Laser-absorption mass flux sensor for high-speed airflows

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Abstract
We present a laser-based concept for an air mass flux sensor based on the simultaneous measurement of density and velocity of O2. The sensor uses wavelength modulation absorption spectroscopy at 10 MHz of an AlGaAs laser diode in the b1∑g+(v′ = 0)−X3∑g−(v″ = 0) system of O2, combined with second-harmonic detection. The apparatus is demonstrated on a high-speed oxygen flow generated behind the incident shock wave in a shock tube. The measurement time is 0.5 ms for typical absorption levels of 0.5%.