Metrology of thin film optical components for high power continuous wave lasers

The increase of continuous wave laser power is an important topic in various new industrial and defence applications. One of the important limitation is due to the thin film component absorption (intrinsic and defects-related) that induces beam distortions and eventually laser-induced damages. In order to study this absorption, it is of prime importance to accurately measure low absorption levels and to determine the origin of this absorption. In this work, we present the use of Lock-In Thermography (LIT) to absorption measurement. This technique relies on the use of a pump laser beam at 1 μm that is modulated at low frequency and an infrared camera that images the thin-film sample that is being heated. By applying a lock-in treatment on thermal images, we show that we can obtain an image of the temperature increase over the optical component with low noise level. A LIT setup with a sensitivity of a few ppm and a ten times better accuracy is demonstrated. We also show that this setup can be used to make mappings of local absorption and can easily reveal local defects with absorption that can be one order of magnitude higher that intrinsic one. This setup is finally implemented to make measurements on different single layer thin-films. Layers made with different materials (Nb2O5, SiO2, TiO2, HfO2) and deposited by plasma ion assisted deposition or plasma-assisted reactive magnetron sputtering are studied. We explore also the effect of annealing on these dense coatings. Finally, we investigate how these intrinsic absorption levels can be used to investigate the absorption of multilayers structures.