Sensitive detection of methane with a 1.65 μm diode laser by photoacoustic and absorption spectroscopy

In this paper we present two different schemes - resonant photoacoustic (PA) and direct absorption - for the sensitive detection of gaseous methane employing recently commercially available low-power distributed feedback (DFB) diode lasers at 1.65 mu m. The sensitivities achievable and the advantages and disadvantages of these systems are compared and discussed. The PA spectrum measured in the range of 1655.5 nm +/- 1.5 nm is compared to the data extracted from the HITRAN 92 database. A minimum detection limit of 60 ppm V or 6 ppm m at 1.6537 mu m and a practical sensitivity of 120 ppm V has been achieved. III addition to PA detection, absorption spectroscopy was employed to detect methane. The signal was quantitatively extracted by wavelength-modulation spectroscopy with second-harmonic detection. Combined temperature and current tuning of the diode laser was used to scan the laser over the methane absorption line. Under atmospheric pressure conditions an excellent linearity was achieved. From the scatter of a series of individual concentration measurements at different concentrations between 15 ppm V and 100 ppm V an average detection limit (SNR = 1) of 1.15 ppm m could be deduced. This corresponds to a practical sensitivity of about 7 ppm V (SNR = 3 and 47.5 cm absorption length).