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Data centers collect and process information with a capacity that has been increasing from year to year at an almost exponential pace. Traditional fiber/cable data center network interconnections suffer from bandwidth overload, as well as flexibility and scalability issues. Therefore, a technology-shift from the fiber and cable to wireless has already been initiated in order to meet the required data-rate, flexibility and scalability demands for next-generation data center network interconnects. In addition, the shift to wireless reduces the volume allocated to the cabling/fiber and increases the cooling efficiency. Optical wireless communication (OWC), or free space optics (FSO), is one of the most effective wireless technologies that could be used in future data centers and could provide ultra-high capacity, very high cyber security and minimum latency, due to the low index of refraction of air in comparison to fiber technologies. In this paper we review the main concepts and configurations for next generation OWC for data centers. Two families of technologies are reviewed: the first technology regards interconnects between rack units in the same rack and the second technology regards the data center network that connects the server top of rack (TOR) to the switch. A comparison between different network technologies is presented.

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

In recent years the need for high data rate underwater wireless communication (WC) has increased. Nowadays, the conventional technology for underwater communication is acoustic. However, the maximum data rate that acoustic technology can provide is a few kilobits per second. On the other hand, emerging applications such as underwater imaging, networks of sensors and swarms of underwater vehicles require much faster data rates. As a result, underwater optical WC, which can provide much higher data rates, has been proposed as an alternative means of communication. In addition to high data rates, affordable communication systems become an important feature in the development requirements. The outcome of these requirements is a new system design based on off-the-shelf components such as blue and green light emitting diodes (LEDs). This is due to the fact that LEDs offer solutions characterized by low cost, high efficiency, reliability and compactness. However, there are some challenges to be met when incorporating LEDs as part of the optical transmitter, such as low modulation rates and non linearity. In this paper, we review the main challenges facing the incorporation of LEDs as an integral part of underwater WC systems and propose some techniques to mitigate the LED limitations in order to achieve high data rate communication

Modulating retro-reflectors (MRR) are beneficial for asymmetric free-space optics communication links. An MRR includes an optical retro-reflector and an electro-optic shutter. The main advantage of an MRR configuration is that it shifts most of the power, weight, and pointing requirements onto one end of the link. In this study an innovative device comprising of nanoparticle-embedded ferroelectric thin film is used as an MRR. The new modulator is mounted in front of a passive retro-reflector. In our study we calculated the link budget for lunar exploration scenario. The scenario includes a base station that communicates with several robots or astronauts. In our simulations, the base station illuminates a robot with a continuous-wave beam, i.e. an interrogating beam. The un-modulated beam strikes the MRR, which is located on the robot, and is passively reflected back to the base station carrying the data that has been modulated onto it by the MRR. In this scenario a robot and a base-station are 4km apart, with a clear line of sight. In addition, the innovative MRR is capable of achieving 12dB contrast ratio. Under these assumptions and using the nanoparticle-embedded ferroelectric MRR we calculated the required transmission power for a given bit-rate and BER.

We present an innovative method for modulating light using a ferroelectric thin-film embedded with metal nanoparticles. Due to the electro-optic effect in ferroelectric PZT, changes in refractive index can be controlled by an external electric field. Consequently, the local surface plasmon resonance of embedded noble metal nanoparticles changes with the media's refractive index. As a result, their optical extinction cross-section is shifted and light passing through the film could be controlled. In other words, an external electric field could modulate light. Using Mie theory for spherical particles, we were able to approximate the metallic nanoparticle's diameter that generates the maximum optical contrast, at a given wavelength. In addition, to establish an accurate model, we considered the impact on plasmon resonance resulting from deformation of the nanoparticles. The deformation is caused by the piezoelectric property of the ferroelectric host material. We assumed 20 nm diameter Au or Ag nanoparticles embedded in a 1 μm thick PZT film. Simulations showed that these particles can reach an optical contrast of up to 12 dB, in the visible spectrum. In addition, deformation of particles had negligible impact on the shift in resonance frequency compared to the change in PZT refractive index. In this study we have shown that a nanocomposite comprising of nanoparticles embedded PZT thin film can perform as an optical modulator. This modulator will be able to achieve a high contrast with low power consumption.

