The growing need for underwater observation and sub-sea monitoring systems has stimulated considerable interest in advancing the enabling technologies of underwater wireless communication and underwater sensor networks. This communication technology is expected to play an important role in investigating climate change, in monitoring biological, bio-geochemical, evolutionary and ecological changes in the sea, ocean and lake environments and in helping to control and maintain oil production facilities and harbors using unmanned underwater vehicles (UUVs), submarines, ships, buoys, and divers. However, the present technology of underwater acoustic communication cannot provide the high data rate required to investigate and monitor these environments and facilities. Optical wireless communication has been proposed as the best alternative to meet this challenge. We present models of three kinds of optical wireless communication links a) a line-of-sight link, b) a modulating retro-reflector link and c) a reflective link, all of which can provide the required data rate. We analyze the link performance based on these models. From the analysis, it is clear that as the water absorption increases, the communication performance decreases dramatically for the three link types. However, by using the scattered lighted it was possible to mitigate this decrease in some cases. We conclude from the analysis that a high data rate underwater optical wireless network is a feasible solution for emerging applications such as UUV to UUV links and networks of sensors, and extended ranges in these applications could be achieved by applying a multi-hop concept.

Recent experiments conducted under the Optical RF Communications Adjunct program demonstrate and validate the viability of hybrid free space optical communications links in heavy atmospheric turbulence. Long range air-to-mountain link closures were established under extreme atmospheric turbulence. The system implemented adaptive mechanisms such as adaptive optics, an optical automatic gain controller, forward error correction coding, and link-level retransmission to achieve low packet error rates for long distance links with heavy turbulence. The system, experiments, and results are presented and comparisons are made to statistical prediction models.

MIT Lincoln Laboratory designed and built two free-space laser communications terminals, and successfully demonstrated error-free communication between two ground sites separated by 5.4 km in September, 2008. The primary goal of this work was to emulate a low elevation angle air-to-ground link capable of supporting standard OTU1 (2.667 Gb/s) data formatting with standard client interfaces. Mitigation of turbulence-induced scintillation effects was accomplished through the use of multiple small-aperture receivers and novel encoding and interleaver hardware. Data from both the field and laboratory experiments were used to assess link performance as a function of system parameters such as transmitted power, degree of spatial diversity, and interleaver span, with and without forward error correction. This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the United States Government.

The paper gives an overview of optical communication activities performed by ESA in international and European intersatellite and satellite-to-ground link experiments and describes the evolution of laser communication technology. It introduces the design of a compact and high-speed adaptive optics system specific for laser communication, which - after testing in ESA's optical ground station (OGS) - will be implemented in small ground-based optical receiver stations for Earth observation satellite data. Finally, a novel homodyne BPSK detection scheme is presented, which may become an enabling technology for short distance coherent transmission through atmospheric turbulence.

Parameters characterizing the atmospheric turbulence in a 16 km maritime optical link were measured over a 6 month period in 2007, on a continuous basis as conditions allowed. Both the scintillation index and the atmospheric structure constant were found to have a strong dependence on the air - water temperature difference. Temporal-frequency spectra were also generated from the received intensity fluctuations. In this paper the frequency components of the laser irradiance are studied in order to ascertain the degree of correlation with prevailing meteorological conditions. In addition, the high frequency behavior of the power spectra is documented and shows an extensive range of values for the power-law scaling index.

Small robots are finding increasing use for operations in areas that may be dangerous to humans. These robots often have needs for high bandwidth communications to return video and other data. While radio frequency (RF) links can be used in may cases, in some circumstances they may be impractical due to frequency congestion, reflections off surfaces, jamming or other RF noise. In these cases an optical link may be advantageous, particularly when a clear line of sight exists. However, a conventional optical link has limitations for this application. For example, a conventional optical link operating at rates of megabits per second at ranges of 1 Km requires about a 1 degree pointing accuracy. This implies a need for active pointing and tracking, which maybe be unacceptable for a small platform. We explored an optical modulating retroreflector (MRR) link for these cases. An array of 6 MRRs and photodetectors with a field of view of 180 degrees (azimuth)x 30 degrees (elevation) was constructed and mounted a small robot, the iRobot Packbot^TM. An Ethernet modem designed to work with MRR links was also part of the system. Using a tracking laser interrogator at the other end of the link, a 1.5 Mbps free space optical Ethernet link was established that completely replaced the normal RF Ethernet link. The link was demonstrated out to ranges of 1 Km down a road, exceeding the range of the RF link. Design issues and measurements of performance will be described.

