Intensity fluctuations in a laser beam due to propagation through a plane turbulent jet

A planar heated air jet was constructed. Its flow properties were characterized and shown to be both reproducible and in good agreement with the results of turbulence theory. The optical properties of the jet were studied with the help of a 632.8 nm HeNe laser beam. Measurements of the random phase modulations imposed on the wavefront of the beam traversing the jet were made and the results reported in a previous paper. The intensity fluctuations arising in the beam were also measured and their spectra and variance determined at various distances from the jet. The spectra behave like power laws with a slope of -1 at low temporal frequencies and a slope of -8/3 at higher frequencies. This dual power-law behaviour is attributed to the differential velocities within the planar jet.     

Laser-pulse fluctuations due to thermal self-action in a turbulent atmosphere

Conclusions Refractive-index fluctuations caused by heating alter the intensity and phase fluctuation spectra arising from the turbulence. The large-scale part of the spectra is weakened, whereas the small-scale part is strengthened, and the spatial-frequency range in which the fluctuations are attenuated becomes narrower as the beam energy increases. Correspondingly, the fluctuation structure functions alter. For values of the arguments in those functions less than the radius of the first Fresnel zone , the thermal nonlinearity causes increases in the structural functions D_, with the energy. For , the dependence of D_, on energy is nonmonotone, with the fluctuations at first weakened but then strengthened. The reduction in fluctuation variance and the improvement in coherence is less pronounced over long lines than on short ones. The range in which geometrical optics can be used to derive the fluctuations is independent of the nonlinearity and is defined by k₂z/2k /4. The trend to the diffraction asymptote becomes slower as the nonlinearity increases.Atmospheric Physics Institute, Academy of Sciences of the USSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 32, No. 9, pp. 1063–1071, September, 1989.     

Light-beat method to process phase fluctuations of a laser light beam

A method for real-time processing of phase fluctuations of a laser beam is described. A beat photocurrent whose phase undergoes random fluctuations is generated by two laser beams at different frequencies, propagating through a turbulent atmosphere. The phase fluctuation component of the beat can be recovered in real-time over a wide range of phase angle, not restricted within 2π, by a specially designed circuit. Probability density functions, autocorrelation functions, and variances of the phase fluctuations are displayed in real-time as most basic quantities of the fluctuations. Visiting Fellow at Department of Electronics, The University of Southampton, England, October 1974–September 1975.     

Diagnostics of the scale of turbulence using a divergent laser beam

A method for estimating the parameters of turbulence that was originally developed for collimated laser beams is applied to the case of a divergent beam. The estimate is based on an analysis of the correlation functions of the Zernike expansion coefficients of the phase. The results suggest that the method can be used for the estimation of the inner scale of turbulence in an experiment with a divergent laser beam. In the case of isotropic turbulence, it is also possible to estimate the outer scale of turbulence from the correlation functions of lower-order modes.     

激光大气传输湍流扰动仿真技术

为了研究激光大气传输时湍流效应对激光应用技术的影响,对湍流扰动的仿真进行了分析。介绍了在实验室内进行激光大气传输湍流扰动研究的数值仿真技术和仿真系统。阐述了快速傅里叶变换（FFT）和Zernike多项式两种湍流扰动数值仿真方法,并且对比了两种方法的优劣。利用物理相位屏搭建了实物湍流仿真系统,介绍了其理论模型并进行了仿真实验,对激光经湍流系统传输后的光强能量分布进行了研究分析。结果显示,室内湍流仿真系统能够准确地模拟弱起伏条件下湍流对激光传输的影响。     

Scintillation of a laser beam propagation through non-Kolmogorov strong turbulence

Atmospheric turbulence causes strong irradiance fluctuations of propagating optical wave under the severe weather conditions in long-distance free space optical communication. In this paper, the scintillation index for a Gaussian beam wave propagation through non-Kolmogorov turbulent atmosphere is derived in strong fluctuation regime, using non-Kolmogorov spectrum with a generalized power law exponent and the extended Rytov theory with a modified spatial filter function. The analytic expressions are obtained and then used to analyze the effect of power law, refractive-index structure parameter, propagation distance, phase radius of curvature, beam width and wavelength on scintillation index of Gaussian beam under the strong atmospheric turbulence. It shows that, with the increasing of structure parameter or propagation distance, scintillation index increases sharply up to the peak point and then decreases gradually toward unity at rates depending on power law. And there exist optimal value of radius of curvature and beam width for minimizing the value of scintillation index and long wavelength for mitigating the effect of non-Kolmogorov strong turbulence on link performance.     

Cascades of velocity and temperature fluctuations in buoyancy-driven thermal turbulence

Direct multipoint measurements of the velocity and temperature fields have been made in a turbulent Rayleigh-Bénard convection cell. In the central region of the cell it is found that both velocity and temperature exhibit the same scaling behavior that one would find for the velocity and for a passive scalar in homogeneous and isotropic Navier-Stokes turbulence. This is despite the fact that energy is pumped into the system vertically via buoyancy. Near the cell's sidewall where thermal plumes abound, vertical velocity and temperature exhibit different scalings. A model that takes into account both buoyancy and energy dissipation is proposed and its predictions agree well with the sidewall experimental results.     

