This PDF file contains the front matter associated with SPIE Proceedings Volume 8517, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Quantum Key Distribution (QKD), either fiber based or free-space, allows for provably secure key distribution solely based on the laws of quantum mechanics. Feasibility of QKD systems in aircraft-ground links was demonstrated with a successful key exchange. Experiment flights were undertaken during night time at the site of the German Aerospace Center (DLR) Oberpfaffenhofen, Germany. The aircraft was a Dornier 228 equipped with a laser communication terminal, originally designed for optical data downlinks with intensity modulation and direct detection. The counter terminal on ground was an optical ground station with a 40 cm Cassegrain type receiver telescope. Alice and Bob, as the transmitter and receiver systems usually are called in QKD, were integrated in the flight and ground terminals, respectively. A second laser source with 1550 nm wavelength was used to transmit a 100 MHz signal for synchronization of the two partners. The so called BB84 protocol, here implemented with faint polarization encoded pulses at 850nm wavelength, was applied as key generation scheme. Within two flights, measurements of the QKD and communication channel could be obtained with link distance of 20 km. After link acquisition, the tracking systems in the aircraft and on ground were able to keep lock of the narrow QKD beam. Emphasis of this paper is put on presentation of the link technology, i.e. link design and modifications of the communication terminals. First analysis of link attenuation, performance of the QKD system and scintillation of the sync signal is also addressed.

Tracking and surveillance systems frequently require identification of targets in the field of view of a camera. For example, in the acquisition phase of an FSO system an optical beacon is often used. Pan-tilt-zoom (PTZ) camera networks are also increasingly finding their way into surveillance systems. With advances in image processing and computational efficiency, such systems can track the 3D coordinates of a target in real-time as it traverses through the field of view (FOV) of the master camera. Master-slave relationships between static-dynamic camera pairs are well-studied problems. These systems initially calibrate for a linear mapping between pixels of a wide FOV camera and the PT settings of a dynamic narrow FOV camera. As the target travels through the FOV of the master camera, the slave cameras PT settings are then adjusted to keep the target centered within its FOV. In this paper, we describe a system that allows both cameras to move and extract the 3D coordinate of the target. This is done with only a single initial calibration between pairs of cameras and high- resolution PTZ platforms to keep track of the master camera movement. The mapping between PT settings of the slave and master camera is then adjusted based on the movement by the dynamic camera. This results in a larger coverage area for the master camera to be able to keep track of the target over longer periods of time. Using the information from the PT settings of the PTZ platform as well as the precalibrated settings from a preset zoom lens, the 3D coordinate of the target is extracted and compared to those of a laser range finder and static-dynamic camera pair accuracies.

Even though advances in wireless technology have yielded lower power consumption, higher data rates, and numerous other improvements, the ability to develop a proactive strategy towards handling degradations and failures in directional wireless networks has evaded the research community. In this paper, we introduce a methodology using an analogy to molecular systems in which a directional wireless network utilizing free space optical (FSO) or RF links is modeled as a molecule whose links can grow/retract similarly to bonds. A normal mode analysis (NMA) is performed to identify link instabilities (degradations and failures) and an N-dimensional potential energy surface (PES) is derived with respect to network and environmental parameters to aide in the detection of when a new topology is available ahead of the topology computation stage. Together, the NMA and PES form a basis for a proactive network methodology aimed at improving performance in directional wireless networks.

Near real-time data downlinks from aircrafts, satellites and high altitude platforms via high-speed laser commu- nication links is an important research topic at the Institute of Communications and Navigation of the German Aerospace Center (DLR). Ground stations for such scenarios are usually fixed at a certain location. With a mo- tivation to provide a ground station that is quickly and easily deployed anywhere in the world, a transportable optical ground station (TOGS) has been developed. TOGS features a pneumatically deployable Cassegrain-type telescope with main mirror diameter of 60 cm, including optical tracking and receiving system. For calibration of position and attitude, multiple sensors like dual-antenna GPS and inclination sensors have been installed. In order to realize these systems, robust software that operates and controls them is essential. The software is platform independent and is aimed to be used on both mobile and ground terminals. It includes implementa- tion of accurate pointing, acquisition and tracking algorithms, hardware drivers, and user interfaces. Important modules of the software are GPS tracking, optical tracking, star- and satellite tracking, and calibration of the TOGS itself. Recently, a first successful data-downlink from an aircraft to TOGS using GPS tracking has been performed. To streamline the software development and testing process, some simulation environments like mount simulator, aircraft path simulator, tracking camera simulator and tracking error analysis tool have also been developed. This paper presents the overall hardware/software structure of the TOGS, and gives results of the tracking accuracy improvement techniques like GPS extrapolation and optical tracking.

