An analysis of the ultimate detectivity of ideal optoacoustic cells, based on viscous gas equations, gives rigorous expressions for both signal and noise for a one-dimensional optoacoustic cell. Choice of boundary conditions for the noise calculations is dictated by the dissipation fluctuation theorem. Results of noise equivalent power calculations indicate superior performances near dc frequencies over those obtained at resonant conditions. A simplifying dissipative acoustic transmission line model describing optoacoustic cells of quite general geometric configurations is developed that is particularly useful for noise evaluation. In contrast to ideal cells, current optoacoustic cells’ detectivities are practically limited by interfering signals induced by windows’ absorption of infrared radiation; acoustical resonant conditions can help reduce such interference. A resonant optoacoustic cell exhibiting high immunity to windows interference is described that yields two orders of magnitude interference reduction compared with previously operated optoacoustic cells. The cell uses the longitudinal modes of a narrow open tube. Its minimum detectable concentration of ethylene in nitrogen is less than 0.3 parts in 109 for 1 Hz detection bandwidth using a 1 W, 10.5326 μm CO2 laser beam. Electronic noise limits the detectivity, and is ~15 dB higher than the expected Brownian noise of an ideal cell of the same configuration. Measurements with flowing trace gas are given.
© 1978 Optical Society of AmericaFull Article | PDF Article
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