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This paper presents a novel concept for joint light source identification and localization (JLIL) with subsequent interference suppression using a liquid crystal display (LCD)-based receiver. The JLIL concept is particularly suitable for multiple-input single-output visible-light-communication settings, where an LCD-based receiver must be able to identify a desired light source before suppressing interfering ones. Given a basic visible-light-communication setup, in a first step modifications required both on the transmitter and the receiver side are identified. Subsequently, the concept for LCD-based JLIL is introduced, and its performance is illustrated by means of simulation results. In this context, intersymbol interference effects are investigated and a known ambiguity problem is overcome. Finally, results of an experimental verification are reported as a proof of concept. It is shown that the derived simulation model accurately predicts measurement results. The latter confirm a virtually error-free light source identification and precise localization within system accuracy range. Furthermore, an improved peak detection is reported. Signal-to-noise ratio measurements suggest good performance for up to $3 \,\mathrm{m}$ using the developed hardware demonstrator.

Reservoir computing (RC) is a bio-inspired framework suited for temporal data processing. Here we propose an all optical coherent RC for the equalization of the impairments of fiber optic (FO) system based on coherent detection. We compare the performances of the RC systems based on semiconductor saturable absorber mirror (SESAM) and highly nonlinear waveguide (HNLW). For a dispersion managed long haul FO system in which the nonlinear penalty is dominant, we find that the SESAM outperforms the HNLW due to its larger nonlinear memory. In contrast, for the equalization of dispersive impairments in a short haul FO system such as data center networks, we find that the HNLW has a much superior performance due to its larger linear memory.

Visible Light Communication (VLC) is one of the emerging technologies of Optical Wireless Communications (OWCs) in finding sustainable solutions for the spectrum crunch of beyond 5G access technologies. This research work introduces the NeoCommLight system, a healthcare communication architecture for Neonatal Intensive Care Units (NICU) based on visible light communication, which offers a promising solution for RF-restricted areas and addresses the challenges posed by spectrum crunch and health concerns associated with traditional radio communication technologies. To demonstrate the feasibility and practicality of the proposed system, a prototype design is presented, accompanied by implementation details. Furthermore, the performance analysis of the NeoCommLight system is conducted, shedding light on crucial aspects such as communication delay, transmitter-to-receiver distance, variations in transmitter angle concerning the Line of Sight (LoS) axis, and the impact of diffraction caused by a knife edge obstacle placed between the transmitter and receiver. The performance analysis of the system showed that it could transmit data at a maximum data rate of 3 Mbps at a distance of 5 cm. The system could transmit data at a data rate 800 Kbps to a maximum distance of 2 meters.

In future mobility, robust obstacle detection is imperative for autonomous vehicles, fostering a reliance on vision-based data and the emergence of light detection and ranging (LiDAR) as a pivotal sensor. Despite challenges of cost and novelty, LiDAR’s unique capabilities find favor among automakers, except Tesla. Offering 3D imaging with superior resolution than radar, LiDAR holds potential significance as a perception sensor for safe and efficient transportation. Its 3D distance measurement using laser pulses and waveform analysis is vital for obstacle detection in autonomous vehicles. Techniques like scanning and flash LiDAR optimize point cloud acquisition, albeit with challenges in balancing resolution and frame rates. Because LiDAR measures the distance to an object using the time-of-flight, the idle listening time required to detect reflected waves increases in proportion to the maximum distance. Both, lateral angular resolution and frame rates are dependent on this idle listening time. Lateral angular resolution impacts LiDAR’s ability to detect objects. This study proposes an optical orthogonal frequency-division multiple access (OFDMA) LiDAR with a coded pulse technique to enhance the detection accuracy of objects. Simulation results demonstrate that the proposed OFDMA LiDAR overcoming false positives and mutual interference through pulse stream LiDARs and strategic approaches is crucial in LiDAR’s integration into future mobility systems.

A novel method of optical Nyquist pulse generation based on recirculating frequency shift loop (RFSL) is proposed and experimentally demonstrated. In this article, a large number of flat phase-locked comb lines output from RFSL are used to synthesize Nyquist pulse, which has ultra-narrow and flexible tunable pulse width. It is the first time to theoretically analyze the condition of Nyquist pulse generation by RFSL in detail. In the experiment, the number of flat spectral lines is continuously tuned in the range of 10 to 40 corresponding to the pulse width of 8.9 ps to 2.4 ps. Meanwhile, Nyquist pulses with tunable repetition rate from 7.5 GHz to 15 GHz are obtained experimentally.

A doppler delay-time frequency (DD-TF) cross-domain high-security transmission scheme based on orthogonal time frequency space (OTFS) is proposed, which is compatible with high-speed and large-capacity optical transmission systems. In this scheme, 36 sets of chaotic sequences are mapped out through a two-dimensional chaotic system with variable control parameters and variable initial values for optical physical layer encryption. The data modulated based on OTFS is encrypted by multiple sets of chaotic sequences, achieving cross-domain joint encryption in the DD domains and TF domains. This method improves security performance of the optical transmission system. An experiment is conducted, in which the encrypted 16- quadrature amplitude modulation (QAM) OTFS signals over 25 km standard single-mode fiber (SSMF) is achieved, and the key space is 101140 to effectively prevent malicious attacks from illegal Optical Network Units (ONUs). This key space is the reported maximum key space of the physical layer of two-dimensional chaotic systems. Compared with orthogonal frequency division multiplexing (OFDM) encryption scheme, this scheme obtains better transmission performance. It is shown that OTFS system with cyclic prefix (CP) of 1/16 obtains 2.1 dB gain than the OTFS system with CP of 1/32 in terms of receiver sensitivity when the bit error rate (BER) is 10-3. Due to its comprehensive advantages in error rate and security performance, this chaotic encryption based on OTFS has a high application prospect in low-cost and reliable optical access system.

We proposed a concurrent transmitting-based 128 channels LiDAR using an bipolar optical code scheme called code-inversion keying based prime-permuted code, which is the most suitable method to shorten the chip sequence length, while maintaining the performance characteristics of LiDAR. In each channel, four laser diodes with different wavelengths are bundled with a coupler to direct toward a target direction. The target direction could be recognized after the reception by identifying the received reflected pulse, even if several target directions were measured simultaneously. Due to the very short chip sequence, only a very short signal transmission time is required.

