This PDF file contains the front matter associated with SPIE Proceedings Volume 7685, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

We extend the theory of the on-axis beam scintillations for the off-axis case beam points for weak and strong scintillation conditions. Theory is based on the parabolic equation for the beam wave propagation and Markov approximation for the calculation of the statistical moments of the beam intensity. We use the Feynman path integral technique and asymptotic analysis to analyze the dependence of the scintillation index on the distance from the beam axis for collimated beams under weak and strong scintillation conditions. Both strong and weak scintillation cases are considered. Beam scintillation index naturally separates in the uniform (axial) and inhomogeneous (radial) components that can be examined separately. Axial component carries most of the diffractive scattering effects whereas the radial component is mostly related to the pure geometrical perturbation. We examine the contribution of the beam wander to the scintillation index at the different parts of the beam cross section and show that beam wander alone cannot account got the expected magnitudes of the scintillation index.

The extended Huygens-Fresnel integral, with a quadratic structure function approximation, was used to derive closed form analytical expressions for the mean irradiance and spot size of higher-order annular Laguerre-Gaussian beams propagating in atmospheric turbulence. Comparisons were made to free space propagation of Laguerre-Gaussian modes, in which the spatial distribution is preserved as the beam spreads due to natural diffraction. It was found that the central void in annular Laguerre-Gaussian beams of any order is filled in as the turbulence strength increases. Higher order modes are less sensitive to the turbulence distortions compared to lower order annular beams, since higher order beams have a smaller turbulence induced fractional increase in their spot size compared to the lower order modes.

In 1966, D. L. Fried developed a theoretical framework for describing the modulation transfer function (MTF) of optical turbulence effects for short-exposure imaging, thereby extending Hufnagel and Stanley's model for longexposure imaging through turbulence. In his analysis Fried assumed the independence of phase perturbations about the mean tilt factor and this tilt factor. In this reanalysis this assumption is deleted, resulting in a new term in the derivation. This term exhibits several new features, including superresolution at high spatial frequencies under moderate turbulence levels, and turbulence strength dependent degradation of the MTF at high turbulence levels.

Turbulent motion is nearly isotropic at spatial scales on the order of a few meters or less, but larger scales, which carry more energy, are anisotropic. It is found that the scales of peak contribution to the total turbulent kinetic energy vary from 50 to 5000 meters depending on stability and elevation above the surface. The scales of peak contribution to the heat flux vary 2 meters to 1000 meters depending on stability and elevation above the surface. The temperature variance shows little contribution at night for scales between a few meters and a few kilometers, while daytime peak temperature variances occur at scales on the order of 100 meters to 2000 meters depending on elevation. Also, the natural coordinate system for the turbulent motion is not aligned with the streamwise, cross-stream, vertical coordinate system and varies with the scale of the motion as well as stability.

One of the major challenges to free space laser communications and ladar is the impact of turbulence on beam propagation, one example of which is signal fading. These impacts can be exacerbated on airborne platforms by turbulence in the vicinity of the laser system aperture and the platform wake. There are a number of strategies to mitigate this, including adaptive optics, active flow control, and various dimensions of diversity: wavelength, polarization, temporal, and spatial diversity. In this paper we will discuss spatial diversity implemented in the focal region of optical telescopes. We will briefly compare this with other methods, describe results of requirements analysis of array features and optical configurations for various atmospheric turbulence states, and suggest several attractive configurations. We will also report on the design and test of one configuration, implemented in a prototype, and tested for noise performance, optical transmission, modulation bandwidth, and BER performance with our dynamic turbulence simulator. Early evidence shows significant BER improvements of several orders of magnitude at high turbulence fluctuation frequencies using this technique.

The Naval Research Laboratory has developed a system to optically "playback" recorded atmospheric scintillation data files. Scintillation data files typically consist of voltage versus time data taken during various outdoor field tests. Optical playback of these files involves modulating laser light in an optical fiber such that the output mimics the fluctuations observed in the outdoor field tests. This capability allows greatly simplified testing of lasercomm hardware and software in a laboratory environment without the need for more expensive and complicated outdoor field tests. This system also enables repetitive testing with the same fluctuations which is extremely useful for optimization of lasercomm system performance.

