Conventional Free-space Optical Research Articles

Modeling and investigation on the performance enhancement of hovering UAV-based FSO relay optical wireless communication systems under pointing errors and atmospheric turbulence effects

This paper investigates and enhances unmanned aerial vehicle (UAV) relay-assisted free-space optics (FSO) optical wireless communication (OWC) systems under the effects of pointing errors (PEs) and atmospheric turbulences (ATs). The incorporation of UAVs as buffer-aided moving relays in the conventional FSO (CFSO) relay-assisted systems is proposed for enhancing the performance of PEs through AT. Using M-PSK (phase shift keying) and M-QAM (quadrature amplitude modulation), the impact of PEs on transmission quality is evaluated in this work. We evaluate and optimize the symbol error rate, outage probability (OP), and signal-to-noise ratio (SNR) for the UAV-to-ground station-based FSO communications systems. The spatial diversity-based relay-assisted CFSO systems can enhance the performance of the UAV-UAV FSO links. In this paper, a new FSO (NFSO) channel model for the hovering UAV-FSO OWC fluctuations under the PEs, AT effects, jitter, deviation, receiving an error, and wind resistance effects are established. To improve the performance of hovering UAV-based FSO relay OWC systems. We reduce the influence of UAV-FSO OWC fluctuations under PEs and AT effects. By receiving incoherent signals at various locations, the spatial diversity-based relay-assisted NFSO systems can significantly increase the system's redundancy and enhance connection stability. Numerical results show that to achieve a bit-error-rate (BER) of le 10^{ - 5}, the required SNR is ≥ 23 dB when the wind variance of the UAVs sigma_{alpha }^{2} increases from 0 to 7 mrad with FSO link distance L = 2000 m. The required SNR is ≥ 25 dB when the wind variance sigma_{alpha }^{2} is 1 mrad at an OP of 10^{ - 6}. To obtain an average BER of 10^{ - 6}, the SNR should be 16.23 dB, 17.64 dB, and 21.45 dB when sigma_{alpha }^{2} is 0 mrad, 1 mrad, and 2 mrad, respectively. Using 8-PSK modulation without PEs requires 23.5 dB at BER of 10^{ - 8} while 16-QAM without PEs requires 26.5 dB to maintain the same BER of 10^{ - 8}. Compared with 16-QAM without PEs, the SNR gain of 8-PSK without PEs is 3 dB. The results show the relay-assisted UAV-FSO system with five stationary relays can achieve BER 10^{ - 8} at 25 dB SNR in the ideal case and 10^{ - 5} at 27 dB SNR with AT and PE at FSO length 1000 m. The results show the relay UAV-FSO system outperforms the CFSO at the BER and SNR performance. The effects of UAV’FSO s fluctuation increase when the UAV-FSO link length, {text{L}}_{{{text{fso}}}} increases. The results of the weak turbulence achieve better SER compared with MT and ST. The obtained results show that decreasing {text{L}}_{{{text{fso}}}} can compensate for the effects of UAV-FSO link fluctuation on the proposed system. Finally, we investigated the CFSO relay-assisted UAV-FSO system with aided NFSO-UAVs spatial diversity-based relay-based on NFSO OWC and revealed the benefits of the resulting hybrid architecture.

Arrayed Waveguide Grating Spectrometer on 2-µm-thick SOI Platform

32-channel arrayed waveguide grating spectrometer (AWG) at 1800 nm is demonstrated on a 2-µm-thick silicon-on-insulator (SOI) platform. The design and simulation of the device are performed using the beam propagation method (BPM) and we obtain the 3-dB channel width, channel spacing, and extinction ratio of 1.16 nm, 1.56 nm, and 5.17 dB, respectively. The AWG demultiplexer can be applied in the central part of a spectrometer which is replacing in integrated optics a prism or a grating in conventional free-space optics.

