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

Scintillation is an effect that limits the performance of many optical systems including imagers and free space optical communication links. Scintillation can be especially severe for optical retro-reflectors. This can limit the range of links based on modulating retro-reflectors. In this work we examine the effects of retro-reflector diversity on optical scintillation. This technique uses multiple retro-reflectors, separated by a distance larger than the atmospheric coherence size and illuminated by one interrogating beam, to reduce the scintillation index. We examine the dependence of scintillation index on the number of retro-reflectors, their separation and the degree of coherent interference at the receiver. Data was taken in the field at three different sites: the Naval Research Laboratory's Chesapeake Bay Detachment, where both over land and over water measurements were made, the Fort AP Hill Laser Test Range and China Lake, California. We show that multiple retro-reflectors can reduce the scintillation index on double-pass links. We demonstrate that when retro-reflectors are spaced too closely, coherent interference can increase the scintillation index and greatly expand its frequency content. We also examine the effects of retro-reflector diversity on the margin needed for modulating retroreflector data links.

The U.S. Naval Research Laboratory (NRL) is developing a small lasercomm terminal for both direct lasercomm and communication to a modulated retroreflector (MRR). A gimbal from Cloud Cap Technology serves as the terminal positioner, which is to be integrated onto a small, unmanned airborne vehicle. A lasercomm module must be developed that meets size, weight, power, and optical performance requirements. The final module incorporates the shared optics, the tracking optic and detector, and the lasercomm receiver optics and detector. This module was designed, fabricated, assembled, and integrated into the Cloud Cap T2 gimbal. After integration, the tracking signals were measured in the lab to establish the field of view and response to pointing errors.

This paper reports propagation characteristic of direct single-mode-fiber (SMF) coupled free-space-optical signal. Accumulated data at recent indoor/outdoor demonstration experiments over 30-500 m link distance under weak to medium atmospheric turbulence conditions are analyzed and discussed. The results show asymmetric surge/fade ratios and long lower tail distribution in SMF-coupled signal intensities due to phase interference at the SMF receiving aperture, which is different from the result in the normal irradiance distribution, such as Gamma-Gamma distribution or log-normal distribution. This asymmetry indicates that reliable FSO links should be operated under sufficiently weak atmospheric turbulence environments where scintillation index is less than 0.1. This requirement is also valid to maintain mutual beacon tracking to assure precise tracking/pointing accuracy required for the direct SMF coupling.

The U.S. Naval Research Laboratory (NRL) is developing a small lasercomm terminal for both direct lasercomm and communication to a modulated retroreflector (MRR). A gimbal from Cloud Cap Technology serves as the terminal positioner, which is to be integrated onto a small, unmanned airborne vehicle. Software was developed to allow the terminal to automatically acquire and track ground terminals and manage the lasercomm payload. This software turns the terminal into a completely autonomous lasercom system, able to travel well outside the communications range of its base station and collect data from ground terminals with no user intervention. Data from boat testing will be presented to show pointing accuracies as well as automatic acquisition and tracking of static targets.

A novel receiver and transmitter that utilize fiber bundles are under development to address the challenges associated with acquiring and maintaining a free-space optical link between mobile platforms in the presence of vibration and atmospheric turbulence. The fiber bundles allow for greater misalignment tolerance at the receiver and greater control over the spatial area covered by the transmitted beam upon reaching the receiver. This paper reports on a simulation-based investigation of design choices that optimize the performance of the system under several operating scenarios. The simulation incorporates prior experimental data into the theoretical calculations for optical propagation to better describe the performance of the physical designs. For a given link length and available power, the coverage area and collected power are controlled by the pattern of transmitters, beam divergence, and the receiver construction. The investigation finds that the coverage area of the receiver can be optimized for a given link length by proper choices of these parameters, and that similar parameters may provide near optimal performance at other wavelengths. Trade-offs between choices of the key parameters are explored as a function of link length. The results provide guidance on the further development of the overall system.

