Two-tone frequency modulation spectroscopy for ambient-air trace gas detection using a portable difference-frequency source around 3 μm

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. PACS 42.72.Ai; 42.81.Wg; 42.68.Ca