355-nm high spectral resolution airborne lidar LNG: system description and first results

Optimization of Mach-Zehnder interferometer of high-spectral-resolution lidar for large-scale wind measurement

The optimization design of a quadri-channel Mach-Zehnder interferometer (QMZI) of the high-spectral-resolution lidar is presented for the large-scale wind measurement. The optimized QMZI can discriminate the Doppler frequency shift generated by atmospheric wind from aerosol Mie scattering echo signals and molecular Rayleigh scattering echo signals, and then the wind information can be retrieved. The optimal optical path differences (OPDs) of QMZI are determined by theoretical and simulation analysis. The wind measurement simulation experiments prove that the designed QMZI can measure the large-scale wind with an accuracy of meter level.

355 nm ultraviolet laser output based on a PPMgLN crystal

This paper reports the method of generating a 355 nm ultraviolet (UV) quasicontinuous pulse laser by using two periodically poled Mg-doped lithium niobate (PPMgLN) crystals in a single-pass cascade. In the first PPMgLN crystal with a length of 20 mm and a first-order-poled period of 6.97 µm, the second-harmonic light of a 532 nm laser with 780 mW is generated from the 1064 nm laser with an average power of 2 W; After that, in the second PPMgLN crystal with a length of 15 mm and a third-order-poled period of 5.30 µm, the 532 nm laser generated was combined with the 1064 nm laser remaining through the first PPMgLN crystal to obtain a 355 nm UV laser with a maximum output average power of 20 mW, a repetition rate of 40 kHz, a pulse width of 49 ns, and a peak power of 10 W. Compared with existing reports, we have higher peak power and single pulse energy, which is an important application of the PPMgLN crystal. This paper will provide an important case for the realization of a 355 nm UV quasicontinuous or a continuous laser.

Aeronautics Application of Direct-Detection Doppler Wind Lidar: An Adapted Design Based on a Fringe-Imaging Michelson Interferometer as Spectral Analyzer

We report on the development of a novel direct-detection Doppler wind lidar (DD-DWL) within the strong requirements of an aeronautic feed-forward control application for gust load alleviation (GLA). This DD-DWL is based on fringe imaging of the Doppler-shifted backscatter of ultraviolet laser pulses in a field-widened Michelson interferometer (FW-FIMI) using a fast linear photodetector. The double approach of detailed simulation and demonstrator development is validated by field measurements with reference wind sensing instrumentation. These experiments allow us to establish wind determination precision at a high repeat rate, short range resolution and close distance of approximately 0.5 m/s, which is in accordance with the dedicated simulations. These findings lead us to the conclusion that this FW-FIMI-based Doppler wind lidar is a pertinent development meeting the special requirements of this aeronautics application. Second, the developed simulators are well suited (given their validation) to be used in the overall and full analysis as well as the optimization of the lidar-based GLA control scheme.

Demonstration of aerosol profile measurement with a dual-wavelength high-spectral-resolution lidar using a scanning interferometer

Simple dual-wavelength high-spectral-resolution lidar at 355 and 532 nm with a scanning interferometer was developed for continuous observations of aerosol profiles. Scanning the interferometer periodically over a range of one fringe at 532 nm (1.5 fringes at 355 nm) enabled recording of range-resolved interference signals at these two wavelengths. Reference signals taken from the transmitted laser were used to correct the interference phase shift due to laser frequency variation for every scan. Profiles of aerosol backscatter and extinction coefficients were retrieved from range-resolved interference data. One month of continuous measurements demonstrated the robustness of the system.

Development of a 355-nm high-spectral-resolution lidar using a scanning Michelson interferometer for aerosol profile measurement

A simple 355-nm high-spectral-resolution lidar (HSRL) is developed for continuous observation of aerosol profiles. A scanning Michelson interferometer is used to separate the Rayleigh and Mie scattering components. The interferometer is periodically scanned in the range of one fringe. Interference contrast, which contains aerosol backscatter information, is estimated at each height through fitting analysis of the scan data. The interference contrast and fringe position are calibrated with the reference signals taken from the transmitted laser. Furthermore, the 1-day continuous measurement of aerosol backscatter and extinction coefficients is demonstrated. Comparison with a nighttime Raman lidar indicates a good performance of the scanning method.

