a two-level atom driven by a strong monochromatic laser [@mollow_power_1969]. Such a spectrum can be computed from fluctuations of atomic coherence [@steck_quantum_2011] and is shown in the Fig. \[mollow\]. There is some light at $\sim$ 30 MHz red of the atomic transition. This resonance fluorescence provides the first photon which is amplified by the driven atoms.

Discussion
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To explain and interpret why the gain and lasing in the red-detuned peak from the driven-atom, cavity coupled system occurs we discuss the results of a model developed to understand the system. The model contains all the essential elements required to understand the lasing observed. We consider 2-level atoms coupled to one of the modes of a Fabry-Pérot cavity, which is probed by a monochromatic laser of frequency $\omega_p$ and the atoms are driven by a single drive laser with frequency $\omega_d$. The probe and drive light fields are assumed to be classical.

The average photon number in the cavity is calculated using a semi-classical theory of atom-light interaction. Fig. \[gainsim\] shows cavity transmission spectrum as a function of the frequency of probe laser using Eqn. \[alphafull\]. From Fig. \[gainsim\] it is evident that for red detuning the probe laser experiences gain 