  We have measured the photothermal effect in a single cross-polarized
interferometer at audio frequencies (5 Hz - 4 kHz). In a Fabry-Perot
interferometer, light in one polarization is chopped to periodically heat the
interferometer mirrors, while light in the orthogonal polarization measures the
mirror length changes. Tests of a polished solid metal mirror show good
agreement with relevant proposed theories by Braginsky et al. ["Thermodynamical
fluctuations and photo-thermal shot noise in gravitational wave antennae,"
Physics Letters A 264, 1-10 (1999)] and Cerdonio et al. ["Thermoelastic effects
at low temperatures and quantum limits in displacement measurements," Physical
Review D 63 082003 (2001)] describing uncoated optics.
