Resistance of Scratched Fused Silica Surface to UV Laser Induced Damage

Hui Ye,Qiao Xu,Zhonghou Wang,Yaguo Li,Chen Jiang

doi:10.1038/s41598-019-46048-4

Abstract

Scratches in fused silica are notorious laser damage precursors to UV laser damage initiation. Ductile and brittle scratches were intentionally generated using various polishing slurries. The distribution, profile and the dimension of scratches were characterized. The damage resistance of polished surfaces was evaluated using raster scanning damage testing protocol. The results show that both ductile and brittle scratches greatly increase area proportion of laser damage about one to two orders of magnitude relative to unscratched surface and brittle scratches are more deleterious. Moreover, finite difference time domain (FDTD) simulation was used to numerically calculate the light field distribution around scratches on rear surface (i.e. exit surface for light) which indicates that modulated light intensity is susceptible to the profile and size of scratches. FDTD simulation results also indicate that the light field intensification is elevated with the dimension of scratches and light modulation effects in triangular scratches are usually not as notable as serrated and parabolic scratches.

Highlights

Fused silica samples (50 mm in diameter and 5 mm thick) were polished using the lapping machine (FD-380XL, Fonda, China)
The exist surface of sample was perpendicularly subjected to a Gaussian laser pulse (8 ns@355 nm, beam waist 800 μm) at the repetition rate of 10 Hz, and the irradiated area was inspected by a long-focus microscope equipped with a CCD camera to record damage initiation
The scratches were investigated in order to find out their possible effects on the laser damage performance of fused silica and the light field distribution under laser illumination was modelled

Summary

Introduction

Fused silica samples (50 mm in diameter and 5 mm thick) were polished using the lapping machine (FD-380XL, Fonda, China). The surface morphology of polished samples was observed using an in-house built defect detecting system and the testing spot size is ~14.8 × 14.8 mm[2]. The tripled frequency 3ω Nd:YAG laser damage testing system (Laser Zentrum Hannovere.V., Germany) was used to evaluate the damage performance of polished samples adopting raster scanning testing protocol. The exist surface of sample was perpendicularly subjected to a Gaussian laser pulse (8 ns@355 nm, beam waist 800 μm) at the repetition rate of 10 Hz, and the irradiated area was inspected by a long-focus microscope equipped with a CCD camera (resolution ∼10 μm) to record damage initiation. The surface of each sample was divided into 3–6 sub-regions (dimension 10 mm × 10 mm), and each sub-region was sufficiently irradiated with a fixed laser fluence. Our ultimate goal is to investigate the damage performance of large-aperture optics for Nd:glass laser (tripled frequency 351 nm, 3 ns), so the damage threshold laser fluence is rescaled from 8 ns to 3 ns in the article

Results

Discussion

Conclusion