Effects of refractive turbulence on ground-based verification of coherent Doppler lidar performance

The effects of refractive turbulence on ground-based verification of the far-field performance of coherent Doppler lidar are determined with numerical simulation and compared with the first-order terms of a theoretical expansion. The collimated small-beam far-field test has better performance than the focused-beam test. For typical ground-based conditions, higher-order terms of the theoretical expansion are required for convergence.     

Coherent Doppler lidar signal covariance including wind shear and wind turbulence

The performance of a coherent Doppler lidar is determined by the statistics of the coherent Doppler signal. The derivation and calculation of the covariance of the Doppler lidar signal for random atmospheric wind fields and wind shear are presented. The signal parameters are defined for a general coherent Doppler lidar system in terms of the atmospheric parameters. There are two distinct physical regimes: one in which the transmitted pulse determines the signal statistics and the other in which the wind field and the atmospheric parameters dominate the signal statistics. When the wind fields dominate the signal statistics, Doppler lidar data are nonstationary and the signal correlation time is proportional to the operating wavelength of the lidar. The signal covariance is derived for signal-shot and multiple-shot conditions. For a single shot, the parameters of the signal covariance depend on the random, instantaneous atmospheric parameters. For multiple shots, various levels of ensemble averaging over the t emporal scales of the atmospheric processes are required. The wind turbulence is described by a Kolmogorov spectrum with an outer scale of turbulence. The effects of the wind turbulence are demonstrated with calculations for a horizontal propagation path in the atmospheric surface layer.     

Special relativity corrections for space-based lidars

The theory of special relativity is used to analyze some of the physical phenomena associated with space-based coherent Doppler lidars aimed at Earth and the atmosphere. Two important cases of diffuse scattering and retroreflection by lidar targets are treated. For the case of diffuse scattering, we show that for a coaligned transmitter and receiver on the moving satellite, there is no angle between transmitted and returned radiation. However, the ray that enters the receiver does not correspond to a retroreflected ray by the target. For the retroreflection case there is misalignment between the transmitted ray and the received ray. In addition, the Doppler shift in the frequency and the amount of tip for the receiver aperture when needed are calculated. The error in estimating wind because of the Doppler shift in the frequency due to special relativity effects is examined. The results are then applied to a proposed space-based pulsed coherent Doppler lidar at NASA's Marshall Space Flight Center for wind and aerosol backscatter measurements. The lidar uses an orbiting spacecraft with a pulsed laser source and measures the Doppler shift between the transmitted and the received frequencies to determine the atmospheric wind velocities. We show that the special relativity effects are small for the proposed system.     

Feasibility study for the simulation of beam propagation: consideration of coherent lidar performance

To analyze the effects of atmospheric refractive turbulence on coherent lidar performance in a realistic way it is necessary to consider the use of simulations of beam propagation in three-dimensional random media. The capability of the split-step solution to simulate the propagation phenomena is shown, and the limitations and numerical requirements for a simulation of given accuracy are established. Several analytical theories that describe laser beam spreading, beam wander, coherence diameters, and variance and autocorrelation of the beam intensity are compared with results from simulations. Although the analysis stems from a study of coherent lidar performance, the conclusions of the method are applicable to other areas related to beam propagation in the atmosphere.     

Ultrawideband coherent noise lidar range-Doppler imaging and signal processing by use of spatial-spectral holography in inhomogeneously broadened absorbers

We introduce a new approach to coherent lidar range-Doppler sensing by utilizing random-noise illuminating waveforms and a quantum-optical, parallel sensor based on spatial-spectral holography (SSH) in a cryogenically cooled inhomogeneously broadened absorber (IBA) crystal. Interference between a reference signal and the lidar return in the spectrally selective absorption band of the IBA is used to sense the lidar returns and perform the front-end range-correlation signal processing. Modulating the reference by an array of Doppler compensating frequency shifts enables multichannel Doppler filtering. This SSH sensor performs much of the postdetection signal processing, increases the lidar system sensitivity through range-correlation gain before detection, and is capable of not only Doppler processing but also parallel multibeam reception using the high-spatial resolution of the IBA crystals. This approach permits the use of ultrawideband, high-power, random-noise, cw lasers as ranging waveforms in lidar systems instead of highly stabilized, injection-seeded, and amplified pulsed or modulated laser sources as required by most conventional coherent lidar systems. The capabilities of the IBA media for many tens of gigahertz bandwidth and resolution in the 30-300 kHz regime, while using either a pseudo-noise-coded waveform or just a high-power, noisy laser with a broad linewidth (e.g., a truly random noise lidar) may enable a new generation of improved lidar sensors and processors. Preliminary experimental demonstrations of lidar ranging and simulation on range-Doppler processing are presented.     

Autonomous beam alignment for coherent Doppler lidar with multielement detectors

Autonomous beam alignment for coherent Doppler lidar requires accurate information about optical misalignment and optical aberrations. A multielement heterodyne detector provides the required information without a loss in overall system performance. The effects of statistical variations from the random backscattered field (speckle field) are determined with computer simulations for both ground-based operation with a fixed calibration target and for space-based operation with random target backscatter.     

