Investigation of stress induced by CO2 laser processing of fused silica optics for laser damage growth mitigation

Laurent Gallais,Jean-Luc Rullier,Philippe Cormont

doi:10.1364/oe.17.023488

Abstract

Laser damage mitigation' is a process developed to prevent the growth of nanosecond laser-initiated damage sites under successive irradiation. It consists of re-fusing the damage area with a CO2 laser. In this paper we investigate the stress field created around mitigated sites which could have an influence on the efficiency of the process. A numerical model of CO2 laser interaction with fused silica is developed. It takes into account laser energy absorption, heat transfer, thermally induced stress and birefringence. Residual stress near mitigated sites in fused silica samples is characterized with specific photoelastic methods and theoretical data are compared to experiments. The stress distribution and quantitative values of stress levels are obtained for sites treated with the CO2 laser in various conditions of energy deposition (beam size, pulse duration, incident power). The results provided evidence that the presence of birefringence/residual stress around the mitigated sites has an effect on their laser damage resistance.

Highlights

Laser damage of optical components is a main issue for high power laser systems
The irradiation conditions were adjusted to create 20 μm to 50 μm deep craters, which corresponds to the depth of typical damages that we need to treat for the laser mitigation process of LMJ optics [14]
Laser damage resistance tests performed with our Nd:YAG laser at a fluence of 11 J/cm2, show that only 10 % of crater sites were damaged in the case 5 where the maximum retardance was measured at 190 μm from the crater center, whereas more than 80 % are damaged in the case 1 for which the distance is 130 μm

Summary

Introduction

For ICF class lasers, the laser damage resistance of fused silica surfaces at 351 nm in the nanosecond regime is a major concern. Such facilities involve many large and highcost elements, such as windows, lenses, crystals, diffractive optical elements...etc. The polishing techniques of silica have been considerably improved, defects that can initiate damage are still present in the material [1]. Laser irradiations of these weak points lead to stress, cracks and absorption. Other studies on the downstream intensification effects associated with the perturbation to the optical surface profile following the mitigation process were conducted [16,17]

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