Self-calibration and laser energy monitor validations for a double-pulsed 2-μm CO2 integrated path differential absorption lidar application

Airborne Testing of 2-μm Pulsed IPDA Lidar for Active Remote Sensing of Atmospheric Carbon Dioxide

The capability of an airborne 2-μm integrated path differential absorption (IPDA) lidar for high-accuracy and high-precision active remote sensing of weighted-average column dry-air volume mixing ratio of atmospheric carbon dioxide (XCO2) is demonstrated. A test flight was conducted over the costal oceanic region of the USA to assess instrument performance during severe weather. The IPDA targets CO2 R30 absorption line using high-energy 2-μm laser transmitter. HgCdTe avalanche photodiode detection system is used in the receiver. Updated instrument model included range correction factor to account for platform attitude. Error budget for XCO2 retrieval predicts lower random error for longer sensing column length. Systematic error is dominated by water vapor (H2O) through dry-air number density derivation, followed by H2O interference and ranging related uncertainties. IPDA XCO2 retrieval results in 404.43 ± 1.23 ppm, as compared to 405.49 ± 0.01 ppm from prediction models, using consistent reflectivity and steady elevation oceanic surface target. This translates to 0.26% and 0.30% relative accuracy and precision, respectively. During gradual spiral descend, IPDA results in 404.89 ± 1.19 ppm as compared model of 404.75 ± 0.73 ppm indicating 0.04% and 0.23% relative accuracy, respectively. Challenging cloud targets limited retrieval accuracy and precision to 2.56% and 4.78%, respectively, due to H2O and ranging errors.

High-Precision CO2 Column Length Analysis on the Basis of a 1.57-μm Dual-Wavelength IPDA Lidar

For high-precision measurements of the CO₂ column concentration in the atmosphere with airborne integrated path differential absorption (IPDA) Lidar, the exact distance of the Lidar beam to the scattering surface, that is, the length of the column, must be measured accurately. For the high-precision inversion of the column length, we propose a set of methods on the basis of the actual conditions, including autocorrelation detection, adaptive filtering, Gaussian decomposition, and optimized Levenberg–Marquardt fitting based on the generalized Gaussian distribution. Then, based on the information of a pair of laser pulses, we use the direct adjustment method of unequal precision to eliminate the error in the distance measurement. Further, the effect of atmospheric delay on distance measurements is considered, leading to further correction of the inversion results. At last, an airborne experiment was carried out in a sea area near Qinhuangdao, China on 14 March 2019. The results showed that the ranging accuracy can reach 0.9066 m, which achieved an excellent ranging accuracy on 1.57-μm IPDA Lidar and met the requirement for high-precision CO₂ column length inversion.

High Repetition Rate Mid-Infrared Differential Absorption Lidar for Atmospheric Pollution Detection

Developments in mid-infrared Differential Absorption Lidar (DIAL), for gas remote sensing, have received a significant amount of research in recent years. In this paper, a high repetition rate tunable mid-infrared DIAL, mounted on a mobile platform, has been built for long range remote detection of gas plumes. The lidar uses a solid-state tunable optical parametric oscillator laser, which can emit laser pulse with repetition rate of 500 Hz and between the band from 2.5 μm to 4 μm. A monitoring channel has been used to record the laser energy in real-time and correct signals. Convolution correction technology has also been incorporated to choose the laser wavelengths. Taking NO2 and SO2 as examples, lidar system calibration experiment and open field observation experiment have been carried out. The observation results show that the minimum detection sensitivity of NO2 and SO2 can reach 0.07 mg/m3, and 0.31 mg/m3, respectively. The effective temporal resolution can reach second level for the high repetition rate of the laser, which demonstrates that the system can be used for the real-time remote sensing of atmospheric pollution gas.

Sensitivity analysis and correction algorithms for atmospheric CO2 measurements with 1.57-µm airborne double-pulse IPDA LIDAR