Low complexity deep learning algorithms for compensating atmospheric turbulence in the free space optical communication system

One of the main barriers of free space optical (FSO) communication systems is atmospheric turbulence. Various processing techniques at the transmitter, receiver, and transceiver sides are available for addressing this issue; however, they have either high complexity or low performance. Considering this problem, in this study, deep learning (DL) is deployed at the transmitter, receiver, and transceiver sides of an FSO system for constellation shaping, detection, and joint constellation-shaping detection, respectively. Furthermore, the proposed DL-based structures are deployed in an FSO-multi-input multi-output (MIMO) system. As the first investigation over DL for the FSO-MIMO system, different combining schemes including the maximum ratio combiner, equal gain combiner, and the selection combiner are considered. Considering a wide range of atmospheric turbulence, from the weak to the strong regime, the performance of the proposed structures are compared with that of the maximum likelihood (ML) detection. To the best of the authors' knowledge, the main contributions and novelties of this work include considering transmitter learning in the FSO system, designing low complexity DL structures for FSO system applications, and providing complexity analysis for the proposed DL algorithms. The results indicate that the proposed DL-based FSO systems achieve the optimum performance with lower complexity compared with the state-of-the-art conventional FSO systems. For instance, the proposed DL-based detector is almost 2, 3, and 7.5 times faster than the ML detector for modulation orders of 16, 64, and 256, respectively.

Statistical Modeling of the FSO Fronthaul Channel for UAV-Based Communications

In this paper, we investigate the statistics of the free space optics (FSO) communication channel between a hovering unmanned aerial vehicle (UAV) and a central unit. Two unique characteristics make UAV-based FSO systems significantly different from conventional FSO systems with static transceivers. First, for UAV-based FSO systems, the incident laser beam is not always orthogonal to the receiver lens plane. Second, both position and orientation of the UAV fluctuate over time due to dynamic wind load, inherent random air fluctuations in the atmosphere around the UAV, and internal vibrations of the UAV. On the contrary, for conventional FSO systems, the laser beam is always perpendicular to the receiver lens plane and the relative movement of the transceivers is limited. In this paper, we develop a novel channel model for UAV-based FSO systems by quantifying the corresponding geometric and misalignment losses (GML), while taking into account the non-orthogonality of the laser beam and the random fluctuations of the position and orientation of the UAV. In particular, for diverse weather conditions, we propose different fluctuation models for the position and orientation of the UAV and derive corresponding statistical models for the GML. We further analyze the performance of a UAV-based FSO link in terms of outage probability and ergodic rate and simplify the resulting analytical expressions for the high signal-to-noise ratio (SNR) regime. Finally, simulations validate the accuracy of the presented analysis and provide important insights for system design. For instance, we show that for a given variance of the fluctuations, the beam width should be properly adjusted to minimize the outage probability.

Tractable Optical Channel Modeling Between UAVs

More recently, the research on potential use of free-space optical (FSO) link as a powerful communication link between unmanned aerial vehicles (UAVs) has created much interest in academia and industry. Due to the need of higher number of UAV-based FSO links relative to the conventional ground-based FSO links, the optimum design of UAV-based FSO system parameters (such as optimum values for beam divergence angle, photodetector size, receiver lens radius, and transmit power, among many others) is much more needful and challenging relative to the ground-based counterpart. Moreover, to avoid the time consumed in Monte-Carlo simulations, existence of a simple and tractable channel model is very important and necessary. To address this need, in this paper, for the weak turbulence conditions, we construct a novel channel model for the considered system model under the effect of log-normal atmospheric turbulence channel. Then, for moderate to strong turbulence conditions, a novel closed-form statistical channel model is derived for Gamma-Gamma turbulence channel. The provided channel models, despite being simple and tractable, include the combined effects of atmospheric turbulence as well as the pointing errors. These also include the effects of receiver field-of-view limitation and inherent position, and orientation deviations of UAVs. Subsequently, for better performance analysis, the closed-form expressions for the outage probability and bit error rate (BER) are derived. Finally, the validity of the proposed novel channel models as well as the closed-form expressions for the outage probability and BER are confirmed by employing Monte-Carlo simulations. The developed results can therefore be applied as a benchmark for finding the optimal tunable parameters of UAV-based FSO links under different channel conditions and different levels of UAV instability without resorting to time-consuming Monte-Carlo simulations.

Transmit Laser Selection for Two Hop Decode and Forward FSO Communication With Pointing Errors

In this letter, two systems have been proposed for efficient unidirectional free space optical (FSO) cooperative communication-one with only transmit laser selection (TLS) and the other using optical spatial modulation (OSM) along with TLS. Performance analysis of such systems for Gamma-Gamma (G-G) fading channels both with and without Pointing errors have been carried out in terms of outage probability and average symbol error probability (ASEP). Our proposed TLS system over G-G channel (with pointing error) yields a signal to noise ratio (SNR) gain of at least 10 dBm over the conventional FSO system.

