  When measuring quadratic values representative of random fluctuations, such
as the thermal noise of Atomic Force Microscopy (AFM) cantilevers, the
background measurement noise cannot be averaged to zero. We present a signal
processing method that allows to get rid of this limitation using the
ubiquitous optical beam deflection sensor of standard AFMs. We demonstrate a
two orders of magnitude enhancement of the signal to noise ratio in our
experiment, allowing the calibration of stiff cantilevers or easy
identification of higher order modes from thermal noise measurements.
