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

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 aircraft at large separation distances, including aero-optical effects around the aircraft itself.

The use of high spatial resolution imagery of the far-field irradiance distribution of a 532nm laser is being used to confirm the spatial power spectrum of atmospheric turbulence under a range of different turbulence conditions. The analysis of the temporal and spatial evolution of this irradiance distribution, using a fast framing camera, aims to validate phase screen models and provide an alternate means of measuring a path integrated inner scale. This process will enable the quantification of the impact of boundary layer atmospheric turbulence upon the long-range calibration of Electro-Optic imaging systems and laser communication systems.

Most system analyses of CW high-power lasers propagating in the atmosphere assume a simple additive linear relation of the impact of thermal blooming and optical turbulence in the atmosphere to the propagated laser beam spreading. In other words, both effects are treated as if they would follow Gaussian statistics in an RMS sense. While the statistics of optical propagation in a turbulent atmosphere can be modeled as Gaussian to first order, thermal blooming is a deterministic nonlinear optical phenomenon. To the best of our knowledge, there is no reason for adding linearly the beam spreading due to these two optical effects. In fact, assuming no interplay in the presence of a strong nonlinear optical interaction is counter-intuitive. As a result, we have performed extensive numerical Monte-Carlo optical wave-propagation simulations, >50,000 realizations, in the presence of thermal-blooming and atmospheric turbulence to varying degrees. During the propagation, the amplitude and the phase of a high power laser field are coupled by the interplay of diffraction, refractive turbulence and thermal blooming. In some cases, we have observed in our numerical experiments a strong coupling between turbulence and nonlinear thermal blooming.

Branch points have been shown to cause problems for adaptive optics (AO) systems which attempt to correct for atmospheric distortion over mid-to-long range horizontal paths. Where branch points (or singularities) occur, the phase of the optical wavefront is undefined and cannot be reconstructed by conventional wavefront reconstruction techniques. Branch points occur in pairs of opposite sign (or rotation) and are joined by wavefront dislocations called branch cuts, which have a 2π jump in phase across them. The aim of the project is to construct a branch point sensitive wavefront reconstructor using a Shack Hartmann wavefront sensor which can be used on a 3km line-of-sight (LOS) free space optical (FSO) communications system currently being tested within our group. The first step in our method is to detect the positions of singularities using the branch point potential method first proposed by LeBigot and Wild. The most common zonal reconstruction method used (the least squares reconstructor) is not sensitive to branch points and different methods are being investigated for this part of the project. Results for the detection of singularities using the branch point potential method in simulations are shown here. Some early results for the reconstruction of branch point affected wavefronts are also presented.

We present a simulation of a 200 km air-to-air link in the presence of aero-optical boundary layers. The boundary layer is shown to be the dominant phase aberrator. The random tilt content in the boundary layer is minimal, which reduces the performance gain of a fast steering mirror. Higher order adaptive optics are shown to provide a significant performance improvement provided it can run at high enough bandwidths.

Atmospheric effects deleteriously impact free space laser communications. Beam wander, distortion and beam bending can affect pointing and tracking in particular. Mirages are an example of these effects. In June 2006, a campaign was conducted across the Chesapeake Bay by the Naval Research Laboratory to quantify effects of mirages at the marine layer. We imaged a series of lights positioned strategically on a tower across the bay, at Tilghman Island, approximately ten miles away from NRL's Chesapeake Bay Detachment (NRL-CBD). Recorded images were subject to displacement and distortion as functions of temperature, humidity, dew point, and other meteorological parameters. Results from the experiment will be presented and phenomenology discussed.

The Georgia Tech Research Institute (GTRI) has developed a new type of LIDAR system for monitoring profiles of atmospheric refractive turbulence. The system makes real-time measurements by projecting a laser beam to form a laser beacon at several successive altitudes. The beacon is observed with a multiple-aperture telescope and the motion of the beacon images from each altitude is characterized as the differential image motion variance. An inversion algorithm has been developed to retrieve the turbulence profile. GTRI built a brassboard version of the LIDAR instrument and tested it in October and December 2007, with truth data from scintillometers and from balloon-borne microthermal probes. The tests resulted in the first time-height diagram of the strength of turbulence ever recorded by a LIDAR.

