  Nonlinear magneto-optical (NMO) resonances occurring for near-zero magnetic
field are studied in Rb vapor using light-noise spectroscopy. With a balanced
detection polarimeter, we observe high contrast variations of the noise power
(at fixed analysis frequency) carried by diode laser light resonant with the
5S$_{1/2}(F=2) \to 5$P$_{1/2}(F=1) $ transition of $^{87}$Rb and transmitted
through a rubidium vapor cell, as a function of magnetic field $B$. A symmetric
resonance doublet of anti-correlated noise is observed for orthogonal
polarizations around $B=0 $ as a manifestation of ground state coherence. We
also observe sideband noise resonances when the magnetic field produces an
atomic Larmor precession at a frequency corresponding to one half of the
analysis frequency. The resonances on the light fluctuations are the
consequence of phase to amplitude noise conversion owing to nonlinear coherence
effects in the response of the atomic medium to the fluctuating field. A
theoretical model (derived from linearized Bloch equations) is presented that
reproduces the main qualitative features of the experimental signals under
simple assumptions.
