The Chesapeake Bay Detachment of the Naval Research Laboratory (NRL-CBD) provides an ideal environment for characterizing the effects of the marine atmosphere on free space optical communication links. The site has recently been converted to an operational 10 mile (16.2 km) one-way test range to collect information on propagation statistics in a variety of atmospheric conditions. The results presented here compare the contributions of thermal gradients across the bay to the variations in intensity scintillations across the bay.

Humidity and C²_n data collected from the Chesapeake Bay area during the 2003/2004 period have been analyzed. We demonstrate that there is an unequivocal correlation between the data during the same time periods, in the absence of solar insolation. This correlation manifests itself as an inverse relationship. We suggest that C²_n in the infrared region is also function of humidity, in addition to temperature and pressure.

April 2005, a laser propagation experiment was conducted over a 470m horizontal maritime path. Scintillation measurements of a divergent Gaussian beam wave were taken simultaneously for different receiver aperture sizes. Terrestrial scintillation theory combined with a numerical algorithm was used to infer the atmospheric parameters C_n² and l_o from the optical maritime scintillation measurements. This paper presents the initial results.

NRL's Chesapeake Bay lasercom test facility (LCTF) offers a variety of ranges for researching free-space optical laser communication (FSO lasercom) links in a maritime environment. This paper discusses link performance over the 16 km one-way range at the LCTF. There are several methods to determine the link quality in FSO lasercom. Bit-error-rate (BER) testing and packet testing are two possible methods. Since errors generally tend to occur in bursts in FSO channels, packet testing may offer a better indication of the quality of service (QoS) rather than BER testing. Link performance measured via packet testing is being investigated in a variety of atmospheric conditions. Results of these experiments will be presented.

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 modeling of the probability density function (PDF) for uplink beams. In particular, we show how the PDF transitions from lognormal statistics when w₀/r₀ << 1 to the negative exponential distribution when w₀/r₀ >> 1. The most interesting regime is the transition region near w₀/r₀ = 1, where the statistical behavior of tracked and untracked beams differs significantly.

Low-order turbulence effects dominate the random irradiance fluctuations in a weakly-scintillated Gaussian beam subject to a certain set of initial conditions, leading to a natural departure from log normal irradiance behavior. This departure conflicts with earlier theoretical studies of weakly scintillated beams which have traditionally assumed log normal behavior. The dominance of low order effects leads to an increase in the theoretical scintillation and probability of fade relative to predictions based on the assumption of log normal behavior. This paper recounts a detailed derivation of a low order turbulence model that successfully captures the non-log normal behavior, and reviews theoretical scintillation and probability of fade predictions that follow from the model.

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 size. Atmospheric parameters were inferred from the measurements and used to reproduce the experimental data with numerical simulations and calculate the parameters for the theoretical probability density functions (PDFs). The simulation values agree well with the experimental data for all three aperture sizes, while the support for the theoretical PDFs depends on the size of the receiving aperture.

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 flattened Gaussian beam and compare this with that of the standard TEM₀₀ Gaussian beam. We verify our results by comparison with a computer simulated model for the flattened beam.

We modelled a whirlwind in the atmosphere with the aid of system of the linear [1] and weakly nonlinear 3D system of the Navier-Stokes equations. We investigated Gaussian beam propagation through the modelled whirlwind in the atmosphere. Parabolic equation method has been applied for study of the intensity variations of the beam. We investigated the evolution of the whirlwind velocity field and laser beam propagation through it. The examples of the distorted laser beam are presented as the images in 2D plane. A new method was set forth to the compensation of random distortions. The method was applied for compensation of distortions of a laser beam propagating through the whirlwind in atmosphere. Results may be applied for the compensation distortions of images from modern optical telescopes, in targeting problem and even for control of laser beam focusing on object-target in the case of the random turbulent medium.

We present an analysis of atmospheric propagation effects on current and proposed imaging systems. Effects such as extinction, resolution, beam spreading, anisotropy, and heterodyne efficiency are discussed. We also discuss a new DARPA program that will use atmospheric turbulence to an advantage to achieve super-resolution imaging. This super-resolution imaging phenomenon exploits a micro-lensing effect caused by atmospheric turbulence through use of a fast camera and signal processing to achieve better-than-diffraction limited resolution.

Atmospheric propagation degradation effects including attenuation, aerosol scattering and turbulence have a great impact on the performance of optical systems. Relevant military optical systems include active and passive imaging for target recognition, free-space optical communication and DIRCM/EOCM. This paper will report on experimental work including measurement of retro signals at 1.5 and in the 3-5 μm wavelength regions for evaluation of retro communication links and DIRCM performance. Imaging experiments using a range-gated system both in the active and passive mode at 1.5 μm, will also be carried along the same paths. A dedicated target box and test targets have been fabricated for mounting on a mast at 8 km from our laboratory. The box contains reflectors and receivers in different slots each of which can be opened by a telephone call. A heated target on top simulates a point target in the IR region. The test targets are aimed for the range-gated imaging system. Preliminary experimental data will be presented and discussed.

