  Resonance fluorescence from atomic systems consists of a single spectral peak
that evolves into a Mollow triplet for a strong excitation field. Photons from
different peaks of the triplet show distinct photon correlation that make the
fluorescence a useful light source for quantum information purpose. We
characterize the fluorescence of a single optically trapped $^{87}$Rb atom that
is excited resonantly at different power levels. Second-order correlation
measurements reveal the single photon nature of the fluorescence concurrently
with Rabi oscillations of a strongly excited atom. The asymmetry in
correlations between photons from two sidebands of the fluorescence spectrum
when the atom is exposed to an off-resonant field further indicates that there
is a preferred time-ordering of the emitted photons from different sidebands.
