This PDF file contains the front matter associated with SPIE Proceedings Volume 9224, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Mathematical models for a Gaussian-beam wave propagating through anisotropic non-Kolmogorov turbulence have been developed in the past by several researchers. In previous publications, the anisotropic spatial power spectrum model was based on the assumption that propagation was in the z direction with circular symmetry maintained in the orthogonal xy-plane throughout the path. In the present analysis, however, the anisotropic spectrum model is no longer based on a single anisotropy parameter—instead, two such parameters are introduced in the orthogonal xyplane so that circular symmetry in this plane is no longer required. In addition, deviations from the 11/3 power-law behavior in the spectrum model are allowed by assuming power-law index variations 3 < α < 4 . In the current study we develop theoretical models for beam spot size, spatial coherence, and scintillation index that are valid in weak irradiance fluctuation regimes as well as in deep turbulence, or strong irradiance fluctuation regimes. These new results are compared with those derived from the more specialized anisotropic spectrum used in previous analyses.

In recent research, propagation of plane electromagnetic (EM) waves through a turbulent medium with modified von Karman phase characteristics was modeled and numerically simulated using transverse planar apertures representing narrow phase turbulence along the propagation path. The case for extended turbulence was also studied by repeating the planar phase screens multiple times over the propagation path and incorporating diffractive effects via a split-step algorithm. The goal of the research reported here is to examine two random phenomena: (a) atmospheric turbulence due to von Karman-type phase fluctuations, and (b) chaos generated in an acousto-optic (A-O) Bragg cell under hybrid feedback. The latter problem has been thoroughly examined for its nonlinear dynamics and applications in secure communications. However, the statistical characteristics (such as the power spectral density (PSD)) of the chaos have not been estimated in recent work. To that end, treating the chaos phenomena as a random process, the time waveforms of the chaos intensity and their spectra are numerically evaluated over a (large) number of time iterations. These spectra are then averaged to derive the equivalent PSD of the A-O chaos. For the turbulence problem, an optical beam passing through an input pinhole is propagated through a random phase screen (placed at different locations) to a desired distance (typically near-field) under different levels of turbulence strength. The resulting spatial intensity profile is then averaged and the process repeated over a (large) number of pre-specified time intervals. From this data, once again, the turbulence PSD is calculated via the Fourier spectra of the average intensity snapshots. The results for the two systems are compared.

Under strong scintillation condition the phase of the optical field propagating through turbulence contains phase singularities. Commonly used statistical description of phase in terms of phase structure function is based on smooth perturbation (Rytov) theory, and does not account for singularities. Markov approximation for wave propagation in turbulence predicts asymptotically normal probability distribution with zero mean for the optical field under strong scintillation conditions, and provides an equation for the coherence function. These allow calculation of all statistics of the field including the amplitude and phase. We derive equations for the single and two-point joint and marginal probabilities distributions of the amplitude and phase, and equation for the phase structure function that are valid for the strong scintillation condition when phase singularities are present.

Monte-Carlo models of the turbulent phase are widely used in the studies of the optical propagation through turbulence atmosphere. However all algorithms, that are currently in use, generate continuous smooth phase samples. Meanwhile, it is well-known that under strong scintillation conditions turbulent phase has singularities, and phase is discontinuous across the branch cuts connecting the singularities. Markov approximation for wave propagation through random inhomogeneous media ^{1, 2} predicts that under the strong scintillation conditions optical field asymptotically has normal distribution ³. We propose to generate the phase samples under strong scintillation conditions by first producing a complex normal random field with a given coherence function, and then recovering the random phase as the argument of this field. This approach allows generation of the two-dimensional discontinuous random phase samples that include phase singularities. Phase simulation algorithm is based on the Sparse Spectrum concept that was introduced in our earlier works, and can be modified to fit any desired shape of the turbulence spectrum. We verify the accuracy of the phase samples statistics by comparison with the theoretical results presented in the companion paper.

The alternative Bendersky, Kopeika, and Blaunstein (BKB) model of measuring the refractive structure index parameter, Cn² has proven to be a reliable, well-used means of quantifying and characterizing the atmospheric turbulence in a given environment. This model relies on various meteorological parameters such as temperature, wind speed, relative humidity, and time of day in order to procure the resulting Cn² quantity. Using experimentally confirmed results from a desert environment, the utility of this model may be extended to other climates by adapting temporal hour weights used within the model. The adaptation of these weighted parameters are shown to have a relationship with the unique weather conditions of a given region which are demonstrated by data points collected from two testing ranges located in Florida in addition to archived weather data. The resulting extended model is then compared to commercial scintillometer data for validation.

