This Guide presents tractable mathematical models from which the practitioner can readily determine beam spreading, beam wander, spatial coherence radius Fried's parameter , angle of arrival fluctuations, scintillation, aperture averaging effects, fade probabilities, bit error-rates, and enhanced backscatter effects, among others. Fade statistics obtained from experimental data were compared to theoretical predictions based on the lognormal and gamma-gamma distributions. Under such conditions turbulence-induced beam wander dominates the scintillation at the target. Numerical solutions of the fourth-moment differential equation are obtained for a finite, initially Gaussian beam propagating in a two-dimensional homogeneous and isotropic random medium with a Gaussian correlation function. Strong-turbulence statistics are then obtained from weak turbulence formulas by iterative calculation of the coherence parameters of the beam in a succession of weakly turbulent path intervals.
Near the center of the beam, the variance is somewhat smaller than that of an infinite plane wave; it increases toward the edge of the beam to a value which is greater than that of an infinite plane wave. The gamma-gamma pdf is found to generally provide a good fit to the simulation data in nearly all cases tested. Results are based on Rytov theory and Kolmogorov spectrum model. In this paper, using a non Kolmogorov spectrum and following same procedure already used for horizontal path analysis, we extend free space optical system performance analysis to uplink and downlink paths. Laser communications, Satellite communications, Optical communications, Portable satellite terminals.
The lognormal and gamma-gamma distributions are compared to simulated and experimental data of the irradiance fluctuations of a Gaussian beam wave propagating through the atmosphere on a horizontal path, near ground, in the moderate-to-strong turbulence regime. Such laser communication systems will be used not only for the space communications but also the terrestrial long-distance network. The measurements were made with collection apertures ranging from 1 mm to 1 m in diameter. In both cases the scintillation index agrees with conventional weak-fluctuation-theory results out of zenith angles of 45 to 60 deg. In the current study we develop theoretical models for beam spot size, spatial coherence, and scintillation index that are valid in weak irradiance fluctuation regimes as well as in deep turbulence, or strong irradiance fluctuation regimes. Scintillation is the irradiance fluctuations observed when light wave propagates through a weakly inhomogeneous medium such as the atmosphere. Of particular interest is the variation of turbulent index fluctuations with altitude.
Annular beams are created by unstable optical resonators, which are used as resonant cavities in high power lasers, and by beam expanders that consist of telescopes where the second mirror obstruct the central portion of the circular aperture. Knowledge of the scintillation of the reflected beam could not only help design an appropriate receiving system, but could also give information about the reflecting object that could be utilized in forming its optical signature. . A numerical integration technique is used to extend the results to a more realistic turbulence model. Scintillation is the effect of atmospheric turbulence which is known to disrupt and alter the intensity and formation of a laser signal as it propagates through the atmosphere. This study investigated the technological feasibility of a man- portable satellite laser communications system.
Retrodirective reflections from a corner cube and a simple lens-mirror system, used as a surrogate for a lens-detector optical system, were studied using near-infrared laser illumination. Values of the aperture factor are calculated. © 2005 The Society of Photo-Optical Instrumentation Engineers. Particularly significant in this regard is the signal fading below a prescribed threshold value owing primarily to optical scintillations associated with the received signal. A review is given of recent investigations of the propagation of laser radiation in randomly inhomogeneous media with large-scale fluctuations of the refractive index. Bibtex entry for this abstract Preferred format for this abstract see Preferences Find Similar Abstracts: Use: Authors Title Abstract Text Return: Query Results Return items starting with number Query Form Database: Astronomy Physics arXiv e-prints The interest for free space laser beam propagation has recently increased due to several experiments. It covers basic concepts and state-of-the-art technologies, emphasizing device technology, implementation techniques, and system trades.
In the weak fluctuation regime this analytic expression is found to compare reasonably well with published data. An apparatus for measuring the intensity fluctuations of a distant source and the spectral transmittance of the atmosphere in the infrared is described. The partially coherent beam created by the use of a phase diffuser is modeled as a Gaussian Schell beam. We consider cases of tracked beams and untracked beams, both of which involve a certain amount of beam wander. This book discusses optical scintillation and its impact on system performance in free-space optical communication and laser radar applications, with a detailed look at propagation phenomena and the role of scintillation on system behavior. In this study we develop theoretical models for beam spot size, spatial coherence, and scintillation index that are valid in weak irradiance fluctuation regimes as well as in deep turbulence, or strong irradiance fluctuation regimes. Despite the inevitable fact that turbulence will cause random blurring and deformation of imaging results, it also offers convenient solutions to some remote sensing and machine vision problems, which would otherwise be difficult.
The resulting irradiance pdf takes the form of a generalized K distribution that we term the gamma-gamma distribution. Theory of scintillation: spherical wave model -- Introduction -- Zero inner scale model -- Effective Kolmogorov spectrum -- Nonzero inner scale model -- Effective atmospheric spectrum -- Outer-scale effects -- Comparison with experimental data -- Covariance function of irradiance -- Gamma-gamma distribution -- Comparison with simulation data -- References -- Chapter 5. For this paper, we model atmospheric scintillation using a gamma-gamma probability distribution. Using experimentally confirmed results from a desert environment, the utility of this model may be extended to other climates by adapting temporal hour weights used within the model. This new expression tracked the gamma-gamma distributed data more closely than the existing approximation and resulted in a higher number of fades.
Both parameter pairs are intuitive, and within the context of locally homogeneous and isotropic turbulence they reflect the long-recognized importance of the Fresnel zone size in the behavior of Rytov propagation statistics. The propagation path was instrumented with a commercial scintillometer to measure the Cn 2 and a weather station to monitor temperature and wind velocity. Scintillation models -- Chapter 1. This departure conflicts with earlier theoretical studies of weakly scintillated beams which have traditionally assumed log normal behavior. In Chapter 2 I introduced a method for easily expressing in integral form the second moment of phase and log-amplitude related quantities for infinite colli-mated and focused waves propagating through turbulence. The mean-square fluctuations of log amplitude and phase are analytically obtained for gaussian light-beam propagating through a randomly inhomogeneous medium with gaussian covariance of the refractive-index fluctuations. For those not acquainted with the literature, in addition to providing a fairly in-depth knowledge of the characteristics of propagati- on through a turbulent medium and an indication of the status of our understanding of this subject, the paper should provide a stepping stone for obtaining a more thorough understanding of the subject and for doing work in the field.