United States

Ablation using ultrafast lasers is a commonly used method for creating structures and geometries in various materials, ranging in size from a few micrometers to millimeters. The most widely used system architecture involves the combination of a galvanometer scanner with ultrafast lasers that have pulse durations ranging from a few picoseconds down to a few hundred femtoseconds. In principal, almost any known material can be processed using a laser in this pulse duration regime. However, it is crucial to understand how to machine a specific material to achieve the desired quality, particularly when key parameters, such as the initial surface geometry or material composition, are unknown or may change during the machining process. One way to simplify the process planning and minimize the required knowledge is to use a sensor that can directly measure the surface topography. In recent years, the use of optical coherence tomography for laser ablation has gained significant attention in the field of laser material processing. This technique allows for precise measurement of surface topography and can help with a smart adjustment of the processing parameters to improve quality and to reduced processing time. This paper describes recent advancements in the use of optical coherence tomography to control laser ablation when machining difficult to process parts and materials from additive manufacturing or materials like carbon fiber-reinforced polymer. To achieve this, the OCT was superimposed with the processing laser beam, enabling real-time measurement and closed-loop adjustment of laser and scan parameters with developed software and special hardware.

ICALEO 2023

Application of Closed-Loop Controlled Laser Ablation for High Quality Processing

closed-loop controlled laser ablation

ultrafast laser processing

technical paper

**General Chair Welcome**

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Welcome to ICALEO® 2023!
	
We were excited to host the world’s leading international conference on Lasers and Electro-Optics in Chicago. The Laser has been shown to have the potential to revolutionize solutions for major global challenges like energy usage and supply, global warming, and CO2 footprint. Even the fusion energy breakthrough is based on laser technology. The conference, with peer-reviewed scientific and technical presentations, focused on the newest results in Micro Applications, Macro Applications, Additive Manufacturing, Beam Shaping and Frontiers in Laser Applications. Additionally, the Battery-Manufacturing Track gave an overview of the process combined with the newest advances in laser application results. 

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**Klaus Löffler **  
ICALEO General Chair   
Managing Director, Precitec GmbH

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ICALEO® brings together the brightest minds, seasoned professionals, and pioneering researchers from around the world. With a laser-focused agenda, ICALEO offers an exceptional platform to delve into the cutting-edge advancements, diverse applications, and transformative research that define lasers and electro-optics.

Laser chemical machining (LCM) is a method of laser processing based on a gentle material removal by means of thermal induced chemical dissolution. Since LCM depends predominantly on the surface temperature of the workpiece, the process window is restricted by the appearance of gas bubbles at higher laser powers and their associated shielding effect. In order to extend the process understanding, the influence of the laser power modulation on the removal behavior is investigated in the present work. The experiments were conducted on titanium grade 1 and with phosphoric acid. Based on the response time in experiments with a single step function of the laser power, a threshold special frequency was determined above which a constant removal depth could be expected. To proof this hypothesis, the laser power was modulated rectangularly in time resulting in combination with the process velocity in different spatial frequencies varying from 1 mm-1 to 20 mm-1 . The investigations showed that the removal cavity exhibited sin-shaped variation in depth along the line with a spatial frequency corresponding to the spatial frequency of the laser power. When the spatial frequency exceeded the determined threshold frequency, the cavity depths is constant. This established the basis for generating complex removal profiles by varying the power in the range below the threshold frequency.

