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Terahertz quartz enhanced photo-acoustic sensor
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View: Figures


Image of FIG. 1.
FIG. 1.

Schematic of THz QCL-based QEPAS sensor. PM—paraboloidal mirror; C—power combiner; LM, HM—low-frequency modulation (triangular ramp), high-frequency modulation; CEU—control electronics unit; PC—personal computer.

Image of FIG. 2.
FIG. 2.

(a) Spectral scan of 0.75% methanol in N at P = 10 Torr, acquired with a modulation depth of 12 mA (∼100 MHz) and 500 ms integration time. Inset: QEPAS acquisition of a methanol/N sample with a certified concentration of 100 ppm and lock-in integration time of 3 sec. The noise fluctuations are ∼ ±25 μV. (b) Line strengths of the main methanol transitions (vertical bars), as reported in the JPL database, falling in the energy range corresponding to the frequency span of Fig. 2(a) .

Image of FIG. 3.
FIG. 3.

Mean value of the QEPAS peak signals measured for gas mixture samples with different methanol concentrations in locked mode at 10 Torr pressure. The red line is a linear fit of the data (R = 0.9963). The small deviations from the linear trend are partially due to uncertainty in the gas mixture concentration.

Image of FIG. 4.
FIG. 4.

Allan deviation in ppm of the QEPAS signal as a function of the integration time. The initial growth from 0.1 to 1 s reflects delays due to the signal sampling time (200 ms). The oscillations at integration time larger than 4 s are due to slow mechanical drifts of the ADM mounting.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Terahertz quartz enhanced photo-acoustic sensor