Multi-wavelength Laser Photoacoustics

Report No. ARL-TR-6147
Authors: Kristan P. Gurton, Melvin Felton, and Richard Tober
Date/Pages: September 2012; 18 pages
Abstract: An optical detection method designed to detect and identify the presence of macro-molecular gas species (e.g., organophosphate-based nerve agent simulants) at trace level concentrations is presented. The technique is based on a modified version of conventional laser photoacoustic spectroscopy (LPAS) in which optical absorption is typically measured using a single laser source. We demonstrate the ability to simultaneously measure multiple absorption related parameters that serve as a concentration independent identifier. Three continuous wave (CW) mid-infrared (MidIR) laser sources, operating at 8.68, 9.29, and 10.35μm, are combined and propagated axially through a specially designed flow-through photoacoustic cell. Each laser is modulated at a different frequency and the resultant acoustic signal(s) are detected and deconvolved using a PC-based 24-bit dynamic signal acquisition device. Species detection and identification is achieved by tabulating independent ratios of the acoustic response for each laser source. Quantitative absorption measured is verified using a Fourier transform infrared (FTIR) spectrometer. Results show good detection and species separation/identification at moderately low ppm concentrations.
Distribution: Approved for public release
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Last Update / Reviewed: September 1, 2012