A portable miniaturized laser heterodyne radiometer (mini‑LHR) for remote measurements of column CH4 and CO2

External-cavity diode laser-based near-infrared broadband laser heterodyne radiometer for remote sensing of atmospheric CO2

A near-infrared broadband (1500-1640 nm) laser heterodyne radiometer (LHR) with a tunable external-cavity diode laser as the local oscillator is developed and the relative transmittance, which represents the absolute relationship between the measured spectral signals and the atmospheric transmittance, is derived. High-resolution (0.0087 cm^-1) LHR spectra in the spectral region of 6248.5-6256 cm^-1 were recorded for the observation of atmospheric CO₂. Combined with the relative transmittance, the preprocessed measured LHR spectra, the optimal estimation method, and the Python scripts for computational atmospheric spectroscopy, the column-averaged dry-air mixing ratio of CO₂ of 409.09 ± 8 ppmv in Dunkirk, France on February 23, 2019, was retrieved, which is consistent with GOSAT and TCCON data. The near-infrared external-cavity LHR demonstrated in the present work has a high potential for use in developing a robust, broadband, unattended, and all-fiber LHR for spacecraft and ground-based atmospheric sensing that offers more channel selection for inversion.

Performance Characterization of a Fully Transportable Mid-Infrared Laser Heterodyne Radiometer (LHR)

A fully transportable laser heterodyne radiometer (LHR), involving a flexible polycrystalline mid-infrared (PIR) fiber-coupling system and operating around 8 µm, was characterized and optimized with the help of a calibrated high temperature blackbody source to simulate solar radiation. Compared to a mid-IR free-space sunlight coupling system, usually used in a current LHR, such a fiber-coupling system configuration makes the mid-infrared (MIR) LHR fully transportable. The noise sources, heterodyne signal, and SNR of the MIR LHR were analyzed, and the optimum operating local oscillator (LO) photocurrent was experimentally obtained. The spectroscopic performance of the MIR LHR was finally evaluated. This work demonstrated that the developed fully transportable MIR LHR could be used for ground-based atmospheric sounding measurements of multiple trace gases in the atmospheric column. In addition, it also has high potential for applications on spacecraft or on an airborne platform.

Development of a laser heterodyne radiometer for regional methane leak detection

We have developed and tested a laser heterodyne radiometer (LHR) for detecting methane leaks from upstream oil and gas infrastructure and landfills that uses the Sun as the signal light source, demonstrating here sensitivity sufficient to detect "super-emitter" leaks (>50kg/h, 1166 slm). Tracking optics follow the Sun during its apparent daily transit across the sky, and the system collects direct absorption data and optionally the 1f and 2f wavelength modulation spectroscopy (WMS) signals. The direct absorption data are processed in real time using a retrieval algorithm with a 5 s update rate to reveal the methane concentration versus altitude for each measurement line of sight. The 1f and 2f WMS signals are significantly non-intuitive because of the dramatic change in the methane lineshape as a function of pressure (altitude) but may ultimately provide useful information for leak localization. We describe herein modifications to the RF detection train and data collection system that allow faster and higher signal-to-noise ratio measurements. Preliminary results suggest that leaks giving rise to methane concentrations of the order of 500 ppm-m can be effectively detected-sensitivity similar to current satellites with more continuous temporal coverage and areal coverage of the order of 100s of km² for relatively low cost. We outline a method of using an array of LHRs to localize the leak using lineshape information and tomographic reconstruction techniques that will be tested in future work.

Transportable mid-infrared laser heterodyne radiometer operating in the shot-noise dominated regime

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. CH₄ and N₂O were simultaneously measured in the atmospheric column using the developed mid-IR LHR. The experimental LHR spectrum of CH₄ and N₂O 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.

Development of a laser heterodyne spectroradiometer for high-resolution measurements of CO2, CH4, H2O and O2 in the atmospheric column

We have developed a portable near-infrared laser heterodyne radiometer (LHR) for quasi-simultaneous measurements of atmospheric carbon dioxide (CO₂), methane (CH₄), water vapor (H₂O) and oxygen (O₂) column absorption by using three distributed-feedback diode lasers as the local oscillators of the heterodyne detection. The developed system shows good performance in terms of its high spectral resolution of 0.066 cm^-1 and a low solar power detection noise which was about 2 times the theoretical quantum limit. Its measurement precision of the column-averaged mole fraction for CO₂ and CH₄ is within 1.1%, based on the standard deviation from the mean value of the retrieved results for a clean sky. The column abundance information of the O₂ is used to correct for the variations and uncertainties of atmosphere pressure, the solar altitude angle, and the prior profiles of pressure and temperature. Comparison measurements of daily column-averaged atmospheric mole fractions of CO₂, CH₄ and H₂O, between our developed LHR and a greenhouse gas observing satellite, show a good agreement, which proves the reliability of our developed system.

Standoff Chemical Detection Using Laser Absorption Spectroscopy: A Review

Remote chemical detection in the atmosphere or some specific space has always been of great interest in many applications for environmental protection and safety. Laser absorption spectroscopy (LAS) is a highly desirable technology, benefiting from high measurement sensitivity, improved spectral selectivity or resolution, fast response and capability of good spatial resolution, multi-species and standoff detection with a non-cooperative target. Numerous LAS-based standoff detection techniques have seen rapid development recently and are reviewed herein, including differential absorption LiDAR, tunable laser absorption spectroscopy, laser photoacoustic spectroscopy, dual comb spectroscopy, laser heterodyne radiometry and active coherent laser absorption spectroscopy. An update of the current status of these various methods is presented, covering their principles, system compositions, features, developments and applications for standoff chemical detection over the last decade. In addition, a performance comparison together with the challenges and opportunities analysis is presented that describes the broad LAS-based techniques within the framework of remote sensing research and their directions of development for meeting potential practical use.