Coherent Lidar Research Articles

Coherent Doppler wind lidar signal denoising adopting variational mode decomposition based on honey badger algorithm.

Coherent Doppler wind lidar (CDWL) is used to measure wind velocity distribution by using laser pulses. However, the echo signal is easily affected by atmospheric turbulence, which could decrease the effective detection range of CDWL. In this paper, a variation modal decomposition based on honey badger algorithm (VMD-HBA) is proposed and demonstrated. Compared with conventional VMD-based methods, the proposed method utilizes a newly developed HBA to obtain the optimal VMD parameters by iterating the spectrum fitness function. In addition, the Correlation Euclidean distance is applied to identify the relevant mode and used to reconstruct the signal. The simulation results show that the denoising performance of VMD-HBA is superior to other available denoising methods. Experimentally, this combined method was successfully realized to process the actual lidar echo signal. Under harsh detection conditions, the effective detection range of the homemade CDWL system is extended from 13.41 km to 20.61 km.

Spatial Resolution Enhancement of Coherent Doppler Lidar by Pseudo-Random Phase Coding

A high spatial resolution coherent Doppler wind lidar (CDWL) incorporating pseudo-random phase coding (PRPC) is proposed and demonstrated. Random phase coding of the transmitted lightwave spreads the atmospheric backscattered spectrum and the spatial resolution can be enhanced as only the signal with specific time delay could be despread during decoding. The narrow peak of the despread spectrum provides higher wind velocity retrieval accuracy compared with conventional pulsed CDWL with the same spatial resolution. Spectrum difference is applied to reduce the interference between the spread and despread spectra. The new method's performance is verified in a comparison experiment with a non-coding CDWL. Continuous radial wind profile measurement over 800 m is demonstrated with spatial/temporal resolution of 4.5 m/1 s, at laser peak power of 250 W.

Meter-scale and sub-second-resolution coherent Doppler wind LIDAR and hyperfine wind observation.

Hyperfine wind structure detection is important for aerodynamic and aviation safety. Pulse coherent Doppler wind LIDAR (PCDWL) is a widespread wind remote sensing method with tunable spatial and temporal resolutions. However, meter scale and sub-second resolution are still challenging for PCDWL. This is because of the constraints among short laser pulse duration, spectral broadening, detection accuracy, and real-time processing. In this Letter, to further improve the spatial and temporal resolution of PCDWL, we optimize the optical design of a nanosecond fiber laser and telescope and adopt a new, to the best of our knowledge, algorithm called the even-order derivative peak sharpening technique. During the experiment, all-fiber PCDWL with spatial and temporal resolutions of 3 m and 0.1 s, respectively, is demonstrated. Two-day continuous observation of the wakes of the Chinese high-speed train shows detailed hyperfine wind structures. This is similar to a computational fluid dynamics simulation.

Turbulence Detection in the Atmospheric Boundary Layer Using Coherent Doppler Wind Lidar and Microwave Radiometer

The refractive index structure constant (Cn2) is a key parameter used in describing the influence of turbulence on laser transmissions in the atmosphere. Three different methods for estimating Cn2 were analyzed in detail. A new method that uses a combination of these methods for continuous Cn2 profiling with both high temporal and spatial resolution is proposed and demonstrated. Under the assumption of the Kolmogorov “2/3 law”, the Cn2 profile can be calculated by using the wind field and turbulent kinetic energy dissipation rate (TKEDR) measured by coherent Doppler wind lidar (CDWL) and other meteorological parameters derived from a microwave radiometer (MWR). In a horizontal experiment, a comparison between the results from our new method and measurements made by a large aperture scintillometer (LAS) is conducted. The correlation coefficient, mean error, and standard deviation between them in a six-day observation are 0.8073, 8.18 × 10−16 m−2/3 and 1.27 × 10−15 m−2/3, respectively. In the vertical direction, the continuous profiling results of Cn2 and other turbulence parameters with high resolution in the atmospheric boundary layer (ABL) are retrieved. In addition, the limitation and uncertainty of this method under different circumstances were analyzed, which shows that the relative error of Cn2 estimation normally does not exceed 30% under the convective boundary layer (CBL).

