Free-space Optical Transmission Research Articles

Physically-Secured Ghost Diffraction and Transmission

A new approach to realizing physically-secured ghost diffraction and transmission is proposed in this letter. A series of random 2D arrays of numbers are used as optical information carriers to transmit original data, e.g., analog signals or images as ghosts. Computer-generated magnification factors are applied for optical data encoding, and physically-generated scaling factors are generated with absorptive filters in free-space optical data transmission. The series of computer-generated magnification factors and physically-generated scaling factors serves as security keys, and is explored to realize high-fidelity and high-security free-space optical data (ghost) transmission. It is experimentally demonstrated that the proposed method is feasible and effective in different environments, i.e., without or with scattering media. The proposed physically-secured ghost diffraction scheme offers a new research perspective on secured optical information (e.g., analog signal) transmission in free space.

Improving the spectral efficiency of visible light communications by a super-Nyquist multi-band CAP modulation.

In this Letter, we propose a flexible bandwidth compression scheme for visible light communication (VLC) systems employing multi-band carrierless amplitude and phase (CAP) modulation. The scheme combines a narrow filtering for every subband at the transmitter and an N-symbol look-up-table (LUT) based maximum likelihood sequence estimation (MLSE) at the receiver. The N-symbol LUT is generated by recording pattern-dependent distortions induced by inter-symbol-interference (ISI), inter-band-interference (IBI), and the other channel effects upon the transmitted signal. The idea is experimentally demonstrated on a 1 m free space optical transmission platform. The results show that the proposed scheme can improve the subband overlap tolerance up to 42% in subband overlapping scenarios, that is, 3 bit/s/Hz, which is the highest spectral efficiency (SE) among the experimented schemes.

Optical data transmission through highly dynamic and turbid water using dynamic scaling factors and single-pixel detector.

Free-space optical data transmission through non-static scattering media, e.g., dynamic and turbid water, is challenging. In this paper, we propose a new method to realize high-fidelity and high-robustness free-space optical data transmission through highly dynamic and turbid water using a series of dynamic scaling factors to correct light intensities recorded by a single-pixel bucket detector. A fixed reference pattern is utilized to obtain the series of dynamic scaling factors during optical data transmission in free space. To verify the proposed method, different turbidity levels, different strengths of water-flow-induced turbulence and a laser with different wavelengths are studied in optical experiments. It is demonstrated that the proposed scheme is robust against water-flow-induced turbulence and turbid water, and high-fidelity free-space optical information transmission is realized at wavelengths of 658.0 nm and 520.0 nm. The proposed method could shed light on the development of high-fidelity and high-robustness free-space optical data transmission through highly dynamic and turbid water.

Ergodic Capacity Analysis of Satellite Communication Systems With SAG-FSO/SH-FSO/RF Transmission

Future non-terrestrial networks aim to achieve a throughput of Terabits/s. Therefore, free-space optical (FSO) communications have been adopted as a candidate solution due to their ability to achieve an extremely high data rate. Nonetheless, FSO communications are sensitive to the adverse effects of beam scintillation, beam-wander-induced pointing errors, free-space loss, and weather conditions. Space-air-ground (SAG) FSO transmission and hybrid single-hop (SH) FSO/radio frequency (RF) transmission are promising solutions to improve the performance of FSO links and can be integrated into a satellite communication (Satcom) system. In this work, we carry out a thorough capacity analysis of the resulting integrated SAG-FSO/SH-FSO/RF Satcom system, where Gamma-Gamma and Rician distributions are used to characterize FSO and RF links, respectively. The exact analytical expressions are derived and validated by Monte-Carlo simulations. We also obtain asymptotic expressions for the ergodic capacity in the high signal-to-noise ratio region. The numerical results highlight the significant potential of the integrated SAG-FSO/SH-FSO/RF Satcom system over existing solutions. We also show that the integrated Satcom system with intensity modulation and direct detection can achieve a capacity gain over that with a heterodyne detection technique over all satellite zenith angles.

