Experimental investigation of saturated polarization spectroscopy for quantitative concentration measurements

Polarization-spectroscopy (PS) line shapes and signal intensities are measured in well-characterized hydrogen-air flames operated over a wide range of equivalence ratios. We use both low (perturbative) and high (saturating) pump beam intensities in the counterpropagating pump-probe geometry. The effects of saturation on the line-center signal intensity and the resonance linewidth are investigated. The PS signal intensities are used to measure relative OH number densities in a series of near-adiabatic flames at equivalence ratios (phi) ranging from 0.5 to 1.5. The use of saturating pump intensities minimizes the effect of pump beam absorption, providing more accurate number density measurements. When calibrated to the calculated OH concentration in the phi = 0.6 flame, the saturated PS number density measurements probing the P(1)(2) transition are in excellent agreement with OH absorption measurements, equilibrium calculations of OH number density, and previous saturated degenerate four-wave mixing OH number density measurements.