Fluorescence spectrum of spontaneous emission in cavity.

We study the probe spectrum of light generated by spontaneous emission into the mode of a cavity QED system. We identify the spontaneous emission process by the polarization of the transmitted light when the excitation of the atoms is with linear polarization in a $\Delta m=0$ transition in the $D_2$ line of $^{85}$Rb. The probe spectrum has a maximum on-resonance when the number of inverted atoms for an input drive is maximal. For a larger number of atoms N, the maximum splits and develops into a doublet, but its frequencies are different from those of the so-called vacuum Rabi splitting.