Larry Andrews

• University of Central Florida

Spot radius is dependent upon whether the receiver is in the near or far field which becomes significant when considering beam skewness. We compare methods to calculate beam radii, impact on skewness, and suggest an empirical method to transition between regimes.

Spot radius is dependent upon whether the receiver is in the near or far field which becomes significant when considering beam skewness. We compare methods to calculate beam radii, impact on skewness, and suggest an empirical method to transition between regimes.

Over the years, there has been much interest in the use of optical wavelengths for communication because of the potential for high data rates. However, the performance of these systems can become significantly degraded due to turbulence-induced signal fluctuations. These fluctuations can be minimized by enlarging the receiving aperture, thereby ave...

Beam wander associated with a propagating beam in the atmosphere has impact on the short-term and long-term spot size, and the scintillation index. New theoretical models are presented here for beam wander and beam-wander-induced scintillation index. These models are then compared with computer simulation results.

Optical scintillometers used to characterize turbulence are based on assumptions of isotropic, Kolmogorov turbulence following a spectral power law. However, experimental data suggest that the turbulence may at times be anisotropic and non-Kolmogorov. In this work, consideration is given to converting from the structure function constant, , based o...

The Rytov approximation is used with different initial beam radius of curvature in orthogonal directions to approximate optical refractivity with an anisotropic nonKolmogorov spectrum in weak turbulence. Results are compared to experimental data.

A single pixel infrared camera is developed utilizing digital micromirror technology. The imager is applied to determination of atmospheric extinction and other meteorological parameters. Experimental evidence demonstrates agreement with forward-scatter based visibility measurements.

Recently, the number of optical systems that operate along near horizontal paths within a few meters of the ground has increased rapidly. Examples are LIDAR or optical sensors imbedded in a vehicle, long range surveillance or optical communication systems, a LIFI network, new weather monitoring stations, as well as directed energy systems for defen...

Experimental data are presented that demonstrate the evolution of the anisotropy/isotropy of atmospheric statistics throughout the course of four days (two winter, two summer) near the ground over a concrete runway in Florida. In late January and early February of 2017, a 532 nm near-plane-wave beam was propagated 1 and 2 km at a height of 2 m abov...

Experimental measurements were recently made which displayed characteristics of plane wave propagation through anisotropic optical turbulence. A near-plane wave beam was propagated a distance of 1 and 2 km at a height of 2 m above the concrete runway at the Shuttle Landing Facility, Kennedy Space Center, Florida, during January and February of 2017...

Image distortions caused by atmospheric turbulence are often treated as unwanted noise or errors in many image processing studies. Our study, however, shows that in certain scenarios the turbulence distortion can be very helpful in enhancing image processing results. This paper describes a novel approach that uses the scintillation traits recorded...

Scintillation index for a Gaussian beam calculated from data taken over a runway in August 2017 is presented which shows evidence of anisotropic conditions early and late in the day and isotropic conditions during midday.

The turbulence in the atmosphere can strongly affect propagation optical waves. The turbulence is created by a variety of atmospheric conditions. Using a mathematical model the fluctuations the statistical fluctuations and wave parameters are computed.

The usage of long-range optical systems for tracking applications encounters regions of deep turbulence throughout propagation. Such conditions lead to the inability to remain on target for a tracked object due to scintillation. To mitigate this issue, a double pass optical system is utilized as a means of tracking enhanced backscatter (EBS) and th...

The use of hybrid free-space optical (FSO)/radio frequency (RF) links can provide robust, high-throughput communication networks for both commercial and military applications. The free-space optical communication (FSOC) links can be operated at data rates much greater than that of RF links. However, the deleterious effects of the atmosphere can cau...

Utilizing a retro-reflector from a target point, the reflected irradiance of a laser beam traveling back toward the transmitting point contains a peak point of intensity known as the enhanced backscatter (EBS) phenomenon. EBS is dependent on the strength regime of turbulence currently occurring within the atmosphere as the beam propagates across an...