The growing need for high data-rate communication both through the atmosphere and the ocean (sub-sea) has stimulated considerable interest in optical wireless communication (OWC) technologies. The main advantages of OWC as compared with RF communication in the atmosphere and with acoustic communication in sub-sea applications are a) high achievable data-rate, b) small size of equipment and c) low power-consumption. On the other hand the characteristics of the communication channel in both scenarios are stochastic with high values of variance, which severely degrades OWC communication system performance. In this paper we present a tutorial discussing the effects of random media on OWC and expand on two examples: Monte-Carlo simulation for sub-sea communication and mathematical synthesis using Meijer G-function for OWC through atmospheric turbulence. These two examples demonstrate that it is possible to gain significant insights on the effects of the random channel on system performance. The results of the different analysis methods could also indicate solutions for the improvement of performance using adaptive solutions or for extending the communication range by applying a multi-hop concept. We summarize the paper with a brief review of two emerging research fields that could, surprisingly, benefit from the characteristics of light propagation through random media and its effect on the communication system performance. The first research field is trans-cutaneous OWC and the second is an unguided optical communication bus for next-generation computers.

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.

Sub-sea monitoring of floating oil production platforms is a crucial issue in the interests of security, efficient functioning and pollution prevention. Sensors and sensor networks are essential tools for implementing the monitoring operations and low-error data communication from the sensors and within the network is a critical element in these systems. Free space optics (FSO) has gained recognition in numerous applications as a high bandwidth, energy efficient communication medium and has recently been considered as a viable alternative to acoustic communications for underwater applications when high data rates are required over short transmission ranges. Video recording is a powerful method for gathering extensive data in time and space that requires broadband communication facilities, such as could be provided by FSO. However, the immense variability of background illumination in shallow waters, inducing shot noise at the receiver, presents a challenge for sub-sea FSO. This has stimulated us to investigate the potential of underwater FSO in the UV solarblind spectral range. The potential of UV solarblind optical wireless links for sub-sea FSO is investigated in this paper and compared with performance at 520nm. In clear ocean data rates of 100bps can be transmitted over distances of above 120m using 520nm radiation, but this range is reduced to around 10m in harbour waters for these data rates and to 50m in clear ocean when the data rate is increased to 100Mbps. It is anticipated that ranges of 10m can also be obtained with UV solarblind wavelengths, although experimental corroboration is not yet available.

Networks of sensors are an emerging technology for real-time data gathering in a wide variety of civilian and military applications. Sensor networks comprise a large number of miniature nodes with sensing, computing and wireless communication capabilities and are randomly deployed in an area or volume of interest that may be remote or hazardous especially in defense applications. Using optical wireless communication (OWC) a population of sensor nodes can be located and mapped and encoded data can be transmitted to the base station. In this paper we review some theoretical and experimental work in this area and underline some of the challenges and possible solutions.

Networks of sensors are an emerging technology for real-time data gathering in applications such as pollution monitoring, home security, surveillance, industrial control, etc. Many miniature nodes with sensing, computing and wireless communication capabilities are randomly deployed in an area or volume to be probed. One of the possible communication modalities for sensor networks is optical wireless communication (OWC). Initially, the sensor population must be mapped prior to interrogation by the base station and data communication from the sensor node. In this paper we review some theoretical and experimental work in this area and underline some of the challenges and possible solutions. The specific scenario of wireless sensor networks in a disaster recovery operation is modeled.

Quantum key distribution (QKD) systems are being developed for possible use in future free space optical communication links to provide high security transmissions. However, these systems restrict the communication to a low bit-rate. To increase the QKD bit rate we suggest implementing a communication system which includes both optical detector and transmitter arrays. The mathematical description of this scheme involves a multi-input multi-output (MIMO) model. The optical detector and transmitter arrays enable the receiver to communicate simultaneously with a number of transmitter elements, which consequently increases the QKD bit rate. However, this method may introduce interference effects when a photon transmitted from one transmitter element reaches the wrong detector element in the receiver, due to scattering and turbulence in the atmospheric channel. This crosstalk and interference reduce the overall system bit rate and increase the quantum bit error rate (QBER). Analyzing the effect of crosstalk and interference on the system performance is accomplished by using the optical transfer function (OTF) theory.

High resolution, wide range optical beam scanning methods are sought to meet the ever-increasing demands of optical wireless communication, printers, scanners and laser radar. In this paper we present a precise steering method with high angular resolution and a wide scanning range. The proposed beam-steering device is also, lightweight and compact, which are significant advantages in many applications. The scanning is implemented by means of a pair of diffraction gratings, which can be translated relative to one another, and a tunable-wavelength laser. Thus the beam is steered by the combined effect of two independent mechanisms. The agility of wavelength tuning enhances the performance of a grating-based device, while the sensitivity to chromatic aberrations is minimized by performing the coarse steering with the diffraction grating. The system performance is demonstrated experimentally and the results are reported.

Optical wireless communication performance through multi-scattering channels has been researched widely in recent years due to the increasing interest in laser satellite-ground links and urban optical wireless communication. The predominant sources of performance degradation have been identified as the spatial, angular and temporal spread of the propagating beam in the presence of a multi-scattering medium, which cause reduced power reception and inter-signal-interference (ISI), and noise due to background illumination and to receiver circuitry. In consequence, the signal-to-noise ratio (SNR) and the bit-error-rate (BER) are degraded. However, coherence effects due to multipath interference caused by a scattering propagation channel do not appear to have been treated in detail in the scientific literature. In this paper we attempt a theoretical analysis of coherence interference in optical wireless communication through scattering channels, and try to quantify the resultant performance degradation.

