Experimental determination of the lidar overlap profile with Raman lidar

The range-dependent overlap between the laser beam and the receiver field of view of a lidar can be determined experimentally if a pure molecular backscatter signal is measured in addition to the usually observed elastic backscatter signal, which consists of a molecular component and a particle component. Two methods, the direct determination of the overlap profile and an iterative approach, are presented and applied to a lidar measurement. The measured overlap profile accounts for actual system alignment and for all system parameters that are not explicitly known, such as actual laser beam divergence and spatial intensity distribution of the laser light.     

Correction scheme for close-range lidar returns

Because of the effect of defocusing and incomplete overlap between the laser beam and the receiver field of view, elastic lidar systems are unable to fully capture the close-range backscatter signal. Here we propose a method to empirically estimate and correct such effects, allowing to retrieve the lidar signal in the region of incomplete overlap. The technique is straightforward to implement. It produces an optimized numerical correction by the use of a simple geometrical model of the optical apparatus and the analysis of two lidar acquisitions taken at different elevation angles. Examples of synthetic and experimental data are shown to demonstrate the validity of the technique.     

Atmospheric aerosol profiling with a bistatic imaging lidar system

Atmospheric aerosols have been profiled using a simple, imaging, bistatic lidar system. A vertical laser beam is imaged onto a charge-coupled-device camera from the ground to the zenith with a wide-angle lens (CLidar). The altitudes are derived geometrically from the position of the camera and laser with submeter resolution near the ground. The system requires no overlap correction needed in monostatic lidar systems and needs a much smaller dynamic range. Nighttime measurements of both molecular and aerosol scattering were made at Mauna Loa Observatory. The CLidar aerosol total scatter compares very well with a nephelometer measuring at 10 m above the ground. The results build on earlier work that compared purely molecular scattered light to theory, and detail instrument improvements.     

Validation of a novel ultraviolet lidar system with relative Raman-scattering cross sections determined from atmospheric measurements

We have developed an ultraviolet lidar system in which the upwelled laser beam and the telescope field of view can be made to overlap at any specified location in space. We refer to this system as the Selected Overlap Lidar Experiment. We discuss validation of our system by calculating relative Raman-scattering cross sections (with respect to the nitrogen scattering cross section) for oxygen and water vapor using data collected during field operations of our lidar. Our relative cross sections are consistent with those obtained by other researchers making similar measurements in laboratory environments.     

Wave optics simulation of atmospheric turbulence and reflective speckle effects in CO2 lidar

Laser speckle can influence lidar measurements from a diffuse hard target. Atmospheric optical turbulence will also affect the lidar return signal. We present a numerical simulation that models the propagation of a lidar beam and accounts for both reflective speckle and atmospheric turbulence effects. Our simulation is based on implementing a Huygens-Fresnel approximation to laser propagation. A series of phase screens, with the appropriate atmospheric statistical characteristics, are used to simulate the effect of atmospheric turbulence. A single random phase screen is used to simulate scattering of the entire beam from a rough surface. We compare the output of our numerical model with separate CO(2) lidar measurements of atmospheric turbulence and reflective speckle. We also compare the output of our model with separate analytical predictions for atmospheric turbulence and reflective speckle. Good agreement was found between the model and the experimental data. Good agreement was also found with analytical predictions. Finally, we present results of a simulation of the combined effects on a finite-aperture lidar system that are qualitatively consistent with previous experimental observations of increasing rms noise with increasing turbulence level.     

Assessment of lidar inversion errors for homogeneous atmospheres

The inversion of lidar returns from homogeneous atmospheres has been done customarily through the well-known slope method. The logarithmic operation over the range-corrected and system-normalized received signal used in this method introduces a bias in the statistics of the noise-affected processed signal that can severely distort the estimates of the atmospheric attenuation and backscatter coefficients under measurement. It is shown that a fitting of the theoretically expected exponential signal to the range-corrected received one, using as the initial guess the results provided by the slope method and a least-squares iterative procedure, can yield enhanced accuracy under low signal-to-noise ratios and especially in moderate-to-high extinction conditions.     

Determination of overlap in lidar systems

The overlap profile, also known as crossover function or geometric form factor, is often a source of uncertainty for lidar measurements. This paper describes a method for measuring the overlap by presenting the lidar with a virtual cloud through the use of an imaging system. Results show good agreement with horizontal hard target lidar measurements and with geometric overlap calculated for the ideal aberration-free case.     

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.     

