This PDF file contains the front matter associated with SPIE Proceedings Volume 12878, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

We present an overview on the development and characterization of multiscale laser processing optics for versatile material modifications across more than six orders of magnitude. Starting with solutions for micromachining we present high-NA microscope objectives creating sub-wavelength material modifications on macroscopic scales with highest peak intensities. Moving on to the millimeter range, the adaptability and scalability of scanning optics is examined for large-area machining. Finally, we explore line beam optics in the meter range, evaluating their use in uniform material processing using average powers above 100 kW. This study provides an insight into the design and performance characteristics of such optics and demonstrates their potential in advanced laser processing.

We present the design of a high-power suitable processing-head used for laser material processing with compact piezo-driven kHz-range 3D beam steering in combination with a multiplexed output beam consisting of three different laser sources. The common beam consists of an 8-kW disc laser that is used to provide the bias-level of energy to preheat the work piece during welding or cutting close to the ablation threshold. The second laser is a MHz-switchable polarization-modulated kW-class Yb-doped fiber amplifier, which in combination with a high-power suitable polarizer transfers the polarization-modulation into a fast switchable amplitude-modulation that is finally used for the material processing. Furthermore, for the purpose of process control during cutting and welding a spectrally shifted 10 W-class fs-pulsed laser is incorporated and used for Laser-Induced Breakdown Spectroscopy (LIBS). Dynamic beam-steering and focusing in the range of several kHz at the output of the processing head is included utilizing a piezo-driven tip/tilt-mirror and a deformable mirror. Further details on the construction of the process head as well as on the different laser sources will be discussed in the presentation. Based on previous teaching of an AI-network with the obtained data from various parameter sweeps, such as camera images of the backscattered light, LIBS measurements and the inspection of the cutting edge or the welding seem, online process optimization shall be granted to improve process quality and cost-efficiency along with a decrease in yield during production.

It is known that a ring profile using a double-core fiber is effective for decreasing spatter and improves penetration stability for copper and aluminum welding. We considered creating a ring profile with an axicon lens and a single-core laser instead of a double-core laser. However, the light emitted from the delivery fiber is sometimes biased, and thus our ring profile becomes unbalanced beam. In order to correct unbalanced beam, we made a tilt correction mechanism and an alignment profiler. This alignment profiler is detachable and has an attenuator, ND filter and damper. Two beam splitters used for the attenuator were arranged in V-shape, because we have to measure, not collimated light but condensed light. V-shape is able to cancel the transmittance bias of the beam splitter because it has a different transmittance by the incident angle dependence. And furthermore, in order to correct astigmatism caused by V-shape beam splitters, the scale in the beam splitter direction was corrected by the software for the axial direction perpendicular to the beam splitter. By developing such a device, it was possible to visualize the unbalanced state of the ring, and corrected the unbalanced ring to a balanced ring. Thereby, we studied the spatters and penetration stability about copper welding.

Multi-Cladding (MC) fibers with undoped silica core and alternating highly fluorine doped and undoped silica cladding layers are a new standard for high power laser beam transmission. Such MC-fibers allow tailoring the output beam profile of a single laser source to the desired application. An important factor for these applications is to avoid additional scattering of the guided light in the different regions leading to an increased output divergence, so-called Focal Ratio Degradation (FRD). In this work, we compare two possible measurement concepts to determine the beam divergence angle under controlled launch conditions to quantify FRD of MC fibers. One approach is the pinhole setup historically used in astronomy; another approach is the inverse far field method with selective excitation. The objective is to define a reliable setup for quality inspection. We show measurement results for FRD of MC fibers with both methods and evaluate the differences between the light-guiding sections. To our knowledge, this is the first time such data are presented. The objective described above is best fulfilled by a modified far field method with defined broad angular excitation.

An innovative approach to enhance high-power laser material processing through the integration of a high-speed motion system combining galvo scanner and linear axes is presented. The research focuses on its application in Printed Circuit Board (PCB) depaneling with lasers, showcasing its precision and efficiency. The combined motion system, featuring a galvo laser beam scanner and multi-axis linear actuator structure, is custom designed to meet the specific demands of PCB depaneling. By combining the strengths of both components, complicated cutting patterns are achieved while minimizing thermal and mechanical stresses on the workpiece. Key to the system’s success is the seamless integration of motion components, facilitating precise coordination and synchronization during laser processing. This ensures real-time transmission of control commands and feedback signals across multiple axes, optimizing the system’s accuracy and efficiency. The motion system enables combined kinematic laser processing with fieldbus cycle times of 250 μs and galvo 2D repositioning time each 25 μs. This is achieved with own developed electronics and FIFO buffering of interpolated XY positions to achieve the desired trajectory within microseconds. This paper describes the analytical model used to achieve the combined motion as well as validation of motion and cutting velocities within the system. The experimental results show excellent process efficiency and high cut quality for several tested materials. As a conclusion, a high-speed motion system with coupled kinematics shows substantial advantages in laser materials processing thus revolutionizing manufacturing processes.

