Reflectivity and transmissivity of an optical cavity coupled to   two-level atoms: Coherence properties and the influence of atomic phase noise
  We consider N identical two-level atoms coupled to an optical cavity, which is coherently driven by an external field. In the limit of small atomic excitation, the reflection and transmission coefficients for both fields and intensities are calculated analytically. In addition, the frequency content of the cavity field and hence also the emission spectrum of the cavity is determined. It is discussed in particular how individual collisional dephasing and common atomic energy-level fluctuations prevent the cavity field from being in a coherent state, which in turn affects the outgoing fields. 
Brian Julsgaard and Klaus M{\o}lmer
quant-ph