The development of coherent free-space optical communications (FSOC) systems could provide an opportunity for significantly increased data rates and improved security compared to conventional radio frequency (RF) systems in the domain of wireless communications. One key challenge in the implementation of FSOC systems is the characterization and mitigation of atmospheric turbulence present in the optical channel. In this work, we present experiments demonstrating coherent free-space optical communications over an outdoor free-space link with data rates on the order of gigabits per second (Gbit/s). By simultaneously recording the laser spot of the received signal and the bit error ratio (BER) in the link, we measure both, the strength of atmospheric turbulence and the quality of data transmission, in an attempt to quantify the effect of the former on the latter.

Face to the continuous wireless data throughput increase demand and with the aim of simplification, we carried out an architectural study as part of the European collaborative project WORTECS [1]. This study proposed to find a common architecture between wireless radio and wireless optical systems that will allow us to mutualize as much as possible the processing in order to reduce integration complexity and cost. In this paper, after introducing some characteristics about propagation model simulation, we will present common architecture between wireless radio and optical transmission schemes and argue about Graphic Unit Interface development. The main functionalities are defined through three electronic boards able to provide 2 Gbps data rates over 2 wavelengths. Then, before conclusion, we will present the software tool and be focus on preliminary results.

In deep-space optical communications, one particular challenge encountered at the ground receiver side are the wavefront deformations caused by atmospheric turbulence. This gives rise to reduced signal-to-background ratios and signal fades, especially during daytime links. Background rejection is therefore a fundamental requirement for optical links operating in the low-photon-count regime during the day. Both, spectral and spatial filtering subsystems are essential for this application. To this end, we have analyzed several wavefront sensing modalities including: Shack-Hartmann wavefront sensor, holographic wavefront sensing, and wavefront sensorless adaptive optics. The insights gained through simulations were used to drive the design of the laboratory-based demonstrator, which exhibited very similar performance improvements due to high-order correction as in the simulations, despite noise levels of the same magnitude as the signal.

Free-space optical communication (FSOC) holds an unmatched potential for data communications with high bandwidth and security while minimizing size, weight, and power (SWAP). However, the effects of atmospheric turbulence on an optical wave during propagation limits and degrades communication performance. Although this degradation of beam quality occurs due to many factors, but unwanted aberrations due to scattering and absorption of the propagating electromagnetic wave is typically the primary cause. In this paper, we report experimental results from a free space optical FSO communication data link. Bandwidth up to 10 Gbps at 1550 nm is correlated to 𝑐_𝑛² (refractive index structure parameter), transmission wavelength, transmit and receive parameters. For further random and data dependent analysis, the communication link's transmission and receive data eye with amplitude and time jitter decomposition is performed using multiple NRZ PRBS patterns. Additionally, with the aim of reducing SWAP and cost, the experiment is built and designed mostly using off-the-shelf long-range single mode, small factor pluggable devices.

We analyze and numerically simulate a mechanism for generating directed rf radiation when a laser pulse train propagates and forms a plasma filament in air. The role of background radiation levels plays an important role in the ionization process. Photo-ionization of the background negative ions provides the seed electrons necessary to initiate collisional ionization with the air molecules. In this ionization mechanism, the peak intensity of the laser pulse train is far below the tunneling ionization level. The collisionally-ionized electrons are driven radially outward by the ponderomotive force associated with the laser pulses. The resulting oscillating radial currents generate rf radiation mainly in the direction of the laser pulses. The rf frequency is directly related to the laser pulse separation time in the pulse train. The ionization and rf generation mechanism is analyzed using a non-relativistic fluid model which incorporates, among other things, the effects of background radiation, photo-ionization, collisional ionization, ponderomotive and space charge effects, and attachment/recombination processes. The electron density build-up and rf radiation level and directionality are obtained. The results of our analysis and simulations are in good agreement with experiments employing laser pulse trains ionizing air and generating rf radiation.

This paper introduces a method and experiment using free-space, pulsed, optical time transfer, through turbid water to synchronize spatially distant chip-scale atomic clocks. This experiment uses one-way time transfer. Clock discrepancy is computed by comparing the measured departure time from Module A to the measured arrival time at the Module B, after accounting for the estimated time of flight. The effect of the turbidity is calculated and compared through clock discrepancy and time transfer stability for an empty tank, still water, turbid water, and fog.

We propose the use of orbital angular momentum (OAM) states to form modulation symbols in a free-space laser communication link affected by atmospheric turbulence. A collection of superpositions of 2 and 4 active OAM modes are used for transmitting digital information. To sense and decode the data we compare three candidate architectures, based on a Mode Sorter, a Shack-Hartmann and phase flattening holograms. In this work we use concepts of Optimal Transport, particularly the Wasserstein distance and barycenter, for an optimal selection of OAM superpositions and a more appropriate processing of OAM spectra, leading to more accurate detection, achieving a classification error smaller than 1/1000 in intermediate to strong turbulence conditions.

Object-Oriented Matlab Adaptive Optics (OOMAO) is a Matlab toolbox dedicated to Adaptive Optics (AO) systems. It is based on a small set of classes representing the source, atmosphere, telescope, wavefront sensor, Deformable Mirror (DM) and an imager of an AO system. The original OOMAO toolbox was developed for modeling the performance of adaptive optics systems utilizing plane-waves. We have extended the capabilities of the OOMAO toolbox by adding a new method, a Gaussian-beam source, to an existing class, source. This new source method is needed to use OOMAO for modeling laser communications uplinks that are currently under development by many institutions. In this paper, we describe the method that we newly added, namely, Gaussian-beam source uplink, and present several numerical examples. These include results from the simulation for uplink pre-compensation of lasers. We will also show how well uplink works in and outside of the isoplanatic patch.

