Breakdown of the Born approximation in laser phase-noise to amplitude-noise conversion

When a vapor absorbs laser light, the field's intrinsic phase fluctuations (PM) induce variations in the atoms' absorption cross section, which in turn yield fluctuations in the transmitted light intensity. In the Born approximation, the observed amplitude noise (AM) arises from a single scattering of the input field. Here, we show that the Born approximation breaks down for optically thick vapors and that the observed AM is influenced by scatterings of the 'medium-perturbed' field. With importance for spectroscopy, we find that the multiple field-atom scatterings reduce the PM-to-AM conversion efficiency