Differential Absorption Lidar System Research Articles

2.7 μm backward wave optical parametric oscillator source for CO2 spectroscopy.

In this work, a novel 2.7 µm source used for CO2 and H2O vapor spectroscopy using the backward propagating wave of a backward wave optical parametric oscillator (BWOPO) is demonstrated for the first time to our knowledge. The unique properties of BWOPOs eliminate the need for additional spectral narrowing or wavelength stabilization, enabling the use of a multi-longitudinal mode Q-switched pump laser centered around 1030 nm. A full characterization of the source is presented, revealing a central output at 2712 nm, showcasing a temperature tuning of -1.77 GHz/K, and achieving an output pulse energy of 2.3 µJ. Novel methods are introduced for measuring the linewidth and wavelength stability using the ambient laboratory air. These approaches demonstrate a narrow output of 43 pm and establish an upper limit of stability at 65 MHz, with no active means of stabilization. These findings underscore the potential of BWOPOs as a robust platform for future differential absorption lidar (DIAL) systems.

Simulation and analysis of the CO2 range-resolved differential absorption lidar system at 2 μm

The range-resolved differential absorption lidar is a high-precision device to measure the concentration of carbon dioxide. This paper provides a system-wide theoretical analysis method for the performance analysis and parameter optimization of the lidar system using the given parameter range. The scattered echo signal, signal-to-noise ratio, and detection sensitivity were simulated by setting assumed parameters with the HITRAN 2020 database and the US 1976 standard atmosphere model to analyze the detection distance and concentration resolution of the lidar system. The effects of the laser energy, repetition frequency, and photodetector noise were also discussed. The wavelength selection near the absorption line is critical because it controls the height region of the highest sensitivity and the demands on frequency stability. Recommendations for the selection of absorption lines are provided in this paper. A quantitative analysis of each error source provided reasonable error ranges.

Improving the ozone retrieval accuracy by minimizing the asynchronous offset in differential absorption lidar systems

The differential absorption lidar (DIAL) has been proposed as an effective method for detecting ozone in the atmosphere. An important factor affecting its detecting precision is the asynchronous offset between backscattered signals at different wavelengths. Recently, a DIAL was built by our group, and the impact of the asynchronous offset was tested and studied. The simulation results show that the measurement error caused by the offset is negatively correlated with the altitude. Meanwhile, the offset has an additional effect in areas with sharp ozone concentration changes. Comparative experiments were carried out by our DIAL system and an airborne atmospheric monitoring system in the Nanjing test site. The results show that a 15 m offset in DIAL led to serious errors in retrieval results. These errors are inversely related to the detection altitude and reach up to 13 ppb, which is consistent with the results from simulations. After controlling the relative position of the backscattered signal to minimize the asynchronous offset, the maximum error was reduced to 2 ppb. Then the optimized DIAL was used for 48-hour continuous observation with a proven ozone analyzer. It shows that the optimized DIAL has high detecting accuracy and stability as point-fixed instruments.

Multi-Frequency Differential Absorption LIDAR (DIAL) System for Aerosol and Cloud Retrievals of CO2/H2O and CH4/H2O

We discuss a remote sensing system that is used to simultaneously detect range-resolved differential absorption LIDAR (light detection and ranging; DIAL) signals and integrated path differential absorption LIDAR signals (IPDA LIDAR) from aerosol targets for ranges up to 22 km. The DIAL/IPDA LIDAR frequency converter consists of an OPO pumped at 1064 nm to produce light at 1.6 μm and operates at 100 Hz pulse repetition frequency. The probe light is free space coupled to a movable platform that contains one transmitter and two receiver telescopes. Hybrid photon counting/current systems increase the dynamic range for detection by two orders of magnitude. Range resolved and column integrated dry-air CO2 and CH4 mixing ratios are obtained from line shape fits of CO2 and CH4 centered at 1602.2 nm and 1645.5 nm, respectively, and measured at 10 different frequencies over ≈1.3 cm−1 bandwidth. The signal-to-noise ratios (SNRs) of the IPDA LIDAR returns from cloud aerosols approach 1000:1 and the uncertainties in the mixing ratios weighted according to the integrated counts over the cloud segments range from 0.1% to 1%. The range-averaged DIAL mixing ratios are in good agreement with the IPDA LIDAR mixing ratios at the 1% to 2% level for both CO2 and CH4. These results can serve as a validation method for future active and passive satellite observational systems.

