Trace detection in gases using photoacoustic spectroscopy and Fabry-perot interferometry

Two thermooptic-based detection systems for gases were devel- oped and studied for their performances in monitoring air pollutants. In the first system which is based on the photoacoustic effect, a continuous-wave carbon dioxide laser was adapted for operation in the infrared with its output wavelength modulated between two consecutive vibration-rotation lines. This was accomplished by applying a time-varying electric field to a commercial piezoelectric pusher attached to the grating mount of the laser. Operating under wavelength modulation, the primary limitation on detectability in photoacoustic measurements--window absorption--was removed and the useful range for application was extended. This concept was illustrated by the determination of ppb levels of ethylene in nitrogen;In the second system, the feasibility of using Fabry-Perot interferometry for in situ trace detection of gases was studied. Two experiments were performed with a single frequency helium neon (HeNe) laser acting as a probe beam. The first one was based on a dual-beam arrangement, with direct measurement of the shift of interference fringe as a result of the change in refractive index caused by the absorption of excitation radiation by the species of interest. Because of certain instrumental limitations, the full potential of this scheme as a sensitive device for trace gas detection was never quite realized;The second Fabry-Perot interferometric experiment was based on a single-beam geometry using modulated excitation and coherent detection of the signal. A laboratory built position stabilization circuit was employed to maintain the interferometer at its optimum point of operation. By combining this with wavelength modulation, the;projected detection limit of the resulting scheme for C(,2)H(,4) in nitrogen was found to be 20 ppb (S/N = 3); (\u271)The Ames Laboratory is operated for the U.S Department of Energy by Iowa State University under contract No. W-7405-eng-82. This work was supported by the Office of Basic Energy Sciences