This PDF file contains the front matter associated with SPIE Proceedings Volume 6551, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

The fluid dynamics of airflow through a city controls the transport and dispersion of airborne contaminants. This is urban aerodynamics, not meteorology. The average flow, large-scale fluctuations and turbulence are closely coupled to the building geometry. Buildings create large "rooster-tail" wakes; there are systematic fountain flows up the backs of tall buildings; and dust in the wind can move perpendicular to or even against the locally prevailing wind. Requirements for better prediction accuracy demand time-dependent, three-dimensional CFD computations that include solar heating and buoyancy, complete landscape and building geometry specification including foliage and, realistic wind fluctuations. This fundamental prediction capability is necessary to assess urban visibility and line-of-sight sensor performance in street canyons and rugged terrain. Computing urban aerodynamics accurately is clearly a time-dependent High Performance Computing (HPC) problem. In an emergency, on the other hand, prediction technology to assess crisis information, sensor performance, and obscured line-of-sight propagation in the face of industrial spills, transportation accidents, or terrorist attacks has very tight time requirements that suggest simple approximations which tend to produce inaccurate results. In the past we have had to choose one or the other: a fast, inaccurate model or a slow accurate model. Using new fluid-dynamic principles, an urban-oriented emergency assessment system called CT-Analyst® was invented that solves this dilemma. It produces HPC-quality results for airborne contaminant scenarios nearly instantly and has unique new capabilities suited to sensor optimization. This presentation treats the design and use of CT-Analyst and discusses the developments needed for widespread use with advanced sensor and communication systems.

The directed energy modeling and simulation community can make important direct contributions to the joint warfighting community by establishing clear and fully integrated future program requirements. These requirements are best determined via analysis of the expected variability/uncertainty in system performance arising from spatial, spectral and temporal variations in operating conditions. In this study of atmospheric effects on HEL systems, the parameter space is explored using the Air Force Institute of Technology Center for Directed Energy's (AFIT/CDE) High Energy Laser End-to-End Operational Simulation (HELEEOS) parametric one-on-one engagement level model. HELEEOS is anchored to respected wave optics codes and all significant degradation effects-including optical turbulence and molecular, aerosol, and liquid water drop/droplet absorption and scattering-are represented in the model. Beam spread effects due to thermal blooming caused by the various absorbers are considered when appropriate. Power delivered in a 5 cm diameter circular area normalized by the total transmitted power is the primary performance metric used in the study, with results presented in the form of histograms. The expected performance of laser systems operating at both low and high powers is assessed at 24 wavelengths between 0.355 &mgr;m and 10.6 &mgr;m for a number of widely dispersed land and maritime locations worldwide. Scenarios evaluated include both up and down looking generally oblique engagement geometries over ranges up to 6000 meters in which anticipated clear air aerosols and thin layers of fog, and very light rain are simulated. Seasonal and boundary layer variations (summer and winter) for nighttime conditions for a range of relative humidity percentile conditions are considered to determine optimum employment techniques to exploit or defeat the environmental conditions. Each atmospheric particulate/obscurant is evaluated based on its wavelength-dependent forward and off-axis scattering characteristics and absorption effects on laser energy delivered. In addition to realistic vertical profiles of molecular and aerosol absorption and scattering, correlated optical turbulence profiles in probabilistic (percentile) format are used, a feature unique to HELEEOS.

Free Space Optics (FSO) technology is currently in use to solve the last-mile problem in telecommunication systems by offering higher bandwidth than wired or wireless connections when optical fiber is not available. Incorporating mobility into FSO technology can contribute to growth in its utility. Tracking and alignment are two big challenges for mobile FSO communications. In this paper, we present a theoretical approach for mobile FSO networks between Unmanned Aerial Vehicles (UAVs), manned aerial vehicles, and ground vehicles. We introduce tracking algorithms for achieving Line of Sight (LOS) connectivity and present analytical results. Two scenarios are studied in this paper: 1 - An unmanned aerial surveillance vehicle, the Global Hawk, with a stationary ground vehicle, an M1 Abrams Main Battle Tank, and 2 - a manned aerial surveillance vehicle, the E-3A Airborne Warning and Control System (AWACS), with an unmanned combat aerial vehicle, the Joint Unmanned Combat Air System (J-UCAS). After initial vehicle locations have been coordinated, the tracking algorithm will steer the gimbals to maintain connectivity between the two vehicles and allow high-speed communications to occur. Using this algorithm, data, voice, and video can be sent via the FSO connection from one vehicle to the other vehicle.