In this study, a 1.57-µm airborne double-pulse integrated-path differential absorption (IPDA) light detection and ranging (LIDAR) system was developed for CO₂ measurements. This airborne IPDA LIDAR is integrated with a real-time frequency monitoring system, an integrated sensor for temperature, pressure, and humidity, an inertial navigation system, and a global positioning system. The random errors of the LIDAR system, which are caused by the signal noise, background noise, and detector noise, among other factors, are analyzed for different target reflectivities at a flight altitude of 8 km. After parametric optimization, the signal is unsaturated at high target reflectivity. Further, it can be detected at low target reflectivity by adjusting the detector gain. After the averaging of 148 shots, the relative random error (RRE) was 0.057% for a typical target reflectivity of 0.1 sr^-1. Moreover, the systematic errors caused by the laser pulse energy, linewidth, spectral purity, and frequency drift, as well as the atmospheric parameters related to the flight experiments are also investigated. The relative system error (RSE) was 0.214% as determined based on an analysis of the systematic errors, which are primarily caused by the frequency drift. Two methods are proposed to reduce the RSE caused by the frequency drift. The first is the averaging of 148 shots, which can reduce the RSE to 0.096%. The other method involves calculating the integral weight function (IWF) using real-time frequency. However, this is a time-consuming and computationally intensive process. Hence, look-up tables for the absorption cross-section were created to overcome this issue, resulting in a decrease in the RSE to 0.096%. Using actual aircraft attitude angles, velocity, and position data from flight experiments, the relative errors (REs) in the IWF caused by the uncorrected integral path and Doppler shift were determined to be 0.273% and 0.479%, respectively. However, it was found that corrections to the integral path and Doppler shift based on accurate calculations of the IWF cause the airborne platform to turn in such a way that the REs are eliminated. Hence, this study confirms the validity of system parameters and provides a reference for other researchers who study similar IPDA LIDAR systems. Further, the sensitivity analysis of the airborne IPDA LIDAR system can provide a reference to future data inversions. Moreover, the proposed correction algorithms for the integral path and Doppler shift contribute to more accurate inversion results.

Optical Energy Variability Induced by Speckle: The Cases of MERLIN and CHARM-F IPDA Lidar

In the context of the FrenchGerman space lidar mission MERLIN (MEthane Remote LIdar missioN) dedicated to the determination of the atmospheric methane content, an end-to-end mission simulator is being developed. In order to check whether the instrument design meets the performance requirements, simulations have to count all the sources of noise on the measurements like the optical energy variability induced by speckle. Speckle is due to interference as the lidar beam is quasi monochromatic. Speckle contribution to the error budget has to be estimated but also simulated. In this paper, the speckle theory is revisited and applied to MERLIN lidar and also to the DLR (Deutsches Zentrum für Luft und Raumfahrt) demonstrator lidar CHARM-F. Results show: on the signal path, speckle noise depends mainly on the size of the illuminated area on ground; on the solar flux, speckle is fully negligible both because of the pixel size and the optical filter spectral width; on the energy monitoring path a decorrelation mechanism is needed to reduce speckle noise on averaged data. Speckle noises for MERLIN and CHARM-F can be simulated by Gaussian noises with only one random draw by shot separately for energy monitoring and signal paths.

Analysis of energy monitoring for a double-pulsed CO2 integrated path differential absorption lidar at 1.57 μm

For double-pulsed 1.57 μm integrated path differential absorption lidar, the transmitted pulse energy measurement is an important factor that can influence the uncertainty of CO₂ concentration measurement. An energy monitoring experiment was performed to determine how to improve the measurement precision of the transmitted pulse energy. Ground glass diffusers were used to reduce the speckle effect during energy monitoring. The roughness and rotational speed of the ground glass diffusers were considered and compared. The normalized energy ratios between on-line and off-line echo pulses and on-line and off-line energy monitoring pulses were analyzed, and the Allan deviation was used to evaluate the energy monitoring results. Averaging 148 shots, the standard deviation of the normalized energy ratio reached 0.0757%, whereas the correlation between the energy ratio of the on-line and off-line energy monitoring pulses and the energy ratio of the on-line and off-line echo pulses was higher than 90%.

2-μm double-pulse single-frequency Tm:fiber laser pumped Ho:YLF laser for a space-borne CO2 lidar

In the framework of space-borne CO₂ lidar development, the transmitter is a critical unit. We report on the development and the assessment of performances of a 2-μm single-frequency thulium fiber laser pumped Q-switched Ho:YLF laser. To fulfill the requirements of space-based operation, a master oscillator power amplifier architecture has been chosen, and the oscillator works in double-pulse operation. The transmitter can generate a single-mode dual wavelength emission "ON" and "OFF" around the R30e line of the 20013←00001 band of CO₂12. It delivers a pair of OFF-ON pulses with 12 mJ and 42 mJ energy, respectively, at a pulse repetition frequency of 303.5 Hz. The pulse energy and central frequency stabilities are especially documented as well as pulse duration, polarization, overall efficiency, beam quality, pointing stability, and spectral purity. The possible limitations by light-induced damage or radiation-induced attenuation on the laser performances are also evaluated.

