Free-space Optical Transmission Research Articles

Proposal and Demonstration of Free-Space Optical Communication Using Photonic Crystal Surface-Emitting Lasers

We propose and demonstrate free-space optical (FSO) communication using photonic crystal surface-emitting lasers (PCSELs). Unlike other types of conventional semiconductor lasers, such as edge-emitting lasers (EELs) and vertical-cavity surface-emitting lasers (VCSELs), PCSELs achieve much larger area single-mode coherent lasing, and this unique feature enables high-power (>watt) and lens-free operations at the same time. To date, these advantages have been recognized to be game changing, especially in light detection and ranging (LiDAR) and laser processing applications. In this work, we show that FSO communication can also benefit from these advantages of PCSELs; more specifically, conventional transmitters that include low-power semiconductor lasers, optical lenses, and fiber-based amplifiers could be replaced with a single PCSEL. Since fiber amplifiers usually consist of bulky components and have low conversion efficiencies, PCSELs can offer more space- and power-saving solutions. Moreover, the narrow beam divergence angles directly obtained from large-area single-mode PCSELs can also eliminate the need for lens systems on the transmitter side. To experimentally verify these potential advantages, we performed FSO transmission experiments based on PCSELs and successfully transmitted 480-MHz and 864-MHz orthogonal frequency division multiplexed (OFDM) signals over 1.1 m using a 500-μm PCSEL in a lens-free transmitter configuration. We believe that PCSELs open new possibilities and choices in FSO communication.

Adaptive Beam Divergence Control to Mitigate Scintillation Effect Caused by Pointing Error in Vertical FSO Transmissions.

Free-space optical (FSO) communication has been studied for next-generation network systems. Because an FSO system establishes point-to-point communication links, maintaining alignment among the transceivers is a critical challenge. In addition, atmospheric turbulence causes significant signal loss in FSO vertical links. Even in clear weather conditions, transmitted optical signals suffer significant scintillation losses due to random variations. Thus, the effect of atmospheric turbulence should be considered in vertical links. In this paper, we analyze the relationship between pointing error and scintillation from the aspect of beam divergence angle. Furthermore, we propose an adaptive beam that optimizes its divergence angle according to the pointing error between the communicating optical transceivers to mitigate the effect of scintillation due to pointing error. We performed a beam divergence angle optimization and compared it with adaptive beamwidth. The proposed technique was demonstrated using simulations, which revealed an enhanced signal-to-noise ratio and the mitigation of the scintillation effect. The proposed technique would be useful in minimizing the scintillation effect in vertical FSO links.

Optical analog-signal transmission system in a dynamic and complex scattering environment using binary encoding with a modified differential method.

High-fidelity optical transmission through dynamic scattering media is challenging, since transmission errors are induced due to dynamic scattering media. In this paper, a new scheme is proposed to realize high-fidelity free-space optical analog-signal transmission in dynamic and complex scattering environments using binary encoding with a modified differential method. Each pixel of an analog signal to be transmitted is first divided into two values, and each of them is encoded into a random matrix. Then, a modified error diffusion algorithm is utilized to transform the random matrix into a 2D binary array. Each pixel of the analog signal to be transmitted is eventually encoded into only two 2D binary arrays, and transmission errors and dynamic scaling factors induced by dynamic and complex scattering media can be temporally corrected. Dynamic smoke and non-line-of-sight (NLOS) are created as a dynamic and complex scattering environment to verify the proposed method. It is experimentally demonstrated that analog signals retrieved at the receiving end are always of high fidelity using the proposed method, when average path loss (APL) is less than 29.0 dB. Only the half number of measurements is used compared to that in conventional methods. The proposed method could open up a novel research perspective for high-fidelity free-space optical analog-signal transmission through dynamic and complex scattering media.

Long-Wave Infrared Discrete Multitone Free-Space Transmission Using a 9.15-μm Quantum Cascade Laser

A free-space optical (FSO) transmission system is experimentally demonstrated in the long-wave infrared (LWIR, 9.15 μm) using a directly modulated quantum cascade laser (DM-QCL) and a commercial mercury-cadmium-telluride infrared photovoltaic detector. At room temperature, the DM-QCL is current-modulated by discrete multitone signals pre-processed with bit-/power-loading. Up to 5.1 Gbit/s data rate is achieved with bit error rate performance below the 6.25% overhead hard-decision forward error correction limit of 4.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , enabled by a frequency domain equalizer. The stability study of the FSO system is also performed at multiple temperature values. This study can provide a valuable reference for future terrestrial and space communications.