Laser welding is crucial for manufacturing e-mobility components, particularly copper and aluminum parts. However, their high reflectivity and thermal conductivity present challenges, leading to inadequate penetration and weaker welds. Beam shaping offers a promising solution by modifying the laser beam's intensity distribution. In this study, we demonstrate successful welding of aluminum battery cases, copper busbars, and hairpins using Multi-Plane Light Conversion for beam shaping. Results show improved weld quality, reduced defects, and enhanced mechanical properties. The technique provides a higher depth of field and an extra degree of freedom for optimizing weld quality, promising efficient and reliable manufacturing of e-mobility components.

Laser Transmission Welding (LTW) is a well-known technique for joining conventionally manufactured high-volume thermoplastic parts, such as automotive injection molded parts. When LTW is used for additively manufactured parts (typically prototypes, low-volume production, or one-offs), the technology must be developed to overcome the difficulties in welding the parts, that result from the additive manufacturing process itself. Compared to injection molding, additive manufacturing results in an inhomogeneous structure with entrapped air within the volume. Therefore, there is a change in the transmissivity behavior in the weld area due to the additive manufacturing process. In order to make LTW available for additively manufactured thermoplastic components, a process chain was developed to support manufacturing. This process chain ranges from the optimization of the additive manufacturing process to the welding process and is supported by an expert system. For the evaluation of the manufacturing process chain, welding experiments with additively manufactured samples were performed. The transparent samples were welded to black samples with varying process parameters in overlap configuration and tensile shear tests were performed. The additive manufacturing process parameters were used to predict the transmittance of the transparent sample and the weld seam strength of welded parts using the expert system.

The keyhole produced during deep penetration laser welding emits a plume of hot metal vapor and particles. The interaction between the plume and the incident laser beam results in beam scattering, absorption, and phase front deformation. The combination of scattering and absorption leads to a partial extinction of the laser beam, while the phase front deformation adversely effects the beam quality. In this study we present a measurement setup which allows for diagnostics of the beam characteristics after interaction with the plume. This is achieved by utilizing an additional measurement beam, which is coaxially aligned to the high-power laser beam used for welding. The experimental procedure presented here enables high-frequency measurements of the caustic changes and relative power losses of the measurement beam. The measurements obtained provide a quantification of the various interaction mechanisms between the laser beam and vapor plume. This knowledge is crucial to prevent weld defects, which result from the adverse effects of the vapor plume on the laser beam.

Laser Beam Welding (LBW) finds widespread use in industries like naval and automotive. To meet the demands of complex welding processes, higher power lasers have been developed. However, conventional refractive optics limit power utilization, affecting robustness. Multi-Plane Light Conversion (MPLC), a fully reflective technology, enables complex beam shaping with 16kW lasers. A MPLC-based laser head with an 800µm annular shape at 1µm wavelength has been developed. LBW of 304L stainless steel (6mm thick) at 7kW and HLAW of steel (16kW) with 23mm penetration depth are successfully demonstrated. MPLC's extended depth of field improves welding efficacy, showcasing its potential in advancing laser welding applications.

Laser-ablation is an attractive alternative to abrasive/hydro blasting for the removal of protective coatings from metallic surfaces. However, the ablation process can lead to the formation of potentially hazardous gases, including volatile organic compounds and greenhouse gases. In this work, we investigate femtosecond laser-ablation of marine coatings by analyzing the ultraviolet/visible (UV/VIS) emission from the ablation plasma and performing mid-infrared absorption spectroscopy on the ablation fumes. By identifying the atomic/ionic composition of the plasma and the molecular composition of the fumes, we gain additional insights into the laser-ablation process and potential hazards for human health. This work is supported by VILLUM FONDEN (Villum Investigator project Table-Top Synchrotrons, no. 00037822) and the Danish Maritime Fund (LASER-CLEANR, no. 2022-054).

Controlling broadband light on robust surfaces useful for many applications such as solar receivers. In many of these applications, spectral selectivity is desirable with lower absorptivity and emissivity in the thermal infrared. This paper investigates texturing and direct micromachining method using a femtosecond laser to produce spectrally selective absorbers in the Infrared (IR) spectrum on stainless steel. The relationship between process parameters, surface morphology and optical performance is shown including angular dependent infrared reflectance resulting from coupling to diffractive modes when the feature size approaches the wavelength. The results show that highly effective black surfaces with diffuse reflectances less than 1% in the visible and thermal IR reflectances greater than 90% can be achieved.