Optimal Packet Length for Free-Space Optical Communications with Average SNR Feedback Channel

In this article, a method to enhance data rates of free-space optical (FSO) systems using packet length optimization is proposed. The average signal-to-noise ratio (ASNR) is measured at the receiver and sent back to the transmitter to optimize packet length. In addition, the length of packet is optimized to enhance the average throughput. We concluded that packet length can be reduced at low ASNR. However, packet length should be increased at higher values of received ASNR. For each ASNR, we also choose the optimal modulation and coding scheme (MCS) and optimal packet length to maximize the throughput. Different MCSs are investigated such as 4-pulse amplitude modulation (PAM) with and without channel coding, 8-PAM, 16-PAM, and 32-PAM. The proposed method gives 0.8–1.9 dB gain with respect to conventional FSO with adaptive modulation and coding (AMC) and fixed packet length. This is the first paper to deal with packet length optimization for FSO systems.

$4\times100$ -Gb/s PAM-4 FSO Transmission Based on Polarization Modulation and Direct Detection

For the first time, we experimentally demonstrate a 400-Gb/s [ $4\times100$ -Gb/s wavelength-division multiplexing (WDM)] pulse-amplitude-modulation-4 (PAM-4) transmission based on free space optics (FSO) utilizing the polarization modulation (PolM) and direct detection (DD) scheme. Each WDM channel comprises a polarization modulator, a polarizing beam splitter (PBS), and two photodetectors (PDs). Four wavelengths each carrying a 100-Gb/s PAM-4 signal (aggregate 400 Gb/s) are transmitted over the free space. At the receiver side, the PBS separates the two orthogonal polarizations, and then, the two polarized signals are detected by two PDs. Then, the two detected signals are combined by a subtractor, followed by digital signal processing (DSP). The PolM-DD system can achieve higher data rate while maintaining the required forward-error-correction (FEC) threshold compared with intensity modulation and direct detection (IM-DD) scheme. Moreover, owing to the stable polarization state in the free space and the mitigation of the common mode noise by the subtraction, the PolM-DD system provides enhanced tolerance to the amplitude fluctuations attributable to weather turbulence, which makes it more reliable than the conventional IM-DD FSO system. To the best of our knowledge, this is the first experimental demonstration of a 400-Gb/s FSO system using the PolM-DD scheme with the transmission distance of 1.2 m.

Enhancing Performance of Hybrid FSO/Fiber Optic Communication Link Utilizing Multi-Channel Configuration

Abstract Optical wireless is the alternative approach to complete the deployment of optical cable networks in case of failure in implementation. In optical wave the attenuation due to atmospheric turbulence conditions causes a performance degradation of a free space optical (FSO) system. Thus, it will lead to the deterioration of the signal power in the optical cable. To overcome these challenges, multi-channel FSO transmitter and erbium doped fiber in optical link was adopted with high transmission power. In this paper, a hybrid FSO/fiber optic (FSO/FO) link is simulated and investigated under different weather conditions. The proposed hybrid link is evaluated in terms of BER and Q-factor. The proposed system shows a closely performance as compared with the conventional FSO for whole investigation range from 200 m to 2000 m. While the communication distance with the hybrid link is increased by 40 km as compared to conventional FSO. The results show good agreement with Kim model, especially at high input power of 160 mW.

Performance Analysis of Full Duplex Modulating Retro-Reflector Free-Space Optical Communications Over Single and Double Gamma-Gamma Fading Channels

This paper initially presents the channel modeling for full duplex (FDX) modulating retro-reflector (MRR) free-space optical (FSO) systems under weak-to-strong turbulence conditions. The uplink channel in the MRR FSO communication is a conventional FSO channel that is typically modeled by a Gamma–Gamma ( $\Gamma \Gamma $ ) distribution. The downlink channel is jointly affected by the turbulence-induced fading in two opposite passes as well as the reflection effect of the MRR, and its normalized channel fading can be described as the product of two correlated $\Gamma \Gamma $ random variables (RVs). The channel model of the downlink is approximated by a single $\alpha $ - $\mu $ distribution via moment matching method. Then, we propose an effective signal modulation and detection scheme to achieve the FDX communication. The bit error rates of the MRR FSO systems are calculated by the analytical formulas that are derived based on the channel models of the two links, and also evaluated by the Monte-Carlo simulations to validate the derived formulas. The effect of system configurations and propagation conditions on the system performance is investigated, and the optimal designs to make a trade-off between the uplink and downlink performance are discussed.

