This PDF file contains the front matter associated with SPIE Proceedings Volume 8162, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Exploration of the moon is a stepping stone for further research of our solar system, the galaxy and, ultimately, the universe. Many intriguing questions arise regarding the moon; what is the moon's composition and structure, what is the potential for settlement or colonization and how did our solar system evolve to name a few. New technologies are required in order to answer these questions. The main goal in our project is to develop technologies for optical wireless communication and navigation systems for use in robotic and in human exploration on the moon. These technologies facilitate the exploration of the moon surface by enabling placing scientific equipment at precise locations and subsequently transferring the acquired information at high data rates. The main advantages of optical technology in comparison with RF technology are: a) high data rate transmission, b) small size and weight of equipment, c) low power consumption, d) very high accuracy in measuring range and orientation and e) no contamination of the quiet electromagnetic (EM) environment on the dark side of the moon In this paper we present a mathematical model and an engineering implementation of a system that simultaneously communicates, and measures the location and orientation of a remote robot on the moon.

Free-space optical (FSO) links are an effective alternative to radio frequency (RF) to handle high-rate data transmission in case of a general communication service failure in disaster scenarios. Establishing high data rate links under the impacts of extreme environmental conditions, like the unregulated RF spectrum, is a very challenging issue. This paper focuses on multicopter (e.g., quadrocopter or hexacopter) based earth observation systems. Its main objective is to show that FSO communication can provide a high-rate link for transmitting earth observation data from a multicopter to a ground station, even under environmental disaster conditions. Before analyzing the feasibility of such an optical multicopter system, the main system structure will be discussed in detail. Several system designs will be compared and evaluated based on link budget calculations. In this process, a novel Grating Light Valve (GLV) retroreflector modulator technology will be introduced in this work. The study will emphasize that a reliable 100 Mbit/s FSO data transmission will be feasible to provide a downlink capacity for multicopter based earth observation systems.

Optical turbulence (OT) distorts light in the atmosphere, degrading the quality of service of free-space optical communications links. Some of the degradation due to turbulence can be corrected by adaptive optics. However, the severity of OT, and thus the amount of correction required, can vary considerably from location to location. In many cases, it is impractical and expensive to deploy instrumentation to characterize the climatology of OT, making simulations a less expensive and convenient alternative. The strength of OT is characterized by the refractive index structure function, C_n ², which can be calculated from Numerical Weather Prediction (NWP) simulations. In this work, a modified version of the Weather Research & Forecast (WRF) NWP model is used to generate C_n ² climatologies in the planetary boundary layer and free atmosphere, allowing for both point-to-point estimates of the Fried Coherence length (r₀) and other seeing parameters. Nearly two years of simulations have been performed over various regions including the Desert Southwest and Haleakala and Mauna Kea on Hawaii. The results, which have shown good agreement with in situ turbulence measurements, are being used to assist engineers in free-space optical system design and site selection studies.

In this review paper we discuss a series of typical mistakes and omissions that are made by engineers and scientists involved in the theoretical research and modeling of the optical propagation through atmospheric turbulence. We show how the use of the oversimplified Gaussian spectral model of turbulence delivers the completely erroneous results for the beam wander. We address a series of common omissions related to calculations of the average beam intensity: unnecessary use of the approximations when rigorous result is available, invalid application of the RMS beam size to the turbulence-distorted beams, overlooking the simple theoretical result - average beam intensity is a convolution with the turbulent Point Spread Function (PSF). We discuss the meaning and potential dangers of the use of the quadratic structure function for modeling of the turbulent perturbations. We will also address the issues related to the energy conservation principle and reciprocity that have very important consequences for the turbulence propagation, but are frequently overlooked in the current literature. We discuss a series of misconceptions that very common in of the Scintillation Index (SI) calculations. We will clarify the infamous misunderstanding of the Rytov's approximation: vanishing scintillation at the beam focus, and show the correct weak and strong scintillation solutions for the SI at the beam focus. We discuss the flaws of the Fried model of the short-term PSF, and direct to the more accurate PSF model. We will briefly review the propagation of the polarized optical waves through turbulence and discuss the inadequacy of the recently published calculations of the electromagnetic beams calculations. We discuss some common errors in representation of the calculation results for the non-Kolmogorov turbulence.

