Using a top-hat excimer-laser beam spot and a helium-neon (HeNe) laser probe beam, researchers at Friedrich-Schiller-Universität Jena (Jena, Germany) are measuring the thermoelastic and thermo-optic responses of ultraviolet (UV) dielectric coatings both below and above the laser-induced damage threshold. Such measurements are valuable to improving UV thin-film coatings.
A 475-mm-diameter spot with top-hat intensity profile from a 248-nm krypton fluoride excimer laser briefly produces a thermally or damage-induced bump on the coating's surface. A 1.7-mm-diameter Gaussian HeNe beam reflected off the same area strikes a photodiode with an aperture small enough to pass only the center of the HeNe beam. The bump alters the mode structure of the HeNe beam, causing the central intensity to drop. The signal decay reveals thermal characteristics of the coating and substrate. A permanent offset in the signal shows damage. If the area is probed in transmission, the nonlinear absorption coefficient of the coating can be measured. The technique can detect bump heights as small as 0.002 nm. Optimization of the mode mismatch between the probe beam and the bump produces the best results. Contact Bincheng Li at[email protected].