Shannon's Channel Capacity has long been an elusive and merely theoretical goal, with real code structures and hardware implementations providing performance relatively far from the limit. With the somewhat recent advent of near-capacity-achieving codes, however, we can now actually use capacity calculations as practical metrics in optical link designs. In link budget calculations, we can explicitly show losses with respect to capacity that are directly traceable to engineering choices such as sub-optimum code rate selection, sub-optimum code structures, sub-optimum decoding architectures, and other effects. In this way, engineering elements of modulation and coding design can be compared equally with compromises in optics, tracking, and so on. We can further use capacity calculations to predict performance in fading channels, the bane of atmospheric and imperfectly tracked optical links. Such analysis suggests structures using coding and possibly interleaving that can get very close to the optimum performance. In fact, performance should be related to the average fade depth and not the deepest fades.

An algorithm for haze determination was developed based on the atmospheric optical properties to determine the concentration of particulate matter with diameter less than 10 micrometers (PM10). The purpose of this study was to use digital camera images to determine the PM10 concentration. This algorithm was developed based on the relationship between the measured PM10 concentration and the reflected components from a surface material and the atmosphere. A digital camera was used to capture images of dark and bright targets at near and far distances from the position of the targets. Ground-based PM10 measurements were carried out at selected locations simultaneously with the digital camera images acquisition using a DustTrak^TM meter. The PCI Geomatica version 9.1 digital image processing software was used in all imageprocessing analyses. The digital colour images were separated into three bands namely red, green and blue for multi-spectral analysis. The digital numbers (DN) for each band corresponding to the ground-truth locations were extracted and converted to radiance and reflectance values. Then the atmospheric reflectance was related to the PM10 using the regression algorithm analysis. The proposed algorithm produced a high correlation coefficient (R) and low root-meansquare error (RMS) between the measured and estimated PM10. This indicates that the technique using the digital camera images can provide a useful tool for air quality studies.

This study quantifies the variability in ship defense high energy laser (HEL) and high power microwave (HPM) performance due to atmospheric effects in the marine boundary layer relative to a commonly defined standard atmosphere. The atmosphere effects are defined using the worldwide probabilistic climatic database available in the High Energy Laser End-to-End Operational Simulation (HELEEOS) model. The expected propagation performance is assessed at 4 wavelengths (1.0642 μm, 2.141 μm, 3.16 mm, and 12.2 cm) across the world's oceans and mapped on a 1° × 1° grid. Scenarios evaluated are primarily near-surface and horizontal over ranges up to 9000 meters in which anticipated clear air aerosols and thin layers of fog, light rain, and various cloud types occur. Seasonal (summer and winter) and boundary layer variations for a range of relative humidity percentile conditions are considered to determine optimum employment techniques to exploit or defeat the environmental conditions. Optical turbulence impacts and numerous atmospheric particulate/hydrometeor distributions are evaluated based on their wavelength-dependent scattering and absorption effects on HEL/HPM engagement. HELEEOS includes a fast-calculating, first principles, worldwide surface to 100 km, atmospheric propagation and characterization package. This package enables the creation of profiles of temperature, pressure, water vapor content, optical turbulence, atmospheric particulates and hydrometeors as they relate to line-by-line layer transmission, path and background radiance at wavelengths from the ultraviolet to radio frequencies. Physics-based cloud and precipitation characterizations are coupled with physically correct temperature and moisture vertical lapse rates to create realistic atmospheric boundary layer effects. HELEEOS characterizes maritime aerosol environments using the Advanced Navy Aerosol Model (ANAM) or various representations of maritime particulates from the Global Aerosol Dataset (GADS). In the lowest 50 m, HELEEOS defines maritime optical turbulence with the Navy Surface Layer Optical Turbulence (NSLOT) model.

It is well-known that free space optical communications through a turbulent atmosphere are adversely affected by scintillation noise. This paper reports on the experimental demonstration of a two-colour common mode rejection technique to mitigate atmospheric scintillation noise. Real-time equalisation was achieved for both analogue (amplitude modulated PAL composite video) and digital (quadrature amplitude modulated) signals.

