Showing papers in "IEEE Photonics Journal in 2020"

High-Accuracy Robot Indoor Localization Scheme Based on Robot Operating System Using Visible Light Positioning

Abstract: Visible light positioning (VLP) is widely believed to be a cost-effective answer to the growing demand for service-based indoor positioning. Meanwhile, high accuracy localization is very important for mobile robots in various scenes including industrial, domestic and public transportation workspace. In this paper, an indoor robot VLP localization system based on Robot Operating System (ROS) is presented for the first time, aiming at promoting the application of VLP in mature robotic system. On the basis of our previous researches, we innovatively designed a VLP localization package which contains the basic operation control of the robots, the features extraction and recognition of the LED-ID, cm-level positioning, and robust dynamic tracking algorithms. This package exploited the proposed lightweight algorithm, distributed framework design, the loose coupling characteristics of the ROS, and the message communication methods among different nodes. What's more, an efficient LED-ID detection scheme is proposed to ensure the lightweight and accuracy of the positioning. A prototype system has been implemented on a Turtlebot3 Robot 1 1 Experiment Demonstration is available at: https://kwanwaipang.github.io/Image/ROS.mp4 . . Experimental results show that the proposed system can provide robot indoor positioning accuracy within 1 cm and an average computational time of only 0.08 s.

Effects of Lateral Optical Confinement In GaN VCSELs With Double Dielectric DBRs

Abstract: Two types of GaN vertical-cavity surface-emitting lasers (VCSELs), with and without lateral optical confinement (LOC) structure, were fabricated and their performances were compared. Compared with the VCSEL without LOC, the device with LOC showed a great improvement in threshold current, slope efficiency, output power and differential quantum efficiency, which was mainly due to the reduction of internal loss. Devices with LOC showed clear and multi-transversal mode structures. However, thermal dissipation became worse. The effects of such a design on thermal resistance and transverse modes were discussed.

Multi-Channel Photonic Crystal Fiber Based Surface Plasmon Resonance Sensor for Multi-Analyte Sensing

Abstract: In this paper, we report a unique multi-channel Photonic Crystal Fibre (PCF) sensor based on Surface Plasmon Resonance (SPR) structure comprising of silver and gold doped plasmonic layers for multi-analyte sensing applications. We deployed a Full Vectorial Finite Element Method (FV-FEM) to investigate the sensitivity performance of the proposed PCF sensor. The SPR sensor is fully optimised to ensure propagation features, such as confinement loss, resonance condition, resolution and sensitivity are investigated within various optimised design parameters. According to spectral sensitivity analyses, 2500 nm/RIU and 3083 nm/RIU with 4 × 10−5 RIU and 3.2 × 10−5 RIU resolutions are obtained for Channel 1 (Ch1) (x-polarized) and Channel 2 (Ch2) (y-polarized), respectively.

Performance Analysis of Dual-Hop RF-UWOC Systems

Abstract: In this paper, we analyze the performance of a dual-hop radio frequency-underwater wireless optical communication (RF-UWOC) transmission systems wherein the RF and UWOC links experience Nakagami- $m$ fading and the mixture Exponential-Generalized Gamma fading, respectively. The location of $S$ is uniformly distributed in the space of the hemisphere where the relay is located in the center of the hemisphere. The effect of bubbles level, temperature gradient, water types, and detection techniques are considered. We derive closed-form expressions for outage probability (OP) and average bit error rate (ABER) for both fixed and variable gain relaying schemes with different detection techniques. Furthermore, by utilizing the expansion of Meijer's $G$ -function and Fox's $H$ -function, the closed-form expressions for the asymptotic OP and ABER are derived when the average signal-to-noise ratio of both links tends to infinity. The analytical results are verified by Monte Carlo simulation results. Our results demonstrate that the diversity order of the dual-hop RF-UWOC systems depends on the RF fading parameter and detection technology of the UWOC link.