The alternative Bendersky, Kopeika, and Blaunstein (BKB) model of measuring the refractive structure index parameter, Cn(2) has proven to be a reliable, well-used means of quantifying and characterizing the atmospheric turbulence in a given environment. This model relies on various meteorological parameters such as temperature, wind speed, relative...

Mathematical models for a Gaussian-beam wave propagating through anisotropic non-Kolmogorov turbulence have been developed in the past by several researchers. In previous publications, the anisotropic spatial power spectrum model was based on the assumption that propagation was in the z direction with circular symmetry maintained in the orthogonal...

In order to better understand laser beam propagation through the analysis of the fluctuations in scintillation data, images from a 30 frame per second monochrome camera are utilized. 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 at...

The purpose of this research is to evaluate scintillation fluctuations on optical communication lasers and evaluate potential system improvements to reduce scintillation effects. This research attempts to experimentally verify mathematical models developed by Andrews and Phillips [1] for scintillation fluctuations in atmospheric turbulence using tw...

In the conventional Kolmogorov model of turbulence the turbulent fluctuations of the index of refraction are assumed to be statistically homogeneous and isotropic, and there is a specific mathematical form for the power spectral density of the index of refraction fluctuations. Development of the turbulent theory of passive scalar transfer has shown...

Expressions related to the buffer requirements of an optical communication system in atmospheric turbulence are developed from the channel signal fade time statistics. Laser irradiance data were recorded over the course of one day by a receiving aperture of variable diameter at the Townes Institute Science and Technology Experimentation Facility (T...

A 10 Gbps FSO system implements beam tracking, a high dynamic range optical receiver, and a dynamic buffering packet modem. Performance was characterized at the 4.5 km Shuttle Landing Facility at Kennedy Space Center Florida.

Using a three-aperture scintillometer system (TASS) to measure irradiance fluctuations along a slant path, it is possible to create a Cn2 profile model as a function of altitude up to (and possibly beyond) the maximum altitude of a laser beam along the propagation slant path. This technique was demonstrated recently in June 2011 on a beacon beam tr...

In this paper we show evidence of aero-optic effects on the measured beacon beam as the gimbal angle of a nosemounted turret changes from 0 to 90 degrees and greater with respect to the line of flight. Data from the beacon beam was collected with a new technology 3-aperture scintillometer over a 82km to 104km air-to-ground downlink during field tes...

Irradiance data were collected over an air-to-ground path using several different sized receiving apertures. The data were collected from the Optical RF Communications Adjunct (ORCA) tracking beacon. The receiver system consisted of three telescopes of sizes 51 mm, 137 mm, and 272 mm. Probability of fade, number of fades per second, and mean fade t...

The DARPA Optical RF Communications Adjunct (ORCA) program was designed to bring high data rate networking to the warfighter via airborne platforms. Preliminary testing of the ORCA free-space optical communication (FSOC) laser beam was performed on May 12, 2009 by the Northrop Grumman Corporation (NGC) at Patuxent (PAX) River Naval Air Base in Mary...

Irradiance data were collected over a 1km horizontal terrestrial path using several different sized receiving apertures. The data were collected under moderate-to-strong turbulence conditions. The receiver system consisted of a 154mm (6") refracting telescope outfitted with several removable apertures. The path was instrumented with three 3-axis an...

Evaluation of the methods developed by Bendersky, Kopeika, and Blaunstein1 to predict the refractive index structure parameter from the direct measurement of macroscopic atmospheric conditions were investigated. Measurements of ground-level temperature, relative humidity, wind speed, solar flux, and aerosol loading taken by the University of Centra...

The DARPA Optical RF Communications Adjunct (ORCA) program was created to bring high data rate networking to the warfighter via airborne platforms. Recent testing of the ORCA system was conducted by the Northrop Grumman Corporation (NGC) at the Nevada Test and Training Range (NTTR) at the Nellis Air Force Range near Tonopah, NV. The University of C...

