Resonant holographic interferometry for species concentration measurements with saturated anomalous dispersion

A double-exposure resonant holographic interferometry measurement technique for full-field, instantaneous species is presented. This technique utilizes tuned pulsed dye lasers, separate reference beams, and a corrective holographic optical element. The resulting interferogram fringes represent the species number density of interest with variations in the bulk refractive index due to the temperature field automatically subtracted. Expressions for relating resonant absorption with fringe shift are disclosed. The relations suggest that RHI is well suited for optically dense media and long path lengths, where absorption measurements can be difficult. A saturation related modification of the dispersion near a broadened resonant transition is described. The depopulation of the ground state due to saturation by the probe beam causes an asymmetric modification of the resonant refractive index, which when convolved over the probing beam does not affect the measured fringe shift as measured by RHI. Resonant interferometry is demonstrated as an accurate and quantitative measure of the concentration of sodium vapor in a low pressure cell. Furthermore, as a combustion diagnostic, RHI is demonstrated on a minor combustion species, OH, in a nonpremixed slot burner.