Free-space Optical Transmission Research Articles

Implementation of FSO Network under the Impact of Atmospheric Turbulences

This work is focused to carry out the investigations of Free Space Optical Transmission System under the impact of different atmospheric attenuation factors. In this work A 10 Gbps data is transmitted through free space optical link of 1 km and the performance is measured in terms of SNR, BER and eye diagrams. Further the role of modulation format is also investigated.

Accurate optical parameter extraction procedure for broadband near-infrared spectroscopy of brain matter

Modeling behavior of broadband (30 to 1000 MHz) frequency modulated near-infrared (NIR) photons through a phantom is the basis for accurate extraction of optical absorption and scattering parameters of biological turbid media. Photon dynamics in a phantom are predicted using both analytical and numerical simulation and are related to the measured insertion loss (IL) and insertion phase (IP) for a given geometry based on phantom optical parameters. Accuracy of the extracted optical parameters using finite element method (FEM) simulation is compared to baseline analytical calculations from the diffusion equation (DE) for homogenous brain phantoms. NIR spectroscopy is performed using custom-designed, broadband, free-space optical transmitter (Tx) and receiver (Rx) modules that are developed for photon migration at wavelengths of 680, 780, and 820 nm. Differential detection between two optical Rx locations separated by 0.3 cm is employed to eliminate systemic artifacts associated with interfaces of the optical Tx and Rx with the phantoms. Optical parameter extraction is achieved for four solid phantom samples using the least-square-error method in MATLAB (for DE) and COMSOL (for FEM) simulation by fitting data to measured results over broadband and narrowband frequency modulation. Confidence in numerical modeling of the photonic behavior using FEM has been established here by comparing the transmission mode's experimental results with the predictions made by DE and FEM for known commercial solid brain phantoms.

Optimal Relay Placement and Diversity Analysis of Relay-Assisted Free-Space Optical Communication Systems

Relay-assisted free-space optical (FSO) transmission exploits the fact that atmospheric turbulence fading variance is distance dependent and yields significant performance gains by taking advantage of the resulting shorter hops. In this paper, we investigate how to determine optimal relay locations in serial and parallel FSO relaying so as to minimize the outage probability and quantify performance improvements obtained through optimal relay placement. We further present a diversity gain analysis for serial and parallel FSO relaying schemes and quantify their diversity advantages in terms of the number of relays and channel parameters.

A photon-counting spatial-diversity-and-multiplexing MIMO scheme for Poisson atmospheric channels relying on Q-ary PPM

A novel Photon-Counting Spatial-Diversity-and-Multiplexing (PC-SDM) scheme is proposed for high-speed Free-Space Optical (FSO) transmission over shot-noise limited Poisson channels experiencing turbulence-induced fading. In particular, Iterative Parallel Interference Cancellation (Iter-PIC) aided Q-ary Pulse Position Modulation (Q-PPM) is employed. Simulation results demonstrate that our proposed scheme exhibits a high integrity and a high throughput, while mitigating the effects of multi-stream interference and background radiation noise.

Network-coded multiple-source cooperation aided relaying for free-space optical transmission

A network-coded cooperative relaying aided free-space optical (FSO) transmission scheme is designed. The resultant multiple-source cooperation diversity is exploited by the relay to mitigate the strong turbulence-induced fading experienced in FSO channels. At the destination, an iterative multiple source detection algorithm is proposed in conjunction with a chip-level soft network decoding method. Our performance evaluation results using simulation analysis demonstrate that the proposed FSO multiple source detection is capable of approaching the single-user-bound for transmission over Gamma–Gamma turbulence channels. Also, the network-coded cooperative FSO scheme can achieve a significant BER improvement in comparison with conventional noncooperation schemes. Copyright © 2012 John Wiley & Sons, Ltd.

Asymptotic error-rate analysis of FSO links using transmit laser selection over gamma-gamma atmospheric turbulence channels with pointing errors

Since free-space optical (FSO) systems are usually installed on high buildings and building sway may cause vibrations in the transmitted beam, an unsuitable alignment between transmitter and receiver together with fluctuations in the irradiance of the transmitted optical beam due to the atmospheric turbulence can severely degrade the performance of optical wireless communication systems. In this paper, asymptotic bit error-rate (BER) performance for FSO communication systems using transmit laser selection over atmospheric turbulence channels with pointing errors is analyzed. Novel closed-form asymptotic expressions are derived when the irradiance of the transmitted optical beam is susceptible to either a wide range of turbulence conditions (weak to strong), following a gamma-gamma distribution of parameters α and β, or pointing errors, following a misalignment fading model where the effect of beam width, detector size and jitter variance is considered. Obtained results provide significant insight into the impact of various system and channel parameters, showing that the diversity order is independent of the pointing error when the equivalent beam radius at the receiver is at least 2(min{α,β})(1/2) times the value of the pointing error displacement standard deviation at the receiver. Moreover, since proper FSO transmission requires transmitters with accurate control of their beamwidth, asymptotic expressions are used to find the optimum beamwidth that minimizes the BER at different turbulence conditions. Simulation results are further demonstrated to confirm the accuracy and usefulness of the derived results, showing that asymptotic expressions here obtained lead to simple bounds on the bit error probability that get tighter over a wider range of signal-to-noise ratio (SNR) as the turbulence strength increases.