In a laboratory experiment we generate, propagate, and detect laser vortex beams carrying orbital angular momentum (OAM) by means of spatial light modulators (SLMs). We show that beams with OAM states from - 20 to +20 can be effectively generated using different types of phase gratings, and that excellent contrast between adjacent OAM modes is achieved. A weak turbulent air flow is induced on the propagation path to emulate the effects of atmospheric turbulence. By characterizing the effects of optical turbulence on the modal crosstalk among received OAM states we show that it is possible to distinguish them for the purpose of increasing the data throughput of a laser communication link in weak turbulence. It is also demonstrated that by increasing the complexity of the receiver, optical separation of the OAM modes is possible at stronger turbulence conditions.

Experiments have been carried out over a series of ranges at China Lake in which the irradiance fluctuations, angle-of-arrival variance and images of the scintillation patterns were recorded, together with meteorological parameters. The decrease in scintillation index associated with the lower turbulence conditions at sunset was observed for the shorter ranges but not for the longer ranges of 19 and 24km where the propagation was in the saturated regime.

The NRL Transportable Atmospheric Testing Suite (TATS) system was used to measure time resolved scintillation over a variety of different ranges at China Lake in December 2010. In this paper an analysis of scintillation effects on retro-reflector links is presented. Scintillation index, power spectral density and probability distribution functions are deduced from the measured irradiance fluctuations. Effects of aperture averaging using multiple retro-reflectors is examined as well as the contribution to the irradiance fluctuations due to enhanced backscatter.

The spatial scale of variations of intensity imposed on a propagating beam as it transits the turbulent atmosphere is of importance in designing free space laser communications receivers. Consecutive images of the entrance pupil of a 125mm diameter telescope have been acquired at a rate of 328fps over a series of ranges from 2km to 24km. The images are analyzed with respect to the aperture averaging available to a given receiver system.

Optical scintillation is an effect that limits the performance of many optical systems including imagers and free space optical communication links. The Naval Research Laboratory is undertaking a series of measurement campaigns of optical scintillation in a variety of environments. In December of 2010 measurements were made over a one week period in the desert at China Lake, CA. The NRL TATS system was used to measure time resolved scintillation over a variety of different ranges and terrains. This data has been used to determine fade rate and duration as a function of weather and link margin. Temporal correlation of fades has also been calculated. This data allows simulation of a variety of communication protocols and the effects of those protocols on link throughput. In this paper we present a comparison of different protocols for both direct and retroreflector links.

Free space optical communication uses photodetectors for two purposes: as communications receivers and, in the form of a quadrant cell or a position sensitive detector, for tracking. Generally two separate detectors are used. In this work we describe combining these functions into one device through the use of heterostructure avalanche photodiode (APD) arrays. Combined functionality more efficiently uses the available light and allows for large area communications detector arrays that maintain the bandwidth and sensitivity of smaller, single-element, devices. In this paper we describe a prototype 2x2 arrays and associated electronics and processing. The design tradeoffs in balancing both functions are explored and future geometries that are more effective than square arrays are described.

Free-space optical (FSO) systems are known for providing data rates much higher than RF based systems, however their narrow beams require a method to keep the transceivers precisely aligned. To date, most systems have used a combination of coarse pointing platforms and fast steering mirrors tied to a feedback loop based on received optical power to accomplish this. This method can encounter problems if the alignment of one of the transceivers is disrupted or obstructed. In this paper, we present an approach to mitigating this problem using a low data-rate, omni-directional RF network that disseminates pointing commands to all platforms in the network, thereby if the main FSO channels are disrupted, the network can recover faster than a purely received signal strength (RSS) based approach. Utilizing custommade high precision direct drive servo pan-tilt platforms coupled with position and orientation sensors, we can calculate the appropriate pointing angles for all the transceiver platforms, which are then relayed over the control network. We present theoretical calculations regarding the required performance specifications of the control network and pan-tiltplatforms. Experimental results are then presented for a link where one transceiver is mounted on a coarse vibration platform to simulate disturbances in a real network.

