Explore the vast range of titles available.

Orbital angular momentum (OAM), which describes tailoring the spatial physical dimension of light waves into a helical phase structure, has given rise to many applications in optical manipulation, microscopy, imaging, metrology, sensing, quantum science, and optical communications. Light beams carrying OAM feature two distinct characteristics, i.e., inherent orthogonality and unbounded states in principle, which are suitable for capacity scaling of optical communications. In this paper, we give an overview of OAM and beyond in free-space optical communications. The fundamentals of OAM, concept of optical communications using OAM, OAM modulation (OAM modulation based on spatial light modulator, high-speed OAM modulation, spatial array modulation), OAM multiplexing (spectrally efficient, high capacity, long distance), OAM multicasting (adaptive multicasting, -dimensional multicasting), OAM communications in turbulence (adaptive optics, digital signal processing, auto-alignment system), structured light communications beyond OAM (Bessel beams, Airy beams, vector beams), diverse and robust communications using OAM and beyond (multiple scenes, turbulence-resilient communications, intelligent communications) are comprehensively reviewed. The prospects and challenges of optical communications using OAM and beyond are also discussed at the end. In the future, there will be more opportunities in exploiting extensive advanced applications from OAM beams to more general structured light.

Technologies for efficient generation and fast scanning of narrow free-space laser beams find major applications in three-dimensional (3D) imaging and mapping, like Lidar for remote sensing and navigation, and secure free-space optical communications. The ultimate goal for such a system is to reduce its size, weight, and power consumption, so that it can be mounted on, e.g. drones and autonomous cars. Moreover, beam scanning should ideally be done at video frame rates, something that is beyond the capabilities of current opto-mechanical systems. Photonic integrated circuit (PIC) technology holds the promise of achieving low-cost, compact, robust and energy-efficient complex optical systems. PICs integrate, for example, lasers, modulators, detectors, and filters on a single piece of semiconductor, typically silicon or indium phosphide, much like electronic integrated circuits. This technology is maturing fast, driven by high-bandwidth communications applications, and mature fabrication facilities. State-of-the-art commercial PICs integrate hundreds of elements, and the integration of thousands of elements has been shown in the laboratory. Over the last few years, there has been a considerable research effort to integrate beam steering systems on a PIC, and various beam steering demonstrators based on optical phased arrays have been realized. Arrays of up to thousands of coherent emitters, including their phase and amplitude control, have been integrated, and various applications have been explored. In this review paper, I will present an overview of the state of the art of this technology and its opportunities, illustrated by recent breakthroughs.

This study introduces an event camera‐based optical camera communication system utilising pulse‐shaped waveform modulation leveraging the camera's high temporal resolution and sensitivity. The proposed scheme demonstrates reliable performance under controlled indoor conditions with a fixed transmitter–receiver distance, achieving bit error rate values below 3.13 × $\\times$10−3 $^{-3}$at data rates of 100 bit/s with mid‐range receiver bias sensitivity and mid‐high transmitter illumination. Although the experiments tested only four symbols, the flexible design supports higher‐order modulation, which has the potential for increased data rates and more efficient bandwidth utilisation. This study presents an event camera‐based optical camera communication (OCC) system using a novel pulse‐shaped waveform modulation to exploit the camera's high temporal resolution and sensitivity. The system achieves error‐free links under controlled indoor conditions, with a bit error rate (BER) below 3.13 × $\\times$10−3 $^{-3}$at data rates of 100 Hz. Its flexible design supports higher‐order modulation, offering potential for increased data rates and efficient bandwidth utilisation

There is a continuing growth in the demand for data bandwidth, and the multiplexing of multiple independent data streams has the potential to provide the needed data capacity. One technique uses the spatial domain of an electromagnetic (EM) wave, and space division multiplexing (SDM) has become increasingly important for increased transmission capacity and spectral efficiency of a communication system. A subset of SDM is mode division multiplexing (MDM), in which multiple orthogonal beams each on a different mode can be multiplexed. A potential modal basis set to achieve MDM is to use orbital angular momentum (OAM) of EM waves. In such a system, multiple OAM beams each carrying an independent data stream are multiplexed at the transmitter, propagate through a common medium and are demultiplexed at the receiver. As a result, the total capacity and spectral efficiency of the communication system can be multiplied by a factor equal to the number of transmitted OAM modes. Over the past few years, progress has been made in understanding the advantages and limitations of using multiplexed OAM beams for communication systems. In this review paper, we highlight recent advances in the use of OAM multiplexing for high-capacity free-space optical and millimetre-wave communications. We discuss different technical challenges (e.g. atmospheric turbulence and crosstalk) as well as potential techniques to mitigate such degrading effects. This article is part of the themed issue ‘Optical orbital angular momentum’.

Free-space optical (FSO) communication can be subject to various types of distortion and loss as the signal propagates through non-uniform media. In experiment and simulation, we demonstrate that the state of polarization and degree of polarization of light passed though underwater bubbles, causing turbulence, is preserved. Our experimental setup serves as an efficient, low cost alternative approach to long distance atmospheric or underwater testing. We compare our experimental results with those of simulations, in which we model underwater bubbles, and separately, atmospheric turbulence. Our findings suggest potential improvements in polarization based FSO communication schemes.

