Laser diode wavelength-modulation spectroscopy for simultaneous measurement of temperature, pressure, and velocity in shock-heated oxygen flows

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Abstract
Wavelength modulation at 10 MHz of an AlGaAs laser diode, superposed on repetitive linear scans of wavelength, is applied to measure second-harmonic absorption line shapes of oxygen in the A band. Theoretical expressions of the harmonic line shapes, including the effect of laser amplitude modulation and varying modulation depth, are presented. A least-squares fit of the experimental line shapes to theoretical second-harmonic line shapes permits simultaneous determination of the temperature and the pressure. The use of high-repetition-rate (10-kHz) linear scans of the studied wavelength region permits application of the technique to high-speed unidimensional transient flows generated in a shock tube; velocity is derived from the Doppler shift of the absorption profiles.