KEYWORDS: Spectroscopy, Mid-IR, Signal to noise ratio, Time metrology, Optical parametric oscillators, Spectrometer engineering, Photodetectors, Modulation, Gases, Frequency combs
A dual frequency comb spectrometer is realized by electro-optic modulation of a 1550 nm laser and subsequent conversion to the mid-infrared by difference-frequency generation (DFG). Using an optical parametric oscillator for the DFG the combs can be tuned from 3 μm to 4.7 μm with 440 comb modes covering 220 GHz (< 6 cm-1). Trace gas detection of nitrous oxide, carbon dioxide and methane is demonstrated with a 7.2-m-multi-pass cell while a sufficiently low noise-equivalent absorbance is reached in already 1 s. The bandwidth normalized noise-equivalent-absorption coefficient is consistently below 2.8 × 10-6 Hz-1/2 cm-1 while the precision of the determined concentrations is better 2 % Hz-1/2.
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