An operation scheme using electrical peaking and optical peaking to engineer the modulation band of a Si microring modulator is presented. By incorporating an inductor design at the metal traces of a Si microring modulator, the driving signal can be magnified near the peaking frequency. Although adjusting the wavelength detuning of a ring modulator also introduces optical peaking to extend the 3-dB roll-off frequency, using inductive peaking has no detrimental effect on the low-frequency response. By exploiting both effects, the modulation band can be tailored with more degrees of freedom. We accomplish a Si microring modulator design with a wide and flat transmission band over 95 GHz, which is potentially applied for a non-return-to-zero (NRZ) data transmission over 120 Gbit/s without extra signal post-compensation.

Optical wireless data transmission from an autonomous underwater vehicle (AUV) to a sea-surface buoy has become a viable solution to support the increasing needs for many applications in the field of marine technology. Despite having unique characteristics and advantages, underwater wireless optical communication remains a challenging field due to the difficulties associated with the harsh marine environments and severe attenuation caused by the bubbles and particulates. This study aims to evaluate the uplink UWOC system in the presence of particles and bubbles under various wind speeds. To calculate the link distances and signal loss between a transmitter and a receiver of the UWOC system, simulations were performed with the inputs of the water optical properties obtained from the Bay of Bengal and Southern Ocean waters and bubble properties derived from the Hall-Novarini (HN) model. Consequently, the normalized received power were calculated for the different receiver configurations and oceanic conditions without and with bubbles. Our results showed that the received power decreased with the increasing wind speeds and smaller bubble populations. The angular and spatial distributions of the received beam increased for smaller bubble populations under high wind speeds. When compared to the clean bubbles, the received power slightly decreased for the non-absorbing coated bubbles with different film thicknesses (0.01 ∼ 1.0 μm) and increased for the absorbing coated bubbles with higher thicknesses (≥1 μm). These results will significantly help the system designer to develop and optimize an UWOC system under different bubble populations and wind speed conditions.

This article illustrates the controlling mechanism along with data transmission using optical camera communication in both line-of-sight and non-line-of sight (NLOS) environment. The transmitter of the system is mainly composite of microcontroller (Arduino Mega), DHT sensor, two LEDs, and four push button switches. Here, two LEDs are continuously transmitting the control and data signal. The control signal is operated through push button switches. We have used four different control signals that are transmitted from the LEDs and data transmission by collecting data from the surrounding through the temperature and humidity sensor. The entire transmitter operations are processed and controlled through arduino programming in a microcontroller. The COOK modulation technique is used for this system. For data reception, we have used laptop's camera that is controlled through five different exposure times and operated based on the rolling shutter effect of the camera. A logistic regression algorithm is applied to recognize the data transmitting LED from the normalized intensity of the stripe patterns. The receiver is capable to collect data under certain conditions, if the light source is obstructed by an object through deep neural network. Again, Generative Adversial Network (GAN) is used as a decoder to reduce the BER during moving scenario and NLOS conditions. Different activation functions are used in the GAN model to find the optimum solution for the system. However, we have archived the maximum 2.5 m communication distance with 10-4 BER. The receiver program is designed using Python 3.8.

In this work, we employ two Mach-Zehnder modulators (MZM) to achieve a novel six kinds of reconfigurable optical logic gate in millimeter wave over fiber, in which one MZM is used to generate millimeter wave (mm-wave) signal and the other is used to attain optical logic gate. By adjusting the direct current (DC) bias voltage of the first MZM, a frequency doubling mm-wave signal can be obtained and no filter is needed. By controlling the radio frequency (RF) drive voltage and DC bias voltage of the second MZM, the phase of between upper and lower arms can be converted. By adjusting the phase of the first channel RF signal to $(\pi /2,0)$ , setting the phase of the second channel RF signal to $(0,\pi /2)$ and controlling the bias voltage to let the phase of the upper and lower to be $\pi /2$ and $\pi /4$ , respectively, OR logic gate can be obtained. To obtain AND, NOR, NAND, XOR and NXOR logic gate, we just need to get proper phase by adjusting the RF drive voltage and DC bias voltage of the second MZM. According to the simulation, the millimeter wave signal with a frequency of 50 GHz can be obtained, which has completed the logic operation.

We investigate the spectral and amplification characteristics of Bi/Er co-doped fiber (BEDF) and Er-doped fiber (EDF), prepared by atomic layer deposition (ALD). BEDF features a noise figure (NF) below 5.2 dB, the bandwidth with NF less than 4 dB is approximately 23 nm (1560–1583 nm), and that with a gain exceeding 25 dB is approximately 42 nm (1525–1567 nm). The quantum conversion efficiency (QCE) of BEDF at 1550 nm is 72.1% at a pump power of 300 mW. The saturated output power of BEDF at 1550 nm is 21.4 dBm. Raman spectra indicate that the maximum phonon energy of BEDF is lower than that of EDF. The fluorescence lifetime of the BEDF is 11.64 ms, which is longer than that of the EDF. These results show that co-doping with Bi ions could increase the fluorescence lifetime of Er3+, reduce the NF, and improve the QCE and saturated output power. Therefore, this type of BEDF is a promising gain medium for optical amplifiers and lasers.

In the next generation (6G) wireless networks, the integration of terrestrial and non-terrestrial networks is essential to provide flawless connectivity over the globe. The vital element of this architecture is the high altitude platform station (HAPS) systems, which can provide reliable and ubiquitous connectivity among satellites, unmanned aerial vehicles (UAVs), and terrestrial users. Motivated by the importance of HAPS systems, in this paper, we provide three different use-cases for HAPS eavesdropping scenarios and investigate their physical layer security (PLS) performances. To quantify the PLS performance of the proposed setups, we perform secrecy outage probability (SOP), probability of positive secrecy (PPSC), average secrecy capacity (ASC), and secrecy throughput (ST) analyses. Furthermore, we also provide important design guidelines that can be beneficial for secure HAPS systems.

