Characteristics of the resonant interaction between gaseous media and frequency-modulated radiation under conditions of saturated amplification or absorption

An experimental investigation was made of the amplitude–frequency and phase–frequency characteristics of the first harmonic signal of the amplitude modulation observed as a result of resonant interaction between frequency-modulated radiation and the spectral line of a gas under conditions of a saturated transition. The experiments were carried out for the gain profile of a CO$_2$ laser and the saturated absorption resonance of $\bar S\bar F_6$. It was found that as the saturation increased, the quantum transition began to exhibit properties of a differentiating stage relative to the modulation. A two-level transition model was used to give a quantitative interpretation of the modulation characteristics of $\bar S\bar F_6$ for a weak field when the quantum transition exhibits properties of a slow-response stage.