Effect of oil vapor contamination on the performance of porous silica sol-gel antireflection-coated optics in vacuum spatial filters of high-power neodymium glass laser

The effect of oil vapor contamination on sol-gel porous silica antireflection (AR) coatings deposited on optics used in vacuum spatial filters of high-power lasers is experimentally studied. The contamination results in the reduction of laser-induced damage threshold (LIDT) and the increase in reflectivity of the AR coatings. In comparison, the silica coatings treated with ammonia and hexamethyl-disilazane (HMDS) show almost no increase of reflectivity, and their LIDT value reduces only marginally at the wavelength of laser operation. It is found that the decrease in LIDT of the coatings exposed to oil vapors occurs due to higher absorption of the laser light, as evidenced from the increase in the imaginary part of the refractive index of the coatings.