A series of experiments were conducted to validate the performance of the free-space optical communications (FSOC) subsystem under DARPA's FOENEX program. Over six days, bidirectional links at ranges of 10 and 17 km were characterized during different periods of the day to evaluate link performance. This paper will present the test configuration, evaluate performance of the FSOC subsystem against a variety of characterization approaches, and discuss the impact of the results, particularly with regards to the optical terminals. Finally, this paper will summarize the impact of turbulence conditions on the FSOC subsystem and present methods for estimating performance under different link distances and turbulence conditions.

The U.S. Naval Research Laboratory (NRL) is developing a small size, weight and power (SWaP) free space lasercomm terminal for small unmanned airborne platforms. The terminal is based on a small gimbal developed by CloudCap Technology. A receiver with a large field of view and with sensitivity sufficient to meet the program range goals is required for this terminal. An InGaAs Avalanche Photodiode (APD) with internal structures engineered to reduce excess noise and k_eff in high gain applications was selected as the detector. The detector is a 350 micron diameter impact ionization engineered (I²E) APD developed by Optogration, Inc. Results of development and characterization of the receiver will be presented.

This paper describes key aspects of modem hardware designed to operate in free space optical (FSO) links of up to 200 km. The hardware serves as a bridge between 10 gigabit Ethernet client data systems and FSO terminals. The modem hardware alters the client data rate and format for optimal transmission and reception over the FSO link by applying forward error correction (FEC) processing and differential phase shift keying (DPSK) modulation. Optical automatic gain control (OAGC) is also used. The result of these features provide sensitivities approaching -48 dBm with 60 dB of error-free dynamic range while in the presence of turbulent optical conditions to deal with large dynamic range optical power fades.

Photodiode arrays are instrumental in providing pointing and tracking information for free space optical communication systems. Recent advances in the fabrication and development of low noise, high bandwidth avalanche photodiode (APD) arrays have enabled these devices to be used not only as position sensitive detectors (PSD) for tracking but also as communications receivers. In a collaborative effort with Optogration, Inc., the U.S. Naval Research Laboratory has developed avalanche photodiode arrays with three different geometries: a 3x3 square pixel array, a centered hexagonal pixel array, and a 5 pixel concentric array configuration with a center pixel and four periphery pixels. The characterization and performance of each array geometry will be described along with associated front-end and digital electronics. Design tradeoffs for maximizing the performance of a given array geometry will also be discussed.

In order for mobile optical transceivers to communicate, the transceivers must be able to acquire and maintain a free-space optical link between them despite misalignments due to movement and the effects of atmospheric turbulence. Recently, novel transmitters and receivers, along with specifically designed control algorithms, were proposed that incorporated a fiber-bundle approach for improving the ease of acquiring and maintaining the link between two transceivers. Preliminary transmitter and receiver nodes have been constructed for testing the capabilities of this approach. This paper investigates the performance of a transmitter and receiver pair through experimental methods. The performance is evaluated on several key parameters, including initial acquisition time, up time of the link when perturbed by movement or simulated atmospheric impairment, and the link recapturing time once a connection is lost. The dependence of the key parameters is evaluated for different levels and types of perturbations, as well as design choices at the transmitter and receiver. The results show that the optical control system successfully recovered and maintained the link while the receiver was in motion, although the performance was impacted by the angular misalignment tolerance of the receiver. The strengths and limitations of the approach revealed by the experiments are also discussed, along with paths for further improvements.

