Analysis of laser absorption gas sensors employing scanned-wavelength modulation spectroscopy with 1f-phase detection

摘要

The recently introduced wavelength-modulation spectroscopy with 1f-phase detection (WMS-theta(1f)) technique showed promising results with potentially improved measurement precision over the popular 1f-normalized WMS-nf (WMS-nf/1f) technique. Like WMS-nf/1f, WMS-theta(1f) enjoys the typical benefits of WMS methods, including low-frequency noise rejection, correction for non-absorbing losses, and insensitivity to the broadband absorption spectra of interfering species. In this work, we performed a detailed analysis of the spectrally resolved scanned-wavelength WMS-theta(1f) measurement technique and its direct comparison against the expected performance of scanned-wavelength WMS-nf/1f and scanned-wavelength direct-absorption spectroscopy (SDAS) measurements. This simulation-based analysis identified specific operating regimes in which the performance of WMS-theta(1f) measurements in terms of accuracy is expected to be greater than the performance of WMS-nf/1f or SDAS. Additionally, improved guidelines for the optimal selection of laser-tuning parameters, including an explicit optimization of the optical scan depth parameter, were developed. Experiments with a CO2 static cell perturbed by a high-speed air jet corroborated the simulation-based findings. Finally, a practical demonstration of a WMS-theta(1f) sensor for measuring temperature and H2O mole fraction in the exhaust of a CH4/air flat-flame burner was presented, with the results confirming model predictions of the superior precision of WMS-theta(1f) relative to WMS-nf/1f and SDAS.