Angle-of-Arrival (AOA) Visible Light Positioning (VLP) System Using Solar Cells With Third-Order Regression and Ridge Regression Algorithms

Abstract: We put forward and demonstrate a angle-of-arrival (AOA) based visible-light-positioning (VLP) system using quadrant-solar-cell (QSC) and third-order ridge regression machine learning (RRML) to improve the positioning accuracy.

Comprehensive Physical Layer Security Analysis of FSO Communications Over Málaga Channels

Abstract: In this article, we study the physical layer security of free-space optical (FSO) communications under different eavesdropping scenarios. More specifically, the secrecy performance of FSO communication employing intensity modulation/direct detection detection is analyzed for the well-established Malaga channels. Three different realistic scenarios of eavesdropping are considered by assuming different placement locations for the eavesdropper in the paper. Novel expressions for the average secrecy capacity (ASC) and secrecy outage probability (SOP) are derived for the considered scenarios, and useful insights are also provided through asymptotic analysis. The results show: (1) When the eavesdropper is placed near the transmitter, atmospheric condition imposes a less significant impact on secrecy performance; (2) Certain level of correlation can potentially enhance the secrecy performance for FSO communications; (3) The correlation imposes opposite impacts on the ASC and SOP of FSO communications; and the secrecy performance metrics exhibit a non-monotonic impact with the increase of correlation; (5) When the correlation of the FSO links is too small or too large (i.e., the correlation parameter around 0 or 1), the correlation plays a more significant impact on secrecy performance; and (6) The asymptotic slope of the SOP is 0.5 for all eavesdropping scenarios under practical FSO channels.

Gb/s Underwater Wireless Optical Communications Using Series-Connected GaN Micro-LED Arrays

Abstract: High speed wireless communications are highly desirable for many industrial and scientific underwater applications. Acoustic communications suffer from high latency and limited data rates, while Radio Frequency communications are severely limited by attenuation in seawater. Optical communications are a promising alternative, offering high transmission rates (up to Gb/s), while water has relatively low attenuation at visible wavelengths. Here we demonstrate the use of series-connected micro-light-emitting-diode (μLED) arrays consisting of 6 μLED pixels either 60 μm or 80 μm in diameter and operating at 450 nm. These devices increase the output power whilst maintaining relatively high modulation bandwidth. Using orthogonal frequency division multiplexing (OFDM) we demonstrate underwater wireless data transmission at rates of up to 4.92 Gb/s, 3.22 Gb/s and 3.4 Gb/s over 1.5 m, 3 m and 4.5 m, respectively, with corresponding bit error ratios (BERs) of 1.5 × 10−3, 1.1 × 10−3 and 3.1 × 10−3, through clear tap water, and Mb/s rates through >5 attenuation lengths (ALs) in turbid waters.

Demonstration of a 2.34 Gbit/s Real-Time Single Silicon-Substrate Blue LED-Based Underwater VLC System

Abstract: We develop a real-time discrete multi-tone (DMT) transceiver based on field programmable gate array (FPGA) chips for a single chip silicon-substrate light-emitting diode (LED) based underwater visible light communication (UVLC). On-chip resource usages are analyzed and discussed. To improve bit error rate (BER) performance, a novel channel estimation technique utilizing hybrid inter-symbol frequency-averaging (Inter-SFA) and intra-symbol frequency-averaging (Intra-SFA) is proposed and investigated. The real-time DMT transceiver is experimentally verified in a silicon substrate blue LED-based UVLC system with a 1.2 m underwater link. By using the enhanced channel estimation, a gross bit rate of 2.34 Gbit/s real-time DMT signal over 1.2 m underwater transmission can be achieved with the BER of 3.5 × 10−3. What's more, multiple-symbol interleaved Reed-Solomon (RS) codes are employed to further improve BER performance. The real-time measured post-FEC BER of the DMT-UVLC with multiple-symbol interleaved RS (255, 191) codes can be improved by more than six orders of magnitude. As a result, error-free (less than 1 × 10−9) transmission is observed in our real-time experiment. Furthermore, 1.485 Gbit/s 720p high-definition video underwater transmission is successfully demonstrated. To the best of our knowledge, it is the first time to demonstrate a real-time LED-based UVLC system with DMT modulation beyond Gbit/s.