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 Central Florida weather station were compared against concurrent measurements of the refractive index structure parameter made by Scintec SLS-20 scintillometers positioned near the weather station. Wind measurements were obtained by three, three-axis sonic anemometers (capable of resolving a three-dimensional wind vector) positioned at heights of 1, 1.5, and 2.5 meters above the ground. Temperature measurements were taken at ground level, and at heights of 1 and 1.5 meters. Data were collected for two days atop Antelope Peak, NV. Collection times covered both daytime and nighttime measurements.

In order to track, acquire and maintain a free-space optical link between mobile platforms experiencing misalignment due to movement and atmospheric turbulence requires a different approach than traditional free-space optical transceivers. Recent investigations of alternative receiver configurations found that a lens array performed better than a collimator array as a light collector, and compared methods for summing light collected from different lenses in the array. This paper reports on experimental investigations of receiver performance for a coupling lens array at the front end and an aspheric-lens based summing approach at the back end. The receiver performance is evaluated using two different transmission systems, including a SONET bit-error-rate tester. Key evaluation parameters include the received power and achieved bit-error rate. Measurements are made under perfect alignment and under a variety of misalignment conditions. The investigation finds that size of the lenses used in the array, which dictates the number of collecting fibers used, impacts the effectiveness of the receiver and of the summing approach. Both a single lens and multiple lens solution are evaluated for implementing the summing function. Optimization of the summing optics is required for the receiver to be effective for high data rate communication.

One aspect of the propagation-physics challenge associated with airborne, free-space, optical communications (FSOC), for example, is the characterization and mitigation of link losses due to aero-optic interactions. That is, air-density gradients due to compressibility effects in turbulent boundary layers, separated flows, and freeshear flows can disturb the wavefront in the near field of the transceiver. To better understand these aero-optical mechanisms, a model of a nose-mounted, FSOC transceiver recently was placed in a compressible-flow wind tunnel, and the resulting wavefront degradations, as a function of flow scenario, were recorded. High-speed, time-resolved movies of the aero-optic disturbances have been realized, using a Schlieren-imaging technique, and a very-highframe-rate camera. Discrete, vortical structures (amid otherwise-irregular shedding) were seen to emerge and convect past the clear aperture. The frequencies of these disturbances have been estimated from the movies, and these have been compared with high-speed, time-resolved wavefront reconstructions. Losses of -3.5 dB (for the case of Mach - 0.45 at 10 kft, side view, and λ - 1.55 μm, for example), and disturbance frequencies of - 1200 Hz (and higher) were observed. The system-level impact of the resulting wavefront degradations will be discussed.

The effects of optical turbulence on high energy laser propagation have been well documented. The optical turbulence is typically characterized using the index of refraction structure parameter, C_n². The value and 3-D variation of C_n² can be diagnosed for the surface boundary layer (lowest 50 m of the atmosphere) from values of temperature, pressure, humidity and wind velocity using meteorological similarity theory. Examples of such similarity theory C_n² calculators include the Tunick model for overland applications and the Navy Surface Layer Optical Turbulence (NSLOT) model for ocean scenarios, both of which are implemented in the AFIT CDE's (Center for Directed Energy) HELEEOS (High Energy Laser End-to-End Operational Simulation) and LEEDR (Laser Environmental Effects Definition and Reference) models where they can be assessed from 400 nm to 8.6 m, continuously. The HELEEOS model further allows for the calculation of the irradiance from within a HEL beam that is scattered by molecules and particulates in the atmosphere to an off-axis observation point, while incorporating the spreading effects of the turbulence and thermal blooming. Field and laboratory experiments conducted at Wright-Patterson AFB, Ohio in summer 2009 allowed for validation measurements for the surface layer optical turbulence and off-axis algorithms to be collected. Turbulence strength measurements were made at a wavelength of 1.55 μm using a state of the art bistatic turbulence profiler for both horizontal and vertical paths. Pressure, wind speed, wind direction, relative humidity and aerosol loading data were collected simultaneously with the C_n² measurements. As part of the experiment, the profiler's beams were imaged offaxis with a calibrated camera array and the received irradiance of the off-axis scattering was quantified. Characterization of the aerosol distribution along the laser path and the path to the observer is accomplished by determining the visibility and climatological aerosols for southwestern Ohio. Comparisons between predicted and measured C_n² and off-axis irradiance are made. Additionally an experimental technique to derive C_n² measurements from weather radar is described and compared to the turbulence profiler data.