Coherent imaging with two-dimensional focal-plane arrays: design and applications

Scanned, single-channel optical heterodyne detection has been used in a variety of lidar applications from ranging and velocity measurements to differential absorption spectroscopy. We describe the design of a coherent camera system that is based on a two-dimensional staring array of heterodyne receivers for coherent imaging applications. Experimental results with a single HgCdTe detector translated in the image plane to form a synthetic two-dimensional array demonstrate the ability to obtain passive heterodyne images of chemical vapor plumes that are invisible to normal video infrared cameras. We describe active heterodyne imaging experiments with use of focal-plane arrays that yield hard-body Doppler lidar images and also demonstrate spatial averaging to reduce speckle effects in static coherent images.     

Coherent Doppler lidar measurements of winds in the weak signal regime

In the weak signal regime coherent Doppler lidar velocity estimates are characterized by a localized distribution around the true mean velocity and a uniform distribution of random outliers over the velocity search space. The performance of velocity estimators is defined by the standard deviation of the good estimates around the true mean velocity and the fraction of random outliers. The quality of velocity estimates is improved with pulse accumulation. The performance of velocity estimates from two different coherent Doppler lidars in the weak signal regime is compared with the predictions of computer simulations for pulse accumulation from 1 to 100 pulses.     

Modeling the performance of direct-detection Doppler lidar systems including cloud and solar background variability

Previous modeling of the performance of spaceborne direct-detection Doppler lidar systems assumed extremely idealized atmospheric models. Here we develop a technique for modeling the performance of these systems in a more realistic atmosphere, based on actual airborne lidar observations. The resulting atmospheric model contains cloud and aerosol variability that is absent in other simulations of spaceborne Doppler lidar instruments. To produce a realistic simulation of daytime performance, we include solar radiance values that are based on actual measurements and are allowed to vary as the viewing scene changes. Simulations are performed for two types of direct-detection Doppler lidar system: the double-edge and the multichannel techniques. Both systems were optimized to measure winds from Rayleigh backscatter at 355 nm. Simulations show that the measurement uncertainty during daytime is degraded by only approximately 10-20% compared with nighttime performance, provided that a proper solar filter is included in the instrument design.     

Coherent Doppler lidar signal spectrum with wind turbulence

The average signal spectrum (periodogram) for coherent Doppler lidar is calculated for a turbulent wind field. Simple approximations are compared with the exact calculation. The effects of random errors in the zero velocity reference, the effects of averaging spectral estimates by use of multiple lidar pulses, and the effects of the range dependence of the lidar signal power over the range gate are included. For high spatial resolution measurements the lidar signal power is concentrated around one spectral estimate (spectral bin), and correct interpretation of the contribution from turbulence is difficult because of the effects of spectral leakage. For range gates that are larger than the lidar pulse volume, the signal power is contained in many spectral bins and the effects of turbulence can be determined accurately for constant signal power over the range gate and for the far-field range dependence of the signal power.     

Heterodyne lidar returns in the turbulent atmosphere: performance evaluation of simulated systems

Simulations of beam propagation in three-dimensional random media were used to study the effects of atmospheric refractive turbulence on coherent lidar performance. By use of the two-beam model, the lidar return is expressed in terms of the overlap integral of the transmitter and the virtual (backpropagated) local oscillator beams at the target, reducing the problem to one of computing irradiance along the two propagation paths. This approach provides the tools for analyzing laser radar with general refractive turbulence conditions, beam truncation at the antenna aperture, beam-angle misalignment, and arbitrary transmitter and receiver configurations. Simplifying assumptions used in analytical studies, were tested and treated as benchmarks for determining the accuracy of the simulations. The simulation permitted characterization of the effect on lidar performance of the analytically intractable return variance that results from turbulent fluctuations as well as of the heterodyne optical power and system-antenna efficiency.     

Using the spectral asymmetry of TEA CO2 laser pulses to determine the Doppler-shift sign in coherent lidars with low frequency stability

We develop a method for determinating the relative positions of the lidar transmitter (LT) and the local oscillator (LO) frequencies in Doppler CO(2) lidars. It uses the weak spectral asymmetry of TEA CO(2) laser pulses, defined by a number of secondary peaks at the high-frequency side of the main spectrum peak. Depending on the sign of the beat frequency, these peaks may appear in the demodulated spectrum at either the high- or the low-frequency side. Each laser pulse spectrum is compared with reference spectra with two types of asymmetry, with the cross-correlation coefficients used as criteria. The performance of the method at different values of signal-to-noise ratio is analyzed numerically. The method is also applied to raw data from the lidar reference channel and demonstrates good performance at noise levels lower than the secondary peaks in the pulse spectrum or at a signal-to-noise ratio of >/=20 dB. Application of the pulse spectrum asymmetry for lidar frequency stabilization is analyzed. Lidar operation without frequency stabilization is considered as well. The method offers a simple Doppler lidar hardware for the creation of low-cost coherent lidars, velocimeters-rangefinders, etc.     