Should Analogue Pre-Equalisers be Avoided in VLC Systems?

Abstract: Visible light communication (VLC) systems are highly constrained by the limited 3-dB bandwidth of light-emitting diodes (LEDs). Analogue pre-equalisers have been proposed to extend the LED's bandwidth at the cost of reduced signal-to-noise ratio (SNR). Compared with the pre-equaliser, the multi-carrier modulation with bit-loading can efficiently use the spectrum beyond the LED's raw 3-dB bandwidth without incuring SNR penalties by employing multiple narrow quasi-flat sub-bands to eliminate the need for equalisation. In this work we show by means of theoretical and experimental investigation that VLC with multi-band carrierless amplitude and phase modulation with bit-loading can outperform VLC with analogue pre-equalisers.

Toward Long-Distance Underwater Wireless Optical Communication Based on A High-Sensitivity Single Photon Avalanche Diode

Abstract: In this study, we built a single photon avalanche diode (SPAD) receiver based underwater wireless optical communication (UWOC) system. The bit error rate (BER) and signal-to-noise ratio (SNR) performance of UWOC with different distances and data transmission rates were obtained. Based on the water attenuation coefficient of 0.12 m−1, a series of neutral density (ND) filters were exploited to attenuate the light output power from the blue laser diode (LD) to simulate the long distance UWOC. The maximum estimated distances of 144 m and 117 m with corresponding BERs of 1.89 × 10−3 and 5.31 × 10−4 at data transmission rates of 500 bps and 2 Mbps were acquired in UWOC system using on-off keying (OOK) modulation scheme, respectively. Furthermore, we compared the differences between free-space and underwater channels, and a divergence angle of ∼1.02 mrad was measured experimentally at a distance of 50 m in the free space. The long UWOC distances obtained in this study partly benefit from high sensitivity SPAD, the small laser divergence angle and low light attenuation. This study provides an approach to achieve long distance UWOC using SPAD.

Dual-Bandwidth Linear Polarization Converter Based on Anisotropic Metasurface

Abstract: In this work, a metasurface with the symmetrical double C-shaped narrow ring connected with the central cross structure is investigated by simulation, theory and experiment, which can near-perfectly convert linearly polarized electromagnetic waves into their orthogonal components in the frequency ranges from 9.38 to 13.36 GHz and 14.84 to 20.36 GHz. And the corresponding fractional bandwidths within the two bands are 35.00% and 31.36%, respectively. The influences of structural parameters on the polarization conversion performance are studied. The results show that the central frequencies and bandwidths of the two bands can be easily modulated by varying the structural parameters of r and θ . The high-efficiency and dual-broadband characteristics can also be well maintained in the oblique incidence range of 0–45°. Meanwhile, the mechanisms of polarization conversion are analysed, and several formulas are used to calculate the reflection coefficients of the co- and cross-polarization under the normal incident y -polarized electromagnetic waves based on the phase difference of the reflection coefficients of the u - and v -polarized conversions. The experiment results are in good agreement with those of simulations and theoretical analysis. The proposed metasurface has important applications in novel polarization control devices.

Ultraviolet-Extended Supercontinuum Generation in Zero-Dispersion Wavelength Decreasing Photonic Crystal Fibers

Abstract: Supercontinuum covering the ultraviolet-blue region is highly useful for fluorescence microscopy. Four zero-dispersion wavelength decreasing photonic crystal fibers with different fiber cross structures and taper profiles are fabricated to extend the short wavelength edge of supercontinuum. Both nanosecond and picosecond pump pulses at 1 μm are used to generate supercontinuum. With a 3 ns pump pulse, the short wavelength edge of supercontinuum is extended to below 400 nm in a fiber with high air-hole ratio (named T3). The underlying mechanism of supercontinuum generation is explored. The short and long wavelength edges of supercontinuum are highly related with the phase-matching condition which decided by the group velocity curve of fiber small core end. With a 10 ps pump pulse, the spectral intensity around ∼800 nm increases in all four fibers. However, the intensity in shorter wavelength band decreased in fibers with a high air-hole ratio (named T3, T4). The experimental results imply that a zero-dispersion wavelength decreasing photonic crystal fiber suitable for nanosecond pulse pumping is not necessarily suitable for picosecond pulse pumping, especially for fibers with high air-hole ratio.