Optical Intelligent Reflecting Surface for Mixed Dual-Hop FSO and Beamforming-Based RF System in C-RAN

Optical intelligent reflecting surface (IRS) is an emerging and low-cost technology that can establish a stable communication route in free space optical (FSO) transmission environment with obstacles. In this work, optical IRS-aided dual-hop mixed FSO and RF system is first proposed for cloud radio access network (C-RAN). Specifically, polar codes are introduced to combat turbulence and building sway induced fading in FSO link, and transmit beamforming (TBF) technique is designed to achieve optimal data-rate for RF link. Supposing that the IRS-aided FSO link is subject to exponentiated Weibull distribution with geometric and misalignment loss, whereas the RF link experiences the Fisher-Snedecor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathcal {F}$ </tex-math></inline-formula> composite fading, the analytical closed-form outage probability expression is derived in terms of Meijer G and Lauricella multivariate hypergeometric functions with decode-and-forward strategy. Exact closed-form average bit error rate expressions are obtained when the relay of C-RAN adopts single antenna and multiantenna beamforming techniques. On the basis of moment generating function of end-to-end signal-to-noise ratio, the ergodic capacity is obtained in terms of univariate and multivariate Fox H-functions over independent but not identically distributed Fisher-Snedecor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathcal {F}$ </tex-math></inline-formula> fading channels. The correctness of the analytical results is verified through Monte-Carlo simulations. We further provide an asymptotic analysis and discuss the achievable diversity orders.

High-fidelity temporally-corrected transmission through dynamic smoke via pixel-to-plane data encoding.

We propose a new approach for high-fidelity free-space optical data transmission through dynamic smoke using a series of 2D arrays of random numbers as information carriers. Data to be transmitted in dynamic smoke environment is first encoded into a series of 2D arrays of random numbers. Then, the generated 2D arrays of random numbers and the fixed reference pattern are alternately embedded into amplitude-only spatial light modulator, and are illuminated to propagate through dynamic smoke in free space. Real-time optical thickness (OT) is calculated to describe temporal change of the properties of optical wave in dynamic smoke environment, and transmission noise and errors caused by dynamic smoke are temporally suppressed or corrected. Optical experiments are conducted to analyze the proposed method using different experimental parameters in various scenarios. Experimental results fully verify feasibility and effectiveness of the proposed method. It is experimentally demonstrated that irregular analog signals can always be retrieved with high fidelity at the receiving end by using the proposed method, when average optical thickness (AOT) is lower than 2.5. The proposed method also shows high robustness against dynamic smoke with different concentrations. The proposed method could open up an avenue for high-fidelity free-space optical data transmission through dynamic smoke.

Hybrid of Free Space Optics Communication and Sensor System Using IWDM Technique

In this work, we designed a novel hybrid of free-space optics (FSO) communication and intensity and wavelength division multiplexing (IWDM)-fiber Bragg grating (FBG) sensor system over a single shared FSO transmission channel for both optical and sensor signal transmission. In the proposed system, we use one broadband light source and one FSO link for both optical and FBG sensor signal transmission, which significantly reduces the cost of system installation and complexity. Besides, the FSO transmission channel is used to solve the problem of geographical constraints when the environment is difficult to install the fiber. In addition, IWDM is one of the best multiplexing technique to increase the sensor number by more than two times the conventional wavelength division multiplexing (WDM) method. Besides, to solve the overlap spectra problem of IWDM based FBG sensor system, we proposed the stacked gated recurrent unit (SGRU) algorithm with the neural network (NN) profile method. The results prove that the proposed SGRU with the NN profile method improves the accuracy of wavelength detection techniques with the Gaussian profile method. Moreover, the FSO transmission channel achieves a clear eye diagram even the system is a hybrid of optical communication and FBG sensors system.