The forward error correction (FEC) and interleaver realizations used in a 5.4 km horizontal-path link experiment incorporated several unique elements that were specifically tailored to address turbulence-induced fading. To facilitate optimization studies, this hardware was designed to afford a high degree of flexibility in the FEC code structure and interleaver length. An essential aspect of this structure was the standards-compliant client interface, which provided seamless connectivity to fiber-based terrestrial networks. Through the use of an OTU1 (2.667 Gbaud) architecture with nonstandard interleaving, error-free transmission was achieved in the presence of strong scintillation that produced fade events that frequently exceeded 10 ms in duration. This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the United States Government.

The specific results of a propagation study carried out on a free space optical link working at 850 nm on an 853 m long terrestrial path over a 4-year period from August 2004 to July 2008 are presented. The records of both the observed fade events and the concurrent meteorological conditions were processed. Individual recorded attenuation events were classified according to the types of individual hydrometeors (rain, rain with hail, snow and fog) that occurred. Fog attenuation events were analyzed in detail and were subdivided into the attenuation events due to fog only, and to combinations of fog with rain, fog with snow, and fog with rain and snow, respectively. The cumulative distributions of attenuation due to fog events for four individual year periods, the entire 4-year period, the individual worst months and the average worst month from the four-year period are presented. It is shown that not only attenuation due to fog only but also attenuation due to fog combined with other hydrometeors may have a significant impact on the attenuation of the optical beam. The assessments of availability performance and error performance parameters are given. The use of measured visibility data for the calculation of attenuation is discussed.

Wireless communications is facing the challenges of a predicted 'explosion' in the number of wireless devices, demand for higher capacity, and the need to reduce power consumption in order to lower greenhouse gas emissions. Optical Wireless (OW) communications may have a part to play in helping to achieve these aims. High speed line of sight optical wireless systems have the potential to provide a low complexity alternative to high frequency RF wireless communications, and may offer lower energy consumption. Visible light communications is also a growing area of interest. Low energy solid-state lighting sources can be modulated to provide data communications, and this can augment the communications provided by other wireless techniques. In this paper we review progress in these areas. Examples and results from systems will be reported, together with future directions and challenges..

The optical wireless system is a promising solution for increasing the available communication bandwidth within a room. This technology can give a very high-speed communication between devices and becomes a good alternative with respect to radio systems. For both technologies, the architecture is similar: a base station is installed to cover zones and transmit data with a defined quality of service. A device may be connected to the Wireless Local Area Network (WLAN) with an adapter that emits and receives on this network. The wireless optical technology has advantageous specific characteristics, such as: secure data transmission, immunity, high data rate, wavelength re-use. Nevertheless, the optical system may have a limit on the network management aspect and link budget. The scope of this paper is to present a prototype developed during a collaborative project. This prototype uses a Giga Ethernet chip and components in the 1550 nm band. Based on OOK modulation, the prototype is fully compatible with the direct conversion of fibre-based Giga-Ethernet to an analogue free space version. Moreover, it also proposes a new class 1 high power emission solution with 30 dBm on 45° HP (Half Power) angle and a new large Field Of View (FOV) module on the reception side. This full duplex system, composed by one Base station and two Modules, transmits data on, at least, one meter. The document will present the prototype characteristics with testbed and experimentation results.

