The Space Development Agency (SDA) is developing the Proliferated Warfare Space Architecture (PWSA) - a constellation of hundreds of satellites in low earth orbit delivering space-based capabilities to the joint warfighter. The PWSA is a mesh network of optically-connected satellites providing low-latency data transport and missile warning/tracking capabilities. SDA capitalizes on a unique business model that values speed and lowers costs by harnessing commercial development. The Optical Communications Terminal (OCT) standard was created to provide optical interoperability specifications, enable a strong marketplace, and to drive advancements in optical communication capabilities to terrestrial, maritime, and airborne warfighting elements. As part of the spiral development process, the OCT standard evolves with PWSA deployment phases. SDA has incorporated feedback as well as advancements to the OCT standard, resulting in the release of version 3.1.0. In this paper we discuss key aspects of the OCT standard, such as wavelength, modulation, data rates, polarization, link distance, error correction coding, pointing, acquisition and tracking, and position, navigation, and timing.

Free Space Optical links suffer from atmospheric effects where turbulence causes the beam to break up (scintillation), which could increase the variance in the signal at the receiver and ultimately worsening the optical link. Various techniques to reduce scintillation exist to alter how the atmosphere will affect the beam, one of them being wavelength diversity of the optical source. Diversifying the wavelength can reduce the scintillation of the optical beam due to the wavelength dependence on the refractive index of the atmosphere. An experiment was conducted comparing a broadband laser source and a monochromatic source over an instrumented 13.5km path. Beam profile and scintillation measurements were conducted along with BLS-2000 Cn2 measurements. This experiment investigates the effects of a short-coherence length / broad-band nature of a source and its ability to reduce scintillation in turbulent atmosphere. This paper will discuss the experimental setup, analysis, and conclusions of this novel experiment.

We explore the impacts of adverse weather on the propagation of a pulsed 1.5um laser source over a 1km maritime channel. The propagation path along this channel is well-instrumented with sensors to measure standard weather conditions (wind, temperature, humidity and rainfall), visibility and atmospheric turbulence. Data collected to characterize the propagation path are used to initialize channel modeling and predict performance of 1.5um propagation. A high-speed detector and a camera located at the target board recorded temporal and spatial effects of rain on the propagated laser beam. The data are analyzed for pulse width, beam profile, beam wander as a function of rainfall and compared to the channel model.

Propagation of laser beams through a turbulent atmosphere over extended ranges can cause significant beam scintillation and wander which can degrade the effectiveness of a Free Space Optical (FSO) link. The use of a spectrally broadband laser light source, with a high spatial coherence and short temporal coherence, could lead to improved performance in one or both of these areas. This experiment investigates the effect of temporal coherence on the far-field turbulence induced effects on the beam. Narrow linewidth coherent sources were compared against a broadband source over a 13.5 km slant-path. The path was instrumented with a path averaged turbulence monitoring device during data collection along with a range of other meteorological parameters to predict atmospheric parameters. Target board beam profile data was collected to measure the spatial statistics due to atmospheric turbulence along with silicon detectors to measure the temporal statistics of the atmospheric turbulence effects. This data is analyzed and compared to full diffraction wave propagation simulation results. Our analysis shows the benefit that the broadband source does not suffer as many scintillation effects as the narrow-linewidth sources.

Modern silicon photovoltaic (PV) cells have high external quantum efficiencies (>70%) from 900nm-1070nm, and are ideally suited as laser power receivers to match the wavelength of high power lasers available today. Silicon PV cells are ~300X less expensive than TTT-V photovoltaic cells making them economical alternatives for large area receivers. A large receiver benefits the laser side of a wireless power system by reducing the requirement for maintaining a small beam at a great distance and eases the wireless receiver design by allowing waste heat to be spread over a larger surface area. Finally, a silicon PV array can efficiently combine solar energy harvesting during the day, and laser energy power transfer at night with a single low-cost array. In this paper we study commercially available silicon solar cells, evaluate their suitability for a laser power converter, and discuss some of the system related aspects of fielding a laser power converter.