The atmosphere distorts and degrades Radio Frequency (RF) and Free-Space Optical (FSO) communications signals. Clouds, precipitation, turbulence, and inhomogeneities in atmospheric temperature and moisture all have the potential to disrupt communications through the atmosphere. However, there are strategies that can be employed to mitigate atmospheric impacts on communications networks such as the Free-space Optical Experimental Network Experiment (FOENEX). These strategies require an accurate characterization of the atmosphere through which the communications links travel. Atmospheric measurements provided by local instrumentation are valuable for link characterization, but provide an incomplete picture of the atmosphere. During the FOENEX demonstrations, these in situ measurements were supplemented with Numerical Weather Prediction (NWP) simulations, which provided weather forecasts for experiment planning, as well as time-varying, three-dimensional characterizations of the atmosphere. Forecast decision aids, derived from NWP ensemble forecasts, were provided several times daily to support experiment planning. Additionally, the Weather Research and Forecasting (WRF) NWP model was used to simulate the atmospheric conditions over the FOENEX test regions during the flights, and provide the high-resolution horizontal and vertical structure of the temperature, winds, moisture, and turbulence for the domain. WRF results were dictated by model inputs that included daily weather conditions, terrain, and land usage. The standard WRF model was modified to calculate the refractive index structure function, C_n², directly from the standard NWP model parameters. This has proven to be a valuable tool for link characterization, since WRF can identify thin relatively layers of optical turbulence that are not represented by standard empirical C_n² profiles.

We present results from a fast holographic adaptive laser optics system (HALOS) incorporating a MEMS-based deformable mirror and an off-the-shelf, photon counting avalanche photodiode array. A simple digital circuit has been constructed to provide autonomous control and the entire system is no larger than a shoebox. Our results demonstrate that this device is largely insensitive to obscuration and in principle can run as fast with one actuator as with one million. We further show how HALOS can be used in image correction, laser beam projection as well as phased-array beam combination.

In many military applications that use Adaptive Optics (AO) a point source beacon is ideally required at the target to measure and to correct for the wavefront aberrations caused by propagation through the atmosphere. However, it is rarely possible to create a point source beacon at the target. The "extended beacons" that are created instead have intensity profiles with a finite spatial extent and exhibit varying degrees of spatial coherence. The Gaussian Schell model might be a convenient way to model these extended sources because of its analytical tractability. The present work examines the validity of using such a model by evaluating the scattered field from a rough surface target using a full wave electromagnetic solution (method of moments). The full wave electromagnetic calculation improves the fidelity of the analysis by capturing all aspects of laser-target interaction i.e. shadowing/ masking, multiple reflections etc. A variety of rough surface targets with different roughness statistics has been analyzed. This analysis will ultimately aid in understanding the key parameters of extended beacons and how they impact the Adaptive Optics (AO) system performance.

High contrast imaging, also known as extreme adaptive optics, has been a topic of research in the astronomic community as an approach to image dim objects near bright objects, such as extra-solar planets. There are a variety of techniques ranging from coronagraphs, shaped pupils, pupil apodization, and the use of multiple deformable mirrors that have been employed to improve the contrast between the two objects. We integrated shaped pupils into our adaptive optics system. Here we will present experimental results exploring the viability of using our testbed to perform dim object detection using shaped pupils in the presence of turbulence.

Using multiresolution spectra data and a measure of anisotropy based on the eigen values of the Reynolds stress tensor, the scale of maximum temperature variance is found to usually be greater than the scale at which the turbulence transitions from isotropy to anisotropy. The scale of the maximum temperature variance is just as likely to be equal to the scale of maximum turbulent kinetic energy as it is to be larger or smaller. The magnitude of the maximum temperature variance is not correlated with the scale of maximum variance, but is roughly correlated with the total heat flux. Also, the magnitude of the maximum temperature variance at 5 m above the ground (agl) only roughly correlates with the magnitude at 50 m agl. The anisotropy of the turbulence at the scale of maximum temperature variance tends to be more pancake-like than cigar-like in the variances especially at 5 m agl.

