B. Attal-Trétout

OH detection and spectroscopy by DFWM in flames; comparison with CARS

This paper presents a study of OH detection by degenerate four-wave mixing (DFWM) in a premixed laminar CH4-air flame, at pressures varying from 1 to 7 bar. The P and Q branches of the A 2 Sigma +-X 2 Pi electronic transition are observed. A spectroscopic study is performed and saturation and pressure effects on the lineshapes and concentration profiles are presented. A preliminary comparison between CARS and DFWM is given.

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 three cross-polarized and co-propagating fields having the same frequency. The two pump waves have arbitrary intensities and the third one (the probe wave) is assumed to be non-saturating. Using the radiative renormalization method, density matrix equations are solved analytically. The forward degenerate four-wave mixing (FDFWM) signal is calculated under thermodynamic conditions where collisional and Doppler broadenings are comparable. The calculation also takes into account crossed-polarization effects. The FDFWM intensity is computed as a function of power densities and pressure. FDFWM experimental spectra of the - (0 - 0) band of OH obtained in a welding torch flame are compared to theoretical profiles. For incident laser intensities in the range of a few , a good agreement is obtained between calculated and experimental profiles. Discrepancies arise under stronger conditions 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 particular, we demonstrate that the signal intensity is optimal and the pressure dependence is minimized when the saturation parameter is unity.

Rotational line strengths in degenerate four wave mixing

In this paper we present a derivation of the rotational line strengths involved in fully resonant degenerate four wave mixing processes (DFWM). General expressions are given for diatomic molecules with different Hunds coupling cases. More precisely, we tabulated the rotational line strengths for the electronic transition 2Σ+ -2Π of OH and NO. For the ground state, Hunds cases (b), (a) and the intermediate case (a-b) are considered. We also stress the dependence of the line strengths with respect to the geometry and to the polarization of the incident beams. So, we consider the optical phase conjugation (OPC) geometry and the forward (FDFWM) one. We also give expressions for the line strengths in different polarization arrangements, i.e. parallel or crossed polarized beams. A comparison is made between these cases. Finally, theoretical simulations, using our line strengths calculations, are compared with experimental spectra of OH obtained in a methane-air flame using OPC and FDFWM.

Observation of strong field effects and rotational line coupling in DFWM processes resonant with 2⩞ − 2π electronic system

Abstract Isolated and merged features within the NO A ← X (0,0) and the OH A ← X (0,0) bands have been observed using a crossed-polarization arrangement in forward degenerate four wave mixing (FDFWM). The spectra are interpreted both using the conventional perturbative treatment, which is appropriate in the low-field limit, and using the dressed-molecule formalism when analysing saturation behaviour. The use of the latter formalism is essential in the range of saturation parameters which has been investigated experimentally. Emphasis is placed on reproducing the signal intensity while taking into account strong field effects, rotational line coupling, and laser beam polarization. A simple model of M-level degeneracy is used to depict the various laser polarization arrangements of the three incoming beams. Relative line intensities and spectral broadenings are correctly reproduced for both OH and NO in atmospheric pressure flame conditions, that is, with comparable Doppler and collisional linewidths. In NO, unresolved principal and satellite lines were observed and these could be reproduced using a three J-level model. The validity of this last model will be discussed.

Four-wave mixing in OH: comparison between CARS and DFWM

SummaryA comparison between CARS and Degenerate Four-Wave Mixing experiments on OH is performed in a premixed CH4-air flame. Both optical phase conjugation and forward BOXCARS geometries are studied in DFWM. The relative properties of these two configurations are experimentally investigated from the study of theP andQ branches of the (O−O) band.

Two-color non-linear spectroscopy: application to NO2

Abstract In this paper, we report about an application of the two-color laser-induced thermal gratings technique in the visible region to the detection of nitrogen dioxide in mixtures of nitrogen and argon at total pressure ranging up to 1 bar. We have investigated the dependence of the signal on the gas mixture parameters (pressure, NO 2 concentration, collision partners) and on the laser properties (beams polarization, pulse energy, bandwidth) at various instants during the temporal evolution of the thermal grating. Comparison of experimental results and theoretical predictions is performed. Finally, the detectivity of 4 ppm was obtained at normal conditions of pressure (1 bar) and temperature (300 K).

Atomic oxygen detection in flames using two-photon degenerate four-wave mixing

We report a theoretical and experimental investigation of atomic oxygen detection in an atmospheric-pressure flame by two-photon degenerate four-wave mixing (TPDFWM). We present analytical expressions for the TPDFWM signal that are based on the diagrammatic perturbation theory. The description of several polarization arrangements is given and used in our experiments. The importance of the polarization scheme is emphasized and the most efficient scheme is found by investigating signal and noise contributions. In this case, thermal grating contributions are greatly reduced and this allows quantitative concentration measurements in flame to be performed using TPDFWM.

Simulation of the photon return of a mono and polychromatic sodium guide star

Summary form only given. Adaptive optics can improve the imaging of ground based telescopes near the diffraction limit in spite of atmospheric turbulence. Laser excitation of the mesospheric sodium layer have been proposed and experimentally investigated as a synthetic beacon for wavefront correction. However, this technique suffers from a major drawback: tip-tilt induced by the atmospheric refraction cannot be measured because of the principle of the inverse return of light. A polychromatic guidestar has thus recently been proposed to measure this tip-tilt effect.