We present the theory of “normal mode splitting” (sometimes also called, in the low-intensity limit, “vacuum Rabi splitting”) for a Doppler-broadened two-level medium in an optical cavity, and illustrate it with experimental results. We note that, in the extreme Doppler limit, the absorptive and dispersive properties of the medium are due to almost completely different groups of atoms, and hence it is possible, in principle, to partially saturate the absorbing atoms without changing the dispersion very much; as a result of this, for a sufficiently high probe intensity, transmission peaks well within the Doppler absorption profile are visible, even though the medium is optically dense. Some interesting features exhibited by this system in various regimes are transmission peaks that are unrelated to the “zero-phase” condition, and three-peaked transmission spectra for high atomic density and high cavity input power.

• Received 7 May 2008

DOI:https://doi.org/10.1103/PhysRevA.78.023828