The Pacific Northwest National Laboratory has developed a remote-sensing LIDAR system designed to detect trace chemicals in the atmosphere. Atmospheric optical turbulence is the largest noise source for the system, causing both fluctuations in the returned signal strength and signal loss due to laser beam break-up and wander. Field experiments have been conducted over the past few years in an effort to better understand the impact of atmospheric turbulence and develop strategies for improving the system. Studies have focused on the propagation of infrared laser beams at 1.278 and 9.56 micrometers over double-pass, horizontal path lengths ranging from 2 to 10 kilometers roundtrip under a variety of turbulence conditions. In addition, numerical simulations of our experimental setup have been developed to complement the experimental work. A comparison of results from the simulations with those from the field experiments shows reasonable agreement. Therefore, similar simulations will be used to aid in the design of a next-generation system.

A set of Risley prisms has been designed that will allow an infrared optical and radio frequency (RF) beam to be simultaneously steered through the same aperture (patent pending). Risley prisms have the advantage of allowing such beams to be steered at the skin of a vehicle such as an aircraft. By providing a mechanism for both of these bands to be steered simultaneously, substantial system size, weight, and power reductions result, making integration of these two bands onto any platform more attainable. The prisms are made of single-crystal silicon, and have special coatings applied to them that reduce surface reflections for optical signals. Additionally, an impedance matching dielectric is applied to the prisms that allow efficient transmission of RF signals. The prisms are mounted in direct drive rotary motors with position feedback resolution of 6 microradians. A set of these prisms can steer beams within a conical field of regard of 120°, has a weight of about 6kg, and a size of about 175mm x 175mm x 100mm. We present some preliminary characteristics of this device, including C-band (~1550nm) optical and Ka RF (38GHz) throughput of the beams through the AR-coated and impedance-matched silicon, and an overview of Risley prism beam steering methodology.

We present results of propagation measurements (over 545 m and 5.7 km paths) carried out with a pulsed laser beam (12 ns pulse width) at a wavelength of 3.7 um under sunny and cloudy atmospheric conditions. The objective of the study was to evaluate, for these distances, the effects of atmospheric turbulence on the deposited power levels achievable on targets having diameters of a few tens of mm. Measurements of scintillation levels, probability distributions, and durations of intervals where the power level stays continuously above or below its mean value were carried out. The results show that, for all atmospheric conditions encountered, the lognormal probability distribution can be used to correctly describe the probability of obtaining some given power levels. Also, the statistics of continuous time intervals spent above and below the mean power are shown to follow an exponential probability distribution. Experimental results relating the mean durations of these intervals to the scintillation levels, the wind speeds, and the probability distribution of collected power levels are also presented. These results could help in determining the source power required to ensure a given level of exposure on targets with a pre-determined probability, taking into account the reaction times of the targets.

In this talk we will present a new zonal wavefront sensor. The device is able to take a single incident beam and diffract it into multiple beams with different focal locations, according to the presence and strength of particular aberrations. The Zernike terms in the beam are simply read out according to the location of the foci on the image plane CCD. We have created a hologram design such that the locations of the beams can be quickly and easily determined, providing all necessary information to correct for these aberrations, so that the wavefront information is derived without the need for any computations. It is a very compact and practical device that could potentially have applications for the USAF airborne laser or atmospheric distortion characterization.

This paper describes a method for real-time 'mapping' of atmospheric turbulence. This can be achieved through excitation of a Rayleigh laser-guide star and coherent detection of the wave-function in the back-scattered light. Detailed information about this function is essential for establishing efficient performance of an adaptive optics system and estimation of atmospheric turbulence parameters such as Fried radius, Rytov parameter, and isoplanatic angle. Amplification of low intensity back scattered light can be achieved by coherent detection based on multi-beam interaction in a non-linear medium. The wave-function of the scattered light can then be recorded and reconstructed with phase-shifting interferometry. The proposed method offers a new tool for wavefront assessment in beam control systems for high energy lasers and other applications.

Mesospheric sodium guidestar radiance is plotted vs. wavelength, fasor power, fasor polarization and date. Peak radiance for circular polarization was about 7000 photons/sec/cm² (V₁ magnitude = 5.1) for 30 watts of pump power in November of 2005. Pumping with circular polarization at high power produces about 2 times more return than linear polarization. Pumping D2a at high power produces about 12 times more return than pumping D2b. A lidar equation is used to determine column density. Estimated maximum possible guidestar radiance is about 3 times greater than measured values. Guidestar radiance may be saturated by atoms becoming trapped in F'=1 and atomic recoil.