Laser communication with both high photon information efficiency (many bits/detected-photon) and high spectral efficiency (many bits/sec-Hz) is impossible with a single spatial-mode free-space link. Achieving these high efficiencies in the same system requires operation with 10's to 1000's of high-transmissivity spatial modes. Such systems will likely be restricted to 1 to 10 km line-of-sight terrestrial paths on which turbulence-induced cross talk will be encountered. In this paper we propose a cross-talk simulator for multiple spatial-mode free-space optical communication that could provide valuable information about the relevant merits of different mode sets when they are employed in conjunction with real modal multiplexers and demultiplexers.

Multiple-input multiple-output (MIMO) systems are employed in free space optical (FSO) links to mitigate the degrading effects of atmospheric turbulence. In this paper, we consider a MIMO FSO system with practical transmitter and receiver configurations that consists of a radial laser array with Gaussian beams and finite sized detectors. We formulate the average received intensity and the power scinitillation as a function of the receiver coordinates in the presence of weak atmospheric turbulence by using the extended Huygens-Fresnel principle. Then, integrations over the finite sized multiple detectors are performed and the effect of the receiver aperture averaging is quantified. We further derive an outage probability expression of this MIMO system in the presence of turbulence-induced fading channels. Using the derived expressions, we demonstrate the effect of several practical system parameters such as the ring radius, the number of array beamlets, the source size, the link length, structure constant and the receiver aperture radius on the system performance.

Multiple-input multiple-output (MIMO) technique is now used in most new broadband communication system, and orthogonal frequency division multiplexing (OFDM) is also utilized within current 4^th generation (4G) of mobile telecommunication technology. With MIMO and OFDM combined, visible light communication (VLC) system’s diversity gain is increase, yet system capacity for dispersive channels is also enhanced. Moreover, with the emerging massive MIMO-OFDM VLC system, there are significant advantages than smaller systems’ such as channel hardening, further increasing of energy efficiency (EE) and spectral efficiency (SE) based on law of large number. This paper addresses one of the major technological challenges, system architecture design, which was solved by semispherical beehive structure (SBS) receiver and so that diversity gain can be identified and applied in Massive MIMO VLC system. Simulation results shows that the proposed design clearly presents a spatial diversity over conventional VLC systems.

The deployment of next generation lunar laser retroreflectors is planned in the near future. With proper robotic deployment, these will support single shot single photo-electron ranging accuracy at the 100 micron level or better. There are available technologies for the support at this accuracy by advanced ground stations, however, the major question is the ultimate limit imposed on the ranging accuracy due to the changing timing delays due to turbulence and horizontal gradients in the earth’s atmosphere. In particular, there are questions of the delay and temporal broadening of a very narrow laser pulse. Theoretical and experimental results will be discussed that address estimates of the magnitudes of these effects and the issue of precision vs. accuracy.

This study quantifies the feasibility for a mobile sodium guidestar system. Simulations are run using the High Energy Laser End-to-End Operational Simulation (HELEEOS) software package with global sodium layer climatology data. Sodium layer data used was collected from the Optical Spectrograph and Infrared Imaging System (OSIRIS) sensor package on board the Odin satellite from 2005 through 2011 and provides a detailed global representation of the variable sodium layer occurring at an altitude of approximately 90 km in the atmosphere. This data is used in conjunction with the HELEEOS atmospheric propagation modeling to create realistic sodium guidestar models. The atmospheric effects for the laser propagation scattering model and creation of the sodium guidestar are defined in the worldwide probabilistic climatic database available in the HELEEOS software package. The simulations run evaluated the performance of a guidestar as viewed from along the propagation path and from non-propagation path viewing angles for engagement scenarios in various locations on earth. HELEEOS includes a fast-calculating, first principles, worldwide surface to 100 km, with extensions above 100 km to account for the sodium layer, 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. HELEEOS is able to produce realistic evaluations of laser propagation, imaging, and adaptive optics systems by use of an end to end directed energy propagation model that incorporates probabilistic, climatological data from temporally and spatially variable meteorological, aerosol, and turbulence profiles. Specifically, HELEEOS performs its propagation calculations utilizing the following algorithms, models and datasets: the Scaling for HEL and Relay Systems (SHaRE) scaling law algorithms, High Resolution Transmission (HITRAN) database for molecular absorption, Global Aerosol Dataset (GADS), Advanced Navy Aerosol Model (ANAM), the Adaptive Optical Compensation of Thermal Blooming (AOTB) model, various turbulence models, and other physics based atmospheric propagation algorithms. HELEEOS was developed by the United States Air Force Institute of Technology (AFIT) under the sponsorship of the High Energy Laser Joint Technology Office.

