This PDF file contains the front matter associated with SPIE Proceedings Volume 11133, including the Title Page, Copyright Information, Table of Contents, Author and Conference Committee lists.

Mid-wave (MW) and long-wave infrared (LWIR) spectral bands (3 to 5 μm and 9 to 14 μm) are known for their robust transmission characteristics in free-space optical communications (FSOC) under various weather conditions such as haze, fog, rain, and snow. These bands are also expected to be more tolerant to atmospheric turbulence compared to the shortwave IR region (SWIR) near 1.55 μm. Conversely, low-cost, power efficient laser transmitters (Tx) and receivers (Rx) for the MW-LWIR wavelengths are not as widely available as the 1.55 μm counterpart especially for high bandwidth. Larger aperture sizes are also likely required for MW-LWIR to maintain acceptable beam divergence and adequate receiver signal-to-noise ratios (SNRs). All of these are challenges for the development of the MW-LWIR FSOC technology. In the framework of ARAP DOC-P program (Applied Research for the Advancement of S and T Priorities Defense Optical Channel Program), CCDC-ARL (Combat Capabilities Development Command Army Research Laboratory) has taken on the challenge to investigate and develop ground-to-space FSOC in the MW-LWIR regions with commensurate comparisons of MW-LWIR and SWIR systems. The effort started with a detailed literature survey on the MWIR and LWIR FSOC experiments and the latest progress. CCDC-ARL has conducted investigations of the state-of-the-art MWLWIR laser Tx and MW-LWIR photodetectors including in-house development. An FSOC ground testbed employing MW-LWIR COTS quantum cascade laser (QCL) sources is being developed. The Tx will be directly modulated using electronic circuits built in-house. In a collaborative effort with the Naval Research Laboratory (NRL), CCDC-ARL is testing a free-space link emulator based on 1.55 μm fiber optics components first developed by NRL. CCDC-ARL is also developing atmospheric beam propagation simulation tools based on random phase screens in order to gain insight and compare the performance envelope for MW-LWIR and SWIR.

An Adaptive Optics (AO) system may offer an alternative to compensate and correct for beam degradation by reducing turbulence distortions that affect signal detection over horizontal propagation. Based on an experimental testbed placed in the laboratory, we simultaneously study the effects of the communication signal detection, beam wavefront and image quality using a continuous membrane-type deformable mirror and Shack- Hartmann wavefront sensor. By inducing distorting effects on the beam with a Spatial Light Modulator and turbulence masks that are Rytov variance-equivalent to that of actual atmospheric scenarios, and by employing a Zernike polynomials decomposition, beam correction was achieved and signal detection improved. Our results show that both beam-spreading and beam-wandering were reduced after correction, but more significantly, the beam's intensity percentage over detector surface increased in 164%. Future improvements are discussed as an experimental campaign is being prepared to evaluate a closed-loop AO setup for an FSO communication link over a 400-m range at the university campus to evaluate the effectiveness of such approach at different hours of the day and weather conditions.

This research is based in the analysis of the decomposition of Laguerre-Gaussian modes into Hermite-Gaussian modes when propagating through turbulence. This effect can be clearly observed by cancelling the original Laguerre-Gaussian beam with another of different topological charge. The results of the propagations of these beams through Kolmogorov phase masks were analyzed, identifying which turbulences achieved the decomposition. The turbulences were then decomposed into the first 15 Zernike polynomials and used as new "Zernike turbulences". We discovered that the turbulence internal process that allows the decomposition has a high correlation with a phase distortion based on a single Zernike polynomial or a combination of them.

Free-space optical communications are highly sensitive to distortions induced by atmospheric turbulence. This is particularly relevant when using orbital angular momentum (OAM) to send information. As current machine learning techniques for computer vision allow for accurate classification of general images, we have studied the use of a convolutional neural network for recognition of intensity patterns of OAM states after propagation experiments in a laboratory. The effect of changes in magnification and level of turbulence were explored. An error as low as 2.39% was obtained for a low level of turbulence when the training and testing data came from the same optical setup. Finally, in this article we suggest data augmentation procedures to face the problem of training before the final calibration of a communication system, with no access to data for the actual magnification and level of turbulence of real application conditions.