Atmospheric attenuation is one of the most significant factors in limiting the performance of millimeter-wave and terahertz systems. Although atmospheric propagation is fairly well understood up to 1 THz, major errors have been published in numerous locations showing atmospheric propagation at frequencies from 10 GHz to 1 THz. Some of these errors have been reported in the past by the present author. The topic was also reviewed in an invited plenary presentation by Bruce Wallace at the 2006 SPIE Defense and Security Symposium in Orlando. Several cases are discussed here, involving clear-air conditions, rain, and fog. In one example, the attenuation at 4 km elevation has been mislabeled as 9150 m (or 30,000 feet) for the 10 to 400 GHz range. This error has appeared in several journal articles, vendors' catalogs, short-course notes, and a recently-published book. In another case the attenuation peak near 22.3 GHz (due to water vapor absorption) has been plotted at 20 GHz. The third case deals with errors pertaining to attenuation in fog for frequencies between 10 and 1000 GHz. Specific information and corrections will be given for all three cases. The net result of these errors is that development of sensor and communications applications has been impeded because the errors usually make atmospheric losses appear to be greater than they really are.

Low power Mid-IR laser light exhibits much lower attenuation in propagation through the New York metro area when compared to Near-IR wavelengths. Depending on the type of atmospheric extinction we record a reduction of up to 800% in the exponential Beer-Lambert coefficient for Mid-IR light compared to Near-IR, thereby demonstrating the possibility of significantly increased deployable range and SNR of current communication systems by utilizing the Mid-IR spectrum. We present and analyze transmission data from an outdoor collinear, coaxial, multi-wavelength laser test bed comparing 1.31&mgr;m, 1.55&mgr;m and 8&mgr;m through outdoor atmospheric fog and rain over a 550 m free space optical link across the Stevens Institute of Technology campus. This is achieved using lasers with average power ranging from 1 mW (Mid-IR QCL) to tens of milliwatts which have been normalized under lock-in detection. We also present corroborating results from an indoor fog experiment simulating various fog types. Here we have also deconstructed Beer's attenuation coefficient and distinguish the contribution of scattering and absorption with a purpose-built polar nephelometer. Using Mie predictions we determine and measure the extent by which a Mid-IR system scatters light less under fog than a traditional Near-IR one, hence accounting for the performance enhancement in the metro-air test bed. We conclude finally that the Kruse-Mie prediction of insignificant Mid-IR-over-Near-IR-gain is strongly in error.

Free-space optical (FSO) communication links are envisioned as a viable option for the provision of temporary high-bandwidth communication links between moving platforms, such as a ground station and a mobile aerial platform such as an unmanned aerial vehicle. One of the limitations of FSO links is the transmission of laser beams through various weather phenomena. One technique to attempt to overcome the effects of weather, such as fog, is to implement a wavelength diversity scheme between the FSO transmitter and receiver. This paper investigates the minimization of link acquisition times using a wavelength diversity scheme between mobile FSO platforms. The wavelength diversity scheme consists of three different wavelengths, 1.55 μm, 0.85 μm and 10 μm. Each wavelength has different advantages and disadvantages for transmission depending of prevalent weather conditions and atmospheric turbulence conditions. A model of a ground-to-air FSO link is developed in order to predict the beam profile in the receiver plane. A simulation analysis of the transmission properties of the wavelength diversity schemes will be presented. Based on the transmission properties, a method for minimizing link acquisition times through the exploitation of various properties of each wavelength is presented and analyzed.