Design Analysis of Mid IR DIAL System for Detection of Hazardous Chemical Species

Nowadays, both military and public agencies are concerned with the remote detection of toxic gases and chemical warfare agents in the atmosphere. A promising method for the remote detection of such harmful chemicals in the atmosphere is Differential Absorption Lidar (DIAL). In the current paper, system design analysis has been carried out to build a DIAL system for the detection of toxic chemical warfare agents, chemical warfare simulants, explosive precursors, and pollutants. The proposed DIAL system comprises an Optical Parametric Oscillator (OPO) based tuneable laser, a 203 mm diameter Cassegrain telescope, a TE-cooled MCT detector, suitable data acquisition hardware, etc. The DIAL output parameters like return signals, SNR, and minimum measurable concentrations have been simulated under different weather conditions such as clear sky, moderately hazy sky, and hazy atmospheric conditions for given system input parameters (pulse energy, detectivity, bandwidth, DAQ resolution, etc.). We have considered chemicals such as Sarin, Thiodiglycol (TDG), acetone, and methane to be detected using the system. Analysis has been carried out for these chemicals present at different locations with varying concentrations. Our analysis reveals that the DIAL system with a laser transmitter of 5 mJ energy and 203 mm receiver telescope is capable of detecting a few ppm concentrations of toxic chemicals present anywhere between the ranges from a few tens of meters to 2 km with topographic target present. The sensitivity of the system in terms of minimum detectable concentrations for the considered chemicals is also estimated for different atmospheric conditions. It is seen that the minimum detectable concentration of TDG is 3.2 ppm in clear weather conditions which increases to 9.2 ppm under a hazy atmosphere. A similar analysis has been carried out for other toxic chemicals and has been discussed in the paper.

Differential absorption lidar data acquisition and control system for remote detection of trace chemicals including methane and thiodiglycol.

We report the design and development of a data acquisition and control system for high-speed acquisition of weak, backscattered differential signals and synchronized sequential operation of all subsystems of a tunable, mid-infrared Differential Absorption Lidar (DIAL) system. The presence of a low-level concentration of chemical species results in weak differential return signals. The differential signal also varies dynamically with respect to background atmospheric conditions. The challenge is to measure this low level of differential signal with high resolution and also control the sequence of operation of subsystems such as lasers and scanners for real time testing and evaluation in open field conditions. The concentration spread of the chemical species varies rapidly with distance. In order to capture this spatial variation, the lidar signal should be sampled and digitized at a high sampling rate. A customized Peripheral Component Interconnect based data acquisition of a 12-bit resolution, 30 Mega samples per second sampling rate, and an industrial personal computer-based control system has been realized. Detection algorithms and the firing sequence of the laser have been developed indigenously and implemented in the LabVIEW platform. The developed graphical user interface has various modes of operation as per user requirement and is capable of executing automatic operations for the developed DIAL system in order to detect and identify the chemicals. The performance has been evaluated by detecting the chemical species thiodiglycol at 800m using 3190 and 3300nm (online and offline wavelengths) with a differential cross-section of 2.5 × 10-23m2. Similarly, methane has been detected and quantified with concentration of 2.2 ppm up to 300m using 3316 and 3326nm (online and offline wavelengths, respectively).

Differential Absorption Lidar Transmitter Based on an Indium Phosphide Photonic Integrated Circuit for Carbon Dioxide Sensing

We demonstrate carbon dioxide sensing using a random-modulation continuous-wave differential absorption lidar transmitter based on an indium phosphide photonic integrated circuit. We have designed and characterized the photonic circuit that has been fabricated through an open access generic integration platform based on standard building blocks. It consists of three four-section distributed Bragg reflector lasers, two fast photodiodes, two electro-absorption modulators and five semiconductor optical amplifiers integrated together with several couplers and waveguides. The circuit contains two interrelated subsystems, one for performing the differential absorption lidar measurement, and the other for stabilizing the emission wavelength of the different lasers. We characterize the individual integrated devices, especially lasers, photodiodes and modulators. The carbon dioxide sensing is done by measuring a gas cell in a fiber setup emulating a lidar configuration. Our promising results pave the way to miniaturized differential absorption lidar systems, while highlighting some of the main challenges to overcome.

