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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.
(a) Spectral scan of 0.75% methanol in N2 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/N2 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, 25 falling in the energy range corresponding to the frequency span of Fig. 2(a) .
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 (R2 = 0.9963). The small deviations from the linear trend are partially due to uncertainty in the gas mixture concentration.
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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