Airborne forward-pointing UV Rayleigh lidar for remote clear air turbulence detection: system design and performance

Atmospheric turbulence strength distribution along a propagation path probed by longitudinally structured optical beams

Atmospheric turbulence can cause critical problems in many applications. To effectively avoid or mitigate turbulence, knowledge of turbulence strength at various distances could be of immense value. Due to light-matter interaction, optical beams can probe longitudinal turbulence changes. Unfortunately, previous approaches tended to be limited to relatively short distances or large transceivers. Here, we explore turbulence probing utilizing multiple sequentially transmitted longitudinally structured beams. Each beam is composed of Bessel-Gaussian ([Formula: see text]) modes with different [Formula: see text] values such that a distance-varying beam width is produced, which results in a distance- and turbulence-dependent modal coupling to [Formula: see text] orders. Our simulation shows that this approach has relatively uniform and low errors (<0.3 dB) over a 10-km path with up to 30-dB turbulence-structure-constant variation. We experimentally demonstrate this approach for two emulated turbulence regions (~15-dB variation) with <0.8-dB errors. Compared to previous techniques, our approach can potentially probe longer distances or require smaller transceivers.

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.

Active alignment of receiving beam for coaxial optics in wind sensing coherent Doppler lidar using feedback control based on the processing of heterodyne-detected signal

We have developed an active alignment of receiving beam (AARB) function for coaxial optics in wind sensing coherent Doppler lidar using feedback control based on the heterodyne-detected signal processing of backscattered light from the aerosols. The proposed method needs only the simple alignment components and contributes to the robustness for the coherent lidars with the high-power laser transmitter under the risky condition of misalignment, for example, in the airborne application. The concept, design, and evaluation results of the alignment precision are shown. The effect of the AARB is demonstrated for both cases of the hard target and soft target (i.e., wind sensing). To the best of our knowledge, this is the first demonstration of the AARB concept for the wind sensing coherent lidar.

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.

Modeling method of a ladar scene projector based on physically based rendering technology

The ladar scene projector is a key device in the hardware-in-the-loop (HWIL) simulation system. Ladar scene modeling is a fundamental work of developing a ladar scene projector. A modeling method based on physically based rendering technology and OpenGL is proposed in this paper. This modeling method can quickly generate delay, amplitude, and pulse width data for all return signals in a large-array-scale ladar scene. A 100×100-array-sized ladar scene model with a distance range of 0-3 km is simulated. The average data generation time is only 5.31 ms. Distance resolution is 1.5 m, and the peak-valley error is less than 0.15 m. This method achieves efficient modeling and fast hardware update rates, which greatly improves the real-time performance of the ladar scene projector. It has strong practicality and can be directly applied in the HWIL simulation system.