Quantum cryptography is a new technique for transmitting quantum information. The information is securely transmitted due to the laws of physics. In such systems, the vehicle that transfers quantum information is a single photon. The problem with using photons is that the transmission distance is limited by the absorption of the photons by the optical fiber along which they pass. The maximum demonstrated range so far is approximately 100 km. Using free-space quantum cryptography between a ground station and a satellite is a possible way of sending quantum information farther than is possible with optical fibers. This is because there is no birefringence effect in the atmosphere. However, there is a complication in that the directions of the polarization basis between the transmitter and the receiver must coincide with each other. This polarization changes because the mobile terminals for free-space transmission continuously change their attitudes. If the transmission protocol is based on polarization, it is necessary to compensate for the change in attitude between the mobile terminals. We are developing a scheme to track the polarization basis between the transceivers. The preliminary result is presented.

In order to iteratively improve an atmospheric optical communications system, a benchtop method of testing its behavior must be developed. We have developed a simple table top source of turbulence and instrumented it to accurately measure the current turbulent state. With this tool, the development of optical communication subsystems such as the Boeing Optical Communications Receiver Array (OCRA) can be optimized in the laboratory before they are tested in the atmosphere. The Tabletop Turbulence Generator is uses a small, low speed wind tunnel with a grid of heated rods at the head of the test section. This creates a turbulent flow with a characteristic scale of ~6-30 mm and maximum angular disturbances of ~.1 milliradians. This is a reasonable scale for 1 and 2 inch optical systems. A crucial element in this Turbulence Generator is instrumentation to allow us to measure the turbulent state. We have implemented a Shack-Hartmann sensor operating at 30 Hz and a Malley probe providing time resolved data along two lines of sight with a time resolution of .1 milliseconds. We have used this system to characterize the performance of communications receivers and will present this as a pilot study of the Turbulence Generator's performance.

In this work, we investigate possible designs for a free-space infrared optical receiver for use in a hybrid optical-RF network with airborne platforms. Due to the relative instability of the platforms compared to traditional building placements, the receiver must possess much greater tolerance to misalignment with respect to the receiver. The limited size and payload of the airborne platforms restricts the design process. We present the results of both experimental and theoretical studies of a range of receiver designs, including single lens receivers with fiber bundles for light collection and multiple lens receivers with bundles coupled to each lens. The results show that the combination of a short focal length lens coupled to a bundle constructed from large core fibers provides the best tolerance to both angular and transverse misalignment. This result, combined with the need to collect light from a relatively large area to meet link budgets, favors a design comprised of multiple lenses of shorter focal length and smaller size, with a small bundle coupled to each lens.

Over a thousand measurements of optical refraction across the Chesapeake Bay were made between November 1999 and June 2001. A survey station was placed on the western side of the bay south of Chesapeake Beach MD and used to view lights on Tilghman Island MD on the eastern side at a range of 16.2 km. In addition, the survey station viewed buoys at ranges of 690, 1420, 2050, 2790, 3440, and 4180 m. The heights for the survey station measurements were 2.3, 3.5, 5.4, 12.7, 27.7, and 37 m above the mean water level and the lights were located at 4.7, 9.5, 13.7, 27.0 m above the water level. When weather and work schedules permitted, observations were made twice a day (just after sunrise and at noon). Survey station measurements of the closest buoy are used to estimate the water level. Observations of air-sea temperature difference measured by the mid-bay buoy in the Chesapeake Bay Observing Station (CBOS) are compared with the elevation angles. The elevation angles and air-sea temperature differences (ASTD) are analyzed over an annual cycle. The elevation angles and the non-dimensional curvature of the refractive rays are compared with the ASTD.