Uncertainty Assessment of Differential Absorption Lidar Measurements of Industrial Emissions Concentrations

Differential absorption lidar (DIAL) has been shown to be a very effective technique for the location and quantification of emissions of pollutants and greenhouse gases at industrial facilities. Several field trials have demonstrated the DIAL system performances and contributed to the development of the DIAL methodology, which is the basis of the protocols described in the European Standard EN 17628. While numerous papers have focused on different aspects of DIAL uncertainties, a rigorous propagation of the uncertainties in the DIAL equation has not been found. In this study, all the uncertainty sources contributing to a DIAL concentration measurement are assessed and the impact they have on the calculation of the mass emission rate. We derive the equations for both a DIAL system path-concentration integral and concentration uncertainties. The results from a methane measurement are presented, showing that for a signal to noise ratio on the backscattered lidar signals of 500, the path-concentration integral standard uncertainty is 2.3 ppb km and the concentration standard uncertainty is 92 ppb over a sampling spacing of 45 m. An equation is also presented enabling calculation of the contribution of the concentration uncertainty to the mass emission rate uncertainty.

Analog and Photon Signal Splicing for CO2-DIAL Based on Piecewise Nonlinear Algorithm

In the CO2 differential absorption lidar (DIAL) system, signals are simultaneously collected through analog detection (AD) and photon counting (PC). These two kinds of signals have their own characteristics. Therefore, a combination of AD and PC signals is of great importance to improve the detection capability (detection range and accuracy) of CO2-DIAL. The traditional signal splicing algorithm cannot meet the accuracy requirements of CO2 inversion due to unreasonable data fitting. In this paper, a piecewise least square splicing algorithm is developed to make signal splicing more flexible and efficient. First, the lidar signal is segmented, and according to the characteristics of each signal, the best fitting parameters are obtained by using the least square fitting with different steps. Then, all the segmented and fitted signals are integrated to realize the effective splicing of the near-field AD signal and the far-field PC signal. A weight gradient strategy is also adopted in signal splicing, and the weights of the AD and PC signals in the spliced signal change with the height. The splicing effect of the improved algorithm is evaluated by the measured signal, which are obtained in Wuhan, China, and the splice of the AD and PC signals in the range of 800–1500 m are completed. Compared with the traditional method, the evaluation parameter R2 and the residual sum of squares of the spliced signal are greatly improved. The linear relationship between the AD and PC signals is improved, and the fitting R2 of differential absorption optical depth reaches 0.909, indicating that the improved signal splicing algorithm can well splice the near-field AD signal and the far-field PC signal.

Electro-optic Q-switched Cr:LiSAF laser

We demonstrate electro-optic Q-switched solid state laser with Cr:LiSAF active medium. A single 50 ns pulse with 14 mJ of output energy is demonstrated. Simultaneous generation of several peaks with a step of 1.4 nm within the spectrum envelope with a full width at half maximum of 10.3 nm is demonstrated. For an electro-optic Q-switched mode a Pockels cell is used. Demonstrated laser can be used in differential absorption lidar systems.

Evaluation of Arctic Water Vapor Profile Observations from a Differential Absorption Lidar

The continuous measuring of the vertical profile of water vapor in the boundary layer using a commercially available differential absorption lidar (DIAL) has only recently been made possible. Since September 2018, a new pre-production version of the Vaisala DIAL system has operated at the Iqaluit supersite (63.74°N, 68.51°W), commissioned by Environment and Climate Change Canada (ECCC) as part of the Canadian Arctic Weather Science project. This study presents its evaluation during the extremely dry conditions experienced in the Arctic by comparing it with coincident radiosonde and Raman lidar observations. Comparisons over a one year period were strongly correlated (r > 0.8 at almost all heights) and exhibited an average bias of +0.13 ± 0.01 g/kg (DIAL-sonde) and +0.18 ± 0.02 g/kg (DIAL-Raman). Larger differences exhibiting distinct artifacts were found between 250 and 400 m above ground level (AGL). The DIAL’s observations were also used to conduct a verification case study of operational numerical weather prediction (NWP) models during the World Meteorological Organization’s Year of Polar Prediction. Comparisons to ECCC’s global environmental multiscale model (GEM-2.5 km and GEM-10 km) indicate good agreement with an average bias < 0.16 g/kg for the higher-resolution (GEM-2.5 km) models. All models performed significantly better during the winter than the summer, likely due to the winter’s lower water vapor concentrations and decreased variability. This study provides evidence in favor of using high temporal resolution lidar water vapor profile measurements to complement radiosonde observations and for NWP model verification and process studies.