In recent years, laser welding has been widely used as an alternative to arc welding because of its high power and faster welding speed with local heating. In the welding process, particularly for e-mobility applications, the demand for quality control via all-point inspection is increasing. The laser process enables real-time observation of the welding area during processing, making all-point inspection possible. In this study, we investigated the possibility of predicting weld bead width from a set of images acquired using a CMOS camera with a band-pass filter. Machine learning was used for the prediction, and the prediction accuracy was determined using the Root Mean Squared Error (RMSE). The laser parameters, such as irradiation power and scan speed, and 13 feature values, such as the area, centroid, and rotation angle of the light emission acquired from the images and were used as training data. The RMSE of 0.16 mm was achieved for a bead width of 0.5-1.5 mm, confirming that the prediction was sufficiently accurate. Furthermore, we conducted an analysis with and without spectroscopic images to verify whether spectroscopic images are effective for the evaluation of laser welding using machine learning.

Laser welding has been actively used in automotive manufacturing and other fields because of its non-contact and easy operation. In particular, the demand for high-quality welding of copper and aluminum is increasing due to the growing popularity of electric vehicles. However, conventional laser welding has led to problems such as spatter and unstable weld beads. Assist gas is one solution to improve welding efficiency and stabilize weld quality. In this study, two new nozzles for laser head scanning and Galvano scanning were developed that can simultaneously flow and suction gas to the laser welding area. In butt welding of steel using an infrared laser, fume removal from the laser optical axis and elimination of undercuts were achieved by airflow through the nozzles. In spot irradiation of copper using a blue laser, it was confirmed that fumes could be controlled by airflow and that the gas flow and suction by the nozzle produced differences in oxygen content at different processing positions.

Laser manufacturing of aluminum and titanium alloys is gaining interest in the automotive, aerospace, and defense industries due to their diverse applications. Laser interaction with these alloys involves heating, melting, and evaporation, with evaporation being critical. Selective vaporization of low-melting-point constituents in multi-element alloys can affect stoichiometry. In-situ monitoring of the ejected plume helps control product quality. Optical Emission Spectroscopy (OES) characterizes plume constituents and provides information about excited states. Our study used high-speed imaging and OES to monitor the laser interaction process with AlMg5 and Ti6Al4V alloys. OES analysis revealed a higher rate of magnesium evaporation compared to aluminum, a trend that intensified with an increase in laser power and a decrease in speed. High-speed imaging revealed an increase in spatter and laser-plume interaction with increased laser power and decreased speed. In-situ monitoring during single-shot laser interactions was conducted in a controlled manner with and without Ar gas shielding. The detection of oxidation in the absence of shielding gas through OES analysis highlights its potential for process monitoring.

The aviation market is rebounding post-COVID, driving the demand for lightweight materials to reduce fuel consumption and CO2 emissions. However, machining Carbon Fiber-Reinforced Plastic (CFRP) is challenging and costly. Microdrilling (⪅1 mm diameter) for acoustic linings, consisting of CFRP skins in a sandwich structure, is widely requested. Laser drilling offers advantages such as smaller hole diameters and wear-free machining. To scale up laser microdrilling, process efficiency and heat control are crucial. This study conducted a thermal evaluation using a short pulse laser and thermal camera. The temperature curves were evaluated taking into account results obtained from studies based on a layout using design of experiments.

Optical absorption of silicon atNnear-Infrared (NIR) wavelengths has high temperature dependence which has been utilized for temperature measurement in the current study. We have demonstrated a technique based on transmitted power measurement utilizing a spectrally shaped supercontinuum source, for temperature measurement in a wide range from room temperature to 700⁰C, with an accuracy of approximately 1 ⁰C for the entire range of temperature measurement with an acquisition speed of milliseconds. It has been observed that shorter wavelengths show high absorption at lower temperatures, whereas longer wavelengths show high absorption at elevated temperatures, and vice versa. As a result, for spectra with exponential roll-off towards longer wavelengths, the transmitted power drops sharply as a function of wafer temperature. Hence, higher accuracy of measurement is possible, which is only limited by the intensity noise of the source. The intensity noise of the source is measured as 1.25%. The intensity noise limited accuracy is approximately 1 ⁰C at 100⁰C and maximum accuracy of 0.16⁰C is achieved at 700⁰C.

