Laser-based sensor for detection of hazardous gases in the air using waveguide CO₂ laser

Abstract

A spectrometer based on the principle of photoacoustic spectroscopy has been developed recently at our laboratory for the detection of hazardous gases such as O₃, C₂H₄, SO₂, NO₂ and SF₆. In most of our earlier works, we employed a mechanical chopper to modulate the laser beam and this chopper modulation has the crucial disadvantage of instability in the chopper frequency. Even a minor shift of about 1 Hz in the modulation frequency could significantly reduce the photoacoustic signal by an order of magnitude at the acoustic resonant mode of the photoacoustic cell. To overcome this problem, we developed a photoacoustic spectrometer where a wave guided CW CO₂ laser beam is modulated electronically with the external frequency generator. Our preliminary results show that the electronic modulation of CO₂ laser beam improved the sensitivity of our spectrometer by a factor of 6. The parametric dependence of photoacoustic signal on laser power, modulation frequency and trace gas concentration, was investigated and the comparison between the two modulation techniques is presented in this paper for detection of trace gases such as C₂H₄.

Keywords:

• Hazardous gas sensors
• trace gas analysis
• gas leak detection
• pollution monitoring
• resonant photo-acoustic cells
• laser photo-acoustic spectroscopy
• modulated spectroscopy
• monitoring of Ozone
• SO₂
• NO₂ and C₂H₄

Acknowledgments

The support by the Physics Department and King Fahd University of Petroleum and Minerals under SABIC Project # 2005/11 is gratefully acknowledged.