Advances in mid-IR Cavity Enhanced Photoacoustic and Photothermal Trace Gas Sensing

Abstract

Resent advances and developments regarding laser based mid-IR gas sensors based on cavity enhanced - quartz enhanced photoacoustic spectroscopy (QEPAS) as well as interferometric cavity assisted photothermal spectroscopy (ICAPS) are reported. A resonant linear Brewster window cavity was added to a commercial prototype of a QEPAS sensor system to increase the power of the employed quantum cascade laser (QCL) at the place of measurement by a factor of more than hundred. In ICAPS, a Fabry-Perot cavity facilitates the sensitive detection of the refractive index change induced by mid-IR absorption. Using a balanced ICAPS detection scheme efficiently eliminates excess noise and resulted in shot noise limited performance. Results obtained when studying small molecules like CO, N2O and SO2 using these techniques are presented and compared. The dependence of the generated signal as a function of the applied modulation frequency will be discussed.