Tunable Anisotropic Perfect Enhancement Absorption in Black Phosphorus-Based Metasurfaces

Abstract: In this work, we theoretically propose an anisotropic metasurface absorber, unit cell of which consists of a continuous monolayer black phosphorus (BP) sheet, sandwiched between a circular silver ring and a dielectric layer stacking on a silver substrate. Numerical results reveal that perfect enhancement absorption can be achieved in both armchair and zigzag directions, in which the resonant absorption peaks occur at different wavelengths due to the anisotropic property of BP. The calculated results by finite-difference time domain (FDTD) simulations have a good agreement with the coupled-mode theory. The physical mechanism of the perfect absorption is attributable to the critical coupling effect. Furthermore, the anisotropic perfect absorber not only exhibits tunable characteristics by varying the electron doping concentration, but also shows a wide-angle tolerance. In addition, our proposed absorber can achieve an arbitrary number of absorption bands by setting a suitable thickness of the dielectric layer. These results may have great potential applications in spectral imaging and photodetectors in the mid-infrared region.

Sand-Dust Image Restoration Based on Reversing the Blue Channel Prior

Abstract: Images acquired under sand-dust weather conditions are severely degraded, with low contrast and severe color shift. The reason is that, due to the influence of sand-dust particles, light is scattered and absorbed, resulting in a blurred image and low contrast; the color shift is caused by the rapid attenuation of blue light. Therefore, to solve the problem of color shift and poor visibility in sand-dust images, this paper proposes a sand-dust image restoration method based on reversing the blue channel prior (RBCP). Under the influence of the blue channel, the dark channel prior (DCP) method will fail. Therefore, the method first reverses the blue channel of the sand-dust image and uses the dark channel prior method, which we call RBCP, and then, RBCP is used to estimate the atmospheric light and transmission map and recover the sand-dust image. The restored image shows significantly improved visibility. When estimating the transmission map, a guiding filter is used to improve the coarse transmission map, and a tolerance mechanism is introduced to modify the transmission map of bright areas in the sky to solve the problem of distortion in the sky. Finally, combined with the gray world, an adaptive color adjustment factor is introduced into the restoration model to remove the color shift. Experimental results via qualitative and quantitative evaluation demonstrate that the proposed method can effectively recover clear sand-dust images and produce results superior to those of other state-of-the-art methods.

A Multipurpose and Highly-Compact Plasmonic Filter Based on Metal-Insulator-Metal Waveguides

Abstract: A multipurpose and ultra-compact nanoplasmonic wavelength filter based on stub structure in a metal-insulator-metal (MIM) waveguide is suggested and numerically investigated. A novel approach of connecting two stepped-like apertures to both input and output ports is applied to form Fabry-Perot (FP) cavities, which enabled the structure to act as a dual band-pass filter at wavelengths 1310 nm and 1550 nm. It is shown that the variation in cavities’ length allows to realize a long-wavelength cutoff filter, and cutoff wavelength can be easily tuned by adjusting the length of the cavities. Furthermore, it is revealed that increasing the gap between the stepped-like apertures and the cavities provides a triple band-pass at telecom wavelengths, e.g., 1267.5 nm, 1414.19 nm, and 1644.7 nm. The tunable broadband high-pass wavelength filter is then achieved while the lengths of stepped-like apertures and stub resonators are set to be identical. Finally, a tunable nearly perfect absorber can be obtained by varying the width of stub resonators. Therefore, because of functionality, size, as well as efficiency the proposed plasmonic filter may greatly contribute to miniaturization of next generation of photonic integrated circuits (PICs), and find applications in on-chip integration and wavelength-division multiplexing (WDM) in optical communication systems.