Toronto Water Vapor Lidar Inter-Comparison Campaign

This study presents comparisons between vertical water vapor profile measurements from a Raman lidar and a new pre-production broadband differential absorption lidar (DIAL). Vaisala’s novel DIAL system operates autonomously outdoors and measures the vertical profile of water vapor within the boundary layer 24 h a day during all weather conditions. Eight nights of measurements in June and July 2018 were used for the Toronto water vapor lidar inter-comparison field campaign. Both lidars provided reliable atmospheric backscatter and water vapor profile measurements. Comparisons were performed during night-time observations only, when the York Raman lidar could measure the water vapor profile. The purpose was to validate the water vapor profile measurements retrieved by the new DIAL system. The results indicate good agreement between the two lidars, with a mean difference (DIAL–Raman) of 0.17 ± 0.14 g/kg. There were two main causes for differences in their measurements: horizontal displacement between the two lidar sites (3.2 km) and vertical gradients in the water vapor profile. A case study analyzed during the campaign demonstrates the ability for both lidars to measure sudden changes and large gradients in the water vapor’s vertical structure due to a passing frontal system. These results provide an initial validation of the DIAL’s measurements and its ability to be implemented as part of an operational program.

Mercury as a Geophysical Tracer Gas - Emissions from the Emperor Qin Tomb in Xi\xb4an Studied by Laser Radar

Mercury is, because of its high vapor pressure and its prevalence in the atmosphere as atoms, an interesting geophysical tracer gas, also with potential archaeological applications. According to historical records dating back 2200 years, the mausoleum chamber of the “Terracotta Army Emperor” Qin in Xi´an, China, contains large amounts of liquid mercury, considered as an elixir of life at the time. We here report on measurements of the atmospheric contents of atomic mercury above the tomb mound performed with a mobile differential absorption lidar (light detection and ranging) system. Our measurements, which were performed from three different locations around the mound, indeed indicate elevated atmospheric mercury levels, with localizations, which correlate with previous in situ soil sampling results. Concentrations up to 27 ng/m3 were observed, significantly higher than the typical general pollutant level in the area which was found to be around 5–10 ng/m3. An out-flux of about 5×10−8 kg/s was estimated. Highly volatile mercury may be escaping through cracks, which developed in the structure over time, and our investigation supports ancient chronicle records on the tomb, which is believed never to have been opened/looted. Our findings also have bearings on the proposed use of mercury as a tracer gas for valuable ores and geothermal resource exploration, and also bring problematics around reliable nuclear waste long-term underground storage to mind.

Compact and movable ozone differential absorption lidar system based on an all-solid-state, tuning-free laser source.

The differential absorption lidar (DIAL) has been proposed as an effective method for detecting polluted gases in the atmosphere. In this paper, we present a compact and movable ozone differential absorption (O3-DIAL) based on an all-solid-state and tuning-free laser source. For the first time, solid-state stimulated Raman scattering technology is used in the emitting source of the lidar for wavelength conversion. A high repetition frequency Innoslab laser is used for pumping SrWO4 crystals to get yellow lasers which can achieve up to 70% light-to-light conversion efficiency. Our results demonstrate that using the SrWO4 crystal as the Raman frequency-shifting media of the lidar laser source for obtaining the vertical profiles of tropospheric ozone in the Planetary Boundary Layer (PBL) is a suitable choice. As a compact movable lidar system, the results demonstrate the reliability and stability.

Remote Analysis of Methane Concentration in the Atmosphere with an IR Lidar System in the 3300–3430 nm Spectral Range

A differential absorption lidar (DIAL) system based on optical parametric oscillators (OPO) with nonlinear KTA and KTP crystals is designed. The crystals allow laser radiation tuning in the IR wavelength region. A series of experiments on remote monitoring of methane along a horizontal surface sounding path in the 3300–3430 nm spectral range was carried out. Based on the experimental results, the CH4 concentrations are retrieved along a surface 800-m path in the spectral range under study with a spatial resolution of 100 m.

Remote Sensing of Atmospheric Methane with IR OPO Lidar System

A differential absorption lidar (DIAL) system designed on the basis of optical parametric oscillators (OPO) with nonlinear KTiOAsO4 (KTA) and KTiOPO4 (KTP) crystals is described. The crystals allow laser radiation tuning in the infrared region (IR) wavelength region. The measurements in the 3.30–3.50 μm spectral range, which includes a strong absorption band of methane, are carried out. Lidar backscattered signals in the spectral band 3.30–3.50 μm has been measured and analyzed along the horizontal path in the atmosphere. Based on the experimental results, CH4 concentrations ~2.085 ppm along a 800 m surface path are retrieved in the spectral range under study with a spatial resolution of 100 m.