Experimental demonstration of a “pin-like” low-divergence beam in a 1-Gbit/s OOK FSO link using a limited-size receiver aperture at various propagation distances

In free-space optical (FSO) communications, there are scenarios (e.g., from a ground station to a drone/airplane) in which: (i) the transmitter (Tx) can have a relatively large aperture whereas the receiver (Rx) aperture should preferably be much smaller, and (ii) the distance between the Tx and Rx can vary such that beam divergence will cause a variation in the recovered signal power. In such cases, transmission using a fundamental Gaussian beam can be significantly degraded due to beam truncation caused by a limited-size Rx aperture. Here, we experimentally demonstrate a 1-Gbit/s on-off keying (OOK) FSO transmission link using a structured “pin-like” beam with a limited-size Rx aperture at various distances. The pin-like beam is generated by passing a Gaussian beam through an “Airy-type” phase pattern in the radial direction. When propagating, this structured beam first narrows and then tends to maintain its narrow beam size over a fairly wide range of distances. In comparison to its Gaussian counterpart, our experimental results show that the pin-like beam has ∼13 to 8 dB less power loss at distances ranging from 0.45 m to 0.8 m with an Rx aperture diameter of 1 mm. Moreover, we simulate the propagation of the pin-like beam and show its relatively lower power loss for a wide distance variation in a longer link (e.g., ∼1 km) with a limited-size Rx aperture. Furthermore, our results show that the pin-like beam can have a wider range of low-power-loss distances than a Gaussian beam that is focused to a given distance.

Free Space Optics Transmission Performance Enhancement for Sustaining 5G High Capacity Data Services.

Enhanced bandwidth issues for 5G system are fruitfully resolved by organizing free space optics (FSO) communication frameworks. The high bandwidth, the maximum number of channel transmission requirements, and high data rate have been boosted during the last years because of the COVID-19 pandemic. The online services and digital applications have increased pressure on installed optical network models. In addition, the optical networks with high capacity transmission produce nonlinear distortions, which degrade system efficiency. This paper presents a mixed FSO and fiber network to tackle the factors of nonlinearities and enrich the system capacity and range. Furthermore, the issues related to radio frequency, FSO pointing, and co-channel interference are considered in this work. The theoretical and simulation structures are validated using advanced measuring parameters, such as bit error rate (BER), peak to average power ratio (PAPR), cumulative distribution function (CDF), and outage probability. The nonlinear factors are addressed successfully, and the capacity is developed from current models. Finally, the proposed model’s limitations and future direction are discussed in this paper.

Characterization of fog-induced attenuation for optimizing optical propagation links in Nigeria

Fog-Induced Attenuation (FIA) is crucial in assessing the performance of Free-Space Optics (FSO) systems for optimal results. The goal of this work is to characterize FIA seasonally and yearly at the selected optical wavelengths over selected locations in Nigeria. FIA has been analyzed using five (5) years (2012–2016) visibility measurements data for Abuja, Ikeja, Jos, Port Harcourt, Ilorin, Makurdi, and Sokoto locations in Nigeria. The visibility re-analysis data were obtained from achieves of the European Center for Medium-Range Weather Forecasts (ECMWF).over the selected locations. The characterization has been based on three different frequencies and their respective optical wavelengths (850 nm/352 THz, 1300 nm/230 THz, and 1550 nm/193 THz). The sensitivity of the FIA to cloud cover rate (information on visibility range) and the comprehensive cumulative distribution rate at the selected frequencies were also examined. The results show that a minimal FIA of about 0.4 dB/km can be achieved at 1550 nm/193 THz for optimal link planning and design of FSO communication links in Nigeria. However, attenuation of 0.25 dB/km can be achieved at link availability of 99.99 % for FSO transmission in the commercial city of Lagos (Ikeja) at the same 1550 nm/193 THz under bad weather conditions. However, due to additional dust in the region, FIA can extend up to 0.6 dB/km in Sokoto (arid region) under the same conditions. The variation pattern in the arid region and the associated ones will require a high-power laser with special mitigation techniques to improve the chances of link availability for optimal performance.

