Observation of radiation-pressure effects and back-action cancellation in interferometric measurements

In the case of gravitational-wave resonant detectors such as dual spheres with optical readout, the sensitivity can be strongly improved by a back-action cancellation effect related to the specific geometry of this kind of detectors. We report the observation of such a back-action cancellation. Our experiment is based on a high-finesse Fabry-Perot cavity where the displacements of both mirrors are monitored by sending a very stable laser beam in the cavity and measuring the phase of the reflected beam with an homodyne detection. Between 100 kHz and 4 MHz, the sensitivity is only limited by the shot noise. We have performed an exhaustive study of the internal thermal noise of mirrors and optomechanical properties of internal acoustic modes: resonance frequency, quality factor, effective mass, and spatial structure.