Broadband optical spectral shift analysis of dielectric coatings

High-energy laser facilities require high reflection multilayer coatings on meter-scale substrates. Due to stringent use specifications, a precise control of deposition parameters is necessary to tailor the optical and mechanical properties of components. The resulting coatings are sensitive to relative humidity variations, leading to a shift of their optical spectra called spectral shift. This spectral shift is generally observed on a narrow range, near the operating wavelength. Here we extend the concept of spectral shift to a broader spectral range. This analysis serves as a tool to study the behavior of a multilayer coating spectrum with relative humidity. To validate the spectral shift determination method, we compared the spectral shift of single layers induced by the relative humidity with simulated optical properties induced by either thickness or refractive index variations. In addition to the validation of the approach, the fitting results and the comparison between spectral shift shapes show that relative humidity variations mainly impact the refractive index of the layers and SiO₂ is more sensitive than HfO₂.