A hot-air turbulence emulator is employed for generating controlled optical clear air turbulence in the weak fluctuation regime in laboratory conditions. The analysis of the first and second-order statistical moments of the fluctuating intensity of a propagating infra-red (IR) laser beam through the turbulence emulator is made and the results are compared with bi-directional shore-to-ship maritime data collected during two 2009 mid-Atlantic Coast field tests utilizing single-mode adaptive optics terminals at a range of 10.7 km, as well as with a 633 nm Helium Neon laser propagating across land and water at the United States Naval Academy.

We demonstrate the communication of three coaxial laser channels using single transmit and receive apertures based on a set of distinct orbital-angular-momentum (OAM)-carrying modes. Each channel is individually modulated using OOK at 40 Mb/s and made incident onto an arrangement of spatial-light modulators (SLMs) that impose OAM. The three generated modes are combined before the transmit aperture. The received beam with superimposed OAM modes is filtered using SLMs to separate the channels, which are subsequently photo-detected. The effects of OAM crosstalk among channels and the selection of OAM modes are analyzed.

In this paper we propose a simulation method using numerical integration, and develop a closed-form link loss model for physical layer channel characterization for non-line of sight (NLOS) ultraviolet (UV) communication systems. The impulse response of the channel is calculated by assuming both uniform and Gaussian profiles for transmitted beams and different geometries. The results are compared with previously published results. The accuracy of the integration approach is compared to the Monte Carlo simulation. Then the path loss using the simulation method and the suggested closed-form expression are presented for different link geometries. The accuracies are evaluated and compared to the results obtained using other methods.

The use of acousto-optic chaos, as manifested via first-order feedback in an acousto-optic Bragg cell, in encrypting a message wave and subsequently recovering the message in the receiver using a chaotic heterodyne strategy, has been reported recently [1-3]. In examining the dynamical system analytically using computer simulation, (expected) modulated chaos waveforms are obtained within specified observation windows. Because of the relatively random nature inherent in chaos waveforms, it is essentially impossible to ascertain from the visual display of the chaotic wave whether a given message signal has in fact modulated the chaotic "carrier". In fact, it has been observed from earlier work that by appropriately controlling the chaos parameters, one may "hide" the silhouette of the message from the envelope of the modulated chaos [1]. This was found to be especially true for low-frequency chaos (in the KHz range). For chaos in the mid-RF (up to 10s of MHz) range, it is seen that the silhouette is more difficult to suppress (even though this does not affect the robustness of the encryption). To adequately determine whether modulation has in fact occurred by passing the AC signal through the sound cell bias input, one needs to examine the spectral content of the chaos wave. In this paper, we discuss the results of such spectral analyses using two different approaches, (i) fast Fourier transforms applied to the displayed waveform; and (ii) transferring the intensity-vs-time data to an Excel spreadsheet, and then applying this information to a laboratory spectrum analyzer with adequate bandwidth. The results are mutually compared and interpreted in terms of encryption and decryption properties.

Free space optical (FSO) communication has attracted tremendous research interest in the recent year. Most existing works focus only on the line-of-sight (LOS) transmission by infrared (IR) or visible light lasers/LEDs, while this article suggested a framework of non-line-of-sight (NLOS) FSO, motivated by our recent experimental results on the successful transmission of NLOS ultraviolet (UV) beams for up to kilometers, which is comparable to the typical distance a LOS FSO transmission. The NLOS provides an alternate path when the LOS path is shadowed or is highly attenuated. In order to mitigate the multipath dispersion of the NLOS FSO, a baseband orthogonal frequency division multiplexing (OFDM) modulation scheme was proposed, based on Discrete Hartley Transform (DHT) and asymmetric clipping to guarantee the positive-realness of the transmitted optical intensity. The proposed system could reduce the hardware complexity of transmitter and receiver. Minimum mean square error (MMSE) precoder was applied before the DHT to remove the crosstalk between subcarriers, i.e. the frequency domain orthogonality of OFDM was preserved. Performance of the BPSK modulated communication system was given under lognormal atmospheric turbulence for demonstration of the feasibility of the proposed method.