Low-noise frequency-agile photonic integrated lasers for coherent ranging

Frequency modulated continuous wave laser ranging (FMCW LiDAR) enables distance mapping with simultaneous position and velocity information, is immune to stray light, can achieve long range, operate in the eye-safe region of 1550 nm and achieve high sensitivity. Despite its advantages, it is compounded by the simultaneous requirement of both narrow linewidth low noise lasers that can be precisely chirped. While integrated silicon-based lasers, compatible with wafer scale manufacturing in large volumes at low cost, have experienced major advances and are now employed on a commercial scale in data centers, and impressive progress has led to integrated lasers with (ultra) narrow sub-100 Hz-level intrinsic linewidth based on optical feedback from photonic circuits, these lasers presently lack fast nonthermal tuning, i.e. frequency agility as required for coherent ranging. Here, we demonstrate a hybrid photonic integrated laser that exhibits very narrow intrinsic linewidth of 25 Hz while offering linear, hysteresis-free, and mode-hop-free-tuning beyond 1 GHz with up to megahertz actuation bandwidth constituting 1.6 × 1015 Hz/s tuning speed. Our approach uses foundry-based technologies - ultralow-loss (1 dB/m) Si3N4 photonic microresonators, combined with aluminium nitride (AlN) or lead zirconium titanate (PZT) microelectromechanical systems (MEMS) based stress-optic actuation. Electrically driven low-phase-noise lasing is attained by self-injection locking of an Indium Phosphide (InP) laser chip and only limited by fundamental thermo-refractive noise at mid-range offsets. By utilizing difference-drive and apodization of the photonic chip to suppress mechanical vibrations of the chip, a flat actuation response up to 10 MHz is achieved. We leverage this capability to demonstrate a compact coherent LiDAR engine that can generate up to 800 kHz FMCW triangular optical chirp signals, requiring neither any active linearization nor predistortion compensation, and perform a 10 m optical ranging experiment, with a resolution of 12.5 cm. Our results constitute a photonic integrated laser system for scenarios where high compactness, fast frequency actuation, and high spectral purity are required.

Real-Time Synchronous 3-D Detection of Air Pollution and Wind Using a Solo Coherent Doppler Wind Lidar

The monitoring and tracking of urban air pollution is a challenging environmental issue. The approach of synchronous 3-D detection of wind and pollution using a solo coherent Doppler wind lidar (CDWL) is developed and demonstrated. The 3-D distribution of pollutant is depicted by the backscatter coefficient based on signal intensity of CDWL. Then, a high-resolution wind field is derived to track the local air pollution source with its diffusion and to analyze transboundary air pollution episodes. The approach is experimentally implemented in a chemical industry park. Smoke plumes caused by point source pollutions are captured well using plan position indicator (PPI) scanning with low elevation. A typical source of pollution is located, combining the trajectory of the smoke plume and the horizontal wind vector. In addition, transboundary air pollution caused by the transport of dust storms is detected in a vertical profile scanning pattern, which is consistent with the results of national monitoring stations and backward trajectory models. Our present work provides a significant 3-D detection approach to air pollution monitoring with its sources, paths, and heights by using a solo-CDWL system.

Dual chirped microcomb based parallel ranging at megapixel-line rates

Laser-based ranging (LiDAR) - already ubiquitously used in industrial monitoring, atmospheric dynamics, or geodesy - is a key sensor technology. Coherent laser ranging, in contrast to time-of-flight approaches, is immune to ambient light, operates continuous-wave allowing higher average powers, and yields simultaneous velocity and distance information. State-of-the-art coherent single laser-detector architectures reach hundreds of kilopixel per second sampling rates, while emerging applications - autonomous driving, robotics, and augmented reality - mandate megapixel per second point sampling to support real-time video-rate imaging. Yet, such rates of coherent LiDAR have not been demonstrated. Recent advances in photonic chip-based microcombs provide a route to higher acquisition speeds via parallelization but require separation of individual channels at the detector side, increasing photonic integration complexity. Here we overcome the challenge and report a hardware-efficient swept dual-soliton microcomb technique that achieves coherent ranging and velocimetry at megapixel per second line scan measurement rates with up to 64 optical channels. Multiheterodyning two synchronously frequency-modulated microcombs yields distance and velocity information of all individual ranging channels on a single receiver alleviating the need for individual separation, detection, and digitization. The reported LiDAR implementation is compatible with photonic integration and demonstrates the significant advantages of acquisition speed afforded by the convergence of optical telecommunication and metrology technologies.