A novel method for measuring the structure constant of the atmospheric turbulence on an arbitrary path has recently been demonstrated by the Air Force Institute of Technology (AFIT). This method provides a unique ability to remotely measure the intensity of turbulence, which is important for predicting beam spread, wander, and scintillation effects on High Energy Laser (HEL) propagation. Because this is a new technique, estimating A novel method for measuring the structure constant of the atmospheric turbulence on an arbitrary path has recently been demonstrated by the Air Force Institute of Technology (AFIT). This method provides a unique ability to remotely measure the intensity of turbulence, which is important for predicting beam spread, wander, and scintillation effects on High Energy Laser (HEL) propagation. Because this is a new technique, estimating C_n² using radar is a complicated and time consuming process. This paper presents a new software program which is being developed to automate the calculation of C_n² over an arbitrary path. The program takes regional National Weather Service NEXRAD radar reflectivity measurements and extracts data for the path of interest. These reflectivity measurements are then used to estimate C_n² over the path. The program uses the Radar Software Library (RSL) produced by the Tropical Rainfall Measuring Mission (TRMM) at the NASA/Goddard Flight Center. RSL provides support for nearly all formats of weather radar data. The particular challenge to extracting data is in determining which data bins the path passes through. Due to variations in radar systems and measurement conditions, the RSL produces data grids that are not consistent in geometry or completeness. The C_n² program adapts to the varying geometries of each radar image. Automation of the process allows for fast estimation of C_n² and supports a goal of real-time remote turbulence measurement. Recently, this software was used to create comparison data for RF scintillation measurements. In this task it performed well, extracting thousands of measurements in only a few minutes.using radar is a complicated and time consuming process. This paper presents a new software program which is being developed to automate the calculation of C_n² over an arbitrary path. The program takes regional National Weather Service NEXRAD radar reflectivity measurements and extracts data for the path of interest. These reflectivity measurements are then used to estimate C_n² over the path. The program uses the Radar Software Library (RSL) produced by the Tropical Rainfall Measuring Mission (TRMM) at the NASA/Goddard Flight Center. RSL provides support for nearly all formats of weather radar data. The particular challenge to extracting data is in determining which data bins the path passes through. Due to variations in radar systems and measurement conditions, the RSL produces data grids that are not consistent in geometry or completeness. The C_n² program adapts to the varying geometries of each radar image. Automation of the process allows for fast estimation of C_n² and supports a goal of real-time remote turbulence measurement. Recently, this software was used to create comparison data for RF scintillation measurements. In this task it performed well, extracting thousands of measurements in only a few minutes.

The performance of optical systems is degraded by atmospheric turbulence. Over propagation distances that exceed several kilometers, it is difficult to evaluate its impact because of terrain variability - a factor that should be taken into account. However, to optimize performance, the turbulence characteristics and its effect on optical wave propagation along the propagation path should be known. The understanding of turbulence impact is one of the main objectives of the NATO group SET 165: "Adaptive Optics (AO) for laser beam delivery, passive and active imaging and turbulence mitigation". In this paper we describe experiments performed by the NATO SET 165 research group, namely, a set of atmospheric experiments over a 7 km distance, and discuss some preliminary results of data processing. The experiments were conducted at the University of Dayton Intelligent Optics Laboratory (UD/IOL) in October 2011. It benefited significantly from the available optical setups and the infrastructure on the UD/IOL site.

A study is underway to address effects of aero-optic (AO) flow components added to the typically modeled clean, dry flow. This is the first of multiple runs to address effects of three-dimensional turrets, moist air and aerosols for a turret in airflow at 0.4 Mach with parameters matching atmosphere at 1 km. The resulting analysis will be used as a baseline for comparison with airflow with the same parameters but including moisture, and again including a realistic population of aerosol particles of varying size. Atmospheric properties were determined using High Energy Laser End-to-End Operational Simulation (HELEEOS).

The role of phase Strehl (as a random variable) in adaptive optics performance modeling is discussed. The characteristic function, general form of the cumulative distribution function (CDF), and results for two and three degrees of freedom are presented.