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 range of beam diameters. We find that certain well-known models fit the simulation data in one of the regimes defined by the ratio of beam radius W₀ to Fried's parameter r₀ (W₀/r₀ <<1, W₀/r₀ ~ 1, W₀/r₀ >> 1), but not generally in the other regimes. This is true for tracked beams as well as untracked beams. Two new pdf models, developed here as a modulation of either the gamma-gamma pdf or the gamma pdf, are shown to provide excellent fits to the simulation data over all three regimes defined above.

The first bi-directional laser communication demonstration between an optical ground station and the Optical Inter-orbit Communication Engineering Test Satellite (OICETS) was successfully conducted in March, May, and September, 2006, with an uplink of 2 Mbps and a downlink of 50 Mbps. The optical ground station, located in Koganei, Tokyo, Japan, is operated by the National Institute of Information and Communications Technology (NICT), Japan. Four laser beams were transmitted from the optical ground station to the OICETS satellite in order to reduce the optical signal's intensity fluctuation due to atmospheric turbulence. The optical scintillation as a function of the number of beams and the frequency response were measured, and the uplink and downlink laser transmission results were obtained.

Optical communications is a key technology to meet the bandwidth expansion required in the global information grid. High bandwidth bi-directional links between sub-orbital platforms and ground and space terminals can provide a seamless interconnectivity for rapid return of critical data to analysts. The JPL Optical Communications Telescope Laboratory (OCTL) is located in Wrightwood California at an altitude of 2.2.km. This 200 sq-m facility houses a state-of- the-art 1-m telescope and is used to develop operational strategies for ground-to-space laser beam propagation that include safe beam transmission through navigable air space, adaptive optics correction and multi-beam scintillation mitigation, and line of sight optical attenuation monitoring. JPL has received authorization from international satellite owners to transmit laser beams to more than twenty retro-reflecting satellites. This paper presents recent progress in the development of these operational strategies tested by narrow laser beam transmissions from the OCTL to retro-reflecting satellites. We present experimental results and compare our measurements with predicted performance for a variety of atmospheric conditions.

The two approximate solutions to the stochastic wave equation governing propagation through atmospheric turbulence applicable in weak scintillation conditions are reviewed. Then, an extensive set of numerical solutions are shown to test the ability of the 2 approximate solutions in predicting scintillation and the irradiance probability density function for a wide variety of beam propagation examples. The non-log normal irradiance behavior associated with one of the approximate solutions is noted and verified by the numerical data.

Current mathematical models describing laser propagation through the atmosphere were developed for terrestrial environments. An atmospheric index of refraction power spectrum specifically tailored to the marine environment has been created and applied to scintillation theory. Optical measurements of a diverge laser beam propagating in a marine environment, in combination with scintillation theory and a numerical scheme, were used to infer the refractive index structure parameter, C_n², along the propagation paths. The analysis was repeated for both marine and terrestrial theoretical scintillation expressions, each resulting in one set of inferred C_n²-values. In the moderate-to-strong fluctuation regime, the inferred C_n²-values based on marine theory were about 20% smaller than those based on terrestrial theory, but a minimal difference was observed in the weak fluctuation regime.

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 model. Unfortunately several experiments showed that Kolmogorov theory is sometimes incomplete to describe atmospheric statistics properly. In this paper, for horizontal path and weak turbulence, we carry out analysis of angle-of-arrival fluctuations using a non Kolmogorov power spectrum which uses a generalized exponent factor instead of constant standard exponent value 11/3 and a generalized amplitude factor instead of constant value 0.033. Also our non Kolmogorov spectrum includes both inner scale and outer scale effects.

The US Naval Research Laboratory has an ongoing research effort in the continuous observation of the maritime environment for free space optical communications. One of the goals of our research program is to characterize the behavior of the maritime environment for lasercomm systems, and use the data gathered from the Lasercomm Test Facility (LCTF) at NRL-Chesapeake Bay Detachment (CBD) to develop a method of predicting the global availability of maritime lasercomm. The LCTF has provided volumes of information about maritime laser propagation and atmospheric turbulence. Highlights of the work on the characterization of the maritime atmosphere are provided in this paper.

