Resonant optoacoustic cells for trace gas analysis

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.

[1]  E. Burkhardt,et al.  Spectroscopic Measurements of Stratospheric Nitric Oxide and Water Vapor , 1974, Science.

[2]  P. Kelley,et al.  Molecular absorption of infrared laser radiation in the natural atmosphere , 1976 .

[3]  E Max,et al.  A study of laser-acoustic air pollution monitors. , 1974, Journal of physics E: Scientific instruments.

[4]  A. Bell.  LXVIII. Upon the production of sound by radiant energy , 1881 .

[5]  M. Golay,et al.  Theoretical consideration in heat and infra-red detection, with particular reference to the pneumatic detector. , 1947, The Review of scientific instruments.

[6]  Y. Pao,et al.  Laser optoacoustic detection of explosive vapors , 1975, IEEE Journal of Quantum Electronics.

[7]  J. Margolis,et al.  Water vapor absorption of carbon dioxide laser radiation. , 1976, Applied optics.

[8]  J. Atwood,et al.  The laser illuminated absorptivity spectrophone: a method for measurement of weak absorptivity in gases at laser wavelengths. , 1968, Applied optics.

[9]  H. Callen,et al.  Irreversibility and Generalized Noise , 1951 .

[10]  L. Rosengren,et al.  Optimal optoacoustic detector design. , 1975, Applied optics.

[11]  L. Rosengren,et al.  Characteristics of a resonant opto-acoustic gas concentration detector , 1974 .

[12]  G. Diebold,et al.  Observation of the optoacoustic effect in the microwave region , 1976 .

[13]  W. Röntgen,et al.  On tones produced by the intermittent irradiation of a gas , 1881 .

[14]  Roger D. Kamm,et al.  Detection of weakly absorbing gases using a resonant optoacoustic method , 1976 .

[15]  W. Schnell,et al.  Detection of air pollutants with a tunable CO2 laser , 1975 .

[16]  John Tyndall,et al.  Action of an Intermittent Beam of Radiant Heat Upon Gaseous Matter , 1881, Nature.

[17]  T. F. Deaton,et al.  Absorption coefficient measurements of nitrous oxide and methane at DF laser wavelengths , 1975 .

[18]  C. E. Hackett,et al.  Acoustic amplifier for detection of atmospheric pollutants , 1973 .

[19]  C. Patel.  Spectroscopic measurements of the stratosphere using tunable infrared lasers , 1976 .

[20]  W. Schnell,et al.  Carbon dioxide laser absorption coefficients of various air pollutants. , 1975, Applied optics.

[21]  L. Rosengren.  A new theoretical model of the opto-acoustic gas concentration detector , 1973 .

[22]  N D Kenyon,et al.  Air Pollution: Sensitive Detection of Ten Pollutant Gases by Carbon Monoxide and Carbon Dioxide Lasers , 1972, Science.

[23]  J. G. Parker,et al.  Optical absorption in glass: investigation using an acoustic technique. , 1973, Applied optics.

[24]  M. Tobin,et al.  Far ir laser with metal-dielectric waveguide to observe the Stark effect. , 1976, Applied optics.

[25]  M. Golay,et al.  The theoretical and practical sensitivity of the pneumatic infra-red detector. , 1949, The Review of scientific instruments.

[26]  L. Kreuzer,et al.  Ultralow Gas Concentration Infrared Absorption Spectroscopy , 1971 .

[27]  Kenya Goto,et al.  An acoustically resonant system for detection of low‐level infrared absorption in atmospheric pollutants , 1974 .