We study communication over atmospheric turbulence channels based on LDPC-coded, multidimensional OAM signal constellations. Multidimensional signal constellation is obtained as the Cartesian product of one-dimensional signal constellation X={(i-1)d, i=1,2,...,M} (where d is the Euclidean distance between neighboring signal constellation points and M is the number of amplitude levels) as X^N={(x₁, x₂,...,x_N)|x_i is from X, for every i}. This scheme represents an energy efficient alternative, since log₂(M ^N) bits/symbol can be transmitted. We describe two possible implementations of N-dimensional OAM modulator and demodulator: (1) volume holograms based, and (2) multimode fibers (MMFs) based. We evaluate the performance of this scheme by determining conditional symbol probability density functions (PDFs) from numerical propagation data. Two cases of practical interest are studied: (i) when conditional PDFs are known on the receiver side, and (ii) when conditional PDFs are not known and Gaussian approximation is used instead. We show that in case (ii) an early error floor occurs because of inaccurate PDF assumption, which is caused by OAM crosstalk introduced by the atmospheric turbulence. We also show that the OAM modulation is more sensitive to atmospheric turbulence as the number of dimensions increases. Finally, we evaluate the BER performance for different amplitude levels and different number of OAM dimensions.

Optical wireless communications over long-range atmospheric links experiences strong fading that heavily influences the performance of communication systems. Most research on this topic is focused on simulation or measurement of the link performance in terms of the bit error ratio. In this work a statistical channel model derived from measurements is used for simulations of the link performance on packet layer. For analysis of a possible improvement of packet layer performance by error protection techniques like forward error correction and automatic repeat request, additional simulations are done. All simulations are done for several communication scenarios like the maritime environment, land mobile and air-to-ground links.

The nonlinear dynamics of a hybrid acousto-optic device was examined from the perspective of the Lyapunov exponent (LE) and bifurcation maps. The plots for LE versus system parameters and bifurcation maps have recently been examined against known simulation results including chaotic encryption experiments [1]. It is verified that the "loop gain" (feedback gain (β) times incident light amplitude (I_in) needs to be greater than one as a necessary, but not sufficient condition for the onset of chaos. It is found that for certain combinations of β, I_in, net bias voltage (α_tοt), and the initial value of the first-order scattered light (I₁(0)), there are pronounced regions of chaos in the parameter field, while for others, chaos is minimal. It is also observed that in some cases, the negative "spikes" in the LE are far larger than its positive amplitudes, hence indicating a greater tendency to become non-chaotic. Additionally, we have examined the bifurcation plots versus the two most salient system parameters, α_tοt and β. These maps have revealed behavior that is by no means uniformly chaotic. It is found that the system moves in and out of chaos within distinct bands along the α_tοt and β axes. These results imply strong sensitivity vis-à-vis these parameters around the passbands and stopbands, and may indicate control of chaos by appropriate parameter adjustment. Such control may have applications in biological chaos, such as arresting malignant, chaotic cell multiplication. Overall, the dynamical results compare favorably with time-domain characteristics of encrypted chaotic waveforms in signal modulation and transmission applications.

In this paper we describe an angle diversity optical wireless system that operates at 280Mbits/s and provides bidirectional data transmission over a wide coverage area. The system uses commercially available components and operates at a wavelength of 860nm. Three terminals, each using seven transmitter and receiver channels were implemented, and the system was successfully tested in a wide range of different conditions. Implementation challenges, design and performance are also discussed, together with future directions for this work.

