Line shapes of saturated absorption spectroscopy in optically thick media

This is the authors' abstract. We don't add key points for this paper.

The line shapes of saturation spectroscopy in optically thick two-level media are calculated for atomic and molecular transitions. Line narrowing and distortion effects due to propagation are presented. For Doppler-broadened media it is shown that, due to multiphoton processes, the natural linewidth limits the ultimate narrowing that can be reached. Thus, up to α₀L=20, only when the rate-equation approximation is valid subhomogeneous linewidths are expected.