détecteur optoacoustique

CONT: Optoacoustic detector. Light which is absorbed by a molecule is generally reradiated from the molecule in the form of heat. If a laser, at an absorbing wavelength of a gas of interest, is passed through a closed cell, the temperature of the entire gas in the cell will rise and thus the pressure of the gas in the cell will rise. If the laser is turned on and off at a frequency of 100 - 20000 Hz, the resulting pressure rises and drops can be picked up as a signal by sensitive microphones. This somewhat odd combination of optics and acoustics turns out to have several advantages over systems which measure the absorption directly. The absorption will be quite small and thus the change in intensity of the laser beam is nearly impossible to detect. Because pressure in the cell rises more quickly for small length cells, this effect exactly cancels out gains in overall absorption by making the cell longer. Thus, a small optoacoustic cell will accomplish the same as a larger cell making its size a great asset.... The device is being tested as a sniffer for bomb vapours and for ambient air analyses.