Dual-gas sensor for ultra-close overlapping spectra based on second harmonic peak shift

Abstract

In spectral absorption techniques, overlapping spectra cause distortion in absorption line heights, leading to inaccuracies in concentration measurements, which poses a significant challenge for measuring multi-component gases. In this paper, an innovative method that utilizes the displacement of the second harmonic (2f) peak to mitigate 2f height errors resulting from cross-interference is presented. Our research findings reveal that cross-interference distorts symmetrical absorption spectra, and by adjusting the modulation depth, a predictable shift in the 2f peak can be induced, with the magnitude of this shift directly correlated to the concentration of the interfering gas. This discovery enables us to correct 2f height errors, thereby enabling precise concurrent analysis of multi-component gases while mitigating the effects of cross-interference. This approach holds promise in addressing the challenge of accurate dual-parameter gas measurement in scenarios of ultra-close spectral overlap. In a proof-of-concept experiment, we selected C₂H₂ and CO₂ (two gases with closely spaced and overlapped absorption spectra) as example gases for detection. Notably, this method achieved high-precision measurements with an accuracy of 27 ppb for C₂H₂ and 5 ppm for CO₂ when dealing with ultra-close spectra of 1 ppm C₂H₂ and 4350 ppm CO₂, whose centerline offset is merely 0.0859 cm⁻¹. This represents a 15-fold improvement in measurement accuracy for C₂H₂. The minimum detection limits for C₂H₂ and CO₂ are 3.2 ppb and 13.6 ppm, respectively. Furthermore, this work offers a novel solution to mitigate cross-interference, demonstrating good versatility for application in other absorption spectroscopy techniques without additional costs or structural changes.