Visible light communication in conjunction with solid state lighting has become an emerging area of interest to achieve lighting and wireless communication simultaneously in an indoor environment. It is anticipated to be a low cost supplement to existing wireless communication technologies. Most existing work has primarily focused on a unidirectional downlink using visible light spectra. The appropriate choice of an uplink to achieve bidirectional communication is a big challenge. In this paper, candidate options of the uplink are compared in terms of device performance, light safety, background interference, and path loss. In visible light communication, white light emitting diodes as optical transmitters are also characterized in terms of impulse response and electrical spectrum. A digital preequalization idea to increase their bandwidth is proposed. Performance of the downlink visible light communication system is also experimentally studied in order to demonstrate the feasibility of the proposed design.

We have proposed a space- and wavelength-division-multiplexing (WDM) indoor optical wireless LAN system based on a custom CMOS image sensor to realize a compact, high-speed, and intelligent nodes and hub. The CMOS image sensor can detect multiple fast optical data concurrently as well as captures ordinary images from which positions of communication nodes or the hub is obtained. In this paper, with the CMOS image sensor, we demonstrate an application of WDM technique to downlinks. We fabricated a 64x64-pixel custom CMOS image sensor with 4-channel concurrent data acquisition function. Experimental results showed that the CMOS sensor received 10Mbps×3ch WDM data while capturing ordinary images.

Free space optical (FSO) links for indoor sensor networks can provide data rates that can range from bits/s to hundreds of Mb/s. In addition, they offer physical security, and in contrast with omnidirectional RF networks, they avoid interference with other electronic systems. These features are advantageous for communication over short distances in fixed infrastructure sensor networks. In this paper the system architecture for a fixed infrastructure FSO sensor network is presented. The system includes a network of small, low power (mW), sensor systems, or "motes," that transmit data optically to a central "cluster head," which controls the network traffic of all the motes and can aggregate the sensor information. The cluster head is designed with multiple vertical cavity surface emitting lasers oriented in different directions and controlled to diverge at 12º in order to provide signal coverage over a wide field of view. Both the cluster head and motes form a local area network. Our system design focuses on low-power wireless motes that can maintain successful communication over distances up to a few meters without having to use stringent optical alignment techniques, and our network design focuses on controlling mote sleep cycles for energy efficiency. This paper presents the design as well as the experimental link and optical communications performance of a prototype FSO-based sensor network.

Light finds a path from a source to a receiver even along a partially occluded channel, which may therefore involve diffuse and specular reflections to allow a signal to arrive at the receiver. Natural FSO channels often exist inside closed structures such as aircraft, satellites, and buildings. Direct, diffusely-reflected, and specularly-reflected paths can be analyzed to assess received intensities in various geometries, such as ducts, rooms, and multi-compartment structures. These calculations are important in choosing sensor network architectures for infrastructure sensing, determining impulse response to estimate usable channel bandwidth, and studying light leakage through openings in multicompartment structures. This presentation will describe both geometrical optics models for carrying out these analyses, as well as a "photon-kinetic-theory" technique for estimating light coupling from compartment to compartment in multicompartment structures connected by openings. The surface absorption and angular reflectance characteristics of surfaces are included in these analyses. Specific results for cylindrical ducts of various aspect ratios will be presented as well as for rectangular coupled compartments.

Rich atmospheric scattering in the ultraviolet (UV) enables non-line-of sight (NLOS) communications, opening a new optical paradigm. In this paper, we incorporate an experimentally validated NLOS channel path loss model and quantitatively predict the performance of a combination of modulation and coding techniques. Our study includes OOK, PPM, MPPM, repetition coding and trellis coding. We consider both Poisson and Gaussian noise assumptions, appropriate for different operating regimes, with a focus on achievable range as a critical parameter in NLOS operation.

Non-line-of-sight (NLOS) ultraviolet (UV) scattering channel impulse response in the deep UV solar-blind spectrum band is investigated. Taking into account a light source power angular distribution and applying a photon tracing technique, Monte Carlo simulation is performed to obtain the channel impulse response and associated path loss. Some comparisons are carried out with an existing single scattering channel model, as well as field measurements in both the impulse response model and path loss, to demonstrate modeling accuracy. The importance of considering multiple versus single-scattering in the analysis is shown. The results provide guidelines for study of limitations to data rate and communication range in NLOS UV communications.

