ABSOLUTE INFRARED-ABSORPTION MEASUREMENTS IN OPTICAL COATINGS USING MIRAGE DETECTION

Absolute measurements of the optical absorption at lambda = 10.6 mu m of BaF2 thin film grown on a ZnSe substrate and bare ZnSe substrate su rface were performed by collinear photothermal deflection technique. A bsorption values were obtained in an absolute manner by fitting experi mental data to the theoretical expressions of beam deflection for two particular cases: when thermal wave extends far from the laser spot an d the opposite, when the heated region does not stretch beyond the las er spot. The validity of the theoretical models were tested in the ran ge of modulation frequencies from 20 Hz to 2700 Hz. This method was al so applied to the analysis of fused silica and sapphire bulk samples i n order to be used as reference absorbing media in the infrared spectr al range. Optical absorption of BaF2 thin film and ZnSe substrate surf ace were also deduced from the same theoretical approach using sapphir e as a bulk reference medium. From both calculation methods an absorpt ance of 1200 ppm for RaF2 thin film and of 425 ppm for ZnSe plate surf ace were measured. (C) 1998 Elsevier Science S.A, All rights reserved.