  The spin noise signal in the Faraday-rotation-based detection technique can
be considered equally correctly either as a manifestation of the spin-flip
Raman effect or as a result of light scattering in the medium with fluctuating
gyrotropy. In this paper, we present rigorous description of the signal
formation process upon heterodyning of the field scattered due to fluctuating
gyrotropy. Along with conventional single-beam experimental arrangement, we
consider here a more complicated, but more informative, two-beam configuration
that implies the use of an auxiliary light beam passing through the same
scattering volume and delivering additional scattered field to the detector. We
show that the signal in the spin noise spectroscopy arising due to heterodyning
of the scattered field is formed only by the scattered field components whose
wave vectors coincide with those of the probe beam. Therefore, in principle,
the detected signal in spin noise spectroscopy can be increased by increasing
overlap of the two fields in the momentum space. We also show that, in the
two-beam geometry, contribution of the auxiliary (tilted) beam to the detected
signal is represented by Fourier transform of the gyrotropy relief at the
difference of two wave vectors. This effect can be used to study spin
correlations by means of noise spectroscopy.
