Laser damage density measurement of optical components in the sub-picosecond regime

International audienceA rasterscan procedure adapted to the sub-picosecond regime is set to determine laser-induced damage densities as function of fluences. Density measurement is carried out on dielectric high-reflective coatings operating at 1053 nm. Whereas laser-induced damage is usually considered deterministic in this regime, damage events occur on these structures for fluences significantly lower than their intrinsic damage threshold. Scanning electron microscope observations of these " under-threshold " damage sites evidence ejections of defects, embedded in the dielectric stack. This method brings a new viewpoint for the qualification of optical components and their optimization for a high resistance in the sub-picosecond regime. The past fifty years have seen several generations of high-power laser facilities devoted to laser/matter interaction and fusion study in particular. From L5, Janus, Shiva [1,2], to the National Ignition Facility (NIF) or the Laser Megajoule (LMJ) [3,4], the output power raised from gigawatts to hundreds of terawatts with the NIF using flash-pumped Nd:glass technology to produce nano-second pulses. The chirped pulse amplification technique proposed by Strickland and Mourou in 1985 [5] paved the way for ultra-intense high-energy systems. Since then, laboratories have developed short-pulse high-power laser facilities all around the world [6]. While considering long-pulse or short-pulse high-power laser facilities, optical components performances and in particular laser damage are always factors limiting the overall system performances. A common practice is to use large beams and therefore large optical components in order to reduce the energy density. For questions of manufactur-ability, ease of maintenance, and cost, optics size has been limited to meter scale since the mid-90s. When larger sizes are needed, mosaic of meter-size optics can be used following the same trend as for astronomical mirrors. In this context, getting a precise knowledge of the laser optics behavior under high fluence is of major importance. It is a warranty that the facility can be operated at its highest energy while preserving the optics lifetime. The question that arises is how can we estimate the optics lifetime? In regards to the nanosecond regime, the first trend was to estimate laser damage resistance through a threshold measurement on a representative sample or a small area of the full scale part. Standardized procedures are widely used in many laboratories such as the 1/1 or S/1 tests [7]. If this approach can bring information to optimize the damage resistance of the optics, it showed up to be insufficient to predict the optics behavior on large facilities such as NIF or LMJ. In this regime, damage is mainly defects driven, and damage sites can grow dramatically under iterative shots. Consequently, optics lifetime is lowered by rare defects. To overcom