A free-space communication scheme is proposed based on orbital angular momentum (OAM) states and pulse position modulation (PPM). It can not only improve the channel capacity, but also be suitable for single-photon detection to realize the communication in the case of ultra-weak light. A parallel coding method is proposed to realize high-capacity and low bit error rate (BER) communication under high background. Theoretical simulation and experimental results show that the scheme can effectively suppress the background noise. The BER is as low as 1.27 × 10$^{-5}$ under the condition of 16 OAM states, 16 single-photon detectors (SPDs) coincidence, and the background noise of 10$^{7}$ counts per second. The BER performance is an order of magnitude better than the traditional PPM, and the channel capacity is increased by 12 times. This work provides an effective solution for deep space high-speed and low BER communication.

Color shift keying (CSK) communicates information by emitting light that hops instantaneously from one chromaticity to another. This rapid hopping is too fast to be noticeable by human vision, which senses only the time-averaged color, as the human vision’s critical fusion threshold and critical color fusion threshold both lie under 100 Hz. That is, color shift keying exploits the human vision’s temporal irresolution of instantaneous fluctuations across chromaticities. Left unexploited, up to now, is the intensity-fluctuation tolerance of the human vision, that humans do not perceive small intensity fluctuations of $\pm 4\%$. This newly proposed constellation-design scheme can raise by $49 \%$ the designed color-shift-keying constellation’s $d_{\min }^\circ$ (i.e. the maximally achievable “minimum distance” $d_{\min }$). Moreover, this enhanced $d_{\min }^\circ$ can be approximated as a simple mathematical expression in terms of the design parameters to serve as a rule-of-thumb to enhance color-shift-keying systems.

In dual-carrier (DC)-paired fiber optic long-reach transmission, which extends a point-to-multipoint coherent system to a cascaded reconfigurable optical add-drop multiplexing (ROADM) system, a narrow passband causes severe performance degradation of the edge subcarriers. In this study, we proposed two-dimensional frequency and polarization domain coding and multicarrier-frequency control for cascaded ROADM filtering of robust multicarrier transmissions at a single wavelength. The proposed coding scheme reduced fiber nonlinearity, polarization-dependent loss (PDL), and passband narrowing (PBN) simultaneously. The multicarrier frequency control algorithm, which feeds back the received signal quality at multiple points, reduced PBN owing to long-term laser frequency drifts. With the proposed coding, the Q value of 56 Gbit/s × DC-paired dual-polarized (DP)-4QAM and 112 Gbit/s × DC-paired DP-16QAM signals improved by 1 dB and 0.2 dB, respectively, compared to that when no coding was used. Furthermore, the long-term frequency control emulated through the emulation model of laser frequency drifts and the performance degradation of average Q value were less than 0.4 dB and 0.1 dB, respectively. To the best of our knowledge, no other study has evaluated point-to-multipoint transmission over ROADM-induced optical filtering systems that apply frequency and polarization domain coding and multicarrier frequency control for DC-paired DP-M-QAM signals. The proposed methods enable stable operation while suppressing the effects of PDL and PBN, which are problems in multicarrier transmission, in long-reach optical fiber transmission systems that integrate access and metro networks.

In visible light communication systems, the ability to suppress interference caused by other light sources is a major benefit towards performance improvements. Especially for large transmitter arrays or even multi-cell arrangements, the interference problem needs to be handled. In previous work, we have presented a liquid crystal display (LCD) used as an adaptive interference-suppression filter mounted in front of each photodetector. The display elements are switched on and off in such a way that light emitted by unwanted light sources ideally is blocked, but light emitted by desired light sources reaches the detector. The pattern generated by the LC display has strong impact on the system performance. In this paper, we propose combined precoding in conjunction with LCD-based interference suppression in order to increase the signal-to-interference-plus-noise ratio and to ensure user fairness in massive MIMO scenarios. The suggested precoding strategy uses a new heuristic optimization approach based on the Santa Claus problem on unrelated machines known from computer sciences, and employs only binary entries in the weighting matrix. Corresponding results are compared with a genetic evolutionary optimization strategy and with conventional zero-forcing precoding. Regarding performance evaluation, we perform numerical ray-tracing simulations and present a room-scale VLC testbed for experimental verification.

The long-existing problem of high positioning inaccuracy by triangulation in the far region in short-range vehicle-to-vehicle (V2V) communication environments is investigated in this study. The triangulation is achieved by sensing angle of arrival with two optical signal-direction sensors developed in our previous works. The measured signal-direction error can be categorized into two different kinds, namely systematic error and random error. The former is mainly incurred by imperfect calibration of hardware and misalignment of signal-direction sensors as well as multipath scattering of the signal. The latter is caused by random noises. Both of them lead to positioning error. By utilizing the specific characteristics pertaining to the triangulation measurements, it is possible to extract the target location and increase the accuracy with an optimization algorithm. With the aid of a general-purpose optimization subroutine together with two proper objective functions developed in this study, this work shows the viability of this idea. With the technique proposed in this paper, precise hardware calibration and careful alignment of signal-direction sensors are unnecessary. The problem of multipath scattering can also be mitigated. This work indicates a possible direction to treat the problem of high inaccuracy in noisy and ill-geometric environments in triangulation measurement. The abovementioned signal-direction sensors are currently constructed for infrared light. By changing the signal sources and detectors, they can also be fabricated for visible light. Such an alteration of transmission medium is beneficial for the currently emerging short-range visible-light intervehicular communications.

Free-space optics (FSO) is attracting renewed attention in the frame of 5G, which demands wireless backhauling technologies with extremely high data rates over distances up to a few kilometers. Link availability is a FSO well-known issue due to the large path loss expected during poor visibility, such as in case of fog. Several path loss models, based on the visibility, are assessed through an analytical approach. The simple relationship valid in the visible spectrum can be extended to the first optical window, while, at 1.550 μm, the effect of fog microphysics cannot be ignored. Fog is definitively less challenging in the mid-IR (10.6 µm) even at very low visibility values (less than 0.5 km), which was not evident from older studies. Lower and upper bounds for the extinction coefficient are calculated and substituted into the FSO link budget equation. The combined effect of the microphysics and of the accuracy of visibility measurements may result in large uncertainties on the maximum achievable path length. The latter is also influenced by the microclimate, as links are usually deployed in urban areas while visibility data are collected outside. Path lengths in city areas can double, for the same link availability.