Performance Analysis on 16-Channels Wavelength Division Multiplexing in Free Space Optical Transmission under Tropical Regions Environment

Problem statement: Wavelength-Division-Multiplexing (WDM) is a promising technique for meeting the growing demand for increased bandwidth and various types of services in the optical access network. For wide area or metropolitan networks, fibers are deployed to provide huge bandwidth. In access networks, the fiber-to-the-home will partially solve the last mile problem. However, some environmentally sensitive area such as housing areas, tower buildings and national parks are not allowed to deploy fibers. Therefore, Radio Frequency (RF) is normally used to overcome this problem. The incompatibility of RF and optical channels is now widely believed to be the limiting factor in efforts to further increase transport capabilities. Free Space Optical (FSO) communication is the technology that can address any connectivity needed in optical networks, such as core, edge, or access networks. Approach: In this project, the simulation software namely Optical System version 7 is used to simulate the design of WDM in FSO transmission. The total losses that have been considered in this design are geometric loss, transmitter and receiver loss and atmospheric attenuation which focus on nonselective scattering during heavy rainfall condition in Malaysian environment. Malaysian weather data are used to reflect the conditions particularly in tropical regions. Results: We have presented the results of 16-channels WDM at 100-GHz channel spacing. The simulated results show that this system can support a higher bit rate up to 2.5 Gbps over 2.4 km distance. Conclusion: Simulation results showed that WDM FSO system may be a good candidate to solve the last mile problem and also it has capability to accommodate the channels more than 16. By introducing the error correction code or balance detection, the transmission distance might be increased further.

Capacity scaling in free-space-optical mobile ad hoc networks

Wireless networking has conventionally been realized via radio-frequency-based communication technologies. Free-space-optical (FSO) communication with an innovative multi-element node design leverages spatially-diverse optical wireless links; making it a viable solution to the well-known diminishing per-node throughput problem in large-scale RF networks. Although it has the advantage of high-speed modulation, maintenance of line-of-sight between two FSO transceivers during a transmission is a challenge since FSO transmitters are highly directional. In this paper, we present our simulation efforts to make high-level assessments on throughput characteristics of FSO-MANETs while considering properties of FSO propagation and existence of multiple directional transceivers. We identify the intermittent connectivity problem that is caused by the relative mobility of nodes with multiple directional transceivers. We propose two cross-layer buffering schemes to remedy this problem and present their performance results. We conclude that sophisticated buffering mechanisms are required to properly buffer a packet during the misalignment period of two communicating nodes to avoid negative effects of this intermittency on the transport layer.

Outage performance of MIMO FSO links over strong turbulence and misalignment fading channels

Atmospheric turbulence produces fluctuations in the irradiance of the transmitted optical beam, which is known as atmospheric scintillation, severely degrading the performance over free-space optical (FSO) links. Additionally, since FSO systems are usually installed on high buildings, building sway causes vibrations in the transmitted beam, leading to an unsuitable alignment between transmitter and receiver and, hence, a greater deterioration in performance. In this paper, the outage probability as a performance measure for multiple-input/multiple-output (MIMO) FSO communication systems with intensity modulation and direct detection (IM/DD) over strong atmospheric turbulence channels with pointing errors is analyzed. Novel closed-form expressions for the outage probability as well as their corresponding asymptotic expressions are presented when the irradiance of the transmitted optical beam is susceptible to either strong turbulence conditions, following a negative exponential distribution, and pointing error effects, following a misalignment fading model where the effect of beam width, detector size and jitter variance is considered. Obtained results show that the diversity order is independent of the pointing error when the equivalent beam radius at the receiver is at least twice the value of the pointing error displacement standard deviation at the receiver. Simulation results are further demonstrated to confirm the analytical results. Additionally, since proper FSO transmission requires transmitters with accurate control of their beamwidth, asymptotic expressions here obtained for different diversity techniques are used to find the optimum beamwidth that minimizes the outage performance.

