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

Although the concept of non-line-of-sight (NLOS) ultraviolet (UV) communications has been studied for decades, recent advances in the design and manufacturing of light-emitting diodes, filters, and sensors have ignited new interest. In this paper, we discuss a Monte Carlo channel model for NLOS UV communications that accounts for the possibility that a transmitted photon experiences multiple scattering events before being received. By simulating the propagation of many photons based on probabilistic rules derived from physics considerations, a computationally efficient algorithm is obtained that allows for the study of the contribution of various orders of scattering to the received signal and to the system impulse response function. We then demonstrate the use of this channel model in the exploration of several system configurations. In particular, we examine the effect of the transmitter beam shape and receiver sensitivity function on the faithfulness of a well-known linear model of path loss versus distance for short-range NLOS UV systems, and we explore geometry design for interference reduction in a full-duplex link. The use of the model to study such diverse system implementations demonstrates its general applicability.

Laser weapon systems comprised of tiled subapertures are rapidly emerging in importance in the directed energy community. Performance models of these laser weapon systems have been developed from numerical simulations of a high fidelity wave-optics code called WaveTrain which is developed by MZA Associates. System characteristics such as mutual coherence, differential jitter, and beam quality rms wavefront error are defined for a focused beam on the target. Engagement scenarios are defined for various platform and target altitudes, speeds, headings, and slant ranges along with the natural wind speed and heading. Inputs to the performance model include platform and target height and velocities, Fried coherence length, Rytov number, isoplanatic angle, thermal blooming distortion number, Greenwood and Tyler frequencies, and atmospheric transmission. The performance model fit is based on power-in-the-bucket (PIB) values against the PIB from the simulation results for the vacuum diffraction-limited spot size as the bucket. The goal is to develop robust performance models for aperture phase error, turbulence, and thermal blooming effects in tiled subaperture systems.

Asymptotic theory of the finite beam scintillations (Charnotskii, WRM, 1994, JOSA A, 2010) provides an exhaustive description of the dependence of the beam scintillation index on the propagation conditions, beam size and focusing. However the complexity of the asymptotic configuration makes it difficult to apply these results for the practical calculations of the scintillation index (SI). We propose an estimation technique and demonstrate some examples of the calculations of the scintillation index dependence on the propagation path length, initial beam size, wavelength and turbulence strength for the beam geometries and propagation scenarios that are typical for applications. We suggest simple analytic bridging approximations that connect the specific asymptotes with the accuracy sufficient for the engineering estimates. Proposed technique covers propagation of the wide, narrow, collimated and focused beams under the weak and strong scintillation conditions. Direct numeric simulation of the beam waves propagation through turbulence expediently complements the asymptotic theory being most efficient when the governing scales difference is not very large. We performed numerical simulations of the beam wave propagation through turbulence for conditions that partially overlap with the major parameter space domains of the asymptotic theory. The results of the numeric simulation are used to confirm the asymptotic theory and estimate the accuracy of the bridging approximations.

The turbulent atmosphere causes wavefront distortion, beam wander, and beam broadening of a laser beam. These effects result in average power loss and instantaneous power fading at the receiver aperture and thus degrade performance of a free-space optical (FSO) communication system. In addition to the atmospheric turbulence, a FSO communication system may also suffer from laser beam pointing error. The pointing error causes excessive power loss and power fading. This paper proposes and studies an analytical method for calculating the FSO channel fading probability density function (pdf) induced by both atmospheric turbulence and pointing error. This method is based on the fast-tracked laser beam fading profile and the joint effects of beam wander and pointing error. In order to evaluate the proposed analytical method, large-scale numerical wave-optics simulations are conducted. Three types of pointing errors are studied , namely, the Gaussian random pointing error, the residual tracking error, and the sinusoidal sway pointing error. The FSO system employs a collimated Gaussian laser beam propagating along a horizontal path. The propagation distances range from 0.25 miles to 2.5 miles. The refractive index structure parameter is chosen to be C_n² = 5×10^-15m^-2/3 and C_n² = 5×10^-13m^-2/3. The studied cases cover from weak to strong fluctuations. The fading pdf curves of channels with pointing error calculated using the analytical method match accurately the corresponding pdf curves obtained directly from large-scale wave-optics simulations. They also give accurate average bit-error-rate (BER) curves and outage probabilities. Both the lognormal and the best-fit gamma-gamma fading pdf curves deviate from those of corresponding simulation curves, and they produce overoptimistic average BER curves and outage probabilities.