Near-threshold reflectivity fluctuations in the independent-convective-hot-spot-model limit of a spatially smoothed laser beam

In the framework of the independent-hot-spot model, it is shown that the reflectivity resulting from scattering instabilities when a spatially smoothed laser beam interacts with a plasma exhibits large statistical fluctuations near threshold. The importance of the fluctuations is discussed in terms of a confidence interval for the reflectivity, which is more relevant to experimental measurements than the average reflectivity. An analytical model for the fluctuating reflectivity is developed and shown to be in good agreement with numerical simulations. The influence of the transverse size of the interaction region is studied.     

Intensity fluctuations of spherical acoustic waves propagating through thermal turbulence*

The intensity fluctuations of acoustic waves that propagate through thermal turbulence are investigated under well controlled laboratory conditions. Two heated grids in air are placed horizontally in a large anechoic room and the mixing of the free convection plumes above them generates a homogeneous isotropic random thermal field. The spectrum of refractive index fluctuations is accurately described by a modified von Karman model which takes into account the entire spectrum of turbulence. Experimental data are obtained by varying both the frequency of the spherical wave and the distance of propagation. In this paper we concentrate on the variance of the normalized intensity fluctuations and on their probability distributions. These measurements cover all the regimes from weak scattering to strong scattering including the peak of the intensity variance. Experimental values of the scintillation index are compared with classical theoretical predictions and also with the results of recent numerical simulations. The classical probability density functions (log-normal, exponential, I-K) are tested against the measured probability distributions. The generalized gamma distribution, which varies smoothly from log-normal to exponential as a function of two parameters, appears m represent the experimental data for a very large range of scattering conditions.     

Influence of non-kolmogorov turbulence on intensity fluctuations in laser satellite communication

Satellite laser communication holds the potential for high-bandwidth communication, but the atmosphere can significantly affect the capability of this type of communication systems for satellite-toground and ground-to-satellite data links to transfer information consistently and operate effectively. Usually the influence of atmosphere on satellite laser communication is investigated based on the Kolmogorov turbulence model. However, both increasing experimental evidence and theoretical investigations have shown that the Kolmogorov theory is sometimes incomplete to describe the atmospheric statistics properly, in particular, in some portions of the atmosphere. Considering a non-Kolmogorov turbulent power spectrum with power law −5 that describes the refractive-index fluctuations in the atmosphere above 6 km, we calculate the scintillation index of a lowest-order Gaussian-beam wave under the weak-fluctuation condition. Then, considering a combined power spectrum of refractiveindex fluctuations and using the expression obtained, we analyze the joint influence of the Kolmogorov turbulence from the ground to 6 km and non-Kolmogorov turbulence above 6 km on the scintillation indices of laser beams used in ground-to-satellite and satellite-to-ground laser communication links. We show that the scintillation index in satellite laser communication is equal to the sum of the scintillation indices induced by the Kolmogorov turbulence from ground to 6 km and that caused by the non-Kolmogorov turbulence above 6 km. Also we investigate variations of the scintillation index with the beam radius on the transmitter, wavelength, the radial distance, and zenith angle. Finally, comparing the scintillation index induced by these two turbulences with the conventional results, we show that the scintillation index induced by these two turbulences is a bit smaller than the conventional results.     

A multiple laser beam probe system for turbulence diagnostics

Deatils are given of a multiple laser beam probe for sizing and velocity measurements in turbulent media. Sample results are presented in the case of thermal turbulent convection in a model buble chamber.     

On the suppression of laser beam geometry fluctuations by spatial filtering

The possibility of reducing fluctuations in the geometry of a laser beam by spatial filtering is analysed, the merits of a double aperture system being pointed out. It is shown that the amount of improvement attainable is quite models, though possibly useful.     

Light-beat measurement of phase fluctuations of a laser beam propagating through a turbulent atmosphere

Real-time processing of phase fluctuations of a laser beam has been considered in association with the phase of a beat signal of light. Two fine light beams, included in the expanded laser beam propagating through an artificially turbulent atmosphere, are shifted in frequency by two successive ultrasonic light modulators at slightly different frequencies, so that they can be photomixed to generate a beat signal whose phase is randomly modulated. The fluctuating phase difference of the two fine beams can be revealed in real-time from the phase of the beat signal. It is immediately processed by a conventional wave-height analyzer, from which a probability density and its variance are simply obtained. When the atmosphere is perturbed by the blower with a heater alone, the probability density takes the Gaussian form and the variance obeys approximately the 5/3-power law as the two beam separation is increased. When the turbulent atmosphere is further disturbed by some metallic mesh, the probability density does not always obey Gaussian statistics, but it depends greatly on the turbulent state of the atmosphere.