Random Temporal Signals (RATS) Method for Measuring High-Reflectance Coatings via a Cavity Ring-Down Configuration

Manufacturing surfaces with high-reflectance (HR) coatings is important for applications where energy losses need to be minimized, such as laser resonators or calibration systems. However, measuring reflectances <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$R&gt;99.99 \%$</tex> is already problematic for a standard photometer. Therefore, multi-reflectance techniques such as cavity ring-down (CRD) measurements are used. Cavities can have different configurations, and each of them brings specific advantages. Nevertheless, typically, HR-CRD systems use a time-resolved approach comprising pulses entering the cavity and detecting cavity response (decay curve <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$I_{\mathrm{D}}$</tex>) represented with lifetime and corresponding reflectance [1]. However, the use of pulses has several drawbacks imposed by the fact that only one pulse must be present in the cavity. This restriction sets a limit to the pulse repetition rate, which has to be very low for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$R&gt;99.9995 \%$</tex>. In addition, the standard CRD experiment needs to find a compromise between the need for a high input pulse energy to overcome high losses during the pulse input into the cavity and the laser-induced damage threshold of the entrance cavity mirror. As a result, a high number of laser pulses with a low repetition rate are averaged, resulting in long acquisition times reaching several hours.