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This paper reports on progress on gas sensing using real-time correlation spectroscopy, where a gas is used to 'recognize' its own spectral absorption lines. Our recent results on methane detection using pressure modulation spectrometry are shown, and a new method of modulating the absorption of the gas in the reference cell section of an ammonia detection system is reported for the first time. 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 sequentially through the cells will suffer absorption in each of the cells. 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. If the absorption lines of the gases do not correlate strongly, then the modulation index of the optical signal is essentially unaffected by the presence of the different gas in the sampling cell. Therefore, the concentration of a specific gas in the sampling cell can be quantified by this means. Pressure modulation of the gas within the reference cell is achieved by the use of a novel acoustic resonator. This device provides a reasonable pressure ratio while being compact and easily driven. Unlike previous piston-compressors, the high modulation frequency of the resonator improves the tolerance to transients in the optical signal produced by the passage of dust through the optical beam. The system can be tailored to detect a desired gas by changing the reference gas and a broadband section filter. Results for methane are presented in this paper. A novel method for detection of ammonia gas is reported for the first time. This involves Stark modulation of the gas in the reference cell, by the application of an electric field. This method is applicable to gases with strong electric dipole moments, such as NH3, CO, NOx, and HC1.
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