Influence of Fluence and Pulse Number on Laser‐Cleaning of Atmospheric Pressure Plasma Jet Etched Optical Glasses

The use of beam‐based technologies to process optical elements with nano‐scale precision enables the fabrication of freeform surfaces. Especially, atmospheric pressure plasma jets (APPJ) have desirable properties such as atmospheric pressure machining, dry processing and direct writing capabilities. However, the presence of metal oxides in optical glasses leads to the formation of non‐volatile reaction products during APPJ etching with fluorine‐containing gas mixtures, forming a residual layer that increases surface roughness and alters etching behavior. To prevent the formation of the residual layer the APPJ process is combined with laser‐cleaning. For a possible future in‐situ cleaning of the residual layer during the plasma process, laser parameter ranges need to be found to remove the residual layer without damaging the glass surface. Therefore, planar etchings are performed by APPJ on N‐BK7 and Zerodur and the etched planes are subsequently laser‐irradiated with varying pulse numbers and fluences. The processed samples are then examined by SEM. For both N‐BK7 and Zerodur, a parameter range is identified that results in clean surfaces. The best machining results are achieved with 2 ‐ 16 pulses and laser fluences of 0.6 ‐ 1.2 J cm^‐2 for N‐BK7 and 0.56 ‐ 0.7 J cm^‐2 for Zerodur.

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