Photo-Acoustic Detection of Laser Induced Damages

Abstract

Photo-acoustic (PA) signals induced by Kr F laser irradiations on to the optical thin layers have been measured to detect the small absorptions and the damage thresholds of dielectric coatings. Extremely low absorption with the extinction coefficient of 10_-4 is detectable for the single-layer coatings with optical thickness of 1.5 wavelength at 248nm. The pre-damage detections have been studied with the assistance of laser scattering damage detection techniques. Thechanges of the slope and the variation of the PA signals have been observed at the irradiation of Kr F laser of lower fluence compared with the damage thresholdsby microscopic measurements. The pre-damage thresholds detected by the PA signaldetection agreed well with the laser scattering method for low index fluoridecoatings. Typical features of the laser induced damages related to the linear absorption and the refractive index will be discussed about the coating materials forshort wavelength lasers.