Local refusion of silica by a continuous CO 2 laser for the mitigation of laser damage growth

On the 3ω part of the LIL laser many optical components will have to sustain fluences above 10J/cm². Even if current progress in silica substrate technology decreases the number of defects/cm² which can induce a damage under such a laser flux, tens of damaged sites will appear on large surface optics. Knowing that these surface damaged sites grow exponentially with the number of laser shots, it is necessary to stop the growth of these defects before the use of the optical component is impaired. In this paper we use localized re-fusion of silica, induced by a continuous CO₂ laser, as a means to reshape the damaged site and circumvent the growth of laser-induced surface damages. In a first part we compare the 1 and 2D model of the interaction of a gaussian laser beam with an homogeneous material and deduce that the 1D model is convenient down to a laser beam radius waist of 100 μm in silica. We show that at atmospheric pressure total mitigation might not be achieved due to silica evaporation and peripheral redeposit in air. This risk cannot be managed with predetermined laser power and interaction time, because thermal conductivity of silica is not homogeneous. In order to keep the process “vacuum free”, a radiometry diagnostic has been mounted to monitor the surface temperature of silica. Real time retroaction on silica exposure to laser radiation enables us to control surface silica evaporation.