LASER-DIODE WAVELENGTH-MODULATION SPECTROSCOPY FOR SIMULTANEOUS MEASUREMENT OF TEMPERATURE, PRESSURE, AND VELOCITY IN SHOCK-HEATED OXYGEN FLOWS

Wavelength modulation at 10 MHz of an AlGaAs laser diode, superposed o n repetitive linear scans of wavelength, is applied to measure second- harmonic absorption line shapes of oxygen in the A band. Theoretical e xpressions of the harmonic line shapes, including the effect of laser amplitude modulation and varying modulation depth, are presented. A le ast-squares fit of the experimental line shapes to theoretical second- harmonic line shapes permits simultaneous determination of the tempera ture and the pressure. The use of high-repetition-rate (10-kHz) linear scans of the studied wavelength region permits application of the tec hnique to high-speed unidimensional transient flows generated in a sho ck tube; velocity is derived from the Doppler shift of the absorption profiles.