Cavity ring-down measurements of OH radical in atmospheric premixed and diffusion flames.: A comparison with laser-induced fluorescence and direct laser absorption

Cavity ring-down spectroscopy (CRDS) is tested in two atmospheric burners: a premixed flat flame burner and a Wolfhard–Parker burner. The quantitative nature and the spatial resolution of CRDS are compared with those of laser-absorption and laser-induced fluorescence by recording OH concentration profiles. Losses per pass due to the abundant OH sample in the CRD cavity need to be carefully controlled to obtain an exponential ring-down decay. Index refraction gradients can be responsible for important random off-resonance losses which perturb CRDS measurements. In contrast, line-of-sight CRD measurements performed along the axis of the gradients are found to be very accurate.