J. Eur. Opt. Society-Rapid Publ. 21, 12( 2025) 137
Table
2. PJP parameters( surface temperature and PJ scan velocity) with reference to the ground samples.
Sample Surface temperature set point PJ scan velocity 2000 ° C 2050 ° C 1 mm / s 0.5 mm / s
# S2 X X # S4 X X # S5 X X
( A)
( B)
Figure 22. Surface roughness measurement of FS freeform lenses:( A) waviness and( B) roughness of ground and PJP state [ n = 2 ].
The results of Figure 21( B, C) demonstrate the effectiveness of the polishing process, which reduced the roughness by approx. 99.5 % to values of Sq R =( 0.69 ± 0.14) nm in the high frequency range( see Fig. 5). Additionally, the waviness was reduced by more than 50 % yielding values of Sq W =( 53.59 ± 20.45) nm. The best result was observed for sample # S2, which demonstrated a 75 % improvement in waviness. For # S3, the best nominal waviness was achieved( Sq W = 37.5 nm), though this came with an expense of the grinding process time. From the perspective of an efficient process chain, smoothing is not significantly better compared to sample # S2 ground in half of the time.
4.3 PJP process optimization
Accordingly, two additional samples(# S4 and # S5) were ground with the same parameters used for # S2 to further optimize the PJP process by varying PJ scan velocity and temperature set point. The nomenclature is provided in Table 2. Depending on PJ scan velocity polishing time is 24 min and 46 min, respectively.
4.3.1 Form measurement
All samples show no appreciable change in figure error, which proves the surface form preservation being an important property of the thermal polishing process. Characteristic values such as PV, RMS and power error are retained.
The OPD result in fulfilling the benchmark value of rms-OPD < 5 nm / cm is corresponding to the requirement for precision optics all after PJP including annealing.
( A)
( B)
Figure 23. Waviness of FS freeform lenses presented in mean isotropic PSD functions given for the( A) position P and( B) position V( according to Fig. 21( A)).