Wavelength-modulation spectroscopy near 1.4 µm for measurements of H₂O and temperature in high-pressure and -temperature gases

The development, validation and demonstration of a two-color tunable diode laser (TDL) absorption sensor for measurements of temperature and H₂O in high-pressure and high-temperature gases are presented. This sensor uses first-harmonic-normalized wavelength-modulation spectroscopy with second-harmonic detection (WMS-2f/1f) to account for non-absorbing transmission losses and emission encountered in harsh, high-pressure environments. Two telecommunications-grade TDLs were used to probe H₂O absorption transitions near 1391.7 and 1469.3 nm. The lasers were frequency-multiplexed and modulated at 160 and 200 kHz to enable a measurement bandwidth up to 30 kHz along a single line-of-sight. In addition, accurate measurements are enabled at extreme conditions via an experimentally derived spectroscopic database. This sensor was validated under low-absorbance (<0.05) conditions in shock-heated H₂O–N₂ mixtures at temperatures and pressures from 700 to 2400 K and 2 to 25 atm. There, this sensor recovered the known temperature and H₂O mole fraction with a nominal accuracy of 2.8% and 4.7% RMS, respectively. Lastly, this sensor resolved expected transients with high bandwidth and high precision in a reactive shock tube experiment and a pulse detonation combustor. (paper)