Measurement of optical coatings absorption at 1570 nm and simulations of photo-induced modifications of spectral function under high power laser exposure

Free space communications between ground stations and geostationary satellites offer high-speed and secure data transmission, but compensating for atmospheric absorption poses a significant challenge. The need for high levels of beam power to overcome atmospheric losses calls for optics that can withstand strong flux, especially in the continuous wave (CW) regime. In this context, the absorption of optics at 1.5 µm is a critical parameter that must be accurately measured and understood to develop efficient and reliable photonic systems. This study focuses on the absorption of optical coatings at 1570 nm. Lock-In Thermography (LIT) has been developed to measure the total absorption of the coatings with high sensitivity under 1 ppm. A modulated 100 W CW laser is used to induce heating into the coating stack, and the resulting rise in internal temperature is measured with a thermal camera. The LIT experimental setup offers a non-destructive and non-contact measurement technique, making it ideal for assessing the absorption of delicate thin-film coatings. To understand the photo induced effects in a stack of thin film layers subjected to high-power laser heating, a finite element model is developed using COMSOL. The model simulates the index and thickness variations of each layer and predicts the shift in optical function resulting from photo induced effects. The results offer valuable insights into the impact of laser-induced heating on the optical properties of the coatings and provide guidelines for designing robust and reliable photonic systems.