Experiments involving laser communications between an optical ground station and a low earth orbit (LEO) satellite were successfully conducted from 2006 to 2009. The optical ground station is located in Koganei, Tokyo, and was developed by the National Institute of Information and Communications Technology (NICT), Japan. Four laser beams were transmitted from the optical ground station to the LEO satellite in order to reduce the intensity of the fluctuation of the optical signal caused by atmospheric turbulence. The frequency characteristics of the downlink are evaluated on the basis of theory and the measurements, where the sampling rate was 20 kHz. The speckle patterns were averaged and the frequency response of the received optical signal was filtered by the telescope aperture. The basis of the temporal aperture filtering function is derived and compared with the measured results.

The performance of a free-space optical (FSO) communications link in a maritime environment was evaluated during two field trials conducted off the mid-Atlantic coast near Wallops Island, VA, in July and September 2009. Bidirectional, ship-to-shore data links operating at 2.5 Gbps and utilizing commercial, single-mode adaptive optics terminals were set up between a lookout tower located on Cedar Island, VA and a Johns Hopkins University Applied Physics Laboratory research vessel over a range of 2-22 km (optical horizon). This paper presents the test configuration, discusses the overall performance of the FSO channel, and compares it to the available turbulence and weather measurements. Additionally, modeling of the link configuration is presented, and comparisons are made to data measured throughout the experiment such as received power, received beam diameter, and local The performance of a free-space optical (FSO) communications link in a maritime environment was evaluated during two field trials conducted off the mid-Atlantic coast near Wallops Island, VA, in July and September 2009. Bidirectional, ship-to-shore data links operating at 2.5 Gbps and utilizing commercial, single-mode adaptive optics terminals were set up between a lookout tower located on Cedar Island, VA and a Johns Hopkins University Applied Physics Laboratory research vessel over a range of 2-22 km (optical horizon). This paper presents the test configuration, discusses the overall performance of the FSO channel, and compares it to the available turbulence and weather measurements. Additionally, modeling of the link configuration is presented, and comparisons are made to data measured throughout the experiment such as received power, received beam diameter, and local C_n².

The Office of Naval Research has begun a five year technology development program in high bandwidth lasercom. The program is designed to mature enabling technologies and demonstrate a notional high bandwidth capability to the warfighter. This is a spiral development program, with increasingly automated, capable, and compact systems being demonstrated at the end of each phase. Both direct lasercom links and modulating retroreflector links will be matured during this program.

The Defence Science & Technology Organisation (DSTO), in collaboration with the US Naval Research Laboratory (NRL), has performed long distance experiments on analogue modulated free space optical links across Chesapeake Bay, Maryland. In the present work, pulse frequency modulation was used to transmit audio signals over a distance of 32 km (folded path across the Bay). Still images were transmitted using slow scan television (SSTV) techniques, and a novel technique to decrease the transmission time of SSTV images is presented.

Effects of beam wander on uncoded bit-error-rate (BER) of direct-detection OOK modulated FSO communication systems using collimated and focused Gaussian beams are studied. Channel fading statistics are obtained from large-scale wave optics simulations and compared with the closed-form log-normal and gamma-gamma models. The avalanche photodiode (APD) is chosen for photodetection. The accurate McIntyre-Conradi APD model is adopted for performance evaluation. Results show that large performance gain (more than 15dB) can be achieved with fast-tracked focused beams. The upper bound of higher-order adaptive optics gain beyond tracking gain is also studied.

