Absorption measurements of optical coatings at 1.1 and 1.5  µm with lock-in thermography and an associated thermal model

To address the increasing demands of high-power laser applications, this paper investigates absorption phenomena in optical components and the photo-induced effects influencing the optical response of thin film stacks. A lock-in thermography (LIT) setup was developed to enable precise, non-destructive, and non-contact absorption measurements. By utilizing modulated high-power lasers operating at 1080 and 1557 nm, the setup induces heating in the coating stacks, with the resulting surface temperature rise captured by a thermal camera. This technique enables measurement sensitivity down to 0.1 ppm. Complementary to the experimental setup, a finite element model (FEM) was established to simulate the thermal behavior during LIT acquisitions, providing insights into the influence of critical physical parameters and aiding the calibration process through sensitivity analysis. Comparative absorption measurements of single-layer and multilayer dielectric stacks were performed at both wavelengths. Furthermore, a numerical model based on complex admittance computations was developed to estimate the absorption in thin film stacks. This integrated experimental and numerical approach enhances the understanding of absorption mechanisms, offering a significant step forward in characterizing and optimizing optical coatings for high-power laser systems.