The analysis of free-space optical system performances requires the calculation of several metrics of the turbulence-affected collected light. One of the most useful of these metrics is the long-term beam spread in the focal plane, where the detector (or optical fiber) should be located. In this paper, we use the ABCD ray-matrix formalism to show the impact of Kolmogorov and non-Kolmogorov turbulence on the long-term beam spread in the focal plane.

Optical link prediction capabilities are important for planning and operating a free-space, optical communication link outdoors. The impact of the atmospheric channel on the optical link is dependent on factors like geographic location and local atmospheric conditions. Currently, link predication and analysis tools, like the Navy Atmospheric Vertical Surface Layer Model (NAVSLAM) and the Path Characterization System (PCS), are used to predict the atmospheric characteristics and received power. Additionally, the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) is a model that has recently been integrated into PCS and needs to have its prediction of refractive index structure parameter (Cn2) validated from outdoor range data. The bit error rate is a measure of error probability and when combined with baud rate, can be used to predicted the data transmission rate. In this experiment, the characterization of a one-way laser communication link was conducted outdoors on a 1 km range in a littoral environment. During communication, the bit error rate and received power were measured on the receiver side of the link. Additionally, the local atmospheric conditions were measured and utilized by link analysis tools, including the NAVSLaM, COAMPS, and PCS. These tools have been used to quantify the turbulence and path losses and evaluated against range measurements. The bit error rate and received power were also computed and compared with the measurements on the range to evaluate link predictive capabilities.

A 1.55µm Laser Propagation Testbed (LPT) has been deployed over a 3.5 km stretch of open water between the port of Den Helder and the island of Texel in The Netherlands. The laser intensity and beam profile have been measured after propagation and reflection off a capture plate. Supplementing data was provided by large-aperture scintillometers, aerosol counters, visibility meters and standard meteorological equipment. The LPT will be presented, as well as a selection of data to demonstrate the potential of this setup. The LPT is expected to validate and verify laser propagation models and thereby to contribute in the field of atmospheric propagation, optical communication and high energy laser systems.

This publication will demonstrate recent advances of a self-referencing homodyne interferometry technique for mitigating atmospheric turbulence. The results will be quantified by using QR codes to document the machine-readable performance gain by using Digital Adaptive Optics when compared to a traditional imaging camera.

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 C_n² 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 present an experimental campaign consisting on the propagation of laser modes with orbital angular momentum (OAM), carried out in our campus, for a total propagation distance of 1 kilometer. In this proceedings article we describe our experiment and describe some preliminary results, using topological charges up to |ℓ| = 45. We also demonstrate that a Shack-Hartmann sensor may be used for OAM sensing in the presence of weak to intermediate turbulence without the help from adaptive optics.

Sonic anemometers have been used extensively to measure virtual temperature fluctuations associated with turbulence and thereby determine the temperature structure function parameter. While it is common to utilize the temperature power spectrum in such an analysis, it is similarly possible to use a structure function based approach. In this work, we consider the details involved and benefits/disadvantages of processing by each method.

The TISTEF Camera and Anemometer Turbulence System (T-CATS) is a single ended instrument for estimating the refractive index structure parameter (C² _n ) along a given path. This instrument was developed by the Wave Propagation Research Group and utilizes a wide-angle camera, a thermal camera, a raspberry pi zero w, and an Applied Technologies SATI-3A sonic anemometer. Kolmogorov’s equations for spatial temperature difference are used to estimate C² _n within the field of view of the thermal camera. T-CATS was deployed and evaluated at the TISTEF laser range. These data were compared to a SATI-3A sonic anemometer.

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.

The generation of orbital angular momentum (OAM) in laser light has been the subject of study for various applications including the improvement of laser transmission in turbid environments. NIWC Atlantic has collaborated with Clemson University to build an experiment to test rapidly tunable OAM system for higher order Bessel beams integrated in time (HOBBIT) and OAM cross-correlator at the communications C-band. This report details the construction of the bench prototype and summarizes future research avenues.

In the field of space optical communication, coherent communication system has the advantages of high sensitivity, compatibility with a variety of modulation formats and strong anti-interference ability compared with intensity modulation direct detection (IM-DD) system. The typical disadvantage of coherent communication system is the complexity of the receiver, which requires carrier recovery. Space optical communication is also developing from point-to-point communication to optical communication network. In this paper, a fast phase locking method based on direct phase control is proposed. After the auxiliary frequency acquisition is completed, the time to recover the carrier is equal to the loop delay. The realization of this method depends on the direct control of the local oscillator carrier phase. Different from the traditional second-order loop method, this method does not need to consider the laser linewidth. This method can greatly save the time of carrier recovery, so it can be used for fast switching of optical network channels and can improve the utilization of optical networks.

Free space optical communications are highly dependent on atmospheric conditions to ensure successful transmission from transmitter to receiver. The integrity of an optical communications link can degrade depending on the atmospheric conditions during propagation. This reduced signal strength of an optical communications link can be directly associated with atmospheric conditions that cause scattering, absorption, and optical turbulence. Our main goal is to characterize the atmosphere for modeling and analysis of atmospheric conditions for optimizing optical communications in a coastal environment. Instruments including an anemometer, scintillometer, and a weather station hub collected local atmospheric and weather data. These data generated plots describing temperature, wind speed and direction, humidity, pressure, and path averaged scintillation index throughout the seasons. We began analyzing the path averaged scintillation index against the other data points to identify trends and correlations in the path averaged scintillation index and how it is affected by the other atmospheric conditions.

This PDF file contains the front matter associated with SPIE Proceedings Volume 12237 including the Title Page, Copyright information, Table of Contents, and Conference Committee Page.