Non-Destructive Testing (NDT) is vital for product safety, quality, and compliance in various industries. It enables early flaw detection, cost-effective maintenance, and customer satisfaction. NDT encompasses technologies like vision testing, radiation testing, ultrasonic testing, and magnetic testing, which are tailored to specific requirements such as environment, size, and material of the specimen. We present concurrent Photoacoustic Imaging (PAI) and Optical Coherence Tomography (OCT) for non-destructive inspection of Printed Circuit Boards (PCBs). PCBs are integral to electronic products, contributing to their miniaturization and portability. Due to the small size of RLC components within PCBs, high-resolution NDT methods are essential. PAI combines optical excitation and ultrasonic detection based on light absorption, while OCT is a purely optical imaging technique based on interference of backscattered signals. By combining PAI and OCT, we can simultaneously obtain complementary information on light absorption and scattering. However, the development of a dual-modality system combining PAI and OCT is challenging due to the opacity of conventional ultrasonic transducers. To overcome this challenge, we have developed an optically transparent ultrasound transducer for coaxial combination of PAI and OCT. Our approach successfully detects anomalies such as internal thin layer delamination and internal wire disconnection that are difficult to visually observe within PCBs, providing high-resolution PA and OCT images. In particular, unlike conventional pure optical imaging methods, PAI and OCT are depth-resolved 3D volumetric images so that internal defects can be investigated through comprehensive information in the depth direction of the sample. This dual modality using PAI and OCT for NDT holds great promise for inspecting various specimens, including PCBs, and contributes to enhanced quality control and reliability.

This study was conducted to observe the effect of the laser on the microstructure of the cement-based materials by laser cutting the cement-based materials with various material composition. The performance of multi-mode fiber laser cutting on cement-based materials have been investigated in the downward the laser direction with a laser power of 9kW. The experimental variables were laser cutting speed and material compositions including cement paste, cement mortar, and Ultra-High-Performance Concrete (UHPC). In addition, the composition of the materials used in the experiments was analyzed by X-Ray Fluorescence (XRF). In order to compare and evaluate the microstructure of cement-based materials before and after the laser interaction, the microstructure of the cutting side of the specimen was observed through SEM. In addition, changes in the chemical composition of the microstructure were evaluated using EDX analysis. After the laser interaction, the Material Removed Zone (MRZ) and Heat Affected Zone (HAZ) were observed on the cutting side of the specimen. In MRZ, two additional areas of re-solidified zone and glass layer were observed. Unlike cement mortar and UHPC, the cement paste did not have a glass layer because of the low SiO₂ content. Also, it was found that the glass layer is thickened by increasing amount of silicate-based materials through cement mortar and UHPC. Furthermore, chemical analysis indicated the distribution of the largest amount of calcium and silicon among the components of cement-based materials in MRZ and HAZ using EDX mapping.

We have determined the potassium penetration cross-section in chemically tempered glasses by nondestructive nonlinear refraction measurements. The nonlinear refraction as a function of the depth was measure by the Nonlinear Ellipse Rotation (NER) signal using ultrafast laser pulses (40 fs, 1 kHz, ~2000 GW/cm² at 780 nm) from an amplified laser system. For local NER measurements, we have used a long distance objective (20x, 1.5 cm WD) which provides a relatively good penetration resolution (~5.5 microns). We characterized several glasses with different ion exchange treatment time. The potassium penetration depth and cross-section could be correlated with the materials’ hardness.

Small hole structures are widely used in aerospace fields, but they are difficult to fabricate because of stringent quality requirements and the difficult-to-cut materials. In this paper, the shaped tube electrochemical and laser synchronous composite machining method are used to fabricate the small holes on the surface of titanium alloy with high quality and efficiency by taking advantage of the local thermal effect and fast removal rate of laser, and high surface quality of electrochemical machining. The relationship between the size of the entrance and exit of small holes and the machining process parameters was studied, and the influencing factors of the entrance and exit of small holes morphology were analyzed. After optimizing the machining process parameters, the through hole machining with a feed speed of 5mm/min was finally realized on the surface of the material with a thickness of 5mm. Overall, this work has shown that the combination of tubular electrode electrolysis and laser synchronous machining has high potential to improve the efficiency and quality of small hole machining, and this method can be further applied to deep small hole machining.

Online 3D depth profiling of molten pool/keyhole during laser processing is of great importance to evaluate the metrics of the live process. The indirect methods based on visual image and thermal radiation suffer low correlations with pool/keyhole behavior, thus the accuracy is typically low. Optical Coherence Tomography (OCT) shoots a light probe coaxially with the processing beam into the pool/keyhole, hence being able to provide a direct depth measurement. When it comes to profiling the depth of an area, a galvanometer is typically used to scan the area of interests. However, the pool/keyhole intrinsically flows in a highly dynamic mode, the mechanical scanned image suffers blurs and deformation, as a rolling-shutter camera suffers when imaging object is moving fast. To address this issue, a global shutter imaging method is proposed to image the pool area synchronously. A low coherent light is split into multiple fibers, which are then bundled into a core-array fiber and guided parallelly into the laser head, resulting a multiple of interfering pairs captured and imaged at the same moment. The theoretical model of this global shutter imaging method was created and analyzed in terms of the image performance and limitations. A two-core fiber global shutter imaging system was built to demonstrate the imaging performance on molten pool/keyhole. It shows a great potential to capture high quality 3D points of keyhole/molten pool for further fine closed loop control.