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sample, where the surface was ablated and structured using an ultra-short pulse( USP)-laser. This ensures that no grinding, lapping, polishing particles or other contaminants are present on the surface. The lenses were irradiated again with max. laser output power, and the temperature reached was measured using a contact thermometer. As shown in Figure 12, it can be observed that significant temperature can also be introduced into the glass in this case. This confirms, that indeed the surface roughness can cause laser absorption and not only surface contaminations.
The ability to achieve laser-induced heating with the IR-Laser can also be extended to glass or glass ceramic samples made of different materials( Fig. 13). The materials used in this experiment were SF6, ULE and Zerodur, which exhibited comparable ground surface roughness values ranging from 1 to 1.6 lm and were subjected to 30 s of laser irradiation for all measurements. The laser power, wavelength, and beam parameters were kept constant for all samples. Differences in the maximum temperature are due to the material-dependent absorption and thermal conductivity of the glass.
As part of a follow-up investigation, stress measurements were conducted on SF6, ULE and Zerodur as well. As shown in Figure 14, the results confirmed that no significant residual stress was induced in the glass, even after laser heating at 500 W for 4 min followed by rapid cooling with a coolant lubricant. This outcome is particularly noteworthy for temperature-sensitive materials such as SF6, which showed low signs of thermal stress( not more than 13 nm / cm) despite their susceptibility to temperatureinduced effects. These findings indicate that laser-assisted heating does not introduce detrimental effects, such as stress in the glass material, and can be effectively utilized in grinding processes.
7 Conclusion
This study demonstrated a clear correlation between the laser absorption and the surface roughness of fused silica glass. The observed behavior can be explained by multiple reflections of the laser beam within the peaks and valleys of rougher surfaces, which significantly increase overall absorption compared to polished, smooth surfaces. As a result, higher surface roughness leads to reduced laser transmission and a measurable temperature increase of up to 80 – 100 K after 30 s of laser application. However, when the roughness values are higher than about 2 lm Rqa saturation or decrease in absorption becomes apparent.
Among the temperature measurement techniques used in this study, the focus was placed on the results with the contact thermometer. Since scattering and reflection phenomena were not investigated so far, there is an error potential regarding the absolute measured temperature values obtained by the thermometer module. On the other hand, the thermography camera presented challenges in determining absolute temperatures due to its sensitivity to the material’ s emissivity. Nevertheless, thermography remained valuable for the qualitative assessment of the areal temperature distribution. Further investigations are planned to further reduce deviations between measured temperature values and actual temperature values.
Furthermore, the IR laser wavelength employed in this study was found to exhibit minimal absorption in water or coolant liquids. This characteristic is particularly relevant for machining processes and offers promising potential for the development of novel hybrid manufacturing methods that combine laser heating with cooling lubricants.
Additionally, it was observed that other optical glasses or glass ceramics exhibit behavior comparable to fused silica in terms of roughness-dependent absorption of IR laser radiation. However, the absolute temperature values achieved during laser exposure varied depending on the material’ s properties, such as absorption and thermal conductivity.
These findings provide a strong foundation for future research into laser-assisted grinding processes. Further investigations should explore the influence of laser parameters on a wider range of glass materials to optimize laserassisted machining techniques. Measurements of reflected and scattered laser radiation components are also planned. In a next development step, several grinding regimes with simultaneous application of laser radiation will be investigated during a planned research project.
Acknowledgments
The authors wish to thank Michael Heinicke( Carl Zeiss Jena GmbH), Lars Schönemann( OptoTech Optikmaschinen GmbH) and Oliver Fähnle( Pandao GmbH) for their support and expertise during the experiments.
Funding
The authors gratefully acknowledge financial support by the DFG in the project“ TOOLS”. Funded by the Deutsche Forschungsgemeinschaft( DFG, German Research Foundation) Project-lD 528591139 – FIP 31.
Conflicts of interest
The authors declare that they have no competing interests to report.
Data availability statement
The raw data supporting the conclusions of this article can be made available by the authors, without undue reservation. Author contribution statement
Conceptualization: Jens Bliedtner Data curation: Sebastian Henkel Formal analysis: Daniel P. Knoche, Sarah Koch, Sebastian
Henkel Funding acquisition: Jens Bliedtner Investigation: Daniel P. Knoche, Sarah Koch, Christian
Schulze, Michael Güpner, Sebastian Henkel Methodology: Jens Bliedtner, Michael Güpner, Christian
Schulze, Sebastian Henkel, Thekla Boeckh Project administration: Jens Bliedtner Resources: Software: Supervision: Jens Bliedtner Validation: Daniel P. Knoche, Sarah Koch