The DARPA Optical RF Communications Adjunct (ORCA) program was created to bring high data rate networking to the warfighter via airborne platforms. Recent testing of the ORCA system was conducted by the Northrop Grumman Corporation (NGC) at the Nevada Test and Training Range (NTTR) at the Nellis Air Force Range near Tonopah, NV. The University of C...

The interest for free space laser beam propagation has recently increased due to several experiments. These experiments have shown that optical links through the atmosphere can be effectively established, regardless of the several deleterious negative effects of refractive index fluctuations. The most deleterious effect is the scintillation which,...

Combined RF and optical communication within a heavily scintillated atmosphere requires special modems that can accommodate significant signal fading. A hybrid network (10 Gbps 1550 nm FSO and RF transmission) has been developed and a link quality parameter is used to assist the network routers with the path cost calculation algorithm. Fading stati...

The use of hybrid free-space optical (FSO)/radio-frequency (RF) links to provide robust, high-throughput communications, fixed infrastructure links, and their associated networks have been thoroughly investigated for both commercial and military applications. The extension of this paradigm to mobile, long-range networks has long been a desire by th...

Atmospheric turbulence has been described for many years by Kolmogorov's power spectral density model because of its simplicity. Unfortunately several experiments have been reported recently that show Kolmogorov theory is sometimes incomplete to describe atmospheric statistics properly, in particular in portions of the troposphere and stratosphere....

The most commonly used altitude-dependent model for refractive index fluctuations, over long high-altitude slant paths or ground/space links, is the Hufnagel-Valley model. For the near-ground turbulence portion of the path, this model uses an exponential decay term suggested by Valley to connect ground level turbulence with the original Hufnagel mo...

In this paper we present experimental beam wander measurements and compare them with theory developed from the Kolmogorov model for the atmosphere. Data from a collimated beam over a 1km horizontal terrestrial path were recorded with a high-speed camera. Several sets of image data were taken before, during, and after the quiescent period of the atm...

B-PPM formatting for trans-atmospheric optical communication is compared experimentally to OOK (NRZ) at a single channel data rate of 1.25Gbps in deep fading conditions. Unlike low data rate transmission using M-ary PPM formatting, high-speed B-PPM formatting does not benefit from the theoretical improvement that has been realized at low data rate....

Free-space optical communications (FSOC) has become an important application area because of the increasing need for larger bandwidths and high-data-rate transfer of information that is available at optical wavelengths. In this paper we discuss some of the atmospheric issues that are important to consider in the design of a FSOC link between aircra...

In this paper we discuss some recent results concerning beam wander-induced scintillation of a Gaussian-beam wave along a horizontal path with constant C2n and also from ground to space, the latter based on the Hufnagle-Valley C2n. We consider cases of tracked beams and untracked beams, both of which involve a certain amount of beam wander. Theoret...

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 random fluctuations in the refractive index. Here, a model is developed for the field of a flatt...

Free space laser system performance is limited by atmospheric turbulence that has been described for many years by Kolmogorov's power spectral density model because of its simplicity. Unfortunately several experiments have been reported recently that show Kolmogorov theory is sometimes incomplete to describe atmospheric statistics properly, in part...

It is well know that free-space laser system performance is limited by atmospheric turbulence. Most theoretical treatments have been described for many years by Kolmogorov’s power spectral density model because of its simplicity. Unfortunately, several experiments have been reported recently that show that the Kolmogorov theory is sometimes incompl...

An optical plane wave propagating through atmospheric turbulence is affected by irradiance fluctuations known as scintillation. The scintillation index of an optical wave in strong turbulence can be analyzed by extended Rytov theory, which uses filter functions to eliminate the effect of cell turbulence sizes that do not contribute to scintillation...