On the Error Probability of Cognitive RF-FSO Relay Networks Over Rayleigh/EW Fading Channels With Primary-Secondary Interference

Abstract: Free space optical (FSO) communication has emerged to provide line of sight connectivity and higher throughput over unlicensed optical spectrums. Cognitive radio (CR), on the other hand, can utilize the radio frequency (RF) spectrum and allow a secondary user (SU) to share the same spectrum with the primary user (PU) as long as the SU does not impose interference on the PU. Owing to the potential of these emerging technologies, to provide full spectrum efficiency, this paper focuses on the mixed CR RF-FSO transmission scheme, where RF communication is employed at one hop followed by the FSO transmission on the other hop in a dual-hop decode-and-forward (DF) configuration. To quantify the performance of the proposed scheme, closed form error probability is derived over Rayleigh/Exponentiated Weibull (EW) fading distributions by considering the statistical and instantaneous feedback channel of the primary network. We also employed an asymptotic analysis to illustrate the diversity gain of the overall system. We believe that the proposed scheme can be applicable to the 5G+ networks where an unlicensed university student connects to the home computer with the aid of an FSO path.

Demonstration of Non-Hermitian Symmetry (NHS) IFFT/FFT Size Efficient OFDM Non-Orthogonal Multiple Access (NOMA) for Visible Light Communication

Abstract: We propose and experimentally demonstrate a non-orthogonal multiple access (NOMA) visible light communication (VLC) system using non-Hermitian symmetry (NHS) inverse-fast-Fourier-transform (IFFT)/FFT size efficient (SE) orthogonal frequency division multiplexing (OFDM). Fix power allocation and successive interference cancellation (SIC) algorithms are realized. The power ratio between different users, transmission distance, network cell coverage are investigated. The proposed scheme is also compared with the HS based direct-current optical (DCO)-OFDM NOMA-VLC system.

Using Linear Interpolation to Reduce the Training Samples for Regression Based Visible Light Positioning System

Abstract: We put forward and experimentally demonstrate a second order machine-learning (ML) based visible-light-positioning (VLP) system using simple linear interpolation algorithm to reduce the training samples required in the ML algorithm. Algorithms of the second order regression ML model using 2,430 training samples; and using the reduced training samples of 570 with and without the proposed linear interpolation are compared and discussed. We can observe that the positioning accuracy of using training samples of 570 with the proposed interpolation can have similar performance when compared with using 2,430 training samples. The training samples are reduced by ∼76.5%. Here, off-the-shelf LED lamps and low bandwidth electrical and optical components are employed; and the system is cost-effective. Good quality on-off keying (OOK) identifier (ID) signals are retrieved after frequency down-conversion from 20 kHz, 40 kHz and 60 kHz without and with optical background noises respectively.

Enhancing Secrecy Capacity in FSO Links via MISO Systems Through Turbulence-Induced Fading Channels With Misalignment Errors

Abstract: It has recently been proved that the free-space optical (FSO) communication links are susceptible to interceptions. Due to this reason, the optics community shows a special interest in studying these high-speed links in greater detail from a physical layer security (PLS) point of view. Therefore, in this paper, we propose, for the first time, enhancing the average secrecy capacity (ASC) in FSO links via multiple-input/single-output (MISO) systems. It is well-known that the fading effects in FSO channels can be significantly mitigated by exploiting spatial diversity techniques at the transmitter end. Thus, we develop a new asymptotic closed-form solution at high signal-to-noise-ratio (SNR) to accurately compute the ASC for MISO based FSO communication systems with equal gain combining (EGC) reception through generalized misalignment and atmospheric turbulence-induced fading channels. As a key feature, we investigate the impact of the eavesdropper's orientation along with its location in the pointing error model. We can conclude that the influence of the eavesdropper on recollecting radiated power is diminished considerably by increasing not only the normalized beam width at the receiver end, but also by increasing the number of laser sources. Numerical results are tested by exact Monte Carlo simulations.

