Abstract

Non-intrusive spatially resolved measurements of gas concentrations are important in fluid flow, combustion, and heat transfer research. By combining multi-angle absorption measurements with tomographic reconstruction, quantitative and spatially resolved measurements may be performed.¹ Due to the low absorbance of most gaseous species in the visible wavelength region, previous attempts in optical absorption tomography have been restricted to specially selected species in order to obtain a sufficiently high signal-to-noise ratio for the tomographic reconstruction. We combine tomography with two-tone frequency modulation spectroscopy (TTFMS), a highly sensitive absorption technique, extending the possibilities for quantitative and spatially resolved measurements to a vast number of species.² Our TTFMS experiment uses laser modulation at 647 ± 5.2 MHz and has an absorption sensitivity of 1:10⁶. The tomographic method is demonstrated by mapping the concentration in a section of a weakly absorbing O₂ gas flow, using a GaAlAs diode laser operating around 760 nm.

© 1994 IEEE