The high data-rate satellite-to-ground coherent optical communication link is limited because the phase integrity of a beam is impaired when passing through the atmospheric turbulence. Based on the interference of two successive data bits in an unequal arm-length Mach-Zehnder delay interferometer, the differential phase shift keying receiver is suited for high data-rate satellite-to-ground coherent optical communication links due to its immunity of the wave front impairment when passing through the atmospheric turbulence. In the time-delay self-homodyne interferometric detection used in 2×4 90 degree optical hybrid, the optical path difference corresponds to the duration of one bit. The optical path difference is stabilized to below one thousandth of the wavelength by moving a finely motorized platform with the close-loop control using the phase feedback from the outputs of the 90 degree hybrid. The 2.5 Gbps optical communication link has already been verified between two buildings over a distance of 2.4km in the worst-case atmospheric conditions. The design and experimental results are given in this paper.

Reciprocity principle for the optical wave propagation in turbulence suggests that scintillations in the focal point of a coherent optical beam and in the center of the point spread function (PSF) of the imaging system are identical, provided that the imaging aperture and initial beam irradiance are matched. Rigorous weak and strong scintillation asymptotes of the scintillation index (SI) in the beam focus indicate that the relatively simple extended Huygens-Fresnel (HF) approximation is accurate in both asymptotic cases. This motivated us to use the HF approximation for calculation of the SI in the moderate turbulence case when SI reaches its maximum. The 8 - fold integral representing the SI was calculated using Mont-Carlo technique. We compare the HF results to the direct numeric wave optics simulation results and find some discrepancies that can be attributed to the finite grid sampling used in simulation. In practical situation the exact position of the beam focal point at the end of the long propagation path is rarely available, but instantaneous, short-term (ST) beam center can be estimated by the beam centroid position. For imaging problems, the short-exposure (SE) PSF and its scintillation are of great interest. We used the combination of the HF approximation and available SE imaging model to calculate the short-term SI for the focused beams under weak strong and intermediate turbulence conditions using the same numeric integration technique as for the Long-Term (LT) case. Calculations show up to 500% increase in the average irradiance and substantial reduction of scintillation for the SE case.

Simulation of beam propagation through turbulent media has always been a tricky subject when it comes to moderate-to-strong turbulent regimes. Creating a well controlled turbulent environment is beneficial as a fast and a practical approach when it comes to testing the optical wireless communication systems in diverse atmospheric conditions. Turbulent media is created using multiple phase screens each having controlled random variations in its frequency and power while the propagated beam is calculated using Fresnel diffraction method. The effect of the turbulent media is added to the propagated beam using modified Von Karman spectrum. Created scintillation screens are tested and compared with the experimental data which are gathered in different turbulence regimes within various atmospheric conditions. We believe that the general drawback of the beam propagation simulation is the difference in terms of spatial distribution and sequential phase textures. To overcome these two challenges we calculate the Aperture Averaging Factors to create more realistic results. In this manner, it is possible create more viable turbulent like scintillations thus the relationship between the turbulence strength and the simulated turbulence parameters are distinctly available. Our simulation gives us an elusive insight on the real atmospheric turbulent media. It improves our understanding on parameters that are involved in real time intensity fluctuations that occur in every wireless optical communication system.

