The construction of a spectrophone and a CO2 laser waveguide excited by a radiofrequency field for application to the detection of atmospheric trace species

The realization of an optoacoustic device capable of registering the spectra of minor atmospheric gases is reported. Optoacoustic trace constituent detection comprises laser emission of a monochromatic light beam corresponding to the absorption line of the gas under study. Absorption by the gas augments the pressure, a factor which can be detected by an acoustic detector operating with a sensitivity of 1 ppb. Emplacement of the detector in the interior of a CO2 laser waveguide is shown to lower the bounds of attainable sensitivity in terms of the concentration of the species under study. A theoretical model was developed for a signal modulated in intensity and frequency. A minimal detectable concentration is demonstrated for the presence of ethylene. Design features for a circular section CO2 waveguide are presented, and comparisons are made between results from placing the spectraphone inside and outside of the cavity.