The European project "hOME Gigabit Access Network" (OMEGA) targeted various wireless and wired solutions for 1 Gbit/s connectivity in Home Area Networks (HANs). One objective was to evaluate the suitability of optical wireless technologies in two spectral regions: visible light (visible-light communications - VLC) and near infrared (infrared communications - IRC). Several demonstrators have been built, all of them largely relying on overthe- shelf components. The demonstrators included a "wide-area" VLC broadcast link based on LED ceiling lighting and a laser-based high-data-rate "wide-area" IRC prototype. In this paper we discuss an adapted optical-wireless media-access-control (OWMAC) sublayer, which was developed and implemented during the project. It is suitable for both IRC and VLC. The VLC prototype is based on DMT signal processing and provides broadcasting at ~ 100 Mbit/s over an area of approximately 5 m2. The IRC prototype provides ~300 Mbit/s half-duplex communication over an area of approximately 30 m2. The IRC mesh network, composed of one base station and two terminals, is based on OOK modulation, multi-sector transceivers, and an ultra-fast sector switch. After a brief discussion about the design of the optical-wireless data link layer and the optical-wireless switch (OWS) card, we address the card development and implementation. We also present applications for the VLC and IRC prototypes and measurement results regarding the MAC layer.

Institute of Atmospheric Physics of Czech Academy measures atmospheric attenuation on 60 m experimental FSO link on 830 and 1550 nm for more than three years. Visibility sensors and two 3D sonic anemometers on both transmitting and receiving site, rain gauge and many sensors enabling the refractivity index computation are spaced along the optical link. Meteorological visibility, wind turbulent energy, sonic temperature, structure index and rain rate are correlated with measured attenuation. FSO link attenuation dependence on the above mentioned parameters is analyzed. The paper shows also basic statistical behavior of the long-term FSO signal level and also the simulation of hybrid link techniques.

Space to ground laser communication is limited by the effects of atmosphere. Differential phase shift keying (DPSK) is suited for high data rate space to ground communication links due to its immunity of the wave front of a beam passing atmospheric turbulence, which carry the information in optical phase changes between bits. The benefit of DPSK over OOK is the 3 dB lower optical signal to noise ratio to reach a given bit error rate (BER) using a balanced receiver. An aperture-matched phase-compensated DPSK receiver with a 90° hybrid is proposed here. The receive optics are based on free-space optics. A Mach-Zehnder delay interferometer is used for differential delay which is equal to the bit period. The input pupil is imaged onto the same place by two afocal systems composed of 4f imaging lenses. The optical path difference is stabilized to a faction of the wavelength with a fine phase adjustment which is measured by a 2×4 90° optical hybrid and closed loop electric circuit. The design and experiments are given in this paper. optical hybrid and closed loop electric circuit. The design and experiments are given in this paper. optical hybrid and closed loop electric circuit. The design and experiments are given in this paper.

In the inter-satellite laser communication, the diffraction-limit wavefront is required. To test the wavefront, we have developed a Jamin double-shearing interferometer. The interferometer is consists of two Jamin plates to form lateral shearing and four wedge plates to divide the aperture. The laser beam to be test is incident on the first Jamin plate and gives rise to two beams. One is reflected from the front surface of the first Jamin plate, then passes two wedge plates and is reflected from the rear surface of the second Jamin plate. The other is reflected from the rear surface of the first Jamin plate, then passes other two wedge plates and is reflected from the front surface of the second Jamin plate. The two beams are recombined to form the interferogram. For the interferometer, simple phase-shifting method to improve the measurement accuracy of the wavefront by moving four wedge plates is proposed in this paper. When the wedge plates are moved along its surface or in the incidence direction of the interferometer, the optical path difference of two interferometric beams is changed to form phase shift. The added optical path difference introduced by the movement of wedge plates is liner function of displacement, thus the phase-shifting amount is easy to be control. Wedge plates can be moved in four steps with interval of quarter wavelength and the phase can be unwrapped using the four-step phase-shifting algorithm. In experiments, phase-shifting interferograms are obtained. The usefulness of the phase-shifting methods is verified.