Measurement of integrated speckle statistics for CO2 lidar returns from a moving, nonuniform, hard target

A pulsed, dual CO(2) laser lidar was used to measure return signal statistics as a function of the number of speckles integrated by the lidar receiver per laser pulse. A rotating target generated statistically independent speckle patterns on each laser pulse. Data were collected for a wide range of receiver aperture sizes. A statistical model is developed that predicts the probability density of the return lidar pulse energy, which includes speckle, depolarization by the target, and albedo sampling. The predictions of this model are compared with the measured probability density function of the return pulse energies. Very good agreement is found between the geometrically calculated number of integrated speckles and the number predicted by the model.     

Boundary layer scattering measurements with a charge-coupled device camera lidar

A CCD-based bistatic lidar (CLidar) system has been developed and constructed to measure scattering in the atmospheric boundary layer. The system uses a CCD camera, wide-angle optics, and a laser. Imaging a vertical laser beam from the side allows high-altitude resolution in the boundary layer all the way to the ground. The dynamic range needed for the molecular signal is several orders of magnitude in the standard monostatic method, but only approximately 1 order of magnitude with the CLidar method. Other advantages of the Clidar method include low cost and simplicity. Observations at Mauna Loa Observatory, Hawaii, show excellent agreement with the modeled molecular-scattering signal. The scattering depends on angle (altitude) and the polarization plane of the laser.     

Ultraviolet Rayleigh-Mie lidar by use of a multicavity Fabry-Perot filter for accurate temperature profiling of the troposphere

A UV Rayleigh-Mie scattering lidar system at 355 nm has been upgraded for more-accurate temperature profiling of the troposphere by use of a new multicavity Fabry-Perot etalon (MCFPE) filter. The MCFPE filter, which was designed to improve the stability and operational characteristics of the lidar system, has three filter bandpass functions and separates one Mie scattering and two Rayleigh scattering signals from the lidar return signal and simultaneously acts as a laser frequency discriminator to lock the laser frequency. Moreover, a high-resolution grating is employed to block signal interference from Raman scattering and the solar background. A practical lidar system, which features strong system stabilization and high measurement accuracy, has been built, and the performance of the lidar system has been verified by comparison of temperature profiling between the lidar and a radiosonde. Good agreement between the two instrument measurements was obtained in terms of lapse rate and inversion layer height. Statistical temperature errors of less than 1 K up to a height of 3 km are obtainable with 5 min observation time for daytime measurements.     

Scanning lidar with a coupled radar safety system

A small scanning three-wavelength lidar system at NASA Langley Research Center in Hampton, Virginia, has been used since 1992 to make atmospheric measurements on stratospheric and upper tropospheric aerosols and on the evolution of aircraft exhaust plumes. Many of these measurements have been made away from the zenith, and, to reduce the hazard to air traffic produced by the laser beam, a radar safety device has been installed. The radar application is original in that the radar beam is made collinear with the laser beam by use of a dichroic mirror that transmits the laser radiation and reflects the microwaves. This mirror is inserted into the outgoing optical path prior to the radiation from both the radar and the laser passing through the independent scanning unit. Tests of the complete system show that the lidar and radar beams remain collocated as they are scanned and that the radar can be used to inhibit the laser prior to an aircraft passing through the beam.     

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.     

Measurement of glitter-point velocities on the sea surface using circular scanning with a collimated narrow laser beam

Alaser device (lidar) for remotely measuring the parameters of the glitter-point motion on the sea surface has been developed. The system employs a He-Ne laser to provide the probe beam. Circular scanning was performed by means of a piezoelectrically controlled mirror that had a frequency of rotation of 2 kHz. The positions of glitter-point echo pulses were compared with the reference signal of the mirror controller to trace the time evolution of the glitter-point coordinates on the circumference of the circular laser beam track. The measurements yielded the velocity distribution of the glitter points. The results were obtained under natural conditions and indicate an anisotropy of the glitter-point motion that is related to the sea conditions and the wind direction.     

Combined Raman-elastic backscatter lidar method for the measurement of backscatter ratios

A variation of the conventional combined Raman-elastic backscatter lidar method, the 1-2-3 lidar method, is described and analyzed. This method adds a second transmitter wavelength to the conventional combined Raman-elastic backscatter lidar. This transmitter wavelength is identical to that of the Raman receiver. One can generate the transmitted beam at this wavelength by Raman shifting the laser radiation in molecular nitrogen or oxygen. Measuring a second elastic lidar signal at the Raman-shifted wavelength makes it possible to eliminate differential transmission effects that can cause systematic errors in conventional combined Raman-elastic backscatter lidar.     