We present a numerical analysis of noise performance improvement of the erbium-doped fiber amplifier by flatting the mode field intensity distribution within the interaction region of erbium ions. Three different refractive index types of erbium-doped fiber were used to study the effect of the pump mode intensity distribution on noise performance. The results show that taking advantage of the depressed refractive index erbium-doped fiber which has a flat mode distribution in the fiber core, the maximum reduction of nearly 0.53 dB for noise figure was achieved across the full C-band. Based on the center depressed refractive index profile, the noise figure was further decreased by about 0.1 dB by reducing the doping radius to 0.71 μm. Finally, maintaining the same gain level of ∼38.3 dB, a NF reduction of ∼0.63 dB was realized by introducing the center depressed refractive index profile and narrowing the erbium doping region. It is suggested that our investigation shows great potential for improving the noise performance of the erbium-doped fiber amplifier.

Optical channel decorrelation is a key aspect in designing a multi-user visible light communication system. As light-emitting diodes (LEDs) are commonly used for illumination and data transmission in such a system simultaneously, the interference between them is a major problem for suitable modulation schemes. We have proposed to position liquid-crystal (LC) matrix elements in front of the receivers in order to block unwanted light – so effectively acting as filter and reducing the interference. To do so, the appropriate pixels of the LC filter are switched either to a transmissive state or to an opaque state. The selection of the correct pixel patterns is a high-order optimization problem, which makes it challenging for classic algorithms. In this paper, we suggest applying a genetic optimization to find suitable pixel patterns. We combine this kind of receiver-side optimization with conventional convex precoding at the transmitter side in an iterative manner. The overall optimization goal is to maximize the sum rate over all users. To prove the effectiveness, we use a numeric ray-tracing simulation with different settings. Also we analyze the impact of the main parameters for the genetic algorithm.

Optical vortices (OVs) have received much attention in recent years and are widely explored in communication applications. The on-chip vortex emitter/modulator is preferred when it comes to photonic integration and chip-scale communication. However, the existing emitters/modulators suffer from either narrow bandwidths or extensibility problems, which actually degrade their ability to improve communication capacity. Here, we propose a metasurface OV emitter based on wave vector synthesis for vortex generation and modulation. A thorough theoretical analysis of the OV generation by two approximate methods was carried out, which suggests the superiority of Wave Vector method over Phase method. Afterwards, the wave vector manipulation implemented by Phase gradient metasurfaces was studied for controlling the topological charges of generated OVs. Finite element method simulations showed that, in the topological charge range of −6 to 6 and the wavelength range of 1.45 μm to 1.55 μm, the effective vortex emission efficiency of the proposed emitter is around 45%. The on-chip vortex generation and control by wave vector manipulation is a very promising technology for high-capacity inter-chip optical communication.

Electroabsorption modulated lasers (EMLs) is sensitive to the residual facet reflection (RFR), which can cause a low-frequency drop (LFD) in the modulation response. The LFD may result in output waveform distortion, especially for high-data rate and multi-level modulation. We verify by simulation and experiments that the effect of LFD on the EML performance and propose to use LFD as one of the key performance indices for qualifying EMLs in high-speed optical transceivers. LFD can be suppressed by incorporating partial-corrugated-grating (PCG) in the laser section. With PCG-DFB structure, the EMLs can maintain high single-mode yield (SMY) and an excellent quality factor even with a strong modulator reflection from EAM section. This provides the robustness in applying the PCG-DFB to achieve a flat intensity modulation response or to enhance the output waveform under large signal modulation. By designing the PCG-EML to have about 60% of grating region, it can reach >80% of SMY and improve the average Q-value from the reduced LFD in the modulation response. The PCG-EML with reduced LFD can enhance the eye-opening through reduced waveform undershoot and overshoot for transmitting 56-Gbaud/s or beyond PAM-4 signals.

Underwater wireless optical communications are knowing a high interest in recent years, with many military and civil applications. The design of efficient systems relies on powerful simulation tools. The most used simulation algorithms are based on a paper from 1989, written by Prahl and designed for light propagation through human tissue. This method relies on Monte Carlo Simulation, following the path of a photon in the participating media. As such, it is difficult to propose evolution or optimization. In addition, a large amount of photons are needed to obtain good results with low error, especially for turbid water. With Monte Carlo Integration method, there exist a lot of optimization techniques to reduce the variance. This paper proposes a mathematical formalization of the propagation of light in water or any other participating medium as an integral problem. Therefore, it opens the way for a large number of future optimizations. A very straightforward and simple variance reduction technique is proposed as an example. Our simulation results show that this new technique has a lower sample variance as expected, and thus better convergence rates.

In the evolution of optical networks, spectral efficiency (SE) enhancement has been the most cost-efficient and thus the main driver for capacity increase for decades. As a result, the development of optical transport systems has been focused on the $C$ - and $L$ -bands, where silica optical fiber exhibits the lowest attenuation, and erbium-doped fiber amplifiers provide an efficient solution to compensate for the optical loss. With a gradual maturity in the SE growth, however, the extension of the optical bandwidth beyond the $C + L$ -band is expected to play a significant role in future capacity upgrades of optical networks and, thus, attract increasing research interests. In this article, we discuss the merits and challenges of ultrawideband optical transport systems and networks beyond conventional bands. [ABSTRACT FROM AUTHOR]

High-speed, high-photoresponse, low-dark current photodetectors with monolithic integration compatibility on a single silicon chip have a vital role in fast optical communication. Schottky barrier junctions provide ultrahigh frequency bandwidth, which makes them suitable for a high-speed data rate optical transition system. In this article, we proposed the novel design of a fast silicon Schottky photodiode (SPD) with reduced dark current and enhanced photocurrent to produce an acceptable photo-to-dark current ratio. A differential Schottky diode is used in series with the designed SPD to reduce the final dark current delivered to the next signal conditioner electronics. A periodic plasmonic array is applied on the metallic Schottky contact layer to localize the illuminated light in the depletion region of Schottky contact and increase light absorption, which results in enhanced photocurrent. The simulation of the device shows a photo-to-dark current ratio of ${2.7} \times {10}^{{4}}$ at room temperature and 1-V biasing voltage, which is much greater than the reported data. Moreover, the design results in a complementary metal–oxide–semiconductor (CMOS)-compatible photodiode (PD) with 28-time reduced dark current and 3.3-time photocurrent enhancement with an ultrafast impulse response of 3 ps and a response time of about 18 ps that are attractive for ultra-fast data communication applications in 850-nm region. [ABSTRACT FROM AUTHOR]

