A theoretical study of amplitude (AM) and frequency (FM) modulation components of an optical wave interacting resonantly with a homogeneously broadened two-level system is presented. It is shown that the AM component is usually less attenuated than the FM component; however, under conditions which depend on the modulation frequency, relaxation times, and the saturation parameter, the reverse may occur. In strongly saturated media the AM component can be amplified, a process limited by the depletion of the carrier intensity as the wave propagates in the absorber. The two- and three-wave cases of unidirectional saturation spectroscopy are considered and the effects of depletion are discussed. In the two-wave (single-probe) case a third wave is generated which substantially modifies the absorption profile for the probe. An analysis is included of the sideband evolution in a laser with an intracavity saturable absorber. By analyzing the problem in the frequency domain, some new insights concerning the role of the saturable absorber in passively mode-locked lasers are presented.

• Received 4 May 1983

DOI:https://doi.org/10.1103/PhysRevA.29.2716