The use of 1.55μm lasers for free space optical links has become well established. While one expects there to be advantages to operating at 3.6μm, namely higher transmission through water vapor and reduced scintillation, the availability of both lasers and detectors at the mid IR wavelength is not as mature as those available at the telecomm choice of 1.55μm. However, there are potential schemes for using frequency conversion to probe the atmosphere in the mid-ir but to detect back in the near-ir. A sequence of experiments has been conducted, over a 16km one-way link across the Chesapeake Bay, to directly compare the intensity variances and the power spectrum imposed by the atmosphere at the two wavelengths. An interband cascade laser was used which operated at a wavelength of 3.6μm and had an output power of 100mW. The 1.55μm system used standard telecomm parts. Data were recorded simultaneously from both systems at 4kHz and were digitized using a 16-bit card. A telescope measured the angle-of-arrival variance of the 1.55mm beam in order to give a measure of the atmospheric structure constant C_n². In addition a visibility monitor and weather station were usually operational at each end of the link.

The US Naval Research Laboratory has undertaken a multi-year research effort in free space optical communications. The goals of this research are to understand and quantify the propagation of optical beams through the maritime atmosphere, assess the global maritime availability and performance of Naval lasercomm, and research methods to improve the robustness of Naval lasercomm links. In support of these goals, NRL has continued to add monitoring and testing capabilities to the Lasercomm Test Facility (LCTF) at NRL-Chesapeake Bay Detachment (CBD). The LCTF has provided volumes of information about maritime laser propagation and atmospheric turbulence. Highlights of recent research collected at the LCTF are presented in this paper.

The availability of new solid-state laser sources in the mid-wave infrared (MWIR) spectral band prompts questions about the utility of these lasers for free-space applications such as gated imaging and communications. We report here on the development of such MWIR laser sources operating near a wavelength of 4 μm, based on laser media employing holmium doping in crystalline hosts, and on the atmospheric propagation characteristics of laser radiation from these new laser sources. Our analysis indicates that these novel laser sources are near a peak in the transmittance of atmospheric water vapor, and suffer relatively low losses both from aerosol scattering and molecular absorption. They also have certain advantages in the areas of laser eye safety and susceptibility to scintillation due to atmospheric turbulence. We illustrate the potential of these new laser sources with field test results of a gated imaging system using such sources for pulsed illumination.

The U.S. Naval Research Laboratory has been examining propagation of a 1550nm laser at its free-space lasercomm test facility at Chesapeake Bay Detachment (NRL-CBD). NRL-CBD offers a ten mile free-space optical laser communication (FSO lasercomm) path over water. Atmospheric propagation data and as well as bit error rate and packet error rate data has been collected along the one-way ten mile link and a round-trip twenty mile link using passive retroreflectors. Long term 24/7 data collection on the one-way range at the lasercomm test facility (LCTF) provides insight into availability and packet error rates of maritime FSO lasercomm. Results from this study will be presented.

The U.S. Naval Research Laboratory (NRL) established a one-way Gigabit Ethernet lasercomm link during the Seahawk exercise in August, 2007 to transfer data ~8 miles across the inlet of San Diego Bay from Point Loma to the Imperial Beach base camp. The data transferred over the link was from an NRL developed, wide field of view (90 degrees), high resolution, mid-wave infrared camera operating at 30 frames per second. Details of the high speed link will be presented as well as packet error rate data and atmospheric propagation data taken during the two week long exercise.

Information superiority, where for the military or business, is the decisive advantage of the 21st Century. While business enjoys the information advantage of robust, high-bandwidth fiber optic connectivity that heavily leverages installed commercial infrastructure and service providers, mobile military forces need the wireless equivalent to leverage that advantage. In other words, an ability to deploy anywhere on the globe and maintain a robust, reliable communications and connectivity infrastructure, equivalent to that enjoyed by a CONUS commercial user, will provide US forces with information superiority. Assured high-data-rate connectivity to the tactical user is the biggest gap in developing and truly exploiting the potential of the information superiority weapon. Though information superiority is much discussed and its potential is well understood, a robust communications network available to the lowest military echelons is not yet an integral part of the force structure, although high data rate RF communications relays, e.g., Tactical Common Data Link, and low data SATCOM, e.g, Ku Spread Spectrum, are deployed and used by the military. This may change with recent advances in laser communications technologies created by the fiber optic communications revolution. This paper will provide a high level overview of the various laser communications programs conducted over the last 30 plus years, and proposed efforts to get these systems finally deployed.