Full mechanical characterization of a cold damped mirror

We describe an experiment in which we have used a cold damping feedback mechanism to reduce the thermal noise of a mirror around its mechanical resonance frequency. The monitoring of the Brownian motion of the mirror allows one to apply an additional viscous force without any extra thermal fluctuations. This scheme has been experimentally implemented with the radiation pressure of an intensity-modulated laser beam. Large noise reductions, up to 30 dB, have been obtained. We have also checked the mechanical response of the cold damped mirror, and monitored its transient evolution between the cooled regime and the room-temperature equilibrium. A simple theoretical model allows us to fully explain the experimental results. A possible application to the active cooling of the violin modes in a gravitational-wave interferometer is discussed.