Recent satellites of the European Data Relay System (EDRS) are established with free-space optical (FSO) terminals. Operational FSO links in space are commercially available. Links between ALPHASAT or EDRS-A as GEO terminals and Sentinel LEO terminals are established with TRL 9. However laser links from the satellites through the atmosphere have a low Technical Readiness Level. Starting 2015 an Airborne Optical Communication (AOC) Demonstrator was developed by Hensoldt, which provides Air-to-Space unidirectional communication capabilities over approximately 40000 km distance (Geo orbit) at a data rate of 1.8 Gbit/s at a wavelength of 1064 nm with a laser power of approx. 15 W. The demonstrator was flown on an Airbus A310 aircraft. Successful links with user data streams from Tenerife to the GEO Alphasaat proved the function of AOC.

Future deep space missions will tax the ability of existing radio frequency systems to return all the data. In addition, these missions must be able to communicate around the clock, including when the spacecraft is near the Sun. This is especially true for crewed missions. Optical communication can solve these problems, and also offers the promise of being more compact and using less power and mass. Traditional deep space optical communication concepts require large diameter telescopes (<8m) to collect enough photons to provide adequate signal-to-noise ratio. Systems operating during the day will experience strong turbulence which results in large point spread functions. This necessitates photon counting detectors with a large field-of-view which are difficult to build. The large field-of-view also lets in excessive sky background which degrades the communication performance. Adaptive optics (AO) can mitigate this degradation by concentrating the light and thus not needing a large field of view. We present an AO system architecture capable of operating in the daytime and requiring only moderate performance. We present the architecture along with performance predictions of the system for different size telescopes. Finally we include a technology gap list, which will guide future development of the component technologies.

We report on a monolithic indium phosphide photonic integrated transmitter capable of generating high-speed return-tozero differential phase shift keying (RZ-DPSK) data streams for space optical communications as high as 5 Gbps. The integrated transmitter includes a sampled grating distributed Bragg reflector laser continuously tunable over 30 nm in the C-band, a semiconductor optical amplifier for amplification, a Mach-Zehnder modulator for encoding phase-shift-keying data, and electro-absorption modulator for return-to-zero pulse carving. The transmitter is situated in a custom electronics test bed for biasing various PIC sections and driving the modulators. Furthermore, this transmitter can also be utilized for 10 Gbps DPSK or NRZ-OOK.

We propose an FSO receiver with adaptive-gain stage and associated filtering to reduce signal amplitude excursion and noise for decoding the received symbols with low error rate. A digital synchronization method together with a sampling strategy is developed for a FPGA-based receiver. The proposed techniques are evaluated through simulation and laboratory experiments under real turbulence.

Atmospheric aerosols, containing water, constitute most of the air during non-ideal weather conditions including fog, haze, and mist, and are present in a lower volume density during ideal weather conditions. These aerosols cause light to be attenuated while propagating through the atmosphere, which can be described by Lambert-Beer’s law. The extinction coefficient is dependent on the cross-sectional geometry of the scattering volume which can be found using Mie theory. In the case of a real environment a distribution of particle sizes must be considered where the particles present are described by a weighted value relative to the number density and distribution function of particle radii chosen. We have built a point visibility meter, which measures the amount of scattered light at a specific forward scattering angle under the assumption that the scattered light is linearly related to the extinction coefficient of different weather conditions. To validate our design, it will be compared against a commercial visibility meter.

Aerosol content of the atmosphere is an important factor in the propagation of high-energy laser (HEL) beams. Aerosol absorption leads to the thermal blooming effect, where the laser beam heats the air and thereby creates a diverging lens. We have used three methods to estimate the aerosol absorption coefficient: (1) a Mie calculation on experimentally determined size distributions, (2) the parametric Advanced Navy Aerosol Model (ANAM), and (3) the chemical transport model LOTOS-EUROS. Individual estimates of the absorption coefficient differ significantly, which in turn impacts greatly on the extent of thermal blooming and HEL-beam propagation.

The performance of certain free space optical applications such as laser communication, LIDAR, target designation and astronomical observations may be improved by using beams of different wavelengths for the auxiliary actions of pointing/tracking or turbulence correction. Thus, wavelength dispersion in the atmosphere is a topic of concern for such applications. The chromatic effects of refraction in the atmosphere are generally well-understood and are a function of temperature, pressure, humidity and altitude, as well as the refractive index gradients. In applications such as astronomical observations, chromatic effects are typically predicted based on standard atmospheric models. However, for long horizontal or near-horizontal paths near the Earth’s surface, significant refractive index gradients can be encountered that are associated with features such as inverse temperature layers and ducts. In this study, we explore the wavelength dependence of optical propagation through these temporary and reoccurring refractive index profiles. A ray tracing approach is implemented and the chromatic divergence of the rays through an inverse temperature layer is studied and compared with the behavior expected for the standard atmosphere.

