Local absorption measurement by laser-induced fluorescence

The method consists of determining the local absorption of a tuned laser pulse between two points close together along the laser beam by measuring the fluorescence emitted from these two points. In this way a nonintrusive local and instantaneous sampling of the laser spectral intensity is achieved at these points where the system-absorption line and subsequent linear fluorescence emission-acts as an in situ spectrometer. Problems due to fluorescence efficiency, fluorescence trapping, and detector sensitivity that could be different for the two points are overcome by using a second beam emitted by the same pulsed laser running collinearly but in the opposite direction to the first beam and with a short delay. Finally a ratio between the two pairs of fluorescence signals provides the optical depth of the medium between the two points involved. Feasibility is demonstrated by the measurement of OH absolute concentration in a homogeneous premixed flame.