Estimation and performance analysis of multiple incident beam misalignment in spatial diversity based FSO transmissions

A novel approximated misalignment model of an alignment error with multiple incident beams in the position detection of the pointing, acquisition, and tracking (PAT) system is developed for spatial diversity based free-space optical (FSO) communication system. A centroid algorithm used in position detection of the incident beam to align transceivers for the PAT system can make additional pointing errors when multiple beams are incident on the position detector. The centroid algorithm measures the alignment errors through the ratio of the segmented received powers from the position detector. Therefore, the distant arrangement of transmitters and the atmospheric fading affect the alignment error, and the pointing loss of FSO communication increases. We propose a statistical misalignment model with multiple incident beams and analyze the transmission performance using this model to achieve power-efficient FSO communication. The optimum beam width with the misalignment model can reduce the transmit power required to satisfy system demands and outage probability with redundant power for reliable FSO communication.

Direct generation of 2D arrays of random numbers for high-fidelity optical ghost diffraction and information transmission through scattering media

In this paper, we propose a new approach for high-fidelity optical ghost diffraction and information transmission through scattering media in free space using a series of 2D arrays of random numbers as information carriers. A series of 2D arrays of random numbers are directly generated to encode the ghosts, i.e., original signals or images. The generated 2D arrays of random numbers are sequentially embedded into amplitude-only spatial light modulator (SLM), and are illuminated to propagate through scattering media in free space. Experimental results show feasibility and effectiveness of the proposed method, and different types of ghosts (e.g., analog signals and grayscale images) can be encoded and retrieved with high fidelity. In addition, comparisons using different algorithm parameters and different experimental parameters (e.g., different diffraction distances and wavelengths) are conducted to analyze and illustrate performance of the proposed method. It is found that the proposed method possesses high robustness against scattering media, and can realize high-fidelity ghost diffraction and optical information transmission. The proposed high-fidelity optical ghost diffraction could open up an avenue for many applications, e.g., free-space optical transmission through scattering media. @ 2022.

Independent dual-single-sideband QPSK signal detection based on a single photodetector.

The two sidebands of the independent dual-single-sideband (dual-SSB) signal can carry different information to achieve higher spectral efficiency. However, the two sidebands of the independent dual-SSB vector signal are received independently. Generally, the receiver divides the signal into two channels. For each channel, we use an optical bandpass filter (OBPF) to select the left sideband (LSB) or right sideband (RSB), respectively. Then a photodetector (PD) is used for photoelectric conversion, followed by subsequent digital signal processing (DSP). To reduce the complexity and cost of the receiver, we propose a new independent dual-SSB vector signal detection scheme based on a single PD combined with conventional DSP. An electric bandpass filter (EBPF) filters out high-frequency components after photoelectric conversion, and then the signal is quadrature demodulated and processed by the DSP algorithm. The LSB and RSB are quadrature phase-shift keying (QPSK) modulated with an initial phase difference of π/4. Simulation results show that the proposed scheme performs better bit error rate (BER). For back-to-back (BTB) transmission, the BER of 2-Gbaud independent dual-SSB vector signal (1-Gbuad RSB and 1-Gbaud LSB) can reach the hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10-3 when the input optical power into PD is -20 dBm. For 1-km and 2-km weak turbulence free-space optical (FSO) channel transmission, the BER of 2-Gbaud independent dual-SSB vector signal can reach the HD-FEC threshold when the input optical power into PD is -18.8 and -17 dBm, respectively. For 1-km weak turbulence FSO channel transmission, the BER of 4-, 8-, and 16-Gbuad independent dual-SSB vector signal can reach the HD-FEC threshold when the input optical power into PD is -17.8, -16, and -15 dBm, respectively.