Strong turbulence measurements that are taken using real time optical wireless experimental setups are valuable when studying the effects of turbulence regimes on a propagating optical beam. In any kind of FSO system, for us to know the strength of the turbulence thus the refractive index structure constant, is beneficial for having an optimum bandwidth of communication. Even if the FSO Link is placed very well-high-above the ground just to have weak enough turbulence effects, there can be severe atmospheric conditions that can change the turbulence regime. Having a successful theory that will cover all regimes will give us the chance of directly processing the image in existing or using an additional hardware thus deciding on the optimum bandwidth of the communication line at firsthand. For this purpose, Strong Turbulence data has been collected using an outdoor optical wireless setup placed about 85 centimeters above the ground with an acceptable declination and a path length of about 250 meters inducing strong turbulence to the propagating beam. Variations of turbulence strength estimation methods as well as frame image analysis techniques are then been applied to the experimental data in order to study the effects of different parameters on the result. Such strong turbulence data is compared with existing weak and intermediate turbulence data. Aperture Averaging Factor for different turbulence regimes is also investigated.

The application of speckle imaging techniques to the reconstruction of scenes degraded by atmospheric turbulence requires knowledge of the underlying atmospheric blurring function. This requirement is especially important in horizontal imaging of extended objects where the use of generalized inverse filters may result in reconstruction artifacts. In this work, a variety blind image quality metrics are evaluated to the task of optimizing the blurring function used for object amplitude recovery when is used C²_n as a free parameter. Effectiveness is evaluated by comparing the optimum images selected by each metric to the minimum mean squared error image. After establishing a baseline to the MSE using simulated imagery metrics are applied to field-acquired imagery and the results evaluated subjectively.

Analysis of wave propagation in the visible and near infrared (IR) has to take into account the influence of optical turbulence due to variations of temperature. Especially the operation area of most electro-optical systems in the lower atmospheric boundary layer is affected by atmospheric turbulence. The influence of thermal and mechanical turbulence on the height dependency of the structure function parameter of the refractive index Cn² is investigated. Cn² is used for the characterization of optical turbulence. The main focus is set on seasonal and diurnal variations. The variations dependent on atmospheric stability in the surface layer and the nocturnal residual layer are analysed. Results are presented from the long-term experiment VerTurm (Vertical Turbulence Measurements). The experiment is continuously performed since June 2009 in rural country at north-western Germany. The vertical structure of optical turbulence is explored using three different measurement techniques to cover the altitude range between the surface and about 250 m height. Model comparisons are carried out and discussed.

The refractive index structure parameter C 2/n(z) as a function of vertical height z, is a key parameter describing the turbulent intensity of the atmosphere, and is important for modeling and predicting beam propagation behavior. Over the past several decades many vertical C 2/n models have been developed, many based on empirical data from field test campaigns involving difficult in situ measurements by radiosondes, or remote-sensing using scidar/lidar/radar techniques. Each model has its own set of limitations and caveats. We have developed an improved C 2/n parametric model for the maritime environment, which uses the Navy Surface-Layer Optical Turbulence model for the low-altitude surface boundary layer, and merges with a generalized Hufnagel-Valley model for the middle- and upper-altitudes (up to 24 km elevation). It takes inputs of local bulk meteorological measurements and forms an estimate of C 2/n based on Monin-Obukhov similarity theory. We present phase-screen wave-optics propagation simulations comparing our improved model with previous models, in terms of turbulence metrics such as Fried's atmospheric coherence width r₀, the scintillation index, and the Strehl ratio for both the weak and strong turbulence regimes, for vertical and slant paths, and for various characteristic regimes of the ratio w₀=r₀, where w₀ is the Gaussian beam waist radius.

Turbulence-induced scintillation is the principal impairment to Gbps laser communication over clear-weather atmospheric paths. This paper, plus its companion [A. Puryear, J. H. Shapiro, and R.R. Parenti, “Reciprocity- Enhanced Optical Communication through Atmospheric Turbulence—Part II: Communication Architectures and Performance”], introduce and analyze the exploitation of atmospheric reciprocity for combating turbulence. Part I presents reciprocity proofs that apply under rather general conditions and underlie the communication performance analysis in Part II.

