Laser Polishing Using Quasi-Beamforming to Enhance Optical Quality of Miniaturized Optics
Sven Lämmler, David Bischof, Markus Michler
Laser-based manufacturing of optics is a promising technology with significant potential for the efficient production of high-precision and complex optical components. At the Institute for Microtechnology and Photonics (IMP), we focus on the selective laser etching (SLE) process chain combined with laser polishing, particularly for miniaturized optical elements. This paper presents an enhanced laser polishing strategy based on a Quasi-Beamforming approach, developed as an extension of the One-Shot polishing strategy. The method dynamically modulates the energy input via high-speed galvanometric scanners to generate a tailored "Sombrero" thermal distribution. By redistributing thermal energy from the center toward the edges, a quasi-uniform thermal influence is achieved across the entire convex lens surface. This controlled heating minimizes mid-spatial frequencies and improves both surface roughness and shape accuracy—especially at the lens periphery. Experimental results demonstrate that the method not only enhances optical quality but also improves energy efficiency by enabling shorter exposure times and reducing thermal stress within the material. The approach is particularly suitable for rotationally symmetric mini-optics such as convex lenses, where conventional methods often fail to provide uniform laser polishing.
Keywords: laser polishing, selective laser etching, mini optics, CO₂ laser, surface roughness, asphere, non-contact processing, beamforming
_______
Polishing of metals with femtosecond GHz-burst laser
Théo Guilberteau, Manon Lafargue, John Lopez, Laura Gemini, Inka Manek-Hönninger
In this work, we explore the use of a femtosecond laser operating in the GHz-burst regime for metal polishing, both with and without ablation, to enhance micromachining quality. The high number of pulses per burst (800 ppb) combined with high burst repetition rate (800 kHz) allows for ideally distributing the laser energy over metallic samples. As surface melting and smoothing is driven by surface tension forces, this is a critical factor in this process. Recent studies have demonstrated that GHz-bursts of femtosecond pulses are particularly effective for metal polishing, as the pulse-to-pulse delay within the burst is shorter than the material's characteristic heat relaxation time. We will present our results on stainless steel and titanium and point out the best process windows.
Keywords: ultrafast laser processing; GHz-burst mode; laser polishing; metals
_______
Minkeshkumar Ramoliya, Annika Bohlen, Thomas Seefeld
Cavitation erosion (CE) is detrimental to a number of engineering components, including ship propellers and pump impellers. Due to deformation and mass loss, CE significantly impairs the serviceability of these components and results in tremendous economic loss. Therefore, the demand for a reduction in material damage due to CE is imperative. Material damage can be minimised by using protective layers tailored to resist CE. In this work, laser melt injection (LMI) was performed to improve the cavitation erosion resistance. A metal matrix composite (MMC) is formed on the aluminium-bronze substrates by injecting spherical fused tungsten carbide (SFTC), or niobium carbide (NbC). The impact of different particle sizes on the CE behavior of the reinforced surface layer was investigated. CE tests of produced layers and untreated aluminium bronze are carried out as per ASTM G32-16 standard and the results are compared. The dependency of coating initial surface roughness, microhardness, and fracture toughness of particles on the cavitation erosion is discussed. It was determined that the small particle-reinforced NbC coating was the most effective in shielding substrates from cavitation damage.
Keywords: laser melt injection, metal matrix composite, cavitation erosion, aluminium bronze, spherical fused tungsten carbide, niobium carbide
_______
Ley, Vincent Benedikt; Linden, Sven; Kuepper, Moritz; Willenborg, Edgar
Laser polishing of metals is a multi-step process requiring detailed and individual parameter optimization for each processing step. The experimental approach to parameter optimization for new materials and initial surface roughness is not optimized itself. Therefore, high numbers of experiments are necessary. Furthermore, established approaches neglect that laser polishing is a multi-step process, resulting in redundant experimentation and suboptimal roughness. In this work, a new approach for optimizing and benchmarking the parameter optimization process is developed. This approach is based on a surrogate model using a dataset of 2,560 conducted experiments on laser polishing of AISI H11. By using the surrogate model, efficient and low-cost optimization and comparison of parameter optimization strategies are enabled. In the benchmark, a wide range of conditions, such as sample limitation and target roughness, were tested. As a result, the domain-based approach could find parameters meeting the criteria in less than 50 experiments even for a suboptimal choice of initial process parameterization fed to the algorithm.
Keywords: Laser polishing, metals; process parameter development; neural networks; low-cost optimization
_______
Applying laser welding in power plant part repair
Isarawit Chaopanich, Vissaruud Preebut, Watit Luanngam, Chattapat Khamenkit, Wirat Sirisappaibun
Laser welding, acknowledged for the low heat input process, was initiated to refurbish a steam turbine rotor. The refurbishment involved applying a superalloy filler metal to the damaged areas on the turbine disk caused by heavy rubbing. The post weld heat treatment was established to minimize the residual stress after welding. The machining work was performed to remove cracks and finish the welded areas to original dimensions. The liquid penetrant test was conducted and revealed no defects. A notable characteristic of high hardness on the machined weldment was observed and attributed to a significant degree of work hardening. The maximum runout of the rotor was lower than that from the as received condition. The rotor mass unbalance was minimized and accepted according to the standard limit. The rotor then resumed operation for power generation. In addition, the consecutive projects showcased the success of laser welding in repairing critical power plant components.
Keywords: laser welding, steam turbine rotor, refurbishment, residual mass unbalance
_______