Laser-absorption mass flux sensor for high-speed airflows

We present a laser-based concept for an air mass flux sensor based on the simultaneous measurement of density and velocity of O(2). The sensor uses wavelength modulation absorption spectroscopy at 10 MHz of an AlGaAs laser diode in the b(1)Sigma(g)(+)(v' = 0)-X(3)Sigma(g)(-)(v'' = 0) system of O(2), 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%.