The detection and measurement of gas concentrations using the characteristic optical absorption of the gas species is important for both understanding and monitoring a variety of phenomena from industrial processes to environmental change. This study reviews the field, covering several individual gas detection techniques including non-dispersive infrared, spectrophotometry, tunable diode laser spectroscopy and photoacoustic spectroscopy. We present the basis for each technique, recent developments in methods and performance limitations. The technology available to support this field, in terms of key components such as light sources and gas cells, has advanced rapidly in recent years and we discuss these new developments. Finally, we present a performance comparison of different techniques, taking data reported over the preceding decade, and draw conclusions from this benchmarking.

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Optical gas sensing: a review

Introductory Fourier transform spectroscopy

A static polarization imaging spectrometer based on a Savart polariscope

A compact, static hyperspectral imaging linear polarimeter (HILP) based on a Savart interferometer (SI) is conceptually described. It improves the existing SI by replacing front polarizer with two Wollaston prisms, and can simultaneously acquire four interferograms corresponding to four linearly polarized lights on a single CCD. The spectral dependence of linear Stokes parameters can be recovered with Fourier transformation. Since there is no rotating or moving parts, the system is relatively robust. The interference model of the HILP is proved. The performance of the system is demonstrated through a numerical simulation, and the methods for compensating the imperfection of the polarization elements are described.

Static hyperspectral imaging polarimeter for full linear Stokes parameters

Snapshot imaging Fourier transform spectrometers include a lens array that produces sub-images that are directed through a birefringent interferometer in orthogonal polarization eigenstates that acquire an optical path difference. Interference patterns based on this OPD can be Fourier transformed to obtain a spectral image. In some examples, polarizing gratings provide a spatial heterodyne frequency and offset the spectra.

Ultra-compact snapshot imaging fourier transform spectrometer

We present several novel designs of static Fourier-transform spectrometers based on Wollaston prisms. By numerical modeling we show the increased field of view that can be obtained when an achromatic half-wave plate is included between the prisms or when prisms fabricated from positive and negative birefringent materials are combined. In addition, we model how a single Wollaston prism with an inclined optic axis produces a fringe plane localized behind its exit face, thus enabling the design of a static Fourier-transform spectrometer based on a single Wollaston prism.

Design of a static Fourier-transform spectrometer with increased field of view.

An ultra-compact static Fourier-transform spectrometer based on a single birefringent component

We report the design, construction, and evaluation of a static Fourier-transform ultraviolet spectrometer. The spectrometer is based on Wollaston prisms that form an interferogram in the spatial domain, which is recorded with a detector array. We demonstrate the application of the spectrometer to gas detection. Using a deuterium light source, we measured a detection limit, with a 1-s integration time, for hydrogen sulfide and sulfur dioxide, corresponding to 0.2 ppm over a 5-m path length.

Static Fourier-transform ultraviolet spectrometer for gas detection

Releases of benzene and other gases have been detected and quantified using a novel optical, open-path instrument based on a deuterium light source and a static Fourier-transform spectrometer. The spectrometer uses Wollaston prisms to form an interferogram in the spatial domain that is recorded by use of a detector array. The instrument is designed to operate in the ultraviolet region of the spectrum between 200 and 270 nm, which coincides with strong absorption features in the spectra of many gases of environmental and health interest. Using the instrument with a 5-s measurement period provides a path-integrated concentration sensitivity to benzene of 2 parts in 10(6) times meter, which corresponds to a 20-parts in 10(9) detection limit over a typical path length of 100 m.

Detection of benzene and other gases with an open-path, static Fourier-transform UV spectrometer

The development of a novel static Fourier-transform spectrometer based on two Wollaston prisms, two polarizers, and a compact two-dimensional detector array is described. The wavelength calibration is fixed by the geometry of the prisms and the detector array and is therefore inherently stable. The Wollaston prisms are fabricated from materials with opposite sign of birefringence which gives a significantly increased field of view compared with existing Wollaston prism based Fourier-transform spectrometers. The spectrometer operates in the visible region of the spectrum, has a resolution of 350 cm−1, an aperture of 6×4.6 mm, and a field of view of ±10°. The optical assembly is interfaced to a laptop computer resulting in a rugged, portable instrument with no moving parts.

B A Patterson

Papers

Design of a static Fourier-transform spectrometer with increased field of view.

An ultra-compact static Fourier-transform spectrometer based on a single birefringent component

Static Fourier-transform ultraviolet spectrometer for gas detection

Detection of benzene and other gases with an open-path, static Fourier-transform UV spectrometer

Wide field of view, ultracompact static Fourier-transform spectrometer