Many literatures have focused on the design, fabrication and intrinsic damage of multilayer dielectric gratings (MLDGs). However, contamination may be a key factor of the MLDGs lifetime. In this paper, we compare the laser damages of the clean and contaminated MLDGs, and damage threshold of clean MLDGs is 3.12J/cm2 . We obtain the curve of the critical particle size at various laser fluences, above which the MLDGs will be damaged. Intrinsic LIDT of clean MLDGs is larger than that of contaminated ones, so the cleanness control is a crucial problem for the MLDGs. The scanning electron microscope (SEM) morphologies of intrinsic and contaminant induced damages are different. The intrinsic damage is cold ablation, while contaminant induced damage is thermal ablation.
In high power laser facility, contaminants on optics surfaces reduce damage resistance of optical elements and then decrease their lifetime. By damage test experiments, laser damage induced by typical metal particles such as stainless steel 304 is studied. Optics samples with metal particles of different sizes on surfaces are prepared artificially based on the file and sieve. Damage test is implemented in air using a 1-on-1 mode. Results show that damage morphology and mechanism caused by particulate contamination on the incident and exit surfaces are quite different. Contaminants on the incident surface absorb laser energy and generate high temperature plasma during laser irradiation which can ablate optical surface. Metal particles melt and then the molten nano-particles redeposit around the initial particles. Central region of the damaged area bears the same outline as the initial particle because of the shielding effect. However, particles on the exit surface absorb a mass of energy, generate plasma and splash lots of smaller particles, only a few of them redeposit at the particle coverage area on the exit surface. Most of the laser energy is deposited at the interface of the metal particle and the sample surface, and thus damage size on the exit surface is larger than that on the incident surface. The areas covered by the metal particle are strongly damaged. And the damage sites are more serious than that on the incident surface. Besides damage phenomenon also depends on coating and substrate materials.
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