Laser damage mitigation process

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Although the polishing techniques of optical components have
been considerably improved, defects that can initiate damage are
still present in the material. Laser irradiations of these weak points
lead to stress, cracks and absorption. The created damage can grow
under subsequent irradiations and makes the component unsuitable. One of the problem that arise is that it cannot be predicted why, when and
where an optical component will be damaged. This is a main concern in high power laser chains such as fusion class laser, because of the huge
numbers, the large dimensions and the cost the optics. To avoid the growth of damage sites under successive irradiations some mitigation process are being developped in the framework of collaboration with the CEA since 2005.

Example of a damage site created on the rear side of a fused silica optic. The observation with luminscence confocal microscopy evidences cracks (in green) and absorbing defects (red).

To avoid damage site growth, one of the most promising methods uses a CO2
laser operating at a 10.6 μm wavelength to locally melt and evaporate the silica surface by producing typically smooth, Gaussian shaped pits. In order to implement this process our research activities include the modelling of thermo-mechanical interactions, the analyis of laser damage sites, the developme,nt of experiments for effective damage mitigation, the study of downstream modulations of the wavefront induced by the mitigated sites,...

Process principle : a nanosecond damage site exbibits fractures, stress and an absorbing area. A focus CO2 laser is used to melt and partially evaporate the damage area. Afetr the process, the clean and smooth crater with not grow under nanosecond UV irradiation and the damage threshold will be comparable to pristine silica.

Example of application. On the left : observation of a typical damage site. On the right : its transformation after CO2 laser heating (LIDT > 12J/cm², 3ns, 355nm).

References of our work on this subject :

  • Combis P., Cormont P., Gallais L., Hebert D., Robin L., Rullier J.-L., ’Evaluation of the fused silica thermal conductivity by comparing infrared thermometry measurements with two-dimensional simulations’, Applied Physics Letters, 101 211908 (2012).
  • Robin L., Combis P., Cormont P., Gallais L., Hebert D., Mainfray C., Rullier J.-L., ’Infrared thermometry and interferential microscopy for analysis of crater formation at the surface of fused silica under CO2 laser irradiation’, Journal of Applied Physics, 111 063106 (2012).
  • Cormont, P., Gallais, L. Lamaignère, L. Rullier J.L., Combis P., Hebert D. ’Impact of two CO2 laser heatings for damage repairing on fused silica surface’, Optics Express, 18 25 26068-26076 (2011)
  • Gallais L., Cormont P., Rullier J.-L., ’Investigation of stress induced by CO2 laser processing of fused silica optics for laser damage growth mitigation’, Optics Express, 17 26 23488–23501 (2009)
  • Palmier S., Gallais L., Commandre M., Cormont P., Courchinoux R., Lamaignère L., Rullier J.-L., Legros P., ’Optimization of a laser mitigation process in damaged fused silica’, Applied Surface Science, 255 10 5532–5536 (2009)
  • Cormont P., Gallais L., Rullier J.L, ’Procédé de traitement correctif d’un défaut sur la surface d’un composant optique pour laser de puissance’, French patent N° 09 56443 (2009).