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

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, NO ₂ 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.