Transient radiative transfer equation applied to oceanographic lidar

We estimate the optical signal for an oceanographic lidar from the one-dimensional transient (time-dependent) radiative transfer equation using the discrete ordinates method. An oceanographic lidar directs a pulsed blue or green laser into the ocean and measures the time-dependent backscattered light. A large number of parameters affect the performance of such a system. Here the optical signal that is available to the receiver is calculated, rather than the receiver output, to reduce the number of parameters. The effects of albedo of a uniform water column are investigated. The effects of a school of fish in the water are also investigated for various school depths, thicknesses, and densities. The attenuation of a lidar signal is found to be greater than the diffuse attenuation coefficient at low albedo and close to it at higher albedo. The presence of fish in the water is found to have a significant effect on the signal at low to moderate albedo, but not at high albedo.     

Oceanographic lidar attenuation coefficients and signal fluctuations measured from a ship in the Southern California Bight

We measured the attenuation coefficient of the National Oceanic and Atmospheric Administration lidar from a ship in the Southern California Bight in September 1995. The region from approximately 5 to 30 m in depth was covered. The laser was linearly polarized, and the receiver was operated with the same polarization and the orthogonal polarization. The measured values were between 0.08 and 0.12 m(-1) and were highly correlated with in situ measurements of the beam attenuation coefficient. Fluctuations of the lidar signal were found to be induced primarily by surface waves whose wavelengths are approximately three times the lidar spot size at the surface.     

Ultraviolet high-spectral-resolution Doppler lidar for measuring wind field and aerosol optical properties

An ultraviolet incoherent Doppler lidar that incorporates the high-spectral-resolution (HSR) technique has been developed for measuring the wind field and aerosol optical properties in the troposphere. An injection seeded and tripled Nd:YAG laser at an ultraviolet wavelength of 355 nm was used in the lidar system. The HRS technique can resolve the aerosol Mie backscatter and the molecular Rayleigh backscatter to derive the signal components. By detecting the Mie backscatter, a great increase in the Doppler filter sensitivity was realized compared to the conventional incoherent Doppler lidars that detected the Rayleigh backscatter. The wind velocity distribution in a two-dimensional cross section was measured. By using the HSR technique, multifunction and absolute value measurements were realized for aerosol extinction, and volume backscatter coefficients; the laser beam transmittance, the lidar ratio, and the backscatter ratio are derived from these measurements.     

Aerosol lidar intercomparison in the framework of the EARLINET project. 2. Aerosol backscatter algorithms

An intercomparison of aerosol backscatter lidar algorithms was performed in 2001 within the framework of the European Aerosol Research Lidar Network to Establish an Aerosol Climatology (EARLINET). The objective of this research was to test the correctness of the algorithms and the influence of the lidar ratio used by the various lidar teams involved in the EARLINET for calculation of backscatter-coefficient profiles from the lidar signals. The exercise consisted of processing synthetic lidar signals of various degrees of difficulty. One of these profiles contained height-dependent lidar ratios to test the vertical influence of those profiles on the various retrieval algorithms. Furthermore, a realistic incomplete overlap of laser beam and receiver field of view was introduced to remind the teams to take great care in the nearest range to the lidar. The intercomparison was performed in three stages with increasing knowledge on the input parameters. First, only the lidar signals were distributed; this is the most realistic stage. Afterward the lidar ratio profiles and the reference values at calibration height were provided. The unknown height-dependent lidar ratio had the largest influence on the retrieval, whereas the unknown reference value was of minor importance. These results show the necessity of making additional independent measurements, which can provide us with a suitable approximation of the lidar ratio. The final stage proves in general, that the data evaluation schemes of the different groups of lidar systems work well.     

Cloud-droplet-size distribution from lidar multiple-scattering measurements

A method for calculating droplet-size distribution in atmospheric clouds is presented, based on measurement of laser backscattering and multiple scattering from water clouds. The lidar uses a Nd:YAG laser that emits short pulses at a moderate repetition rate. The backscattering, which is composed mainly of single scattering, is measured with a detector pointing along the laser beam. The multiple scattering, which is mainly double scattering, is measured with a second detector, pointing at a specified angle to the laser beam. The domain of scattering angles that contribute to the doublescattering signal increases monotonically as the pulse penetrates the cloud. The water droplets within the probed volume are assumed to have a constant size distribution. Hence, from the double-scatteringmeasured signal as a function of penetration depth within the cloud, the double-scattering phase function of the scattering volume is derived. Inverting the phase function results in a cloud-droplet-size distribution in the form of a log-normal function.