Backscatter Amplification Effect Research Articles

Backscatter amplification effect in a non-Kolmogorov anisotropic turbulent medium

The paper presents a tool for numerical simulation of the backscatter amplification effect associated with double-pass propagation of laser radiation passing through an anisotropic non-Kolmogorov turbulence. It is shown that for variety of power-law exponents of refractive-index fluctuations, all other conditions being equal, the value of amplification factor also changes: the amplification factor increases when the fluctuation spectrum becomes more and more low-frequency. For the anisotropic medium, the two-dimensional distribution of the amplification factor allows remotely determining the spatial orientation of anisotropic inhomogeneities in a randomly inhomogeneous medium.

Amplification of laser echo signal in a turbulent atmosphere

Laser beam backscatter in a turbulent atmosphere is analyzed. The mean intensity of radiation scattered in turbulent atmosphere is shown to increase in the backward direction in comparison with the intensity of radiation scattered in the same direction in absence of turbulence, due to the effect of backscatter amplification in random media. As the strength of refractive turbulence increases, the amplification factor first grows, achieves the maximum, and then decreases. The area of manifestation of the backscattering amplification effect in the plane, transverse to the propagation direction, increases in size with intensification of refractive turbulence. It is found that the amplification factor depends on the inner scale of turbulence and the regime of diffraction of optical radiation at the transmitting aperture. It is maximal for spherical wave and grows with an increase of the inner scale. The data on the amplification effect for the mean power of the lidar echo signal as a function of refractive turbulence strength, regime of diffraction at the transmitting aperture, inner scale of turbulence, and size of the receiving aperture are reported for the first time.

Turbulent Lidar: I−Design

Two designs of a laser radar system based on the backscatter amplification effect (BSA) are suggested. The system is a micropulse aerosol lidar with two receiving channels, one of which records an increase in the lidar returns at the laser beam axis as the atmospheric turbulence intensifies. The second channel does not respond to the BSA and is required for calibration. The BSA effect manifests itself in a narrow spatial region around the laser beam axis; so, the receiver aperture should be small enough and comparable with the Fresnel zone. The creation of the turbulent lidar became possible with the advent of compact diode-pumped micropulsed lasers with high pulse repetition rates. The lidar is intended for continuous long-term unattended operation. It is eye-safe. Two designs of the turbulent lidar based on an afocal Mersenne telescope (mirror collimator) are suggested. BSA-2 and BSA-3 turbulent lidars are described. An algorithm is suggested for retrieval of the structure parameter of optical turbulence C 2 from lidar data based on the Vorob’ev’s approximation for a statistically homogeneous turbulent medium.

Statistical characteristics of a Gaussian beam reflected by a retro-reflector in atmospheric turbulence

The statistical characteristics of a retro-reflector reflecting a Gaussian beam is analyzed, which shows that the retro-reflector can be approximately characterized by an ABCD ray-transfer matrix, and a monostatic system in atmospheric turbulence with a retro-reflector as the target is processed with ABCD ray matrix theory. Analytical expressions are developed for the basic statistical moments, the degree of coherence and the scintillation index of the retro-reflected wave are calculated in terms of three fundamental statistical moments, and then the backscatter amplification effect associated with these properties are discussed. As comparisons, the results of both monostatic and bistatic cases of a plane mirror are also presented. All the results are based on the weak turbulence with von Karman spectrum.

Lidar using the backscatter amplification effect

Experimental data proving the possibility of lidar measurement of the refractive turbulence strength based on the effect of backscatter amplification (BSA) are reported. It is shown that the values of the amplification factor correlate with the variance of random jitter of optical image of an incoherent light source depending on the value of the structure constant of the air refractive index turbulent fluctuations averaged over the probing path. This paper presents the results of measurements of the BSA factor in comparison with the simultaneous measurements of the BSA peak, which is very narrow and only occurs on the laser beam axis. It is constructed the range-time images of the derivative of the amplification factor gives a comprehensive picture of the location of turbulent zones and their temporal dynamics.

Refractive turbulence strength estimation based on the laser echo signal amplification effect.

Experimental data proving the possibility of lidar measurement of the refractive turbulence strength based on the effect of backscattering amplification are reported. It is shown, for the first time to our knowledge, that the values of the amplification factor correlate with the variance of random jitter of the optical image of an incoherent light source depending on the value of the structure constant of the air refractive index turbulent fluctuations averaged over the probing path.

Laser echo signal amplification in a turbulent atmosphere.

