Intensity correlations of frequency-filtered light fields

The theory of the intensity correlations between photons of different frequencies is re-examined from basic principles and is found to lead to an expression for the correlation function containing operator products which are normally ordered in time. This expression leads to results which are free of certain extra contributions that arise in another version of this theory which contains non-normally ordered products. The analysis presented is based on a description of the frequency-filtering process in terms of Fabry-Perot interferometers. The mirrors of the interferometers are modelled as classical dielectrics. The EM field in the presence of these dielectrics is then quantised and the intensity correlation function defined in terms of these modified field operators. The results are examined in detail for the case of a light field radiated by a two-level atom.