Evolution of highly-selective gas sensing methods using correlation spectroscopy

This review describes the recent history and latest developments in gas detection at Southampton University, using real-time correlation spectroscopy. The general approach has been to use a gas sample as a matched optical filter, to selectively detect similar absorption spectra of the same gas in a measurement region. All variations of the method exhibit excellent selectivity whenever the gases have narrow line spectra, even when simple broadband sources are used for illumination. Several have been employed in laboratory tests, for remote detection over optical fibre leads. The paper describes work in four main areas. Firstly, our earliest work on a pressure modulation for methane detection is briefly reviewed, secondly a Stark modulation method to form an optical hygrometer is described, thirdly a multi-line light source (formed by combining a broadband optical source with a Michelson interferometer, having a gas cell in one arm) is discussed and, finally the most recent progress with a complementary modulated beam system, using either alternately-chopped or intensity modulated light sources, is described