Adaptive optics methods have long been used by researchers in the astronomy field to retrieve correct images of celestial bodies. The approach is to use a deformable mirror combined with Shack-Hartmann sensors to correct the slightly distorted image when it propagates through the earth’s atmospheric boundary layer, which can be viewed as adding relatively weak distortion in the last stage of propagation. However, the same strategy can’t be easily applied to correct images propagating along a horizontal deep turbulence path. In fact, when turbulence levels becomes very strong (C_n ²>10^-13 m^-2/3), limited improvements have been made in correcting the heavily distorted images. We propose a method that reconstructs the light field that reaches the camera, which then provides information for controlling a deformable mirror. An intelligent algorithm is applied that provides significant improvement in correcting images. In our work, the light field reconstruction has been achieved with a newly designed modified plenoptic camera. As a result, by actively intervening with the coherent illumination beam, or by giving it various specific pre-distortions, a better (less turbulence affected) image can be obtained. This strategy can also be expanded to much more general applications such as correcting laser propagation through random media and can also help to improve designs in free space optical communication systems.

In this study, we present a brief review on the existing approaches for optical turbulence estimation in various layers of the Earth’s atmosphere. The advantages and disadvantages of these approaches are also discussed. An alternative approach, based on mesoscale modeling with parameterized turbulence, is proposed and tested for the simulation of refractive index structure parameter (C²_n ) in the atmospheric boundary layer. The impacts of a few atmospheric flow phenomena (e.g., low-level jets, island wake vortices, gravity waves) on optical turbulence are discussed. Consideration of diverse geographic settings (e.g., flat terrain, coastal region, ocean islands) makes this study distinct.

A plenoptic camera is a camera that can retrieve the direction and intensity distribution of light rays collected by the camera and allows for multiple reconstruction functions such as: refocusing at a different depth, and for 3D microscopy. Its principle is to add a micro-lens array to a traditional high-resolution camera to form a semi-camera array that preserves redundant intensity distributions of the light field and facilitates back-tracing of rays through geometric knowledge of its optical components. Though designed to process incoherent images, we found that the plenoptic camera shows high potential in solving coherent illumination cases such as sensing both the amplitude and phase information of a distorted laser beam. Based on our earlier introduction of a prototype modified plenoptic camera, we have developed the complete algorithm to reconstruct the wavefront of the incident light field. In this paper the algorithm and experimental results will be demonstrated, and an improved version of this modified plenoptic camera will be discussed. As a result, our modified plenoptic camera can serve as an advanced wavefront sensor compared with traditional Shack- Hartmann sensors in handling complicated cases such as coherent illumination in strong turbulence where interference and discontinuity of wavefronts is common. Especially in wave propagation through atmospheric turbulence, this camera should provide a much more precise description of the light field, which would guide systems in adaptive optics to make intelligent analysis and corrections.

This paper discusses an advanced target in the loop (ATIL) system with its performance based on a nonlinear phase conjugation scheme that performs rapid adjustment of the laser beam wavefront to mitigate effects associated with atmospheric turbulence along the propagation path. The ATIL method allows positional control of the laser spot (the beacon) on a remote imaged-resolved target. The size of this beacon is governed by the reciprocity of two counterpropagating beams (one towards the target and another scattered by the target) and the fidelity of the phase conjugation scheme. In this presentation we will present the results of the thorough analysis of ATIL operation, factors that affect its performance, its focusing efficiency and the comparison of laboratory experimental validation and computer simulation results.

This paper describes a novel technique for the detection of contaminants in air using the process of laser-induced filamentation. This work is focused primarily on the visible and infrared spectrum. Characterization of the temporal and spatial evolution of laser-generated plasma in solvent aerosols is necessary for the development of potential applications. Atmospheric aerosols impact capabilities of applications such as range from laser-induced ionized micro channels and filaments able to transfer high electric pulses over a few hundreds of meters, to the generation of plasma artifacts in air, far away from the laser source.

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.