Free-space optical communication provides rapidly deployable, dynamic communication links that are capable of very high data rates compared with those of radio-frequency systems. As such, free-space optical communication is ideal for mobile platforms, for platforms that require the additional security afforded by the narrow divergence of a laser beam, and for systems that must be deployed in a relatively short time frame. In clear-weather conditions the data rate and utility of free-space optical communication links are primarily limited by fading caused by micro-scale atmospheric temperature variations that create parts-per-million refractive-index fluctuations known as atmospheric turbulence. Typical communication techniques to overcome turbulence-induced fading, such as interleavers with sophisticated codes, lose viability as the data rate is driven higher or the delay requirement is driven lower. This paper, along with its companion [J. H. Shapiro and A. Puryear, “Reciprocity-Enhanced Optical Communication through Atmospheric Turbulence–Part I: Reciprocity Proofs and Far-Field Power Transfer”], present communication systems and techniques that exploit atmospheric reciprocity to overcome turbulence which are viable for high data rate and low delay requirement systems. Part I proves that reciprocity is exhibited under rather general conditions, and derives the optimal power-transfer phase compensation for far-field operation. The Part II paper presents capacity-achieving architectures that exploit reciprocity to overcome the complexity and delay issues that limit state-of-the art free-space optical communications. Further, this paper uses theoretical turbulence models to determine the performance—delay, throughput, and complexity—of the proposed architectures.

It is well known that optical signals propagating through the atmosphere are subject to random fluctuations in phase and amplitude. These fluctuations are caused by random temperature distributions in the atmosphere, which manifests themselves as a random index of refraction changes along the propagation path. We introduce a simulation method for modeling atmospheric turbulence effects, which is based on a split-step approach to numerically solve the parabolic wave equation. Atmospheric turbulence effects are modeled by a number of phase screens. These phase screens are generated on a numerical grid of finite size, which corresponds to a narrow spatial area of atmospheric turbulence.

Gaussian beam propagation through a thin screen and an extended random media has been studied using a beam propagation method. We use the modified von Karman spectrum model to describe the phase screen statistics. The scintillation index is analyzed as a function of the structure constant, phase screen location, the initial width and curvature of the Gaussian beam, etc. The numerical simulations are extended using a pair of Gaussian beams. We examine the interference of the beams and measure the fringe visibility at the target. The results are correlated with the scintillation index.

Ideally, as planar wave fronts travel through an imaging system, all rays, or vectors pointing in the direction of the propagation of energy are parallel, and thus the wave front is focused to a particular point. If the wave front arrives at an imaging system with energy vectors that point in different directions, each part of the wave front will be focused at a slightly different point on the sensor plane and result in a distorted image. The Hartmann test, which involves the insertion of a series of pinholes between the imaging system and the sensor plane, was developed to sample the wavefront at different locations and measure the distortion angles at different points in the wave front. An adaptive optic system, such as a deformable mirror, is then used to correct for these distortions and allow the planar wave front to focus at the point desired on the sensor plane, thereby correcting the distorted image. The apertures of a pinhole array limit the amount of light that reaches the sensor plane. By replacing the pinholes with a microlens array each bundle of rays is focused to brighten the image. Microlens arrays are making their way into newer imaging technologies, such as “light field” or “plenoptic” cameras. In these cameras, the microlens array is used to recover the ray information of the incoming light by using post processing techniques to focus on objects at different depths. The goal of this paper is to demonstrate the use of these plenoptic cameras to recover the distortions in wavefronts. Taking advantage of the microlens array within the plenoptic camera, CODE-V simulations show that its performance can provide more information than a Shack-Hartmann sensor. Using the microlens array to retrieve the ray information and then backstepping through the imaging system provides information about distortions in the arriving wavefront.

New results for investigating optical propagation through the random wavy air-water interface relevant to underwater optical communications are presented. A laser beam propagating through the air-water interface reaching a receiver below the water surface, as well as propagated through the water towards an airborne receiver, is significantly distorted due to the high geometric phase aberrations introduced by the random motion of the water surface waves. This causes a significant reduction in the received communications signal resulting in limiting the data transfer capability and the transmitting and receiving data rates. This research develops probabilistic models for optical propagation at the random air-water interface for both reflection and transmission cases under various wind speed conditions. Preliminary results from a laboratory water tank experiment provide information about histograms or the probability density functions of intensity fluctuations measured by a CCD camera for both reflection and transmission cases. Angular displacements of the centroid of the fluctuating laser beam generates the beam wander effects. Finally preliminary results for BER estimates for an on-off keying (OOK) for air-water interface only are presented for a communication system where random air-water interface is a part of communication channel.

