Quantitative analysis of dilute mixtures of SO2 in N2 at 7.4 μm by difference frequency spectroscopy
We describe a 7.4-μm source based on difference frequency generation with 6.5 mW of 1278-nm radiation from an extended cavity laser and 66 mW of 1544-nm radiation from another extended cavity laser, amplified in an erbium-doped fibre amplifier. Optimum focusing of the input beams in the 5×5×10-mm3 AgGaSe2 crystal, and the spatial and temporal characteristics of the output beam, are determined. The source is used for accurate determination of line parameters for selected lines of the ν3 band of SO2, centred at 1361 cm-1. Subsequently, these lines are used for performing quantitative analysis of gas mixtures containing SO2 at concentration levels down to 4 ppm without relying on any calibration with certified gas mixtures. This demonstrates the potential of infrared spectroscopy as a primary method for low-concentration gas analysis.