Multi-Harmonic Wavelength Modulation Spectroscopy for the Detection of Atmospheric Methane and Nitrous Oxide in mid-IR region*

A high precision and sensitive wavelength modulation spectroscopy (WMS) multi-harmonic detection technique has been utilized in various fields including chemical sensing for environment, defense and industrial applications. The multi-harmonic WMS is an important tool to probe weak molecular rotational-vibrational transitions of overlapping atmospheric line transitions to determine atmospheric broadening, density etc. In this paper, we present quantum cascade laser based multi-harmonic WMS detection to simultaneously probe atmospheric broadened transitions of methane (CH₄) and nitrous oxide (N₂O) in the mid-infrared region (7.8 μm) of the spectrum. We show that for a given experimental SNR, the spectral line resolution of disparate line transitions can be improved by higher harmonic WMS detection. We also show that Rayleigh resolution criterion can be extended to quantify spectral resolution and the structures of WMS signals.