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.     

Validation of satellite-retrieved oceanic inherent optical properties: proposed two-color elastic backscatter lidar and retrieval theory

Recent radiative transfer models show that: (1) regardless of elastic lidar receiver field of view (FOV), at vanishing lidar depth the lidar-derived attenuation coefficient klidar --> a, where a is the total absorption coefficient per meter of depth; and (2) for a wide FOV as the lidar sensing depth approaches some large value (depending on water type), klidar --> Kd, where Kd is the diffuse attenuation for downwelling irradiance. As a result, it is shown that a time-resolved, dual-wavelength-laser, elastic-backscattering lidar can retrieve the three principal oceanic optical properties: (1) the absorption coefficient of phytoplankton aph, (2) the absorption coefficient of chromophoric dissolved organic matter (CDOM) aCDOM, and (3) the nonwater total constituent backscattering coefficient bbt. The lidar-retrieved aph, aCDOM, and bbt inherent optical properties can be used to validate corresponding satellite-derived products such as those from terra moderate-resolution imaging spectroradiometer (MODIS), Aqua MODIS, Sea-viewing Wide Field-of-view Sensor, (SeaWiFS), and other ocean color sensors.     

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.     

Investigation of the effect of scattering agent and scattering albedo on modulated light propagation in water

A recent paper described experiments completed to study the effect of scattering on the propagation of modulated light in laboratory tank water [Appl. Opt.48, 2607 (2009)APOPAI0003-693510.1364/AO.48.002607]. Those measurements were limited to a specific scattering agent (Maalox antacid) with a fixed scattering albedo (0.95). The purpose of this paper is to study the effects of different scattering agents and scattering albedos on modulated light propagation in water. The results show that the scattering albedo affects the number of attenuation lengths that the modulated optical signal propagates without distortion, while the type of scattering agent affects the degree to which the modulation is distorted with increasing attenuation length.     

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.     

Differences arising in the determination of the atmospheric extinction coefficient by transmission and target reflectance measurements

Measurements of the optical extinction at a wavelength of 1.06 microm have been made in water droplet clouds. The extinction coefficient has been measured in the laboratory using two different methods simultaneously. In the first, measurements of the transmitted signal attenuation over a known path length were used. In the second the extinction coefficient was derived from the two-way attenuation of the signal reflected from a target on the opposite side of the cloud from the laser source and detector. It is found that in general the two values of the coefficient derived differ considerably, and the magnitude of the difference depends on the cloud density, the target size, and the system's optical parameters. The difference is shown to originate in the off-axis forward scattering caused by the cloud droplets, and the implications of the results on the measurement of the atmospheric extinction by reflection (lidar) techniques are discussed.     

Lidar system model for use with path obscurants and experimental validation

When lidar pulses travel through a short path that includes a relatively high concentration of aerosols, scattering phenomena can alter the power and temporal properties of the pulses significantly, causing undesirable effects in the received pulse. In many applications the design of the lidar transmitter and receiver must consider adverse environmental aerosol conditions to ensure the desired performance. We present an analytical model of lidar system operation when the optical path includes aerosols for use in support of instrument design, simulations, and system evaluation. The model considers an optical path terminated with a solid object, although it can also be applied, with minor modifications, to cases where the expected backscatter occurs from nonsolid objects. The optical path aerosols are characterized by their attenuation and backscatter coefficients derived by the Mie theory from the concentration and particle size distribution of the aerosol. Other inputs include the lidar system parameters and instrument response function, and the model output is the time-resolved received pulse. The model is demonstrated and experimentally validated with military fog oil smoke for short ranges (several meters). The results are obtained with a lidar system operating at a wavelength of 0.905 microm within and outside the aerosol. The model goodness of fit is evaluated using the statistical coefficient of determination whose value ranged from 0.88 to 0.99 in this study.     