Effect of Rain Attenuations on Free Space Optic Transmission in Kuala Lumpur

This research paper describes the effects of rain attenuation on the free space optical (FSO) transmission system. The entire research was conducted in the heart of Kuala Lumpur city- year 2001, as the city was using such systems (LAN-to-LAN & WLAN-to-WLAN) widely to connect the metropolitan areas for data transmission. The preliminary experiments were done using PAVLight product modeled PL-1G/1Tx/GigE which leaded to maximum reach of 700m distance with 810nm wavelength and also laser power of 14dBm. The data for rain intensities were obtained from measuring unit at UTM Jalan Semarak, Kuala Lumpur following the ITU protocols such as ITU-R P.1814 and ITU-R P.1817. The transmission and reception were achieved by using Continuous Wave (CW) Laser signals as transmitting signals and APD photo detector to receive it on the other end. The experiments were fully carried out for two conditions (heavy-rain and lighter rain or drizzle) with multiple repetitions. At last, the collected data were compiled and compared with other researcher's results. It was found to be the same as how the other researchers have obtained before for various cities worldwide. The results were found to be very satisfactory. It obeyed the theoretical aspects as higher the rain, higher the attenuations then. These results the degradation on the received signals. As a guide for future implementation of FSO systems for various atmospheric conditions in the metropolitan areas of Kuala Lumpur; we believed that this preliminary works may help.

Free-space optical transmission with orbital angular momentum division multiplexing

The experimental demonstration of a free-space optical transmission system with orbital angular momentum (OAM) division multiplexing is reported. Two beams have been multiplexed with different values of the azimuthal index l, and hence carrying different OAM, at a wavelength of 1550 nm. The first beam (l = 0) is nearly Gaussian and it features an OAM equal to zero. The second beam (l = 1) is an optical vortex produced by means of a suitable computer-generated hologram, which shows an intensity pattern with a doughnut shape and possesses an OAM of –h per photon. A binary intensity modulation at 1.25 Gbit/s has been applied to each OAM channel and an interferometer with Dove prisms has been exploited for OAM demultiplexing.

Free Space Optical Communication Systems and Image Transmission

Free Space Optical Communication Systems and Image Transmission

Optical Wireless Communication Systems in the Mb/s to Gb/s Range, Suitable for Industrial Applications

For future short- and mid-range industrial applications, optical wireless (OW) communication systems are expected to play a major role. When moderate transmission rates (100 Mb/s range) are required, OW communications present a viable and promising technology, supplemental to conventional radio wireless systems. Advanced approaches based on diversity techniques and adaptive signal processing show potential to achieve both high spatial coverage and high bit rates of more than 100 Mb/s. Visible-light communication systems using white phosphorescent LEDs equally present an interesting application potential, combining illumination with data transfer. When high data volumes (100 Gb/s range) need to be transmitted, tailored optical data links provide a solution of choice. Exemplarily, a scalable (24-140 Gb/s) optical data link is presented, developed for future implementation in maskless lithography systems. The link comprises a high-speed data buffer with synchronizable architecture and scalable throughput (N × 24 Gb/s), an optical free-space transmission solution, and finally, a 45-channel low-noise optical receiver chip based on BiCMOS 0.6 μm technology.

Polarization-Multiplexed Optical Wireless Transmission With Coherent Detection

In this paper, we propose, analyze, and experimentally verify polarization-multiplexed optical wireless transmission with coherent detection as a means of improving performance and increasing the per-wavelength data rate in the presence of atmospheric turbulence. It is first shown that in terrestrial last-mile applications, the polarization state changes of the optical wireless signal are governed by optical source properties both in the absence and in the presence of mild to moderately severe turbulence. Under these conditions, it is shown that the polarization evolution is reduced to a polarization state rotation wherein the cross-polarization interference can be equalized with well-known blind algorithms. We also experimentally verify the analytical findings, demonstrating 112 Gb/s POLMUX-QPSK transmission with coherent detection in a non-fading free-space channel, and numerically extend the experimental results to turbulent settings by invoking the lognormal model for turbulence-induced fading. Performance gains of 5-14 dB along with a doubling of the data rate through polarization multiplexing are exhibited compared to legacy single-polarization intensity modulation/direct detection (IM/DD) optical wireless systems. The presented analytical and experimental results indicate that the proposed approach is promising for notably increasing the data rate in last-mile free-space optical transmission.

