Abstract

Asynchronous optical sampling (ASOPS) is a pump–probe method for the measurement of species concentrations in turbulent high-pressure flames. We show that rapid measurement of species number density can be achieved in a highly quenched environment by maintaining a constant beat frequency between the mode-locking frequencies of the pump and the probe lasers. A model for the ASOPS method based on rate equation theory for three- and four-level atoms is presented. A number of improvements are made to the basic ASOPS instrument, which result in a greatly enhanced signal-to-noise ratio. Atomic sodium is aspirated into an atmospheric pressure C₂H₄/O₂/N₂ flame and detected with the ASOPS instrument. When excited-state lifetimes are fitted by using the ASOPS theory, a 3P_1/2,3/2 → 3S_1/2 quenching-rate coefficient of 1.72 × 10⁹ s⁻¹ and a 3P_3/2 → 3P_1/2 doublet-mixing rate coefficient of 3.66 × 10⁹ s⁻¹ are obtained, in excellent agreement with literature values. ASOPS signals obtained over a wide range of pump and probe beam powers validate the rate equation theory. Improvements are suggested to improve the signal-to-noise ratio, since the present results are limited to laminar flows.

© 1992 Optical Society of America

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