Degenerate four-wave mixing (DFWM) line shapes and signal intensities are measured experimentally in well-characterized hydrogen–air flames operated over a wide range of equivalence ratios. We use both low (perturbative) and high (saturating) beam intensities in the phase-conjugate geometry. Resonances in the <i>A</i><sup>2</sup>Σ<sup>+</sup>–<i>X</i><sup>2</sup>Π (0, 0) band of OH are probed with multiaxial-mode laser radiation. The effects of saturation on the line-center signal intensity and the resonance linewidth are investigated. The DFWM signal intensities are used to measure OH number densities in a series of near-adiabatic flames at equivalence ratios ranging from 0.5 to 1.5. Use of saturating pump intensities minimizes the effects of beam absorption, providing more-accurate number density measurements. The saturated DFWM results are in excellent agreement with OH absorption measurements and equilibrium calculations of OH number density. The polarization dependence of the <i>P</i><sub>1</sub>(2) and <i>R</i><sub>2</sub>(1) resonances is investigated in both laser intensity regimes. There is a significant change in relative reflectivities for different polarization configurations when saturated.
© 1999 Optical Society of America
Thomas A. Reichardt, William C. Giancola, Christopher M. Shappert, and Robert P. Lucht, "Experimental Investigation of Saturated Degenerate Four-Wave Mixing for Quantitative Concentration Measurements," Appl. Opt. 38, 6951-6961 (1999)