Rateless coding for hybrid free-space optical and radio-frequency communication

Free-space optical (FSO) transmission systems enable high-speed communication with relatively small deployment costs. However, FSO suffers a critical disadvantage, namely susceptibility to fog, smoke, and conditions alike. A possible solution to this dilemma is the use of hybrid systems employing FSO and radio frequency (RF) transmission. In this paper we propose the application of a rateless coded automatic repeatrequest scheme for such hybrid FSO/RF systems. The advantages of our approach are (a) the full utilization of available FSO and RF channel resources at any time, regardless of FSO or RF channel conditions and temporal variations, and (b) no need for a-priori rate selection at the transmitter. In order to substantiate these claims, we establish the pertinent capacity limits for hybrid FSO/RF transmission and present simulation results for transmission with off-the-shelf Raptor codes, which achieve realized rates close to these limits under a wide range of channel conditions. We also show that in conditions of strong atmospheric turbulence, rateless coding is advantageous over fixed-rate coding with rate adaptation at the transmitter.

All-optical Ti:PPLN wavelength conversion modules for free-space optical transmission links in the mid-infrared

Data-format-independent all-optical transmitter and receiver modules for free-space optical communications in the 3.8 mum region have been developed, essentially consisting of Ti-indiffused, periodically poled LiNbO(3) waveguides. Using these modules, conversion of C-band radiation to/from the mid-infrared (MIR) via difference-frequency generation was demonstrated. More than 10 mW of MIR power could be generated with the transmitter unit; the internal low-power conversion efficiency was 69%/W. A free-space MIR link was set up with a -41 dB fiber-to-fiber loss; only -15 dB are due to the parametric processes.

BER Performance of Free-Space Optical Transmission with Spatial Diversity

Free space optical (FSO) communications is a cost-effective and high bandwidth access technique, which has been receiving growing attention with recent commercialization successes. A major impairment in FSO links is the turbulence- induced fading which severely degrades the link performance. To mitigate turbulence-induced fading and, therefore, to improve the error rate performance, spatial diversity can be used over FSO links which involves the deployment of multiple laser transmitters/receivers. In this paper, we investigate the bit error rate (BER) performance of FSO links with spatial diversity over log- normal atmospheric turbulence fading channels, assuming both independent and correlated channels among transmitter/receiver apertures. Our analytical derivations build upon an approximation to the sum of correlated log-normal random variables. The derived BER expressions quantify the effect of spatial diversity and possible spatial correlations in a log-normal channel.

Multilevel coding in free-space optical MIMO transmission with Q-ary PPM over the atmospheric turbulence channel

We propose an efficient coded modulation scheme for multiple input/multiple output free-space optical systems with Q-ary pulse-position modulation. It is based on multilevel coding and low-density parity-check codes as component codes. The proposed scheme provides excellent coding gain in the regime of strong atmospheric turbulence.

Optical networks, last mile access and applications

Free Space Optical (FSO) links can be used to setup FSO communication networks or to supplement radio and optical fiber networks. Hence, it is the broadband wireless solution for closing the “last mile” connectivity gap throughout metropolitan networks. Optical wireless fits well into dense urban areas and is ideally suited for urban applications. This paper gives an overview of free-space laser communications. Different network architectures will be described and investigated regarding reliability. The usage of “Optical Repeaters”, Point-to-Point and Point-to-Multipoint solutions will be explained for setting up different network architectures. After having explained the different networking topologies and technologies, FSO applications will be discussed in section 2, including terrestrial applications for short and long ranges, and space applications. Terrestrial applications for short ranges cover the links between buildings on campus or different buildings of a company, which can be established with low-cost technology. For using FSO for long-range applications, more sophisticated systems have to be used. Hence, different techniques regarding emitted optical power, beam divergence, number of beams and tracking will be examined. Space applications have to be divided into FSO links through the troposphere, for example up- and downlinks between the Earth and satellites, and FSO links above the troposphere (e.g., optical inter-satellite links). The difference is that links through the troposphere are mainly influenced by weather conditions similar but not equal to terrestrial FSO links. Satellite orbits are above the atmosphere and therefore, optical inter-satellite links are not influenced by weather conditions. In section 3 the use of optical wireless for the last mile will be investigated and described in more detail. Therefore important design criteria for connecting the user to the “backbone” by FSO techniques will be covered, e.g., line of sight, network topology, reliability and availability. The advantages and disadvantages of different FSO technologies, as well as the backbone technology are discussed in this respect. Furthermore, the last mile access using FSO will be investigated for different environment areas (e.g., urban, rural, mountain) and climate zones. The availability of the FSO link is mainly determined by the local atmospheric conditions and distance and will be examined for the last mile. Results of various studies will complete these investigations. Finally, an example for realizing a FSO network for the last mile will be shown. In this example FSO transmitters with light emitting diodes (LED) instead of laser diodes will be described. By using LEDs, problems with laser-and eye safety are minimized. Some multimedia applications (like video-conferences, live TV-transmissions, etc.) will illustrate the range of applications for FSO last mile networks.

Free-Space Optical MIMO Transmission with$Q$-ary PPM

Free-Space Optical MIMO Transmission with$Q$-ary PPM