Mid-infrared laser heterodyne radiometer (LHR) based on a 3.53 μm room-temperature interband cascade laser

Simultaneous detection of multi-component greenhouse gases based on an all-fibered near-infrared single-channel frequency-division multiplexing wavelength-modulated laser heterodyne radiometer

The performance of an all fibered near-infrared (NIR) single-channel frequency-division multiplexing wavelength-modulated laser heterodyne radiometer (FDM WM-LHR) is demonstrated in ground-based solar occultation mode. The system modulates the laser through the high-frequency signal output by the lock-in amplifier to replace the traditional chopper modulation, making it more stable and compact. Moreover, personal computers are used to simultaneously control the operating current of two distributed feedback (DFB) lasers through a general purpose interface bus-universal serial bus (GPIB-USB), thereby controlling the central wavelength of the laser at 1602.88 and 1653.727 nm, which serve as the absorption lines for the local oscillator detection of the two main greenhouse gases: CO₂ and CH₄. Firstly, the performance of traditional laser heterodyne radiometer (LHR) and the wavelength-modulated laser heterodyne radiometer (WM-LHR) are compared. The results reveal that both the radiometers have an optimized 2f signal when the modulation amplitude m = 2.2. In the actual measurement, 0.25 V and 0.21 V are selected as the modulation amplitude of the laser for the detection of CH₄ and CO₂. Under the same experimental parameters, at 1602.88 nm, the signal-to-noise ratio (SNR) for the 2f signal of CO₂ in the WM-LHR system is 500.24, while that for the direct absorption signal (DAS) of CO₂ in the traditional LHR system is 337.94. At 1653.727 nm, the SNR for the 2f signal in the WM-LHR system and the DAS of CH₄ in the traditional LHR system are 512.04 and 389.58, respectively. Obviously, the SNR for the WM-LHR system is greatly improved. Finally, the application of frequency-division multiplexing (FDM) technology in the WM-LHR system is discussed. The modulation frequency of the two lasers should be appropriately selected to avoid interference between the signals. Overall, the results show that the FDM WM-LHR system can not only detect multiple gases simultaneously but also reduce the implementation cost of the ground-based radiometer. In addition, this study provides useful insights on planetary atmosphere exploration.

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.

High-resolution oxygen-corrected laser heterodyne radiometer (LHR) for stratospheric and tropospheric wind field detection

We developed a near-infrared (NIR) dual-channel oxygen-corrected laser heterodyne radiometer (LHR) in the ground-based solar occultation mode for measuring vertical profile of wind field in the troposphere and low stratosphere. Two distributed feedback (DFB) lasers centered at 1.27 µm and 1.603 µm were used as local oscillators (LO) to probe absorption of oxygen (O₂) and carbon dioxide (CO₂), respectively. High-resolution atmospheric O₂ and CO₂ transmission spectra were measured simultaneously. The atmospheric O₂ transmission spectrum was used to correct the temperature and pressure profiles based on a constrained Nelder-Mead's simplex method. Vertical profiles of atmospheric wind field with an accuracy of ∼5 m/s were retrieved based on the optimal estimation method (OEM). The results reveal that the dual-channel oxygen-corrected LHR has high development potential in portable and miniaturized wind field measurement.

Optical amplification enables a huge sensitivity improvement to laser heterodyne radiometers for high-resolution measurements of atmospheric gases

A novel, to the best of our knowledge, performance-enhanced laser heterodyne radiometer has been developed by utilizing a semiconductor optical amplifier to amplify the collected weak solar radiation in an optical fiber. High-spectral-resolution measurements of atmospheric carbon dioxide column absorption are used to validate the technique and performance of the developed instrument. The implementation of optical amplification led to a 9-times improvement in sensitivity according to the Allan variance analysis for noise fluctuations, and resulted in a 7.7-times enhancement in measurement precision for atmospheric carbon dioxide. The promising results showed the great potential of employing this type of compact fiber-optics-based spectral radiometer for applications such as atmospheric greenhouse gas sensing.

A MEMS modulator-based dual-channel mid-infrared laser heterodyne radiometer for simultaneous remote sensing of atmospheric CH4, H2O and N2O

The performance of a micro-electro-mechanical system (MEMS) modulator-based dual-channel mid-infrared laser heterodyne radiometer (MIR-LHR) was demonstrated in ground-based solar occultation mode for the first time. A MEMS mirror was employed as an alternative modulator to the traditional mechanical chopper, which makes the system more stable and compact. Two inter-band cascade lasers (ICL) centered at 3.53 µm and 3.93 µm, were employed as local oscillators (LO) to probe absorption lines of methane (CH4), water vapor (H₂O) and nitrous oxide (N₂O). The system stability greater than 1000 s was evaluated by Allan variance. The experimental MIR-LHR spectra (acquired at Hefei, China, on February 24th 2022) of two channels were compared and were in good agreement with simulation spectra from atmospheric transmission modeling. The mixing ratio of CH₄, H₂O and N₂O were determined to be ∼1.906 ppm, 3069 ppm and ∼338 ppb, respectively. The reported MEMS modulator-based dual-channel MIR-LHR in this manuscript has great potential to be a portable and high spectral resolution instrument for remote sensing of multi-component gases in the atmospheric column.

