Ljuan L. Gurdev

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.

Lidar technique for simultaneous temperature and pressure measurement based on rotational Raman scattering

Abstract A possibility for simultaneous determination of the atmospheric temperature and pressure on the basis of interferometric analysis of the rotational Raman scattering spectrum of the atmosphere is pointed out and discussed.

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.

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.

Gamma-Ray Backscattering Tomography Approach Based on the Lidar Principle

An approach is proposed, and its potentialities are studied, for single-sided gamma-ray in-depth sensing and tomography of dense opaque media. The approach is based on lidar (LIgth Detection And Ranging) principle or, in the present case, graydar (Gamma RAY Detection And Ranging) principle, that is, time-to-range resolved detection of the backscattering-due radiative returns from the probed object irradiated by pulsed gamma-photon pencil beams. The basic analysis and data processing delta-pulse single-scattering graydar equation is formulated by analogy with the lidar equation and is shown to be applicable, under some determinate conditions, to the problems of gamma-ray in-depth profiling of dense media. It is shown analytically and by computer simulations that the approach developed in the work would enable one, at large-enough but reasonable sensing photon fluxes and measurement time intervals, to determine with good controllable accuracy and resolution the location, the material content, and the mass density of different homogeneous ingredients inside the probed object as well as the mass (or electron) density distribution within one-material objects. This approach can be widely applied, e.g., for nondestructive material examination in industry and aviation, detection of landmines and explosives, investigating the constitution of archeological artifacts, etc

Lidar Profile Deconvolution Algorithms for Some Rectangular-Like Laser Pulse Shapes

Simple lidar profile deconvolution algorithms are developed for some laser pulse shapes. The possibility is pointed out and discussed to use these algorithms for determination of the fine spatial structure of atmospheric and other objects whose size is less than the pulse length.