Development of a near-infrared external-cavity laser heterodyne radiometer with balanced detection for the measurement of atmospheric water vapor/δD and CO2/δ13C

A near-infrared external-cavity laser heterodyne radiometer (EC-LHR) with balanced detection was developed for remote sensing of water vapor/δD and CO₂/δ¹³C in the atmospheric column. This tunable external cavity laser, with a center wavelength of 1.56 µm, serves as local oscillator and offers a tuning range of hundreds of wavenumbers by scanning work temperature and current. By optimizing the optical heterodyne balanced detection configuration, the EC-LHR achieved quasi-shot-noise-limited performance. High-resolution atmospheric transmission spectra of water vapor, HDO, CO₂, and ¹³CO₂ were simultaneously measured using the developed EC-LHR operating in ground-based solar occultation mode. Within the framework of the optimal estimation algorithm, three inversion strategies were employed: single peak retrieval, multiple peaks joint retrieval, and isotopic ratio-constrained retrieval. This approach allows for the determination of the column abundance and profiles of CO₂ and water vapor, as well as δD and δ¹³C. The reported EC-LHR has broad application potential in the fields of anthropogenic gas emission monitoring and water vapor transport research.