PHASE-SHIFTS AND INTENSITY DEPENDENCE IN FREQUENCY-MODULATION SPECTROSCOPY

We discuss experimental observations of an induced phase shift of the rf signal produced in frequency-modulated saturated-absorption measure ments under the conditions of large probe-beam intensities. This phase shift can be understood in terms of the phase difference between the phase modulation of the optical driving field and the steady-state mac roscopic polarization of the medium. We also present theoretically cal culated spectra that are based on the optical Bloch equations and that are in excellent agreement with the experimental results.