It is well known that free space laser system performance is limited by atmospheric turbulence. Most theoretical treatments have been described for many years by Kolmogorov's power spectral density model because of its simplicity. Unfortunately several experiments have been reported recently that show the Kolmogorov theory is sometimes incomplete t...

Intensity fluctuations from a 532nm CW laser source were collected over an outdoor 1km path, 2m above the ground, with three different receiving apertures. The scintillation index was found for each receiving aperture and recently developed theory for all regimes of optical turbulence was used to infer three atmospheric parameters, Cn2, l0, and L0....

Failure of the first-order Rytov approximation to properly predict the scintillation index of a large-aperture focused beam, or an uplink collimated (or focused) beam, has been discussed in several recent publications, which cite beam wander effects as the main reason for this failure. We use computer simulations to examine several aspects of beam...

The performance of lasercom systems operating in the atmosphere is reduced by optical turbulence, which causes irradiance fluctuations in the received signal. The result is a randomly fading signal. Fade statistics obtained from experimental data were compared to theoretical predictions based on the lognormal and gamma–gamma distributions. The prob...

Atmospheric turbulence induces significant variation on the angle-of-arrival of laser beams used in free space laser communication. Angle-of-arrival fluctuations of an optical wave in the plane of the receiver aperture can be described in terms of the phase structure function that already has been calculated by Kolmogorov's power spectral density m...

In this paper we discuss several models for the probability density function (pdf) of the irradiance of a Gaussian-beam wave from ground to space. We consider cases of tracked beams and untracked beams, both of which involve a certain amount of beam wander. The various pdf models that we introduce are all compared with simulation data over a broad...

The lognormal (LN) and gamma–gamma (GG) distributions are compared to simulated and experimental data of the irradiance fluctuations of a Gaussian beam wave propagating through the atmosphere along a horizontal path, near the ground, in the moderate-to-strong turbulence regime. Irradiance data were collected simultaneously at three receiving apertu...

We report on a set of measurements made in December 2005 by researchers from the University of Central Florida, SPAWAR's Innovative Science and Technology Experiment Facility (ISTEF), Harris Corporation, NASA Kennedy Space Center, and Northrop Grumman. The experiments were conducted on the Shuttle Landing Facility (SLF) at Kennedy Space Center (KSC...

First-order weak-fluctuation Rytov theory predicts that the longitudinal (on-axis) component of the scintillation index of an uplink collimated beam will become significantly smaller as the size of the transmitter aperture increases up to around 100 cm. However, the results of recent computer simulations are at odds with this behavior, and we belie...

In February 2005 a joint atmospheric propagation experiment was conducted between the Australian Defence Science and Technology Organisation and the University of Central Florida. A Gaussian beam was propagated along a horizontal 1500 m path near the ground. Scintillation was measured simultaneously at three receivers of diameters 1, 5, and 13 mm ....

In an attempt to mitigate the effects of the atmosphere on the coherence of a 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. We develop a model for the scintillation index of a flatte...

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. Irradiance data was collected simultaneously at three receiving apertures of different s...

Recent studies of ground-to-space beam propagation have revealed significant departures from Rytov theory when the beam diameter w0 is on the order of the atmospheric coherence width r0. It has been shown that such departures from Rytov theory are primarily a consequence of beam wander and other low-order aberrations. In this paper we discuss model...

Interest in high-data-rate free-space optical (FSO) laser communication systems has grown significantly in recent years because of the advantages offered by FSO systems over radio frequency (RF) systems. Most advantages are simple consequences of the short wavelengths associated with optical waves. However, there are some drawbacks that arise from...

The Shuttle Landing Facility runway at the Kennedy Space Center in Cape Canaveral, Florida is almost 5 km long and 100 m wide. Its homogeneous environment makes it a unique and ideal place for testing and evaluating EO systems. An experiment, with the goal of characterizing atmospheric parameters on the runway, was conducted in June 2005. Weather d...

