We examine the potential of subsea free-space optics (FSO) for sensor network applications leveraging the emerging technologies of highly sensitive photon-counting detectors and semi-conductor LED and laser light sources in the UV solar blind. Monitoring oil and gas production installations is the niche application discussed. The merits of FSO include the capacity for broadband communication that would enable the transmission of video data in real time, which is not possible with other technologies at present. However, subsea FSO is challenged by high extinction and the immense variability of background illumination in shallow waters. This has stimulated us to investigate the potential of underwater FSO in the UV solar-blind spectral range, where background illumination is nearly nonexistent and considerable scattering occurs. The achievable performance is compared to transmission at 520 nm, where, in Clear Ocean, data rates of 100 Mbps can be transmitted over distances of ~170 m, falling to under 15 m in harbor waters. It is anticipated that ranges of 12 m can also be obtained with UV solar-blind wavelengths, although experimental corroboration is not yet available.
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
In this paper, we evaluate the possibility of a wireless sensor network concept operating in the oceanic environment. A population of underwater sensor nodes, termed "optical plankton," form a distributed sensor network characterized by the deployment of many miniature and low-cost sensors capable of profiling the particulate composition in the immediate marine environs. The stimulation of backscatter signals from the surrounding medium is one principle of operation studied, but the potential for additional probing methods is discussed. The optically probed signal is communicated to a base station, where the large number of received signals is fused to obtain an accurate estimate of the nature of the local aqueous medium. The many similarities and differences between the atmospheric and oceanic sensing and wireless communication environments are discussed and the distinctive features of an oceanic probing system are underlined. Specific scientific applications are briefly reviewed. The challenges to be met are addressed in general and a focused analysis of the specific issue of multi-access interference (MAI), common to all optical wireless-based sensor networks, is presented. Novel analytical approaches have been employed to evaluate and quantify the MAI. It is the object of this paper to assess the feasibility of a novel miniaturized oceanic probing system and explore some of the challenges. It would appear that the proposed scheme could be the basis of an innovative oceanic probing method and fill a niche not as yet catered for with existing technologies.
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.
A compact and mobile distributed sensing system which can monitor contaminant content in the ocean and initiate an alert system when high contaminant levels are detected would be useful in the surveillance of ports and harbors. Oceanic data acquisition takes many forms, but miniature and low cost platforms are not available for widespread monitoring and surveillance tasks. In this paper we present a novel sensing system, adapted from a similar concept designed for atmospheric probing and evaluate its feasibility in the ocean environment. Both the probing task itself and the data communication from the sensor nodes to the base station are based on free space optics. We also develop a probabilistic model of multi-access interference in a system using spectral diversity, which may be applicable in many distributed sensor multihop networks.
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.
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 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.
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.
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.
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.