The low frequency facility Fabry-Perot cavity used as a speed-meter

Fabry-Perot cavities have many different applications as scientific instruments. In the gravitational waves research field they are extensively used to frequency stabilize lasers and to measure very small distance variations. In the present Letter a method to evaluate from the transmitted power only the relative speed and position of the mirrors of a cavity, having finesse F > 40, is described. A displacement spectral sensitivity of the order of about 3 x 10(-10) m/Hz(-1/2) at 10 Hz is obtained with the cavity of the low frequency facility.