Development and Test of a Fringe-Imaging Direct-Detection Doppler Wind Lidar for Aeronautics

DLR currently investigates the use of Doppler wind lidar as sensor within feedforward gust alleviation control loops on fast-flying fixed-wing aircraft. Such a scheme imposes strong requirements on the lidar system such as sub-m/s precision, high rate, high spatial resolution, close measurement ranges and sensitivity to mixed and pure molecular backscatter.

We report on the development of a novel direct-detection Doppler wind lidar (DD-DWL) within these requirements. This DD-DWL is based on fringe-imaging of the Doppler-shifted backscatter of UV laser pulses in a field-widened Michelson interferometer using a fast linear photodetector.

A prototype for airborne operation has been ground-tested in early 2018 against a commercial coherent DWL, demonstrating its ability of measuring close-range wind speeds with a precision of 0.5 m/s, independent of the actual wind speed.

Operation of the Airborne 355-nm High Spectral Resolution and Doppler Lidar LNG

High spectral resolution lidar (HSRL) are known to offer capabilities of separating attenuated aerosol and molecular backscattering so that particle extinction and backscattering can be separately retrieved. UV operation provides high energy in eye-safety conditions. Further to that, it could be important for most meteorological or environmental studies to get wind measurements at the same time. LNG is now the only HSR Doppler Lidar (HSRDL) system capable of this. Results obtained during ground-based and airborne measurements show that the backscatter and extinction coefficients at 355 nm can be measured with a relative precision better than 10% (adjusting altitude and time resolution from 60 m to 240 m and 30s to 2mn, respectively) in aerosol layers of 0.5 10−6 m−1 sr−1 backscatter coefficient from ground and aircraft. The same relative precision is obtained in cirrus clouds of a 10−5 m−1 sr−1 backscatter coefficient. The capacity of the system to perform wind velocity measurements has also been demonstrated with precisions in the range of 1 to 2 ms−1 in same conditions. We present the main characteristics and illustrate observational capabilities from ground-based and airborne measurements.

Calibration of a high spectral resolution lidar using a Michelson interferometer, with data examples from ORACLES

The NASA Langley airborne second-generation High Spectral Resolution Lidar (HSRL-2) uses a density-tuned field-widened Michelson interferometer to implement the HSRL technique at 355 nm. The Michelson interferometer optically separates the received backscattered light between two channels, one of which is dominated by molecular backscattering, while the other contains most of the light backscattered by particles. This interferometer achieves high and stable contrast ratio, defined as the ratio of particulate backscatter signal received by the two channels. We show that a high and stable contrast ratio is critical for precise and accurate backscatter and extinction retrievals. Here, we present retrieval equations that take into account the incomplete separation of particulate and molecular backscatter in the measurement channels. We also show how the accuracy of the contrast ratio assessment propagates to error in the optical properties. For both backscattering and extinction, larger errors are produced by underestimates of the contrast ratio (compared to overestimates), more extreme aerosol loading, and-most critically-smaller true contrast ratios. We show example results from HSRL-2 aboard the NASA ER-2 aircraft from the 2016 ORACLES field campaign in the southeast Atlantic, off the coast of Africa, during the biomass burning season. We include a case study where smoke aerosol in two adjacent altitude layers showed opposite differences in extinction- and backscatter-related Ångström exponents and a reversal of the lidar ratio spectral dependence, signatures which are shown to be consistent with a relatively modest difference in smoke particle size.