Video imagery is extremely useful in developing an intuitive sense for the effects of atmospheric turbulence in long, horizontal-path imaging. This is especially true when attempting to understand the effects of Non-Kolmogorov and anisotropic turbulence. We have created simulated video sequences featuring a static scene where parameters such as turbulence strength, power-law exponent, direction and degree of anisotropy are varied. Because these image sequences are simulated, it is possible to explore the effects of these parameters using non-subjective measures such as the Mean Squared Error. We find that changing the power-law exponent while leaving the Fried parameter fixed results in more blurring but less anisoplantic tip-tilt distortions at lower power laws. Images have subjectively less blurring and more global tip-tilt distortion when the power law is higher than the Kolmogorov value. We also observe that Mean Squared Error increases approximately linearly with the degree of anisotropy.

In military applications that use adaptive optics, an extended beacon instead of a point source beacon is created at the target due to atmospheric turbulence and other factors. These beacons, which have a finite spatial extent and exhibit varying degrees of coherence, are typically modeled in existing literature as a Gaussian Schell Model (GSM) due to its analytical tractability. Earlier, we used a full wave computational technique to evaluate the scattered field from a rough impedance surface in vacuum. The results showed some deviations from GSM behavior. The present work uses a simulation approach based on Physical Optics (PO) approximation to study the scattering behavior in presence of atmospheric turbulence. A fully coherent Gaussian beam is propagated through atmospheric phase screens to the rough surface target plane. The PO current is computed on the rough surface and the scattered field right above the surface is determined. The scattered light is propagated through a second set of atmospheric phase screens and thus the double passage through the atmosphere is realized. The rough surface is simulated using statistical parameters derived from profilometer measurements of standard targets. Through multiple realizations of the atmosphere and the rough surface, the statistics of the scattered field is determined. The simulations are done with different strengths of turbulence and different roughness scales of the target. The results are compared with a GSM. An effects model where the rough surface is modeled as a phase screen has also been implemented in order to verify the nature of the speckle returns.

The recently introduced class of Multi-Gaussian Schell-model [MGSM] beams is investigated via simulations and experiments with regards to its intensity fluctuations on propagation in atmospheric turbulence. The results indicate that the scintillation index of the MGSM beam is reduced for high values of the summation index, in agreement with previous theoretical results.

We conduct an initial experimental study of implementing partially coherent beams with super-Gaussian far-field intensity distributions with the help of a reflection-type spatial light modulator. Using a recently proposed random screen approach for any Schell model type of beam, various super-Gaussian far-field intensity patterns are generated, although with an expected diffraction limited core (bright spot) in the center of each pattern. It is demonstrated that the experimental results agree well with the theoretical predictions. Our work is beneficial for creating and implementing exotic beams in various applications and can be useful for improving link performance in free-space optical communications.

Plenoptic functions are functions that preserve all the necessary light field information of optical events. Theoretical work has demonstrated that geometric based plenoptic functions can serve equally well in the traditional wave propagation equation known as the “scalar stochastic Helmholtz equation”. However, in addressing problems of 3D turbulence simulation, the dominant methods using phase screen models have limitations both in explaining the choice of parameters (on the transverse plane) in real-world measurements, and finding proper correlations between neighboring phase screens (the Markov assumption breaks down). Though possible corrections to phase screen models are still promising, the equivalent geometric approach based on plenoptic functions begins to show some advantages. In fact, in these geometric approaches, a continuous wave problem is reduced to discrete trajectories of rays. This allows for convenience in parallel computing and guarantees conservation of energy. Besides the pairwise independence of simulated rays, the assigned refractive index grids can be directly tested by temperature measurements with tiny thermoprobes combined with other parameters such as humidity level and wind speed. Furthermore, without loss of generality one can break the causal chain in phase screen models by defining regional refractive centers to allow rays that are less affected to propagate through directly. As a result, our work shows that the 3D geometric approach serves as an efficient and accurate method in assessing relevant turbulence problems with inputs of several environmental measurements and reasonable guesses (such as C_n ² levels). This approach will facilitate analysis and possible corrections in lateral wave propagation problems, such as image de-blurring, prediction of laser propagation over long ranges, and improvement of free space optic communication systems. In this paper, the plenoptic function model and relevant parallel algorithm computing will be presented, and its primary results and applications are demonstrated.

The fundamental principles of laser beam detection from atmospheric scattering are well understood and have been used to make successful predictions of received laser power at off-axis detectors. Furthermore, models have been developed that can correlate atmospheric conditions to these predicted received powers. However, in addition to the first-order scattering effects, other "higher-order" effects (multiple scattering, non-spherical aerosols, and optical turbulence) will also play a role in determining the received power and therefore will affect laser beam detection. Using a validated model created by SPAWAR Systems Center, Pacific, an assessment of the relative impacts of these "higher-order" effects is given by model modifications and comparisons with the fundamental prediction. A summary analysis of the "higher-order" effects is presented in an hierarchy, applying each relative contribution to the detection problem.