Atmospheric turbulence imparts phase distortions on propagating optical waves. These distortions couple into amplitude uctuations at the pupil of a telescope, which, for strong enough phase distortions, produce branch points (zeros in the amplitude). In our earlier work we have presented the case that branch points can be utilized as a source of information on the turbulent atmosphere. Using our bench-top data, we have demonstrated several properties of branch points including motion, density, persistence and separation. We have identied empirical relationships for density and separation as functions of the strength and altitude for a single layer. However, this work was done using a bench-top adaptive optics system utilizing a two-layer atmospheric turbulence simulator. In this rst paper, we use independently anchored wave optics simulations to verify these results. This simulation provides a means to further examine how the turbulence conditions contribute to the branch point distribution. Additionally, we look at the role of the inner scale in the formation of branch points within the optical simulation. The companion paper will examine the properties of branch point velocity and persistence.

In our first work in this research thread, we demonstrated that turbulence-created optical vortices are created innitesimally close together in pairs of opposite helicity--call these creation pairs. In that rst work, we postu- lated that creation pairs separate as they propagate, and that they carry both the velocity of, and distance to, the turbulence layer that created them. Subsequent experimental papers have demonstrated this to be true. Our purpose here is to ll two gaps in our original theoretical results and demonstrate both how, in a mathematical treatment, turbulence-created optical vortices can have the velocity of the turbulence layer that created them, and also, present calculation of their separation velocity.

Some special functions of the Mathematical Physics are a very helpful resource in problems involving the propagation of coherent light beams, which will suffer dispersion in a turbulent media such as the Earth's ionosphere waveguide. Unfortunately, these tools are difficult to use because it involves very complex mathematical developments. For this reason it is interesting to find a friendly method to make the implementation of these special functions possible. Using Maple I will be able to overcome the mathematical difficulty of solving these equations and get to the understanding of these phenomena. Specifically I will consider the excitation of the Earth's ionosphere as a cavity or a waveguide by satellite borne current sources in the form of satellite-based antennas when the medium inside the waveguide is turbulent. As a result, three kinds of coherent light beams will be derived: Bessel beams, for relatively low turbulence, Whittaker beams, for moderate turbulence, and Heun beams, for strong or fully developed turbulence. These beams are represented by the corresponding electric fields but the associated magnetic fields can be derived as well. It is verified that Maple is a very powerful tool in the study of the propagation of an input field through axially symmetric systems using the methods of the Mathematical Physics. It is expected that Maple will have important applications for more general models concerning propagation trough turbulent environments.

Clouds are key driver in the performance of free space optical communication (FSOC) systems. Clouds are composed of liquid water and/or ice crystals and depending on the physical thickness can produce atmospheric fades easily exceeding 10 dB. In these more common cases, impacts on FSOC systems may be severe. On the other hand, there are times when cloud fades may be as low as 1 or 2 dB as a result of thin, ice crystal based cirrus clouds. In these cases, the impacts on FSOC communication collectors may be limited. The ability to characterize the distribution and frequency of clouds are critical in order to understand and predict atmospheric impacts. A cloud detection system has been developed and applied to produce high resolution climatologies in order to investigate these impacts. The cloud detection system uses geostationary, multi-spectral satellite imagery at horizontal resolutions up to one kilometer and temporal resolutions up to fifteen minutes. Multispectral imagery from the visible wavelengths through the longwave infrared is used to produce individual cloud tests which are combined to produce a composite cloud analysis. The result represents a high spatial and temporal resolution climatology that can be used to derive accurate Cloud Free Line of Sight (CFLOS) statistics in order to quantify atmospheric effects on optical communication systems. The Lasercom Network Optimization Tool (LNOT) is used along with a mission CONOPS and the cloud database to find configuration of geographically diverse ground sites which provide a high availability system.

