This PDF file contains the front matter associated with SPIE Proceedings Volume 8732, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

The Naval Research Laboratory (NRL) has extensively investigated atmospheric effects on maritime lasercomm in both long term research studies and short term system demonstrations. A current effort is underway in the ONR Tactical Line-of-Sight Optical Communications (TALON) program to better characterize atmospheric transmission at 1550 nm in multiple climates to enable prediction of communication link performance. In order to allow long term unmanned measurements in these multiple climates, NRL developed a scattering-based 1550 nm transmission sensor similar in function to commercial visible wavelength visibility monitors. After the system was built and calibrated, it was deployed to a number of sites with various climate types to collect data over a 1-month period for each site. This paper describes the development of the instrument and the software. It also presents some of the collected data.

This paper investigates the performance of a recently proposed pointing, acquisition and tracking algorithm for mobile FSO node alignment through experimental methods that include multi-wavelength operation. The performance is evaluated on key parameters including power availability at the receiver, switching time, link recapturing time, and coverage area at the receiver plane. The dependence of performance on the fiber-lens distance at transmitter was examined. The results show that the alignment control system successfully recovered and maintained the link at each selected wavelength while the receiver was in motion.

The presence of intervening clouds in the optical channel is a major factor limiting high availability of Free-Space Optical Communications (FSOC) systems for airborne-to-airborne or airborne-to-ground links. Modern Earth-observing satellite systems are a valuable source of information for characterizing the horizontal stratification of cloud structures that represent the most disruptive source of channel fades. We have created a tool for the estimation of Cloud-Free Lineof- Sight probability based on geographical location, altitude of the participants, the range between them, and the time of year. Using this tool, we explore several scenarios related to previous FSOC work. This tool can be used to develop concept-of-operations for future Air-to-Air and Air-to-Ground FSOC.

Recent studies have been carried out, in which the statistical properties of the received signal of free space communication systems are assessed by means of simulations. The obtained results, however, have not brought information about the temporal behavior of the generated photocurrent. This work describes how this can be accomplished through stochastic processes simulations. Although most of the theory on stochastic field simulation has been developed for application in mechanics and fluids engineering, it is well suited for signal analysis in wireless optical systems. Basically, if it is possible to artificially create a signal with realistic properties that is transmitted over the atmospheric channel, it can be used for system analysis and design without the need of mounting a real transceiver. This dramatically reduces development costs and time. A technique that has been widely applied in engineering is the spectral representation method. The concept of spectral representation of Gaussian random processes was introduced in 1944. Its use, however, in generating simulations of random processes was only proposed in 1972. This method deals with the summation of a large number of weighted trigonometric functions. This work shows how to apply this technique in order to assess the generation of turbulence-corrupted signals using the above mentioned method and its application in simulating data transmission over the atmospheric channel. As figure of merit, system Bit Error Rate and the eye-figure of the received signal are obtained.

Current Strehl ratio models for actively compensated free-space optical communications terminals do not accurately predict system performance under strong turbulence conditions as they are based on weak turbulence theory. For evaluation of compensated systems, we present an approach for simulating the Strehl ratio with both low-order (tip/tilt) and higher-order (adaptive optics) correction. Our simulation results are then compared to the published models and their range of turbulence validity is assessed. Finally, we propose a new Strehl ratio model that is valid for general turbulence conditions independent of the degree of compensation.

Monte-Carlo simulation of phase front perturbations by atmospheric turbulence finds numerous applications for design and modeling of the adaptive optics systems, laser beams propagation simulations, and evaluating the performance of the various optical systems operating in the open air environment. Accurate generation of two-dimensional random fields of turbulent phase is complicated by the enormous diversity of scales that can reach five orders in magnitude in each coordinate. In addition there is a need for generation of the long “ribbons” of turbulent phase that are used to represent the time evolution of the wave front. This makes it unfeasible to use the standard discrete Fourier transform-based technique as a basis for the Monte-Carlo simulation algorithm. We propose a novel concept for turbulent phase – the Sparse Spectrum (SS) random field. The principle assumption of the SS model is that each realization of the random field has a discrete random spectral support. Statistics of the random amplitudes and wave vectors of the SS model are arranged to provide the required spectral and correlation properties of the random field. The SS-based Monte-Carlo model offers substantial reduction of computer costs for simulation of the wide-band random fields and processes, and is capable of generating long aperiodic phase “ribbons”. We report the results of model trials that determine the number of sparse components, and the range of wavenumbers that is necessary to accurately reproduce the random field with a power-law spectrum.

