(a) Raw signals detected with photodiode (a) in the absence of probe laser beam due to the scattering of the pump laser, light gray signals correspond to the 15% of highest intensity background signals and are not considered to determine the Brillouin gain signal (see text). (b) Statistics of the intensity of the scattering signal (realized over a set of 500 signals),events to the right of the dotted line correspond to the 15% highest intensity background signals.