We report on measurements made at the Shuttle Landing Facility (SLF) runway at Kennedy Space Center of receiver aperture averaging effects on a propagating optical Gaussian beam wave over a propagation path of 1,000 m. A commercially available instrument with both transmit and receive apertures was used to transmit a modulated laser beam operating...

In May 2004 a joint atmospheric propagation experiment was conducted between the Australian Defence Science and Technology Organisation, the Office of Naval Research and the University of Central Florida. A 45 mm divergent Gaussian beam was propagated along a horizontal 1500 meter path approximately 2 meters above the ground. At the receiver were 3...

Mean irradiance data from a field experiment conducted jointly by the Australian Defence Science and Technology Organisation, the Office of Naval Research, and the University of Central Florida is presented. The experiment was conducted in May 2004 in Adelaide, Australia. The propagation path was characterized by conditions of moderate to strong ir...

First-order weak-fluctuation Rytov theory predicts that the longitudinal (on-axis) scintillation component of an untracked focused beam projected along a horizontal path will become significantly smaller as the size of the transmitter aperture increases. At the same time, the radial component near the diffractive edge of the beam is predicted to in...

Mean irradiance data from a field experiment conducted jointly by the Australian Defence Science and Technology Organisation, the Office of Naval Research, and the University of Central Florida is presented. The experiment was conducted in May 2004 in Adelaide, Australia. The propagation path was characterized by conditions of moderate to strong ir...

Since publication of the first edition of this text in 1998, there have been several new, important developments in the theory of beam wave propagation through a random medium, which have been incorporated into this second edition. Also new to this edition are models for the scintillation index under moderate-to-strong irradiance fluctuations; mode...

In May 2004 a joint atmospheric propagation experiment was conducted between the Australian Defence Science and Technology Organisation, the Office of Naval Research and the University of Central Florida. A 45 mm divergent Gaussian beam was propagated along a horizontal 1500 meter path approximately 2 meters above the ground. At the receiver were 3...

A new user-friendly software product called the Atmospheric Laser Turbulence Model (ALTM) has been developed at the University of Central Florida and is distributed through Ontar Corporation (www.ontar.com) to provide Gaussian-beam wave propagation models for calculating various beam characteristics in the presence of atmospheric turbulence. The ca...

Free space optical (FSO) communication systems offer several advantages over conventional radio frequency (RF) systems but, because of shorter wavelength, are subject to various atmospheric effects. Particularly significant in this regard is the signal fading below a prescribed threshold value owing primarily to optical scintillations associated wi...

Renewed interest in the propagation characteristics of a partially coherent beam has led to several recent studies that have extended theoretical developments started in the 1970s and 1980s. In this paper we use a model developed by the authors for single-pass propagation of a partially coherent beam and extend it to the case of a Gaussian-beam wav...

Atmospheric scintillation causes fading to a free-space optical communications link. Optical communication links can be improved by the correct application of coding schemes customized to meet the atmospheric conditions. For this paper, we model atmospheric scintillation using a gamma-gamma probability distribution. From the scintillation model, th...

Fading due to scintillation has been measured at path lengths of up to approximately 13.3 Km. Theoretical fading and aperture averaging models based on the gamma-gamma distribution were used to generate predictions of the fade parameters. Data collected at the optical receiver was compared to the theoretical values to determine if the fade theory r...

A laser beam with a different irradiance profile may have favorable scintillation and diffraction characteristics, which are important qualities in the area of optical wireless communication. The propagation in random media (atmosphere) of a laser beam with an annular, donut-shaped, irradiance cross section was examined. Annular beams are created b...

The Molniya orbit can be exploited for communications using laser technology. Since the Molniya orbit repeats, exactly, every 24 hours, such a system will allow high latitude and ground based lasers to use Molniya satellite links much the same way that equatorial located lasers can use geostationary satellites. Thus the benefits of a geostationary...