Optical spatial modulation over Gamma–Gamma turbulence and pointing error induced fading channels

The installations of free space optical (FSO) systems on many structures may account to link availability degradation and poor system performance due to vibration in the transmitted optical beam between the transmitter and the receiver coupled with atmospheric turbulence fading within the channel. In this paper, we study the performance of free space optical spatial modulation system (FSO-SM) under the influence of Gamma–Gamma turbulence channel and misalignment effect. We derived the average bit error rate (ABER) for the system using a power series approach by eliminating the difficulty incur with Meijer-G function adopted in many related researches. The effect of pointing error on the system performance is therefore analyzed under the normalized transmitter beam waist and jitter variance conditions and the numerical results reveal how receive diversity significantly improved the system error rate. It was found that the proposed system outperformed the conventional FSO systems. Furthermore, convolutional coding is employed to enhance the system performance and the study discovered that a very low optical power is required by the system to achieve a minimum error rate.

On-chip continuous-variable quantum entanglement

AbstractEntanglement is an essential feature of quantum theory and the core of the majority of quantum information science and technologies. Quantum computing is one of the most important fruits of quantum entanglement and requires not only a bipartite entangled state but also more complicated multipartite entanglement. In previous experimental works to demonstrate various entanglement-based quantum information processing, light has been extensively used. Experiments utilizing such a complicated state need highly complex optical circuits to propagate optical beams and a high level of spatial interference between different light beams to generate quantum entanglement or to efficiently perform balanced homodyne measurement. Current experiments have been performed in conventional free-space optics with large numbers of optical components and a relatively large-sized optical setup. Therefore, they are limited in stability and scalability. Integrated photonics offer new tools and additional capabilities for manipulating light in quantum information technology. Owing to integrated waveguide circuits, it is possible to stabilize and miniaturize complex optical circuits and achieve high interference of light beams. The integrated circuits have been firstly developed for discrete-variable systems and then applied to continuous-variable systems. In this article, we review the currently developed scheme for generation and verification of continuous-variable quantum entanglement such as Einstein-Podolsky-Rosen beams using a photonic chip where waveguide circuits are integrated. This includes balanced homodyne measurement of a squeezed state of light. As a simple example, we also review an experiment for generating discrete-variable quantum entanglement using integrated waveguide circuits.

Improving Performance of Free Space Optics Link Using Array of Receivers in Terrible Weather Conditions of Plain and Hilly Areas

Free-space optical (FSO) communication is a cost effective and high data rate access technique, which has been proving itself a best alternative to radio frequency technology. FSO link provides high bandwidth solution to the last mile access bottleneck. However, for terrestrial communication systems, the performance of these links is severely degraded from atmospheric loss mainly due to fog, rain and snow. So, a continuous availability of the link is always a concern. This paper investigates the dreadful weather effects such as rain, fog, snow, and other losses on the transmission performance of FSO systems. The technique of using an array of receivers for improving the performance of FSO links is explored in this paper. It involves the deployment of multiple photo detectors at the receiver end to mitigate effects of various weather conditions. The performance of the proposed system is evaluated in terms of bit error rate, received signal power, Q- factor and height of eye diagram. The influence of various weather conditions of plain and hilly areas are taken into consideration and results are compared with conventional FSO links.

Free-Space Optical Communication With Spatial Modulation and Coherent Detection Over H-K Atmospheric Turbulence Channels

The use of optical spatial modulation (OSM), which has been recently emerged as a power and bandwidth efficient pulsed modulation technique for indoor optical wireless communication, is proposed as a simple, low-complexity means of achieving spatial diversity in coherent free space optical (FSO) communication systems. In doing so, this paper makes several novel contributions as follows. It presents a generic analytical framework for obtaining the Average Bit Error Probability (ABEP) of uncoded OSM with coherent detection in the presence of turbulence-induced fading. Although the framework is general enough to accommodate any type of models based on turbulence scattering, the focus in this paper is the H-K distribution. Although this distribution represents a very general scattering model valid over a wide range of atmospheric conditions, it is has not been considered in the past in conjunction with FSO systems possibly because of its mathematical complexity. The proposed analytical framework yields exact performance evaluation results for MIMO systems with two transmit- and an arbitrary number of receive apertures. In addition, tight upper bounds are derived for the error probability for OSM systems with an arbitrary number of transmit apertures as well as for convolutionally encoded signals. The performance of OSM is compared to that of well established coherent FSO schemes, employing spatial diversity at the transmitter or the receiver only. Specifically, it is shown that OSM can offer comparable performance with conventional coherent FSO schemes while outperforming the latter in terms of spectral efficiency and hardware complexity. Various numerical performance evaluation results are also presented and compared with equivalent results obtained by Monte Carlo simulations which verify the accuracy of the derived analytical expressions.

