  The spin-fluctuations-related Kerr rotation noise of the optical beam
reflected from a microcavity with a quantum well in the intermirror gap is
studied. In the regime of anti-crossing of the cavity polariton branches, the
several hundred times enhancement of the noise signal, or giant noise, is
observed. The effect of the amplification of the noise signal is explained by
the nonlinear instability of the microcavity. In the frame of the developed
model of built-in amplifier, the non-trivial properties of the noise signal in
the regime of the negative detuning of microcavity are described.
