Characterization of unwanted noise in realistic cavities
  The problem of the description of absorption and scattering losses in high-Q cavities is studied. The considerations are based on quantum noise theories, hence the unwanted noise associated with scattering and absorption is taken into account by introduction of additional damping and noise terms in the quantum Langevin equations and input--output relations. Completeness conditions for the description of the cavity models obtained in this way are studied and corresponding replacement schemes are discussed. 
A.A. Semenov, D.Yu. Vasylyev, W. Vogel, M. Khanbekyan, and D.-G.   Welsch