Current Search: Andrews, Larry (x)
View All Items
 Title
 APPROXIMATING THE SPECTRAL WIDTH OF IRRADIANCE FLUCTUATIONS WITH QUASIFREQUENCY.
 Creator

Reel, Andrew, Andrews, Larry, University of Central Florida
 Abstract / Description

Under weak turbulence theory, we will use the random thin phase screen model and the Kolmogorov powerlaw spectrum to derive approximate models for the scintillation index, covariance function of irradiance fluctuations, and temporal spectrum of irradiance fluctuations for collimated beams. In addition, we will provide an expression for the quasifrequency of a collimated beam and investigate the relationship between the quasifrequency and the maximum width of the normalized temporal...
Show moreUnder weak turbulence theory, we will use the random thin phase screen model and the Kolmogorov powerlaw spectrum to derive approximate models for the scintillation index, covariance function of irradiance fluctuations, and temporal spectrum of irradiance fluctuations for collimated beams. In addition, we will provide an expression for the quasifrequency of a collimated beam and investigate the relationship between the quasifrequency and the maximum width of the normalized temporal spectrum of irradiance for a collimated beam.
Show less  Date Issued
 2008
 Identifier
 CFE0002047, ucf:47595
 Format
 Document (PDF)
 PURL
 http://purl.flvc.org/ucf/fd/CFE0002047
 Title
 ASYMPTOTIC FORMULAS FOR LARGE ARGUMENTS OF HYPERGEOMETRICTYPE FUNCTIONS USING THE BARNES INTEGRAL.
 Creator

Heck, Adam, Andrews, Larry, University of Central Florida
 Abstract / Description

Hypergeometric type functions have a long list of applications in the field of sciences. A brief history is given of Hypergeometric functions including some of their applications. A development of a new method for finding asymptotic formulas for large arguments is given. This new method is applied to Bessel functions. Results are compared with previously known methods.
 Date Issued
 2004
 Identifier
 CFE0000235, ucf:46263
 Format
 Document (PDF)
 PURL
 http://purl.flvc.org/ucf/fd/CFE0000235
 Title
 EFFECTS OF ATMOSPHERIC TURBULENCE ON THE PROPAGATION OF FLATTENED GAUSSIAN OPTICAL BEAMS.
 Creator

Cowan, Doris, Andrews, Larry, University of Central Florida
 Abstract / Description

In an attempt to mitigate the effects of the atmosphere on the coherence of an optical (laser) beam, interest has recently been shown in changing the beam shape to determine if a different power distribution at the transmitter will reduce the effects of the random fluctuations in the refractive index. Here, a model is developed for the field of a flattened Gaussian beam as it propagates through atmospheric turbulence, and the resulting effects upon the scintillation of the beam and upon beam...
Show moreIn an attempt to mitigate the effects of the atmosphere on the coherence of an optical (laser) beam, interest has recently been shown in changing the beam shape to determine if a different power distribution at the transmitter will reduce the effects of the random fluctuations in the refractive index. Here, a model is developed for the field of a flattened Gaussian beam as it propagates through atmospheric turbulence, and the resulting effects upon the scintillation of the beam and upon beam wander are determined. A comparison of these results is made with the like effects on a standard TEM00 Gaussian beam. The theoretical results are verified by comparison with a computer simulation model for the flattened Gaussian beam. Further, a determination of the probability of fade and of mean fade time under weak fluctuation conditions is determined using the widely accepted lognormal model. Although this model has been shown to be somewhat optimistic when compared to results obtained in field tests, it has value here in allowing us to compare the effects of atmospheric conditions on the fade statistics of the FGB with those of the lowest order Gaussian beam. The effective spot size of the beam, as it compares to the spot size of the lowest order Gaussian beam, is also analyzed using Carter's definition of spot size for higher order Gaussian beams.
Show less  Date Issued
 2006
 Identifier
 CFE0001377, ucf:46969
 Format
 Document (PDF)
 PURL
 http://purl.flvc.org/ucf/fd/CFE0001377
 Title
 ON THE USE OF GAUSSIAN FILTER FUNCTIONS FOR ADAPTIVE OPTICS.
 Creator

Assad, Merfit, Andrews, Larry, University of Central Florida
 Abstract / Description

For adaptive optic systems, the use of aperture filter functions calculated using various Zernike modes can be useful in removing lowerorder aberrations caused by atmospheric turbulence. Traditionally, these filter functions are calculated using the step function depicting a hard aperture that introduces integrals that are sometimes difficult to integrate and must be done numerically. The Gaussian method can be used in place of the conventional method for calculating the aperture filter...
Show moreFor adaptive optic systems, the use of aperture filter functions calculated using various Zernike modes can be useful in removing lowerorder aberrations caused by atmospheric turbulence. Traditionally, these filter functions are calculated using the step function depicting a hard aperture that introduces integrals that are sometimes difficult to integrate and must be done numerically. The Gaussian method can be used in place of the conventional method for calculating the aperture filter functions. Evaluation of the Gaussian approximation for modeling a finite receiver aperture can be made by comparison of reduction in phase variance with results achieved using the conventional method. The validity of Gaussian approximation in this application is demonstrated by the consistency of results between the two methodologies. Comparison of reduction in scintillation by the two methodologies reveals several benefits derived from utilization of Gaussian approximation. The Gaussian approximation produces data that can be interpreted analytically. It further produces greater scintillation reduction. This paper will first examine the use of statistical models for predicting atmospheric turbulence and then the use of Zernike polynomials in adaptive optics. Next, this paper compares the reduction of phase variance and scintillation using the conventional method with the Gaussian approximation to evaluate the effectiveness of the new filter functions. The results of these comparisons are presented both as mathematical expressions and graphically.
Show less  Date Issued
 2006
 Identifier
 CFE0001436, ucf:52885
 Format
 Document (PDF)
 PURL
 http://purl.flvc.org/ucf/fd/CFE0001436
 Title
 Effect of Nonclassical Optical Turbulence on a Propagating Laser Beam.
 Creator

