On the definition of absorption for a Fabry-Perot interferometer

Abstract

The concept of absorption in a Fabry-Perot (FP) interferometer is revisited. The FP is modelled as a medium in the cavity formed by two semi-reflecting absorbing films (typically, two metallic coatings with finite thickness) and not merely by two interfaces. Starting from this model the concept of absorption for an isolated metallic film and for each one of the coatings of the FP is discussed. Due to the coupling between the reflected waves in the coatings, the expressions for both cases are essentially different. Furthermore, even if the two coatings of the FP were identical, the absorption takes different values in each of them. The phase lag in the reflection from the coatings depends on the angle of incidence and on their thickness, causing a shift in the position of the maxima of the transmitted and reflected irradiances.