  In the framework of the signal processing approach to single-atom resonance
fluorescence with spectral resolution, we diagrammatically derive an analytical
formula for arbitrary-order spectral correlation functions of the scattered
fields that pass through Fabry-Perot interferometers. Our general expression is
then applied to study correlation signals in the limit of well separated
spectral lines of the resonance fluorescence spectrum. In particular, we study
the normalized second-order temporal intensity correlation functions in the
case of the interferometers tuned to the components of the spectrum and obtain
interferential corrections to the approximate results derived in the secular
limit. In addition, we explore purely spectral correlations and show that they
can fully be understood in terms of the two-photon cascades down the dressed
state ladder.