Impact of Lock-In Time Constant on Remote Monitoring of Trace Gas in the Atmospheric Column Using Laser Heterodyne Radiometer (LHR)

The time constant selected for lock-in amplification (LIA) has a crucial impact on observed line shapes in laser heterodyne spectroscopy, in particular in the case of ground-based remote monitoring of trace gas in the atmospheric column using laser heterodyne radiometer (LHR). Conventional simulation could not allow validation of LHR spectra measured in a real and complex atmospheric environment exhibiting large temporal and spatial variability (humidity, temperature, pressure, etc) that impact significantly the measured LHR spectra profiles. High-precision spectral measurement is thus crucial to avoid any spectral distortion resulting from the measurement. In this paper, the impact of LIA time constant on spectral line shape is investigated for LHR operating in continuous laser tuning mode, based on analysis of laboratory heterodyne spectra, in terms of signal-to-noise ratio (SNR), line width broadening, absorption depth and line shift. With respect to the given frequency scanning speed in continuous mode and to the halfwidth of the absorption feature to scan, a reasonable scanning time ΔTscan, the time needed for scanning laser frequency through the halfwidth ΔνHWHM of the absorption line, equal to or longer than 14 times of the LIA time constant τ is concluded in order to efficiently reduce the noise while without significant shift and distortion of the line shape. Experimental validation was carried out using a laser heterodyne absorption spectroscopy approach in the laboratory. Four different combinations of time constants τ and scanning time ΔTscan were used to record heterodyne spectra of a CH4 absorption line near 1242.00 cm−1 in continuous laser tuning mode. An optimal combination of a scanning time of 137 ms with a time constant of 1 ms was obtained. This optimal combination was used for ground-based measurements of CH4 and N2O in the atmospheric column by LHR. The extracted LHR spectrum is in good agreement with a referenced TCCON (Total Carbon Column Observing Network) FT-IR (Fourier-transform infrared) spectrum.

Distributed Feedback Interband Cascade Laser Based Laser Heterodyne Radiometer for Column Density of HDO and CH4 Measurements at Dunhuang, Northwest of China

Remote sensing of HDO and CH4 could provide valuable information on environmental and climatological studies. In a recent contribution, we reported a 3.53 μm distributed feedback (DFB) inter-band cascade laser (ICL)-based heterodyne radiometer. In the present work, we present the details of measurements and inversions of HDO and CH4 at Dunhuang, Northwest of China. The instrument line shape (ILS) of laser heterodyne radiometer (LHR) is discussed firstly, and the spectral resolution is about 0.004 cm−1 theoretically according to the ILS. Furthermore, the retrieval algorithm, optimal estimation method (OEM), combined with LBLRTM (Line-by-line Radiative Transfer Model) for retrieving the densities of atmospheric HDO and CH4 are investigated. The HDO densities were retrieved to be less than 1.0 ppmv, while the CH4 densities were around 1.79 ppmv from 20 to 24 July 2018. The correlation coefficient of water vapor densities retrieved by LHR and EM27/SUN is around 0.6, the potential reasons for the differences were discussed. Finally, in order to better understand the retrieval procedure, the Jacobian value and the Averaging Kernels are also discussed.

Statistical Characterization of Temperature and Pressure Vertical Profiles for the Analysis of Laser Heterodyne Radiometry Data

The statistical analysis of historic pressure and temperature profiles from radiosonde launches for use in the fitting of molecular oxygen line shapes is presented. As the O₂ mixing ratio is nearly constant throughout the lower atmosphere, only variations in pressure and temperature profiles will affect the fit of observed O₂ features in Laser Heterodyne Radiometry (LHR) spectra. Radiosonde temperature and pressure data are extracted from the Integrated Global Radiosonde Archive (IGRA) for a given station, date, and launch time. Data may be extracted for a single launch, for the same date over several years, and/or within a window centered on a target date. The temperature and pressure profiles are further characterized by the statistical variation in coefficients of polynomial fits in altitude. The properties of the probability distributions for each coefficient are used to constrain fits of O₂ line shapes through Nelder–Mead optimization. The refined temperature and pressure profiles are then used in the retrieval of vertically resolved mixing ratios for greenhouse gases (GHGs) measured in the same instrument. In continuous collections, each vertical profile determination may be treated as a Bayesian prior to inform subsequent measurements and provide an estimate of uncertainties.

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.

Laser heterodyne spectroradiometer assisted by self-calibrated wavelength modulation spectroscopy for atmospheric CO2 column absorption measurements

We have developed a laser heterodyne spectroradiometer in combination with self-calibrated wavelength modulation spectroscopy based on a software-based lock-in amplifier to observe the atmospheric carbon dioxide (CO₂) column absorption near wavelength 1.57 μm in solar occultation mode. This combination facilitates miniaturization of laser heterodyne radiometer. Combined with our developed retrieval algorithm, the atmospheric carbon dioxide column concentration is measured to be 413.7 ± 1.9 ppm, in agreement with GOSAT satellite observation results. This system offers high spectral signal-to-noise ratio of ~333 for the zeroth harmonic (0f) normalized second harmonic (R2f) signal of CO₂ transition (R22e), with a measurement averaging time of 8 s, which can be further improved by increasing averaging time in accordance to the Allan deviation analysis for the noise fluctuation. This demonstrates the feasibility of the system for atmospheric investigation and the potential of ground-based, airborne and spaceborne observations for the variation of the global greenhouse gases.