For enhancing channel capacity and spectrum efficiency, vortex beam carrying orbital angular momentum (OAM) has been ardently investigated in optical communication. However, the crosstalk among OAM modes induced by mode coupling has seriously hindered the development of vortex beam communication (OBC). A novel concept of mitigating crosstalk within an OAM-mode group for a short-haul optical fiber communication (OFC) is proposed by reversing the transmission matrix. To support more OAM modes propagation, a step-index OAM fiber with ring-shaped profile is also designed and fabricated, which can support 6 vector/OAM modes propagation. A proof-of-concept experiment is performed for verifying the feasibility of the proposed concept, where a quadrature-phase-shift-keying (QPSK) signal is utilized as an excitation signal. Bit-error-ratio (BER) and constellation diagram (CD) are employed to evaluate the performance of the proposed system. The captured intensity profiles and interference diagrams demonstrate that the propagated four OAM beams within the 2rd OAM-mode group are successfully received. The measured BER and CD illustrate that the crosstalk within the mode group can be significantly mitigated. Further, although the data speed or fiber length is increased, the system performance using the proposed concept is still superior to the case without increasing fiber length or data speed when the proposed concept is missed. Therefore, the proposed concept is effective for mitigating the crosstalk and can be then used for a short-haul OAM-based OFC. [ABSTRACT FROM AUTHOR]

In visible light communication systems, the ability to suppress interference caused by other light sources is a major benefit towards performance improvements. Especially for large transmitter arrays or even multi-cell arrangements, the interference problem needs to be handled. In previous work, we have presented a liquid crystal display (LCD) used as an adaptive interference-suppression filter mounted in front of each photodetector. The display elements are switched on and off in such a way that light emitted by unwanted light sources ideally is blocked, but light emitted by desired light sources reaches the detector. The pattern generated by the LC display has strong impact on the system performance. In this paper, we propose combined precoding in conjunction with LCD-based interference suppression in order to increase the signal-to-interference-plus-noise ratio and to ensure user fairness in massive MIMO scenarios. The suggested precoding strategy uses a new heuristic optimization approach based on the Santa Claus problem on unrelated machines known from computer sciences, and employs only binary entries in the weighting matrix. Corresponding results are compared with a genetic evolutionary optimization strategy and with conventional zero-forcing precoding. Regarding performance evaluation, we perform numerical ray-tracing simulations and present a room-scale VLC testbed for experimental verification. [ABSTRACT FROM AUTHOR]

Underwater optical wireless communications (UOWCs) are promising and potential wireless carriers to envisage underwater phenomenal activities for various applications towards the futuristic 5G and beyond (5GB) wireless systems. The main challenges to deploy underwater applications are the physicochemical properties and strong turbulence channel conditions. In this regard, the end-to-end (E2E) performance analysis of a dual-hop mixed FSO/UVLC system under the intensity modulation/direct detection (IM/DD) technique in consideration of pulse amplitude modulation (PAM) scheme is investigated. Throughout this study, to tackle the issues of moderate-to-strong turbulence channel conditions, this work deploys the Gamma-Gamma (GG) distribution fading model and the links are designed by unifying plane wave models in the corresponding links, respectively. This investigation outperforms higher achievable data rate with minimal delay response and enhance network connectivity in real-time monitoring scenarios as compared with the traditional underwater wireless communication technologies. In more contrast, the probability distribution function (PDF), cumulative distribution function (CDF), and closed-form expression of the system are derived and presented in terms of Meijer-G function as well as Extended Generalized Bivariate Meijer-G Function (EGBMGF). The significant E2E performance metrics are obtained by employing the decode-and-forward (DF) relay protocol in hostile channel conditions. In aggregating this work, we combine the analytical expressions that present an efficient tool to depict the impact of channel parameters on the system. The simulation results are plausible of the system performance metrics as average BER (ABER) and outage probability $(P_{out})$ in the presence of pointing and without pointing error events. Finally, in this work, we use the Monte-Carlo approach for the best fitting curves and validate the numerical expression yields simulation results. [ABSTRACT FROM AUTHOR]

To achieve on-chip lasers for Si-photonics, monolithic integration using selective epitaxy is a favorable option due to the unique defect engineering and resultant bufferless structure. Among the intensively investigated selective epitaxy methods, lateral aspect ratio trapping generates III-V devices in the same plane as the Si layer enabling efficient coupling with Si waveguides. Here, we demonstrate telecom InGaAs/InP quantum well lasers selectively grown on commercial silicon-on-insulator substrates with an in-plane structure using a simple structural design and growth scheme without any regrowth steps. Leveraging on the lateral aspect ratio trapping technique, uniform quantum wells with high crystalline quality were obtained and shown by comprehensive material characterizations. Room temperature pulsed lasing was achieved on the fabricated micro-ring lasers with a low threshold of 20 μJ/cm2. The results present a crucial step towards electrically pumped lasers at telecom band as well as fully integrated Si-photonics. [ABSTRACT FROM AUTHOR]

AlGaN-based UV LEDs have proven to have broad applications in many fields including sterilization, disinfection, purification, and phototherapy, but the performance still needs to be further improved. Benefiting from advantages such as better current spreading, lower self-heating effect and higher light extraction efficiency achieved by the reduced LED size, UV micro-LEDs are expected to improve quantum efficiency and thus can expand more potential applications. In this article, performance enhancement techniques and applications of UV micro-LEDs will be reviewed, providing an outlook for further development of AlGaN-based UV-LEDs. The micro-LED format offers possibilities to address fundamental issues that UV LEDs confront in general, but also demonstrates specific characteristics and performance advantages opening up new areas of application including high-speed optical communication, time-resolved fluorescence lifetime measurement, optical pumping, direct writing and charge management. [ABSTRACT FROM AUTHOR]

