The principles of operation of Fabry–Pérot interferometers are well known, and their application in spectroscopy has established their status as one of the most sensitive instruments ever invented. However, the behavior of a Fabry–Pérot device in polarized light, especially when birefringence and optical activity are present within the mirrors or in the cavity, is less well known. We devote this chapter to a description of some of these phenomena, in the hope of clarifying their physical origins and perhaps suggesting some new applications.
The dielectric mirror
A multilayer stack mirror is shown schematically in Figure 15.1. The substrate is a transparent slab of glass, and the layers are made of high- and low-index dielectric materials. In the examples used in this chapter the low-index layers will have (n, k) = (1.5, 0) and thickness d = 105.5 nm, and the high-index layers will have (n, k) = (2, 0) and d = 79.125 nm. (At the operating wavelength, 633 nm, both these layers will be one quarter-wave thick.) Figure 15.2 shows computed plots of amplitude and phase for the reflection coefficients of a 10-layer mirror. At normal incidence (θ = 0) both p- and s-components of polarization have an amplitude reflectivity |ρ| = 0.844. The mirror, therefore, reflects about 71% of the incident optical power and transmits the remaining 29%. Ignoring any loss of light at the uncoated facet of the substrate, the amplitude transmission coefficient (outside the substrate) turns out to be |τ| = 0.536.