The outdoor 3.2km, partially over water, turbulence measurement and monitoring communication link has being developed with the goal to statistically describe atmospheric turbulence using results derived from the experimentally collected data. The system described in this paper has two transmitters and a receiver. The transmitter side is equipped with the laser and the bank of 20 horizontally, in-line mounted LEDs. The receiver side consists of the two-channel receiver allowing performing simultaneous wavefront sensor and point spread function measurements.The data collected from both channels are used for the Fried parameter estimations. In this paper we emphasize out attention on the data collection and analysis via point spread function channel only. The results presented in this paper are based on the 6Tb of data collected through 40 days time interval, and under various day and night atmospheric conditions.

Laser weapon systems comprise of tiled subapertures are rapidly emerging in the directed energy community. The Air Force Institute of Technology Center for Directed Energy (AFIT/CDE), under sponsorship of the HEL Joint Technology Office has developed performance models of such laser weapon system configurations consisting of tiled arrays of both slab and fiber subapertures. These performance models are based on results of detailed waveoptics analyses conducted using WaveTrain. Previous performance model versions developed in this effort represent system characteristics such as subaperture shape, aperture fill factor, subaperture intensity profile, subaperture placement in the primary aperture, subaperture mutual coherence (piston), subaperture differential jitter (tilt), and beam quality wave-front error associated with each subaperture. The current work is a prerequisite for the development of robust performance models for turbulence and thermal blooming effects for tiled systems. Emphasis is placed on low altitude tactical scenarios. The enhanced performance model developed will be added to AFIT/CDE's HELEEOS parametric one-on-one engagement level model via the Scaling for High Energy Laser and Relay Engagement (SHaRE) toolbox.

During the second half of the '70's Ben-Shalom et al.1 developed a system for research of the spectral transmittance of the atmosphere in the infrared range on nearly horizontal atmospheric paths between 2.5 and 14 microns. The system configuration is bi-static: a source (of known emission) and a measuring sensor station linked by line of sight. Comparison of the measured radiation with the known emission gives the fraction of transmitted radiation. The longest path published there was a respectable 44 Km. and provided significant data. However, the system required a very high temperature (2400K) short lived (~3 hours) emitter element, home-built projection optics, and a cumbersome closed cycle water cooling system for the 6 KWatt source. The sensor end of the system was a Circular Variable Filter (CVF) based spectroradiometer using liquid nitrogen (LN2) cooled detectors. The signal processing electronics was based on a synchronous detection method using a chopper at the source and a radio-transmitted reference signal in phase with the chopper. In this paper we describe the Atmospheric Transmittance Measurement System (ATMS) recently built by CI and presently undergoing reliability and accuracy tests. Its main advantages over the old system are: it is built of only commercial off-the-shelf items (COTS), it can measure in both the IR and visible ranges simultaneously, it is cost effective and easy to use and maintain. The calibration method and transmittance measurement algorithm are also described. A transmittance measurement of a nearly horizontal sea level path of 6.5 Km. is shown here as an example for the whole spectral range of 0.4 to 14 microns in less than very clear and dry weather conditions. The result shows agreement with the MODTRAN model on the spectral behavior but at the time of publication we are still investigating the accuracy of the actual transmittance values. We believe that the ATMS can be used for longer paths, based on the signal to noise ratio encountered at 6.5 Km. In addition and in contrast to the old '70's system, which could be used only for measurements above 1 Km. paths, the ATMS is built and calibrated so that it can measure transmittance through short paths of few tens of meters.

A common assumption used in describing the turbulence structure present during atmospheric propagation is turbulent isotropy. Yet low frequency fluctuations obviously cannot exhibit this property since vertically oriented fluctuations are impacted by surface boundary conditions. To gain insight into the anisotropic nature of the surface boundary, a measurement campaign was carried out in the spring of 2008 at White Sands Missile Range, NM. A 2D array of 3D sonic anemometers was constructed. This array, a 14m wide by 10m high grid of sensors was used to sample wind and temperature fluctuations over a two-month period. More than 300 hours of data were collected that fit the criteria of sensor availability, mean winds directed from a desired sector, and steady wind speed. This paper describes the grid, the overall experiment, and considers aspects of correlations present in the temperature data fields that may be useful in characterizing an anisotropic refractive index structure function.

