Auto calibration of OH laser induced fluorescence signals by local absorption measurement in flame

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The local absorption of a tuned laser pulse between two points close together along the laser beam is instantaneously determined by measuring the fluorescence emitted from these two points in order to realize an in situ calibration of these single shot fluorescence signals. At these two points the system—absorption line and subsequent linear fluorescence emission—acts as punctual non-intrusive spectrometer to probe the laser spectral intensity. To make such a sampling independent of local absorbing concentration, fluorescence efficiency and trapping that could be different for the two probed points, a dual pulsed laser beam configuration is used; a second beam emitted from the same pulsed laser is focused in the flame colinearly but opposite in direction to the first beam and with a short delay. Finally, a cross ratio between the two pairs of signals successively delivered by the two fluorescence detectors provides the optical depth between the two points at each laser shot independently of all other properties of the medium. Experiments performed with OH in a premixed flat flame show the advantages and limitations of this technique; the accuracy for mean absolute concentration is 20% and the single shot reproducibility is about 20%. The spatial resolution (>2 mm in the laser direction) and the detectivity (>4 10−4 OH mole fraction) are not very high but this technique is thought as an in situ calibration of the fluorescence signals which are simultaneously available for high resolution and sensitivity detection.