SATURATED FDFWM LINESHAPES AND INTENSITIES - THEORY AND APPLICATION TO QUANTITATIVE MEASUREMENTS IN FLAMES

The present paper deals with a three-level model of interaction with t hree cross-polarized and co-propagating fields having the same frequen cy. The two pump waves have arbitrary intensities and the third one (t he probe wave) is assumed to be non-saturating. Using the radiative re normalization method, density matrix equations are solved analytically . The forward degenerate four-wave mixing (FDFWM) signal is calculated under thermodynamic conditions where collisional and Doppler broadeni ngs are comparable. The calculation also takes into account crossed-po larization effects. The FDFWM intensity is computed as a function of p ower densities and pressure. FDFWM experimental spectra of the (2) Sig ma(+)-(2) Pi (0-0) band of OH obtained in a welding torch flame are co mpared to theoretical profiles. For incident laser intensities in the range of a few MW cm(-2), a good agreement is obtained between calcula ted and experimental profiles. Discrepancies arise under stronger cond itions of saturation, showing the limits of application of our model. However, because there is little virtue in using higher laser powers, the model should be extremely useful in practical applications. In par ticular, we demonstrate that the signal intensity is optimal and the p ressure dependence is minimized when the saturation parameter is unity .