In dual-carrier (DC)-paired fiber optic long-reach transmission, which extends a point-to-multipoint coherent system to a cascaded reconfigurable optical add-drop multiplexing (ROADM) system, a narrow passband causes severe performance degradation of the edge subcarriers. In this study, we proposed two-dimensional frequency and polarization domain coding and multicarrier-frequency control for cascaded ROADM filtering of robust multicarrier transmissions at a single wavelength. The proposed coding scheme reduced fiber nonlinearity, polarization-dependent loss (PDL), and passband narrowing (PBN) simultaneously. The multicarrier frequency control algorithm, which feeds back the received signal quality at multiple points, reduced PBN owing to long-term laser frequency drifts. With the proposed coding, the Q value of 56 Gbit/s × DC-paired dual-polarized (DP)-4QAM and 112 Gbit/s × DC-paired DP-16QAM signals improved by 1 dB and 0.2 dB, respectively, compared to that when no coding was used. Furthermore, the long-term frequency control emulated through the emulation model of laser frequency drifts and the performance degradation of average Q value were less than 0.4 dB and 0.1 dB, respectively. To the best of our knowledge, no other study has evaluated point-to-multipoint transmission over ROADM-induced optical filtering systems that apply frequency and polarization domain coding and multicarrier frequency control for DC-paired DP-M-QAM signals. The proposed methods enable stable operation while suppressing the effects of PDL and PBN, which are problems in multicarrier transmission, in long-reach optical fiber transmission systems that integrate access and metro networks. [ABSTRACT FROM AUTHOR]

A free-space communication scheme is proposed based on orbital angular momentum (OAM) states and pulse position modulation (PPM). It can not only improve the channel capacity, but also be suitable for single-photon detection to realize the communication in the case of ultra-weak light. A parallel coding method is proposed to realize high-capacity and low bit error rate (BER) communication under high background. Theoretical simulation and experimental results show that the scheme can effectively suppress the background noise. The BER is as low as 1.27 × 10 $^{-5}$ under the condition of 16 OAM states, 16 single-photon detectors (SPDs) coincidence, and the background noise of 10 $^{7}$ counts per second. The BER performance is an order of magnitude better than the traditional PPM, and the channel capacity is increased by 12 times. This work provides an effective solution for deep space high-speed and low BER communication. [ABSTRACT FROM AUTHOR]

Multi-band optical fiber transmission is generally proposed for capacity upgrades in optical transport networks. To comprehensively assess the potential of multi-band transmission, key metrics such as the potential capacity increase, energy consumption, and the number of required interfaces must be evaluated for different transmission scenarios. We consider progressive spectral exploitation, starting from the C-band only and up to C+L+S+U-band transmission, for both transparent and translucent solutions that exploit optical signal regeneration. By considering accurate state-of-the-art physical layer models, we derive a networking performance metric that enables the comparison of different solutions in terms of capacity allocation and energy consumption. For a translucent network design, different regenerator placement algorithms are compared, with the aim of minimizing energy consumption. The proposed network-wide numerical analysis shows that, for spectral occupations exceeding the C+L-band, translucent solutions can significantly increase network capacity, while leading to a similar energy consumption per transmitted bit as in the transparent design case, but they require the deployment of additional line interfaces. Significantly, these results provide evidence that the transparent exploitation of an additional transmission band produces a capacity increment that is at least comparable to that of a translucent solution based on already-in-use bands. Since this is attained at the expense of fewer line interfaces, it is a key finding suggesting that extending the number of bands supported is a cost-effective approach to scaling the capacity of existing fiber infrastructures. [ABSTRACT FROM AUTHOR]

Transceiver imperfections become the primary source of impairment as baud rate and modulation order grow in advanced optical coherent communications. Thus, transceiver imperfections, both linear and nonlinear, need to be appropriately characterized, measured, and specified. Treatments for linear imperfections are relatively mature. This study reviews the transceiver's linear imperfection modeling, characterization, and measurement technologies. In practical applications, in-field measurement using the transceiver and a few low speed additional devices is preferred. In the case of nonlinear imperfections, the situation is complex. One important task is to estimate the nonlinear system performance from the nonlinear characteristics of the devices. In this study, we attempt to establish a connection between them by examining different technologies. Although the orthogonal component has a good correlation with nonlinear system performance, its measurement is prohibitively complex. In the nonlinear noise to power ratio measurement, a certain frequency component of the input signal is notched, and the re-growth component at the notch frequency is measured at the nonlinear device output. The ratio between the re-growth component power and the output signal power is the noise to power ratio. While this method is easy to carry out, it does not correctly estimate nonlinear impairment in general. The reason for this is that the signal incurs different nonlinear responses in two conditions, i.e., with or without a notch. This method accurately estimates the nonlinear impairment in some special but useful cases, such as Gaussian input signals and nonlinear systems whose dominant nonlinear term is the even order term. [ABSTRACT FROM AUTHOR]

We analyze the potential suitability or strength of arguments for the application of multicore optical fibers in high capacity submarine cable systems via transmission and techno-economic modeling. We consider hypothetical multicore fibers (MCFs) with 2–4 weakly coupled cores and compare capacity and cost/bit against conventional single-core fibers (SCFs). The analysis is performed in the context of a trans-Atlantic link length system and we evaluate the relative fiber performance with three different, but related, system design approaches. Two SCF coating diameters are assessed in terms of how this parameter affects the cost/bit through fiber density in submarine cables and resulting cable cost. We find that MCFs may enable higher cable capacity when fiber pair limits are imposed, but likely not at lower cost/bit unless optimistic and best case assumptions are made with respect to MCF relative fiber cost. We also find that reduced diameter SCFs can deliver much of the density and cable cost savings that motivates interest in MCF without the challenges of a new eco-system as required by MCF. However, MCF may enable the design of the largest cable capacities such as 1 Pb/s or more that might not be attainable with SCFs without significant cable changes. [ABSTRACT FROM AUTHOR]

Wedemonstrate a high-speed light fidelity (LiFi) communication system deploying ultra-high brightness laser-based white light illumination sources in a surface mount device (SMD) packaging platform. The LiFi transmitter SMD source provides 450 lumens of white light output with a brightness of 1000 cd/mm2 in a dual wavelength configuration comprising blue and infrared (IR) emitting laser diodes within the SMD. First, we present high-speed data transmission beyond 25 Gbit/s over a 3 meter channel distance with the combined data rates from the blue and IR lasers in the single SMD. Next, we present a 2.8 Gbit/s data rate with the dual wavelength laser SMD light source using a side-emissive fiber as the transmitter. This work proves the viability of LiFi systems deploying laser-based white light sources operating at very high data rates with the visible light and communication signal delivered in conventional free-space transmission architectures along with novel configurations using side-emitting fibers. [ABSTRACT FROM AUTHOR]