Net-centric warfare in todays dynamically changing military environments and the need for low-cost gigabit intra-city communication present severe challenges for current free-space optical systems. Enabled by high-speed electronics and advances in wavefront control, we develop an architecture that provides free-space coherent optical links with information capacity, security, network robustness and power management performance that exceed the current state-of-the-art, including commercially deployed systems, R&D test-beds, and alternative theoretical architectures proposed. The deleterious effects of the turbulent atmosphere are mitigated with several diversity transmitters and receivers. We allow the phase and the amplitude of each transmitter to be controlled independently and assume, through coherent detection, that the phase and amplitude of the received wave is measured. Thus we can optimally allocate transmit power into the diffraction modes with the smallest propagation losses to increase channel capacity and mitigate turbulence-induced outages. Additionally, spatial mode modulation and rejection provides robust communication in the presence of denial of service via interference by adversaries with a priori knowledge of the system architecture. Some possible implementations of this system are described. New results, including asymptotic singular value distribution, expected bit error rate, interference performance, and performance in the presence of inhomogeneous turbulence, are given. Finally, performance of this system is compared with the performance of optical diversity systems without wavefront control and optical systems without diversity, both current state-of-the-art systems.

In this paper, the concept of Chase Combining (CC) is extended to photon communications systems. It is assumed that intensity modulation (IM) along with on-off-keying (OOK) schemes are utilized to generate signals at the transmitter. At the receiver, direct-detection (DD) mechanisms are utilized to recover the transmitted signal. In particular, it is assumed that the optical detection is achieved via an avalanche photo-detector (APD). It is assumed that the photon channel is impacted by clear-air (log-normal) turbulence and that the major impairments of the receiver include receiver thermal noise, shot noise due to optical detection, background radiation, and random gain distortion caused by APD. Performance is established in terms of bit error rate (BER) using analytical and simulation means. The numerical results demonstrate that CC technique offers a significant improvement in performance at a cost of increased hardware complexity.

We discuss the design of an acousto-optic cell based free space optical communication link where the data beam is made secure through chaos encryption. Using external signal modulation of the diffracted light from a hybrid acousto-optic cell chaos (or directly via incorporation in the sound-cell driver's bias voltage) encryption of data is possible. We have shown numerically that decryption of the encoded data is possible by using an identical acousto-optic system in the receiver.

For terrestrial free space optical (FSO) systems, we investigate the use of multipulse pulse position modulation (MPPM), which has the advantage of bandwidth efficiency compared to the classical PPM. We first discuss the upper-bound on the information transmission rate for the case of a Gaussian (turbulence-free) channel. We next consider the channel coding issue for MPPM. We propose to use a simple binary convolutional code and to perform iterative soft demodulation and channel decoding at the receiver. We study the performance of this scheme by presenting some numerical results for the cases of Gaussian and weak-turbulence channels. We also show that, in contrast to PPM, the bit-symbol mapping is an important point for MPPM, especially regarding the proposed iterative receiver. In this view, we propose design rules for optimal bit-symbol mapping.

Combined RF and optical communication within a heavily scintillated atmosphere requires special modems that can accommodate significant signal fading. A hybrid network (10 Gbps 1550 nm FSO and RF transmission) has been developed and a link quality parameter is used to assist the network routers with the path cost calculation algorithm. Fading statistics, determined by field experiments, are emulated in the laboratory network by a statistically-driven VOA. COTS hardware (FEC and a special amplifier) enable a 35 dB dynamic range. The special modem and its performance within a multi-node network are presented.

