THE BALANCE OF LOSSES IN OPTICAL COATINGS BY ABSORPTION AND GUIDED PROPAGATION ATTENUATION MEASUREMENTS

We use free space and guided propagation techniques to evaluate the le vels of absorption and to determine the absorbing areas in the thickne ss of a single layer deposited on a glass substrate. The model takes i nto account absorption localized in the layer bulk, at the interfaces and in the substrate. Conclusions on absorption losses in the thicknes s of various coatings (single layers of Ta2O5 and TiO2) are presented and discussed. The two techniques of measurement (free space and guide d propagation) lead to results which are in good agreement. We note a strong absorption localized in the substrate bulk (about 70% of the ov erall measured absorption). This absorption is photoinduced by the rad iation emitted in the coating plant during deposition. It can be partl y reversed by thermal annealing after coating. On the other hand, the studied coatings have interfaces presenting much higher absorption (ab out 25% of the overall absorption) than that of the bulk material (abo ut 5%).