This study quantifies the impacts on high energy laser (HEL) air defense performance due to atmospheric effects in the marine boundary layer driven by varying elevated aerosol layers. The simulations are run using several different engagement geometries to more completely show the effects of aerosols. High adaptive optics are applied to reduce the turbulence effects. The atmospheric effects are defined using the worldwide probabilistic climatic database available in the High Energy Laser End-to-End Operational Simulation (HELEEOS) model. The anticipated effects on HEL propagation performance is assessed at 1.0642 μm across the world's oceans, mapped on a 1° × 1° grid, and at 573 land sites. The scenarios evaluated are primarily near-surface and horizontal over ranges up to 10000 meters. Seasonal and boundary layer variations (summer and winter) for a range of relative humidity percentile conditions are considered. In addition to realistic vertical profiles of molecular and aerosol absorption and scattering, correlated optical turbulence profiles in probabilistic (percentile) format are used. Results indicate profound effects of elevated aerosol layers on HEL engagements as compared to standard scenarios without elevated layers. Also, results suggest changing optical properties to have additional significant effects. 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 a probability of cloud free line of sight (CFLOS) algorithm for air-to-air, air-tosurface, and surface-to-air (or space) look angles. HELEEOS characterizes 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 optical turbulence with the Navy Surface Layer Optical Turbulence (NSLOT) model. HELEEOS was developed under the sponsorship of the High Energy Laser Joint Technology Office.

Detection and characterization of space objects require the capability to derive physical properties such as brightness temperature and reflectance. These quantities, together with trajectory and position, are often used to correlate an object from a catalogue of known characteristics. However, retrieval of these physical quantities can be hampered by the radiative obscuration of the atmosphere. Atmospheric compensation must therefore be applied to remove the radiative signature of the atmosphere from electro-optical (EO) collections and enable object characterization. The JHU/APL Atmospheric Compensation System (ACS) was designed to perform atmospheric compensation for long, slant-range paths at wavelengths from the visible to infrared. Atmospheric compensation is critically important for airand ground-based sensors collecting at low elevations near the Earth's limb. It can be demonstrated that undetected thin, sub-visual cirrus clouds in the line of sight (LOS) can significantly alter retrieved target properties (temperature, irradiance). The ACS algorithm employs non-traditional cirrus datasets and slant-range atmospheric profiles to estimate and remove atmospheric radiative effects from EO/IR collections. Results are presented for a NASA-sponsored collection in the near-IR (NIR) during hypersonic reentry of the Space Shuttle during STS-132.

Atmospheric propagation properties of various laser systems, including diode pumped alkali lasers (DPALs) and the Chemical Oxygen Iodine Laser (COIL), are of importance. However, there appears to be a lack of highly accurate transmission characteristics of these systems associated with their operating conditions. In this study laser propagation of the rubidium-based DPAL and the COIL has been simulated utilizing integrated cavity output spectroscopy. This technique allowed for the simulation of laser propagation approaching distances of 3 kilometers on a test stand only 35 cm long. The spectral output from these simulations was compared to the HITRAN database with excellent agreement. The spectral prole and proximity of the laser line to the atmospheric absorbers is shown. These low pressure spectral proles were then extrapolated to higher pressures using an in-house hyperne model. These models allowed for the comparison of proposed systems and their output spectral prole. The diode pumped rubidium laser at pressures under an atmosphere has been shown to interact with only one water absorption feature, but at pressures approaching 7 atmospheres the D1 transition may interact with more than 6 water lines depending on resonator considerations. Additionally, a low pressure system may have some slight control of the overlap of the output prole with the water line by changing the buer gases.

Free Space Optics (FSO) technology was originally envisioned to be a viable solution for the provision of high bandwidth optical connectivity in the last mile of today's telecommunications infrastructure. Due to atmospheric limitations inherent to FSO technology, FSO is now widely envisioned as a solution for the provision of high bandwidth, temporary mobile communications links. The need for FSO communications links will increase as mobility is introduced to this technology. In this paper, a theoretical solution for adding mobility to FSO communication links is introduced. Three-dimensional power estimation studies are presented to represent mobile FSO transmission under various weather conditions. Three wavelengths, 0.85, 1.55 and 10 um, are tested and compared to illustrate the pros and cons of each source wavelength used for transmission, depending on prevalent weather conditions and atmospheric turbulence conditions. A simulation analysis of the transmission properties of the source wavelengths used in the study is shown.