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Proceedings Paper

Concentration measurements of complex mixtures of broadband absorbers by widely tunable optical parametric oscillator laser spectroscopy
Author(s): K. Ruxton; N. A. Macleod; D. Weidmann; G. P. A. Malcolm; G. T. Maker
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Paper Abstract

The ability to obtain accurate vapour parameter information from a compound’s absorption spectrum is an essential data processing application in order to quantify the presence of an absorber. Concentration measurements can be required for a variety of applications including environmental monitoring, pipeline leak detection, surface contamination and breath analysis. This work demonstrates sensitive concentration measurements of complex mixtures of volatile organic compounds (VOCs) using broadly tunable mid wave infrared (MWIR) laser spectroscopy. Due to the high absorption cross-sections, the MWIR spectral region is ideal to carry out sensitive concentration measurements of VOCs by tunable laser absorption spectroscopy (TLAS) methods. Absorption spectra of mixtures of VOCs were recorded using a MWIR optical parametric oscillator (OPO), with a tuning range covering 2.5 μm to 3.7 μm. The output of the MWIR OPO was coupled to a multi-pass astigmatic Herriott gas cell, maintained at atmospheric pressure that can provide up to 210 m of absorption path length, with the transmission output from the cell being monitored by a detector. The resulting spectra were processed by a concentration retrieval algorithm derived from the optimum estimation method, taking into account both multiple broadband absorbers and interfering molecules that exhibit narrow multi-line absorption features. In order to demonstrate the feasibility of the concentration measurements and assess the capability of the spectral processor, experiments were conducted on calibrated VOCs vapour mixtures flowing through the spectroscopic cell with concentrations ranging from parts per billion (ppb) to parts per million (ppm). This work represents as a first step in an effort to develop and apply a similar concentration fitting algorithm to hyperspectral images in order to provide concentration maps of the spatial distribution of multi-species vapours. The reported functionality of the novel fitting algorithm makes it a valuable addition to the existing data processing tools for parameter information recovery from recorded absorption data.

Paper Details

Date Published: 8 November 2012
PDF: 12 pages
Proc. SPIE 8537, Image and Signal Processing for Remote Sensing XVIII, 85370I (8 November 2012); doi: 10.1117/12.971184
Show Author Affiliations
K. Ruxton, M Squared Lasers Ltd. (United Kingdom)
N. A. Macleod, Rutherford Appleton Lab. (United Kingdom)
D. Weidmann, Rutherford Appleton Lab. (United Kingdom)
G. P. A. Malcolm, M Squared Lasers Ltd. (United Kingdom)
G. T. Maker, M Squared Lasers Ltd. (United Kingdom)

Published in SPIE Proceedings Vol. 8537:
Image and Signal Processing for Remote Sensing XVIII
Lorenzo Bruzzone, Editor(s)

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