The selection of the “optimal” operating wavelength for Free Space Optical (FSO) systems has been a subject of some ongoing controversy over the past several decades. Practical FSO systems have been found to suffer severe performance degradation in adverse atmospheric visibility conditions (high extinction/low-transmission) such as fog, haze, and other atmospheric aerosols (smoke, dust). Claims have been made that certain wavelengths offer generally superior performance and reduced attenuation for FSO system operation. We will revisit the problem of optical propagation through atmospheric particulates, and will show that the specific details of the selected aerosol size distribution function (SDF), which specifies the aerosol number density distribution by radius, and the corresponding wavelength-dependent complex refractive indices can significantly influence the total extinction/transmission behavior of various wavelengths and hence the choice of “optimal” wavelength. We will use a variety of realistic atmospheric SDFs to highlight the sensitivity of the “optimal” wavelength to the SDF composition details. A primary result will be a comparison illustrating extinction performance at selected wavelengths across the spectrum of visible to LWIR for a variety of realistic and clearly-defined atmospheric scenarios: urban, desert, maritime, with fogs, hazes, smoke, and dust.

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.

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.

Looking through the atmosphere, it is sometimes difficult to see the details of an object. Effects like scintillation and blur are the cause of these difficulties. Exhaust plumes of e.g. a ship can cause extreme scintillation and blur, making it even harder to see the details of what lies behind the plume. Exhaust plumes come in different shapes, sizes, and opaqueness and depending on atmospheric parameters like wind speed and direction, as well as engine settings (power, gas or diesel, etc.). A CFD model is used to determine the plume’s flow field outside the stack on the basis of exhaust flow properties, the interaction with the superstructure of the ship, the meteorological conditions and the interaction of ship’s motion and atmospheric wind fields. A modified version of the NIRATAM code performs the gas radiation calculations and provides the radiant intensity of the (hot) exhaust gases and the transmission of the atmosphere around the plume is modeled with MODTRAN. This allows assessing the irradiance of a sensor positioned at some distance from the ship and its plume, as function of the conditions that influence the spatial distribution and thermal properties of the plume. Furthermore, an assessment can be made of the probability of detecting objects behind the plume. This plume module will be incorporated in the TNO EOSTAR-model, which provides estimates of detection range and image quality of EO-sensors under varying meteorological conditions.

Atmospheric effects limit the performance of any electro-optical (EO) system. Tasks such as laser communications or horizontal-path imaging for long-range surveillance are highly affected by environmental effects. In the majority of cases, effects like turbulence impose a fundamental limitation to the capability of EO systems. In this paper, we give an overview of the limiting factors and we will show possibilities for restoration of images degraded by atmosphere.

Mitigating the effects of turbulence in imaging is an important capability for surveillance systems. For image capture applications, atmospheric turbulence causes global blur in isoplanatic conditions which prevents detection and identification of objects due to loss of important features. Free-space communication applications additionally suffer from these artifacts. The knowledge of the atmospheric characteristics can help improve the process of turbulence mitigation by applying enhancement filters designed according to optics parameters and turbulence characteristics. The additional problem is that estimating turbulence parameters require controlled equipment and known sources such that a transmitter and receiver pair are required. In this paper we investigate a method that addresses both common problems where only a single imaging system (with known parameters) is used for observation in horizontal paths through the atmosphere. We first desire to investigate a method for automating the process of selecting the correct modulation transfer function such that the observed image can be deblurred; thus enhancing the received images. Secondly, we wish to investigate a method for estimating the refractive-index structure parameter. We demonstrate the performance of this method with simulated data such that the camera and atmospheric conditions are known. Experiments were conducted and data collected in Point Loma, San Diego where atmospheric conditions were measured along with captured images of static scenes. We present the results of our approach with the simulated and real-world data. We discuss the issues with this type of approach and suggest plans for improving the method in the future.