Beason, Melissa, Phillips, Ronald, Atia, George, Richardson, Martin, Andrews, Larry, Shivamoggi, Bhimsen, University of Central Florida
 Abstract / Description

Theory developed for the propagation of a laser beam through optical turbulence generally assumes that the turbulence is both homogeneous and isotropic and that the associated spectrum follows the classical Kolmogorov spectral power law of . If the atmosphere deviates from these assumptions, beam statistics such as mean intensity, correlation, and scintillation index could vary significantly from mathematical predictions. This work considers the effect of nonclassical turbulence on a...
Show moreTheory developed for the propagation of a laser beam through optical turbulence generally assumes that the turbulence is both homogeneous and isotropic and that the associated spectrum follows the classical Kolmogorov spectral power law of . If the atmosphere deviates from these assumptions, beam statistics such as mean intensity, correlation, and scintillation index could vary significantly from mathematical predictions. This work considers the effect of nonclassical turbulence on a propagated beam. Namely, anisotropy of the turbulence and a power law that deviates from . A mathematical model is developed for the scintillation index of a Gaussian beam propagated through nonclassical turbulence and theory is extended for the covariance function of intensity of a plane wave propagated through nonclassical turbulence. Multiple experiments over a concrete runway and a grass range verify the presence of turbulence which varies between isotropy and anisotropy. Data is taken throughout the day and the evolution of optical turbulence is considered. Also, irradiance fluctuation data taken in May 2018 over a concrete runway and July 2018 over a grass range indicate an additional beam shaping effect. A simplistic mathematical model was formulated which reproduced the measured behavior of contours of equal mean intensity and scintillation index.?
Show less  Date Issued
 2018
 Identifier
 CFE0007310, ucf:52646
 Format
 Document (PDF)
 PURL
 http://purl.flvc.org/ucf/fd/CFE0007310
 Title
 Variance in FadeTime of a GammaGamma Distributed Irradiance Signal.
 Creator

Leclerc, Troy, Phillips, Ronald, Weeks, Arthur, Richardson, Martin, Marinescu, Dan, Andrews, Larry, University of Central Florida
 Abstract / Description

Freespace optical communications are predominantly hindered by optical turbulence, an effect caused by temperature and pressure variations within the atmosphere. The result is an optical wave interfering with itself due to multipath propagation via tiny refractiveindex fluctuations across the wavefront. Optical communication systems are affected when the channel conditions induce fading in the irradiance signal that is received at the detector. The nature of optical interference imparted...
Show moreFreespace optical communications are predominantly hindered by optical turbulence, an effect caused by temperature and pressure variations within the atmosphere. The result is an optical wave interfering with itself due to multipath propagation via tiny refractiveindex fluctuations across the wavefront. Optical communication systems are affected when the channel conditions induce fading in the irradiance signal that is received at the detector. The nature of optical interference imparted by the atmosphere is a random process and therefore the received irradiance signal is often characterized by an appropriate probability density function (PDF). Data collected during past freespace optical experiments in the atmosphere support the gammagamma distribution as a practical PDF model for received irradiance fluctuations, although the irradiance fluctuations do occasionally tend towards a lognormal distribution.Utilization of the gammagamma irradiance PDF allows for calculation of statistical moments of the irradiance threshold levelcrossing distribution. Presented analysis focuses on the results of the gammagamma irradiance PDF. Previously, expressions were developed for the expected number of gammagamma distributed irradiance threshold levelcrossings. Expressions for the mean square number of gammagamma distributed irradiance threshold levelcrossings are derived and presented. The derived expressions lead to the mean and variance of signal fade time. Outcomes of the derived expressions are presented in relation to freespace optical communication system performance.Comparisons are made between the theoretical analysis and experimental data taken at the Innovative Science and Technology Facility (ISTEF) located at the Kennedy Space Center in Cape Canaveral, Florida. The strength of the atmospheric turbulence is often characterized by three measurable parameters: the refractive index structure constant Cn2, the inner scale l0, and the outer scale L0. The optical path (L~1km) was instrumented such that direct comparisons could be drawn between the measured atmospheric turbulence parameters and the parameters of the gammagamma irradiance model. Variance of fade time data were found to agree well for smaller apertures where effects of aperture averaging are not present and in cases where scintillation is weak to moderate. It is suggested that a more appropriate PDF, with a heavier focus on aperture averaging, may be applied in future studies of these fade statistics.
Show less  Date Issued
 2012
 Identifier
 CFE0004397, ucf:53153
 Format
 Document (PDF)
 PURL
 http://purl.flvc.org/ucf/fd/CFE0004397