Dual Polarization DQPSK ROF System with Duo Binary Precoding and Hybrid Amplification for Effective Hybrid SMF FSO Transmission Medium

This article demonstrates the utilization of a hybrid amplification technique on the transmission part of the proposed hybrid medium transmission system which includes both using Single Mode Fiber (SMF) with 60 km and Free Space Optic (FSO) with a range between 5 km and 25 km. It has included four weather situations with an investigation of the FSO medium-based system performance. The system uses 64 channels based on Dual Polarization-Differential Quadratic Phase Shift Keying (DP-DQPSK) Dense Wavelength Division Multiplexing (DWDM) for the Radio over Fiber (RoF) system using the Duo Binary coding scheme to improve the system efficiency in general and the cross-talk in particular. The investigations will include using the hybrid amplifiers of Erbium Doped Fiber Amplifier (EDFA) and Optical Amplifier (OA) for transmission. The analysis results show that using the hybrid amplification could improve the transmission to reach 25 km for light air case, meanwhile, for light haze, it could reach up to 20 km. Moderate haze could improve transmission efficiency for a distance of 7 km. Finally, for moderate rain situations, the transmission could be further improved to cover a distance of up to 5 km.

Aiming for high-capacity multi-modal free-space optical transmission leveraging complete modal basis sets

Free-space optical (FSO) communication has gained much interest due to its capability of providing not only a cost-efficient solution where the optical fiber is too expensive or too difficult to deploy but also a much higher system capacity than its radio-frequency competitor. In addition to conventional polarization-division multiplexing and coherent detection, combining with mode-division multiplexing can unlock the full potential of FSO communications and achieve ultra-high spectral efficiency. In this paper, we review the recent progress of multi-modal FSO communication systems leveraging complete modal basis sets. Some practical issues for system design are discussed. We also present two latest demonstrations that achieved the record-high single-wavelength data rate using a commercial transponder and a pair of commercial multiplexer/demultiplexer for multi-modal FSO transmission without turbulence and in atmospheric turbulence, respectively, showing the potential of such a technique.

Beyond 5G Fronthaul Based on FSO Using Spread Spectrum Codes and Graphene Modulators.

High data rate coverage, security, and energy efficiency will play a key role in the continued performance scaling of next-generation mobile systems. Dense, small mobile cells based on a novel network architecture are part of the answer. Motivated by the recent mounting interest in free-space optical (FSO) technologies, this paper addresses a novel mobile fronthaul network architecture based on FSO, spread spectrum codes, and graphene modulators for the creation of dense small cells. The network uses an energy-efficient graphene modulator to send data bits to be coded with spread codes for achieving higher security before their transmission to remote units via high-speed FSO transmitters. Analytical results show the new fronthaul mobile network can accommodate up to 32 remote antennas under error-free transmissions with forward error correction. Furthermore, the modulator is optimized to provide maximum efficiency in terms of energy consumption per bit. The optimization procedure is carried out by optimizing both the amount of graphene used on the ring resonator and the modulator's design. The optimized graphene modulator is used in the new fronthaul network and requires as low as 4.6 fJ/bit while enabling high-speed performance up to 42.6 GHz and remarkably using one-quarter of graphene only.

Experimental demonstration of an 8-Gbit/s free-space secure optical communication link using all-optical chaos modulation.

For the first time, to the best of our knowledge, we experimentally demonstrate a high-speed free-space secure optical communication system based on all-optical chaos modulation. The effect of atmospheric turbulence on optical chaos synchronization is experimentally investigated via a hot air convection atmospheric turbulence simulator. It is shown that, even under moderately strong turbulent conditions, high-quality chaos synchronization could be obtained by increasing the transmission power. Moreover, a secure encryption transmission experiment using a high bias current induced chaotic carrier for 8-Gbit/s on-off-keying data over a ∼10-m free-space optical link is successfully demonstrated, with a bit-error rate below the FEC threshold of 3.8 × 10-3. This work favorably shows the feasibility of optical chaotic encryption for the free-space optical transmission system.

Space-Air-Ground FSO Networks for High-Throughput Satellite Communications

The next generation of satellite communication systems aims to achieve terabit-per-second throughput. Because of the limited spectrum available, traditional radio frequency (RF) communication links cannot provide such high throughput. Free-space optical (FSO) transmission is a possible alternative that has recently gained increased attention in the satellite community. However, FSO communications are vulnerable to the severe effects of atmospheric turbulence, such as beam-wandering-induced pointing errors and beam scintillation. To successfully remedy such effects, we propose a space-air-ground (SAG) FSO network with a strategically deployed high-altitude platform acting as a relay. We show that such a design can substantially mitigate the effects of atmospheric turbulence, especially when the satellite zenith angle is relatively high. Then we present a novel SAG satellite communication network that integrates the suggested SAG-FSO transmission and conventional hybrid single-hop FSO/RF transmission to improve overall system reliability and performance even further. The numerical results clearly show the potential of the proposed, highly innovative SAG-FSO network architecture.

