Non-destructive evaluation of UV pulse laser-induced damage performance of fused silica optics

Abstract

The surface laser damage performance of fused silica optics is related to the distribution of surface defects. In this study, we used chemical etching assisted by ultrasound and magnetorheological finishing to modify defect distribution in a fused silica surface, resulting in fused silica samples with different laser damage performance. Non-destructive test methods such as UV laser-induced fluorescence imaging and photo-thermal deflection were used to characterize the surface defects that contribute to the absorption of UV laser radiation. Our results indicate that the two methods can quantitatively distinguish differences in the distribution of absorptive defects in fused silica samples subjected to different post-processing steps. The percentage of fluorescence defects and the weak absorption coefficient were strongly related to the damage threshold and damage density of fused silica optics, as confirmed by the correlation curves built from statistical analysis of experimental data. The results show that non-destructive evaluation methods such as laser-induced fluorescence and photo-thermal absorption can be effectively applied to estimate the damage performance of fused silica optics at 351 nm pulse laser radiation. This indirect evaluation method is effective for laser damage performance assessment of fused silica optics prior to utilization.

Figure 1

Damage probability (up) and corresponding zero probability damage thresholds (down) of different post-process samples.

Figure 2

Damage density contrast of different samples at 8 J/cm² fluence.

Figure 3

Comparison of the same position of a mechanical polishing fused silica sample surface under bright field micro imaging(left) and UV laser induced fluorescence imaging (right).

Figure 4

Comparison of typical fluorescence defect distribution at the sample surface treatment with DCE and MRF.

Figure 5

Results of fluorescence defects area percentage for the nine samples.

Figure 6

Comparison of typical 355 nm laser absorption distribution on sample surface after dynamic chemical etching and MRF.

Figure 7

Quantitative 355 nm absorption results for all samples.

Figure 8

Fitting relationship between the fluorescence defect percentage and zero probability damage thresholds.

Figure 9

Fitting relationship between average absorption and zero probability damage thresholds.

Figure 10

Fitting relationship between fluorescence defect percentage and damage density at 8 J/cm² test fluence.

Figure 11

Fitting relationship between 355 nm average absorption and damage density at 355 nm 8 J/cm² test fluence.

Figure 12

Fitting relationship between 355 nm average absorption and damage point numbers of 1000 cm² fused silica optics at 8 J/cm² test fluence.

Figure 13

Schematic of photo-thermal deflection for weak absorption induced by laser.

Figure 14

Layout of laser-induced damage test system and temporal and spatial profile of target plane.

Figure 15

Schematic of Raster scan and regular hexagon beam overlap model.