This research paper presents the development and implementation of a real-time communication system that utilizes LEDs for transmitting text, image, and audio through Free Space Optical Communication (FSOC) and Underwater Wireless Optical Communication (UWOC) channels. The impact of atmospheric and oceanic attenuation and turbulences are considered for the performance analysis of the proposed system. The various atmospheric weather conditions like light fog, dense fog, and heavy rain are considered for FSOC while in the case of UWOC, effects like pure water, and seawater with turbulence and without turbulence are considered. The outcomes of the experiments and simulations, including quality factors, eye diagrams, and bit error rates, are plotted and discussed in the results.

Due to the increasing demand for higher bandwidth in modern communication systems, conventional networks are continuously expanded with new technologies to improve coverage. Free space optical communications (FSOC) shows some significant advantages concerning system setup time in comparison with the classical fiber optical systems on one hand, substantial spectral bandwidth and performances in comparison with the wireless systems under certain conditions on the other hand. This makes this technology not only a reasonable extension for metropolitan area networks but also provides the capability to set up a network after an outage in case of natural disaster quickly. But transmitting data by using FSOC involves some limiting factors that have to be considered prior to each installation. Since the atmospheric channel is not static, the influence of changing weather conditions or industrial smog have a significant impact on the available bitrate. A simulation platform is developed and presented in this paper for investigation of FSOC considering these circumstances. Regarding the atmospheric channel, turbulence, distance-dependent beam divergence, and applied modulation schemes, a general overview of the capabilities is presented and discussed. The insight of this paper should help to make a decision under which preconditions either the FSOC provides a meaningful application possibility, or the limiting factors become too crucial and other technologies must be considered.

Presently employed underwater communication systems suffer from issues such as low data rates and short communication ranges. In this paper, a novel hybrid underwater intelligent communication system is proposed and modeled to address both issues. The proposed system provides higher data rate, longer communication range and secure data transmission by employing optical fiber, underwater free space optics (UFSO) and free space optics (FSO). An intelligent routing mechanism governed by Tabu search (TS) algorithm is used for optical path selection based on data traffic, data rate and communication length to maintain a desired quality of service (QoS) while ensuring security of data transmission. For, modeling, optical fiber channel, priority aware (PA) scheduling algorithm for packet-switched optical network (PSON) for data transmission is used whereas for the FSO channel modeling, orthogonal frequency division multiplexing (OFDM) and On–Off Keying (OOK) are used.To simulate underwater turbulent conditions for UFSO channel, scintillation model with gamma-gamma distribution (GGD) is applied. A data rate of 10 Gbps with BER of 10 –9 has been achieved for communication ranges of 140 and 200 m for the cases of turbulent and non-turbulent water conditions. In each of the cases, QoS meeting IEEE standards is ensured.

The deployment of non-terrestrial networks (NTNs) is envisioned to achieve global coverage for 6G and beyond. In addition to space nodes, aerial NTN nodes such as high-altitude platform stations (HAPSs) and rotary-wing unmanned aerial vehicles (UAVs) could be deployed, based on the intended coverage and operational altitude requirements. NTN nodes have the potential to support both wireless access and backhauling. While the onboard base station provides wireless access for the end users, the backhauling link connects the airborne/space-borne base station to the core network. With its high data transmission capability comparable to fiber optics and its ability to operate in the interference-free optical spectrum, free space optical (FSO) communication is ideally suited to backhauling requirements in NTNs. In this paper, we present a comprehensive tutorial on airborne FSO backhauling. We first delve into the fundamentals of FSO signal transmission and discuss aspects such as geometrical loss, atmospheric attenuation, turbulence-induced fading, and pointing errors, all of which are critical for determining received signal levels and related link budget calculations. Then, we discuss the requirements of airborne backhaul system architectures, based on use cases. While single-layer backhaul systems are sufficient for providing coverage in rural areas, multi-layer designs are typically required to establish connectivity in urban areas, where line of sight (LoS) links are harder to maintain. We review physical layer design principles for FSO-based airborne links, discussing both intensity modulation/direct detection (IM/DD) and coherent modulation/coherent demodulation (CM/CD). Another critical design criteria for airborne backhauling is self-sustainability, which is further discussed in our paper. We conclude the paper by discussing current challenges and future research directions. In this context, we discuss reconfigurable intelligent surfaces (RIS) and spatial division multiplexing (SDM), for improved performance and an extended transmission range. We emphasize the importance of advanced handover techniques and scalability issues for practical implementation. We also highlight the growing role of artificial intelligence/machine learning (AI/ML) and their potential applications in the design and optimization of future FSO-based NTNs.

Free Space Optical (FSO) communication systems are gaining popularity due to its tremendous speed, advanced capacity, cost effectiveness, secure and easy to deploy wireless networks. This technology has proven an effective choice for last mile applications and hard to reach areas where deployment of optical fibre links is not feasible. But FSO link is highly weather dependent and as signal passes through the atmospheric channel, the main impairments are the atmospheric turbulence, which induce fading and deteriorate the system performance. Delhi has a great potential for FSO communication because of its clear skies. Since there is no analysis for weather condition found in Delhi, this work provides analysis of typical Delhi weather condition ranging from heavy to light rain, fog and clear sky. The performance of FSO link is analysed in terms of attenuation and link length margin under different weather condition. The results are concluded to identify which atmospheric condition influences more on FSO link performance.