Twin scanning lidars for accurate measurement of lower tropospheric aerosols by numerical approximation

In order to improve accuracy of aerosol measurements, a novel method using twin scanning lidars is presented; this method is able to overcome the incomplete overlap range of vertical lidar as well as provide 2D spatial distributions. The scanning lidar setups in the opposite directions are employed as remote sensing tools. Aerosol measurements are performed with cross scanning from the ground to the height of interest. Aerosol optical properties are retrieved using numerical approximation, in which differences between the measured values and the constructed values of the logarithmic range-square-corrected lidar data in the cross-scanning region are minimized. In the data retrieval, we utilize a matrix formulation, in which a Cartesian 2D range-height-indicator diagram is constructed. To verify this method, scanning measurements by ultraviolet Mie scanning lidar performed at different time intervals were taken as the cross-scanning measurements from the twin scanning lidars. With the retrieved spatial distributions of aerosol optical properties, such as aerosol backscatter, aerosol extinction, and lidar ratio, the regional aerosol studies showed that aerosol loading was relatively small and in the presence of multiple layers, which may be influenced by airflow from long-range transportation and cause a large impact on the local environment. To conclude, the presented method using twin scanning lidars is feasible for aerosol measurement in the application of horizontally atmospheric inhomogeneity.

Spaceborne Lidar in the Study of Marine Systems

Satellite passive ocean color instruments have provided an unbroken ∼20-year record of global ocean plankton properties, but this measurement approach has inherent limitations in terms of spatial-temporal sampling and ability to resolve vertical structure within the water column. These limitations can be addressed by coupling ocean color data with measurements from a spaceborne lidar. Airborne lidars have been used for decades to study ocean subsurface properties, but recent breakthroughs have now demonstrated that plankton properties can be measured with a satellite lidar. The satellite lidar era in oceanography has arrived. Here, we present a review of the lidar technique, its applications in marine systems, a perspective on what can be accomplished in the near future with an ocean- and atmosphere-optimized satellite lidar, and a vision for a multiplatform virtual constellation of observational assets that would enable a three-dimensional reconstruction of global ocean ecosystems.

High spectral resolution lidar based on quad mach zehnder interferometer for aerosols and wind measurements on board space missions

We present the measurement principle and the optical design of a Quad Mach Zehnder (QMZ) interferometer as HSRL technique, allowing simultaneous measurements of particle backscattering and wind velocity. Key features of this concept is to operate with a multimodal laser and do not require any frequency stabilization. These features are relevant especially for space applications for which high technical readiness level is required.

EUREC4A: A Field Campaign to Elucidate the Couplings Between Clouds, Convection and Circulation

Trade-wind cumuli constitute the cloud type with the highest frequency of occurrence on Earth, and it has been shown that their sensitivity to changing environmental conditions will critically influence the magnitude and pace of future global warming. Research over the last decade has pointed out the importance of the interplay between clouds, convection and circulation in controling this sensitivity. Numerical models represent this interplay in diverse ways, which translates into different responses of trade-cumuli to climate perturbations. Climate models predict that the area covered by shallow cumuli at cloud base is very sensitive to changes in environmental conditions, while process models suggest the opposite. To understand and resolve this contradiction, we propose to organize a field campaign aimed at quantifying the physical properties of trade-cumuli (e.g., cloud fraction and water content) as a function of the large-scale environment. Beyond a better understanding of clouds-circulation coupling processes, the campaign will provide a reference data set that may be used as a benchmark for advancing the modelling and the satellite remote sensing of clouds and circulation. It will also be an opportunity for complementary investigations such as evaluating model convective parameterizations or studying the role of ocean mesoscale eddies in air-sea interactions and convective organization.

Measurement method of high spectral resolution lidar with a multimode laser and a scanning Mach-Zehnder interferometer

A simple high spectral resolution lidar technique using a multi-longitudinal mode laser is proposed for measuring aerosol extinction and backscattering coefficients. A scanning interferometer having the same free spectral range as the mode spacing of the laser is used to separate Rayleigh from Mie scattering. Scanning the interferometer in the span of one fringe, the lidar signals at the minimum and maximum Mie-scattering transmission are measured. The Rayleigh scattering signal is analyzed from these signals, and the aerosol extinction coefficient is derived. The interferometer transmittance for Mie scattering is calibrated with the reference signals taken with a portion of the transmitted laser beam.