High-Speed 9.6-μm Long-Wave Infrared Free-Space Transmission With a Directly-Modulated QCL and a Fully-Passive QCD

Free-space optics (FSO) in the mid-infrared (mid-IR) contains rich spectral resources for future ultrahigh-speed wireless communications yet is currently under-exploited. Two atmospheric transmission windows at the mid-IR, namely, the mid-wave IR (MWIR, 3–5 μm) and the long-wave IR (LWIR, 8–12 μm), show great potential in supporting free-space communications for both terrestrial and space application scenarios. Particularly, the LWIR signal with a longer wavelength has high intrinsic robustness against aerosols’ scattering and turbulence-induced scintillation and beam broadening effects, which are the main concerns hindering the wide deployment of practical FSO systems. In this context, high-bandwidth semiconductor-based mid-IR FSO transceivers will be desirable to meet the requirements of low energy consumption and small footprints for large-volume development and deployment. Quantum cascade devices, including quantum cascade lasers (QCLs) and quantum cascade detectors (QCDs), appear promising candidates to fulfill this role. In this work, we report a high-speed LWIR FSO transmission demonstration with a 9.6-μm directly-modulated (DM)-QCL and a fully passive QCD without any active cooling or bias voltage. Up to 8 Gb/s, 10 Gb/s, and 11 Gb/s signal transmissions are achieved when operating the DM-QCL at 10 °C, 5 °C, and 0 °C, respectively. These results indicate a significant step towards an envisioned fully-connected mid-IR FSO solution empowered by the quantum cascade semiconductor devices.

Average symbol error rate analysis of reconfigurable intelligent surfaces-assisted free-space optical link over log-normal turbulence channels

&lt;span&gt;Optical wireless communication (OWC) has attracted significant interest recently in academia and industry. Free-space optical (FSO) communication systems are where free space acts as a communication channel between transceivers that are line of sight (LOS) for the successful transmission of optical signals. The FSO transmissions through the atmosphere, nevertheless, bring significant challenges, besides the uncertainty of atmospheric channels, especially the signal fading due to the atmospheric turbulence, attenuation and pointing errors caused by the random beam misalignments between transceivers, signal obstruction due to buildings or trees can pre-vent the transmitted message to reach the destination. This study theoretically investigates the average symbol error rate (ASER) of reconfigurable intelligent surfaces (RIS) assisted FSO link over log-normal turbulence channels. The RIS effect is examined by considering the influence of link distance, transmitted optical power, and quadrature amplitude modulation (QAM) scheme on the ASER.&lt;/span&gt;

Hybrid SAG-FSO/SH-FSO/RF Transmission for Next-Generation Satellite Communication Systems

Recent advances demonstrate satellite communication (SatCom) as a potent enabler for future Terabit/s global connectivity. Existing SatCom systems, however, rely mostly on radio frequency (RF) transmission, whose limited available bandwidth is the main bottleneck for further data rate increases. Free-space optical (FSO) communication links, with huge license-free bandwidth, have emerged as an attractive alternative. Despite their ability to deliver high-throughput transmissions, FSO communications are weather-dependent and susceptible to atmospheric turbulence. Hybrid FSO/RF and space-air-ground (SAG) FSO transmissions are possible solutions to increase the reliability of FSO-based transmission systems. A strategically deployed unmanned-aerial vehicle, acting as a relay, can successfully mitigate the adverse effect of atmospheric turbulence, while the hybrid FSO/RF transmission can address weather-dependent effects. These solutions can also be integrated to create a system with significantly improved performance and reliability. To evaluate the performance of the resulting integrated transmission system, we analyze the outage probability and average symbol error probability in this work. Asymptotic expressions are also derived to get further insight into the system behavior and calculate the overall diversity gain. Furthermore, we consider the optimal design of switching thresholds. The numerical results show that the integrated transmission system achieves about 10 dB performance gain over existing solutions for both downlink and uplink scenarios.