Monolithic coherent LABS lidar based on an integrated transceiver array.

We demonstrate a monolithic frequency-modulated continuous-wave (FMCW) lidar chip with an integrated transceiver array based on lens-assisted beam steering (LABS) technology. It enables beam emitting, steering, receiving, and coherent detecting on a single chip with simultaneous distance and velocity detection. An integrated transceiver is designed with a composite structure of a Bragg grating in the middle and a U-shaped photodetector (PD) surrounding it. For a proof-of-concept demonstration, a chip with 2 × 2 switchable transceiver array is fabricated. A monolithic coherent LABS lidar system with a scanning angle of 2.86° and a scanning speed of 5.3 µs is implemented for 5 m ranging and 0.45 m/s velocity detection.

Simultaneous Extraction of Planetary Boundary-Layer Height and Aerosol Optical Properties from Coherent Doppler Wind Lidar.

Planetary boundary-layer height is an important physical quantity for weather forecasting models and atmosphere environment assessment. A method of simultaneously extracting the surface-layer height (SLH), mixed-layer height (MLH), and aerosol optical properties, which include aerosol extinction coefficient (AEC) and aerosol optical depth (AOD), based on the signal-to-noise ratio (SNR) of the same coherent Doppler wind lidar (CDWL) is proposed. The method employs wavelet covariance transform to locate the SLH and MLH using the local maximum positions and an automatic algorithm of dilation operation. AEC and AOD are determined by the fitting curve using the SNR equation. Furthermore, the method demonstrates the influential mechanism of optical properties on the SLH and MLH. MLH is linearly correlated with AEC and AOD because of solar heating increasing. The results were verified by the data of an ocean island site in China.

Testing a Drop of Liquid Using Smartphone LiDAR

We present the first system to determine fluid properties using the LiDAR sensors present on modern smartphones. Traditional methods of measuring properties like viscosity require expensive laboratory equipment or a relatively large amount of fluid. In contrast, our smartphone-based method is accessible, contactless and works with just a single drop of liquid. Our design works by targeting a coherent LiDAR beam from the phone onto the liquid. Using the phone's camera, we capture the characteristic laser speckle pattern that is formed by the interference of light reflecting from light-scattering particles. By correlating the fluctuations in speckle intensity over time, we can characterize the Brownian motion within the liquid which is correlated with its viscosity. The speckle pattern can be captured on a range of phone cameras and does not require external magnifiers. Our results show that we can distinguish between different fat contents as well as identify adulterated milk. Further, algorithms can classify between ten different liquids using the smartphone LiDAR speckle patterns. Finally, we conducted a clinical study with whole blood samples across 30 patients showing that our approach can distinguish between coagulated and uncoagulated blood using a single drop of blood.

Microburst, Windshear, Gust Front, and Vortex Detection in Mega Airport Using a Single Coherent Doppler Wind Lidar

Accurate wind shear detection is crucial for aviation safety, especially in landing and departure. A new approach for windshear alerting is proposed and demonstrated. This approach monitors orthogonal wind components in multiple runways using single coherent Doppler wind lidar (CDWL). First, the two orthogonal components of the wind field are retrieved from radial speed by an updated variational method. Then, the heading wind and cross wind on different runways are calculated simultaneously, without the location restriction of the single lidar. Finally, a windshear alerting message is generated through quantitatively evaluating the distribution of shear ramps over the monitoring area. The new CDWL-based approach for windshear alerting is implemented at the Beijing Daxing International Airport. The retrieved horizontal wind from the lidar is consistent with that from anemometers. Thanks to its high spatial/temporal resolution, some meteorological phenomena of aviation hazards, including microburst, windshear, gust front, and vortex are well captured. Particularly, all 10 windshear cases reported by crews are successfully identified during the windshear verification experiment, demonstrating the effectiveness and robustness of the new approach.

