Fabry-Perot with an absorbing etalon cavity

The properties of the Fabry-Perot interferometer with an absorbing medium in the cavity have been investigated. When very large reflective finesse etalons are used, the interferometer behaves as a long-path absorption cell with a gain, defined as the ratio of the measured absorption over the sample absorption for the same path as the etalon spacer, equal to C1/2. (C is the etalon contrast.) Practical considerations make the plane Fabry-Perot unsuitable for this use because of the rather large area and surface finesse requirements. The spherical Fabry-Perot, on the other hand, is not constrained in the area requirement and thus appears to be the choice etalon for a long-path absorption cell. The results also show that interference filters, because of their high reflective finesse, have absorption effects which degrade their ultimate performance.