Dimitar V. Stoyanov

Deconvolution techniques for improving the resolution of long-pulse lidars

Deconvolution techniques are developed for improving lidar resolution when the sampling intervals are shorter than the sensing laser pulse. Such approaches permit the maximum-resolved lidar return in the case of arbitrary-shaped long laser pulses such as those used in CO2 lidars. The general algorithms are based on the Fourier-deconvolution technique as well as on the solution of the first kind of Volterra integral equation. In the case of rectangular pulses a simple and convenient recurrence algorithm is proposed and is analyzed in detail. The effect of stationary additive noise on algorithm performance is investigated. The theoretical analysis is supported by computer simulations demonstrating the increased resolution of the retrieved lidar profiles.

Effect of pulse-shape uncertainty on the accuracy of deconvolved lidar profiles

The effect of random and deterministic pulse-shape uncertainties on the accuracy of the Fourier deconvolution algorithm for improving the resolution of long-pulse lidars is investigated theoretically and by computer simulations. Various cases of pulse uncertainties are considered including those that are typical of Doppler lidars. It is shown that the retrieval error is a consequence of two main effects. The first effect consists of a shift up or down (depending on the sign of the uncertainty integral area) of the lidar profile as a whole, proportionally to the ratio of the pulse uncertainty area to the true pulse area. The second effect consists of additional amplitude and phase distortions of the spectrum of the small-scale inhomogeneities of the lidar profile. The results obtained allow us to predict the order and the character of the possible distortions and to choose ways to reduce or prevent them.

EARLINET coordinated lidar observations of Saharan dust events on continental scale

EARLINET, the European Aerosol Research Lidar Network, is the best tool to investigate the horizontal and vertical transport of aerosols over Europe. Within the network, particular attention is devoted to Saharan dust events monitoring. An alert system has been established in order to perform devoted measurements in case of intrusions of desert particles on European continent. Starting from data collected within EARLINET since May 2000, a first statistical analysis of the aerosol vertical distribution on European scale during Saharan dust outbreaks, has been performed. These results highlights the fundamental role that EARLINET can have for the study of impact of Saharan dust on European scale. The current 5-year EU project EARLINET-ASOS, started in March 2006, will enhance the operation of the network through the improvement of the instruments and of the temporal coverage, and of the data analysis procedures.

High-range-resolution velocity-estimation techniques for coherent Doppler lidars with exponentially shaped laser pulses

On the basis of an analysis of the autocovariance of the complex heterodyne signal, some novel algorithms are derived and investigated for recovering the nonuniform Doppler-velocity coherent-lidar profiles within the lidar resolution interval conditioned by the sensing laser-pulse length. The case of exponentially shaped sensing laser pulses is considered. The algorithm performance and efficiency are studied and illustrated by computer simulations (based on the use of pulse models and real laser pulses), taking into account the influence of additive noise and radial-velocity fluctuations. It is shown that, at some reasonable number of signal realizations used and with appropriate data processing to suppress the noise effects, the Doppler-velocity profiles can be determined with a considerably shorter resolution interval in comparison with that (usually accepted as a lower bound) determined by the pulse length.

Lidar receivers without overlap of the photomultiplier's single pulses.

Lidar receivers of linear performance without overlap of the single pulses are analyzed. The statistics of output signals are investigated by using a model of the photoreceiving system that is based on the conversion of secondary electron trains into single-frequency decayed oscillations. I show that the lidar profile can be separated into nonuniform and uniform parts and a background and sampled by low-speed analog-to-digital converters (12-16 bits/0.01-100 kHz) to provide high amplitude and temporal resolution. The nonstationary background is reduced without chopping to the square root of its intensity. The use of the lidar receivers is discussed.

Scattering of a laser beam in turbid media with forward-peaked Henyey?Greenstein indicatrices

The propagation of a continuous laser beam through homogeneous tissue-like liquid turbid media having sharply forward-directed Henyey–Greenstein indicatrices is studied. The in-depth on-axis and cross-sectional radial distributions of the detected forward-propagating light power are experimentally determined. The spatial distribution of the detected power is also described analytically by solving the radiative transfer equation in the so-called small-angle approximation. The experimental results are consistent with the analytical expressions obtained that are shown to allow one to estimate the extinction, reduced-scattering and absorption coefficients and the g-factor of the investigated media.