Material Removal in Laser Chemical Processing With Modulated Laser Power

Ultrashort pulsed lasers have been proven to foster micro-manufacturing processes such as drilling, cutting, structuring and even welding, respectively. Using ultrashort pulsed laser, these processes are associated to a significantly reduced heat-affected zone, high precision and the possibility to process various materials via single or multi-photon absorption. However, in typical laser machine setups using galvanometer scanners or conventional processing optics, the processing area is limited to a 2 - 2.5 D micromachining with limited workpiece dimension. To expand the potential of ultrashort pulsed lasers for micromachining to a more flexible and large-area 3D micro processing tool, we report for the first time on the design, realization and application of an ultrashort pulsed laser equipped to a 6-axis robot. In addition, we show and discuss a sophisticated beam guidance and focus positioning concept, that are required for such a combination of laser-robot. For the fundamental characterization of the laser robot system, the dynamic laser beam position is evaluated under movement of the robot by CMOS cameras. As a particular result, for the compensation of occurring position deviations of the laser beam during and after the movement of the robot axes, the development of an innovative and high dynamic beam stabilization concept is necessary. In addition, differently pronounced influences of individual axes must be considered on beam positioning. Moreover, first laser cutting and structuring applications are demonstrated to highlight the enormous potential of the laser robot system for a flexible and large 2D and 3D laser micromachining using ultrashort laser pulses.

Realization of an Ultrashort Pulsed Laser Robot System: Characterization, Limits and Application

In laser metal deposition (LMD), the powder is fed into the laser-induced melt pool using different powder nozzles for the purpose of additive manufacturing and the generation of wear and corrosion protection coatings. So far, there are no industrially established in-process monitoring systems for the powder stream, but mainly measuring systems that examine the powder stream propagation offline and without the processing laser. A challenge in implementing an image-based in-process monitoring system is the process illumination, for the segmentation of the powder particles from the background radiation caused by the processing laser and the melt pool. To overcome this challenge, filtering is needed to attenuate the process emissions and simultaneously brighten the powder stream. Therefore, this work focuses on generating a continuous high contrast between the powder and the background. The powder particles are illuminated by a light source mounted laterally to the powder stream in the horizontal plane below the nozzle opening to make the reflecting powder particles visible to the camera. The optical process emissions were characterized during LMD with respect to the influence of an increasing laser power which was presented in correlation to the increasing process emissions. The evaluation of the spectrograms has made it possible, due to the adapted illumination and filtering, to ensure a constantly high contrast between the process emissions and the powder, so that online monitoring of the powder stream was implemented successfully during the LMD process despite the active processing laser.

Characterization of Optical Emissions During Laser Metal Deposition for the Implementation of an In-Process Powder Stream Monitoring

Controlling heat transfer in casting tools is a key quality aspect. It can be improved by selectively applying volumetric aluminium bronze (CuAl9.5Fe1.2) sections in the core of the tools and subsequently depositing these cores with hard-facing H13 tool steel. Directed energy deposition (DED) can be used for both additive manufacturing of aluminium bronze and hard-facing by depositing the filler-material onto a substrate-surface or previously manufactured bodies. A sufficient metallurgical bonding of the deposited filler-material and the underlying layer must be ensured. However, high dilution of the underlying layer and the filler-material negatively affects the desired properties and must be monitored. Optical emission spectroscopy of the DED process-emissions, is investigated by comparing the emission-lines of the individual elements comprising the base and the filler-materials. 
Multiple single tracks using aluminium bronze as the filler-material are laser cladded with varying power, onto the two different types of substrates i.e. mild steel (S355) and hot working tool steel (1.2343). Additionally, single tracks of H13 are deposited with varying laser powers onto an additively manufactured core of aluminium bronze. Both resulting in deposition-tracks with varying dilution values. Multiple emission-lines of Cr, Fe, Cu, Al and Mn are detected and measured (line-intensity). Line‑intensity‑ratios using the element-emission-lines are calculated and correlated with the respective metallographic results of the deposition-tracks (dilution and chemical composition). Deposition-tracks with a higher dilution (CuAl9.5Fe1.2 onto S355/1.2343 as well as H13 onto CuAl9.5Fe1.2) showed an increased line‑intensity-ratio of the underlying-material to the filler‑material. Moreover, this technology was transferred in a multilayer industrial application.