This work details the analysis of time-lapse images with a point-tracking image processing approach along with the use of an extensive numerical weather model to investigate image displacement due to refraction. The model is applied to create refractive profile estimates along the optical path for the days of interest. Ray trace analysis through the model profiles is performed and comparisons are made with the measured displacement results. Additionally, a supervised machine learning algorithm is used to build a predictive model to estimate the apparent displacement of an object, based on a set of measured metrological values taken in the vicinity of the camera. The predicted results again are compared with the field-imagery ones.

We report on additional analysis of the Laser Communication System (LCS) experiment. The LCS experiment consisted of transmitter and receiver stations separated by 3 km over a partially land and water horizontal path. An array of fourteen LEDs operating at 808 nm were placed at the transmitter evenly spaced on a 2.4 m bar. The transmitter was observed by a 0.28 m aperture imaging system. Data was collected over fifty days in the summer of 2009 in a variety of conditions. In previous works we reported on the estimated Fried parameter, isoplanatic angle, and turbulence strength estimated along the imaging path. In this work, inspired by other works, we report on the differential tilt-variance observed and how these measurements may be used to estimate other atmospheric parameters. A reanalysis of the original LCS data is presented in that context.

Free space optical communication utilizing modulating retro-reflectors (MRR) can greatly reduce the complexity of a system in both pointing requirements as well as the necessity for a laser transmitter at both ends of the link. Retroreflectors are susceptible to the same atmospheric turbulence effects of scintillation and beam wander of any laser communication system. An MRR link using an array (N>1) of retroreflectors is affected by self-interference of the return beams. This self-interference can create additional fluctuation that compound to increase the apparent scintillation of the received signal. Data were collected over a 1km outdoor path on the interference pattern returned from a pair of 7mm and 12.5mm retro-reflectors, with multiple spacing distances, in varying turbulence regimes with a 1550nm and 1070nm laser. The interference data of the retroreflectors were correlated with Cn2 data collected simultaneously over the same 1km horizontal path. Under weak turbulence, the self-interference fringes matched diffraction theory, under stronger turbulence regimes the self-interference fringes were either visibly reduced or completely destroyed. We also analyze the contrast of the interference fringes as a function of wavelength for varying turbulence regimes as well as the ability to measure Fried’s parameter from the retroreflector spacing and the returned self-interference pattern.

Electro-optical and laser systems are operated world-wide. Their performance in the outside atmosphere is mainly governed by the strength of optical turbulence C_n² . The predictability of C_n² using weather-forecast models is investigated by performing simulations with the Weather Research and Forecast Model (WRF). The WRF output data were combined with a micrometeorological parametrization to derive C_n² . Simulation runs were performed for locations and times included in our worldwide data set of C_n² obtained in several field trials over land and over the sea. Experimental data of point and integrated path measurements in the surface layer were compared to model calculations of C_n² . The regions include different climatic conditions from South Africa, the US, as well as Central and Northern Europe. The applicability of WRF to predict C_n² at the different locations will be discussed. It will be shown that WRF in a 1.1-km resolution is adequate to provide a first estimate of C_n².

The Space Shuttle Landing Facility (SLF) at The Kennedy Space Center is an immense slab of concrete. This flat and uniform surface has little to no vegetation or structure surrounding. This environment is ideal for the study of laser propagation through atmospheric turbulence. The assumption of homogeneity is satisfied by the runway and the surrounding environment. In our experimentation visible and infrared lasers are propagated across this homogenous environment. Temperature spectra generated by sonic anemometers along with additional MET and scintillometer data will be analyzed and presented.

Atmospheric boundary layer optical turbulence has a strong, and often dominant, impact on laser system performance. Optical turbulence is normally quantified by the refractive index structure parameter (C²_n ). Numerical weather prediction (NWP) models do not produce direct forecasts of C²_n , but gridded, multi-level forecasts of standard meteorological parameters, such as wind speed, air temperature, humidity, pressure and sea surface temperature, can be input to atmospheric optical turbulence models to indirectly forecast three-dimensional C²_n conditions from the surface up through the troposphere. This paper describes methods and issues involved with integrating the Naval Postgraduate School’s Navy Atmospheric Vertical Surface Layer Model (NAVSLaM) and Tropospheric Optical Turbulence Ensemble of Models (TOTEM) with European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis data fields to produce C²_n forecasts for laser performance predictions.