Comparison of CO₂ Vertical Profiles in the Lower Troposphere between 1.6 µm Differential Absorption Lidar and Aircraft Measurements Over Tsukuba

A 1.6 μm differential absorption Lidar (DIAL) system for measurement of vertical CO₂ mixing ratio profiles has been developed. A comparison of CO₂ vertical profiles measured by the DIAL system and an aircraft in situ sensor in January 2014 over the National Institute for Environmental Studies (NIES) in Tsukuba, Japan, is presented. The DIAL measurement was obtained at an altitude range of between 1.56 and 3.60 km with a vertical resolution of 236 m (below 3 km) and 590 m (above 3 km) at an average error of 1.93 ppm. An in situ sensor for cavity ring-down spectroscopy of CO₂ was installed in an aircraft. CO₂ mixing ratio measured by DIAL and the aircraft sensor ranged from 398.73 to 401.36 ppm and from 399.08 to 401.83 ppm, respectively, with an average difference of -0.94 ± 1.91 ppm below 3 km and -0.70 ± 1.98 ppm above 3 km between the two measurements.

Energy calibration of integrated path differential absorption lidars

The stringent requirements for energy reference measurement represent a challenging task for integrated path differential absorption lidars to measure greenhouse gas columns from satellite or aircraft. The coherence of the lidar transmitter gives rise to speckle effects that have to be considered for accurate monitoring of the energy ratio of outgoing on- and off-line pulses. Detailed investigations have been performed on various measurement concepts potentially suited for deployment within future satellite missions.

Evaluation of 2-μm Pulsed Integrated Path Differential Absorption Lidar for Carbon Dioxide Measurement-Technology Developments, Measurements, and Path to Space

The societal benefits of understanding climate change through the identification of global carbon dioxide sources and sinks led to the recommendation for NASA's Active Sensing of Carbon Dioxide Emissions over Nights, Days, and Seasons space-based mission for global carbon dioxide measurements. For more than 15 years, the NASA Langley Research Center has developed several carbon dioxide active remote sensors using the differential absorption lidar technique operating at 2-μm wavelength. Recently, an airborne double-pulsed integrated path differential absorption lidar was developed, tested, and validated for atmospheric carbon dioxide measurement. Results indicated 1.02% column carbon dioxide measurement uncertainty and 0.28% bias over the ocean. Currently, this technology is progressing toward triple-pulse operation targeting both atmospheric carbon dioxide and water vapor-the dominant interfering molecule on carbon dioxide remote sensing. Measurements from the double-pulse lidar and the advancement of the triple-pulse lidar development are presented. In addition, measurement simulations with a space-based IPDA lidar, incorporating new technologies, are also presented to assess feasibility of carbon dioxide measurements from space.

Development of an advanced Two-Micron triple-pulse IPDA lidar for carbon dioxide and water vapor measurements

An advanced airborne triple-pulse 2-μm integrated path differential absorption (IPDA) lidar is under development at NASA Langley Research Center that targets both carbon dioxide (CO2) and water vapor (H2O) measurements simultaneously and independently. This lidar is an upgrade to the successfully demonstrated CO2 2-μm double-pulse IPDA. Upgrades include high-energy, highrepetition rate 2-μm triple-pulse laser transmitter, innovative wavelength control and advanced HgCdTe (MCT) electron-initiated avalanche photodiode detection system. Ground testing and airborne validation plans are presented.

Airborne Two-Micron Double-Pulse IPDA Lidar Validation for Carbon Dioxide Measurements Over Land

An airborne double-pulse 2-μm Integrated Path Differential Absorption (IPDA) lidar has been developed at NASA LaRC for measuring atmospheric CO2. IPDA was validated using NASA B-200 aircraft over land and ocean under different conditions. IPDA evaluation for land vegetation returns, during full day background conditions, are presented. IPDA CO2 measurements compare well with model results driven from on-board insitu sensor data. These results also indicate that CO2 measurement bias is consistent with that from ocean surface returns.

Double-pulse 1.57 μm integrated path differential absorption lidar ground validation for atmospheric carbon dioxide measurement

A ground-based double-pulse integrated path differential absorption (IPDA) instrument for carbon dioxide (CO₂) concentration measurements at 1572 nm has been developed. A ground experiment was implemented under different conditions with a known wall located about 1.17 km away acting as the scattering hard target. Off-/offline testing of a laser transmitter was conducted to estimate the instrument systematic and random errors. Results showed a differential absorption optical depth (DAOD) offset of 0.0046 existing in the instrument. On-/offline testing was done to achieve the actual DAOD resulting from the CO₂ absorption. With 18 s pulses average, it demonstrated that a CO₂ concentration measurement of 432.71±2.42  ppm with 0.56% uncertainty was achieved. The IPDA ranging led to a measurement uncertainty of 1.5 m.