Monitoring the Degree of Dilution During Directed Energy Deposition of Aluminium Bronze and H13 Tool Steel Using Optical Emission Spectroscopy

Directed Energy Deposition is an additive manufacturing process that allows the production of near net shape structures. Moreover, the process can also be applied for the repair of high value components. To obtain structures with consistent good characteristics, the Directed Energy Deposition process requires the implementation of a control system. The current applied approaches for control that are discussed in literature have specifically focus on melt pool temperature control. Pyrometers have been used for such purpose; however, they provide only a single scalar value without any spatial information. In this paper the implementation of a high-speed hyperspectral camera-based system is discussed with a high spatial resolution unlike the pyrometers. Different calibration and temperature estimation procedures for this camera-based system are evaluated and analyzed. The number of effective wavelengths needed for temperature estimation will be discussed in detail and provide an outlook to the potential of this hyperspectral camera-based system. In addition to the number of wavelengths, another important aspect of the temperature estimation methods is the stability with respect to disturbances. Within this paper the impact of different parameters such as: the scanning strategy, the nominal laser power will be evaluated on the stability of the temperature signals for a control system.

Comparison and Analysis of Hyperspectral Temperature Data in Directed Energy Deposition

The rapid growth of electric vehicle industry requires reliable high-speed and high-quality manufacturing processes. One of the important components of the electric vehicles is the motor which incorporates a large quantity of welds to join wires. These wires are insulated with a variety of polymers to prevent shorts in the motor. Insulation removal and stripping are essential to achieve welds with acceptable electrical and mechanical properties. However, conventional mechanical stripping of insulated hairpin wires is inadequate for high-speed mass production due to the slow process speed, poor surface quality, long-term tool wear, and production line interruptions. The use of a CO2 laser for stripping the polymer followed by laser welding as an alternative can address speed limitations of conventional mechanical methods. However, single-step CO2 laser stripping is known to leave a thin polymeric residue on the copper surface that may result in welding defects such as excessive porosity formation, brittleness, and excessive spatter and therefore not acceptable. Herein, a two-step stripping and cleaning process using CO2 and UV lasers is introduced that enables effective insulator removal to achieve low-defect low-spatter welds. Analytical techniques are utilized to assess the surface quality and correlate that with weld quality. Results will be presented.

Novel Laser Stripping and Cleaning Approach to Enhance Laser Welding Quality of Electric Motor Hairpins

High power lasers have become an industry standard for electric drive manufacturing as has the common design of such motors, called ‘hairpin motors’. The term refers to the structure of the windings and&nbsp;has been adapted by all relevant automotive OEM and suppliers. TRUMPF is by far technology leader in this specific manufacturing area and a vital part to all new developments. 
Highly integrated lasersystems are utilized, including high power cw lasers with beam shaping and high beam qualities, scanning optics and camera based as well as OCT sensors for weld joint and seam tracking. 
In this work we are presenting the latest advancements meant for moving away from static processing strategies and parameter sets towards adaptive and automated processing taking into account the challenges specifically evolving from parts clamping and tolerances. By adjusting the processing strategy based on acquired measurement data the welding results were successfully stabilized under the presence misalignments. Typical misalignment types ‘gap between pins’, ‘lateral offsets’ and ‘height differences’ were simulated and welded in different variations. To prove the effectiveness of adaption, a comparison of static welding results to the ones of adapted welding has been carried out. The process of compiling the adapted strategies to level misalignments will be demonstrated. Evaluations have been carried out looking into joint strength, weld sections, porosity formation and weld seam shape and will be shown in this presentation. Finally, a generalizable systematic on adapted processing strategies will be derived based on results of further, different wire types.