The Georgia Tech Research Institute (GTRI) is developing a transportable multi-lidar system known as the Integrated Atmospheric Characterization System (IACS). The system will comprise three lidars: an imaging lidar for profiling refractive turbulence, a Raman lidar for profiling water vapor, and an aerosol lidar operating at 0.355, 1.064, and 1.625 microns for profiling aerosol extinction. All of the lidar transmit/receive optics will be co-aligned on a common mount, pointable at any elevation angle from below horizontal to vertical. The entire system will be computer controlled to facilitate pointing and automatic data acquisition. The purpose of IACS is to characterize optical propagation paths during outdoor tests of electro-optical systems. The tests are anticipated to include ground-to-ground, air-to-ground, and ground-to-air scenarios, so the system must accommodate arbitrary slant paths through the atmosphere with maximum measurement ranges of 5-10 km. Elevation angle scans will be used to calibrate the atmospheric extinction profiles and data from the three wavelengths will be used to determine the aerosol Angstrom coefficient, enabling interpolation of results to other wavelengths in the 0.355 to 1.6 micron region. Some of the lidar engineering challenges and solutions are presented here.

This paper demonstrates the capability of AFIT/CDE's Laser Environmental Effects Definition and Reference (LEEDR) model to accurately characterize the meteorological parameters and radiative transfer effects of the atmospheric boundary layer with only surface observations of temperature, pressure, and humidity. The LEEDR model is a fastcalculating, 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-to-surface, and surface-to-air (or space) look angles. In general, LEEDR defines the well-mixed atmospheric boundary layer with a worldwide, probabilistic surface climatology based on season and time of day, and then computes the radiative transfer and propagation effects from the vertical profile of meteorological variables. However, the LEEDR user can also directly input surface observations. This research compares the LEEDR vertical profiles created from input surface observations to actual observations from balloon launches. Results are then compared to the LEEDR ExPERT climatological sounding for the same time of day and season. RMSE are calculated and it was found that closer for those profiles made from surface observations than those made from climatological data for the same season and time. Impacts of those differences are shown with a relevant tactical scenario in AFIT/CDE HELEEOS program.

Phase Screen simulations for laser propagation through non-Kolmogorov turbulence are presented and the results for scintillation index and correlation functions for the intensity are compared with the theory at low turbulence levels at selected non-Kolmogorov exponents. Additional simulation results are presented the strong turbulence region. In particular, effects of transitioning from Kolmogorov to non-Kolmogorov turbulence using their spectral equivalence at the Fresnel scale (as suggested in the literature) on the scintillation index and correlation functions at the receiver are examined for two example paths.

We discuss the SOR Atmospheric Monitor's (SAM's) algorithms for estimating a number of atmospheric parameters from Shack-Hartmann wave front sensor measurements. In addition to the previously reported Fried parameter work, we report on estimates of Greenwood frequency and isoplanatic angle. We also report on a few months of statistics of these parameters at Kirtland AFB.

In this paper we show evidence of aero-optic effects on the measured beacon beam as the gimbal angle of a nosemounted turret changes from 0 to 90 degrees and greater with respect to the line of flight. Data from the beacon beam was collected with a new technology 3-aperture scintillometer over a 82km to 104km air-to-ground downlink during field testing of the ORCA system in Nevada in May 2009. In this paper we present data analysis on the impact of an aero-optic boundary layer on a laser link between an aircraft and a ground-based stationary node. Particularly we look at the impact of an aero-optic boundary layer on the mean, variance, scintillation, probability density function (PDF), power spectral density (PSD), and fading of the received irradiance. We find that the most compelling argument for the presence of strong aero-optic effects comes from calculating the PSD of the received beacon intensity. We also find the cumulative effect of the aero-optic boundary layer differs depending on the transmitted beam parameters, i.e. collimated or divergent.

