Tunable diode laser spectroscopy for analysis of gas impurities

Abstract

To determine the concentration of a given component of the gas mixture, separate absorption lines of this component are selected. The main criterion for choosing a line is the condition of minimal intersection with the absorption lines of other analyzed components of the gas mixture. The near-infrared range makes it possible both to analyze the selected absorption lines of the studied substances and to use a sufficiently accessible element base in the form of wavelength-tunable laser diodes and photodetectors. In the range of 1-2 microns, a fairly wide range of substances (water, methane, hydrogen sulfide, carbon dioxide, etc.) have an absorption cross section of at least 10-23...10-22 cm2/molecule, which allows using laser diodes with a power of about 100 mW to analyze at the ppm level. Herriot's gas cell makes it possible to significantly increase the length of the optical path (up to 30 m or more) and move to the sub-ppm level. The use of fiber-optic connections makes it possible to reduce the influence of the surrounding atmosphere and avoid the need to manufacture protective covers, as for open-path systems. To improve the signal-to-noise ratio, a multiplex factor is used (similar to the Felget gain) when not a difference signal is analyzed, but an interpolation of the measured spectral line by a Lorentz profile. Wavelength-modulated spectroscopy (WMS) will be used to offset power instability and improve the sensitivity of the method. The linearity of the 2f/1f signal from the methane concentration with a coefficient of determination of at least 0.99 is shown. The described gas analyzer is a modular design, when a separate measuring channel is supplemented for each gas under study.

Speaker

Igor Fufurin
Bauman Moscow State Technical University
Russia

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