Generating highly validated and well-resolved vertical profiles of water vapor is crucially important to understand short and long term global climate changes. Latest results of a newly developed water vapor Raman lidar instrument at the Environment Canada’s Centre for Atmospheric Research Experiments (CARE) (44°14'02" North, 79°45'40" West) will be presented. The CARE Raman lidar setup utilizes third harmonic (355 nm) output of employed YAG laser to probe aerosols, water vapor, and nitrogen profiles. By manipulating inelastic backscattering lidar signals of the Raman nitrogen channel (386.7 nm) and Raman water vapor channel (407.5 nm), vertical profiles of water vapor mixing ratio (WVMR) from the near ground up to 9 km geometrical altitude are routinely deduced, calibrated, validated, and compared against WVMR profiles obtained from simultaneously performed and collocated radiosonde launches. Seasonal effects and variations of WVMR will be also discussed and related to Raman lidar setup efficiency.

The detection and characterization of a laser by an off-axis receiver has been demonstrated and has been modeled to varying degrees of complexity and application. Within these models, many factors contribute to the received intensity at an off-axis receiver, but none is more important than the aerosol distribution; it is the aerosols that are the primary scatterers. Here the dependency of off-axis scattering on the type of aerosol distribution used is explored and the relative importance of this distribution in different geometries is evaluated. The effort uses a single-scatter off-axis model to simulate the received intensity at an off-axis sensor for the inter-comparisons.

Adaptive optics is used in applications such as laser communication, remote sensing, and laser weapon systems to estimate and correct for atmospheric distortions of propagated light in real-time. Within an adaptive optics system, a reconstruction process interprets the raw wavefront sensor measurements and calculates an estimate for the unwrapped phase function to be sent through a control law and applied to a wavefront correction device. This research is focused on adaptive optics using a self-referencing interferometer wavefront sensor, which directly measures the wrapped wavefront phase. Therefore, its measurements must be reconstructed for use on a continuous facesheet deformable mirror. In testing and evaluating a novel class of branch-point- tolerant wavefront reconstructors based on the post-processing congruence operation technique, an increase in Strehl ratio compared to a traditional least squares reconstructor was noted even in non-scintillated fields. To investigate this further, this paper uses wave-optics simulations to eliminate many of the variables from a hardware adaptive optics system, so as to focus on the reconstruction techniques alone. The simulation results along with a discussion of the physical reasoning for this phenomenon are provided. For any applications using a self-referencing interferometer wavefront sensor with low signal levels or high localized wavefront gradients, understanding this phenomena is critical when applying a traditional least squares wavefront reconstructor.

ABSTRACT Free space optical (FSO) communication experiences severe fading due to optical scintillation in long-range links. Adaptive schemes based on channel variation can utilize the channel capacity efficiently. In this paper, we propose a truncated variable rate adaptive scheme for FSO channel. We determined the improvement in channel capacity for perfect channel side information (CSI) at the transmitter and receiver using M-ary pulse position modulation (M-PAM) format. We also determined the improvement in capacity for an erroneous feedback channel using one link of a correlated bi-directional FSO link as a feedback link to the transmitter. Both synthetic and experimental channel samples are used to determine the gain. For perfect CSI we have 1-2 dB improvement in channel capacity compared to non-adaptive schemes using 2-PAM and 4-PAM modulation. For an erroneous feedback channel the improvement depends on the channel correlation coefficient. With medium to high correlation it gives good capacity improvement.

Achieving a minimal size laser spot (beacon) on a remote object is a major objective of the adaptive optics-based phase conjugations methods used in laser communications and directed energy systems. Achieving this goal not only requires a high quality conjugation of the laser beam wave front relative to the object-returned beacon beam, but also fulfillment of the reciprocity condition. The latter can be defined as precise matching of the intensities and wave fonts of two contrapropagating beams in each cross section along the propagation path. This condition is central for effectively focusing a laser beam on a remote object. Violation of the conditions of reciprocal propagation occurs, for example, when the receiving aperture is of limited size compared to the size of the object-returned beacon beam. Such size disparity between the receiving aperture and beacon returned beam results in decreased power density on the focused spot and reduced intensity of the target-returned beam. When the beacon wave is formed on a rough-surface object there is an additional decrease in the efficiency of the beam focusing. The paper provides a detailed discussion of these phenomena.