A free-space optical communications link spanning 147 km between the islands of Hawaii and Maui was studied as part of AFRL's IRON-T2 program and in support of risk reduction efforts for DARPA's ORCA program in September/October 2008. Over 14 days, the performance of a 10-Gbps bi-directional link was tested during different periods of the day. This paper will present the test configuration, discuss the effects of atmospheric turbulence on the 10 Gbps link, and compare its performance to the available turbulence measurements. Additionally, modeling of the link configuration will be presented and comparisons will be made to collected data including local C_n ² to understand the impact of atmospheric turbulence on future long distance links.

The U.S. Naval Research Laboratory (NRL) is characterizing InGaAs avalanche photodiodes (APDs) with internal structures engineered to reduce dark counts and ionization coefficient ratio (keff). Recently, much progress has been made in the use of APDs in linear mode for photon counting applications.¹ However, the best results in linear mode single photon counting in InGaAs devices have been obtained by cooling the devices well below 200 K to reduce dark current. The single photon counting capability is due to the high gain available in the tail of the APD gain distribution, and this high gain tail is enhanced by reducing the ionization ratio (keff) and dark noise.² Since recent promising results in linear mode APD photon counting have involved engineering the APDs to reduce keff, it is likely that these devices will also perform much better than standard APDs in free space lasercomm applications at temperatures which can easily be reached by thermoelectric coolers, or even uncooled, due to the keff reduction. NRL has obtained several InGaAs APDs of both the standard design and of a new design using impact ionization engineering from OptoGration, Inc. of Wilmington, MA. Some results of characterization of these APDs will be presented.

Irradiance data were collected over an air-to-ground path using several different sized receiving apertures. The data were collected from the Optical RF Communications Adjunct (ORCA) tracking beacon. The receiver system consisted of three telescopes of sizes 51 mm, 137 mm, and 272 mm. Probability of fade, number of fades per second, and mean fade time was computed for various intensity levels for irradiance data collected on all three telescopes. These measured statistics are compared to fading models derived from lognormal and gamma-gamma probability density function (PDF) models. Discussion is centered on the viability of these models under various conditions and on the presence of aero-optic effects. The gamma-gamma and lognormal model are found to be insufficient to model all fading statistics.

This paper investigates the performance of a recently proposed control algorithm for mobile FSO node alignment through experimental methods. The performance is evaluated on coverage area at the receiver plane, the link recapturing time and the connection up-time during tracking. The dependence of performance on the transmitted power, switching time and the threshold power for beam steering was examined. The results show that the optical tracking system successfully recovered and maintained the link while the receiver was in motion and the signal coverage at the target area was enhanced significantly.

Free-space optical communication terminals have been designed and extensively tested in various configurations. The FALCON terminals are designed to operate on large unmanned airborne vehicles (UAVs) or piloted aircraft. They provide a secure, two-way air-to-air and air-to-ground data link. In the latest flight test a successful 132km link was established. The beacon lasers operated at half of their available power, which was sufficient to establish and maintain link for the full flight track. The data and beacon links remained locked for approximately 30 minutes during which both aircraft turned, banked, and experienced air turbulence. This demonstration proved that laser communications is possible with tip-tilt correction as the primary control system compensation. It further demonstrated that compact, low cost free-space optical communications are now available for test and evaluation of operational scenarios.

