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.
Highlights
Besides tunable laser absorption spectroscopy (TLAS), photoacoustic spectroscopy (PAS) is the most widely employed laser spectroscopic technique for trace gas sensing [1]
We present intracavity quartz enhanced photoacoustic spectroscopy (I-quartz-enhanced photoacoustic spectroscopy (QEPAS)) measurements of CO under highly saturated conditions targeting the fundamental R9 ro-vibrational transition of CO at 2179.77 cm−1
intracavity QEPAS (I-QEPAS) spectra were recorded by stepping the laser wavelength via temperature in increments of the cavity’s free spectral range (FSR)
Summary
Besides tunable laser absorption spectroscopy (TLAS), photoacoustic spectroscopy (PAS) is the most widely employed laser spectroscopic technique for trace gas sensing [1]. Direct absorption spectroscopic sensing is commonly performed in the mid-IR range well below 10−9 cm−1Hz−1/2 using multipass absorption cells [5, 6] or even below 1 0−10 cm−1 Hz−1/2 using different techniques of cavity-enhanced spectroscopy [7,8,9]. A quantitative treatment of saturated absorption PAS is necessary to fully exploit the sensitivity enhancement provided by high finesse cavities. In this contribution, the influence of saturated absorption on intracavity PAS is quantitatively discussed both theoretically and experimentally. Linearization is performed using the measured saturation intensity and theory of saturated absorption This allows taking full advantage of the power enhancement in I-QEPAS beyond the linear absorption regime. The obtained figures of merit are compared with standard 2f-wavelength modulation QEPAS (2f-WM QEPAS) measurements without cavity enhancement
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