Design of a lidar receiver with fiber-optic output

Design considerations for a coaxial lidar receiver are examined, including details of coupling to an optical fiber for transfer of return light to a remote detector box. Attention is concentrated on the influence of fiber position on return-light capture efficiency and dynamic range of the return signal. The effect of a central obstruction on short-range signals is included. The analysis is augmented with simulations of lidar receiver performance.     

Numerical evaluation of the possibilities of remote laser sensing of fish schools

The operation of an airborne lidar intended for the detection of fish schools is numerically simulated by the Monte Carlo method. The calculations are performed for schools located at small depths in order to study the regularities in the shaping of the lidar return accurately. Three models of the phase function of scattering of laser radiation in sea water are used. The signals reflected from surface waters that contain a school of fish are determined as a function of the lidar parameters, light scattering and absorption coefficients in the water, stratification of light scattering layers, and fish-school depth. The results obtained can be used for interpreting the signals of the fish-detection lidar.     

Time-dependent attenuator for dynamic range reduction of lidar signals

A time-dependent variable attenuator to reduce the dynamic range of lidar signals is introduced. The attenuator consists of a Pockels cell between two crossed polarizers that is incorporated into the receiving optic. The transmission is controlled electronically to attenuate the large signals from close ranges but to transmit far-range signal returns to their full extent. The signal dynamic range has been reduced by more than a factor of 100. Reproducibility and the effect of different rise times on the variable transmission are investigated. It is found that the attenuation is highly reproducible, and the associated statistical error remains below the detection limit of 10(-3). Systematic errors in differential absorption lidar (DIAL) measurements are negligible for relative wavelength differences between on-line and off-line Dlambda/lambda < 0.1%. Otherwise it is shown how these can be corrected. We used the attenuator to adapt the measured range to the heights of interest by increasing the electronic gain or to extend the range considerably to lower heights. It is estimated that with this variable attenuator a height range of 0.2-10 km can be covered with one data-acquisition channel only.     

Standoff determination of the particle size and concentration of small optical depth clouds based on double scattering measurements: concept and experimental validation with bioaerosols

A multiple-field-of-view (MFOV) lidar is used to characterize size and optical depth of low concentration of bioaerosol clouds. The concept relies on the measurement of the forward scattered light by using the background aerosols at various distances at the back of a subvisible cloud. It also relies on the subtraction of the background aerosol forward scattering contribution and on the partial attenuation of the first-order backscattering. The validity of the concept developed to retrieve the effective diameter and the optical depth of low concentration bioaerosol clouds with good precision is demonstrated using simulation results and experimental MFOV lidar measurements. Calculations are also done to show that the method presented can be extended to small optical depth cloud retrieval.     

Analysis of the receiver response for a noncoaxial lidar system with fiber-optic output

The return signal of a noncoaxial lidar system with fiber-optic output is examined. The dependence of the overlap regions and the overlap factor of the system on the fiber diameter is calculated for several inclination angles between the laser beam and the optical receiver axes. The effect of central obstruction is included and both cases of Gaussian and quasi-Gaussian laser beam profiles are treated. The irradiance spatial distribution on the focal plane of the system is calculated and experimentally determined. Finally, an alignment procedure of the lidar system is described based on the comparison between the range-corrected lidar signal and the range-corrected exponentially attenuated Rayleigh backscattered coefficient.     

Ozone and water-vapor measurements by Raman lidar in the planetary boundary layer: error sources and field measurements

A new lidar instrument has been developed to measure tropospheric ozone and water vapor at low altitude. The lidar uses Raman scattering of an UV beam from atmospheric nitrogen, oxygen, and water vapor to retrieve ozone and water-vapor vertical profiles. By numerical simulation we investigate the sensitivity of the method to both atmospheric and device perturbations. The aerosol optical effect in the planetary boundary layer, ozone interference in water-vapor retrieval, statistical error, optical cross talk between Raman-shifted channels, and optical cross talk between an elastically backscattered signal in Raman-shifted signals and an afterpulse effect are studied in detail. In support of the main conclusions of this model study, time series of ozone and water vapor obtained at the Swiss Federal Institute of Technology in Lausanne and during a field campaign in Crete are presented. They are compared with point monitor and balloon sounding measurements for daytime and nighttime conditions.     