A lidar system for registration of laser return amplification in a turbulent atmosphere due to backscatter amplification effect is considered in this paper. In the system, two receiving channels are used. One of them (axial) coincides with the transmitter channel, while another channel (nonaxial) receives the backscattered radiation at a small angle to the probing beam axis. The power ratio of the echo signal recorded in the axial channel to that recorded in the nonaxial one is a measure of the return amplification. The results of long-duration lidar atmospheric experiments show that the power of the echo signal registered in the axial channel usually exceeds that in the nonaxial one.

Backscatter amplification effect for a reflected partially coherent Gaussian beam in turbulent medium

The extended Huygens–Fresnel principle is used to develop a formulation for the backscattered intensity enhancement of a Gaussian Schell-model source beam through a weak turbulence. The results are shown that backscattered intensity enhancement factor of the reflected GSM beam is concerned with the coherence length of source, the wavelength, the size of target and wave structure function. In addition, the closed-form expressions can interpret backscattered intensity enhancement of plane and spherical wave scattered from a diffuse target. The results are illustrated by examples and compared with the previous work.

Backscattering amplification of laser radiation in a mediumwith fluctuations of the imaginary part of permittivity

The effect of backscattering amplification of laserradiation with respect to the radiation intensity reflected froman ordinary mirror in a medium with fluctuations of the real(refractive index) and the imaginary (absorption or amplification coefficient) parts of the permittivity is considered.Formulas for the backscattering amplification coefficient andthe variance of the intensity fluctuations of the reflected wavepropagating in a random dissipative (amplifying) medium arederived. Asymptotic expressions derived for the saturationregion of intensity fluctuations take into account the effect offluctuations of the refractive index and absorption (amplification) coefficient, as well as their correlation. The contribution of fluctuations of the complex permittivity parts andthe characteristic spatial scale of the problem to thebackscattering amplification coefficient is analysed. It isshown that for uncorrelated fluctuations of the real andimaginary parts of the permittivity of a random medium, thebackscattering amplification coefficient in the region ofstrong fluctuations is larger than in a transparent randommedium. It is also found that the correlation of pulsations ofthe real and imaginary parts of the permittivity suppresses thebackscattering amplification effect in an absorbing mediumand increases this effect in an amplifying medium.

Numerical simulation of the effect of refractive turbulence on coherent lidar return statistics in the atmosphere

We propose an algorithm and the results of a numerical study of random realizations and statistics of a pulsed coherent lidar return that allow for refractive turbulence. We show that, under conditions of refractive turbulence, the relative variance of the lidar return power can exceed unity by a factor of as much as 1.5. Clear manifestations of the turbulent effect of backscattering amplification have been revealed from simulations of space-based lidar sensing of the atmosphere with coherent lidar. Under conditions of strong optical turbulence in the atmospheric boundary layer, as a result of the backscattering amplification effect, the mean lidar return power can exceed the return power in the absence of turbulence by a factor of 3.

An estimation of sea surface influence on radar reflectivity of ships

A theoretical and numerical method is presented for the analysis of sea surface influence on the radar reflectivity of ships. The method of analysis is based on presentation of the field density scattered by the ship as a sum of echoes from different backscatterers located on the surface of the ship. The influence of sea surface is accounted for under the physical optics assumption. An analytical expression is obtained for the propagation factor with the ship in line-of-sight (LOS) range. Plots are illustrated for the dependence of the propagation factor on ship range for different heights of the radar antenna. The effect of microwave backscattering amplification from the ship is predicted for grazing angles of observation within LOS range.

The mutual coherence function and the backscatter amplification effect for a reflected Gaussian-beam wave in atmospheric turbulence

Abstract The influence of a modified spectrum of refractive-index fluctuations (that includes a high wavenumber rise as well as inner- and outer-scale parameters) on the backscatter amplification effect, arising from double passage of an optical wave through statistically dependent inhomogeneities of a random medium, is studied for the case of a Gaussian-beam wave reflected by a mirror of finite size. A formal expression is first developed for the mutual coherence function, which subsequently leads to tractable analytic models for the mean irradiance in the strictly backward direction. When the inner scale and Fresnel zone are of comparable size, the modified spectrum predicts significantly larger values of the enhancement factor than predicted by the Kolmogorov power-law spectrum. It is also shown in this analysis that by varying the focal length of the mirror the enhancement effects can be greater or less than those of a plane mirror, depending on focus adjustment. All calculations are based on weak irradiance fluctuations using complex ABCD ray-matrix representations for the propagation channel and a generalized spectral representation theory for the complex phase perturbations.

Propagation of reflected radiation in a randomly inhomogeneous medium

Equations are derived for the n-th-order moments of the complex amplitudes of a reflected radiation field in a randomly inhomogeneous medium. By solving an equation for the second moment of the field, an analysis is made of the backscattering amplification effect and its range of existence is established.

Effect of backscattering amplification during large intensity fluctuations

Effect of backscattering amplification during large intensity fluctuations