On the pump intensity dependence of probe noise, absorption and dispersion in a driven two-level system at points of zero absorption and in the dead-zone

We investigate the quantum noise gained by a weak probe laser interacting with a driven two-level system at points of zero absorption and in the dead-zone. We find that in the Doppler-limit, maximum noise is obtained at the extremum point of the probe dispersion curve as a function of the pump intensity. This implies that in this limit, the noise is maximal at the transition point between focusing and defocusing. On the other hand, in the absence of Doppler broadening, this is true only for the case of a resonant pump.