Expressions related to the buffer requirements of an optical communication system in atmospheric turbulence are developed from the channel signal fade time statistics. Laser irradiance data were recorded over the course of one day by a receiving aperture of variable diameter at the Townes Institute Science and Technology Experimentation Facility (TISTEF) 1km laser range located within the Kennedy Space Center at Cape Canaveral, FL. Fade statistics of collected data and scintillometer measurements were compared to the derived model gamma-gamma fade model. Parallel to the laser instrumentation was a commercial scintillometer unit which reported the refractive index structure coefficient, C_n² and the inner-scale of atmospheric turbulence, l₀. The atmospheric parameters inferred from the collected laser data and the commercial instruments were compared. Mean and variance of the fade times were found to agree well with theory for smaller apertures where effects of aperture averaging are not present and in cases where scintillation is weak to moderate. It is suggested that a more appropriate PDF, with a heavier focus on aperture averaging, may be applied in future studies of free space optical communication system fade statistics.

This paper reports the result of follow-on study about the propagation characteristics of direct single mode fiber (SMF) coupled free-space optical (FSO) signals which have been obtained during recent outdoor propagation experiments over 100-500 m link distance. We observed good agreements of the surge height distribution of SMF-coupled optical signal intensity with those of the lognormal prediction. However, the same comparison for the fade depth of SMF-coupled signal indicated 2-3 times larger results. These statistical analyses have validated the method used in this paper to compare atmospheric turbulence model with real experimental data and predict the SMF-coupled signal quality based on the scintillation index determined by C_n² profiles.

We have constructed a fiber-bundle based FSO receiver and investigated its performance as a function of transmission misalignment, turbulence, and weather. We also investigate a wavelength diversity scheme, which consists of switching between multiple transmission wavelengths, for reducing the impact of turbulence. Three wavelengths, 850nm, 1310nm, and 1550nm, are emitted by one or more transmitting fibers, and the effects of turbulence and misalignment experimentally evaluated in an indoor environment. The receiver retained the link for a reduced range of misalignment at all wavelengths without adjustments. Initial outdoor experiments were conducted using 1550nm under various lighting, wind, and turbulence conditions.

A viable beam control technique is critical for effective laser beam transmission through turbulent atmosphere. Most of the established approaches require information on the impact of perturbations on wavefront propagated waves. Such information can be acquired by measuring the characteristics of the target-scattered light arriving from a small, preferably diffraction-limited, beacon. This paper discusses an innovative beam control approach that can support formation of a tight laser beacon in deep turbulence conditions. The technique employs Brillouin enhanced fourwave mixing (BEFWM) to generate a localized beacon spot on a remote image-resolved target. Formation of the tight beacon doesn’t require a wavefront sensor, AO system, or predictive feedback algorithm. Unlike conventional adaptive optics methods which allow wavefront conjugation, the proposed total field conjugation technique is critical for beam control in the presence of strong turbulence and can be achieved by using this non-linear BEFWM technique. The phase information retrieved from the established beacon beam can then be used in conjunction with an AO system to propagate laser beams in deep turbulence.

A Gaussian Schell Model (GSM) might be a convenient way to model extended beacons created on diffuse targets. Earlier, we used a full wave computational technique called the Method of Moments (MoM) to evaluate the scattered field from a rough impedance surface in vacuum. The MoM model showed several deviations from GSM. The present work uses a simulation approach based on physical optics approximation to study the scattering behavior in presence of atmospheric turbulence. A fully coherent beam is propagated through weak turbulence and is incident on the rough surface. The light scattered from the rough surface is again propagated through turbulence back to the source plane and the properties of the scattered radiation are studied through numerical simulations. The simulation results are compared with a GSM.

Aimpoint acquisition and maintenance is critical to high energy laser (HEL) system performance. This study demonstrates the development by the AFIT/CDE of a physics-based modeling package, PITBUL, for tracking airborne targets for HEL applications, including atmospheric and sensor effects and active illumination, which is a focus of this work. High-resolution simulated imagery of the 3D airborne target in-flight as seen from the laser position is generated using the HELSEEM model, and includes solar illumination, laser illumination, and thermal emission. Both CW and pulsed laser illumination are modeled, including the effects of illuminator scintillation, atmospheric backscatter, and speckle, which are treated at a first-principles level. Realistic vertical profiles of molecular and aerosol absorption and scattering, as well as optical turbulence, are generated using AFIT/CDE’s Laser Environmental Effects Definition and Reference (LEEDR) model. The spatially and temporally varying effects of turbulence are calculated and applied via a fast-running wave optical method known as light tunneling. Sensor effects, for example blur, sampling, read-out noise, and random photon arrival, are applied to the imagery. Track algorithms, including centroid and Fitts correlation, as a part of a closed loop tracker are applied to the degraded imagery and scored, to provide an estimate of overall system performance. To gauge performance of a laser system against a UAV target, tracking results are presented as a function of signal to noise ratio. Additionally, validation efforts to date involving comparisons between simulated and experimental tracking of UAVs are presented.