The growth of optical communication has created a need to correctly characterize the atmospheric channel. Atmospheric turbulence along a given channel can drastically affect optical communication signal quality. One means of characterizing atmospheric turbulence is through measurement of the refractive index structure parameter, C-n(2). When calculating C-n(2) from the scintillation index, sigma(2)(I), the point aperture scintillation index is required. Direct measurement of the point aperture scintillation index is difficult at long ranges due to the light collecting abilities of small apertures. When aperture size is increased past the atmospheric correlation width, aperture averaging decreases the scintillation index below that of the point aperture scintillation index. While the aperture averaging factor can be calculated from theory, it does not often agree with experimental results. Direct measurement of the aperture averaging factor via the pupil plane irradiance covariance function allows conversion from the aperture averaged scintillation index to the point aperture scintillation index. Using a finite aperture, camera, and detector, the aperture averaged scintillation index and aperture averaging factor are measured in parallel and the point aperture scintillation index is calculated. A new instrument built by SSC Pacific was used to collect scintillation data at the Townes Institute Science and Technology Experimentation Facility (TISTEF). This new instrument's data was then compared to BLS900 data. The results show that direct measurement of the aperture averaging factor is achievable using a camera and matches well with ground-truth instrumentation.

The usage of long-range optical systems for tracking applications encounters regions of deep turbulence throughout propagation. Such conditions lead to the inability to remain on target for a tracked object due to scintillation. To mitigate this issue, a double pass optical system is utilized as a means of tracking enhanced backscatter (EBS) and thus keeping alignment while characterizing turbulent conditions. EBS is detected through image processing algorithms that capture the returning constructive interference from the target. This paper evaluates EBS optical systems using a retro-reflector at a 1 kilometer distance in order to validate theoretical models that typify atmospheric turbulence regarding low-ground propagation. Meteorological conditions are also included in the empirical data obtained for the analysis of atmospheric conditions that contribute to non-homogenous turbulent conditions along the path.

Laser beam speckle resulting from atmospheric turbulence contains information about the propagation channel. The number and size of the speckle cells can be used to infer the spatial coherence and thus the C-n(2) along a path. The challenge with this technique is the rapidly evolving speckle pattern and non-uniformity of the speckle cells. In this paper we investigate modern blob counting techniques used in biology, microscopy, and medical imaging. These methods are then applied to turbulent speckle images to estimate the number and size of the speckle cells. Speckle theory is reviewed for different beam types and different regimes of turbulence. Algorithms are generated to calculate path C-n(2) from speckle information and path geometry. The algorithms are tested on speckle images from experimental data collected over a turbulent 1km path and compared to C-n(2) measurements collected in parallel.

In a previous paper, the feasibility and trade space of a CubeSat with modulated retroreflector optical communications was discussed. In this paper, the design and testing of a surrogate retroreflector payload to assess key components of a modulated retroreflector payload is presented. In addition to passive retroreflectors, the surrogate payload has an optical beacon to aid in acquisition and tracking as well as an optical detector. The payload design is comprised of six subsystems: mechanical structure, optical electrical circuits, flight computer, retroreflectors, optical beacon, and optical detector. The mechanical structure has been tailored to accommodate the retroreflectors, while preserving roughly a 1.5U size. The circuits are designed to perform electrical power conversion, drive a high current optical source, and drive an optical detector. The flight computer will be comprised of a commercially available FPGA and/or a microcontroller on a custom circuit board to interface with the optical beacon and optical detector as well as collect telemetry. Previously developed models are used to design custom retroreflectors to provide a specific returned optical intensity pattern. The optical source and optical detector are commercially available and designed according to link analysis and electrical power restrictions. Individual components will be benchmarked, environmentally tested, and reassessed for performance prior to integration into the mechanical structure.

The Navy is actively developing diverse optical application areas, including high-energy laser weapons and free-space optical communications, which depend on an accurate and timely knowledge of the state of the atmospheric channel. The Optical Channel Characterization in Maritime Atmospheres (OCCIMA) project is a comprehensive program to coalesce and extend the current capability to characterize the maritime atmosphere for all optical and infrared wavelengths. The program goal is the development of a unified and validated analysis toolbox. The foundational design for this program coordinates the development of sensors, measurement protocols, analytical models, and basic physics necessary to fulfill this goal.

The growth of optical communication has created a need to correctly characterize the atmospheric channel. The measurement of turbulence, due to its ability to drastically effect signal quality, is an important part of this characterization and can be partially accomplished via calculation of the scintillation index. However, proper calculation of the scintillation index requires that the background (specifically the diffuse solar background) be accurately subtracted from the transmitted signal. While there are many methods to remove this background we introduce a hardware based method which seeks to overcome the weaknesses of traditional approaches while adding its own strengths. The corrected signal is allowed a greater dynamic range and atmospheric background variations are accounted for during transmission. We begin by discussing the scintillation index and traditional means of background subtraction followed with an introduction of our proposed optical design. We provide details of the experimental setup, data collection over a maritime location in San Diego, and analysis. Finally, we compare scintillation index calculations using our new method and a traditional method of background subtraction. Our results ranked our method favorably alongside common methods of background subtraction.