Evaluation of coherent Doppler lidar velocity estimators in nonstationary regimes

We evaluate the mean velocity estimator performance for coherent Doppler lidar measurements of wind fields with wind shear and nonuniform system response as a function of target range. Performance of the velocity estimates is characterized by the bias and standard deviation that are determined by computer simulations. Results are for solid-state lasers with a Gaussian transmitted pulse. We consider data with high signal energy that produces negligible random outliers.     

Lidar calibration technique using laboratory-generated aerosols

A new calibration technique for continuous-wave Doppler lidars that uses an aerosol scattering target has been developed. Calibrations with both single- and many-particle scattering were performed at the same lidar operating conditions as in atmospheric measurements. The calibrating targets, simulating atmospheric aerosols, were laboratory-generated spherical silicone oil droplets with known complex refractive indices and sizes, hence with known single-particle backscatter cross sections as obtained from Mie theory. Measurements of lidar efficiency with the conventional hard target calibration method were consistently higher by a factor of ~2 than measurements with the aerosol calibration technique. This result may have important implications for lidar backscatter estimates both for aerosol modeling efforts and for optimal design of future lidar systems. The aerosol calibration method provides a validation of basic lidar theory for particle scattering for coherent detection.     

Extracting vertical winds from simulated clouds with ground-based coherent Doppler lidar

The performance of mean velocity estimators is determined by computer simulations for solid-state coherent Doppler lidar measurements of wind fields at a cloud interface with deterministic profiles of velocity and aerosol backscatter. Performance of the velocity estimates is characterized by the standard deviation about the estimated mean and the bias referenced to the input velocity. A new class of estimators are required for cloud conditions, as traditional techniques result in biased estimates. We consider data with high signal energy that produces negligible random outliers.     

Four-Element Receiver for Pulsed 10-mum Heterodyne Doppler Lidar

A four-element photomixer receiver has been tested in a 10-mum heterodyne Doppler lidar. It addresses a reduction of the variance of the power scattered off distributed aerosols targets at ranges as long as 8 km. An improvement in performance is expected when the four independent signals recorded on every single shot are combined. Two summation techniques of the four signals have been implemented: a coherent summation of signal amplitude and an incoherent summation of intensities. A phasing technique for the four signals is proposed. It is based on a more suitable correlation time with discernible self-consistent packets (SCP's). The SCP technique has been successfully tested, and the results obtained with a coherent summation of the four signals, i.e., variance reduction, carrier-to-noise ratio improvement, and velocity accuracy improvement, are in agreement with theory.     

Analysis of the performance of a coherent pulsed fiber lidar for aerosol backscatter applications

The antenna and the Doppler estimation characteristics of a coherent pulsed lidar intended for short-range aerosol backscatter applications have been analyzed. The system used fiber-optic interconnects and operated at a wavelength of 1.548 microm. The range dependence of the signal for various bistatic and monostatic antenna configurations has been determined. The system operated in a low-pulse-energy, high-pulse-repetition-rate mode, and the Doppler estimates from the return signal were achieved with a multipulse accumulation procedure. The expected performance of the accumulation in this low-photocount regime was compared with the data obtained from the system, and a reasonable level of agreement was demonstrated.     

Numerical simulation of the effect of refractive turbulence on coherent lidar return statistics in the atmosphere

We propose an algorithm and the results of a numerical study of random realizations and statistics of a pulsed coherent lidar return that allow for refractive turbulence. We show that, under conditions of refractive turbulence, the relative variance of the lidar return power can exceed unity by a factor of as much as 1.5. Clear manifestations of the turbulent effect of backscattering amplification have been revealed from simulations of space-based lidar sensing of the atmosphere with coherent lidar. Under conditions of strong optical turbulence in the atmospheric boundary layer, as a result of the backscattering amplification effect, the mean lidar return power can exceed the return power in the absence of turbulence by a factor of 3.     

直接探测激光雷达系统的仿真软件设计

如何合理而有效地设计激光雷达系统,是进行激光雷达系统研究的关键,设计时必须充分考虑各种参数之间的相互关系,根据所提出的指标,选择激光器和探测器及其它元器件。建立了激光雷达系统的数值仿真模型,模型以激光雷达原理为基础,考虑了激光雷达距离方程、噪声模型、接收信噪比模型,利用LOWTRAN软件分析了各种天气和系统条件下,对激光雷达性能的影响,编制了计算机仿真软件进行雷达系统模拟,结果表明,数值仿真有助于实验系统的方案设计和性能改进。     

Effect of aerosol particle microstructure on cw Doppler lidar signal statistics

Analysis of signal statistical characteristics is carried out, and estimation errors of the radial wind velocity are calculated by use of numerical simulation of a cw Doppler lidar return, taking into account the atmospheric aerosol microstructure. It has been found that, at small sounded volume, the large particles contribute significantly to the scattered field. As a result the lidar return probability density function distribution can differ significantly from a Gaussian distribution. Neglect of the aerosol microstructure effect results in considerable underestimation of the error of cw Doppler lidar velocity estimates at small sounded volume.