The free-space communications community has only recently recognized the complexity of atmospheric channel interactions, which are highly dependent on the turbulence profile, beam propagation geometry, and transceiver design. The search for models that accurately describe link performance and overall availability is currently an active field of research. This paper describes a method for defining link availability based on statistical channel models, which can be derived from measured signal fluctuations during periods of stable atmospheric conditions. Measurements made during an extended communication link experiment conducted during the summer of 2008 indicate that the intervals of channel stability, which impact the length of link outages, can vary in duration from a few minutes to several hours. This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the United States Government.

Free space optical communication systems require robust pointing and tracking to establish and maintain lineof- sight (LOS). Atmospheric scintillation can present a challenge to the LOS tracking systems located at each end of the link. This paper describes a pointing, acquisition, and tracking (PAT) approach for single-mode fiber coupling, which was successfully demonstrated over a 5.4 km lasercom link that was subject to severe turbulence conditions. One of the primary advantages of the scheme described is its compensation for thermo-mechanical drift, which simplifies optomechanical design and allows use of simple COTS hardware. An overview of the PAT system and performance data are presented.

Performance of quad-APD spatial tracking loop (STL) in the presence of scintillation is investigated for freespace optical (FSO) channels impaired by optical turbulence. The atmospheric turbulence is assumed to follow the weak turbulence model, described by Rytov approximation, which in turn suggests log-normal statistics for the received optical signal intensity. It is assumed that the pointing error in large part is due to atmospheric wander and that the correlation time of the beam wander is comparable to the correlation time of the amplitude variations due to amplitude scintillation. Computationally-efficient extensions of a recently proposed spatial tracking model are investigated, resulting in two alternative implementation. The proposed algorithms enable one to adjust the gain of the tracking loop, resulting in an adaptive bandwidth adjustment scenario in the presence of correlated fading. The performance of the proposed tracking loops along with that of the standard tracking loop are assessed and compared via simulation in terms of the mean square tracking error (MSTE). Simulation results reveal the effectiveness of the computationally-efficient algorithms proposed here in reducing the MSTE as compared with the standard tracking loop, while offering a realizable solution as compared to the tracking loop suggested by the previous study on the subject.

We present recent studies on the impact of atmospheric turbulence on free-space links using either synchronous or nonsynchronous detection. We compare options for atmospheric compensation, including phase compensation and diversity combining techniques. We consider the effects of log-normal amplitude fluctuations and Gaussian phase fluctuations, in addition to local oscillator shot noise, for both passive receivers and those employing active modal compensation of wave-front phase distortion. We compute error probabilities for M-ary phase-shift keying, and evaluate the impact of various parameters, including the ratio of receiver aperture diameter to the wave-front coherence diameter, the number of diversity channels, and the number of modes compensated.

We consider general theoretical aspects of level crossings of multidimensional fluctuating functions. Examples of such functions are turbulent fields such as refractive-index functions or turbulence-aberrated fields such as laser intensity functions in free-space laser communications. From a practical point of view, it is important to consider level crossings because they correspond to the temporal instances or spatial occurrences of when or where a signal of interest reaches, exceeds, or falls below a particular threshold. For example, the statistics of level crossings for a laser communication signal at a threshold corresponding to the minimum detection signal are important in order to study the probability density of the extent of intervals of down-time for communication links. For 1-D signals, the concept of the level crossing scale is clear and well established as it is the extent of the interval between successive level crossings. However, for multidimensional fields, this concept cannot be utilized directly because it is not clear how to define or identify successive level crossings, and therefore level crossing scales, in multiple dimensions. We describe a theoretical formulation which enables a consistent definition of level crossing scales for multidimensional fields, i.e. consistent with the traditional 1-D definition. We use the recently-developed concept of the shortest-distance scale because the latter applies naturally to multiple dimensions. We define the probability density function of level crossing scales, in any number of dimensions, in terms of a derivative of the probability density function of shortest-distance scales. Analytically, we illustrate this approach using exact theoretical examples with 2-D objects and we also provide results for exponential, lognormal, and power-law level crossing statistics which are basic models for applications involving turbulence and free-space laser communications.

