Damage Thresholds of Antireflection Coatings at 790 nm

Damage thresholds of antireflection coatings on sapphire and alexandrite substrates have recently been measured. Optimization of AR coatings was necessary to improve damage thresholds on alexandrite laser rods, over that observed on the bare material. Eight different coatings supplied by seven different vendors plus uncoated sapphire and alexandrite were tested. Thresholds from 7 J/cm² to 80 J/cm² were observed. Sapphire was used because it has an index of refraction similar to alexandrite and is more readily available in sizes suitable for testing. An alexandrite laser at a wavelength of 790 nm with a pulse duration of 200 ns at 30 Hz for two seconds was used. The mean spot diameter of the nearly Gaussian beam was 0.380 mm. All of the damage testing in the AR screening survey was done at normal incidence.