ABSTRACT
The metal reflecting layers of the previous chapter suffer from a considerable absorption loss which, although unfortunate, still permits a high level of performance in most simple systems. There are applications where the absorption in metal layers is too high and the reflectance too low. These include multiplebeam interferometers and resonators, where the large number of successive reflections magnifies the effects of absorption, and high-power systems where the energy absorbed can be sufficient to damage the coating. One way of increasing the reflectance of an opaque metal coating, as we have seen, is to boost the reflectance by adding dielectric layers. This also reduces the absorptance, but the transmittance remains effectively zero. For high-reflecting coatings which must transmit what they do not reflect, all-dielectric multilayers are required. The description which follows is built around the most successful of the multiplebeam interferometers, the Fabry-Perot interferometer. As we shall see later, this interferometer is also of considerable importance in the development of thin-film band-pass filters, and this is a further reason for dealing with it in some detail here.