We report 108 Gb/s Duo-Binary PAM-8 (DB-PAM-8) transmission using direct-detection aided by Volterra equalizer. The bit-error-rate (BER) performance of the DB-PAM-8 signal has been evaluated through error counting by varying the bit rate, received optical power, and reach. The experimental results suggest that up to 108, 102, and 84 Gb/s signals can be propagated in back-to-back (B2B), and over 10, and 20 km of standard single-mode fiber (SMF), respectively, below the low-density parity-check forward error correction (LDPC-FEC) threshold. Furthermore, we derive a probabilistic model of the BER for multilevel DB-PAM-M signaling that is verified using our measurements. The model of this important performance metric turned out to be accurate enough, especially at low BER values, where its use is more convenient.

In this work, the design and fabrication of a Cu/p-Si/Pt Schottky photodetector with a simple structure is investigated. The mechanism of electron flow is explained using internal photoemission theory, which is further applied to make the fabricated devices reach a high responsivity of 0.542 mA/W at 0–V bias. The investigation revealed that the rapid thermal annealing process could significantly influence the device characteristics. Variations in Schottky barrier height and series resistance of the photodetectors were also analyzed and correlated with the device responsivity. With proper conditions to improve the Pt/Si ohmic contact, the device performance can be enhanced.

Suitable composable data center networks (DCNs) are essential to support the disaggregation of compute components in highly efficient next generation data centers (DCs). However, designing such composable DCNs can be challenging. A composable DCN that adopts a full mesh backplane between disaggregated compute components within a rack and employs dedicated interfaces on each point-to-point link is wasteful and expensive. In this paper, we propose and describe two (i.e., electrical, and electrical-optical) variants of a network for composable DC (NetCoD). NetCoD adopts a targeted design to reduce the number of transceivers required when a mesh physical backplane is deployed between disaggregated compute components in the same rack. The targeted design leverages optical communication techniques and components to achieve this with minimal or no network performance degradation. We formulate a mixed integer linear programming (MILP) model to evaluate the performance of both variants of NetCoD in rack-scale composable DCs that implement different forms of disaggregation. The electrical-optical variant of NetCoD achieves similar performance as a reference network while utilizing fewer transceivers per compute node. The targeted adoption of optical technologies by both variants of NetCoD achieves greater (4 - 5 times greater) utilization of available network throughput than the reference network which implements a generic design. Under the various forms of disaggregation considered, both variants of NetCoD achieve near-optimal compute energy efficiency in the composable DC while satisfying both compute and network constraints. This is because marginal concession of optimal compute energy efficiency is often required to achieve overall optimal energy efficiency in composable DCs.

Visible light communication (VLC) has emerged as a promising technology for wireless communication as it offers higher data rates and secure data transmission along with providing indoor illumination. However, VLC is restricted by the line of sight (LoS) nature of the optical channel that consequently results in light path blockages. Therefore, an effective solution would be to combine VLC with a radio frequency (RF) system to form a hybrid VLC/RF network that would take into account the preferences of an end-user with the practicality of implementation. In such networks, an efficient vertical handover (VHO) technique is the most critical element as it ensures a seamless transition between the two networks. In this work, we propose a vertical handover technique that utilizes the user’s location information to make a handover decision. We found that the frequency of light path blockages increases with the increasing number of users in a confined space, resulting in significant performance deterioration. This additional information is then utilized so that the VHO algorithm effectively selects the most feasible network. The proposed algorithm has been tested against the immediate vertical handover algorithm (I-VHO) and the dwell vertical handover algorithm (D-VHO) with two different dwell times. The average number of handovers, quality of experience (QoE), and packet loss have been set as performance metrics. We show from several simulation scenarios that the proposed method results in a fewer number of handovers while maintaining higher QoE and lower packet loss.

The charge-coupled device-based Shaker–Hartmann wavefront sensor has been widely used to compensate for wavefront distortion in coherent optical communication. However, the influence of strong background light causes large errors when restoring the signal light wavefront. This study proposes a wavefront recovery method by combining a detector array with an optical nutation of the local oscillator. By adding a small rotation angle to the local oscillator, the wavefront slope is estimated according to the output of the change law of the mixed signal and the local oscillator light, and the multi-order Zernike mode is used to reconstruct the signal light wavefront. Simulation studies show that the root-mean-square value (RMS) and the coherence efficiency of the signal light wavefront were significantly optimized. Under direct sunlight, the estimation compensation error of the wavefront slope reached 0.008. Under strong turbulence, the mixing efficiency was increased by nearly 97.5 times, and the RMS was reduced to the original value of 0.048 The designed system has good background light compensation and wavefront recovery capabilities and is suitable for coherent optical communication under direct sunlight.

Optical Wireless Communication (OWC) refers to transmission in unguided propagation media through the use of optical carriers, i.e., visible, Infrared (IR), and Ultraviolet (UV) bands. In this paper, we focus on indoor Visible Light Communication (VLC)-based Medical Body Sensor Networks (MBSNs) which allow the Light Emitting Diodes (LEDs) to communicate between on-body sensors/subdermal implants and on-body central hubs/monitoring devices while also serving as a luminaire. Since the Quality-of-Service (QoS) of the communication systems depends heavily on realistic channel modeling and characterization, this paper aims at presenting an up-to-date survey of works on channel modeling activities for MBSNs. The first part reviews existing IR-based MBSNs channel models based on which VLC channel models are derived. The second part of this review provides details on existing VLC-based MBSNs channel models according to the mobility of the MBSNs on the patient’s body. We also present a realistic channel modeling approach called site-specific ray tracing that considers the skin tissue for the MBSNs channel modeling for realistic hospital scenarios.