Towards Developing a Micropulse Differential Absorption Lidar to Measure Atmospheric Temperature

It has generally been assumed that differential absorption lidar (DIAL) systems are incapable of measuring atmospheric temperature with useful accuracy. This assumption is a direct result of errors that arise in standard DIAL retrievals due to differential Rayleigh-Doppler broadening from aerosols and molecules. We present here, a combined high spectral resolution (HSRL) and DIAL system that addresses this identified source of uncertainty by measuring quantitative aerosol parameters as well as oxygen absorption parameters. This system, in combination with a perturbative retrieval method, accounts for the Rayleigh-Doppler broadening effects on the oxygen absorption. We describe this combined DIAL/HSRL system and retrieval to evaluate the first retrieval parameters exploring the likelihood that it is possible to measure atmospheric temperature using a DIAL system.

Measurement of CO2 rectifier effect during summer and winter using ground-based differential absorption LiDAR

The rectifier effect refers to the exchange of CO2 near the boundary layer of the atmosphere. This effect is an important part of the carbon cycle because it affects the vertical distribution of CO2 and indirectly affects global CO2 distribution. However, the intensity of this effect is difficult to measure directly. A differential absorption LiDAR (DIAL) system can observe the rectifier effect because this device can accurately measure planetary boundary layer height (PBLH) and CO2 profile concentration (Quan et al., 2014; Guo et al., 2016). Accordingly, we conducted experiments in Huainan, China using a DIAL system for CO2 detection developed by our group. We selected two cases, namely, summer and winter, for the analysis. Firstly, we calculated CO2 profile, average CO2 concentration and PBLH. Secondly, we analysed the rectifier effect. Results showed a negative correlation between PBLH and the rectifier effect. Simultaneously, we observed a special phenomenon in which CO2 concentration is lower near the ground than in high altitudes. This phenomenon may be explained by the activities of plants.

Open field testing of mid IR DIAL for remote detection of thiodiglycol vapor plumes in the topographic target configuration

A tunable mid-infrared differential absorption lidar (DIAL) mounted on a mobile platform has been built for long range remote detection of thiodiglycol (TDG) vapor plumes. The IR spectra of TDG have been analyzed from available spectroscopy database and from the laboratory measurement. Based on the analysis, laser wavelengths at 3.190 and 3.300 μm have been selected as λon (strong absorption) and λoff (weak absorption) in the measurements. The influence of interfering molecules at these wavelengths has been also studied. The lidar return signal and sensitivity of the system have been numerically simulated based on the system parameters. The experimental DIAL system has been built using an optical parametric oscillator (OPO) based tunable laser operating in the wavelength band 3.0–3.45 μm as a transmitter and Cassegrain telescope having 200 mm diameter mirror integrated with thermoelectric cooled mercury cadmium telluride (MCT) detector as a receiver. The trans-receiver system is mounted on a pan-tilt that enables the aerial scanning operations. The lidar subsystems have been assembled and integrated on a mobile platform for conducting field experiments. An initial field testing of the system has been carried out at a remote site using the enclosed chamber containing the TDG vapor. The chamber was erected at a distance of about 950 m acted as a topographic target that reflected the laser radiation. The differential absorption signal in terms of signal ratio has been monitored continuously in real time during the trial period. The varying column content ranging from 10 to 200 ppm × m under dynamic atmospheric conditions has been calculated and discussed in this paper.

Development and application of an airborne differential absorption lidar for the simultaneous measurement of ozone and water vapor profiles in the tropopause region.

A new, combined, lidar system has been developed that is able to simultaneously measure profiles of ozone and water vapor onboard aircraft. The concurrent measurement of these complementary trace species in the upper troposphere and lower stratosphere allows inferring exchange processes in the tropopause region. Whereas an advanced H2O differential absorption lidar at 935 nm has successfully been developed and extensively tested at DLR in the past, we describe here an amendment of this lidar by the addition of an ultraviolet (UV) channel to measure ozone. The transmitter of the ozone differential absorption lidar (DIAL) is based on a near-IR optical parametric oscillator that is frequency-converted into the UV spectral range by intracavity sum frequency mixing. Hereby, a continuous UV tuning range of ∼297-317 nm has been achieved. The average output power in this range is higher than 1 W corresponding to more than 10 mJ per pulse at a repetition rate of 100 Hz. The ozone DIAL system has been carefully characterized both on the ground and in flight. The first simultaneously measured two-dimensional cross-sections of ozone and water vapor in the upper troposphere and lower stratosphere have been recorded during the Wave-driven Isentropic Exchange (WISE) field campaign in 2017 demonstrating the high potential of this system for studying exchange processes in this region of the atmosphere.