In order to solve capacity and energy-efficiency problems of future Internet technologies simultaneously, in this paper, we propose the use of energy-efficient N-dimensional (ND) orbital angular momentum (OAM) coded-modulation. The energy-efficient signal constellation is obtained by employing the energy-efficient signal constellation design algorithm. This scheme can achieve beyond 100 Gb/s transmission while employing the state-of-the-art 10 Gb/s technology. The proposed scheme significantly outperforms conventional M-ary PAM. The proposed scheme represents a promising candidate for indoor optical wireless communication, terrestrial free-space optical (FSO) communication, data center applications and can be used as enabling technology for heterogeneous optical networking, thanks to its transparency to both free-space optical and few-mode/multimode fiber links.

We consider n independent disruptive channels with random availability periods and make use of channel diversity to maintain a link as often as possible. This situation arises when an optical link is to be maintained between a spacecraft and n ground stations affected by cloud coverage. Statistical independence between the stations and equal (un)availability duration distributions are assumed. Based on a given single-station (un)availability duration distribution, we derive analytically the network (un)availability duration distributions. Also derived is the distribution of the station operation duration within a network. Derived expressions allow evaluating the increase of the continuous operation duration of a station and the decrease of the network unavailability durations.

Modern wireless optical communication systems in many aspects overcome wire or radio communications. Their advantages are license-free operation and broad bandwidth that they offer. The medium in free-space optical (FSO) links is the atmosphere. Operation of outdoor FSO links struggles with many atmospheric phenomena that deteriorate phase and amplitude of the transmitted optical beam. This beam originates in the transmitter and is affected by its individual parts, especially by the lens socket and the transmitter aperture, where attenuation and diffraction effects take place. Both of these phenomena unfavourable influence the beam and cause degradation of link availability, or its total malfunction. Therefore, both of these phenomena should be modelled and simulated, so that one can judge the link function prior to the realization of the system. Not only the link availability and reliability are concerned, but also economic aspects. In addition, the transmitted beam is not, generally speaking, circularly symmetrical, what makes the link simulation more difficult. In a comprehensive model, it is necessary to take into account the ellipticity of the beam that is restricted by circularly symmetrical aperture where then the attenuation and diffraction occur. General model is too computationally extensive; therefore simplification of the calculations by means of analytical and numerical approaches will be discussed. Presented model is not only simulated using computer, but also experimentally proven. One can then deduce the ability of the model to describe the reality and to estimate how far can one go with approximations, i.e. limitations of the model are discussed.

Turbulence is one of the key factors responsible for light beam distortions while its propagation through randomly inhomogeneous medium such as the atmosphere. Many common methods of turbulence study are based on the phase or amplitude analyses of the lightwave that have passed through turbulent medium. The significant role of explicit account of the inner and the outer scales in experimental data description is well known. We propose an optical method of turbulence characteristic scales estimation using phase data from Shack-Hartmann sensor obtained of a single laser beam. The method is based on the sequential analysis of normalized correlation functions of Zernike coefficients. It allows the excluding of the structural constant of refractive index value from the analysis and reduces the solution of a two-parameter problem to sequential solution of two single-parameter problems. The method has been applied to analyze the results of measurements performed in a water cell with created turbulence. A horizontal flow was induced to simulate turbulence driftage. It is shown that taking into account the inner scale is necessary for fitting of correlations of the third-order Zernike modes in the experimental error limits for l_m/D=0.5 or higher values (l_m - the inner scale, D- aperture diameter). Inner scale estimations did not depend on the flow or changes in the temperature difference. We have shown also that taking into account the outer scale is necessary for fitting of experimental correlations of the first-order Zernike modes in the experimental error limits when L0/D<50 (L0 – the outer scale).

