Laser thin films with very high damage threshold

Abstract

New developments of laser thin films for high power laser systems were reviewed. Special attention was paid to several key coatings which bottlenecked the higher power laser system, such as pick-off mirrors, large aperture polarizer and YAG harmonic wavelength splitter, as well as AR coatings. Defects were deemed to be the initial and essential source of laser-induced damage. The investigation of the original control of defects was focused on the deposition process control, such as substrate cleaning, purity of deposition materials, deposition temperature, oxygen pressure in vacuum chamber and so on. Several methods of evaluation of laser damage were employed, which included Laser Induced Damage Threshold (LIDT) determination and absorption detection based on Surface Thermal Lensing (STL) technique, as well as Total Integrated Scattering (TIS) measurement. The LIDT of pick-off mirrors reached to 20 J/cm² (1064 nm, 1 ns), meanwhile the LIDT of the ultraviolet coatings did to 8 J/cm² (355 nm, 1 ns) because of its comprehensive mechanism. The LIDT of third harmonic mirror could be improved effectively with post-treatment methods, such as oxygen-plasma post-treatment and vacuum annealing. Actually the situation of the laser thin films with very high damage threshold was complicated due to their uniformity of optical properties and stress induced surface deformation in the large aperture.