Improving the signal-to-noise ratio of GM-APD coherent lidar system based on phase synchronization method

Geiger mode avalanche photodiode (GM-APD) coherent lidar is a new coherent detection system for obtaining multidimensional information. The system can increase the detection sensitivity to the level of a single photon, which is the main technical direction of long-distance laser detection. The system noise of GM-APD follows a Gaussian distribution. Hence, the time-domain cumulative signal-to-noise ratio is much higher than that of the spectrum cumulative, but the jitter of the multipulse echo phase limits the system’s ability to accumulate time-domain signals. This paper proposes a phase synchronization method to solve the abovementioned problem. First, the system achieves multipulse phase consistency by introducing a clock stable and synchronized radio frequency source as well as a pulse signal externally triggered by an acousto-optic modulator. A theoretical model of the method is established. Second, the phase distribution of the system is monitored using a double-balanced detector, and the period of the system’s phase distribution is obtained, thereby verifying the abovementioned theoretical analysis. To verify the feasibility of the method, the GM-APD coherent lidar experimental system is used to detect the all-fiber system and targets. This work addresses the problem of the system being unable to perform time-domain accumulation because of the randomness of the system’s multipulse phase. This framework reduces the required time for the subsequent algorithm, yielding improved real-time performance.

Coherent Doppler wind lidar with real-time wind processing and low signal-to-noise ratio reconstruction based on a convolutional neural network.

Multi-classification using a convolutional neural network (CNN) is proposed as a denoising method for coherent Doppler wind lidar (CDWL) data. The method is intended to enhance the usable range of a CDWL beyond the atmospheric boundary layer (ABL). The method is implemented and tested in an all-fiber pulsed CWDL system operating at 1550 nm wavelength with 20 kHz repetition rate, 300 ns pulse length and 180 µJ of laser energy. Real-time pre-processing using a field programmable gate array (FPGA) is implemented producing averaged lidar spectrograms. Real-world measurement data is labeled using conventional frequency estimators and mixed with simulated spectrograms for training of the CNN. First results of this methods show that the CNN can outperform conventional frequency estimations substantially in terms of maximum range and delivers reasonable output in very low signal-to-noise (SNR) situations while still delivering accurate results in the high-SNR regime. Comparing the CNN output with radiosonde data shows the feasibility of the proposed method.

Marine Mixed Layer Height Detection Using Ship-Borne Coherent Doppler Wind Lidar Based on Constant Turbulence Threshold

Marine mixed layer height (MLH) detection using a ship-borne coherent Doppler wind lidar (CDWL) based on a constant turbulent kinetic energy dissipation rate (TKEDR) threshold is realized and experimentally demonstrated. The MLH can be retrieved from the TKEDR estimated by the CDWL via setting an appropriate threshold. Here, the value of threshold is determined by a reference MLH retrieved from aerosol backscattered signal. The threshold of 10−4 m2 s−3 is found to be applicable in retrieving both inland and marine MLHs. In the experiments, to validate the reliability of the constant threshold, the MLH diurnal cycles at inland and marine sites are retrieved by using a ground-based CDWL. The MLH retrieval result at the marine site shows good agreement with radiosonde-derived MLH. After that, by using a ship-borne CDWL, the marine MLH along the ship’s route in South China Sea is successfully detected in real time.

Complex moving target recognition based on optical phase-locked loop in LIDAR

At present, laser vibration measurement technology is widely used based on the micro-Doppler effect. Micro-Doppler information contains a specific target feature, which provides an effective method for target recognition. Complex translation of the target is not conducive to detecting and collecting of the beat signal and increases the difficulty of extracting vibration information. In this paper, we propose to utilize an optical phase-locked loop (OPLL) operating in the carrier tracking mode at the receiver of coherent LIDAR, which can adaptively compensate the variable Doppler frequency shift (DFS) caused by the complex translational to obtain a detectable beat signal with the stable carrier frequency. On this basis, the micro-Doppler information can be accurately obtained from the variable DFS.A laboratory experiment for vibration targets with our OPLL method has been conducted, and it revealed that our approach worked well to extract the micro-Doppler information, thus the complex vibration contains two vibration frequencies of 100 Hz and 300 Hz, with amplitudes of 48.1 μm and 44.3 μm, respectively. This micro-motion information extraction method has universal applicability in recognizing the complex moving target, such as space debris recognition.

