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Stabilisation de dommages laser et de défauts sur composants optiques de silice par procédés laser CO2

Thomas Doualle 1, 2 
2 ILM - ILM
FRESNEL - Institut FRESNEL
Abstract : One limitation of the operation of large power lasers chains such as Laser MegaJoule, is the issue of laser damage of optical components. Different physical phenomena which depend on both the properties of materials, their conditions of manufacture / preparation and laser irradiation parameters can lead to damage initiation on the surface or in the volume, which will grow under successive irradiation. This effect limits the output power, affects the lifetime of the optical components and the maintenance cost of the laser. It can also cause serious safety problems. To address this issue and increase the lifetime of fused silica components, a laser process called "stabilization" is studied in this thesis, the aim being to treat the damage sites to stop their growth under repeated pulses for recycling damaged optics. This process consists of melting, followed by local evaporation by localized energy deposition by a CO2 laser beam of the damage site. We focused particularly on the stabilization of silica components by a micromachining process using a CO2 laser in order to treat millimeter size damages. This technique is based on fast micro-ablation of the silica during which the laser beam is scanned on the component surface to form an adjustable form of crater (typically conical) including the damaged site. A bench of experiments has been set up at the Fresnel Institute to develop and study this process. Various numerical and experimental works were also conducted to validate and optimize the technique. Our work has shown the efficiency of this micro-machining process by CO2 laser to stop the growth of damage to several hundred microns wide and deep. To achieve this goal we relied on modeling of physical phenomena involved in stabilization experiments using the COMSOL Multiphysics simulation software. First, the thermal model developed in this thesis is used to calculate the temperature distribution in the material during laser irradiation with or without movement of the beam. Combined with a thermodynamic approach, these thermal simulations can describe the transformation of silica during irradiation and predict the morphology of craters formed in the glass. Secondly, the mechanical part of the model can simulate the position and value of residual stress generated in the material around the crater after the temperature rise followed by rapid cooling. Other experiments on the treatment of fractures related to polishing on silica surfaces, or manufacturing defects on silica gratings are covered in this manuscript.
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Submitted on : Friday, May 19, 2017 - 12:09:12 PM
Last modification on : Wednesday, November 3, 2021 - 4:01:54 AM
Long-term archiving on: : Monday, August 21, 2017 - 12:28:30 AM

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Thomas Doualle. Stabilisation de dommages laser et de défauts sur composants optiques de silice par procédés laser CO2. Optique [physics.optics]. Ecole Centrale Marseille, 2016. Français. ⟨NNT : 2016ECDM0010⟩. ⟨tel-01525042⟩

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