In this Chapter, we solve the exciton-cavity photon coupling analytically with the linear model and analyze its properties of emission in the presence of decoherence (dissipation and incoherent pump). The results presented here have been published in the references 4, 6, 10, 16 and 17 of the list of my publications .3.1 The last one is a Mathematica plotter of the spectra of emission, that might be useful to follow the argumentation.