Design Methodology for Wireless Backhaul/Fronthaul Using Free Space Optics and Fibers

The recent increase in data rates for Free space optics (FSO) transmission technology, means they could be used for designing the backhaul/fronthaul for 5G and beyond cellular networks. The flexibility and cost-effectiveness provided by FSO are the primary reasons for the mobile operators to investigate the potential of the technology as a mobile backhaul/fronthaul. Unfortunately, the reliability of FSO links is weather dependent, especially if the link covers considerable distance. Optical fibers, on the other hand, are expensive but more reliable. Hence, optimally designing a hybrid network consisting of both fiber and FSO connections can bring in the cost-effectiveness of FSO as well as the robustness of fibers. In such a design, the more important links are connected using fibers while the links with higher tolerance towards failure are designed using FSO. Therefore, in this paper, we propose a hybrid FSO/fiber backhaul/fronthauling methodology for connecting wireless base stations (BSs) to the network core. We first formulate a mixed integer non-linear program (MINLP) for determining the number of splitter/FSO distribution points required in the network that optimally provide connectivity to the BSs. The MINLP is designed to identify the locations of the splitters/FSO distribution points as well. Thereafter, we solve the MINLP with the help of particle swarm optimisation (PSO) and mixed integer linear programming (MILP) techniques. We also derive a heuristic for solving the MILP. Finally, we propose another method for determining the number of splitters required in a relatively shorter time: K-means cluster based method. The results verify that the hybrid network is cost-effective while conforming to the data rate and reliability requirements of the links. The proposal allows evaluation of a seamless design solution of the hybrid network with practical time-complexity.

Variable antennas positions solution to reduce pointing errors due to wind speed and temperature coupled effects during free space optical link using matrix Rician pointing error model

Pointing errors (PE) during free space optical (FSO) transmission can be caused by laser beam wander due to thermal and wind dynamic instability. The aim of this work is to study the coupled effects of temperature and wind speed on PE using matrix Rician pointing error (MRPE) model; then show how variable antennas height can reduce PE due to wind speed and temperature coupled effects. To achieve this purposes, average PE expression was established using MRPE model. Then considering a Gaussian beam wave and Monin–Obukhov similarity functions for the structure parameters of temperature, explicit relationship was established between average PE, temperature and wind speed. It comes out of this study that under dynamic turbulence, one can appropriately modify temperature to reduce PE due to dynamic instability and reciprocally. Depending on turbulence large cells or frozen turbulence eddies distribution, PE can be reduced by appropriately modified antennas height or the distance between transmitter and receiver. That is why this work suggests to install variable or dynamic antennas (rather than fixed ones) which could intelligently modify its positions according to laser beam wander created by atmospheric turbulence.

Performance analysis of MPPM FSO transmission over Gamma–Chi-square strong atmospheric turbulence

In this paper, performance analysis of free space optical (FSO) system operating in conditions of strong atmospheric turbulence over Gamma–Chi-square turbulence model, has been carried out. We have observed reception over multi-pulse pulse-position (MPPM) modulation format for the case of strong atmospheric turbulence conditions modeled with Gamma–Chi-square turbulence model and have compared it with turbulence modeling distributions such are: Gamma–Gamma distribution, K-distribution, negative exponential distribution, log–normal distribution. First, we have provided closed-form analytical expressions for average bit error rate (ABER) at the reception for each observed case and then based on them, we have obtained numerical and Monte Carlo simulation results in order to observe turbulence level impact on system performance.

6G enabling FSO communication system employing integrated PDM-OAM-OCDMA transmission: impact of weather conditions inIndia.

Currently, free space optics (FSO) transmission has gained attention due to its capability to deliver high data rates with a high level of security. In addition, using multiplexing techniques with FSO makes it capable of handling the excessive increase in traffic data and supporting the 5G and 6G wireless network requirements. Accordingly, in this paper, a novel, to the best of our knowledge, FSO system integrating three multiplexing techniques-orbital angular momentum (OAM) (four OAM beams are used) polarization-division multiplexing (two polarization states are used), and optical code division multiple access (four channels assigned with permutation vector codes are used)-is proposed. Additionally, the effects of rainy, foggy, dusty weather and the real weather for four different Indian cities that have different geographical locations are studied and investigated. The system performance is evaluated based on the bit error rate (BER), quality factor (Q-factor), maximum FSO range, and eye diagram opening. The simulation results show successful transmission of 320Gbps overall capacity with a maximum achievable FSO range of 7km under clear weather. On the other hand, the shortest FSO range of 0.105km is achieved when there are heavy dust storms. As for the Indian cities, Srinagar (hilly area of India) achieves the shortest FSO link, which is 4.2km while the largest range of 7km is observed for Chennai city (coastal area of India). All these ranges are evaluated for a log(BER) value <-7. Consequently, this proposed transmission model is suggested for use in 6G applications of FSO communication systems.

