Optical EngineeringProgress in fiber-remoted gas correlation spectrometry
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The paper reports on the progress in gas sensing using real-time correlation spectroscopy, where a gas is used as a matched optical filter to "recognize" its own spectral absorption lines. The basic concept of correlation spectrometry involves the passage of light sequentially through two gas cells: a reference cell containing a known quantity of the gas to be detected, and a sampling cell where the presence of the gas is to be determined. An optical signal passing through both cells will suffer absorption due to the gas in each. If the absorption in the reference cell is periodically modulated, then the total absorption depends on whether the gas absorption lines in the sampling cell correlate with those in the reference cell gas. Two methods of modulating the reference cell absorption are reported, pressure and Stark modulation. Results are presented for methane detection employing pressure modulation. The pressure fluctuations are generated within a compact resonant acoustic cell driven by a piezoelectric transducer. Also given are results for cross-sensitivity measurements with ethane as the contaminant gas. The Stark technique is applied to ammonia detection here, but can be used with many gases that exhibit a strong dipole moment.