Scanned- and fixed-wavelength absorption diagnostics for combustion measurements using multiplexed diode lasers

A multiplexed diode-laser sensor system comprising two diode lasers and fiber-optic components has been developed to nonintrusively monitor temperature and species mole fraction over a single path using both scanned- and fixed-wavelength laser absorption spectroscopy techniques. In the scanned-wavelength method, two InGaAsP lasers were current tuned at a 2-kHz rate across H2O transitions near 1343 mn and 1392 mn in the 2 nu(1) and nu(1) + nu(3) bands. Gas temperature was determined from the ratio of single-sweep integrated line intensities. Species mole fraction was determined from the measured line intensity and the calculated line strength at the measured temperature. In the fixed-wavelength method, the wavelength of each laser was fixed near the peak of each absorption feature using a computer-controlled laser Line-locking scheme. Rapid measurements of gas temperature were obtained from the determination of peak line-intensity ratios. The system was applied to measure temperature and species concentration in the postflame gases of an H-2-O-2 flame. The good agreement between the laser-based measurements obtained using scanned- and fixed-wavelength methods with those recorded with thermocouples demonstrates the flexibility and utility of the multiplexed diode-laser sensor system and the potential for rapid, continuous measurements of gasdynamic parameters in high-speed or transient flows with difficult optical access.