Measuring the shape of randomly arriving pulses shorter than the acquisition step

In this paper we have developed and tested a novel method for measuring precisely the shape of pulses shorter than the acquisition step, which is effective for random delays of the input pulses with respect to the start pulse of the analogue-to-digital converter (ADC). The method is based on conversion of the short pulses to be measured into longer damped oscillations and their correct acquisition (sampling) with saving the pulse information, rearranging of the sampled oscillations with respect to some reference time instant to form a finer-discretization high-precision oscillation, and retrieving the pulse shape by inverse algorithms. We demonstrated experimentally the good performance (5–7% rms error) of this method (by using 20 MHz/8 bits ADC) when measuring the shape of randomly arriving pulses, shorter than the ADC sampling step (50 ns), with an equivalent sampling frequency up to 2 GHz (0.5 ns equivalent sampling step). The resolving of shapes in a pulse pair with an inter-pulse delay shorter than the ADC sampling interval has also been demonstrated. The limiting equivalent sampling frequency is estimated to be up to 500 GHz. This method can be effectively applied for creation of some novel short-pulse measuring techniques, avoiding the problem of time synchronization to the start pulses in lidar and radar, nuclear experiments, tomography, communications, etc.

High-resolution Doppler-velocity estimation techniques for processing of coherent heterodyne pulsed lidar data

On the basis of an analysis of the autocovariance of the complex heterodyne signal, some novel algorithms are derived and are investigated for use in determining, with high spatial resolution, Doppler-velocity coherent-lidar profiles in the case of rectangular and rectangularlike sensing laser pulses. These algorithms generalize other known Doppler-velocity estimators for the more complex case of nonuniform scattering and Doppler-velocity distribution within the pulse length. Algorithm performance and efficiency are studied and are illustrated by computer simulations. It is shown that the Doppler-velocity profiles can be determined with essentially better resolution in comparison with the use of other known estimation approaches, but at the expense of some increase in the number of statistical realizations (number of laser shots) required to reduce the speckle-noise effect. The minimum achievable resolution interval is shown to be much shorter than the pulse length.

Pulse backscattering tomography based on lidar principle

It is shown that using sufficiently short pulses of sensing radiation (physical δ-pulses) one can determine in a simple, stable and fast contactless way the spatial distribution of the backscattering and extinction coefficients within a translucent scattering object. One should only measure, in combination with a lateral scan, the backscattering signal profile and the pulse energy passing through the object along each current line of sight at both the mutually opposite directions of sensing. It is also shown that deconvolution techniques may be employed to avoid the necessity of ultrashort sensing pulses. The possibilities of some signal-registration techniques to ensure ultrashort spatial sampling intervals for various types of sensing radiation are discussed.

Optimization of lidar data processing: a goal of the EARLINET-ASOS project

EARLINET-ASOS (European Aerosol Research Lidar Network - Advanced Sustainable Observation System) is a 5-year EC Project started in 2006. Based on the EARLINET infrastructure, it will provide appropriate tools to improve the quality and availability of the continuous observations. The EARLINET multi-year continental scale data set is an excellent instrument to assess the impact of aerosols on the European and global environment and to support future satellite missions. The project is addressed in optimizing instruments and algorithms existing within EARLINET-ASOS, exchanging expertise, with the main goal to build a database with high quality aerosol data. In particular, the optimization of the algorithms for the retrieval of the aerosol optical and microphysical properties is a crucial activity. The main objective is to provide all partners with the possibility to use a common processing chain for the evaluation of their data, from raw signals to final products. Raw signals may come from different types of systems, and final products are profiles of optical properties, like backscatter and extinction, and, if the instrument properties permit, of microphysical properties. This will have a strong impact on the scientific community because data with homogeneous well characterized quality will be made available in nearly real time.