Two-tone frequency modulation spectroscopy for ambient-air trace gas detection using a portable difference-frequency source around 3 μm
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- Maddaloni, P., Malara, P., Gagliardi, G. et al. Appl. Phys. B (2006) 85: 219. doi:10.1007/s00340-006-2299-6
We report a portable, widely tunable, mW-power mid-infrared spectrometer based on difference-frequency generation in a periodically poled lithium-niobate crystal, realized in a compact and robust design. The analytical performance for real-time monitoring of natural-abundance trace gases in ambient air is evaluated, pointing out the possibility of field applications. In a direct-absorption scheme, a minimum detectable concentration of 3 ppb Hz-1/2 is demonstrated around 3.3 μm for methane at atmospheric pressure. The sensitivity is further improved by using a two-tone frequency modulation spectroscopy technique that provides an enhancement of a factor of 100 in the signal-to-noise ratio, thus yielding a minimum absorption coefficient of 5.3×10-9 cm-1 Hz-1/2.