Orlin I. Vankov

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.

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

Application of a lidar-type gamma-ray tomography approach for detection and identification of buried plastic landmines

The efficiency is studied of some applications of a recently developed lidar-type gamma-ray tomography approach for non-destructive evaluation of dense media. The approach consists in time-to-range resolved detection of the Compton returns from the probed object (irradiated by annihilation gamma-photon sensing beams) and data processing based on a lidar-type equation and intended for determination of the extinction and backscattering profiles along the line of sight. The concrete purpose of the work is to reveal by statistical modeling the capabilities, under Poisson noise conditions, of investigating underground layers and detecting low-contrast ingredients such as plastic landmines in soil. The results from simulations show that the method is capable of finding and identifying down to 5 % density-contrast ingredients in soil, at depths to 20 cm, with spatial resolution of 1 to 10 mm, for measurement time of 10 to 1000 s and activity of the gamma-ray source of 50 - 300 mCi. So, the method could be successfully used for examination of ground for landmines.

High-precision laser range measurements using convolution and deconvolution of reflected pulses

A novel high-precision pulsed ranging method is developed. It involves preliminary transformation of photon detector signals (convolution), analog-to-digital sampling, software processing by deconvolution, digital filtering, pulse shape retrieving and pulse center determination. The method is effective for arbitrary pulse durations (shorter or larger than the sampling step) and is low sensitive to the shapes of the reflected pulses. Using 20 MHz/8bits ADC, an experimental timing accuracy is achieved of approximately 600 ps (approximately 10-2 times the ADC-sampling step) for single measurements and of approximately 30 - 50 ps (approximately 10-3 - 10-4 times the sampling step) in averaging regime.

Lidar profiling by long rectangular-like chopped laser pulses

A novel, simple method is developed and tested (experimentally and by simulations) for effective, high-resolution retrieving of time-resolved lidar profiles for long rectangular-like laser pulses with arbitrary shapes of their leading and trailing edges. Such pulses are typically created by chopping cw optical radiation. The processing algorithm is based on differentiation and iteration procedures and avoids the use of divisions or deconvolutions that are often responsible for some errors and increases in the noise. Comparisons with pulsed and pseudo-random noise modulation lidar methods are given. The method enables a simplification of the entire lidar hardware. No powerful pulsed supplies, high driving pulsed voltages, complicated optical modulations, etc. are required. It could be very attractive for high-resolution, wide-spectral-band lidar measurements in the atmosphere, ocean, etc., using arbitrary optical emitters, especially for closer distances.

Features of the attenuation and single-sided imaging potential of near-infrared laser radiation in tissue-like liquid turbid media

Tissue-like phantoms are important tools in studying light propagation and scattering in biological tissues and in the development, testing and calibration of novel optical diagnostic and therapeutic methods and instruments. This motivates the interest in characterizing the optical properties of tissue-simulating turbid media. In the present work, using an original approach, the specific features have been revealed experimentally and interpreted physically (as owing to the polydispersity of the ensemble of scatterers) of the generally non-linear behaviour of the extinction (total attenuation) coefficient of Intralipid-20% dilutions in distilled water, depending on the Intralipid concentration, for near-infrared laser radiation of different wavelengths. At relatively low incremental concentrations, the values obtained of the extinction coefficient are shown to increase linearly being in agreement at each wavelength of concern with results for the integral scattering coefficient predicted by empiric formulae found by other authors. Comparative estimation has been performed as well, depending on the radiation wavelength, of the potentialities of single-sided optical sensing of tissue-like media. The estimates obtained outline the advantage of the wavelengths around the upper limit of the tissue optical window.

On the Efficiency of a Lidar‐Type Single‐Sided Gamma‐Ray Tomography Approach

The efficiency is investigated by computer simulations of a LIDAR‐type approach for non‐destructive single‐sided gamma‐ray probing and tomography of dense optically‐opaque media. The approach is based on time‐to‐range resolved detection of the backscattering returns from the probed object (irradiated by pulsed gamma‐photon beams) and determination of the internal distribution of the attenuation and Compton‐backscattering coefficients. The image‐reconstruction accuracy of sensing homogeneous and multifragment objects is investigated as a function of the depth of sensing. The results obtained show that this approach allows one to reliably establish the presence, the material content, and the disposition of different ingredients and flaws without noticeable influence of masking effects.

Spatial intensity distribution of the radiative return from scattering media irradiated by a cw laser beam

Experimental measurements and theoretical description have been performed of the spatial intensity distribution of the backward radiative response of tissue-like Intralipid-20% dilutions in distilled water irradiated by a collimated near-infrared cw laser beam. The investigations performed are a first step toward a complete estimation of the feasibility and potentialities of a stationary one-sided linear-strategy biomedical tomography approach to detecting characteristic inclusions (inhomogeneities, say ill places) in homogeneous highly-scattering host media (healthy tissues). The experimental results obtained are in good agreement with the derived theoretical expressions that thus would be of importance for the development and numerical modeling of stationary tomography algorithms ensuring optimally accurate data processing and interpretation.

Total attenuation coefficient of intralipid dilutions for discrete laser wavelengths between 405 and 1315 nm

The experimental investigations on different aspects of optical tomography require the knowledge of the optical parameters of tissues and tissue-like phantoms in order to unambiguously interpret the experimental data and specify characteristic inhomogeneities in tissue diagnostics. The main optical parameters of interest are the absorption coefficient, the scattering, backscattering, and reduced-scattering coefficients, the total attenuation (extinction) coefficient and the anisotropy factor. In this work, we extend our investigations of the optical properties of tissuemimicking phantoms, such as Intralipid-20% fat emulsion, using an approach we have developed recently based on the peculiarities of laser radiation beams propagating through semi-infinite turbid media. The dependence of the total attenuation coefficient on the Intralipid concentration, for laser radiation wavelengths λ=405, 672, 850, and 1314 nm, is studied, by using a set of phantoms consisting of different dilutions of Intralipid in distilled water. The experimental results for the extinction are in agreement with our previous results and with empiric formulae found by other authors concerning the wavelength dependence of the scattering coefficient of Intralipid -10% and Intralipid - 20%. They are also in agreement with known data of the water absorptance. As a whole, the results obtained in this work confirm the consideration of the experimental phantoms as semi-infinite media. They also confirm and extend theoretical and experimental results obtained previously, and reveal advantages of using longer wavelengths for deeper diagnostics of tissues and mimic turbid media.