Both the plenoptic sensor and the light field camera can be used to correct images distorted by turbulence. The underlying principle involves using the redundant light field information collected by these devices to discriminate and suppress random distortion in the images. The light field camera records multiple light rays that converges to each spatial point on the image plane, and the plenoptic sensor records multiple views per sub-angular space. Correspondingly, image filters and synthetic methods that are used in the two approaches are significantly different. To the best of the authors’ knowledge, we are the first to build a hybrid system to compare the differences between the two devices in their effectiveness for imaging through turbulence. We show through analysis and case-by-case experimental studies that the turbulence scenarios that fit the employment of a plenoptic sensor or a light field camera are significantly different. Based on our studies, we have summarized the rule of thumb to wisely use light field technology in imaging through turbulence.

Wavefront reconstruction of Laguerre-Gaussian (LG) beams presents a great challenge due to the singularities in their phase profiles. Interferometric methods are difficult to implement since they require a reference beam and they are very sensitive to mechanical perturbations. Deterministic methods such as those based on the phase-transport or intensity-transport equation are limited to the paraxial approximation. Furthermore, if such beams propagate in atmospheric turbulence, complex dynamic characteristics are added to the problem. We seek to find an implementation of a technique that is capable of dynamically recovering the singular phases of LG beams under such random conditions. In this work, we will demonstrate a phase-retrieval technique that allows the recovery of LG wavefronts in turbulence and will characterize its effectiveness under a range of atmospheric parameters and propagation length. This technique is based on binary amplitude modulation and is suitable for dynamic applications.

For applications involving optical beam propagation through the atmosphere, knowledge of the path extinction can be a critical measurement. Measurement of this parameter has been performed in a variety of ways in the past. These approaches have been effective but require careful calibration and expensive Focal Plane Arrays (FPAs) for imaging. In this work, we describe the development of a Near-Infrared (NIR) imaging device utilizing a single detector element, forming a type of single-pixel camera specialized for extinction measurements. This approach somewhat simplifies the calibration process and avoids the expense of an FPA. We demonstrate the capability of the device to measure path extinction through extended real-world testing.

I have generated a simulated light fields of a distant scene imaged by a hypothetical light-field capture system through volumetric turbulence over a long horizontal path. Imaging scenes in this way allows for single-shot scene reconstruction using Multi-Frame Blind Deconvolution (MFBD) where different angular views are substituted for the frames. In this work, I examine the MFBD estimated Point Spread Function (PSF), parameterized in terms of Zernike polynomials, for each of the angular viewpoints. The aim being to understand the degree to which different angular viewpoints are correlated and the rate of decorrelation. The latter rate being important in the design of light-field capture systems designed specifically to deal with extreme anisoplanatism. I find that rate of angular decorrelation is qualitatively very rapid allowing resulting in MFBD reconstructions that are equivalent to fifteen or more independent frames from a single light-field.

Cross correlations between mode coefficients in the Zernike decomposition of atmospheric path aberrations induce correction errors and limit temporal and spatial receiver bandwidths. In this work we propose an optimal modal decomposition technique for gradient-based wavefront sensorless adaptive optics. We implement statistically independent Karhunen-Loeve functions for iterative blind correction and analyze their optimal correction performance in static turbulence conditions.

Effective performance of laser systems intended for power delivery on a distant object requires an Adaptive Optics System (AOS) to correct distortions of the laser beam caused by atmospheric perturbations along the propagation path. The turbulence-induced effects are responsible for beam wandering and intensity scintillation, resulting in degradation of the beam quality and waning of the power density on the target. Adaptive optics methods are used to compensate these negative effects, though an effective AOS performance requires a reference wave formed, for example, by the beacon on the target. One way of forming such a beacon is by using an ultra-short laser pulse (USLP) delivered a tight light spot. Multiple physical phenomena play a key role at USLP propagation in turbulent atmosphere, including: (a) atmospheric perturbations and random amplitude-phase modulation of the propagating beam; (b) spatial modulation of the wavefront and beam shape expressed as its self-focusing caused by the non-linear effects in the atmosphere; (c) a laser noise at USLP propagating in dispersive and nonlinear media. This presentation discusses the requirements to the USLP-based beacon that can support optimal operation of the AOS for effective correction of the wavefront of the outgoing power beam.