We analyze some recent publications addressing propagation of the partially coherent polarized beams and beam arrays in the turbulent atmosphere. We show that the published results are limited to the scalar propagation model, and are not particular to the beam polarization. Therefore these results are equally relevant for the scalar beam pairs and arrays discriminated by some parameters such as small frequency shift, time delay or geometry, but not necessary the polarization. We use the virtual incoherent source model to derive the general form of the mutual coherence function of the two Schell-type beams. We discuss some physical stochastic models that result in the creation of the Schell-type beams and beam arrays. New classes of the uniformly, nonuniformly and nonlocally coherent beam pairs emerge naturally from this analysis. Rigorous, Markov approximation-based, propagation model provides relatively simple analytic results for the second-order moments of the optical field of the partially-coherent individual beams and beam pairs. We examine the changes of the beam mutual coherence in the process of the free-space propagation and propagation through the turbulent atmosphere.

The orbital angular momentum of photons in paraxial beams offers the possibility of arbitrary base-N digits for freespace laser communications. Atmospheric turbulence can cause the orbital angular momentum of photons in a propagating beam to scatter from its original azimuthal mode. The probability of obtaining correct or incorrect measurement of the transmitted orbital angular momentum state after propagation through atmospheric turbulence is calculated from the rotational field correlation (second order field moment). A previously published model of the rotational field correlation for Laguerre-Gaussian beams is limited to the weak turbulence regime and assumes that the turbulence effects can be considered a pure phase perturbation. This model is validated by calculating the same quantity with the extended Huygens-Fresnel integral, valid in all regimes of turbulence. To obtain closed form expressions, a quadratic structure function approximation was applied. The probability of receiving the transmitted orbital angular momentum state was calculated and compared to the existing model. The results indicate that the quadratic structure function approximation leads to a slight overprediction of the probability in the weak turbulence regime. For finite transmitter apertures, the previously published model, with a spherical wave structure function, rather than the plane wave structure function used in the original work, is believed to be the most accurate model in the weak turbulence regime.

In the paper the description of the last version of code MaexPro (Marine aerosol extinction Profile) for calculation spectral and vertical profiles of aerosol extinction coefficient α(λ), aerosol sizes distribution, area distribution, volumes distribution, modes aerosol extinction spectra is submitted. Code MaexPro is a computer program under constant development to estimate of EO systems signal power at a location place in which a fetch is key entrance parameter. The program carries out calculation α(λ), as functions of atmospheric effects using standard meteorological parameters, aerosol microphysical structure, a spectral band and a height of the sensor location. Spectral behavior α(λ) can be submitted as graphically, and as tables. Commands overplot for superposition or change of figures; profiles extrapolation; a lens; all kinds of possible copying; the data presentation, convenient for an input in code MODTRAN, and etc. are stipulated. The code MaexPro is a completely mouse-driven PC Windows program with a user-friendly interface. Calculation time of spectral and vertical profiles of α(λ) depends on the necessary wave length resolution, radius of aerosol particles and the location place height, and does not exceed tens seconds for each new meteorological condition. Other calculations characteristics, such as aerosol sizes distribution, area distribution, volumes distribution, modes aerosol extinction spectra, are performed in a few seconds.

The indirect way using meteorology factors to estimate the atmospheric turbulence has been taken to describe the characteristic of atmospheric turbulence in marine surface layer in large spatial and temporal scale as for the limitation of direct observation method. The algorithm based on the fluctuation of temperature wind speed and humidity offered by friehe is taken into account, the data we selected relative to this research is from 10° to 21° of north latitude and 110° to 120° east longitude time span from 1965 to 1997. Firstly, we treat all the data of 33 years an average for every moment according to observing point, in this way the totally average of all the routine meteorology factors of typical marine surface layer are obtained, then we make a statistic for air sea temperature differences (ASTD) by month, and 12 groups of data about fluctuation of ASTD are obtained, We chose the maximum and minimum month (December and March) in the fluctuation of ASTD to indicate the spatial distribution of the strength of atmospheric turbulence. It turns out that the range of mean turbulence is from 10^-17 to 10^-14, there are significant seasonal changes about atmospheric turbulence in marine surface of South China Sea, in general turbulence is much stronger in winter than it in summer, and in seacoast the turbulence is stronger than it in mare liberum, it may be related to the effect of land frontier to wind speed.