Feasibility study of a space-based high pulse energy 2 μm CO2 IPDA lidar

Sustained high-quality column carbon dioxide (CO2) atmospheric measurements from space are required to improve estimates of regional and continental-scale sources and sinks of CO2. Modeling of a space-based 2 μm, high pulse energy, triple-pulse, direct detection integrated path differential absorption (IPDA) lidar was conducted to demonstrate CO2 measurement capability and to evaluate random and systematic errors. Parameters based on recent technology developments in the 2 μm laser and state-of-the-art HgCdTe (MCT) electron-initiated avalanche photodiode (e-APD) detection system were incorporated in this model. Strong absorption features of CO2 in the 2 μm region, which allows optimum lower tropospheric and near surface measurements, were used to project simultaneous measurements using two independent altitude-dependent weighting functions with the triple-pulse IPDA. Analysis of measurements over a variety of atmospheric and aerosol models using a variety of Earth's surface target and aerosol loading conditions were conducted. Water vapor (H2O) influences on CO2 measurements were assessed, including molecular interference, dry-air estimate, and line broadening. Projected performance shows a <0.35  ppm precision and a <0.3  ppm bias in low-tropospheric weighted measurements related to column CO2 optical depth for the space-based IPDA using 10 s signal averaging over the Railroad Valley (RRV) reference surface under clear and thin cloud conditions.

Development of 1.6 μm DIAL using an OPG/OPA transmitter for measuring atmospheric CO2 concentration profiles

An experiment for the measurement of atmospheric CO₂ vertical profiles up to a 7 km altitude was successfully conducted using a 1.6 μm ground-based differential absorption Lidar developed by Tokyo Metropolitan University. To achieve a high pulse repetition rate, large power output, and high frequency stabilization, we developed a new 1.6 μm Lidar system using an optical parametric generator (OPG) transmitter. Unlike the previous system's transmitter, OPG does not need a resonator. We amplified its output with two optical parametric amplifiers. We validated our system against an in situ sensor and found the difference between their CO₂ concentration measurements to be 0.06 ppm.

Double-pulse 2-μm integrated path differential absorption lidar airborne validation for atmospheric carbon dioxide measurement

Field experiments were conducted to test and evaluate the initial atmospheric carbon dioxide (CO₂) measurement capability of airborne, high-energy, double-pulsed, 2-μm integrated path differential absorption (IPDA) lidar. This IPDA was designed, integrated, and operated at the NASA Langley Research Center on-board the NASA B-200 aircraft. The IPDA was tuned to the CO₂ strong absorption line at 2050.9670 nm, which is the optimum for lower tropospheric weighted column measurements. Flights were conducted over land and ocean under different conditions. The first validation experiments of the IPDA for atmospheric CO₂ remote sensing, focusing on low surface reflectivity oceanic surface returns during full day background conditions, are presented. In these experiments, the IPDA measurements were validated by comparison to airborne flask air-sampling measurements conducted by the NOAA Earth System Research Laboratory. IPDA performance modeling was conducted to evaluate measurement sensitivity and bias errors. The IPDA signals and their variation with altitude compare well with predicted model results. In addition, off-off-line testing was conducted, with fixed instrument settings, to evaluate the IPDA systematic and random errors. Analysis shows an altitude-independent differential optical depth offset of 0.0769. Optical depth measurement uncertainty of 0.0918 compares well with the predicted value of 0.0761. IPDA CO₂ column measurement compares well with model-driven, near-simultaneous air-sampling measurements from the NOAA aircraft at different altitudes. With a 10-s shot average, CO₂ differential optical depth measurement of 1.0054±0.0103 was retrieved from a 6-km altitude and a 4-GHz on-line operation. As compared to CO₂ weighted-average column dry-air volume mixing ratio of 404.08 ppm, derived from air sampling, IPDA measurement resulted in a value of 405.22±4.15  ppm with 1.02% uncertainty and 0.28% additional bias. Sensitivity analysis of environmental systematic errors correlates the additional bias to water vapor. IPDA ranging resulted in a measurement uncertainty of <3  m.