Cavity ring-down measurements in flames using a single-mode tunable laser system

Abstract

High-resolution pulsed cavity ring-down spectroscopy (CRDS) of OH in a flame has been demonstrated using a novel single-mode tunable laser (STL). This system operates by pulse amplification of the output of a single-mode diode laser in a modeless dye laser. Ring-down curves obtained using the narrow-bandwidth STL, for both strong and weak transitions, are shown to be well fitted by single exponentials. These results are demonstrated to be in direct contrast with those obtained using a standard dye laser, for which the bandwidth is comparable to the transition linewidths and ring-down curves require multi-exponential fits. Accurate lineshape analysis is thus made possible using the STL, allowing the temperature to be derived from the measured Doppler width. The resulting measurement is in good agreement with the value derived from a Boltzmann plot of data obtained using a conventional laser in a similar flame. The advantages of using the STL system for quantitative CRDS measurements are discussed, together with a suggestion for quantitative measurements of the ASE content of narrowband lasers using CRDS.