The characterization of atmospheric effects on a propagated laser beam is important to applications ranging from free-space optical communications to high-energy laser systems for ship defense. These applications are frequently developed for a dynamic propagation environment in which either one or both ends of the optical link are moving. The instruments are often constrained by size, weight, and power limitations due to the platforms on which they will be installed. The dynamic nature of the optical link induces several difficulties in link-path instrumentation: turbulence statistics on a continuously changing path are hard to interpret, and the optical instruments must be designed to maintain a high-quality link between beacon and receiver. We will review some of the scintillometer designs and we examine the associated data produced by these different instruments.

The authors have recently developed an optical transmissometer device used for estimation of the visibility and atmospheric extinction coefficient along a horizontal or slant terrestrial path of ranges from 500m out to 6 km. This is a bistatic device using a modulated LED beacon transmitter and an 8” (200mm) primary receiver lens with a silicon (Si) photodetector. We discuss how this device can be used to simultaneously obtain an estimate of the atmospheric turbulence characteristics along the same propagation path, using the optical intensity scintillation effect, without requiring any hardware modifications to the existing device. Device principles of operation are presented, followed by the results of a preliminary proof-of-concept field test which yielded encouraging results showing validity of the basic system design but indicating that additional engineering work is required to resolve some implementation details, and further field testing needed to verify and validate the system.

Machine learning methods improve performance of a novel atmospheric parameter estimation algorithm for characterizing local atmospheric propagation channel optical turbulence, extinction, and scattering. Results of lab experiment, field test, and wave-optics propagation simulations are presented.

Free-space laser communications are subjected to performance degradation when heavy fog or smoke obscures the line of sight (high-loss optical media). On the other hand, it has been demonstrated that laser-induced plasma filaments (LIPF) can propagate for long distances (up to a few kilometers) through clouds and/or turbulent (lossy) atmosphere. Here we propose to use LIPF to improve and/or restore laser communication in adverse, high-loss and/or denied conditions. This work is focused on demonstrating the proof of concept and is dedicated primarily to gaseous, optically transparent media.

Current transmissometer designs can be physically bulky, electronically complex, and susceptible to background light; ultimately limiting performance. We describe a novel transmissometer design based upon a modulated LED source and an AC-coupled receiver to improve upon the aforementioned shortcomings. The design aims to reduce both complexity and SWAP through the use of a high frequency modulation technique, while ultimately improving SNR and measurement range over a variety of atmospheric conditions. The instrument is a dynamic atmosphere and range transmissometer (DART). First we discuss the theory associated with our technique; particularly addressing how the effects of atmospheric turbulence are handled. Next, we describe the radiometry and calibration procedures for the transmitter and the receiver. We describe the instrument hardware and how the DART was built and tested in the laboratory. Finally, we discuss the field experiment to test the DART against a commercial unit over a 700m coastal path in San Diego. The processed data are compared with concurrent measurements from the Optec LPV-3 commercial transmissometer. Transmission data from the DART tracks the commercial instrument very well over varying atmospheric conditions.

Obtaining accurate, precise and timely information about the local atmospheric turbulence and extinction conditions and aerosol/particulate content remains a difficult problem with incomplete solutions. It has important applications in areas such as optical and IR free-space communications, imaging systems performance, and the propagation of directed energy. The capability to utilize passive imaging data to extract parameters characterizing atmospheric turbulence and aerosol/particulate conditions would represent a valuable addition to the current piecemeal toolset for atmospheric sensing.Our research investigates an application of fundamental results from optical turbulence theory and aerosol extinction theory combined with recent advances in image-quality-metrics (IQM) and image-quality-assessment (IQA) methods. We have developed an algorithm which extracts important parameters used for characterizing atmospheric turbulence and extinction along the propagation channel, such as the refractive-index structure parameter C-n(2), the Fried atmospheric coherence width r(0), and the atmospheric extinction coefficient beta(ext), from passive image data. We will analyze the algorithm performance using simulations based on modeling with turbulence modulation transfer functions. An experimental field campaign was organized and data were collected from passive imaging through turbulence of Siemens star resolution targets over several short littoral paths in Point Loma, San Diego, under conditions various turbulence intensities. We present initial results of the algorithm's effectiveness using this field data and compare against measurements taken concurrently with other standard atmospheric characterization equipment. We also discuss some of the challenges encountered with the algorithm, tasks currently in progress, and approaches planned for improving the performance in the near future.