2.5 kW Narrow Linewidth Linearly Polarized All-Fiber MOPA With Cascaded Phase-Modulation to Suppress SBS Induced Self-Pulsing

Abstract: As a major limitation for power scaling of high power narrow linewidth fiber master oscillator power amplifiers (MOPAs), Stimulated Brillouin Scattering (SBS) induced self-pulsing in polarization maintaining (PM) fiber amplifiers is well characterized and analyzed in this paper by comparing different white noise signal (WNS) phase-modulated modes in experiments. It is found that the self-pulsing effect is not observed in the PM-amplifier with single-frequency laser seed injection, and cascaded WNS modulation provides superior self-pulsing suppression than single WNS modulation with similar output linewidth. Moreover, the experimental results indicate that the self-pulsing threshold can hardly be predicted only by the output linewidth or the defined SBS threshold in a WNS phase modulated fiber amplifier system. As self-pulsing is originated from the spectral spikes in WNS modulated system, we theoretically analyzed characteristics of these spikes in different phase-modulation modes. It indicates the spectral peak intensity can be reduced by cascaded modulation, for which self-pulsing can be suppressed. The theoretical predictions agree well with the experimental results. At the same time, in order to suppress the mode instability effect, a plum blossom shaped bending mode selection device is used in this high-power narrow linewidth fiber amplifier system. Finally, a 32 GHz cascaded WNSs modulated, over than 2.5 kW linearly polarized all-fiber amplifier with a slope efficiency of 86.7% is demonstrated. The polarization extinction ratio (PER) is measured larger than 14 dB and the beam quality factor M2 maintains lower than 1.3 in the power scaling process.

Tunable Electro- and All-Optical Switch Based on Epsilon-Near-Zero Metasurface

Abstract: A novel design of a tunable optical switch based on epsilon-near-zero (ENZ) metasurface is proposed, which can work as an electro-optical or an all-optical switch, and be tuned by gate-voltages, incident angles, and intensity of pump light. The result shows that the coupling of the ENZ mode and plasmon resonance lead to an obvious Rabi splitting which can be observed in the transmission spectrum. Numerical analysis also demonstrates that the coupling belongs to the ultra-strong coupling regime. The proposed design can achieve electro-optical switching with a large modulation depth of up to ∼ 17 dB, all-optical switching with an extinction ratio exceeding 5 dB and an ultrafast response time of 650 fs.

Vertical Second Harmonic Generation in Asymmetric Dielectric Nanoantennas

Abstract: High-permittivity III–V semiconductor nanocavities have shown huge potential for enhanced nonlinear light–matter interactions at the nanoscale. In particular, Second Harmonic (SH) generation in AlGaAs nanoantennas can be extremely efficient; however, vertical emission is difficult to achieve, due to the zincblende χ(2) tensor and epitaxially growth on (100) substrates. Here, we demonstrate that we can shape the second harmonic radiation pattern from a single AlGaAs nanostructure by exploiting a geometrical symmetry breaking optimization approach. The optimized design allows to redirect the SH signal toward the normal direction and to increase the SH power collection efficiency by 2 orders of magnitude in a small numerical aperture of 0.1 with respect to the symmetrical counterpart structure.

Optical Multipath RF Self-Interference Cancellation Based on Phase Modulation for Full-Duplex Communication

Abstract: Optical multipath RF self-interference cancellation (SIC) based on phase modulation for full-duplex communication is proposed and demonstrated experimentally Phase modulation is utilized to convert the RF signal into optical domain, in which the time delay tuning, amplitude tuning and phase inversion for multipath RF SIC are completed The comprehensive theoretical model of the optical multipath RF SIC system is established, and the factors affecting SIC performance including the time delay, amplitude and phase deviations are analyzed The experimental results verify the feasibility of the proposed scheme for full-duplex communication with the cancellation depth of 26 dB and 28 dB over 100 MHz at central frequency of 6 GHz and 10 GHz, respectively A figure of merit of the maximum interference to signal of interest ratio is defined to characterize the SOI recovery capability of optical RF SIC system

