deep penetration welding and therefore on occurring weld defects. To determine the reasons, the use of synchrotron radiation during the welding process enables the visibility of the keyhole shape. This investigation contains an analysis of the keyhole and melt pool shape based on the high-speed video images of the synchrotron radiation. The variation of the power ratio of center beam to ring allows a comparison of the influence of the beam shaping method. To stabilize the keyhole, a temporal power modulation is used for the ring. The results show an influence of the ring mode on the keyhole depth and the melt pool shape. The videos allow an analysis of different mechanisms for pore formation, such as the collapse of the keyhole.

The Laser assisted double wire with non-transferred arc surfacing process (LDNA) is based on an electric arc to melt the filling wires and a laser beam to shape the melt pool. This work investigates the welding seam characteristics for different oscillation amplitudes with differing laser output power. Therefore, three laser powers between 1,000 W and 2,000 W in combination with a power of 4,800 W brought by the arc are investigated regarding the resulting seam geometry and the dilution ratio. A linear oscillation pattern is used with a constant oscillation frequency of 10 Hz. The investigations are conducted using AISI 316L welding wires of 1.2 mm in diameter and sandblasted AISI 1024 plates of 20 mm thickness. Cross-sections are analyzed to investigate the occurrence of cracks and pores as well as to determine the dilution ratio with the base material. The welding seam geometry is measured with a laser scanning microscope Keyence VK-X1100. The topology of adjacent welding seams is examined showing an increase of the welding seam width with an increase of the oscillation amplitude and the laser output power. Thus, a higher maximal seam width can be applied by adjusting the laser power.

The coaxial Laser Double-wire Directed Energy Deposition (LD-DED) process is capable of providing two wire materials simultaneously into a common processing zone. Thus, in-situ production of alloys in a local manner or across the entire sample can be realized with the characteristic high material utilization of the laser wire Directed Energy Deposition (DED) processes. Fabricated samples show a homogenous distribution of alloying elements across single welding seams enabling a functionally graded transition zone along multi-layer samples. This work shows the potentials of the LD-DED process for the production of Functional Graded Materials (FGM). Therefore, the process is displayed and single welding seams are examined regarding the element distribution along the seam with a graded material distribution. The samples are produced with a horizontally graded material transition using 1.4430 and 1.4718 stainless steel wires. All samples are fabricated using the multiple Diode Coaxial Laser (DiCoLas) processing head of the Laser Zentrum Hannover e.V. The processing head provides the materials under a small angle of incidence and utilizes three fiber coupled laser diodes to supply the necessary thermal energy for the melting process of the base and wire materials. Using Energy-Dispersive X-ray spectroscopy (EDX) line-scans and mappings to determine the element constituents along the cross-section, a graded transition of elements in the horizontal direction can be detected. Images captured with a Keyence VK-X1100 3D-laser-scanning microscope provide information of the cross-section quality regarding material defects and surface quality. Furthermore, the Vickers hardness progression along the building direction is measured.

The available (average) power of high-power lasers is steadily increasing. This poses the challenge of providing this power dynamically tailored to the respective laser processing application, be it surface structuring, cutting or 3D printing, in order to ensure efficient and high-quality processing. In dynamic high-power laser beam shaping, a compromise usually has to be made between the applicable amount of (average) laser power and the degrees of freedom for the beam shaping device. In general, the higher the damage threshold is, the fewer are the degrees of freedom for available beam shaping devices[1,2]. One way to overcome this deficit is to first shape the beam with a high resolution and low power output and then amplify the beam to the necessary laser power. The subsequent amplification introduces unwanted changes in the desired beam shape, which needs to be compensated. The current method to compensate the amplification induced changes is to exactly simulate the optical system at hand as well as the amplification process. For this purpose, an Iterative-Fourier- Transformation-Algorithm (IFTA) combined with an additional optimization is used. This method requires prior knowledge of all system and amplification defining parameters, which are non-trivial to determine. Another approach, pursued in this paper, is the use of an artificial neural network (ANN). The ANN is trained through the combinations of different phase masks and the resulting beam shape profiles. This training method should allow the ANN to indirectly map any optical system without any regard to its complexity. Through an appropriate choice of training samples and subsequent training the ANN is able to approximate the mapping function of the optical system including the amplification. The fully trained ANN generates phase masks for the beam shaping process in one step and thus allows highly dynamic beam shaping of arbitrary beam shape profiles.

We have developed a high-throughput laser marking system using a programmable multi-spot modulated line beam capable of >13x throughput enhancement over a single-spot system. While commercially available lasers have been rapidly growing in output energy and power, single-spot marking systems cannot take full advantage of higher laser outputs without causing damage to the material. This system provides high throughput, high resolution marking on a variety of surfaces including stainless steel and polymer. This high productivity system is enabled by a high-power MEMS spatial light modulator called the Planar Light Valve (PLV^TM). The PLV is a 1088-pixel device in a linear configuration supporting up to 200 kHz modulation. The PLV supports pulse energies of 920 μJ with pulse widths down to 200 femtoseconds and CW power of 1 kW at wavelengths of 355-1070 nm. In this system the PLV is imaged onto the work surface to create roughly 100 segments which are individually addressed to select the laser fluence in each spot with grayscale control. The linear array is scanned across the media using precision X-Y stages. The resulting feature size is 20 μm yielding high resolution 1270 dpi images. Each spot on the work surface is made up of several PLV pixels, which allows precise edge placements. In this demonstration a 100 W laser is used for a 13x throughput enhancement over a single-spot system with 4x better resolution. This optical system can be adapted for many laser processing applications such as additive manufacturing, lithography, and micromachining.

The quality, efficiency and robustness of laser materials processing significantly benefits from an application-adapted beam shaping. For a successful realization of tailored beam shapes that utilizes state-of-the-art optic concepts, research in two different steps is necessary. First, information about the required beam shape (especially in terms of intensity distribution) is needed. During the last years, we have developed and validated a method to solve the so-called inverse heat conduction problem which enables the derivation of an intensity distribution that specifically tailors the induced temperature profile within the processed material. Here, we present the latest enhancement of the algorithm to complex time-dependent scenarios and new applications, e.g. from the field of surface treatment and tape placement. With the knowledge of the target beam shape, in a second step, optical systems must be designed that enable the realization of the achieved, highly complex intensity distributions. We further demonstrate the potential and limits of novel optics such as freeform mirrors, LCoS-SLMs or diffractive neural networks as well as VCSELs for application-adapted beam shaping. We especially present specific design methods that can contribute to a robust, flexible, and practical realization in various applications.

Optical fibers made of fused silica have many applications like telecom, industrial, medical, or spectroscopy. These applications are as varied as are the requirements for the fibers. Large core step index multimode fibers made of high purity fused silica core and fluorine doped silica cladding are common standard in industrial high-power laser, minimal invasive medical and spectroscopic applications. One of the biggest factors influencing fiber performance is the utilized fused silica core material and its composition. Hydroxyl groups and trace impurities, for example, can influence the transmission properties of the fiber. But also, defect centers created by strong UV radiation or drawing induced absorption bands define the performance over time. Therefore, the choice of the right material is key for high performing fibers. In addition, the height of the refractive index step between the core and the cladding, which defines the numerical aperture, as well as the cladding thickness and cross section design of the fiber are important factors which should be balanced against performance and costs. Depending on the application wavelengths and performance requirements, different fused silica materials and fiber designs are recommended tailored to the application. We will present deeper insights in the optical properties of different fused silica materials and new silica material developments dedicated for the increasing utilization of blue and green lasers to give a guideline to choose the best fiber type depending on the application wavelengths.