Systematics for Increasing the Robustness of the Welding Process of Electric Drive Copper Hairpin Windings

We investigate the impact of femtosecond laser wavelengths on the properties of inscribed Nd: Y3Al5O12 (Nd: YAG) waveguide lasers. The flexibility and adaptability of femtosecond laser inscription have garnered significant interest due to numerous possibilities for 3D micrometric scale modification within dielectric materials. By controlling writing parameters such as pulse energy, translation velocity, and track separation, we can control the laser-matter interaction process and the resulting waveguide properties. However, the impact of the source wavelength on the resulting micro-modifications inside crystalline materials has been relatively unexplored until now. 
We conducted a comparative study of double-track Nd: YAG waveguide lasers inscribed with femtosecond laser pulses at 515 and 1030 nm, examining the effects of the source wavelength on micro-modifications. Our results indicate that the modification threshold was around 0.06 µJ for the 515-nm source, while it increased by five times to 0.3 µJ for the 1030-nm source. We also observed differences in track shapes and depths for varying pulse energy. A propagation loss lower than 0.2 dB/cm was achieved for both wavelengths – it is the lowest value reported for similar waveguides inside Nd: YAG crystal. The lasing efficiency is 41 % and 15% for the wavelength of 1030 and 515nm, respectively, at 20 % output coupler; the corresponding pump-power thresholds are 9 and 22 mW. The impacts of scanning speeds and track separations on propagation loss, refractive index change, and lasing performance will be presented.

Guiding and Lasing Comparison of Nd: YAG Waveguide Lasers by Femtosecond Laser Inscription at 515 and 1030 NM

In this study, we develop mid-infrared (MIR) waveguides and beamsplitters for applications such as spectroscopy, chemical sensing, and remote sensing using ultrafast laser inscription (ULI) within chalcogenide glass IG2 (Ge33As12Se55). MIR photonic integrated circuits (PICs) enable compact and efficient optical layouts. IG2 glass is ideal for MIR PICs because of its broad transparency window, low absorption, and compatibility with atmospheric windows at 3-5 and 8-12 µm. 
The waveguiding properties at 1.064 µm (Near-IR) and 4.55 µm (MIR) were investigated by examining the effects of inscription parameters and geometries, particularly the impact of pulse energy and the number of layers for a multi-scan strategy on the mode confinement and the morphology the waveguide cross-section. We determine that higher pulse energy or a higher number of layers is required to achieve guided modes at the longer wavelength. For example, for the single-layer configuration, the threshold pulse energy that enables the guided modes at the 1.064 and 4.55-µm wavelength is 3 and 6 nJ, respectively. Mode confinement at MIR was further achieved at 6nJ when two or more vertical layers were applied, achieving a minimum propagation loss of 1.8 dB/cm with a four-layer configuration. Waveguides with cross-sections larger than two layers exhibited higher-order modes at MIR in addition to the fundamental mode. We further demonstrate MIR 1x4 beamsplitters, achieving uniform splitting ratios of over 96 %. Our results highlight the potential of ULI-inscribed IG2 waveguides and beamsplitters for developing mid-infrared photonic devices and integrated circuits.

Mid-IR Waveguide Beamsplitters in IG2 Glass by Ultrafast Laser Inscription

Ultrashort pulse (USP) lasers, with pulse widths measured in femtosecond to picosecond, are widely used for machining or cutting materials of all kinds.&nbsp; Today’s newest USP lasers offer high output power (&gt;100W), with high pulse energy and high pulse repetition rate.&nbsp;&nbsp; When machining or marking metals, which have low ablation threshold and high thermal conductivity, low pulse energy and high pulse repetition rate typically give the best results.&nbsp; In contrast, when processing materials such as glass or polymers which can have high ablation threshold and low thermal conductivity, it is best to use high laser pulse energy and low pulse repetition rate. &nbsp;However, regardless of the material being processed, when total output power of the laser exceeds about 50W, thermal damage to the processing material (HAZ) can easily occur.&nbsp; Spreading laser power distribution across the target material becomes critical.&nbsp; This is typically achieved using higher scan speed, split beam processing, diffractive optics, or a combination of these. We discuss methods currently being used to effectively distribute &gt;100W laser power across a processing surface, to utilize the full power capacity of today’s USP lasers, and we present experimental results achieved with our newest 140W HyperRapid picosecond laser, and 150W Monaco femtosecond laser.

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Application of Closed-Loop Controlled Laser Ablation for High Quality Processing

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Material Removal in Laser Chemical Processing With Modulated Laser Power