Broadband High-Efficiency Electromagnetic Orbital Angular Momentum Beam Generation Based on a Dielectric Metasurface

Abstract: Electromagnetic waves carrying orbital angular momentum (OAM) have motivated a wealth of applications. Achromatic or broadband OAM beams are preferred in many applications and have been extensively explored in previous research endeavors. In view of the low efficiency of the previous reported broadband OAM generators, this work proposes a metasurface composed of high-refractive-index dielectric resonators to achieve broadband OAM beam generation with high efficiency. An efficient method for designing the resonators is presented, which indicates that proper adjustment of the dimensions and rotation of the dielectric resonators can render broadband and high-efficiency full phase control. A broadband high-efficiency OAM generator is engineered based on the resonators from 18 to 28 GHz as a proof-of-concept example and validated by both simulation and experiments. Above 70% polarization conversion efficiency and above 65% operation efficiency are measured throughout the entire design band. Such structure provides an alternative mechanism for realizing broadband high-efficiency microwave, millimeter-wave, terahertz or optical OAM generation and can find potential applications in high-capacity communications and imaging.

AquaE-lite Hybrid-Solar-Cell Receiver-Modality for Energy-Autonomous Terrestrial and Underwater Internet-of-Things

Abstract: Our goal is to develop an energy-autonomous solar cell receiver that can be integrated with a variety of smart devices to implement the Internet of Things in next-generation applications. This paper details efforts to develop such a prototype, called AquaE-lite. Owing to the capability of detecting low-intensity optical signals, 20-m and 30-m long-distance lighting and optical wireless communication with data rates of 1.6 Mbit/s and 1.2 Mbit/s have been achieved on a laboratory testbed, respectively. Moreover, field trials on an outdoor solar cell testbed and in the turbid water of a harbor by the Red Sea have been conducted. Under bright sunlight, energy autonomy and 1.2-Mbit/s optical wireless communication over a transmission distance of 15 m have been implemented, which demonstrated that AquaE-lite with an elaborate receiver circuit has excellent performance in energy harvesting and resistance to background noise. In a more challenging underwater environment, 1.2-Mbit/s signals were successfully received over a transmission distance of 2 m. It indicates that energy-autonomous AquaE-lite with large detection area has promising prospects in future underwater mobile sensor networks to significantly relieve the requirement of pointing, acquisition and tracking while resolving the energy issues.

A Novel Hybrid Secure Method Based on DNA Encoding Encryption and Spiral Scrambling in Chaotic OFDM-PON

Abstract: This paper proposes a novel hybrid secure method based on improved deoxyribonucleic acid (DNA) encoding encryption and spiral scrambling in chaotic OFDM-PON for enhancing the physical-layer security. In the improved DNA encoding encryption, the odd-even cross-bit DNA encoding, base scrambling and base-level substitution are determined by chaotic sequences. For each binary and base, the selected encoding rules and base scrambling methods are dynamically changing, which enhances the robustness against malicious attacks by attackers. In the spiral scrambling process, the QAM symbol matrix is divided into several blocks, and these blocks are scrambled. In the scrambling of the plural matrix, the position where the spiral starts and the orientation and direction of the traversal are also controlled by the chaotic sequences. By employing DNA encoding encryption and spiral scrambling, a key space of $\sim \!{10^{135}}$ can be achieved in the multi-fold encryption of the proposed scheme, which can improve the physical-layer security. The encrypted 16-QAM OFDM data are successfully transmitted over a 60-km SSMF in OFDM-PON. The simulation results demonstrate that it has better BER performance at the BER of ${10^{ - 3}}$ than other schemes. The proposed encryption method can effectively protect data from attacks by eavesdroppers or illegal users.

Digital Holographic Reconstruction Based on Deep Learning Framework With Unpaired Data

Abstract: Convolutional neural network (CNN) has great potentials in holographic reconstruction. Although excellent results can be achieved by using this technique, the number of training and label data must be the same and strict paired relationship is required. Here, we present a new end-to-end learning-based framework to reconstruct noise-free images in absence of any paired training data and prior knowledge of object real distribution. The algorithm uses the cycle consistency loss and generative adversarial network to implement unpaired training method. It is demonstrated by the experiments that high accuracy reconstruction images can be obtained by using unpaired training and label data. Moreover, the unpaired feature of the algorithm makes the system robust to displacement aberration and defocusing effect.