Performance Evaluation and Security Analysis of UAV-Based FSO/CV-QKD System Employing DP-QPSK/CD

Unmanned aerial vehicles (UAV) based on free-space optical (FSO) systems combine the benefits of both the high data rate of the FSO and the mobility of UAV. However, cumulative effects of laser beam divergence and turbulence-induced fading on the received irradiance in the FSO link might allow an external eavesdropper located near the authorized receiver to break the transmission under certain conditions. Quantum key distribution (QKD) is a technique of secure communication that employs quantum mechanics-based cryptography principles. A combination of a continuous-variable quantum key distribution (CV-QKD) and Gaussian modulation of quantum coherent states (GMCS) enables a relatively secure communication system against collective attacks over FSO transmission links. However, the performance is restricted and degraded due to the influence of atmospheric turbulence and excess noise. This paper aims to evaluate the performance of a secured UAV-based FSO system utilizing prepare and measure CV-QKD protocol based on GMCS under collective attack in terms of quantum bit error rate (QBER), outage probability, and secret key rate (SKR) considering the impairments of the FSO channel, link budget, deviations in the position and attitude angles of the UAV, channel fluctuating transmittance, and excess noise. The system parameters, including transmit power, receiver’s field-of-view, and beam divergence angle, are optimized to satisfy the system’s design criteria of threshold-QBER <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$=10^{-3}$</tex-math></inline-formula> and threshold outage probability <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$=10^{-2}$</tex-math></inline-formula> . The proposed system tolerates boresight displacement up to 7 cm with the optimized values. The Closed-form expressions and analytical results are confirmed by Monte-Carlo simulations.

Demonstration of 10-channel mode- and polarization-division multiplexed free-space optical transmission with successive interference cancellation DSP.

We experimentally demonstrate 10-channel mode-division multiplexed free-space optical transmission with five spatial modes, each carrying 19.6925-Gbaud dual-polarization quadrature phase shift keying signals. Strong inter-mode cross talk is observed in our commercially available photonic lantern based system when using a complete orthogonal mode set as independent channels. A successive interference cancellation based multiple-input multiple-output digital signal processing (DSP) algorithm is first applied to mitigate the inter-mode cross talk in mode-division multiplexed systems. The DSP also supports unequal transmit and receive channel numbers to further improve the cross talk resiliency. Compared to the conventional minimum mean square error DSP, the required optical signal-to-noise ratio of the successive interference cancellation DSP is decreased by approximately 5 dB at the hard-decision forward error correction limit. As a result, this system demonstrates a record-high independent channel number of 10 and spectral efficiency of 13.7 b/s/Hz in mode-division multiplexed free-space optical systems.

Coherent Free-Space Optical Communications: Opportunities and Challenges

The ever-increasing data rate demand for wireless systems is pushing the physical limits of standalone radio-frequency communications, thus fostering the blooming of novel high-capacity optical wireless solutions. This imminent penetration of optical communication technologies into the wireless domain opens up a set of novel opportunities for the development of a new generation of wireless systems providing unprecedented capacity. Unlocking the full potential of free-space optics (FSO) transmission can only be achieved through a seamless convergence between the optical fiber and optical wireless domains. This will allow taking advantage of the staggering progress that has been made on fiber-based communications during the last decades, namely leveraging on the latest generation of Terabit-capable coherent optical transceivers. On the other hand, the development of these high-capacity optical wireless systems still faces a set of critical challenges, namely regarding the impact of atmospheric turbulence and pointing errors. In this work, we provide an in-depth experimental analysis of the main potentialities and criticalities associated with the development of ultra-high-capacity FSO communications, ultimately leading to the long-term (48-hours) demonstration of a coherent FSO transmission system delivering more than 800 Gbps over <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 42 m link length, in an outdoor deployment exposed to time-varying turbulence and meteorological conditions.

Colloidal Quantum Dot Light Emitting Diodes at Telecom Wavelength with 18% Quantum Efficiency and Over 1MHz Bandwidth.