In this work, the propagations of unpolarized and polarized beams are modeled using wave optics simulations. The second-order correlation properties and the degree of polarization were examined for various scenarios. Our results are consistent with a theoretical analysis available in the literature. The average intensity profiles in the receiver plane show excellent consistence with the theory. The on-axis scintillation indices are investigated and we find that when source transverse correlation distance is much smaller than the atmospheric correlation distance, then the unpolarized beam reduces the normalized intensity fluctuation up to a factor of two.

We study the effects of optical turbulence on the energy crosstalk among constituent orbital angular momentum (OAM) states in a vortex-based multi-channel laser communication link and determine channel interference in terms of turbulence strength and OAM state separation. We characterize the channel interference as a function of C²_n and transmit OAM state, and propose probability models to predict the random fluctuations in the received signals for such architecture. Simulations indicate that turbulence-induced channel interference is mutually correlated across receive channels.

We present a design methodology for free-space laser communications systems. The first phase includes a characterization through numerical simulations of the channel to evaluate the range of extinction and scintillation. The second phase is the selection of fade mitigation schemes, which would incorporate pointing, acquisition, tracking, and communication system parameters specifically tailored to the channel. Ideally, the process would include sufficient flexibility to adapt to a wide range of channel conditions. We provide an example of the successful application of this design approach to a recent set of field experiments. This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the United States Government.

This paper describes a lasercom terminal using spatial diversity to mitigate fading caused by atmospheric scintillation. Multiple receive apertures are separated sufficiently to capture statistically independent samples of the incoming beam. The received optical signals are tracked individually, photo-detected, and summed electrically, with measured diversity gain. The terminal consists of COTS components. It was used in successful demonstrations over a 5.4km ground-ground link from June through September 2008, during which it experienced a wide temperature range. Design overview and hardware realization are presented. This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the United States Government.

Understanding the relative contribution of beam wander and scintillation to the power fluctuations due to atmospheric turbulences is essential to improving the performance of Free Space Optical Communication systems. In prior work [5], we briefly mentioned that cone modulation could be used to measure the beam wander. We presented a few experimental results showing high correlation between the power fluctuation and the measured beam wander. Here we present the method in details, as well as the results of a more extensive set of measurements. The measurements are done using an optical interrogator tracking a passive retro-reflector over distances of 800 and 8km, using a cone modulation frequency of 625Hz, a rate significantly higher than the Greenwood frequency. We recorded the data at 10 kHz sampling rate, and ran off-line analysis to evaluate the correlation between the position of the beam relative to the retro-reflector and the fluctuations of the power received by the interrogator. We also recorded the angle of arrival fluctuations, which allows for an evaluation the correlation between the beam wander and the angle of arrival. The same data was used to calculate the C_n² structure constant from the statistics of the angle of arrival, in order to correlate the amplitude of the beam wander to the strength of the turbulence.

The implementation of cone tracking to optimize the beam pointing in a retro-reflective link was reported in prior work [1]. One parameter required to close the loop is the phase difference between the scanning signal and the reflected modulated power, which gives the direction of the error. The phase difference is easy to measure in a retro-reflective link because the beam is reflected back to the transmitter making the feedback signal locally available. In a direct mode configuration, this phase difference is unknown. The problem was avoided in prior work [2] by using two detectors a few inches apart. This paper presents the results of the evaluation, using simulation, of an algorithm that estimates and tracks the phase difference between the two ends of the link making it possible to close the loop and insure convergence. It also shows how to implement the feedback path without using an additional beam as opposed to using an additional high-power wide-divergence beam.

The design of an optical birefringent 90° hybrid for coherent receiver in free-space optical communication system is presented. The bulk optical design of 2×4 90° hybrid is a six-port device with two inputs and four outputs. For the balanced receiver configuration two pairs of 180° phase shifted outputs are obtained and one pair has a phase shift of 90° with respect to the other. The optical birefringent 90° hybrid includes two pairs of birefringent plates, an analyser plate and a λ/8 wave plate. The scheme of phase compensation is also presented.