Conventional techniques used to model optical wave propagation through the Earth’s atmosphere typically as- sume flow fields based on various empirical relationships. Unfortunately, these synthetic refractive index fields do not take into account the influence of transient macroscale and mesoscale (i.e. larger than turbulent microscale) atmospheric phenomena. Nevertheless, a number of atmospheric structures that are characterized by various spatial and temporal scales exist which have the potential to significantly impact refractive index fields, thereby resulting dramatic impacts on optical wave propagation characteristics. In this paper, we analyze a subset of spatio-temporal dynamics found to strongly affect optical waves propagating through these atmospheric struc- tures. Analysis of wave propagation was performed in the geometrical optics approximation using a standard ray tracing technique. Using a numerical weather prediction (NWP) approach, we simulate multiple realistic atmospheric events (e.g., island wakes, low-level jets, etc.), and estimate the associated refractivity fields prior to performing ray tracing simulations. By coupling NWP model output with ray tracing simulations, we demon- strate the ability to quantitatively assess the potential impacts of coherent atmospheric phenomena on optical ray propagation. Our results show a strong impact of spatio-temporal characteristics of the refractive index field on optical ray trajectories. Such correlations validate the effectiveness of NWP models as they offer a more comprehensive representation of atmospheric refractivity fields compared to conventional methods based on the assumption of horizontal homogeneity.

The focus of this paper is on the estimation of optical turbulence (commonly characterized by C²_n ) near the land-surface using routinely measured meteorological variables (e.g., temperature, wind speed). We demonstrate that an artificial neural network-based approach has the potential to be effectively utilized for this purpose. We use an extensive scintillometer-based C²_n dataset from a recent field experiment in Texas, USA to evaluate the accuracy of the proposed approach.

In this paper, we reconstruct the meteorological and optical environment during the time of Titanic’s disaster utilizing a state-of-the-art meteorological model, a ray-tracing code, and a unique public-domain dataset called the Twentieth Century Global Reanalysis. With high fidelity, our simulation captured the occurrence of an unusually high Arctic pressure system over the disaster site with calm wind. It also reproduced the movement of a polar cold front through the region bringing a rapid drop in air temperature. The simulated results also suggest that unusual meteorological conditions persisted several hours prior to the Titanic disaster which contributed to super-refraction and intermittent optical turbulence. However, according to the simulations, such anomalous conditions were not present at the time of the collision of Titanic with an iceberg.

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.

Optical beams propagating through the atmosphere acquire phase distortions from turbulent fluctuations in the refractive index. While these distortions are usually deleterious to propagation, beams reflected in a turbulent medium can undergo a local recovery of spatial coherence and intensity enhancement referred to as enhanced backscatter (EBS). Using simulations, we investigate the EBS of optical beams reflected from mirrors, corner cubes, and rough surfaces, and identify the regimes in which EBS is most distinctly observed. Standard EBS detection requires averaging the reflected intensity over many passes through uncorrelated turbulence. Here we present an algorithm called the “tilt-shift method” which allows detection of EBS in static turbulence, improving its suitability for potential applications.

Enhanced backscatter effects have long been predicted theoretically and experimentally demonstrated. The reciprocity of a turbulent channel generates a group of paired rays with identical trajectory and phase information that leads to a region in phase space with double intensity and scintillation index. Though simulation work based on phase screen models has demonstrated the existence of the phenomenon, few experimental results have been published describing its characteristics, and possible applications of the enhanced backscatter phenomenon are still unclear. With the development of commercially available high powered lasers and advanced cameras with high frame rates, we have successfully captured the enhanced backscatter effects from different reflection surfaces. In addition to static observations, we have also tilted and pre-distorted the transmitted beam at various frequencies to track the dynamic properties of the enhanced backscatter phenomenon to verify its possible application in guidance and beam and image correction through atmospheric turbulence. In this paper, experimental results will be described, and discussions on the principle and applications of the phenomenon will be included. Enhanced backscatter effects are best observed in certain levels of turbulence (C_n ²≈10^-13 m^-2/3), and show significant potential for providing self-guidance in beam correction that doesn’t introduce additional costs (unlike providing a beacon laser). Possible applications of this phenomenon include tracking fast moving object with lasers, long distance (>1km) alignment, and focusing a high-power corrected laser beam over long distances.

Underwater optical wireless communication is an emerging technology, which can provide high data rate. High data rate communication is required for applications such as underwater imaging, networks of sensors and swarms of underwater vehicles. These applications pursue an affordable light source, which can be obtained by light emitting diodes (LED). LEDs offer solutions characterized by low cost, high efficiency, reliability and compactness based on off-the-shelf components such as blue and green light emitting diodes. In this paper we present our recent theoretical and experimental results in this field.