Inter-comparison of wind measurements in the atmospheric boundary layer and the lower troposphere with Aeolus and a ground-based coherent Doppler lidar network over China

Abstract. After the successful launch of Aeolus, which is the first spaceborne wind lidar developed by the European Space Agency (ESA), on 22 August 2018, we deployed several ground-based coherent Doppler wind lidars (CDLs) to verify the wind observations from Aeolus. By the simultaneous wind measurements with CDLs at 17 stations over China, the Rayleigh-clear and Mie-cloudy horizontal-line-of-sight (HLOS) wind velocities from Aeolus in the atmospheric boundary layer and the lower troposphere are compared with those from CDLs. To ensure the quality of the measurement data from CDLs and Aeolus, strict quality controls are applied in this study. Overall, 52 simultaneous Mie-cloudy comparison pairs and 387 Rayleigh-clear comparison pairs from this campaign are acquired. All of the Aeolus-produced Level 2B (L2B) Mie-cloudy HLOS wind and Rayleigh-clear HLOS wind and CDL-produced HLOS wind are compared individually. For the inter-comparison result of Mie-cloudy HLOS wind and CDL-produced HLOS wind, the correlation coefficient, the standard deviation, the scaled mean absolute deviation (MAD) and the bias are 0.83, 3.15 m s−1, 2.64 m s−1 and −0.25 m s−1, respectively, while the y=ax slope, the y=ax+b slope and the y=ax+b intercept are 0.93, 0.92 and −0.33 m s−1. For the Rayleigh-clear HLOS wind, the correlation coefficient, the standard deviation, the scaled MAD and the bias are 0.62, 7.07 m s−1, 5.77 m s−1 and −1.15 m s−1, respectively, while the y=ax slope, the y=ax+b slope and the y=ax+b intercept are 1.00, 0.96 and −1.2 m s−1. It is found that the standard deviation, the scaled MAD and the bias on ascending tracks are lower than those on descending tracks. Moreover, to evaluate the accuracy of Aeolus HLOS wind measurements under different product baselines, the Aeolus L2B Mie-cloudy HLOS wind data and L2B Rayleigh-clear HLOS wind data under Baselines 07 and 08, Baselines 09 and 10, and Baseline 11 are compared against the CDL-retrieved HLOS wind data separately. From the comparison results, marked misfits between the wind data from Aeolus Baselines 07 and 08 and wind data from CDLs in the atmospheric boundary layer and the lower troposphere are found. With the continuous calibration and validation and product processor updates, the performances of Aeolus wind measurements under Baselines 09 and 10 and Baseline 11 are improved significantly. Considering the influence of turbulence and convection in the atmospheric boundary layers and the lower troposphere, higher values for the vertical velocity are common in this region. Hence, as a special note, the vertical velocity could impact the HLOS wind velocity retrieval from Aeolus.

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.

Gain switched linearly polarized single-frequency pulsed fiber laser

Single-frequency pulsed fiber lasers have aroused intense interest due to their excellent performances in terms of good coherence, compact structure and good beam quality, which have been widely used in different areas, such as coherent LIDAR, nonlinear frequency conversion, and remote sensing. In this paper, a linearly polarized single-frequency pulsed fiber laser is reported. The narrow linewidth all-fiber oscillator is built by using an all-polarization-maintaining ring cavity structure. A section of unpumped polarization-maintaining ytterbium doped fiber is fused in the cavity as a saturated absorber to produce ultra-narrow bandwidth dynamic grating, which can be used for longitudinal mode selection. Thus, the laser can realize single-frequency operation. A 976-nm semiconductor laser is used as a pump source to generate a hybrid pump laser, which contains both pulsed component and continuous component. As a result, a long-pulse laser is achieved with a repetition rate of 10–90 kHz and a pulse duration of 1–8 μs. At the same time, the effects of pump power and repetition rate on the time domain, the frequency domain and the power characteristics of the output laser are investigated. It is found that there is an optical bistability in the frequency domain characteristic of the output laser within a certain power range. And the influence of the laser power characteristic on the longitudinal mode selection mechanism of dynamic grating is analyzed. Finally, through parameter optimization, single-frequency pulsed fiber laser is achieved with a center wavelength of 1064 nm, a linewidth of about 23.5 MHz, a repetition rate of 10–90 kHz, a pulse duration of 4–8 μs, and a polarization extinction ratio of about 29 dB.

Random Phase Compensation Method of Coherent Lidar Based on Symmetrical Double Harmonic Signals

Random Phase Compensation Method of Coherent Lidar Based on Symmetrical Double Harmonic Signals

Multi-Pulse Coherent Accumulation in Frequency Domain Based on Frequency-Modulation Continuous-Wave Coherent Lidar

Multi-Pulse Coherent Accumulation in Frequency Domain Based on Frequency-Modulation Continuous-Wave Coherent Lidar