At present inter-satellite laser communications have made great success, such as SILEX, TerraSAR-X LCT etc. But satellite to ground laser communications still at the experimental stages because of the atmosphere turbulence channel and the clouds. Once the satellite to ground laser communication technology obtains a breakthrough, the all laser spacebased communication era is coming. In this paper, we suggest a DPSK modulation/self-coherent homodyne reception scheme to overcome the atmosphere turbulence. The key in the scheme lies in the phase error compensation with the external environment change. In our experiment, we use two parallel plates rotating to compensate the phase error. The communication data rate reaches 2.5Gbps in the field experiment. The real time bit error rate was obtained with the variation of the communication channel’s turbulence.

Laser communication links between satellite and ground remains a bottleneck problem that has not been solved in free space communication network now. Atmospheric disturbances have badly influenced the wave-front of signal light and destroyed the integrality of optical phase, so the bit error rate (BER) is increased at the receiving terminal in the space-to-ground laser communication. With conventional coherent reception, the contrast of coherent light increased dramatically, and transmission efficiency of Space to ground laser communication decreased. Receiving technology based on differential phase shift keying (DPSK) is proposed here to overcome the effects of atmosphere here. Differential phase shift keying without the integrality and compensation of the optical phases, is suited for high rate space to ground communication links due to its immunity of the wavefront of a beam passing atmosphere. A Mach-Zehnder delay interferometer is used for differential delay which is equal to the one bit period. The differential data information can be obtained from the optical phase changes. Differential phase modulation technique can be a promising optical receiving technology.

In the inter-satellite laser communication, the laser beam transmitted from the optical terminals is required to be highly collimated and its divergence approaches diffraction-limit. For testing the diffraction-limit wavefront, a polarization phase-shifting cyclic Jamin shearing interferometer is proposed. It is composed of a Jamin plate with a PBS film coated on its front surface, a right-angle prism reflecting beams two times, a shearing plate shearing beams by its rotation and a polarization phase shifter. The laser beam to be test is incident on the Jamin plate and gives rise to two interference beams with mutually perpendicular polarization directions by the PBS film. The two beams falls on the right-angle prism before or after passing through the shearing plate. With reflection of the right-angle prism, a cyclic Jamin shearing interferometric light path is formed. Two emitted beams go into the polarization phase shifter to obtain phase-shifting interferograms. In this interferometer, the cyclic interferometric light path can eliminate error of the surface profile of the optical element and the effect of environment. The interferometer has polarization phase shifting function and its fringe visibility is high. Therefore the interferometer can obtain high accuracy with variable shearing amount. In experiments, phase-shifting interferograms are obtained and the usefulness of the interferometer is verified.

Atmospheric hydrometeors such as rain and fog may cause attenuation of an optical signal and degrade the performance of free-space optical (FSO) systems. For efficient design of the FSO links, attenuation characteristics must be predicted by sufficiently reliable models that have been tested on experimental data. A long term experiment on the FSO links operating at 850 and 1550 nm wavelengths is conducted in Prague. The path lengths are 100 and 853 m. Received power fluctuations on the FSO links and relevant meteorological quantities such as rain intensity and liquid water content of fog are measured simultaneously. The relationships between the physical parameters of hydrometeors and path attenuation are analyzed and compared with theoretical relations derived using the Mie scattering theory together with some natural assumptions about the physical properties of scattering particles such as droplet size distribution of different types of hydrometeors. Long term statistics of attenuation are obtained and availability performance of the experimental FSO links is assessed. The method for predicting attenuation statistics based on physical and statistical models is introduced and the errors of the proposed models with respect to measured data are analyzed. The models are compared with the existing empirical relationships derived from other FSO experiments and differences are discussed.

The phase-space description of signals is a joint space-spatial frequency representation in local domain. It employ the phase-space representation functions to describe the property of optical signals between two variables that form a Fourier transform pair and provide a valuable analysis tool for signals when this new mathematical analysis tool applied to an optical system. This method can also be applied to the analysis of propagation of laser beam within free space or atmosphere condition. In this paper, we give the analytical formula of optical field distribution of laser beam after propagation within turbulence in the domain of phase-space. The results show an instructing aspect of optical signal propagation and are helpful to the future application of phase-space method to the laser communications.