Beam In Turbulent Medium Research Articles

Exact Analytic Representations for the Integral Characteristics of a Four-Point Coherence Function for Laser Beams in Turbulent Media

A new integral-functional equation is derived for the four-dimensional Fourier transform of the four-point coherence function of laser beams in turbulent media and two families of exact analytical solutions of this equation are found. These solutions hold for any level of fluctuations of the refractive index of air. They are used to obtain exact analytic representations of the integral characteristics of the four-point coherence function. In particular, the truncated spectral characteristics of the spatial correlation function of the intensities are found. These representations can be used to test asymptotic, numerical, and other methods for finding this function and to describe its integral properties.

On-axis average intensity of a hollow Gaussian beam in turbulent ocean

On the basis of the extended Huygens–Fresnel principle and the spatial power spectrum of the refractive index fluctuations of ocean water, a propagation formula for on-axis average intensity of hollow Gaussian beams (HGBs) in oceanic turbulence is analytically obtained. Furthermore, the propagation properties of an HGB in oceanic turbulence are studied numerically in detail. The numerical results indicate that the initial beams with a higher beam order, longer blue–green wavelength, and larger waist width are helpful in mitigating the influence of turbulent ocean. Meanwhile, an HGB may propagate a much longer distance in weak oceanic turbulence by decreasing the dissipation rate of mean-square temperature and the ratio of temperature and salinity, as well as increasing the dissipation rate of turbulent kinetic energy per unit mass of fluid. The simulations also verify that HGBs have more resiliency to oceanic turbulence effects than ordinary Gaussian beams. Our research is expected to provide useful guidance for understanding the propagation characteristics of structured light beams in turbulent media.

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.

Spreading of apertured partially coherent beams in turbulent media

The closed-form expression for the mean-squared width of apertured partially coherent beams propagating through turbulent media is derived by using the integral transform technique. The influence of turbulence on the spreading of apertured partially coherent beams is studied quantitatively by examining the relative mean-squared width, which is defined as the ratio of the mean-squared width of an apertured partially coherent beam in turbulence to the mean-squared width of the same beam in free space. On the other hand, the range of turbulence-independent propagation, also a reasonable measure of the resistance of a beam to turbulence, is obtained by examining the mean-squared width. It is shown that the spreading of apertured partially coherent beams is less affected by turbulence with smaller truncation parameter δ and coherence parameter α than with larger δ and α. In addition, the influence of turbulence on the spreading of apertured partially coherent beams increases first and then decreases due to increasing waist width w0. The results obtained are explained physically.

Spectral shifts of general beams in turbulent media

Using the concept of generalized beam, by applying the extended Huygens–Fresnel principle, we derive the spectrum for partially coherent cosh-Gaussian, cos-Gaussian, Gaussian and annular Gaussian beams propagating in turbulent media. From this formulation, graphical outputs are constructed illustrating the variation of receiver plane spectra against propagation distance, turbulence strength, degree of partial coherence and transverse coordinate. The dependence of spectral shifts on source and propagation parameters is observed. Spectrum invariance conditions are deduced analytically and discussed for the particular case of a fundamental Gaussian beam.

Spreading of partially coherent flattened Gaussian beams propagating through turbulent media

Based on the extended Huygens–Fresnel principle, the spreading of partially coherent flattened Gaussian beams propagating through turbulent media is studied by using the incoherent superposition of higher-order Hermite–Gaussian beams. The mean-squared width of partially coherent flattened Gaussian beams in turbulence is derived. It is shown that the spreading of partially coherent flattened Gaussian beams increases with increasing turbulent parameter and beam order N. However, the difference between the relative spreading of partially coherent flattened Gaussian beams in turbulent media and in free space reduces as N increases. Therefore, partially coherent flattened Gaussian beams with high N are less sensitive to the effects of turbulence than those with lower N.

Inverse source problem for partially polarized beams in turbulent media

The propagation of a class of partially polarized beams though atmospheric turbulence is treated using the coherent-modes-decomposition approach. This approach allows specifying the properties of sources generating radiation with a narrow polarization distribution that could be employed in active polarimetric remote sensing applications.

White-Noise and Geometrical Optics Limits of Wigner-Moyal Equation for Wave Beams in Turbulent Media

Starting with the Wigner distribution formulation for beam wave propagation in Hölder continuous non-Gaussian random refractive index fields we show that the wave beam regime naturally leads to the white-noise scaling limit and converges to a Gaussian white-noise model which is characterized by the martingale problem associated to a stochastic differential-integral equation of the Itô type. In the simultaneous geometrical optics the convergence to the Gaussian white-noise model for the Liouville equation is also established if the ultraviolet cutoff or the Fresnel number vanishes sufficiently slowly. The advantage of the Gaussian white-noise model is that its n-point correlation functions are governed by closed form equations.

The diffusion of partially coherent beams in turbulent media

We study the spreading of the rms spatial and angular characteristics of partially coherent beams in turbulent media. The angular broadening of a beam is shown to be diffusion-like. The dynamics of the rms width of the beam is found to be determined by the interplay of free-space diffraction and turbulent diffusion. Our results indicate the conditions under which partially coherent beams are less sensitive to distortions caused by the atmospheric turbulence than are fully coherent beams.

Thermal blooming of laser beams in turbulent media

This paper presents the results of laboratory experiments that studied the joint effect of turbulence and thermal blooming on laser beam propagation. The effect of turbulence on thermal defocusing, changes in the intensity fluctuations, and the coherence of radiation were investigated under the conditions of the self-action of narrow and broad beams both in the cw and pulse regimes of laser generation.

Thermal self-stressing of laser beams in a turbulent medium

Thus, in this paper the results of experimental and theoretical investigations of thermal self-stressing of laser beams in turbulent media were discussed. The following problems were considered: 1) The effect of turbulent agitation on thermal defocusing; 2) reduction of intensity fluctuations of wide and narrow, in comparison with the dimensions of turbulent inhomogeneities, laser beams due to defocusing by a thick thermal lens; 3) reduction of fluctuations due to defocusing by random temperature inhomogeneities induced by intensity fluctuations; 4) fluctuations in the presence of thermal self-stressing of pulsed radiation, the effect on it of spatial coherence of the field; 5) fluctuations in the presence of selfstressing in a moving medium.