The U.S. Naval Research Laboratory, Chesapeake Bay Detachment (NRL-CBD), has a ten mile free-space optical laser communication (FSO lasercom) maritime testbed. Over the past year, a comparison study between packet error rates and bit error rates has been performed. These are the two most common methods to characterize the quality of an FSO lasercom link. Bit error rate (BER) testing and packet error rate (PER) testing are measured in a variety of atmospheric conditions on the one-way range at the lasercom test facility (LCTF). Results from this study will be presented.

We are developing a new type of lidar for measuring range profiles of atmospheric optical turbulence. The lidar is based on a measurement concept that is immune to artifacts caused by effects such as vibration or defocus. Four different types of analysis and experiment have all shown that a turbulence lidar that can be built from commercially available components will attain a demanding set of performance goals. The lidar is currently being built, with testing scheduled for summer 2007.

We have measured the optical turbulence structure parameter, C²_n, in two extremely different locations: the first being the littoral region on the southwest coast of Puerto Rico. The second location is over the dry desert in central New Mexico. In both cases, the horizontal beam paths are approximately 0.6 km long, within 2 meters of the local surface (Puerto Rico) and varying between 2 to 100 meters (New Mexico). We present our findings from the two datasets.

The NATO RTG-40 Active Imaging Land Field Trials were conducted at the High Energy Laser System Test Facility at White Sands Missile Range, NM, during November of 2005. This experiment intercompared six active imager systems operating in the visible, near-infrared, and short-wave infrared sensing bands. To characterize the atmospheric turbulence structure present during the optical measurements eight scintillometers were arranged along or near the atmospheric path to characterize the vertical and temporal structure of scintillation, and inner and outer scales of turbulence. A met mast, two 32-m met towers, and an 8-m tower complemented the scintillometer data. This report focuses on analysis of data from four 3-D sonic anemometers positioned at midrange on the 8-m tower and on four of the scintillometers arranged along the 2-km propagation path. First and second order statistics from the sonic sensors are illustrated, along with an analysis of the turbulence spectrum measured by the sonic temperature sensors. The analysis of this data should support both estimating turbulence strength using sonic anemometers as well as outer scale. The data acquired throughout the 10-day measurement period and have proved useful in characterization of the overall weather conditions present during testing and in prediction of various surface layer characteristics.

Accuracy of alignment is a key factor for successfully establishing and maintaining connections in networks of freespace optical links, and is particularly critical when one or both of the transceivers are moving. Scintillation and other atmospheric effects create beam deflections that further complicate the alignment process by creating rays that enter the receiver at an angle to the optical axis. This paper theoretically studies the effective angular misalignment that can be caused by such deflections and mitigation methods for a traditional free-space optical link. The theory uses Gaussian beam propagation and system theory to determine the optical power distribution at the receiver lens and the position of the beam at the lens focal point. Coordinate transformation and overlap integrals are used to assist in calculating the amount of power collected by the lens and incident on the collecting core of the fiber. The use of a fiber bundle at the focal plane of the lens is investigated as a possible method for reducing the receiver sensitivity to misalignment. The simulation results show that some reduction in misalignment sensitivity within some practical system design limits.

Measurements of C_n² time series using unattended commericial scintillometers over long time intervals inevitably lead to data drop-outs or degraded signals. We present a method using Principal Component Analysis (also known as Karhunen-Loève decomposition) that seeks to correct for these event-induced and mechanically-induced signal degradations. We report on the quality of the correction by examining the Intrinsic Mode Functions generated by Empirical Mode Decomposition.