Networked sensors are an emerging technology with the purpose of gathering data, profiled in time and space, for monitoring and surveillance in medical, environmental, home security and other applications. Typically, a large number of miniature sensing and communicating nodes are distributed ad hoc at the location of interest, where they establish a network and wirelessly communicate sensed data to one another, or to a base station. The optical modality is a potential solution for the links, due to the small and lightweight hardware and low power consumption, despite the drawback of alignment limitations. We propose an experimental concept to investigate the multi user interference effects arising when a number of nodes operate in a multi-scattering environment simultaneously. The laboratory experiment simulates a single internode link and the multi-scattering environment is modeled by a fog chamber. Linear and angular perturbations of transmitter-receiver line-of-sight (LOS) simulate geometries wherein multiply scattered light from the transmitted beam may interfere with the desired signal at the receiver causing multi-access interference. The potential multi-access interference is evaluated at differing optical densities and wavelengths. Conversely, the possibility for communication in the absence of LOS is inferred. Lastly, interference due to side-scattering from a link in the proximity of a receiver is modeled and assessed. In conclusion, guidelines are outlined for future experimental validation.

Clustering micro satellites in cooperative fly formation constellations leads to high-performance space systems. The only way to achieve high-speed communication between the satellites is by a laser beam with a narrow divergence angle. In order to make the communication successful three types of focal plane detector arrays are required in the communication terminal: acquisition, tracking and communication detector arrays. The acquisition detector array is used to acquire the neighbor satellite using a wide field-of-view telescope. The tracking detector provides fast, real time and accurate direction location of the neighbor satellite. Based on the information from the acquisition and tracking detectors the receiver and transmitter maintain line of sight. The development of large, fast and very sensitive focal plane detector arrays makes it possible to implement the acquisition, tracking and communication with only one focal plane detector array. By doing so it is possible to reduce dramatically the size, weight, and cost of the optics and electronics which leads to lightweight communication terminals. As a result, the satellites are smaller and lighter, which reduces the space mission cost and increases the booster efficiency. In this paper we will present an overview of the concept of integrated focal plane arrays for laser satellite communication. We also present simulation results based on real system parameters and compare different implementation options.

In this paper we summarize work done on the crosstalk effect of aerosol backscatter on the performance of an urban optical wireless communication (UOWC) system. The communication link is a segment within a metropolitan area network (MAN), where a WDM transmitter and receiver are housed in one transceiver unit with parallel, or near-parallel, optic axes. The crosstalk at the receiver is caused by light from the transmitted signal of the same transceiver, which has been backscattered by molecules and aerosols in the atmosphere. This is exacerbated in the presence of fog and haze, when both the desired signal from another transceiver is attenuated by scattering and the backscatter-induced crosstalk increases. In our research we derive a bit error rate (BER) model which takes into consideration the dominant noise sources, which include the backscatter-induced crosstalk and the signal mixing with amplified stimulated emission (ASE) from an optical pre-amplifier at the receiver. Our numerical calculations indicate that in a moderate fog the BER may increase by an order of magnitude, due to backscatter.

One of the most promising remote sensing technologies for the coming years is a network of sensors. The system under consideration includes a network of sensors and a remote base-station. The base-station communicates with the sensors using optical wireless links and acquires and identifies sensors based on the unique subcarrier frequency. The sensors use active retro-reflectors to communicate with the base-station in order to make the sensor simple, cost-effective, and with minimum power consumption. The base-station employs an imaging receiver (detector matrix), in which signals arriving from different directions are detected by different pixels. The imaging receiver mitigates ambient light noise and interference between simultaneous uplink transmissions from different sensors, provided that the transmissions are imaged onto disjoint sets of pixels. Our scheme allows simultaneous acquisition and identification of a sensor in a network by an imaging receiver. A probability model of erroneous acquisition due to noise is derived.

In this paper, we present an improved concept of “Laser Firefly Clustering” for atmospheric probing, elaborating upon previous published work. The laser firefly cluster is a mobile, flexible and versatile distributed sensing system, whose purpose is to profile the chemical and particulate composition of the atmosphere for pollution monitoring, meteorology, detection of contamination and other aims. The fireflies are deployed in situ at the altitude of interest, and evoke a backscatter response form aerosols and molecules in the immediate vicinity using a coded laser signal. In the improved system a laser transmitter and one imaging receiver telescope are placed at a base station, while sophisticated miniature distributed sensors (fireflies), are deployed in the atmosphere. The fireflies are interrogated by the base station laser, and emit non-coded probing signals in response. The backscatter signal is processed on the firefly and the transduced data is transmitted to the imaging receiver on the ground. These improvements lead to better performance at lower energy cost and expand the scope of application of the innovative concept of laser firefly clustering. A numerical example demonstrates the potential of the novel system.

