Determination of relative number density and decay rate for atomic sodium in an atmospheric premixed flame by asynchronous optical sampling

Asynchronous Optical Sampling (ASOPS) is a pump/probe method that utilizes a dual timebase scheme to determine number densities and decay rates. In the ASOPS technique, two lasers are mode-locked at slightly different frequencies, causing a relative phase walkout between the pump and probe lasers. This allows the dynamic measurement of subnanosecond excitation processes in a time scale on the order of milliseconds. The current ASOPS instrument consists of two dye lasers synchronously pumped by two frequency-doubled Nd:YAG lasers at a repetition rate of ∼82 MHz. The ASOPS signal is obtained by AC amplification of the probe beam voltage, which is measured by a single photodiode at a beat frequency of 10 kHz. In this paper we report the first use of ASOPS in a flame environment. The relative number density and decay time of atomic sodium are measured by aspirating a sodium chloride solution into an atmospheric premixed CH 4 /O 2 /N 2 flame. The pump and probe beams are each tuned to the D 2 transition (589.0 nm). Both ASOPS and laser-induced florescence are used to determine relative concentration profiles and saturation curves. Measurements obtained using these techniques are found to be in close agreement, demonstrating the viability of the ASOPS method as a combustion diagnostic and its potential for use in high-pressure, turbulent flames.