Visible light communication (VLC) is a new emerging technology that uses standard visible light to transmit broadband data streams in addition to illumination. In our research we have theoretically studied an innovative device that can serve as a modulating retro-reflector (MRR) for VLC applications. The device comprises of a nanocoposite of ferroelectric thin-film embedded with noble metal nano-shells. In comparison to the nano-spheres, the nano-shells provide more flexibility in the design of the device. This MRR can be used in asymmetric communication links as an optical transceiver for mobile devices. The main conclusion from our study is that a nanocomposite based MRR can save power, complexity, dimensions and weight in comparison to standard communication links. this fact is very important for mobile platforms.

New results for characterizing laser intensity fluctuation statistics of a laser beam transmitted through a random air-water interface relevant to underwater communications are presented. A laboratory watertank experiment is described to investigate the beam wandering effects of the transmitted beam. Preliminary results from the experiment provide information about histograms of the probability density functions of intensity fluctuations for different wind speeds measured by a CMOS camera for the transmitted beam. Angular displacements of the centroids of the fluctuating laser beam generates the beam wander effects. This research develops a probabilistic model for optical propagation at the random air-water interface for a transmission case under different wind speed conditions. Preliminary results for bit-error-rate (BER) estimates as a function of fade margin for an on-off keying (OOK) optical communication through the air-water interface are presented for a communication system where a random air-water interface is a part of the communication channel.

Oceanic turbulence is simulated with the help of a series of 2D phase-screens and used to investigate beam propagation in the ocean. Individual phase screens are created from a model for the spatial power spectrum of oceanic turbulence that includes both temperature and salinity fluctuations. Numerical simulation is compared with the experiment and with previous results in the literature. Intensity profiles as well as scintillation data is explored for several different turbulence parameters as well as for several propagation distances to examine a wide range of naturally occurring situations.

Wirelessly transmitting large volumes of information at high data rates underwater is becoming increasingly important for such applications as environmental monitoring and petroleum exploration and maintenance. Underwater free-space optical (FSO) communication addresses the aforementioned need by providing wireless high-data-rate links. Visible light transmission through seawater typically peaks in the blue-green spectrum (475 nm–575 nm), but local clarity conditions, which are dynamic, strongly influence the actual maximum. We describe the development of a new laser-wavelength auto-selection algorithm and system for optimized underwater FSO communication. This system has the potential to improve underwater optical link reliability for high-data-rate communications. First, we describe the laser system and water tube setup for performing optical experiments. Next, we present research on recreating various seawater types (from clear to turbid) in the laboratory using particle suspensions and dye, which will enable wavelength-dependent transmission tests. Finally, we show experimental results from optical water tube tests, and describe the development of the autoselection algorithm.

Free space optical communication has become an attractive solution for data transmission due to its cost-effective, license-free and high bandwidth characteristics. The performance of such systems, however, is highly affected by the effects of optical turbulence. Multiple laser transmitters and/or multiple aperture receivers can be used to mitigate the turbulence fading by exploiting the advantages of spatial diversity. In this work, the performance of a link employing spatial diversity at the receiver was investigated by analyzing two figures of merit: Bit Error Rate and Link Availability. The statistical properties of the photocurrent generated at the receiver – required for evaluating Bit Error Rate and Link Availability – were obtained by means of computer simulation using the Monte Carlo method. The results presented here can be very helpful in designing a free space optical link with spatial diversity, since they are given in function of system configurations and impairment conditions.

This paper describes an optical ranging and communication method based on all-phase fast fourier transform (FFT). This kind of system is mainly designed for vehicle safety application. Particularly, the phase shift of the reflecting orthogonal frequency division multiplexing (OFDM) symbol is measured to determine the signal time of flight. Then the distance is calculated according to the time of flight. Several key factors affecting the phase measurement accuracy are studied. The all-phase FFT, which can reduce the effects of frequency offset, phase noise and the inter-carrier interference (ICI), is applied to measure the OFDM symbol phase shift.

Short range non-line-of-sight (NLOS) ultraviolet (UV) communication, with its solar blind and Non-Line-of- Sight characteristic, received grant interest. However as the communication range increases, the communication performance deteriotes due to NLOS UV turbulence, even with special UV turbulence mitigation. In this work, we conducted a series of outdoor experiments to investigate the received signal energy distribution, which is the product of the complex interaction of transmitted UV radiation, by utlizing both a UV LED array and a UV laser, with the atmosphere. Separation distance, pointing angles and UV light source were taken into considerate as key parameters to affect the distribution. These experimental results will be valuable for studying NLOS UV communication performance.