Characterization of Field of View in Visible Light Communication Systems for Intelligent Transportation Systems

Abstract: This paper reports a detailed experimental characterization of non Line-of-Sight (LoS) optical performances of a Visible Light Communication (VLC) system using a real traffic light for ultra-low latency, infrastructure-to-vehicle (I2V) communications for intelligent transportation systems (ITS) protocols Despite the implementation of long-sought ITS protocols poses the crucial need to detail how the features of optical stages influence the overall performances of a VLC system in realistic configurations, such characterization has rarely been addressed at present We carried out an experimental investigation in a realistic configuration where a regular traffic light (TX), enabled for VLC transmission, sends digital information towards a receiving stage (RX), composed by an optical condenser and a dedicated amplified photodiode stage We performed a detailed measurements campaign of VLC performances encompassing a broad set of optical condensers, and for TX-RX distances in the range 3–50 m, in terms of both effective Field of View (EFOV) and Packet Error Rate (PER) The results show several angle-dependent nontrivial behaviors for different lens sets as a function of position on the measurement grid, highlighting critical aspects for ITS applications as well as identifying most suitable optical configurations depending on the specific application and on the required EFOV We also provide a theoretical model for both the signal intensity and the EFOV as a function of several parameters, such as distance, RX orientation and focal length of the specific condenser To our best knowledge, there are no optical and EFOV experimental analyses for VLC systems in ITS applications in literature Our results could be very relevant in the near future to assess a most suited solution in terms of acceptance angle when designing a VLC system for real applications, where angle-dependent misalignment effects play a non-negligible role, and we argue that they could have more general implications with respect to the pristine I2V case mentioned here

A Fast and High-Accuracy Real-Time Visible Light Positioning System Based on Single LED Lamp With a Beacon

Abstract: With the advantages of high positioning accuracy and low cost, visible light positioning (VLP) is becoming a promising solution for practical indoor positioning system. However, most of the VLP systems require at least two VLP LED lamps for accurate position calculation. Therefore, the application of VLP in practical scenarios may be restricted due to this limitation. In this paper, we propose a fast and high-accuracy single-LED based VLP system. Firstly, an unbalanced single-LED VLP algorithm is proposed to increase the positioning accuracy and reduce the computational complexity. Secondly, a fast beacon searching algorithm is proposed to further reduce the processing time for each captured image. Finally, since the proposed algorithms have the advantages of high accuracy and low complexity, the proposed system can also be implemented on a low-end hardware platform. Experimental results show that the average positioning error of the proposed system is decreased to 2.26 cm at the height of 3 m, and the average positioning time is reduced to 6.3 ms on a laptop and 60ms on a low-end embedded platform.

Impulse Response Modeling of Underwater Optical Scattering Channels for Wireless Communication

Abstract: Despite the fact that underwater optical wireless communication (UOWC) systems are able to provide high-data rate links with high security, the performance of these systems presents several limitations related to the maximum achievable distance due to attenuation, and scattering effects. Hence, quantifying the signal attenuation, and the time-dispersion produced by such effects represents a crucial work in channel modeling. Motivated by this, we present, for the first time, a novel, and unified impulse response modeling of underwater optical scattering channels based on the superposition of one impulsive component, and one dispersive component with two degrees of freedom. We provide analytical results for channel path loss, and channel impulse response (CIR) which are validated through Monte-Carlo simulations based on photon-tracing for clear ocean, coastal, and harbor waters. In order to provide a physical insight, the developed CIR is used to compute the root-mean-square (RMS) delay spread as a function of distance, and type of water, as well as to analyze in greater detail the impact of inter-symbol interference (ISI) on the data rate. These outcomes can be used for high-speed systems design, and optimization.