This paper investigates binary wavefront control in the focal plane to compensate for atmospheric turbulence in fiber-coupled free-space laser communication (LaserCom) systems. Traditional approaches to turbulence compensation (i.e., adaptive optics) modify optical phase in the pupil plane to improve the focal plane image or increase energy on target in the far field. For high-energy laser applications, focal plane phase modulation is problematic due to high power densities and device damage thresholds. However, LaserCom systems aim to use minimal power for reasons such as eye safety and covert communication. Thus, focal plane wavefront control is a reasonable approach for this application. Numerical results show that in an air-to-air scenario, binary phase modulation provides mean fiber coupling efficiency nearly identical to that resulting from ideal least-squares adaptive optics, but without the requirement for direct wavefront sensing. The binary phase commands are derived from a single imaging camera and an assumption about the nature of spot breakup. The use of binary wavefront control suggests that existing ferro-electric spatial light modulator technology may support real-time correction. Coupling efficiency results are also compared to those for the Strehl ratio, highlighting the importance of metric-driven design.

Accuracy of alignment is a key factor for successfully establishing and maintaining connections in networks of freespace optical links, and is particularly critical when one or both of the transceivers are moving. Scintillation and other atmospheric effects create beam deflections that further complicate the alignment process. This paper studies the use of a fiber-optic bundle at the transmitter and receiver to mitigate the atmospheric effects on the link up-time. The bundle at the transmitter allows fast, non-mechanical steering of the optical beam to track and correct for relative motion. The bundle at the receiver allows for a significant improvement in misalignment tolerance, particularly to angular misalignment. Laboratory experiments and theoretical analyses were conducted on a free-space link to determine the inter-relationship between spacing of the fibers within the bundle, the focal lengths of the transceiver lenses, the beam deflection angle, and the misalignment tolerance for varying atmospheric conditions. A shorter focal length lens at the transmitter provides greater coverage, while a moderate focal length lens at the receiver reduces the bundle size required to improve misalignment tolerance. A smaller overall system size is possible, provided that sufficient power is used to overcome the greater spatial spreading and subsequent loss of peak power at the receiver.

Free-space optics (FSO) is a technology that utilizes modulated light beam to transmit information through the atmosphere. Line-of-sight connection between both FSO transceivers is a necessary condition to maintain continuous exchange of voice, video, and data information. To date, the primary concentration of mobile FSO research and development has been toward the accurate aligning between two transceivers. This study introduces an advanced FSO receiver that provides wider receiving angle compared with that of conventional FSO systems. We present data from measurements of optical power, which were very promising, and indicated that these advanced FSO receivers are suitable for FSO alignment applications and perform favorably with similar FSO receivers.

The U. S. Naval Research Laboratory (NRL) and OptoGration, Inc. have collaborated in the development and testing of large area, high speed InGaAs avalanche photodiode (APD) receivers for use in free-space lasercom systems. A 200 micron diameter InGaAs APD receiver has been tested in a free-space lasercom testbed and has demonstrated sensitivities of -42.4 dBm at 622 Mbps and -44.8 dBm at 155 Mbps. A 100 micron diameter receiver has been tested with a resulting sensitivity of -35.75 dBm at 2.4883 Gbps. These receivers are made possible due to OptoGration's capability to manufacture a large area, high speed InGaAs APD with an effective ionization ratio of < 0.2 and by matching the APD device with an appropriate transimpedance amplifier and limiting amplifier. Development and testing of the APD receivers will be described below.

Imaging through turbulence using adaptive optics is limited by scintillation, even with perfect wavefront sensing and reconstruction. Such errors can be mitigated in closed loop by multi-conjugate adaptive optics systems consisting of two phase correctors, each of which is driven by a pair of wavefront sensor phase measurements, along with an internal probe beam that samples the beam train along a common path while propagating in the opposite direction as the external signal beam or beacon wavefront that samples the turbulence. With this arrangement, not only direct measurement and feedback of irradiance but also intensive and/or highly coupled nonlinear control algorithms can be avoided in favor of more conventional, simple, decentralized linear control laws. Linear stability analysis of such systems is feasible in spatial frequency domain, and nonlinear wave-optic simulations in time domain suggest that, given sufficient temporal bandwidth, rejection of combined phase and amplitude disturbances can be enhanced by a factor of two or more (as quantified by error variances or Strehl ratio logarithms). Previous studies by other authors are extended using simplified regularization methods.