Abstract: A device for preventive treatment of an optical surface designed to be exposed to a laser flow including a thermal excitation source for providing a localized thermal annealing of a site of the optical surface) by means of a beam applied to the site. The device further includes a measuring member for measuring, in real time said during said localized thermal annealing, a quantity representing the temperature of the site of the optical surface) and at least one control member for increasing the linear power density of the beam applied to the site by the excitation source and, when the quantity reaches a predetermined set point, for gradually decreasing the linear power density.