Mid-infrared sensing of CO at saturated absorption conditions using intracavity quartz-enhanced photoacoustic spectroscopy

Jakob Hayden; Bettina Baumgartner; Johannes P Waclawek; Bernhard Lendl

doi:10.1007/s00340-019-7260-6

Mid-infrared sensing of CO at saturated absorption conditions using intracavity quartz-enhanced photoacoustic spectroscopy

Appl Phys B. 2019;125(9):159. doi: 10.1007/s00340-019-7260-6. Epub 2019 Aug 5.

Authors

Jakob Hayden¹, Bettina Baumgartner¹, Johannes P Waclawek¹, Bernhard Lendl¹

Affiliation

¹ Institute of Chemical Technologies and Analytics, Technische Universität Wien, Getreidemarkt 9/164, 1060 Vienna, Austria.

Abstract

The sensitivity of quartz-enhanced photoacoustic spectroscopy (QEPAS) can be drastically increased using the power enhancement in high-finesse cavities. Here, low noise resonant power enhancement to 6.3 W was achieved in a linear Brewster window cavity by exploiting optical feedback locking of a quantum cascade laser. The high intracavity intensity of up to 73 W mm^-2 in between the prongs of a custom tuning fork resulted in strong optical saturation of CO at 4.59 µm. Saturated absorption is discussed theoretically and experimentally for photoacoustic measurements in general and intracavity QEPAS (I-QEPAS) in particular. The saturation intensity of CO's R9 transition was retrieved from power-dependent I-QEPAS signals. This allowed for sensing CO independently from varying degrees of saturation caused by absorption induced changes of intracavity power. Figures of merit of the I-QEPAS setup for sensing of CO and H₂O are compared to standard wavelength modulation QEPAS without cavity enhancement. For H₂O, the sensitivity was increased by a factor of 230, practically identical to the power enhancement, while the sensitivity gain for CO detection was limited to 57 by optical saturation.