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Figure 15. PSD comparison generated from telecentric WLI measurement for the same Alvarez sample after D30 fine grinding and after D16 ultra-fine grinding.
Figure
16. Roughness comparison of the fine and ultra-fine ground surface condition on Alvarez lenses of fused silica glass, measured by WLI with 20 objective [ n = 4 ].
4.2.1 Form measurement
As the objective is to maintain the surface form throughout the PJP process, an assessment was conducted after ultra-fine grinding and compared to the results after PJP, as illustrated in Figure 18 for the samples listed in Table 1. After fine grinding the PV ranged from 4 lmto8.5lm. The comparison to the form measurement taken directly after the PJP process and after the subsequent annealing step reveals an improvement in the PV, as presented in Figure 18( A). It is evident that the annealing process does not result in a significant change in PV values. A notable improvement can be detected in the case of both # S1 and # S2. To ensure a fair comparison, the root mean square( RMS) values, shown in Figure 18( B), were used to eliminate the impact of outliers in the measurement. There are minimal variations, but no significant alteration to the initial surface form. The discrepancies could be attributed to measurement errors, such as thermal imbalances in the measurement environment or deviations in the freeform fit routine.
Birefringence due to internal stress was investigated by OPD measurements. The measurements normalized to local sample thickness are shown in Figure 19 for sample # S2. It is evident that internal stress can already be detected in the ground sample depicted in Figure 19( A). Internal stress arises during the glass forming step, which is a thermal process. Defined cooling curves ensure that this is kept to a minimum. The grinding process can also generate further internal stresses due to the clamping of the sample. The birefringence was less than 5 nm / cm, which meets the requirements for precision optics.
As shown in Figure 19( B) the PJP causes a significant increase in OPD. At x + 10 mm there is a notable surge in the measured values. This marks the start position of the PJ polishing process, where the hot PJ rapidly heats the sample from room temperature to the set point of 2000 ° C. As the process continues, the sample base temperature remains at an elevated level, resulting in a reduced increase in OPD. However, the process-related local heating and subsequent natural cooling after PJP, which does not follow a definedcoolingcurve, resultsinanincreaseinstress birefringence. The subsequent controlled annealing procedure reduces the stress birefringence through a programmed heating and cooling cycle that includes dwell times for relaxation. Afterwards, the samples again meet the requirements for precision optics, as demonstrated by the example of # S2 in Figure 19( C).
Further analysis of the material’ s response to the thermal load from the PJ revealed no irregularities, provided that the samples after PJP were subjected to annealing. Figure 20 presents the results of XRD and FT-IR measurements on plasma-polished sample # S2, both without and with annealing, in comparison to a conventionally polished reference FS surface. As shown in Figure 20( A), there is no discernable difference in the XRD measurements with respect to the intensities, indicating that there is no significant change of the amorphous structure. The FTIR measurement shown in Figure 20( B) can detect a change at 3670 cm �1 when the sample is not annealed after PJP. Repeating the measurement at different positions( m01, m02, m03) reveals minimal differences, but the general result was confirmed. It is likely that a change in the bulk material has occurred, as the peak indicates the stretching