A novel photonic-assisted crosstalk cancellation approach for twin-single sideband (twin-SSB) radio over fiber (RoF) transmission is proposed by utilizing a dual polarization-dual derive Mach Zehnder modulator. By properly adjusting polarization controller in the transmitter, the crosstalk between the right side band signal and left side band signal of twin-SSB signal can be cancelled, which indicates the background noise of twin-SSB signal can be much more mitigated. A theoretical analysis is presented and validated by a simulation. The results show that crosstalk suppression ratios of about 42 dB for single frequency signal and about 30 dB for broadband signal are achieved in our proposed twin-SSB RoF system. Compared with the conventional twin-SSB RoF system, the crosstalk suppression ratio of proposed system is increased by about 22 dB for single frequency signal or 10 dB for broadband signal and the proposed system can reduce the error vector magnitude by about 10% and 11% in back-to-back case and 30 km single mode fiber transmission. [ABSTRACT FROM AUTHOR]

Photonic frequency converters are optical systems used to translate data at RF or microwave frequencies to new carrier frequencies for processing or retransmission. The conventional architecture includes RF to optical conversion using Mach-Zehnder modulators, inherently nonlinear devices. The mixing process occurs in photodiodes acting as optical to electrical square law detectors. Therefore, the frequency converter performance metrics are governed by complex nonlinear functions that vary widely with power levels, bias points and specific configurations that require lengthy simulations to assess; this makes understanding the performance impacts of various architectures and components difficult. This work provides analytic expressions for the primary metrics of photonic frequency converters, validated thru comparison with simulations and experiment. The derived equations are useful for the design of photonic frequency converting links, whether integrated or non-integrated. The nominal architecture comprises two Mach-Zehnder modulators in parallel within a larger interferometer. Equations are presented that describe the performance of single drive, dual drive, single detection, and balanced detection implementations. Lastly, this work provides a means of assessing the component metrics necessary for high performance photonic frequency converters as well as the upper limits on system performance. [ABSTRACT FROM AUTHOR]

We derive improved and easily computable upper bounds on the capacity of the discrete-time Poisson channel under an average-power constraint and an arbitrary constant dark current term. This is accomplished by combining a general convex duality framework with a modified version of the digamma distribution considered in previous work of the authors. [ABSTRACT FROM AUTHOR]

The establishment of laser communication relies on high probability acquisition and then stably track with a high precision. However, the limitation of laser divergence and the interference may lead to unstable acquisition and tracking. In this paper, we proposed a novel beaconless acquisition approach to achieve the coverage of a large uncertainty cone with a narrow communication beam, using a composite spiral scanning method based on fast steering mirror and servomechanism system. The modeling and an analytical expression of sub-regions searching in conjunction with spiral skip overlap are described to realize the effective coverage. A detailed analysis of optimal matching for acquisition parameter is presented to quantify the design result. For the tracking procedure, an experiment is presented to investigate the performance of servo system with the combination of coarse-fine tracking consideration, and iteration learning control scheme in additional to the traditional controller based on periodic external disturbance is introduced for dynamic target tracking system. The iterative control law is taken into account to adjust deviation value and further improve the tracking performance. The obtained tracking accuracy is within ±2μrad which demonstrates the effectiveness the control characteristic in the presence of laser fluctuation.

The perfect optical vortex (POV) beam with the characteristic of the topological charge (TC)- independence radial profile is first considered to improve the spatial-mode-based underwater wireless optical communication (UWOC) performance. The evolution model of POV beams is proposed on the basis of Rytov approximation and the mutual coherence function to explore the impact of wave parameters on the orbital angular momentum (OAM) detection probability spectrum. The instantaneous crosstalk coefficients describing the POV-based UWOC systems are simulated by the multiple phase screen method with the signal coefficient modeled by a mixture Gamma-Gamma and normal distribution. A discrete and memoryless channel and a set of noncollaborative channels are modeled to obtain the capacity performances of the spatial-mode modulating and multiplexing systems, respectively. Results show the transmission quality of POV beams in oceanic turbulence is most related to the beam radius, while it is nearly free from the wavelength, TC, and radius-thickness ratio. Enjoying the advantage of the TC-independence beam radius, POV beams-based UWOC systems could achieve better bit error rate and capacity performances than the widely used Laguerre-Gauss beams when the higher-order TCs are adopted to modulate and multiplex information, which also confirms the superiority of POV beams in spatial-mode-based UWOC systems. [ABSTRACT FROM AUTHOR]

Precise position information is considered as the main enabler for the implementation of smart manufacturing systems in Industry 4.0. In this article, a time-of-flight based indoor positioning system for LiFi is presented based on the ITU - T recommendation G.9991. Our objective is to realize positioning by reusing already existing functions of the LiFi communication protocol which has been adopted by several vendors. Our positioning algorithm is based on a coarse timing measurement using the frame synchronization preamble, similar to the ranging, and a fine timing measurement using the channel estimation preamble. This approach works in various environments and it requires neither knowledge about the beam characteristics of transmitters and receivers nor the use of fingerprinting. The new algorithm is validated through both, simulations and experiments. Results in an $\text{1}\;{\rm{m}} \times \text{1}\;{\rm{m}} \times \text{2}\;{\rm{m}}$ area indicate that G.9991-based positioning can reach an average distance error of a few centimeters in three dimension. Considering the common use of lighting in indoor environments and the availability of a mature optical wireless communication system using G.9991, the proposed LiFi positioning is a promising new feature that can be added to the existing protocols and enhance the capabilities of smart lighting systems further for the benefit of Industry 4.0. [ABSTRACT FROM AUTHOR]

The charge-coupled device-based Shaker–Hartmann wavefront sensor has been widely used to compensate for wavefront distortion in coherent optical communication. However, the influence of strong background light causes large errors when restoring the signal light wavefront. This study proposes a wavefront recovery method by combining a detector array with an optical nutation of the local oscillator. By adding a small rotation angle to the local oscillator, the wavefront slope is estimated according to the output of the change law of the mixed signal and the local oscillator light, and the multi-order Zernike mode is used to reconstruct the signal light wavefront. Simulation studies show that the root-mean-square value (RMS) and the coherence efficiency of the signal light wavefront were significantly optimized. Under direct sunlight, the estimation compensation error of the wavefront slope reached 0.008. Under strong turbulence, the mixing efficiency was increased by nearly 97.5 times, and the RMS was reduced to the original value of 0.048 The designed system has good background light compensation and wavefront recovery capabilities and is suitable for coherent optical communication under direct sunlight. [ABSTRACT FROM AUTHOR]