The effect of a (spatially) partially coherent quasi-monochromatic lowest order Gaussian-beam on the direct detection system is studied in weak and strong atmospheric turbulence. The analytic expression for the scintillation index has been developed and analysed as a function of the spatial correlation distance (lc) and the correlation time (ts), a...

Free space optical (FSO) communication systems offer several advantages over conventional radio frequency (RF) systems but, because of shorter wavelength, are subject to various atmospheric effects. Particularly significant in this regard is the signal fading below a prescribed threshold value owing primarily to optical scintillations associated wi...

Free space optical (FSO) communication systems offer several advantages over conventional radio frequency (RF) systems but, because of shorter wavelength, are subject to various atmospheric effects. Particularly significant in this regard is the signal fading below a prescribed threshold value owing primarily to optical scintillations associated wi...

Analytic expressions for the mutual coherence function (MCF) and the scintillation index of a partially coherent lowest order Gaussian beam wave propagating through the atmosphere (based on Kolmogorov spectrum model) are developed for the pupil plane of a receiving system. Partial coherence of the beam is modeled as a thin (complex) phase screen wi...

Analytic expressions for the mutual coherence function and the scintillation index of the lowest order Gaussian beam as a function of target roughness are developed for a bistatic configuration in weak and strong atmospheric turbulence. Results are based on Rytov theory and Kolmogorov spectrum model. The surface roughness is modeled by a thin compl...

By using a complex phase screen model for a diffuser located at the transmitter, analytic expressions are developed for the scintillation index of a lowest order Gaussian-beam wave in the pupil plane of the receiver in weak and strong atmospheric conditions. The effect of partial coherence on the scintillation index is analyzed as a function of the...

Atmospheric scintillation negatively affects point-to-point laser communication requiring either an increase in power to maintain link throughput rate or decreasing the data rate at a constant power. The effect of atmospheric scintillation on irradiance can be modeled using a gamma-gamma probability distribution. With the model of irradiance known,...

By using ABCD ray matrix theory and a random phase screen located near the source, analytic expressions are developed for the mutual coherence function and scintillation index of a Gaussian-beam wave propagating through weak atmospheric turbulence in both the pupil plane and image plane of a receiving. The phase screen model that we use is based on...

Based on ABCD ray matrix theory and a random phase screen model for the target surface, analytic expressions are developed for the normalized mutual coherence function (MCF) of a reflected Gaussian-beam wave from a finite target in the presence of atmospheric turbulence. This analysis features both pupil plane and image plane expressions and includ...

Laser communication systems offer several advantages over conventional radio frequency (RF) systems but, because of shorter wavelength, are subject to various atmospheric effects. Particularly significant in this regard is the signal fading below a prescribed threshold value owing primarily to optical scintillations associated with the received sig...

We develop a model for the probability density function (pdf) of the irradiance fluctuations of an optical wave propagating through a turbulent medium. The model is a two-parameter distribution that is based on a doubly stochastic theory of scintillation that assumes that small-scale irradiance fluctuations are modulated by large-scale irradiance f...

A scintillation model previously developed by the authors is extended in this paper to the case of a propagating Gaussian-beam wave. As in the previous model, we account for the loss of spatial coherence as the optical wave propagates through atmospheric turbulence by eliminating effects of certain turbulent scale sizes that exist between the scale...

A heuristic scintillation model previously developed for weak-to-strong irradiance fluctuations of a spherical wave is extended in this paper to the case of a monostatic lidar configuration. As in the previous model, we account for the loss of spatial coherence as the optical wave propagates through atmospheric turbulence by eliminating the effects...

For a Gaussian-beam wave incident on a point target, we use a recently developed scintillation theory and the gamma- gamma probability density function to calculate the fade statistics of the echo wave as a function of threshold below the mean irradiance. We also compare the results with experimental data collected by a coherent receiver array over...