In free space optical homodyne receiver that analyze Residual carrier COSTAS loop, Inter-satellite LEO-GEO laser communication link frequency analysis, result from Doppler frequency shift 10GHz in the maximum range, LEO-GEO inter-satellite laser links between Doppler rate of change in the 20MHz/s. The optical homodyne COSTAS receiver is the application in inter-satellite optical link coherent communication system. The homodyne receiver is the three processes: Scanning frequency, Locked frequency and Locked phase, before the homodyne coherent communication. The processes are validated in lab., and the paper presents the locked frequency data and chart, LO laser frequency with triangle control scanning and receiving optical frequency is mixed less 100MHz intermediate frequency, locked frequency range between 100MHz and 1MHz basically, discriminator method determines mixing intermediate frequency less 1MHz between the signal laser and the LO laser with the low-pass filter due to frequency loop and phase loop noise. When two loops are running, the boundary frequency of laser tuning is fuzzy, so that we must be decoupling internal PID parameters. In the Locked frequency and phase COSTAS loop homodyne receiver gave the eye-diagram with Bit error rate 10E-7.

During the process of laser ranging, the interaction between the target and the laser beam has the close relationship with the light scattering characteristic of the target surface, which can be characterized by the bidirectional reflectivity distribution function (BRDF). This paper discusses the effects of target reflective characteristics by the BRDF model and the Lambertian model. The BRDF is definited by surface parameters, such as surface roughness, correlation lengths and refractive index, incident angle and wavelength, which the Lambertian model does not include the detailed properties of the target. The results show BRDF is more precise than Lambertian model in factual environment. The main work is the research on calculating and comparing the minimal detectable power of laser rangefinder obtained the two models under different incident angles and surface roughness. The angular dependence of the BRDF is related to the microscopic properties of the surface. It showS that when the surface roughness increases the detectable power decreases rapidly. Modeling and simulation of the typical target shape of parabolic is provided in this paper on the bases of the BRDF and the LRCS is calculated. According the above study, it will provide some fruitful reference for further parameters choice of laser rangefinders and laser radar.

The practical demonstration of an optical two-way time transfer based on single photon signal level has been completed. This approach enables to reach extreme timing stabilities and minimal systematic errors using existing electrooptic technologies. The crucial condition to almost eliminate systematic errors and to reach picosecond time transfer accuracy over free space communication channel is the maximum symmetry in experimental setup. In our indoor experiment we have achieved sub-picosecond precison and 3 ps accuracy. The entire system is compact and relatively simple.

Long-distance imaging through atmospheric turbulent medium is affected mainly by blur and spatio-temporal movements in the recorded video, which have contradicting effects on the temporal intensity distribution, mainly at edge locations. For automatic surveillance, a correct model of the background can contribute to a successful background subtraction often applied for the extraction of the moving targets. Following a recent study of modeling the background due to atmospheric effects, we further examine here experimentally the effect of image de-blurring on the model using an automatic image restoration method implemented to real video signals recorded from a variety of imaging distances.

The new dynamic direction of wireless networks development is based on the idea of networks utilizing the optical radiation in the visible spectrum VLC (Visible Light Communications). The impulse of this development direction was improvement in the semiconductor lighting technologies, namely the white power LEDs (Light Emitting Diode). These types of wireless networks are denoted as the optical wireless networks for indoor spaces utilizing optical radiation in the visible spectrum. The paper deals with the issue of deployment of multi-state modulations into the indoor visible light communications in LOS (Line of Sight) configuration. The first part of the paper focuses on design of modulation element (SMD LED matrix 3 × 3) and problems connected to deployment of multi-state modulation M-QAM (subcarrier intensity modulation) through this modulation element into the indoor visible light communications (MER). The second part deals with the irradiation distribution in dark room in comparison with real room during used multi-state modulation scheme in both simulation and real measurement.

Utilizing a retro-reflector from a target point, the reflected irradiance of a laser beam traveling back toward the transmitting point contains a peak point of intensity known as the enhanced backscatter (EBS) phenomenon. EBS is dependent on the strength regime of turbulence currently occurring within the atmosphere as the beam propagates across and back. In order to capture and analyze this phenomenon so that it may be compared to theory, an imaging system is integrated into the optical set up. With proper imaging established, we are able to implement various post-image acquisition techniques to help determine detection and positioning of EBS which can then be validated with theory by inspection of certain dependent meteorological parameters such as the refractive index structure parameter, Cn² and wind speed.