Gaussian beam is very often used for the transmission of information in optical wireless links. The usage of this optical beam has its advantages and, of course, disadvantages. This work focuses on possibilities of using laser beams with different distribution of optical intensity - Top Hat beam. Creation of the optical beam with selected optical intensity profile will be briefly described. Optical beams will propagate through the "clear" and stationary atmosphere in the experimental part of this work. These results will be compared with the data obtained after a laser beam is passed through the turbulent and attenuated atmosphere. We will use an ultrasound fog generator for laser beam attenuation testing. To create the turbulence, infra radiators will be applied. Particular results obtained from different atmospheric conditions will be compared and using different types of optical beams will be assessed.

Acquisition strategy is very important during the inter-satellite laser communications systems. Spatial acquisition of the companion terminals using very narrow beacon laser beams is a very difficult task especially under the laser terminal scanning errors situation. Acquisition is a statistical process. In this paper, we detailed the optimized scanning overlap factor of the beacon laser beam which depends on the scanning accuracy of the laser terminals.

The resistance of signal transmission to atmospheric phenomena is possible to solve partially by means of the special optical intensity distribution at the beam spot, i.e. by means of the special beam profile. In practice the resultant beam profile can take users of the free space optical link by surprise if only computer models of the optical beams are used and any diffraction effect at the transmitting lens is not considered. Two models of diffraction of optical beam radiated from the optical transmitter are presented and two methods of the beam modeling are clarified (the method based on Bessel function integrating and the method based on FFT). Confirmation of the models elaborated is a part of the contribution.

In an indoor optical wireless link laser beam carrying the information propagates through a room and reflects on walls and various objects. Multiple reflections and multipath distortions occur when using this link. A power budget of the indoor optical wireless link and a model of the surface reflectivity are presented in the contribution. The directional properties of the surface reflectivity are simulated by an empirical mathematical function characterizing reflectivity and directional reflectance of the surface. In the last part of the paper an experimental setup for verification of the created empirical function is presented.

In this paper, a simplified decoding algorithm to correct both errors and erasures is used in conjunction with the Euclidean algorithm for efficiently decoding nonsystematic RS codes. In fact, this decoding algorithm is an appropriate modification to the algorithm developed by Shiozaki and Gao. Based on the ideas presented above, a fast algorithm described from Blahut's classic book is derivated and proved in this paper to correct erasures as well as errors by replacing the Euclidean algorithm by the Berlekamp-Massey (BM) algorithm. In addition, computer simulations show that this simple and fast decoding technique reduces the decoding time when compared with existing efficient algorithms including the new Euclidean-algorithm-based decoding approach proposed in this paper.

Range-resolved reflective tomography is one of the most effective high-resolution imaging methods for laser sensing and imaging technologies. In experiments reported earlier by MIT Lab, only the outline of the target was recovered using reflective tomography algorithm. In our experiment, we adopt a novel imaging method which can get an imaging result of the whole region covering the target. A target of letter "E" is placed on a plane with a tilt angle to the horizontal plane and rotated about the axis perpendicular to it, the target is illuminated by parallel light pulses, the range-dependent return signal is collected by a non-imaging optical system. Filtered back-projection and algebraic reconstruction techniques algorithm are used to reconstruct the target, then we get an image result which has clear description of the target. After that, the imaging quality and resolution of this new approach are discussed. Our experiment system reported in this paper can achieve high imaging quality in real two-dimension image construction using reflective tomography algorithm, thus it has a great practical significance for applications in extensive imaging fields.

Process of acquirement and locking-up of compound axis system in inter-satellite laser communication has been studied. The effect of different condition parameters on the process of acquirement and locking-up have also been researched and simulated. Simulation results show that when the system with appropriate bandwidth has been adopted, both fine pointing system and coarse pointing system can satisfy three requirements of step response, stability criterion and dwell time and then finish the acquirement and locking-up of beacon laser beam. The simulation results provide the suited condition parameters for both fine pointing system and coarse pointing system, which is very helpful to the subsequent point and tracking processes.