  By Faraday-rotation fluctuation spectroscopy one measures the spin noise via
Faraday-induced fluctuations of the polarization plane of a laser transmitting
the sample. In the fist part of this paper, we present a theoretical model of
recent experiments on alkali gas vapors and semiconductors, done in the
presence of a {\em static} magnetic field. In a static field, the spin noise
shows a resonance line, revealing the Larmor frequency and the spin coherence
time $T_2$ of the electrons. Second, we discuss the possibility to use an {\em
oscillating} magnetic field in the Faraday setup. With an oscillating field
applied, one can observe multi-photon absorption processes in the spin noise.
Furthermore an oscillating field could also help to avoid line broadening due
to structural or chemical inhomogeneities in the sample, and thereby increase
the precision of the spin-coherence time measurement.
