EFFECT OF DOPPLER BROADENING ON OPTICAL GAIN WITHOUT INVERSION IN A 4-LEVEL MODEL

We discuss the effect of Doppler broadening on the gain and refractive index experienced by an optical probe when passing through a collecti on of atoms in the absence of a population inversion. We calculate the gain-absorption spectrum of the medium when the atoms have a Maxwell velocity distribution and compare the atomic response when the pump an d probe beams are arranged in copropagating and counterpropagating con figurations. The results in these two cases can be qualitatively very different from each other, as we show with the help of numerical calcu lations. A physical interpretation and the conditions for the validity of the results are provided.