During two separate field tests (July and September 2009) the performance of a free-space optical (FSO) communications link was evaluated in the maritime environment off of the mid-Atlantic coast near Wallops Island, VA. During these two field tests, a bi-directional shore-to-ship data link was established using commercially available adaptive optics terminals. The link, which ranged from 2 - 22 km (optical horizon), was established between a lookout tower located on Cedar Island, VA and a Johns Hopkins University Applied Physics Laboratory research vessel. This paper presents statistical analysis of the power-in-the-bucket captured from two detectors placed alongside the adaptive optics terminal during the September 2009 field trial. The detectors ranged in size from 0.25" to 1.0" in diameter. We will present the histogram reconstruction and compare the data for the 0.25" and 1.0" power-in-bucket (PIB), and 1.0" power-in-fiber (PIF) Adaptive Optics (AO) detectors with analytical probability density function (PDF) models based on the Lognormal, Gamma-Laguerre, and Gamma-Gamma distributions. Additionally, dependence of the results on propagation distance, detector aperture size, and varying levels of optical turbulence are investigated.

Near the ground laser communication systems must operate in the presence strong atmospheric turbulence. To model the performance of a laser communication system operating in the real world we have developed an outdoor 3.2 km, partially over water, turbulence measurement and monitoring communication link. The transmitter side is equipped with the laser and the bank of 20 horizontally, in-line mounted light emitting diodes. The receiver side consists of two channels used for wavefront sensor and point spread function measurements. The effects of anisoplanatism on the point spread function and statistics of Fried parameter r₀ are discussed in this article.

This paper reports on a simulation-based investigation of an optical wireless system using fiber-bundle based transmitters and receivers. The simulation incorporates both beam propagation theory and experimental data from evaluations of the individual components. The coverage area at the receiver is evaluated as a function of transmitter power, link length, misalignment, and the number of transmitter fibers employed. An optimum coverage area is achievable with proper selection of transmitter parameters for a given link length. Trade-offs between the number of fibers employed and the transmitter power are explored.

Recently ultraviolet (UV) scattering channels have received renewed interest for non-line-of-sight (NLOS) communication. Monte Carlo simulations and field experiments have yielded valuable results to predict channel path loss and impulse response at relatively short ranges, critical for communication link analysis. However, as communication range increases, the effect of turbulence becomes pronounced and inevitably induces additional impairments to system performance. This paper suggests a turbulence modeling method for NLOS UV channels incorporating the effects of scattering and absorption. The modeling results can be applied to study communication performance.

We report on the free space optical transmission of FM audio/video signals using a 6.3mm diameter InGaAs modulating retro-reflector.

The Orbital Angular Momentum (OAM) states of photons in paraxial beams allow, in theory, an unlimited number of bits per photon to be used for information encoding in lasercom systems. Atmospheric turbulence scatters the transmitted OAM mode to neighboring modes. The probability of receiving the transmitted mode number decreases with increasing turbulence strength. The degradation is more severe for larger transmitted mode numbers due to their bigger spot size, limiting the range of an OAM encoded lasercom system. To compensate for the lower probability of receiving higher order modes, the concept of receiver OAM bandwidth is defined as a range of received neighboring OAM states allocated to the transmitted OAM mode. By increasing the receiver OAM bandwidth for higher order transmitted modes, the probability to determine the transmitted mode number is similar for all transmitted mode numbers. The optimal system design for OAM encoding using higher order Laguerre-Gauss beams, with the suggested transmitted mode numbers and their corresponding receiver OAM bandwidth, is presented. A closed form analytical expression of the probability to determine the transmitted mode number of the system design is developed. It can be used to easily determine the maximum propagation distance for an OAM encoded lasercom system with a probability to determine the transmitted OAM mode number close to unity.

In recent years, optical wave propagation through strong atmospheric turbulence and adaptive optics compensation thereof has received much attention in literature and technical meetings. At the Air Force Institute of Technology, recent simulation-based efforts in strong turbulence compensation are expanding into laboratory experiments utilizing a versatile surrogate turbulence simulator and adaptive optics system. The system can switch between using two different wavefront sensors, a Shack-Hartmann and a self-referencing interferometer. Wavefront reconstruction takes place on field programmable gate arrays, operating at kilohertz frame rates. Further, the system is able to perform reconstruction and control in software for testing of advanced algorithms (at frame rates below 10 Hz). The entire package is compact enough for transportation to other laboratories and live test facilities. This paper describes the optical layout, architecture, and initial results of real-time operation.