Laser beams used in many open space applications, such as in defense, optical communication, and remote sensing, will subject to turbulence distortions that disrupt the intended beam profiles at the end of propagation. To guide the transmitted beam properly through an open space channel, adaptive optics (AO) are often used to implement beam corrections based on the reciprocity principles. In specific, if wave distortion from a remote spot can be determined and field conjugated at the site of the transmitter, the transmitted light will focus to the same spot at the receiver. Many experiments have demonstrated such a principle using a cooperative laser guide star on the target plane. However, finding or creating a well-defined guide star is impractical in real-world applications. The second best beacon choice is temporal glint signals that are relatively refined in geometry and brighter than ambient target illumination. To date, the best approach to extract information from arbitrary glint signals to instruct AO correction is still unknown. We propose the plenoptic sensor technique to extract phase distortion information from glint signals with minimum loss of information. In addition, as the addressed turbulence channel is typically a lateral path near the ground, we also validate the function of the plenoptic sensor in revealing the anisotropic state of turbulence.

Free space optical (FSO) communication is a line of sight technology capable of carrying large volume of data using laser signals through the atmosphere. This unguided propagation of laser beams through the atmosphere confronts with turbulent fluctuations and suspended aerosol particles on its en route to the receiver. Random fluctuations in the atmospheric refractive index causes variations in the propagation constant and thereby affects the optical pulse propagation. We examine the local atmospheric warming effects of absorbing aerosols on the atmospheric refractive index fluctuation statistics and its influence on the group velocity dispersion (GVD) parameter. Black Carbon (BC) aerosols increase local temperature through solar absorption, which will be amplified when they reside in the upper atmosphere for longer duration, owing to the reduced atmospheric density prevailing at higher altitudes. To elucidate the implications of elevated BC layer heating on FSO links, vertical BC mass concentration was measured using an Aethalometer (Model AE-42, of Magee Scientific, USA) mounted on a hydrogen filled balloon. Long term analysis of multi-satellite observations along with in-situ measurements of aerosol parameters show dependence of GVD on aerosol induced local atmospheric warming. Effect of warming on outage probability of FSO systems employing chirped Gaussian pulses are also presented.

Performance of terrestrial and vertical Free-Space Optical (FSO) communication systems are strongly influenced by the atmospheric boundary layer (ABL) dynamics. In addition to the diurnal variations in the refractive index structure parameter (C_n² ) caused by the wind speed and the temperature difference between the surface and the near-surface air mass, any other unprecedented temperature variations can lead to C_n² variations. Even though the prevailing ‘simple’ C_n² models capture the overall trend in vertical C_n² variations, they fail to capture the effects of temperature inversions, especially in tropical regions. Influence of absorbing aerosols like Black Carbon (BC), which can improve the atmospheric stability by forming strong temperature inversion layers, are not considered in these models. BC can reduce the optical beam intensity by scattering and absorption and can also cause variations in refractive index by modifying the local temperature. The uncertainties in the implications of BC will be large, owing to their large spatio-temporal and vertical variations. Using high-resolution balloon measurements and multi-satellite observations coupled with a radiative transfer model, we substantiate the strong influence of absorbing aerosols on the vertical distribution of C_n² . We report how vertical profiling of absorbing aerosols can be used to estimate altitudes with low refractive index fluctuations. The manifestations of high-altitude aerosol-induced atmospheric warming in FSO systems are also pointed out. We conclude by discussing how mass concentration of BC, a good tracer for ABL dynamics, is correlated with the near-surface refractive index fluctuations.

In this paper, an FSO/CV-QKD/QBaudSK system using WCS and coherent detection for dynamical atmospheric conditions emulated by atmospheric turbulence box is presented. In particular, two systems (Alice and Bob) were designed and implemented in order to perform a conventional QKD algorithm using a unidirectional free space channel (private channel) and a bidirectional classical channel (public channel). The preliminary results obtained were related to the performance of the Degree of Polarization of the QWP and PolM considering and non-considering dynamical atmospheric conditions. The results showed the proposed method to be feasible for different weather conditions based on the BER using a 2PolSK-BPSK modulation.

The characteristics of non-line-of-sight (NLOS) ultraviolet (UV) overwater communication channels under shore-to-vessel conditions and multiple scattering effects is modeled using Monte Carlo simulations. The field experiment measured received photon distribution, and calculated path loss at distances of up to 164 meters with the support of a GPSsynchronized accelerometer. Key parameters, such as transceiver elevation angles, water surface reflection index, airborne humidity, pollution, and temperature, were considered and analyzed in path loss simulations to more accurately compare with measured data. These channel modeling and experimental results will serve as the foundation for further study of the NLOS UV overwater and maritime communication system.

Publisher’s Note: This paper, originally published on 6 September 2019, was replaced with a corrected version on 10 October 2019. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.