Design of iodine absorption cell for high-spectral-resolution lidar

Iodine absorption cells are extensively employed by high-spectral-resolution Lidars (HSRLs) for aerosol optical properties and atmosphere state parameters profiling. To the best of our knowledge, the optimal design of the parameters of iodine cells has not been talked about systematically. In this paper, a heuristic method based on multi-objective concept is proposed for the design of iodine cells employed in HSRLs for aerosol profiling, and the method can be also applied to different types of HSRLs. The bi-objective model is established based on the retrieval error analysis of HSRL and then the Pareto optimal solutions are obtained through the Non-dominated Sorting Genetic Algorithm II (NSGA-II). The performance of different absorption lines are compared according to the Pareto solution sets, and the stability of transmittance characteristics of different absorption lines are discussed through sensitivity analysis. The results are expected to provide guidance for the design of HSRLs based on iodine absorption filters.

Design of the interferometric spectral discrimination filters for a three-wavelength high-spectral-resolution lidar

We address design of the interferometric spectral discrimination (ISD) filters for a specific three-wavelength high-spectral-resolution lidar (HSRL) in this paper. Taking into account the strong dependence of the transmittance of the ISD filters on the incident angle of light ray, the optical path of the receiving channel with an ISD filter in HSRL is analyzed. We derive the lidar equation with the angular distribution of backscatter signal, through which Monte Carlo (MC) simulations are then carried out to obtain the optimal parameters of the ISD filters for the HSRL at 1064 nm, 532 nm and 355 nm, respectively. Comparing the retrieval errors of the MC simulations based on different ISD filters, the configuration and parameters of the best ISD filter at each wavelength are determined. This paper can be employed as a theoretical guidance during the design of a three-wavelength HSRL with ISD filters.

Airborne compact rotational Raman lidar for temperature measurement

We developed an airborne compact rotational Raman lidar (CRL) for use on the University of Wyoming King Air (UWKA) aircraft to obtain two-dimensional (2D) temperature disman tributions. It obtained fine-scale 2D temperature distributions within 3 km below the aircraft for the first time during the PECAN (Plains Elevated Convection At Night) campaign in 2015. The CRL provided nighttime temperature measurements with a random error of <0.5 K within 800 m below aircraft at 45 m vertical and 1000 m horizontal resolution. The temperatures obtained by the CRL and a radiosonde agreed. Along with water vapor and aerosol measurements, the CRL provides critical parameters on the state of the lower atmosphere for a wide range of atmospheric research.

Design of a monolithic Michelson interferometer for fringe imaging in a near-field, UV, direct-detection Doppler wind lidar

The low-biased, fast, airborne, short-range, and range-resolved determination of atmospheric wind speeds plays a key role in wake vortex and turbulence mitigation strategies and would improve flight safety, comfort, and economy. In this work, a concept for an airborne, UV, direct-detection Doppler wind lidar receiver is presented. A monolithic, tilted, field-widened, fringe-imaging Michelson interferometer (FWFIMI) combines the advantages of low angular sensitivity, high thermo-mechanical stability, independence of the specific atmospheric conditions, and potential for fast data evaluation. Design and integration of the FWFIMI into a lidar receiver concept are described. Simulations help to evaluate the receiver design and prospect sufficient performance under different atmospheric conditions.

Field-widened Michelson interferometer for spectral discrimination in high-spectral-resolution lidar: practical development

A field-widened Michelson interferometer (FWMI), which is intended as the spectroscopic discriminator in ground-based high-spectral-resolution lidar (HSRL) for atmospheric aerosol detection, is described in this paper. The structure, specifications and design of the developed prototype FWMI are introduced, and an experimental approach is proposed to optimize the FWMI assembly and evaluate its comprehensive characteristic simultaneously. Experimental results show that, after optimization process, the peak-to-valley (PV) value and root-mean-square (RMS) value of measured OPD variation for the FWMI are 0.04λ and 0.008λ respectively among the half divergent angle range of 1.5 degree. Through an active locking technique, the frequency of the FWMI can be locked to the laser transmitter with accuracy of 27 MHz for more than one hour. The practical spectral discrimination ratio (SDR) for the developed FWMI is evaluated to be larger than 86 if the divergent angle of incident beam is smaller than 0.5 degree. All these results demonstrate the great potential of the developed FWMI as the spectroscopic discriminator for HSRLs, as well as the feasibility of the proposed design and optimization process. This paper is expected to provide a good entrance for the lidar community in future HSRL developments using the FWMI technique.