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

We report on a portable, widely-tunable, mW-power mid-infrared spectrometer based on difference frequency generation (DFG) 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 ?m for methane at atmospheric pressure. The sensitivity is further improved by using a two-tone frequency modulation spectroscopy (TTFMS) technique that provides an enhancement of a factor 100 in the signal-to-noise ratio, thus yielding a minimum absorption coefficient of 5.3×10^^-9×cm^-1×Hz^^-1/2.