Developing high performance, low‐cost solid‐state light emitters in the telecom wavelength bandwidth is of paramount importance for infrared light‐based communications. Colloidal quantum dot (CQD) based light emitting diodes (LEDs) have shown tremendous advances in recent times through improvement in synthesis chemistry, surface property, and device structures. Despite the tremendous advancements of CQD based LEDs in the visible range with efficiency reaching theoretical limits, their short‐wave infrared (SWIR) counterparts mainly based on lead chalcogenide CQDs, have shown lower performance (≈8%). Here the authors report on highly efficient SWIR CQD LEDs with a recorded EQE of 11.8% enabled by the use of a binary CQD matrix comprising QD populations of different bandgaps at the emission wavelength of 1550 nm. By further optimizing the optical out‐coupling via the use of a hemispherical lens to reduce optical waveguide loss, the EQE of the LED increased to 18.6%. The CQD LED has an electrical bandwidth of 2 MHz, which motivated them to demonstrate its use in the first SWIR free‐space optical transmission link based entirely on CQD technology (photodetector and light emitter) opening a new window of applications for CQD optoelectronics.

120 Gbps SAC-OCDMA-OAM-based FSO transmission system: Performance evaluation under different weather conditions

A novel 120 Gbps free space optics (FSO) transmission scheme is introduced by combining orbital angular momentum (OAM) multiplexed signals with spectral amplitude coded (SAC)-optical code division multiplexing access (OCDMA) technique. Four OAM beams, each carrying three independent channels with 10 Gbps data rate, are used for increasing the capacity of the FSO system to 120 Gbps. Enhanced double weight (EDW) codes are employed for the SAC-OCDMA system. The proposed system is simulated and its performance is compared for the twelve channels under different weather conditions including clear air (CA), varying levels of rain, haze and fog conditions. The obtained results reveal that longer propagation distances between transmitter and receiver are possible with a bit error rate (BER) of ∼ 10-5. The possible distances are, respectively, 300 m, 160 m, 200 m, and 150 m, under CA, heavy rain (HR), heavy haze (HH), and heavy fog (HF), with a system capacity of 120 Gbps.

Structured Light Transmission under Free Space Jamming: An Enhanced Mode Identification and Signal-to-Jamming Ratio Estimation Using Machine Learning

In this paper, we develop new classification and estimation algorithms in the context of free space optics (FSO) transmission. Firstly, a new classification algorithm is proposed to address efficiently the problem of identifying structured light modes under jamming effect. The proposed method exploits support vector machine (SVM) and the histogram of oriented gradients algorithm for the classification task within a specific range of signal-to-jamming ratio (SJR). The SVM model is trained and tested using experimental data generated using different modes of the structured light beam, including the 8-ary Laguerre Gaussian (LG), 8-ary superposition-LG, and 16-ary Hermite Gaussian (HG) formats. Secondly, a new algorithm is proposed using neural networks for the sake of predicting the value of SJR with promising results within the investigated range of values between −5 dB and 3 dB.

Free-Space Terabit Optical Interconnects

The continuous growth of Internet data traffic is progressively increasing the pressure over wireless access technologies. To overcome the imminent bandwidth bottleneck, free-space optics (FSO) is currently deemed as a key breakthrough towards next-generation ultra-high-capacity wireless links. As this technology matures, it is starting to find its place not only in future mobile access networks, but also in applications such as datacenter interconnections, satellite communications and high-frequency trading. To meet the ever-increasing demand for high-capacity links, future-proof FSO systems should seek maximum compatibility with state-of-the-art optical fiber systems, which nowadays can already provide data rates per channel in the Terabit/s range. However, the ultra-wideband potential of FSO systems comes at the expense of very tight requirements in terms of optical beam alignment, so that reliable end-to-end communications can be ensured. Following this research challenge, in this work we demonstrate a dynamic single-wavelength 1 Tbps FSO link with high pointing error tolerance, boosted by adaptive probabilistic constellation shaping (PCS) and an active gimbal-based acquisition, tracking and pointing (ATP) mechanism. Our results demonstrate successful indoor FSO transmission over 3 m with enhanced resilience towards pointing errors, enabling to adapt the data rate in the range of 0.8–1 Tbps.