Quantize and Encode Relaying Through FSO and Hybrid FSO/RF Links

A relaying scheme that uses both radio frequency (RF) and free space optical (FSO) links is investigated. In the proposed quantize and encode relaying (QER), a mobile source broadcasts the RF signal using quadrature amplitude modulation (QAM). The relay estimates and quantizes the log-likelihood ratio (LLR) of each received bit in the symbol and transmits the corresponding information to the destination through a high-speed FSO link or through a hybrid FSO/RF link. Relaying of multiple bits through high-speed FSO links improves RF spectrum utilization over systems where both the source and the relay transmit using RF signals. Exploiting the high available unlicensed bandwidth of the FSO links, the relay can even encode the LLRs using a channel code. Multiple encoded bits can then be mapped to FSO or hybrid FSO/RF symbols. The performance of the proposed system is analyzed, and it is shown that optimal symbol mapping can provide improvements of several decibels over conventional FSO transmissions that employ ON and OFF for binary ones and zeros, respectively. The system is found to achieve full diversity in fading FSO and RF channels.

Mode-matching metasurfaces: coherent reconstruction and multiplexing of surface waves.

Metasurfaces are promising two-dimensional metamaterials that are engineered to provide unique properties or functionalities absent in naturally occurring homogeneous surfaces. Here, we report a type of metasurface for tailored reconstruction of surface plasmon waves from light. The design is based on an array of slit antennas arranged in a way that it matches the complex field distribution of the desired surface plasmon wave. The approach is generic so that one can readily create more intricate designs that selectively generate different surface plasmon waves through simple variation of the wavelength or the polarization state of incident light. The ultra-thin metasurface demonstrated in this paper provides a versatile interface between the conventional free-space optics and a two-dimensional platform such as surface plasmonics.

High speed, long reach OFDM-FSO transmission link incorporating OSSB and OTSB schemes

Free Space Optics (FSO) link is extremely responsive to the diverse climate state of affairs that bound the FSO range. A demonstration of fading resistant FSO system using a simulated test-bed employing OFDM scheme is reported in this work to realize the prolonged FSO link with acceptable SNR and BER with the highest stream rate of 5Gbps under the impact of diverse weather conditions. Simulations point toward that the proposed hybrid OFDM-FSO transmission system incorporating OTSB- and OSSB-schemes promises significantly enhanced FSO link compared to conventional FSO systems.

EDFA-Based All-Optical Relaying in Free-Space Optical Systems

Free-space optical (FSO) communication has recently gained a lot of interest for last-mile terrestrial applications. Some of its advantages include high data rates, ease of deployment, license-free operation, and high security. However, the weather-dependent optical wireless channel introduces attenuation and intensity variations known as scintillation which impose severe challenges for reliable data transmission. The distance dependence of both attenuation and scintillation motivates the use of relays as a means of improving the system performance and extending the range of communication. In this paper, we advocate the use of all-optical relays equipped with erbium-doped fiber amplifiers (EDFAs), which, in contrast to conventional FSO relays with electrical amplification, avoid optical-to-electrical and electrical-to-optical conversions. We develop accurate signal and noise models for fixed and variable gain all-optical and electrical relaying which include the effects of all relevant system parameters and types of noise. For performance evaluation, we analyze the outage probability of all-optical relaying in lognormal fading for dual-hop and multi-hop transmission. Our results show that all-optical relays, while simpler from an implementation point of view, outperform electrical relays unless the number of relays is very large. Moreover, for a fixed source-destination distance, performance improves as the number of hops (relays) increases up to a certain point beyond which adding more hops deteriorates performance.

Finite-Element Method Simulations of Guided Wave Phenomena at Terahertz Frequencies

As the science and engineering associated with terahertz time-domain spectroscopy and imaging evolves past the use of conventional free-space optics, the continued development of waveguides for terahertz pulses is increasingly relevant. The ability to model and simulate terahertz wave propagation aids in the development, visualization, and understanding of novel terahertz devices and phenomena. We discuss the use of the finite-element method, a powerful computational tool for the modeling of guided wave phenomena and devices at terahertz frequencies.