132 J. Eur. Opt. Society-Rapid Publ. 21, 12( 2025)
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11. Iterative reduction of unwanted grinding structures:( A) removed arch structure and( B) removed ripple structures.
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Figure 12. PSD comparison generated from telecentric WLI measurement of D30 fine ground Alvarez lenses:( A) Alvarez sample shown in Figure 10 before process optimizations and( B) sample shown in Figure 11( B) after process optimizations.
Alvarez shape with a semi-transparent, homogeneous surface on fused silica after ultra-fine grinding, as shown in Figure 13.
Manufactured Alvarez samples were again investigated regarding surface deviations of different spatial frequencies. Qualitative inspection of the sample surfaces with a telecentric WLI and depiction of the measured figure error topography images in Figure 14 show, that apparently, the ultra-fine grinding process does not significantly produce new form deviations and mid-spatial frequency errors in comparison to the fine ground state.
This is confirmed by a quantitative evaluation of the surface topography using the PSD function given in Figure 15. It is apparent that the ultra-fine ground state produces a lower PSD curve along the whole spatial frequency range. This means that all surface deviations from waviness to mid-spatial errors and higher frequency errors get reduced homogeneously by application of the ultra-fine grinding process to the freeform surface.
The achievable roughness is around 100 nm Sq which is an order of magnitude higher than the possible roughness on planar samples described in the introduction. This can be explained by the mentioned differences regarding
tool-sample interactions between planar grinding with big areal tool engagement using cup tools and punctual, subaperture tool engagement using ball tools. Still, the resulting surface quality after ultra-fine grinding is at a suitable level to apply the PJP process afterwards as shown in the next chapters. The ultra-fine grinding process is a relevant connection point in order to achieve a polishable surface starting from the smooth ground state with a D30 metal bond. The diagram in Figure 16 emphasizes the roughness reduction from fine grinding to ultra-fine grinding with D16 resin bond is approx. 86 %.
After the process optimizations have been carried out as described, freeforms are finally ground by varying selected factors, and several samples are produced using the grinding process optimizations found. The final step of the process chain, the PJP, is then examined and optimized on these samples, see Sections 4.2 and 4.3.
4.2 Linking of grinding and PJP process
The implementation of a process chain requires the establishment of appropriate transfer points. This ensures that the surface state generated in the pre-process is sufficiently