Performance analysis of free space optic transmission link for broadband access networks

Abstract Since the last two decades free space optical (FSO) communication transcends the RF communication and microwave systems because of its capability of carrying high bandwidth, license-free, long-range, small size, high bandwidth, low cost and ease of deployment. FSO communication has few limitations as well like beam dispersion, scintillation, weather effects etc. This paper demonstrates the implementation of FSO link and analysis of transmission performance of FSO channel for broadband access networks. The proposed FSO link is simulated in opti-system to analyze the physical characteristics of transmitter and receiver. The transmission performances of FSO link are validated using on/off keying (OOK) and differential phase shift keying (DPSK) at various atmospheric conditions with different data rates and transmission ranges. The transmission performance of the FSO link is analyzed under the influence of key parameters like launch power, beam divergence, atmospheric attenuation, receiver sensitivity and receiver aperture. The proposed model of the FSO link is investigated using key parameters like optical signal to noise ratio (OSNR), noise figure and bit error rate (BER).

On Secure Uplink Transmission in Hybrid RF-FSO Cooperative Satellite-Aerial-Terrestrial Networks

This work investigates the secrecy outage performance of the uplink transmission of a radio-frequency (RF)-free-space optical (FSO) hybrid cooperative satellite-aerial-terrestrial network (SATN). Specifically, in the considered cooperative SATN, a terrestrial source (S) transmits its information to a satellite receiver (D) via the help of a cache-enabled aerial relay (R) terminal with the most popular content caching scheme, while a group of eavesdropping aerial terminals (Eves) trying to overhear the transmitted confidential information. Moreover, RF and FSO transmissions are employed over S-R and R-D links, respectively. Considering the randomness of R, D, and Eves, and employing a stochastic geometry framework, the secrecy outage performance of the cooperative uplink transmission in the considered SATN is investigated and a closed-form analytical expression for the end-to-end secrecy outage probability is derived. Finally, Monte-Carlo simulations are shown to verify the accuracy of our analysis.

Space-Air-Ground FSO Networks for High-Throughput Satellite Communications

The next generation of satellite communication systems aims to achieve terabit-per-second throughput. Because of the limited spectrum available, traditional radio frequency (RF) communication links cannot provide such high throughput. Free-space optical (FSO) transmission is a possible alternative that has recently gained increased attention in the satellite community. However, FSO communications are vulnerable to the severe effects of atmospheric turbulence, such as beam-wandering-induced pointing errors and beam scintillation. To successfully remedy such effects, we propose a space-air-ground (SAG) FSO network with a strategically deployed high-altitude platform acting as a relay. We show that such a design can substantially mitigate the effects of atmospheric turbulence, especially when the satellite zenith angle is relatively high. Then we present a novel SAG satellite communication network that integrates the suggested SAG-FSO transmission and conventional hybrid single-hop FSO/RF transmission to improve overall system reliability and performance even further. The numerical results clearly show the potential of the proposed, highly innovative SAG-FSO network architecture.

Orbital angular momentum state variation of vortex beams propagating in a plasma sheath turbulence

The presence of turbulent media is one of the major problems in the free-space communications. This arises from the sever distortion of spatial modes which leads to data degradation. In recent years, utilizing pseudo-nondiffracting beams has been proposed as an efficient way for mitigating the disturbance induced by turbulence. In this regard, this paper is intended to theoretically investigate anisotropic plasma turbulence (PT) effects on the orbital angular momentum (OAM) spectra and phase evolutions of different kinds of pseudo-nondiffracting vortex beams (VB), including Bessel-Gaussian (BG) and Hypergeometric-Gaussian (HGG) beams. For this purpose, using the modified von Karman spectrum in the frame of Rytov theory, the orbital OAM-spectra of VBs propagating in anisotropic PT are derived, and the effects of anisotropy and source beams parameters on the mode probability of transmitted VBs are studied. Our numerical analysis reveals that when propagating VBs carrying OAM come across PT, under influences of such random medium, their OAM-spectra spread, and several side-band modes which induce phase perturbations are generated. As well as, the propagation performance of pseudo-nondiffracting VBs in an anisotropic PT is better than isotropic one. Furthermore, it is found out that under the equivalent PT conditions BG beams are less influenced by PT in comparison with HGG beams. We expect our results to provide advances on turbulence-resilient communications, free-space optical data transmissions, and imaging.