Absorption measurements in optical coatings by lock-in thermography

We evaluate and apply lock-in thermography as a method to quantitatively evaluate absorption losses of optical coatings. The principle of the method consists of applying periodically modulated laser intensity on the coatings and to monitor the periodic surface temperature evolution with an infrared camera. By application of a lock-in correlation procedure and using calibrated absorption samples, it is possible to obtain quantitative absorption values and to obtain absorption mappings with spatial resolution that depends on the optical configuration. Numerical simulations and experiments were performed in the case of 10-60 W laser irradiation at 1060 nm on different single layer coatings and highly reflective mirrors. In the tested conditions, the measurement of absorption down to 1 ppm level could be reached. The advantages, limitations, and potential applications of the technique are discussed.