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Abstract
A transportable laser heterodyne radiometer (LHR) based on an external cavity quantum cascade laser, operating in the mid-infrared (mid-IR) around 8 µm, was developed for ground-based remote sensing of multiple greenhouse gases. A newly available novel flexible mid-IR polycrystalline fiber was first exploited to couple solar radiation, real-time captured using a portable sun-tracker, to the LHR receiver. Compared to free space coupling of sunlight, the technique usually used nowadays in the mid-IR, such fiber coupling configuration makes the LHR system readily more stable, simpler, and robust. Operation of the LHR with quasi-shot-noise limited performance was analyzed and experimentally achieved by optimizing local oscillator power. To the best of our knowledge, no such performance approaching the fundamental limit has been reported for a transportable LHR operating at a long mid-IR wavelength around 8 µm. ${\rm CH}_4$ and ${\rm N}_2{\rm O}$ were simultaneously measured in the atmospheric column using the developed mid-IR LHR. The experimental LHR spectrum of ${\rm CH}_4$ and ${\rm N}_2{\rm O}$ was compared and is in good agreement with a referenced Fourier-transform infrared spectrum from the Total Carbon Column Observing Network observation site and with a simulation spectrum from atmospheric transmission modeling.
© 2021 Optical Society of America
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