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.     

Simultaneous measurements of particle backscattering and extinction coefficients and wind velocity by lidar with a Mach-Zehnder interferometer: principle of operation and performance assessment

The development of remote-sensing instruments that can be used to monitor several parameters at the same time is important for the study of complex processes such as those that control climate and environment. In this paper the performance of a new concept of lidar receiver that allows for the direct measurement of aerosol and cloud optical properties simultaneously with wind velocity is investigated. This receiver uses a Mach-Zehnder interferometer. Two different configurations, either with four photometric output channels or with fringe imaging on a multichannel detector, are studied. Analytical expressions of the statistical errors are given under the assumption of Gaussian signal spectra. It is shown that similar accuracies can be achieved for both configurations. Performance modeling of the retrieval of semitransparent cloud optical scattering properties and wind velocity was done at different operation wavelengths for a Nd:YAG laser source. Results for such a lidar system onboard an aircraft flying at an altitude of 12 km show that for semitransparent clouds the best results were obtained at 355 nm, with relative standard deviations of 0.5% and 5% for the backscatter and extinction coefficients, respectively, together with a velocity accuracy of 0.2 ms(-1). The accuracy of optical properties retrieved for boundary layer aerosols are comparable, whereas the velocity accuracy is decreased to 1 ms(-1). Finally, an extrapolation to a large 355-nm spaceborne lidar shows accuracies in the range from 2.5% to 5% for the backscatter coefficient and from 10% to 15% for the extinction coefficient together with a vertical wind speed accuracy of better than 0.5 ms(-1) for semitransparent clouds and boundary layer, with a vertical resolution of 500 m and a 100 shot averaging.     

Determination of cloud microphysical properties by laser backscattering and extinction measurements

The extinction and backscattering of 514-nm laser radiation in polydisperse water droplet clouds have been studied in the laboratory. Three cloud size distributions with modal diameters of 0.02, 5, and 12 microm have been investigated. The relationships between the cloud optical parameters (attenuation coefficient sigma and volume backscattering coefficient beta(pi)) and the cloud water content C have been measured for each size distribution. It has been found that a linear relationship exists between sigma and C and between beta(pi) and C for cloud water content values up to 3 g/m3. The linear relationships obtained, however, have slopes which depend on the droplet size distribution. For a given water content both sigma and beta(pi) increase as the modal diameter decreases. The measured data are compared with existing theoretical analyses and discussed in terms of their application to lidar measurements of atmospheric clouds. It is concluded that the empirical information obtained can serve as a basis for quantitative lidar measurements.     

Channel modeling of light signals propagating through a battlefield environment: analysis of channel spatial, angular, and temporal dispersion

Free-space optical communication (FSOC) is used to transmit a modulated beam of light through the atmosphere for broadband applications. Fundamental limitations of FSOC arise from the environment through which light propagates. We address transmitted light signal dispersion (spatial, angular, and temporal dispersion) in FSOC that operates in the battlefield environment. Light signals (photons) transmitted through the battlefield environment will interact with particles of man-made smoke such as fog oil, along the propagation path. Photon-particle interaction causes dispersion of light signals, which has significant effects on signal attenuation and pulse spread. We show that physical properties of battlefield particles play important roles in determining dispersion of received light signals. The correlation between spatial and angular dispersion is investigated as well, which has significant effects on receiver design issues. Moreover, our research indicates that temporal dispersion (delay spread) and the received power strongly depend on the receiver aperture size, field of view (FOV), and the position of the receiver relative to the optical axis of the transmitter. The results describe only specific scenarios for given types of battlefield particles. Generalization of the results requires additional work. Based on properties of the correlation, a sensitive receiver with a small FOV is needed that can find the line-of-sight photons and work with them.