Gas sensing by tunable multimode diode laser using correlation spectroscopy

A novel method to use multimode diode lasers for gas detection is demonstrated. The relationship between the intensity decay of partially absorbed multiple modes and the target gas concentration is presented. The gas concentration is retrieved by a linear-regression-based procedure applied on simultaneously recorded absorption signals in a dual-beam wavelength-modulation configuration. Signals of non-gas origin or disturbed by interfering gases are effectively eliminated by correlation analysis procedure. Light intensity fluctuations and wavelength shifts due to temperature drift are automatically compensated for. Several absorption signals can readily be obtained within a limited tuning range by overlapping the laser multimode chart with the target gas spectrum. This scheme was applied on the detection of trace acetylene mixed in ambient air, and a sensitivity of 3 ppm was achieved with a 48.5 cm path length. The trade-off between sensitivity and reliability for retrieving gas concentration by multiple absorption signals is also presented.