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Development of a passive optical heterodyne radiometer for near and mid-infrared spectroscopy.

We have developed a novel laser heterodyne radiometer using a fiber-coupled distributed feedback laser as the local oscillator to perform spectroscopic measurements of small molecules in the near-infrared (NIR) spectral region. Here, we demonstrate measurement of HCN and CO2 in the lab and CH4 and CO2 in the atmospheric column. In addition, we demonstrate detection of a neutral iron, Fe(I), Fraunhofer line in the spectrum of the sun, at a vacuum wavelength of 1559.252 nm, that can be used to calibrate the wavelength scale of the instrument and enable verification of proper system operation for field applications.