Viktor A. Banakh

Numerical simulation of propagation of laser beams formed by multielement apertures in a turbulent atmosphere under thermal blooming

An algorithm for simulating laser beam propagation in a turbulent atmosphere under conditions of thermal blooming for the case where the beam is formed by a multielement aperture is presented. Coherent and incoherent combining of elementary fields of the multielement aperture is considered. Based on numerical simulation, properties of a combined beam are analyzed in comparison with an equivalent Gaussian beam. A complex diffraction pattern is shown to appear upon coherent combinig as a result of superposition of fields formed by individual elements of the initial aperture. Average values of maximum intensity, both of the Gaussian beam and the combined laser beam, little differ from each other when the nonlinearity parameter Nc > 1. Under conditions of strong turbulence and strong nonlinearity, integral radiation characteristics of combined beams are close to characteristics of the Gaussian beam, the effective size of which is determined by sizes of the combined beam. The intensity of combined laser beams in a turbulent atmosphere fluctuates upon incoherent combining of fields to a lesser extent as compared to coherent combining.

Enhancement of the laser return mean power at the strong optical scintillation regime in a turbulent atmosphere

The results of the analysis of the backscattered radiation mean power enhancement in a turbulent atmosphere are presented. The above enhancement arises because of the correlation of the forward and backward waves in a random media. The analysis was performed for the strong optical scintillation regime in a turbulent atmosphere. It is shown that the laser return mean power in the turbulent atmosphere can two times exceed that in a homogeneous medium because of the correlation of forward and backscattered waves. With an increase in the optical turbulence strength, the enhancement effect decreases according to the power law.

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.

Effectiveness of the subharmonic method in problems of computer simulation of laser beam propagation in a turbulent atmosphere

The results of the analysis of the effectiveness of the subharmonic method for simulating large-scale turbulent inhomogeneities of the refractive index in problems of laser beam propagation in a turbulent atmosphere are presented in comparison with experimental data.

Method of radial velocities for the estimation of aircraft wake vortex parameters from data measured by coherent Doppler lidar

The method of radial velocities (RV) is applied to estimate aircraft wake vortex parameters from measurements conducted with pulsed coherent Doppler lidar (PCDL). Operations of the Stream Line lidar and the 2-µm PCDL are simulated numerically to analyze the accuracy of the estimated wake vortex parameters with the RV method. The RV method is also used to estimate wake vortex trajectories and circulation from lidar measurements at Tomsk and Munich airports. The method of velocity envelopes and the RV method are compared employing data gathered with the 2-µm PCDL. The domain of applicability of the RV method is determined.

Accuracy of estimation of the turbulent energy dissipation rate from wind measurements with a conically scanning pulsed coherent Doppler lidar. Part I. Algorithm of data processing

The procedure of lidar data processing permitting one to estimate the dissipation rate of turbulent kinetic energy from wind measurements with a pulsed coherent Doppler lidar (PCDL) upon conical scanning by a probing beam is presented. The proposed algorithm for the PCDL data processing was tested and the error of the lidar estimate of the dissipation rate was calculated as a function of the number of full scans by a probing beam using numerical simulation.

Efficiency of combined beam focusing under thermal blooming

Thermal defocusing of a laser beam leads to a restriction in the power of radiation that can be efficiently transmitted through the atmosphere. Focusing under thermal blooming can be considered as efficient if the minimum radius of the complete radiation interception circumference in the focal plane is less than the radius of the initial aperture of the radiation source. A method for estimating the efficiency of combined beam focusing under conditions of thermal blooming is considered.

Collimation of the initial wavefront of a partially coherent light beam by the backscattered signal

The possibility of compensating distortions of the initial wavefront of a partially coherent beam by the backscattered radiation power is analyzed. A new algorithm for controlling the flexible mirror by the signal of the volume atmospheric scattering is proposed and its efficiency is studied. It is shown that the developed algorithm makes it possible to considerably reduce the compensation time of distortions compared to the gradient method.

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.

Reconstruction of the structural characteristic of the refractive index and average air density in a shock wave arising in a supersonic flow past obstacles from optical measurements

Average and fluctuation parameters of a shock wave that arises in a supersonic air flow around bodies are studied based on numerical simulation using FLUENT 6.0 models of computational fluid dynamics. A series of closed numerical experiments show that distributions of the structural characteristic of the refractive index and average air density in the region of shock wave localization can be reconstructed from measurements of parameters of the laser beam intersecting the flow.