In this work we derive a model, which optimizes the performance of a laser satellite communication link with an optical preamplifier in the presence of random jitter in the transmitter-receiver line of sight. The system utilizes a transceiver containing a single telescope with a circulator. The telescope is used for both transmitting and receiving and thus reduces communication terminal dimensions and weight. The optimization model for optimal transmitted power and transceiver gain was derived under the assumption that the dominant noise source was amplifier spontaneous emission (ASE) noise. We investigate the effect of the amplifier spontaneous emission noise on the optimal transmitted power and gain by performing an optimization procedure for various combinations of amplifier gains and noise figures. It was shown that the amplifier noise figure determines the transmitted power needed to achieve the desired BER, but does not affect the transceiver telescope gain. We found in our numerical example that for a BER of 10-9, doubling the amplifier noise figure results in an 80% increase of minimal transmitted power for rms pointing jitter of 0.44 μrad.

In this paper we present a mathematical model, which describes the relation between the optical power received in a free space optical communication system and the receiver field-of-view (FOV), when the propagation medium is multiply scattering. The model is investigated at different optical densities and a deterministic relation is developed. Monte-Carlo simulation results and the derived mathematical formulation are compared with experimental results. The ability to maintain a communication link in the absence of strict line-of-sight (LOS) alignment is a major challenge for optical wireless systems as well as for the emerging technology of distributed sensors (such as the "smart dust" or oxygen project). We show that in the presence of multiply scattering media, such as fog and haze, scattered light reception at angles of incidence of several milliradians can render a link functional even when unscattered light is not received because of inadequate LOS alignment. An adaptive FOV receiver is proposed as a solution for maintaining communication in adverse conditions.

With growing interest in terrestrial, inter-building and short distance wireless communication for high data-rate transmissions, solutions are sought for the crippling problems presented by multi-scattering phenomena typified by fog and particulate media. The multiple scattering results in spatial, temporal and angular spread of the light as it propagates through the medium. This both attenuates the total power incident on the receiver and increases the Bit Error Rate (BER) as subsequent pulses are not distinguishable due to Inter-symbol interference (ISI). A model of light transmission through fogs of different optical thicknesses and types is presented at four different wavelengths, using Monte-Carlo simulations. An adaptive field of view (FOV) receiver for optical wireless communication is proposed and the possibility of thus enhancing communication system performances through fog is indicated. Necessarily, the limitations presented by thermal noise in a detector of dimensions affording large fields of view restrict the applicability of the proposed solution. Hence, in this work we investigate optimal FOV settings, taking into consideration thermal noise signal degradation. The essence of the concept is to be configured as a simple design tool whereby environmental data are correlated to optimal FOV settings.

The use of a 2-D optical phased array (OPA) steering antenna in microsatellite clusters is very attractive as it is lightweight and compact, and provides agile and inertia-free 2-D beam steering. In this work, we combined OPA theory with optical wireless communication system concepts to achieve a new perspective on potential technologies for microsatellite cluster communication. The optical wireless communication intersatellite link using 2-D integrated OPA steering antenna was modeled. Then we theoretically analyzed the proposed communication system and illustrated the model by a numerical simulation. Taking into account the phasing errors, the antenna's gain distribution statistics were derived from OPA theory by use of the Monte Carlo method. We then applied these results to make a broad analysis of an optical wireless communication system and to investigate the effect of OPA phasing errors on system performance.

This work presents research in the field of laser satellite communication between a cluster of nano-satellites and a ground station. The scenario under consideration is a cluster of nano-satellites, communicating by means of a laser beam with a detector array receiver, which is located on the Earth's surface and equipped with a common optical system for all incoming beams. A critical parameter, determining the successful receipt of a transmitted signal for a given configuration, is the angular separation between the satellites within the cluster. This separation must be retained to prevent critical overlapping of the spots on the detector. The maximum allowable overlapping is calculated in terms of given BER. The spatial spreading of the beams, caused by scattering from aerosols in different layers of the atmosphere, is calculated for the case of single scattering, as appropriate for the stratified model used. Turbulence influences the beam width especially for the case of short exposure. In this research a new approach is adopted to characterize the atmospheric channel using OTF (Optical Transfer Function) concepts from the field of imaging and remote sensing. We evaluate the effectiveness of this new approach inapplications where spatial spread is very important, and detector array feasibility is currently under investigation.